SSC-328. FRACTURE CONTROL FOR. FIXED OFFSHORE STRUCTURES.
Thisdocument hasbeen ... J. E. GAGORIK. MR. A. E.. ENGLE. MR. ...... p~acticesg
This final report has been modified to reflect the questions, comments, and ...
SSC-328
FRACTURE CONTROL FOR FIXED OFFSHORE STRUCTURES
This document has been approved for publicrelease and sale;its distribution isunlimited
SHIP STRUCTURE COMMITTEE 1985
#
SHIP STRUCTURE COMIITkiESHIP STRUCTURE CUIIMITTEEis constituted to prosecute a research program to improve tk hull 8tructure$ of BhipB ah otkr marina structures ●n extension of krmwledqe ~rtaining to design, materialB and methds of construction. Mr. T. M. Proes Asfiwiate Administrator fok Shipbuilding, Operation & Weearch Uaritime Mmin18tration
RAW C. T. Lusk, Jr., USCG (diaiman) Chga:4t;ffice of Nerctmnt ~“rine tl. s.
mast
GJard
Beadquart&rS
Mr. J. B. Gregory ~ief, Technology AsBemment & Research Branch 14im3ralsManagement Semite
Mr. P. !!.Palenno Sxecuti~ Director Ship &sign k Integration Directorate
Naval sea syst~s
~nd Mr. T. W. Allen Engineering Officer Military Sealift mmmati
Mr. W. M. Barman vice President American Bureau of Shippirq
U. S. ~a6t
CDR D. B. Aderwn,
~ard
(Secretary)
SHIP STRUCTURE SUBC-ITTEE The SHIP STRUCTURE SUBCMMTTEE acts for the Ship Structure bmmittee on technical matters by provfdirq teshnical c-rdination for tk determination of goals ard objectives of the prqram, and by evaluating and interpreting the ad operation. remits in term of structural design, construction D. S. COAST GUARD
MILITARY SEALIFT
CAPT A. E. OENN CAFT J. R. WALLACE MR. J. S. SPENCER MR. R. E. WILLIAMS
NR. D. STEIN MR. T. W. CNAMN MR. A. A!CTERMEYER MR. A. B. STAVOVY
NAVhL SEA SYSTEWS ~
AMERICAN BUREAU OF SHIPPING
MR. J. B. O’BRIEN (CHAIRMAN) CDR R. BUBECK HR. J. E. GAGORIK MR. A. E. ENGLE MR. S. G. ARNTSON (COTR) MR. G. W~DS (Cm)
DR. D. LIU MR. I. L. STERN MR. B. NADALIN
C-D
MINERM.S MANAGEMENT SERVICE MR. R. GIANGERELLI MR. R. C. E. SMITE
~ITIPIE MR. MR. DR. us.
AMINISTRATION
INTERNATIONAL SHIP STRUCTURES CONGRESS HR. S. G. STIANSEN - LIAISON
F. SEIBOLD la.0. BANMER w. ii.MACLEAM H. w. TOlmA
AMERICAN
IRON
k
STEEL INSTITUTE
NR. J. J. SC~IDT
- LIAISON
MkTIOiWiLASSDEXY 0? SCI~CES CCsmlTTEE ON MARINE sTRlmTUREs NM. A. DUDLEY EAFF - LIAISON MR. R. W. R- LIAISON
DR.
# sOCIETY OF WAVAL ARCHITECTS MARINE ENGINEERS
STATS UNIVERSITY OF NY MARITIMR COLLEGE
&
W. R.
KUTTER -
U.6. COAST GuARD
LIAISON
ACAQ~Y
LT J. TUTTLS - LIAISON ilR.N. O. BANNER - LIAISON MR. F. SELLARS - LIAISON
U.S. NAVAL
WELDING RESEARCH COONCIL
DR.
DR. G. U.
U.S. MSRCILA?rT WARINS ACADEMY
OYMR
-
LIAISON
R.
ACAD~Y
B~mACSARYyA
- LIAISON
DS. C. B. KIM - LIAISON
F\
w-. .:~..“‘ COMMITTEE ON MARINE STRUCTURES Commission on Engineering and Technical Systems National Academy of Sciences - National Research Council
●
The COMMITTEE ON hARINE STRUCTURES has technical cognizance of the program. interagency Ship Structure Committee’s research
!
J {
\ Q \
Mr. A. DudleY Haff, Chairman, Annapolis,MD Prof. Alfredo H.-S. Ang, University of Illinois, Urbana, IL Mr. Griff C. Lee, Griff C. Lee, Inc., New Orleans, LA Mr. Peter W. Marshall, Shell Oil Company, Houston, TX Mrs. Margaret D. Ochi, Gainesville, FL Prof. David L. Olson, Colorado School of Mines, Goldon, CO hr. Richard W. Rumke, Executive Secretary, Committee on Marine Structures
The LOADS ADVISORY GROUP
Mr. Peter k. Marshall, Chairman, Shell Oil Company, Houston, TX Prof. kobert F. Beck, University of Michigan, Ann Arbor, MI Prof Colin B. Brown, University of Washington, Seattle, WA Mr. John C. Estes, Zapata Offshore Company, Houston, TX Mr. Joseph P. Fischer, American Steamship Company, Buffalo, NY Prof. Joseph P. Murtha, University of Illinois, Urbana, IL Mr. Robert J. vom Saal, Bethlehem Steel Corporation, Sparrows Point, MD Prof. Paul El.Wirsching, University of Arizona, Tucson, AZ
The MATERIALS ADVISORY GROUP
Prof. David L. Olson, Chairman, Colorado School of Mines Mrs. Nancy C. Cole, Combustion Engineering, Inc., Chattanooga, TN Prof. George T. hahn, Vanderbilt University, Nashville, TN Prof. William T. hartt, Florida Atlantic University, Boca Raton, FL h . Santiago Ibarra, Gulf Science and Technology Company, Pittsburgh, PA Mr. John J. Schmiat, Lukens, Inc., Coatesville, PA Mr. Robert E. Sommers, Welding Consultant, Hellertown, PA Dr. Nicholas Zettlemoyer, Exxon Production Research Company, Houston, TX
SHIP STRUCTURE COhMITTEE PUBLICATIONS .
SSC-317
Determination of Strain Rates in Ship Hull Structures: A Feasibility Study by J. G. Giannotti and K. A. Stambaugh, 1984
SSC-319
Development of A Plan to Obtain In-Service Still-Water Bending Moment Information for Statistical Characterization by J. W. Boylston and K. A. Stambaugh, 1984
SSC-321
Survey of Experience Using Reinforced Concrete in Floating Marine Structures by O.H. Burnside and D.J. Pomerening, 1984
SSC-322
of Major Uncertainties Associated Analysis and Assessment With Ship Hull Ultimate Failure by P. Kaplan, M. Benatar, J. 13entson and T.A. Achtarides, 1984
SSC-323
Updating of Fillet Weld Strength Parameters for Commercial Shipbuilding by R.P. Krumpen, Jr., and C.R. Jordan, 1984
SS(+324
Analytical Techniques ior Predicting Grounded Ship Response by J.D. Porricelli and J.H. Boyd, 1984
SSC-325
Correlation of Theoretical and Measured Hydrodynamic Pressures for the SL–7 Containership and the Great Lakes Bulk Carrier S. J. Cort by H.H. Chen, Y.S. Shin & 1.S. Aulakh, 1984
SSC–326
Long-Term Corrosion Fatigue of Welded Marine Steels by O.H. Burnside, S.J. Hudak, E. Oelkers, K. Chan, and R.J. Dexter, 1984
SSC-327
Investigation of Steels for Improved Weldability in Ship Construction by L.J. Cuady, J-S. Lally and L.F. Porter 1985
SSC-328
Fracture Control for Fixed Offshore Structures by K. Ortiz, J.h. Thomas and S.D. Adams 1985
SSC-329
Ice Loads and Ship Response to Ice and H. hlount, 1985
SSC-330
Practical Guide for Shipboard Vibration Control by G.P. Antonides and W.A. hood, 1985
None
Ship Structure Committee Publications - A Special Bibliography, Ail-A140339
by
P.M. Besuner,
J.k. St. John,
C.
DaleY,
E.F. Noonan
Member Agencies:
). 6
$L
~~ v-
United States Coast Guard Naval Sea Systems Command Maritime Administration American Bureau of Shipping Military Sealift Command Minerals Management Service
&
Adde.orresponenceto: Secretary, Ship Structure Committee US, Coast Guard Headauarters. (G-M/TP 13) -, Washington, D.C. 20593’ ‘ (202) 426-2197
, “’
ship
structure Committee An Interagency Advisory Committee Dedicated to the Improvement of Marine Structures
SR-1288
The authors of this report reviewed numerous documents and discussed the fracture control practices which were in use at the time of their interviews with engineers involved in designing fixed offshore platforms. Based on the aggregate of the information then available to them, the authors summarized the state-of-the-art in material selection, design, con– struct ion and operation. Using their own engineering judgement, they then recommended research in those same general categories. Thus , this report represents the authors’ opinions based on the information gathered at a specific “point in time. ” As our knowledge of the fracture problem continues to increase, we will cent inue to advance the state-of-the-art in preventing detrimental fractures. To those entering the fixed offshore platform fracture control discipline, this report will serve as a sound basis from which to begin.
CL Chairman, Ship” Structure Committee
48v334
. o
2mw’”l-
m Technical ReportDocumentationPage 1. Ropott
No.
2.
Govwtnmont
Accession
No.
3.
Rocipiont”s
5.
Ropmrt
b.
Performing
Catalog
No.
SSC-328 4. Titl-
ond
Subtitl=
Doto
December”1984 Fracture Control for Fixed Offshore Structures
Or9anizotien
Cod.
SR-1288 E.PorforminO 7.
OrBonizotion
Ropart
No.
,_
Author(s)
‘FaAA-83-6-2
P.M. Besuner, K. Ortiz, J,M. Thomas, S.D. Adams 9.Porlerming
Organization
Nnmomd
Address
Failure Analysis Associates 2225 East Bayshore Road Palo Alto, California 943o3 12. Spo. xoring
AgoncY
Nom*
end
Addr*ss
Ship Structures Committee National Research Council 2101 Constitution Ave., NW Washington, D.C. 20412
10.
Wotk
Unit
11.
Contract
No.
(TRAIS)
or Grant
No.
DTCG23-82-C-20015 la.Typo of Ropart and Pariod Cevwt*d Final Report April 1982 – July 1983 14.
Sponsoring
Agency
Cod*
G-M 15.
SuPPlomenturYNot8~
16. Abstroct
Literature and telephone surveys were conducted to determine the current status of fracture control as practiced by U.S. designers, builders, and operators. From this, recommendations are made for strengthening industry practices: promising areas for research are identified and prioritized, as are areas where cost–effective improvements could be made within existing technology. A fracture control checklist is also provided as an example of an unstructured, yet responsive, fracture control plan.
17.
K-y
I 18.
Words
Crack Defect Failure Failure Mode Fracture 19.
SecuritY
Classic.
(of
Di~tribwtion
Fracture Control Fixed Offshore Structur~!s Offshore Offshore Structures this
2Q.
ropnrt)
SccuritY
Classic.
Statcm.nt
Document is available to the public through the National Technical Information Service, Springfield, Virginia 22151. 21.
(ofthi~pn~*)
No.
of Pages
22.
I
I
Unclassified Form DOT F 1700.7 (S-72)
486-334
I
Unclassified
Reproduction
of
completed
03
poge
authotizod
#5447f4
209
I
Price
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“--Table of Content”s
I.
II.
INTRODUCTION .............*****...* ......................*.******== ●
1
SUMMARY OF CURRENT PRACTICES AND TRENDS FOR FRACTURE CONTROL OF FIXED STEEL OFFSHORE STRUCTURES
1.0
Introduction 1.1 1.2 1.3 1.4 1.5
2.0
2.5
4.0
●
●
●
●
●
●
●
● ●
●
Problem . . . . . . . . . . . . . . . . . . . . . . . ...***.
●
***.*
●
................................ ........... Introduction . . . . . . . . . . . . . ...0. . . . . . . . . . . . Sources of Crack Initiation **.****.* Typical Examples . . . . . . . . . . . . . . . . . . . . . . . . ...*.* Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..tm.. httaaat References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ●
●
Material
Practices:
Selection
.......................
4 4 4 6 10 12 13 13 13 14 16 17
18
= .. .. *.
18 19 22 29 32
Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34
Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Practices ....................................... ........................ ............ Design Verification Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principal References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
34 35 54 55 63
.............................
66
Philosophy ... . * . = Current Practices ................. Testing . . . . . ,. **** * * * O Discussion . . . . . . . . . . . . . . . . . . . . . . . . Principal References . . . . . . . . . . . . . ●
●
●
● ● ● ● ●
●
● ● ● ● ●
● ● ● ● ● ●
●
Practices:
=. . . . ................. .* .* . = . . ** . ................. . . . . . . . . . . . ..**
●
● ●
● ●
●
● ●
● ● ●
●
● ●
●
●
●
●
●
● ● ● ● ●
●
. ... . . ... *
● ● ●
●
●
●
●
● ● ●
●
●
Practices:
Construction
Philosophy .. Current Practices Inspection ......... Two Special Topics . Principal References ●
Current 6.1 6.2 6.3 6.4 6.5
●
●
Current 5.1 5.2 5.3 5.4 5.5
6.0
●
●
Current 4.1 4.2 4.3 4.4 4.5
5.0
●
Current 3.1 3.2 3.3 3.4 3.5
●
Scope *.*.**** .**.** *** ***O**** =*=*=*..= ..***=**. ===0=.===== Fracture Control Background . . . . . . . . . *..****** Fracture Control of Fixed Offshore Structures ........... A Summary of Current Practices and Trends . . . . . . . . . . . . . . . Principal References . . . . . . . . . .**. . * ** ***= .= . . . *
Scope of Fracture 2.1 2.2 2.3 2.4
3.0
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..=..=.c~~=~=
Practices:
● ● ● ● ● ● ● ●
* , . . . .* * . . . . .* .................... .................... .................... ................ .. ●
●
●
●
● ●
●
Operation
. ................. . . . . ...* ..***.*.. . . ...* . . . . . . . . . . . .... ............
● ●
● ● ●
● ● ● ● ●
●
Philosophy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Current Practices .............................. Inspection and Monitoring . . . . . . . . . . . . . . . . . . . . Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Principal References . . . . . . . . . . . . . . . . . . . . . . . . . . .
● ●
48fj
-334
@)
66 67 73 77 83
=.....
85
......... ......... .. * * ......... .........
8!5 86 92 100 105
. . . . . . . . . . . . . . . . . . . . . ..=..
ii
● ● ● ● ● ●
●
● ● ● ●
——.
7.0
Comparison 7.1 7.2
with
.. -----
Current
Practices
.----- ““”
in the
North
Sea
Offshore and Operating Environments . . . . . . . . . . . . . . . . . . . . . Major Differences in Current Fracture Control Practices . . . . . . . . . . . . . . . . . . . . ...**** *****=.* . ..=..... Discussion ..* . ...** ******** ******** *=**.... . . . . . . . . . . Principal References . . . . . . . . . . . . . . ...*** *******. =.*=== ●
7.3 7.4 III.
●
1.0
Introduction
2.0
Elements
2.3 3.0
4.0
5.0
6.2 6.3 6.4
a Fracture
●
******** . . . . . . . . . . . . ●
Control .
●
Plan . . . . . . . . . . .
Fracture
.
.
.
.
.
● ● ● ●
. . . ...*.*
Control
.
●
. ..*.****
.
.
.
●
Selection
and Specification
********* .*=.**
120
.
.
.
.
.
.
.
.
.
●
.
.
.
.
120 121 122 126
.........................
129
●
●
..................... ............. . . Technology . . . . . . . . . . ..................... ●
●
● ● ●
● ●
.
● ●
● ● ● ●
Summary of Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevant Documentation . . . . . . . . . . . . . . . . . . .***..*** **O*** Recommendations for Use of Existing Technology . . . . . . . . . . Recommendations for Future Research . . . . . . . . . . . . . . . . . . . . . ●
.................................................
●
●
● ● ●
.
137 138 138 142 156
....................................
162
● ● ● ● ● ●
*. . . . ●
●
● ● ● ●
.
● ● ● ● ●
● ● ● ●
Summary of Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevant Documentation . . . . . . . . . . . . . . . . . . . . . . . . . Recommendations for Use of Existing Technology . Recommendations for Future Research . . . . . . . . . . . .
iii
486”334
137
156 157 158 159
.......................... .......................... Existing Technology . . . . . . Research . . . . . . . . . . . . . . . . .
Summary of Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevant Documentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Recommendations for Use of Existing Technologies Recommendations for Future Research . . . . . . . . . . . . . . . ..... . ....
129 130 130 131
.... .... .... ....
Summary of Status . . . . . . . . . Relevant Documentation . . . . Recommendations for Use of Recommendations for Future
Operation 8.1 8.2 8.3 8.4
.
......... . ....... ......... .........
Summary of Status . . . . . . . . . . . . . . . . . . Relevant Documentation . . . . . . . . . . . . Recommendations for Use of Existing Recommendations for Future Research
and Monitoring
117
● ● ●
.
● ● ● ● ● ● ● ●
●
116
117 117 119
Summary of Status . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevant Documentation . . . . . . . . . . . . . . . . . . . . . . . . . Recommendations for Use of Existing Technology Future Work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Inspection 7.1 7.2 7.3 7.4
8.0
********
Scope and Method of Documentation . . . . . . . . . . . . . . . . . . . . . . . Outline . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . * . . . =
Construction 6.1
7.0
for
●
Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 5.2 5.3 5.4
6.0
. . . . . . . . . . . ...**** and Rationale
Material 4.1 4.2 4.3 4.4
●
109 113 114
FOR IMPROVEMENTS AND FUTURE RESEARCH
Integrated 3.1 3.2 3.3 3.4
●
●
RECOMMENDATIONS
2.1 2.2
●
107
@
...... ...... . ...0. ......
162 163 163 166
●
167
......... ......... ......... .........
167 168 168 168
... .... ●
.—
9.0
Overview 9.1 9.2
10.0 IV.
and Prioritization
. ..*...*****
● *
*
*
*,***....
.
.
.
.
169
...***
General Conclusions ..................................... Prioritization of Recommendations ......................
References
9
● ● ● ● ● ● ●
.......... * ●
●
● ● ● ●
● ●
●
. . . . . . . ~(.. . . . .,.8’. ●
● ● ● ● ● ● ●
CONCLUSIONS AND GENERAL RECOMMENDATIONS ...****.* *....... .......... ●
APPENDIX A:
A.1 A.2
FRACTURE CONTROL:
APPENDIX B:
6.1 B.2
6.3
182 184
STATEMENT OF EFFECTS
Description of Problems . . . . . . . . . . . . . ............... Primary Impact on Fracture Control of Weld-Toe Grinding Improvement . . . . . . . , .. ........................ Secondary Influences of Weld-Toe Grinding on an Integrated Fracture Control Plan . . . . ................ ● ●
●
A.3
169 169
●
● ● ●
●
●
● ● ● ● ●
● ●
A-1
●
A-1
● ● ● ● ● ● ● ● ●
A-2
FRACTURE CONTROL CHECKLIST
Introduction . . . . . . . . . *****,** . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Initial Checklist for Fracture Control Execution *******. . .
B-1 B-1
B.2.1 B.2.2 B.2.3 B.2.4 6.2.5 B.2.6
. . . . . .
B-1 B-1 B-l B-2 B-2 B-3
.........
B-3
Sources of Crack, Defect and Damage Initiation ........ Material Selection and Specification .................. Design . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Construction ******... . . . . . . . . . . ********* ****.*... .. Operation and Inspection . . . . . . . . ,. . . .* .. .
B-3 B-4 B-5 B-8 B-9
●
●
Specific B.3.1 6.3.2 B.3.3 6.3.4 B.3.5
●
General Policy .................................... Objective . . . . . . . . . . . * .................. “Scope . . .................... . ... ,.... Prerequisites and Assumptions ..................... Organization and Responsibilities .................. Fracture Critical Parts Selection Criteria ......... ● ●
●
● ●
● ● ●
● ●
● ● ●
● ● ● ●
Requirements,
● ●
Procedures
● ● ●
● ●
● ●
and Documentation
●
●
●
.. .. .. .. .. ..
●
● ● ●
● ●
● ●
● ● ● ● ●
Listing
of Acronyms
and Symbols
ABS API API X, X’, D’, and K curves
American Bureau of Shipping American Petroleum Institute Stress vs. Number of Cycles (S-N) curves for design contained in API RP-2A (see below)
API RP-2A
Publication of the American Petroleum Institute is the primary design guide for American fixed structures (see reference listings)
ASCE
American
Society
of Civil
ASTM
American
Society
for
AhlS
American
Welding
Society
BIGIF
BOSS
A general purpose computer program for fracture analysis relying heavily upon quantities called ~ntegral equation — Generated ~nfluence~unctions Behavior of Offshore Structures (conference)
BSI
British
Standards
CEGB
Central
Electricity
COD, CTOD
Crack-Tip
Opening
CVA
Certified
Verification
CVE, CVNE
CVN energy
CVN
Charpy
da/dN
Crack propagation rate (most often of crack length per load cycle
daldt
Crack propagation rate (most cracking) in units of length
DAF
Dynamic Amplification
Factor
d/D
Ratio
diameter
DFM
Deterministic
DIRT
Design-Inspection-Redundancy
DNV
Det norske
DT
Dynamic Tear
D/T
Diameter-to-thickness
DWT
Drop-Weight
FAD
Fracture
Analysis
FASD
Failure
Assessment
FCAW
Flux Cored Arc Welding
fatigue
which offshore
Engineers
Testing
and Materials
mechanics Boundary —
Institute
(see
Generating
Board,
Displacement
Un ted
Kingdom
(test)
Agent
(USGS
m)
pmgri
CVN)
V-Notch
(test)
of brace-to-can Fracture
due to fatigue),
often due to stress per unit time in welded
tubular
in units corrosion
joints
Mechanics Triangle
(after
Peter
Veritas (test) ratio
of can or chord
member
Test Diagram Diagram (semiautomatic
v
486-334
a 0
welding)
Marshall)
GMAW
Gas Metal
Arc Welding
HAZ
Heat Affected
Zone (adjacent
IIIAJ
International
Institute
J
Applied J-Integral, a measure elastic-plastic conditions
+,JIC
Critical J-Integral required to initiate crack extension under static loads that cause significant elastic-plastic A property of the material, environment, and deformation. plastic constraint condition
K
Stress under
‘applied AK
Applied
stress
intensity
factor
Applied
stress
intensity
factor
Kc,K1c
Critical
KQ
Critical stress intensity at failure without regard
Kr
K/KQ
LEFM
Linear
MMS
Minerals
NAVSEA
Naval Sea Systems
NDE
Non-Destructive
NDT
Nil-Ductility
n/N
Ratio of applied-to-critical fatigue cycles of specified
NRL
Naval Research
Ocs
Outer
OTC
Offshore
P-A effect
The change of applied large deformation
PFM
Probabilistic
PWHT
Post-Weld
R, R-ratios
Ratio of minimum and maximum stress factor) in a fatigue cycle
RMS
Root-Mean-Square
s
Standard
SCF
Stress
Concentration
SF
Safety
Factor
intensity primarily
of Welding
factor, elastic
stress
Fracture
Management
of near
a measure conditions
intensity
Elastic
to weld)
crack-tip
of near-crack
stress
tip
under
stress
range
factor factor computed from the applied to plastic deformation or failure
loads mode
Mechanics
Service
Examination
or Inspection
Transition number of constant stress
amplitude
Lab
Continental
Shelf
Technology
Conference bending
Fracture
moment upon a column due to its
Mechanics
Heat Treatment
deviation,
as in 2s Factor
Stress
vi
4$6-334
~’ @
(or stress
intensity
Plastic-Collapse Stress and ultimate stresses) SMAW
Shielding
S-N
Stress
SPE
Society
Sr
0/a
o
Brace-to-Chord
TIG
Tungsten-Inert-Gas
t IT
Ratio
UK DOE
United
Kingdom,
USGS
United
States
WI
Welding
Institute
WRC
Welding
Research
z-di recti on
Direction
Metal vs.
(often
taken
as average
of yield
Arc Welding
Number of cycles
of Petroleum
(curve
fatigue
angle
(underwater
of brace-to-can
welding
thickness
Department
Geological
of Energy Survey
Council to plate
rolling
vi i
4$6-334
for
Engineers
intersection
normal
used
a
plane
technique)
design)
w 1:
INTRODUCTION
The Ship Structure Committee sponsored this examination of the technology and practices that constitute the fracture control plans used by designers, builders, and operators of fixed steel offshore structures. This report presents the findings of that study as responsive to four identified tasks:
Task 1:
Determine the current status of fracture control practices through
review
of pertinent U. S. and foreign
literature and interaction with designers, builders, operators, and classification societies in order to identify the extent of and contributors to the fracture problem for fixed offshore structures.
Task 2:
Identi&y the essential elements and rationale of a fracture control plan to provide a framework which could eventually evolve to a fracture control plan for fixed offshore structures.
Task 3:
Identify areas where existing technology would suggest cost-effective improvements in current practices.
Task 4:
Identify promising areas of technical research which would provide a sounder basis for fracture control of fixed offshore structures.
Performance of Task 1 was approached in two ways:
through discussion
(and formal interviews) with experts in the field and through a review of standards, specifications, and fracture control studies and surveys available in the open literature. The authors contacted members of the Ship Structure Subcommittee, Coast Guard, Naval Sea Systems Command (NAVSEA), and other members of the offshore community and requested direction to key publications and noted industry experts as a beginning point for both the literature survey and the interview phases of the project. From this point on, the two approaches becsme complementary as the literature brought forth names of people to contact, and the new people suggested (and sometimes supplied) literature for review.
4$6-334
~)
Extensive telephone interviews were conducted with eighteen experts in the offshore oil industry, most of whom are design engineers or fracture control “generalists.” The interviews covered five key areas:
(1) scope of
the fracture problem, (2) current practices, (3) identification of imediate cost-effective improvements, (4) identification of areas for further research, and (5) additional opinions/references. ‘Thus,while the most direct outcome of the Task 1 survey was the definition of current practices
(Section
II
of
this report), the material gathered during completion of this task formed the basis for defining future needs as well (Tasks 2, 3, and 4).
Since the interviews averaged over two hours in length, and since they were not taped, the authors were concerned about possible misunderstandings and misquotation. The phone notes from these interviews were sent to the participants for correction. Not only did this ensure that the information gathered was an accurate representation of each man’s opinion, but it often resulted in the interviewee including additional material and references.
The report necessarily reflects a U. S. focus, since U. S. participants are the major concern of the Committee, and since only U. S. design, construction, and operating companies were contacted during the telephone survey phase of this contract. Although several of these companies (and interviewees) have experience with European operating environments and regulations, this experience has not been researched as extensively or reported with the same degree of confidence as the information on U. S; practices. Also please note that the report emphasises the occurrence and prevention of structural failures. Detailed discussion of the reduction of their consequences, such as through better evacuation plans or designs to withstand collision damage, is outside the scope of this report.
Once completed in draft form, the “Summary” report (Section II) was sent out for review. It was reviewed by the Project Advisory Group (composed of members of the Committee on Marine Structures and the Ship Structure Subcommittee) in October of 1982 (Sections 1 through 4) and in January of 1983 (Sections 1 through 7), then as a complete draft report in late 1983. It was also sent
to
various overseas experts to verify references to European
2
4$6-334
@[
p~acticesg This final report has been modified to reflect the questions, comments, and corrections received from these various sources.
In reviewing how the procedures currently used
constitute
an
informal
fracture control plan, holes and weaknesses in the practices were identified and indications of new directions in research, particularly in materials and design for frontier areas came to light. Many of the current and future trends identified in the “Recommendations” (Section III) stem from the enthusiastic recommendations (or equally enthusiastic condemnations) voiced by participants in the telephone survey.
Others were gathered from the literature
and from the authors’ own experience in fracture control. The “Recommendations” section refere~ces the “Summary” heavily, so that there is a clear correlation made between the recommended practices and the historical and operating environments from which these have emerged.
II.
A SUMMARY OF CURRENT
PRACTICES AND TRENDS FOR FRACTURE CONTROLOF FIXED STEEL OFFSHORE STRUCTURES
1.0
INTRODUCTION
1.1
Scope This
the
is
fracture
control
designers,
plans
builders,
in the
U.S.
of the
Gulf
brief
a summary of the
Gulf
of Mexico
of Mexico,
as well
and
a discussion
for
D.C.
although
no effort
Edition,
important
through
has the
survey,
has
for
offered
record.
used
for
American
mentioned in the
scope
constitute
used
by their
structures
located
practices
outside
when appropriate.
North
of
been
to
structures
the
will,
references been
was
the
made to
Sea
A
(Norwegian
and
problem
are
fracture
are
listed
list
all
in
is
at
authors’
only
Platforms,”
Petroleum
Institute,
the
each
end
of
references
were
Knowledge
conducted made to what
with
as
contact
little
API
section,
members
confidentially
own experience
as the
used.
with
however,
provided
Offshore
to
interviews
experience;
“API Recommended
be referred
communications
Attempts
was
American
the
telephone
the
Fixed
by the
personal
service authors
from
hereinafter,
incorporated.
the
gathered
and Constructing
document
especially
detailed
Finally,
offshore
the
published
authors’
industry,
operators
used
summary
Designing,
This
Other
offshore
this
Planning,
RP-2A.
gained
for
Thirteenth
Washington,
practices of
structures
however, are
that
completeness.
for
API RP-2A,
The practices
as abroad,
sectors)
and trends
offshore
emphasized;
current
Practice
of
the
part
of
offshore information
and
off-the-
fracture
control
of
branches
and
included.
Fracture Control Background
Fracture engineering, the
are
practices
steel
and operators.
Information
1.2
fixed
with
included
was
for
comparison
British
this
current
control management,
understanding
is’~,the
rigorous
manufacturing,
and prevention
of
application and operations
crack
initiation
4
4g6-334
a
those
technology
dealing
and propagation
of with
leading
to
catastrophic
about
- Fracture
failure.
1940.
Until
then
fracture
and evolving
design
stress
levels
ence.
When fracture
did
degree
of
built
phic
redundancy
(often
would
in
often
use large
Fracture Modern
high but
stresses, in
the
and/or
due
understanding
in past
In
minimized
Liberty
stress
1950s,
to
Study
ships.
led
to
the
use
higher
and less aggressive
environments techniques
have
led
implicit
by
by the
problem
rules,
led
along
of improved
notch-
of fracture
mechanics
and
reduced
problem
in particular
These
to
controls
fracture
of this
working
“forgiveness”
analytic
the
ships,
concentrations.
engineering
brittle
and
fatigue sizes
leading
fracture
are
or proposed in
While
for
of
final
based
pressure and jet and the
is
rare
it
each
still
integration subspecialty
in
brittle
to
with
the
design further
and crack-toughness
engines, space to
of
kinds
of
used ductile)
structures.
in nuclear
to
and critical
of fracture
bridges
largely
For mechanics
power plants,
and ships,
military
shuttle.
find the
has
of
rates,
principles
steel
is
types
propagation
and piping
was born
mechanics
several
many
on the
vessels
fracture
(and
crack
fracture
plans
plants
brittle
subcritical
aircraft,
emphasize to
to
Today
ships.
arrest
control
power
and commercial
in
and other
example, used
application
fracture
initiation
fracture,
attention
of
use
Thus some of the
drawn
consideration.
by some designers.
predict
which
of safety.
designs
stresses.
design
dynamics)
high
catastro-
subsequent
to
to
experi-
were
working
better
(e.g.,
number of World War II
sinking
in the
prevent
turbines
resistance
was
Thus the
crack
in
attention
and
to
ductility
1940s,
fracture
materials
lower
removed.
of a large
research
of
have been
the
which
designer
behavior
factors
a failure
lower
same time,
until
due to the
structures),
the
methods
cracking
rules
At the
structural
and smaller
design
decreases
defects. of
redundancy
to
If
become an important
expense
exist
by low working
catastrophic
and thus
allow
not
on trial-and-error
structures.
recently
the
based
production
did
implicitly
was not
of safety
has
at
crack-like
the
as such
controlled
often
materials
often
material
it
or early
control
plans
procedures
into
factors
strength
was
occur
prototype
control
formal
documentation
subspecialties
been
increasing
of and
and
procedures
fracture
control,
some integration
is
guaranteed
by
the
specialists. govern
negotiations
Thus, design
welding
and
inspections,
fracture
to
growth
stress
whether
material
of
explicit
behind
or
sub-
implicit, procedures,
such
as
fracture
provide
for
redundance
in
the
event
structure
these
the
welding
also
a
satisfy
properties
They
safety
The philosophy
to
concentrations,
and
the
needed
plans,
resistance.
maintain
of a part.
trade-offs
control
levels,
and crack
“fail-safety”
fracture
stress
defects
toughness
and
plans
may be simply
or
of
the
described
as to:
1.
Prevent
cracks
2.
Contain
or tolerate
3.
Contain a fracture the part if a crack
properties
concentrations.
mechanics,.
as
the
inspections
1.3
the
cracking to
a higher
fracture
repairs
forming,
or
plan
cracked
has parts
by critically tolerance
cracking
in the
and fabrication
quality
structure
structure.
into
The ultimate
and a better
both the
is
of
seen
and
elements
based
necessary
be avoided the
it.
Obviously by pre-
severity
structure. is
control
to perform
of
Increased
of a structure
result
among
on fracture
many benefits.
the
of
are
which
often
can
unseen
specification
assessing
and designing design
loss
configurations,
is not always
to
the
uses
control
control
prevented;
Some unseen
joint
expert
not
example,
procedures.
of a fracture
from
for
prequalified
and
plan
a priori
wil 1 lead
a safer,
more
cost-
use of resources.
Fracture Control of Fixed Offshore Structures
the entire risk
steel
of
mechanics
of
cracks
Fracture
has
are,
Thus while
a fracture
The adoption
efficient
use
cracks
control
and inspection
stress
to
a part or tolerate grow critically.
elements
visible
such
attention
within should
Some
those,
the
of those
a fracture
material
venting
growth
When implemented, elements.
costly
when possible;
determined offshore
control of
practices
an offshore
1‘our major Iplatforms.
consider project.
activities First,
the
risk
For ease related
material
to the
of
as a part
of discussion, fracture
control
this
of
report of fixed
selection and quality control are
6
486-334
fracture
/ C?3
aimed
at the
prevention
due to substandard with
resistance
material to
crack
phases are conducted
fabricated
details
residual
of brittle,
growth
used
in the
all
fracture
current
constitute as
such.
industry
for
hereinafter
four
conferences practices
it
activities
substandard
and detrimental are carried out to
the
is
structure
construction
minimize
sizes,
In examining
structures,
major
Building
and
current
practices
necessary
to consider
and see how they
or
(e.g.,
Offshore
in
Structural The most
may relate
to
in
the
fixed
plan,
whether
documentation
Classing
not
of
this
ASCE,
Offshore
professional
proceedings
of
explicitly plan
American
the
Bureau
or
formally
be
found
of Shipping’s
(e.g.,
American of Also
Conference
on
to
Society technical
many of the
in more quantitative
1981
in
(referred
or OTC).
discussed the
can
proceedings
Conference, are
structures
Installations”
and
report
offshore
journals
journals),
Technical
steel
or
API RP-2A and the
in this
basic
American
Practice
Platforms,”
recommendations
to
detail
Fatigue
and
Steels.
Recommended
Offshore
related
ABS Rules**),
Engineers,
Fisher
the
(e.g.,
summarized
P.J.
Offshore
Much of
as the
Civil
issued
defect
the
Third,
a way as to
initial
of these
control
publications
“Rules
“API
provides
design
operation and inspection*
practices
a fracture
stated
by
such
and fatigue
control. The
of
in the
fracture
of damage.
of the structure.
control
used
in
welds),
And fourth,
fracture
practices
and tolerance
and inspected
maintain the integrity
of ductile,
Second,
properties.
(especially
stresses.
and types
which
describing
October
1969,
for is
fracture Planning, not
currently and
control
is
document
Designing,
a code
or
now in
its
and
regulation,
acceptable
is
API RP-2A,
Constructing but
practices. thirteenth
the
Fixed
a compilation It
edition.
was
of first
Because
*Inspection may be treated as a separate activity (see Section 111:4.4) but, because most inspection occurs before and during the operation phase, is considered here as a part of that activity. **As noted in the references, the 1982 draft version of this document was used in preparing this report, and that version has not been checked against the nonavailable 1983 ABS Rules.
American
experience
has
generally
represents
that
document
are
discussed
been
mainly
in
the
Gulf
Two types
experience.
of
Mexico,
the
of structure
API
covered
RP-2A
by this
next.
A template-type
Template Platform: The jacket parts.
platform consists of three
is
a welded tubular space frame which is designed as a template for pile driving and as lateral bracing for the piles. The piles anchor the platform permanently to the sea floor and carry both vertical and lateral loads. The superstructure is mounted on top of the jacket and consists of the necessary to deck and trusses support supporting Generally, operational loads. template-type and other platforms are carried from the fabrication yard to the site on a barge and are either lifted or launched off the barge into the water. After positioning the jacket, the main piles are driven through the jackets’ legs (usually four or eight), one through each leg. Other piles, “skirt piles,” may be driven around the perimeter of the jacket as needed.
Tower Platform: A tower platform is a tubular space frame four, large diameter legs (e.g., 15 which has a few, generally feet ). The tower may be floated to the site on its large legs such a tower is also known as a “selfwithout a barge; Piles, when used, floater.” are usually driven in groups or clusters through sleeves located either inside or outside the large legs. When piles are not used, spread footings may support the tower.
The ABS Rules gravity
apply
structures,
buoyant
several
In steel
and
or
types
interest
to
this
Offshore
this
as pile-supported
other
platforms.
in
gas
also
report
space
and
been
terminals,
requirements) All
summary
of
report.
platforms, to articulated
these
are
Figure
discussed,
1 illustrates
is
drilling used
and
in
and for
other
drilling
production
light
platforms,
stations,
applications.
and production
fixed
oceanographic However,
platforms,
the
main
which
are
frames.
technology
was installed
2000 platforms
and (with
leg
oil
have
supertanker
tubular
template
tension
defined
of platforms.
addition
of
structures”
collectively,
structures
research,
“fixed
guyed towers,
towers
individually
steel
to
installed
is
growing
in twenty in the
feet Gulf
rapidly. of water of Mexico,
8
The first in
1947.
in water
shallow
water
Today there depths
are
steel over
of up to 1050
I a
—
o 0
-
I
-o 0 0 a
E
o
I
.
a -+ -+ -+
I
-@ t!Y
T
d 1
I
a
0 0
+==&=---: I
U7 aJ
03
.
1-
1.
F—+ * -_.
—--——
--
F-4
-_:
m
a 0 0
i. 4 w m
1-
❑
l-----____
-
w i-
48~-$J~A
m
FaAA-83–6-2
feet.
Along
with
have
also
practices
this
growth
changed.
in
technology,
fracture
The API RP-2A is
control
now in its
related
thirteenth
edition
been the
extension
in as many years. The primary of the its
motivation
technology
foreword,
tural
deeper
states
types
terized
to
or
that
Southern
has
all
around
the
in
computer-aided
dynamic
early
tubular
with
Large as
structures
define it this
1.4
the target.
the
average,
must define
been the
and
programs
loads
frames.
Future
involve
North
United
Sea,
States,
concurrent
and growth
large
characterize
loads shell
platforms
or no operating
complex
wave loads
been changing.
transfer
through
charac-
life.
have also
to
those
Today’s
which
and fatigue
through
action,
are
often
For example, gusset
without
in the
more complex
analyses
plates.
the
stiffened
trends
use of
internally
design
of offshore
and more thorough
joints. control
practices
A summary of current
the
design.
in
struc-
in the
has
in deep water
fracture
moving
opened
development
stress,
undoubtedly,
have
little
of the
designed
aircraft
where
Draft,
“unique
are
Coast
computer
transfer
which
East
of designs
were
of tubular
Thus, fast
in
will,
understanding
with
joints
the
aimed at
areas,
or areas
analysis
types
joints
rings,
this
response,
joints
Modern tubular gussets.
Canada,
Aiding
The prevailing
specifically
frontier
depth
has
The Ai3S Rules/1982
waters.
New frontiers
Alaska,
designed
development
is
in
water
structural
are
and analyze
located
world.
this
document
obtained.”
California,
structures
the
great
been
all
and rougher
structures
by relatively
experience
for
or typical, trend,
this
practices
practice
or direction,
of
must,
and the of those
industry
are
clearly
therefore,
variation
about
practices.
This
a
not only the
average,
is a goal
of
project.
A $umnary
of
After
short
a
Current
discussion
(Section 2), the current fracture
Practices and Trends
practices
control will be summarized
placed on Gulf
of Mexico
practices
of
the
of the
of
four major
the
fracture
activities
(Sections 3, 4, 5, and 6).
because
10
486-334
scope
@
most
American
offshore
problem
related to Emphasis
structures
is
are
located
The concluding
there.
Mexico practices The design,
with
four
North
current
current
fracture
particular
control
measures
brief
discussion
short
list
Offshore number
of
into
other
as testing
will
highlight
follows The
physical
These
practices
Design,
and
are
and
Installations:
spread
subject
around
American
made when using Quality And a
of controversy,
will
the
indicated,
be summarized.
conclude
Gulf
etc.
each
In
world.
international
developed,
follows
in
the
North
a different
In a sense,
Construction
DNV Rules),
the
of the
summarized.
also
points
be
goals
be
will
trends,
for
being
used
environment
of Mexico.
will
installa-
A
section.
spite
of
practices
the
tend
of Mexico practices.
to The
Sea practices.
practices
Gulf
of
selection,
philosophy
will
between
practices
current
One type
North
Gulf
and
the
control
or inspection
has
areas
two types.
compare
material
section,
and trade-offs
references
technology
discuss
fracture
current
such
different
to
goals The
of principal
7) will
transportation,
In each
respect
practices.
control
fall
with
will
fabrication,
and inspection.
practices
including
sections
(including
and operation
(Section
Sea practices.
practices
construction
tion),
section
they
documented
the
represent by
United
Guidance
on
have
regulatory
the
and Inspection by
Sea
of Fixed
Design
and
norske
of
than
opposite
a different those
of
Construction,”
for
the
1974
(the
Energy 1977
the
pole.
“Rules
Structures,”
Department
in
philosophical
Veritas
Offshore
Kingdom
out
structure
the
Det
grown
(the
“Offshore UK DOE
Guidance). The final Gulf
of
elsewhere the
other,
Mexico in
the
or will
section
of this
practices world,
or
report
compares
and
North
Sea
the
United
States,
be somewhere
between
the
11
486-334
@r)
the
practices.
two.
will
main differences The probably
between
practices
found
resemble
one or
1.5
Principal
References*
“API Recommended Practice Petroleum Institute, and Constructing Fixed Offshore Platforms,” Edition, Dallas, Texas, January 1982.
1.
American Designing, Thirteenth
2.
Det norske Veritas, “Rules Fixed Offshore Structures,”
3.
United Kingdom Department “Offshore Installations: Majesty’s Stationery Office,
4.
Marshall, P.W., ASCE Convention,
5.
McClelland, B. (cd.), The Design published by Van Nostrand Reinhold,
6.
American Bureau of Shipping, Rules Installations, ABS Special Committee (Draft) 1982.
7.
Fisher, “Fatigue February
for the Design, Construction Oslo, Norway, 1974. Petroleum of Energy, Guidance on Design London, 1977.
Planning, API RP-2A,
and Inspection
of
Engineering Division, and Construction,” Her
“Fixed-Bottom, Pile-Supported, Steel Atlanta, Georgia, October 1979. of Fixed New York,
for
Offshore 1982.
Offshore
Platforms,”
Structures,
for Building and Classing on Offshore Installations,
to
be
Offshore New York,
“Summary of Current Design and Fatigue Correlation,” in P.J., in Offshore Structural Steels,” Conference Proceedings, London, 24-25, 1981.
*All references cited in this able to the authors at the were finalized in 1983.
document time this
were current or the most recent availstudy was undertaken. The ABS rules
12
2.0
THE SCOPE OF THE FRACTURE
2.1
Introduction
While fixed
it
steel
cracked
is
or
joints,
structures.
This
confidence
in
exist
whether
is
ation
missing
due
a desire
In truth, and heat-affected
platforms,
bridges,
exist,
but
is
to
zones
rather,
these
public’s
crack-like
or
of
in
the
of steel
buildings,
reveal
exist
maintain or
of
a reluctance
cracks
cracks
welds
do they
that
fabrication sometimes
there
admit
to
during
inspections
braces,
to
operations.
not,
section
in fixed
nificance
2.2
of these
Sources
offshore tion, the
of
Crack
defects structures,
nuclear
how significant
reactors. and serious
defects
are
are
Poor large
heavy
sometimes, of
its
good
can be prevented
of
crack
examples.
initiThe sig-
be considered.
source
a defect
largest the
the
materials
in the
weld.
normally the
which cool caught
proper
cracks
preheat
in high
@
of
or these
from the welding
welding
technique
configurations
residual
The worst
and other
inclusion
significant
joint
inspections
In steel
used.
poor
Certain
installa-
material.
to initiate
or
of an
encountered
a non-metal
is
and shrink. in
initiation
most
plate
results
13
486-334
or and
of
welds
construction or after
original
region,
for
as the
the
of crack
in the
before
welds,
during
and installation,
opportunities
of the
by using
may initiate
the
defects
are
also
be a porous
weldability
cracking cracking
is
practice
several
restraint
will
transport
and rejected
crack-like
and some typical
The first
might
detected
There process.
defects
operation.
In
common sources
Initiation
during
a defect
the
structures
of any structure
such
examine
of cracking
or crack-like
lamination.
leave
types
its
life
briefly
offshore
structure, during
plate
will
steel
Cracks
type
is
crack
divers’
industry
apparently
of the
often
they? This
to
even
offshore
offshore
welds
and that
or
be offshore
The question are
the
in all
they
known that
structures,
parted of
always
well
offshore
some members
in
PROBLEM
during welding
lead
stresses examples
can
and, of this
fabrication, procedures.
and
Improved with
material
and
embrittled
fracture
when
thickness
ductility
lamellar
of
correctly load,
and hence
the
load
out,
damage
is
object
from
usually
crack
the
the
most are
subtle
these
time;
continued
processes
due to
loading
such
not
anticipated,
be
i.e.,
operator
if
Typical
that
so they
is
might
not
Impact
or
crack
initiation.
corrosion
a dropped
the
operator
and
fatigue.
evolve
over
a period
to find
these
cracks.
after
a severe cracks
not In the
operator
be discovered
case, should
probably until
but
considered
first and
of
environmental
may initiate,
properly
may initiate
known by causing
over-
structure.
for
underdesigned.
loads
ways during
cases,
is
the
to joint
these
required
the
lead
overload
coordinated
collision
However,
cracks
the
balancing
the
cracks
loading
underdesign,
2.3
defects
ordinary
by
of leg
to
In
may be suspected
case,
or make themselves
rejected
is
where
large,
and
the
of
are
and their
knows
initiate,
large
initiation
usually
to
defects
crack
structure
cracks
With
to check
the
second
this.
surveillance
of fracture
in several
A boat
or an earthquake.
if
not
may initiate
where)
slowly
overloads
as a storm
In the
easily.
occur
through-
rolling-plane
could
operation
of
chance
may require
loading
common cause.
of
or periodic
Cracks
and
special
loads.
a crane,
impact
examples
sources
the
to brittle
many opportunities
frames
Cracks
knows when (and perhaps
Since
the
sudden
problems
susceptible
in the
are
the
alleviate
with
out-of-plane
of
launch,
deck
extension
there
initiation.
transport,
probably
More
design,
or
is
material
Mispositioning
cranes,
also
can reduce
crack
assembly
cranes.
between
of
locations
construction,
instance,
several
use
to significant
during
can
Such material
the
brittle
subjected
For
effort
Also,
in critical
Finally,
procedures
by welding.
and of
walls
a joint.
welding
loaded.
tearing,
and brace ..
special
they
in
overload, find
doesn’t become
them expect rather
problems.
Examples
good welding enough during are
to
practice degrade
the
welding
known to
exist
and proper the
structure process.
in a welded
controls, are
usually
However, joint
but
.
14
486-334
cracks
2-G (29
and crack-like
prevented there
are
are allowed
or caught classes to
of
remain.
A small the
defect
joint
are
evaluated
if,
purpose
Lamellar plate
problem. loading (or
occur,
brittle
only
where
the
are
a few years
after
is,
but
either than
to
at
of
and the
remedial
of
whether
another
problem, measures
strike
followers, the
pile
is
the
ten years
restraint
used,
resists
they
are
these
its
cracks
ago.
The extra-
sensitive
to
this
and through-thickness
with
high
lamellar
through-thickness
tearing
used
as
under
crack
cracks,
and
occur?
in-plane
cases,
during
installation
is
case,
case
where
crack
has
suddenly
must
common problem several
estimate
on
with
extension
been
dropped.
to
down.
15
02
that
during
at
the
in
not they
trans-
are
clearly
the
time
of
For this
operator
must
crack
is due
the
available
before
needed. drop For
off
depending
more
the
example,
hammer when the
Obviously,
and discussed and Inspection.
were
a few years.
are
the
welds
existed
is
asked.
initiated
concerns
time,
cases
cracks
fatigue
or whether
way
are
suspected
how much time
their
will be defined *“Fitness-for-purpose” sections on Construction and Operations
486-334
is
is to have heavy objects
water)
have
in
or replacement)
braces
(above
crack
underdesigned,
repair,
it
appeared
degradation is
the
operator’s
no significant
In the
those
or from
the
ultrasonic
installed.
Sometimes
In other
the
underwater
many questions
time.
that
water,
been
possibly
structure
an
has
real
not
(inspection,
or
platform
up with
found;
and then
one
picked
to
structural
the
An apparently and
is
sometimes
were
these
in the
situation
determine
and
accomplish
Thus,
life).
defect
is,
still
was part’ cularly
material
when did the
from an overload
serious
that will
about
of heavy weld
correspond
initiated
installation,
problem
which
indications
thoroughly
different
joint
fatigue
platforms
plate
inspected
If
Sea
ductility,
fabrication,
port.
the
and useful
when conditions
always
during
North
indications
The first
defect,
the
extension.
Crack testing
“fitness-for-purpose,”*
was a serious
a special
crack
When found,
be problems.
in
z-direction)
one example.
the
strength
tearing
Today,
is
their
even with
to
used
root
for
(i.e.,
not considered
thick
weld
considered
to see
intended are
in the
detail
deck pile
top
of
on the
in
the
object
dropped,
the
resulting
crack,
a gouge,
a tear, as
Corrosion, always.
An improper
can
the
turn
highly
mentioned, or
or more severe it
Sea have
was not
fatigue
led
with
the
horizontal
below
the
In
those
2.4
Conclusions
this
are
struction these
by preventing
The other designs.
tearing that
the
the
most
the
problems
cases
falling
of
cases,
first
plates,
problems,
experience the
have been worked
out.
being
technology
or
area,
appropriate
measures.
aware and
of
then
to
has occurred the
first
level
led to fatigue
cracking,
even
of fracture
problems.
human error.
poor
welding
One
Examples technique,
control
is
new technology
gained
weldability in the to
fewer,
the of
of con-
achieved
North
and by the
Fracture unique
by
rapidly
control
2 CD
problem
Sea.
in
new frontier diminishes
Experience
third is
best
generation, achieved and
experience
in
lamellar
in a frontier
characteristics gaining
or
new materials,
be installed
the
5-334
spectrum)
were
Fracture
16
48(!
at
fatigue
probably
types
is
of platforms has
in the
forces
or direct
relates
second
by first
the
load
platforms
wave
weld.
Sea designs
conductors
also
the
rapid,
installed
(wave
later
operation
from occurring.
and fatigue
generation
well
objects.
problem are
for
due to
errors
as
North
basic
initiations
Examples
in thick
isn’t
in
into
In early
vertical
workmanship
the
type
In these
areas. later
and
two
corrosion)
environment
overloads
are
crack
overloads,
cases
joint
poor
the
designs,
there
significant
it
resulting
platforms
problem
supporting
was considered.
of
welding
first
severe
Another
fatigue
because
an offshore
(crevice
fatigue.
and the
though
occurs
however,
metal.
some of the
with
and repeated
type
slices
for
problem;
The damage may be corrosion
framing
that
a subtle
weld
of the
to cracking.
appears
of a joint.
anodes,
underestimated
It
a
electrolytic
into
known that
surface.
in a brace,
welds
analyzed
for
be
a dent
ground
had problems
explicitly
separation
can
knife-edge
is
be negligible,
inadequate
corrosion
Finally, North
or complete
underwater
detectable,
pitting
damage could
has area
has
most of in these
demands and
shown
of
applying
a
2.5
References
1.
Telephone Production
interview Research
with Charles P. Royer Company, Houston, Texas,
and Nick Zettlemoyer, on September 14 and 16,
2.
Telephone interview with Peter Marshall, Shell Oil Company (USA) Houston, Texas, on August 17, 1982, from 12:30 - 2:00 p.m. PDT.
3.
Rolfe, S.T. and J.M. Barson, Fracture Prentice Hall, Inc., New Jersey, 1977.
and Fatigue
Control
Exxon 1982.
in Structures,
3.0
CURRENT PRACTICES:
Of the practices
There
major
for
material
are
disagreement
why?
desired
practices
the
this; are
And, what
are are
and how best
is
current
indus-
there
is
fracture. are
see
current
because
to control
done to
the
the in the
What properties
What tests
properties?
one
needed
questions:
control,
most variation
a major
properties the
fracture
selected
that
the
thoughts
and
materials
on how these
improved?
Philosophy
assure
fundamental
that
the
material
must
without
fracture.
which
might
fracture
To decrease
ture
be
accommodate
service
option
an inverse or
a
trade-off
between
more
creates
costly
steels
a more complex
to
a
to
with for
the
That
defects to
brittle
is,
fracture and toughness
strength-toughness
for
which
both
properties
among strength,
selection.
a higher
yield
joints.
There
the
strength
and the
a stronger
material
the
18
material
in material
be chosen
a stronger
trade-off
the
deformation
susceptible
material
between
strength
design
structure.
often
resistant
in
to
means that
plastic
limit
material
of a material.
less
of
is
assumed this
to be considered
So when choosing
avoid
amounts
selection
as
structures
avoid
correlation
material
material
as well
large
will
toughness,
One way to
least
structure,
material,
in
offshore
of the
the
of
at
and to
temperature
weight
goal
two sub-goals:
the
the
of
are
trade-off
tough.
considered.
there
in
fracture,
often
possible
expense
to
a stronger
less
Therefore,
able Thus,
resistance,
usually
behave
is an important
i.e,,
however,
will
joints
initiate
There
control
For tubular
at the
stress,
fracture
material
calculations.
is
to
exhibit for
specified?
may be deficient
The
is,
reasons
discusses
they
related
selection
several
section
How are
have the
activities
over what material
This
3.1
four
used
try.
MATERIAL SELECTION
is
for also
the
dilemma are toughness,
steel
is
joints,
being
should
fracit
chosen.
be carefully is
to
bear
adequate.
the This
and cost.
3.2
Current
Practices
3.2.1
Desired
Properties
General
properties
tility
are
needed
to prevent
presence
of material
process
ability.
which
defects
assure
and some other
tural
avoid Diagram
transition is
plate.,
the
size
which
dynamic
in tubular
joints.
test;
a specimen
passes
break
the
plotted
for
material
is
of the
material
avoid
materials brittle
different
the
plastic
based
on the
is
weld-
the
of
always
only
if
moderate least
applications
strains
45°F
Based stresses
below
the
to this
19
ductility below which failure
and
is
be similarly philosophy,
Test the
bends
strain
rates
(hot
arrested
well
a
against
dynamically
design
is
to propagate
dynamically
on these
at
can
nil
calibrated
locations
crack
Analysis
of
before
if
above yield,
a of
during
data,
a
spots)
stress-versus-temperature
sizes. flaws
the
To
(NRL) Drop-Weight
is
initiated
the
S-shaped, flaw
according
the
suddenly
Fracture
required
which
highest-stress
are
test
test,
struc-
deformation.
of ductile
Lab
which
occurs
temperature
stress
Research
This
is
probability
temperature,
Cracks
be at
other
fracture,
for
mode in
that
prior
the
dynamic
simulates
initial
to withstand
in
test
at
A family
must
steel
and weld-
measure
failure
element
and the
E208-69).
initiation
is
important
a Naval
closely
welds
as through
nominal
in
of the
plate.
properties
defect,
The NDT represents
the
failure
plate,
fracture
the
process,
of the
relevant
for
Since
manufacturing
catastrophic
selection
brittle
ASTM Standard
sharply-notched
important
levels
set.
composition
or crack-like
most
(NDT).
entirely
to
steel
are
key mechanical
most
such
of
as a function
NDT (see
the
tolerance
plate
The chemical
by a crack
The FAD plots
flaw
rolled
by the
a frequently
material
almost
negligible. given
is
temperature
fracture
may be
key properties.
fracture,
(FAD),
and duc-
ductility
fracture,
in the
material’s
or no warning,
brittle
strength,
through-thickness
controlled
is
initiated
and w-ith little
for
the
fracture
steels,
ultimate
initiate
may be specified. to
Brittle
might
is
Carbon-equivalent
ability
cases,
and laminations
itself
controlled
strength,
tearing.
defects
inclusions,
as yield
special
lamellar
porosity,
also
In
standard.
To limit
the
such
this it
can
curves
is
the
joint the
NDT
temperature.
The NDTs
determined.
Thus,
the
NDT of the
materials
to
should
be
below
operational
3.2.2
temperatures
less
than,
in
terms
of,
minimum
Specifications mill
carbon-equivalent, brittle
elastic
testing
methods
bounds.
This if
toughness
most
levels,
the
toughness
current using
Tests
or
The API RP-2A gives
the
testing
tubular
joint
its
Table
54°F
below
the
in ratios
lowest
anticipated
criteria
call
for
no-break
failure
in the
test
plate,
Charpy specimen
V-notch
fracture
steels,
and at
Charpy
at least
energies
energy
25 ft-lbs.
are thought
that
the
NDT of the
Some operators testing
different
specify
temperatures Charpy
the
FAD (in
for
the
terms
material’s
essentially
This
of flaw lower
size
energy
on flat
cracks
at
implies
energy
below the
(Group lower
for for
impact
underwater joints
plates
not
with
should
be Test
propagate
severe.
NDT.
level),
15 ft-lhs. strength
to
II)
necessary
Other
a different
before (Group
steels.
shelf Tests
I)
These
of the
corresponding
same criteria
either
@j
low
and Drop-Weight
or stress at the
least
for
above the
the
20
486-334
test
for
tests is
energy
do
may be more or less
energies.
absorbed
criteria
i.e.,
the
material
desired
toughness
example,
medium strength
Charpy
low.
to assure
For
to
temperature.
to be slightly
Thus the
equal
NRL Drop-Weight
call
(i.e.,
or
conditions
temperature
for
curves
materials.
to assure
criteria
than
critical
temperature.
specified
specified
energy-versus-temperature these
performance, at the
the
20 and 30,
service
at
too
notch
temperature
conservative
that,
less
V-notch
toughness
as
of KIC tests
2.9.3.
between
except
is already
Charpy
of Tradi-
fracture
dictate
is to specify
tests.
material
KIC)
thicknesses
ability
levels,
standardized.
no value
resistance
defect
The specification
less
practices
practice
NRL Drop-Weight
diameter-to-thickness
or
unsatisfactory
standard
is
(LEFM or
little
fracture
material
so forth.
mechanics
determined
usually
determine and
as
of
brittle
the
either
such
typically be
to
ductility,
because
details, Given
for
are is
used
fracture
KIC can
structural
are
properties
linear
joints,
tests
strength,
fracture
tional
the
defined
temperatures.
Standard
for
and
impact
to
the
NDT)
are
intended
temperature. as the
API,
operators
except specify
interpretation
or a different
expectation
of
Some operators material
and
rely
do not
perform
steels,
such
classes,
A, B, and C.
toughness
toughness
Class
C the
ABS Rules
and
are
discussed
in the
throughout
the but,
over
encountered
the
directions, metal,
indirectly
generic
to have the
a
common
toughness
highest
qualitative
(i.e.,
generic
This
inexpensive
of materials,
plate (that
The Charpy
is,
test
correlation
assure
with
more
which test
for
whether
heats,
results
used of
toughness
in base
loading
metal,
weld
(CVE) cannot
CVE is
quantitative
is
is
a variety
adequate
thicknesses,
However,
computations.
quantitative
test,
test
energy
Both
experience-based)
small-specimen toughness
their
thickness.
impact
to
by
The
V-notch
control
weld
steels
toughness.
Charpy
the
of and plate
the
quality
to
design
way of
displacement,
or
mechanics-based in
a
Properly
specifying
COD (also measure
ductile related
to Thus,
establishing
quantitative
properties,
and crack
specimens
than
material
qualification,
rather toughness
than
the
especially
amount before
stress
this
is the
of
sometimes
parameters
It
Charpy
test,
is,
more
be used
such
as
for
Its
than
among
loads,
its
use
in use
the
is
the
currently
making
specifications
evaluations. 21
0 3
fracture a crack
at
loading. for
Charpy
design
test
geometry,
larger
restricted
defect measure
to
evaluation, of
evaluation its
for
material
and employs
and accepted
a
static
normally
fitness-for-purpose
fitness-for-purpose
486-334
is
is
suitable
more expensive
certification, COD is
practice
COD is
opening
tip
withstood under
useful
however, so that
control.
steels.
occurs the
crack
This
strain
fracture
is
the
material.
and strain,
test
size.
through
plastic
relationships
British
in ductile
of
structural
quality for
of the
local
toughness
CTOD),
material
calculations.
practice,
of
below. Another
defect
for
zone).
for
into
and III)
as a semi-quantitative
spectrum
through
described
defects
upon
and proximities
directly
II,
adequate
section.
or heat-affected
used
tip
based
especially,
purposes
42,
supposed
methods,
assuring
industry
Grade
classifications
(I,
optional
next
are
toughness
The API RP-2A groups
tests.
similar
of Grade
for
specifications
A steels
generic
lowest.
as a function
quantitative
supposed
ASTM A572
contain
ABS and API contain
on the
toughness
ASTM A36 or
and Class
The
the
as
entirely
way into
fracture of
weld
American
and structural
and
3.3
Testing
Quality testing
samples
tests,
and assurance
of the
material
in particular,
Materials
3.3.1
those
(ASTM), that
Testing
Toughness
values
plans, is
and
considerable There
purposes.
to be used
used
four
to
basic
American
quality
types
of
There
assurance,
best
accomp” ished are
standard
to
as
toughness
use
part
for
tests
and
and so forth.
of
calculations.
test
by
Te! ting
for
strength,
assessment
the
is
Society
toughness,
fracture as
selection
in fabrication.
by the
for
detailed debate
are
material
composition,
are
for
for
specified
cover
Toughness
control there
control
fracture However,
these
used
for
varying offshore
structures: .
The Charpy
.
The Drop-Weight Tear Test
.
The Crack Tip Opening
.
Fracture mechanics stress tests to measure critical intensity factors (KC or K ~) or critical values of the Jintegral (Jc or J1 ). i lnce COD results can be used (usually) more effec ! ively in fracture mechanics analyses, and since current trends dictate that offshore materials should be tough enough to invalidate K1 tests of specimens taken from the offshore structure? 11! tle description is provided below for KIC and JIC testing.
As described difficulty,
degree
toughness
or crack
V-notch
Test
below, of
(CVN) impact
(DWT), or the
Displacement
these
tests
familiarity,
propagation
test
vary whether
toughness
closely
related
Dynamic
(CTOD or COD) test
in
a number
they
or both,
of ways:
measure
crack
and whether
they
cost
and
initiation are
static
or dynamic. Charpy used
toughness
(10 mm square swinging
Test.
This
test. in
pendulum.
test
It
cross
is
is the covered
section)
The energy
simplest,
by an ASTM standard,
notched to
most familiar,
break
22
specimen the
is
specimen
A370-77.
broken is
and most widely A small
dynamically
recorded.
This
by a CVN
energy
(CVE) is
results,
the
although
toughness the
fracture
The CVN energy
increases
the
region,
transition
of energy
versus
The directly
related
Its
specified never of
use
is
API
was
for
specifies
specifications
controls
such
selection
guidelines)
widely
used the
usually
results structure.
of
Class
results
with
cannot
structure
15 ft-lb for
parameters that
design
stress
B, Group
were
CVN energy
criterion
inadequate
meet
Fractures
Class
be
without
materials
test.
an
on”the
to as
criteria
is
still
B, Group I steels guarantee
against
levels.
II steels.
Grade
material
the
API RP-2A Some operators
of
the
and
(I,
II,
or III)
quality
fact
that
the
not
very
similar
CVN test
control rate
cannot the
range thickness.
grade,
service
The
include
properties Table
from 15
A.3
direct
and
indirect
for
material
temperature,
and
areas. are
that
it
that is
the
specimen
universally
is
cheap
familiar.
and It
is
purposes.
The disadvantages
of the
CVN test
of
and the
of the
specimen
to
loading those
be directly
energy
also
These
(in
steel
Specified
and plate
Charpy
of
optional
selection.”
controls.
application
and
test,
for
Further,
steel
maximum thicknesses
of material
Charpy-based,
CVN specimens)
toughness
as a function
fabricate
similar
for
transverse-direction
on
test
contains
longitudinal-direction
advantages
the
higher
10.1.3,
“toughness
criticality
to
This
provide
for
Section
other
relating
simple
A plot
CVE and such
in the
minimum level
for
(for
controls
are
specified
or
contain
The
through
World War II when the
greater.
steels
depending
ABS Rules
shelf,
levels.
minimum averages
include
or
during
the in the
between
absorbed
plates
may
and
25 ft-lbs
test
a parameter.
temperatures.
and
experience
energy
ship
the
The ABS Rules,
defined
for
it
higher
as
lower
higher
stresses
on satisfactory
a minimum of 25 ft-lb
may require
at
correlation
However, some
shelf
to allowable
ft-lb
and is
be used
from the
qualitative
the
based
15
RP-2A.
fracture
to
essentially
in Liberty
steel
upper
from the
may be drawn.
minimum standards
commonly used, in
is
may also
temperature
the
demonstrating
observed
the
test
quantitatively
data
as KIC.
with
temperature
most commonly extracted
appearance
up to
CVN test
extensive
parameter
experienced related
absorbed
23
includes
to
thickness by the allowable both
structure. stresses
initiation
The in the
energy
and
energy
to
this
is
for
hull
propagate
a drawback steel
cracks
is
will
in
fracture
argument, crack.
test
This
is
such
crack
of the
the
should
most
In
initiation is
to
initiation.
measure
resistance
argued
toughness
argument
(which
dynamically),
dynamic
Hence,
propa-
according
The DWTtest
the
crack
to a dynamically
philosophy.
property since
this
arrest
that
propagation,
can occur
crack
“fracture-safe”
been
crack
flaws.
plate
has
important
of dynamic
weld
base
dynamic
used the
at
It
specimen.
that
initiate
that
prevent
the
to and arrest
function
to
the
grounds
event
assumes
not
through
resistance
control
gation,
on the
any
conservatively
is
a crack
to
this
propagating
described
below
such a test. Drop-Weight
Laboratory
by
Test.
Pellini
Crack-initiation weld
(or,
pressed
the
energies.
The
(all
x 2 x 5 inches material
than
material
test
they
is covered
chosen
crack
in
a that
the
it
is
and it
is
lower
falls
of
a more
does the
E208-69.
least
one
of is
even
the
a
notched rate,
realistic
the
will
weld, the
the
stress
The
be above
NDT.
gradient
who use
test. the
this
in terms
yielding,
of
plate
levels.
stress
joints,
Charpy
the
For temperatures
industry
limited
where at
bending offshore
nil-ductility
always
of tubular
than
the
plate,
at yield
representation
24
is
at yield.
temperature
representative
strain
than
structures
maximum temperature
in
size thick-
the
service
impor-
material
is
Those
more
uses
This
be arrested
is
specimen
DWT test
surface
specimen
This
the
edge
sharp
and arrest
Charpy
as
The 13WTtest
monotonically-decreasing
presence
thickness, that
so that
a very
from
in the
be arrested.
loading,
feel
will
similar,
therefore consider
stress
the
x 14 inches).
by ASTM Standard
at
crack
than
6957). starter
propagation
in offshore
Research
a brittle
by using
generally
used
extracted
to
nominal
1 x 3.5
equal).
of those
propagates
NDT, the
is
flexural plate
being
(NDT) temperature.
when the
should
up to toughness
factors
parameter
surface,
test,
larger
The test
flaw
tear
considerably
test.
Any
dynamic
Naval
NRL report
by using
is
Charpy
higher
related
low levels
specimen
more representative
initial
to
the
example,
predominantly,
nesses
transition
reduced
for
measures,
other
The main
(see,
at
test
apparent
increases
is
developed
Thus the
from 5/8
since
DWT was
co-workers
closely
notch).
tant
and
energy
in
(ranging
The
of
realistic Furthermore
most
dangerous
failure
mode in the
arrested
in the The
fabricate lated
DWT has
between
test
of
the
the
test
results
the
proposed
to
replace
CVN test
British
initiates
in
the
specimen,
surements
made
philosophy
at
to
are
popular
in
Europe,
a “static”
test,
or propagation
under
loading
the
more
costly
to
in
linked
quantitatively
structure
all
formu-
to be used DWT
accuracy. levels
in the
correlation
with
opening
are
static
is
both
value
control allowable
some degree
used
in
crack Mea-
and
are
to
only
are
design COD
contrast
approach,
dynamic,
as noted
COD values
stresses
and crack
of uncertainty),
to
as “specifica-
However,
specified.
is
initiation
under
used
rapidly
COD test
in marked
values
the
barrier.
The
is
context is
when the
resistance
This
(CTOD
maximum load.
Britain,
measures
has been
displacement
industry.
in this
to
which
initiation
measure
fracture
and directly
(although
in
of test
initiation
conditions.
COD values of
tip
offshore
which
type
type
particularly
a minimum allowable
a different
The
reaches
usually
loading
conditions.
below,
it
DT tests,
i.e.,
been
CVN test
stress
specimen
on the
American i.e.,
slow
CVN, DWT, and
tests,”
and
COD can be measured
crack
placed
normally
the
allowable
crack
5762).
is
in
the
the
reliance
popularity
tion
a weld
have
some cost
Diagram.
or when the
close
gaining
impact
is
Standard
or
allow
A further
where
measurements
in
however.
Displacement.
(see
to
Analysis
is
Correlations
at
related
Tip Opening
or COD) test
specimen
specimen.
NDT, although
Fracture
the
the
and NDT which
measure
and approximate,
Crack
the
Charpy
be approximately
using
qualitative
the
DWT to
can
structure
may be initiated
that
disadvantages
than
Charpy
results
a crack
where
plate.
and test
instead
structure,
unlike
the
can sizes
be in
results
of CVN, DWT, and DT tests. The
British
bending
specimen
of
notch
the
fracture the tip?
differences
American with
“fracture” stress
this
loaded
be translated
occurs.
in
for
to be slowly
can
difficulties does
standard
interpreting
it
the for the
mean “fracture
calls
until
into
standards
gradient
test
this
4E16 -334
(@
or
notched
three-point
The measured the
are
results
specimen
25
at
test
initiation”
for?
a
fractures.
opening
test
between
for
tip
being are:
“final
of
opening
a crack
developed. where
is
fracture?”
and structure
best
when Among
the
crack
how are accounted
Idhile North
Sea
authors with to
COD is
practice,
are this
unaware
yards
inadequate
matches
quantitative
as
procedure
gains
addresses
empirically
results
(~)
(KQ) crack
stresses
inherent
a wide (pressure Note
assessment
that line.
the
al.,
variety vessel) one
I to al.
the
to of
Figures
the
the
ratio
effective
use
According
American
of
different
to
and
such
The
COD tests.
FASD
fact
that
it
be
used
in
can
test
ranging
from
to COD specimens. is
considered
to
of weld defects Electricity
the
be
Generating reproduced
and the
validThe
(m) to
plastic-
Kr of applied
(K) to
critical
safety
(2)
large of
being
factors
in
characterizations the
conservative
experimental fracture
a
and has
FASD procedure.
conditions
and
rapidly
data
specimens
evaluated. the of
FASD both
techniques base which and
full-
simulators.
point
in
to
the
each
figure
Supplement
26
486-334
is
applied
conservative
types
Method)
from the
Sr of the
their
FAD which
2 and 3 have been
elastic-plastic
envelop
the
of
and
Assessment
usually
key FASD format
ratio
properties
structural data
in
supplement
a Central
development
the
and material et
as
evaluation
generality,
(1)
and/or
specimens
document
during
under
come from
by Harrison,
includes
figures
and the
retrospectively
has demonstrated
Diagram
fracture
conservative
et
is
COD procedure,
Supplement
practicality,
loadings
the
of evaluating
toughness
procedure
scale
generated
consists
collapse
used
These
be designed
The Failure
simultaneously
tensile
by Harrison,
I.
used
and generality
modes
like in
to
one that
Analysis
elastic-plastic
for
The
found
acronym
methods
ultimate-strength
(CEGB) report
procedure
any
validated
for
competitor
failure
technique
from Supplement
Fracture
practicality
almost
country.
this
defects
published
fracture
FASD procedure,
formally
applied
the
practical
two
unnotched
state-of-the-art
The
a
been
(FASD) Procedure.
of the
much of its
with
The
ation
that
weld
in
platform.
Diagram
elastic-plastic
conjunction
for
specification
structure
has
a new material
which
and in
offshore
COD testing
criteria specify
method
attention
fixed
However,
(for
testing
now getting
in an actual
acceptance
Board
only
Assessment
unfortunately
been
any American
and to
Procedure
ordinary
of
toughness
Failure
gaining
is
acceptance
fabrication
a routine
it
specification.
determine
Diagram
becoming
3 “CD
falls authors,
slightly “These
within
the
points
are
2.2
I
I
I
I
I
I
CM
I
I
1
2.C
1 .a da *
1.6
u
u
44
1.4
1.2 Kr
v 1.0
v b
0.8
0.6
6
3000
+%+0
o 0.4
0
mu
0
‘n 0.2
I
0
I
0.2
0.4
I
I
0.6
I
0.8
10
1 ~
I
I
I
1.4
1.6
1.8
Sr Results
Symbol
Section
of analyses
of data
Symbol
from
test
Section
specimens.
Symbol v
Section
A
Al
•1
A4
+
A2
o
A5
u
A8
o
A6
x
A9
Figure 2.
Containing
Taken from CEGB “Assessment Defects,” Supplement 1.
of the
Integrity
A?
of Structures
Formerly Figure A
.
FaAA-83-6-2
486-334
m
1.6
I 1.4 %
0
o
1.2
1.0 Kr
0 0.8
0
0.6
0.4
0.2
I
0
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0.4
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1,0
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I
14
, e
sr Results
of analyses
of data
Symbol
from
vessel
tests.
Section
o
B1
❑ El
❑ ❑ El
❑
Figure 3. Taken from CEGB “Assessment Containing Defects,” Supplement 1.
B2 1 B3
of the Integrity Formerly Figure
of Structures B. r
486
-33,4
@
considered these
unreliable
figures
provides
for
confirm
reasons
that
an appropriate
discussed
the
limit
in the
Failure line
text.
Assessment
for
the
It
is concluded
Diagram
avoidance
of
that
Procedure
failure
.
in
.
.
ferritic
structures.” 3.3.2
Test
Samples
In general, For
critical
special,
Samples
pieces.
has
actual
been
requirements material
is
of each
piece
are taken
of
only
plate
samples
to be used
heat
of manufacture.
made
from welding
fabrication,
material
While
results
is
sampled.
procedure
may be taken
in a critical
absorbed
qualiof weld-
joint.
extremely
conservative
Weight
First,
small
the
for
Charpy
actually
and COD tests
design
samples used
are
mechanics
useful
are
in
not
tubular
preferred
and
serious related
therefore, for
fracture
as
necessarily joints.
by most
the
fitness-forCharpy
toughness,
KIC versus
only
a lower
CVE data bound
specialists
this
give
on KIC. of
reason
the
in material
V-
KIC.
representative For
to
correlations
have been made between
analyzed
are
not
empirical
or
Analysis
several
are
COD),
of the
Fracture
are
purposes,
of statistically and
the
results
KIC and
Test
temperature
there
through
correlations
values
from
test
indirectly as
or Drop-Weight
transition
accepted,
CVE, and fracture
bounds
of Charpy
determined
well
usable
energies,
sections
is
Statistical
confidence the
value
parameters
directly
The lower
purpose
nil-ductility
(except
stress-related not
the
the
required
strain
evaluation.
Second,
that
approach.
and
are
purpose
that
practice
this
stress such
the this
to
applied
discussed
below
limitations
heavy
each
during
is to assure
Diagram.
notch
from plates
Discussion
It
with
taken
in general,
Rarely,
made in the
3.4
are
applications,
of weldments,
fication ments
samples
the
Drop-
toughness
evaluations, Third, toughness results, determined
testing
versus even for
at
only
temperature for
one
several
piece pieces
one
temperature
transition of
curve.
material.
of similar
materia”
@
not
- here
Were the
29
486-334
does
define
the
is much scatter transition
Y i.e.,
the
curves
entire in test to
be
same ASTM speci-
fication
NDTs would
be different,
the
S-shaped
than
others.
to
fully
at
a single
accepted,
not
different
terms
only
flaw (in
for
various
not
only
the
transition
ship
are
all
be
nominal
a
fields
in it
hand, of
surface
for
be the
And faster
one that transition
be determined
in the
notably levels
use
by tests
the for
British defects
But,
of the
is
for
future. fracture
most
test
in
so are the
many
is often
in the
is
at the
to
use states
the
CVN tests
immediate used
not
offshore to be too
potentially
three-dimensional
because
The COD, on it
skirts
conservative
documented
joints,”
in
could
complex
through
fracture potential.
considered
moment doubtful. (partly
might
KIC, should
JIC,
on some methods welded
COD testing
various
toughness,
plate
important
486-334(@
to
described,
toward
Of the
the
30
flaw
specifications
thicknesses
in fusion
for
for
which
plate
“Guidance
curves
to be valid
as just
a trend
and
stress
are
of the
FAD be valid
well
there
same material,
The J-integral,
easy
is
the
supposed
qualification
strain
are
propagate
are
COD has the
then
of
temperature
control,
distant
joints
there
to
it
structures?
and applicability
and
a family
NDT.
there
place.
relatively
as well,
a single
quality
steels
three-dimensional
stress)
their
offshore
plane
first
in others
the
pieces
measures,
in tubular
is
what
not too
is measurable, its
versus
arguments,
ductile
while
required
COD material
the
(FAD) --
The same curves
broad
in the
the
stress
should
in steel above
but
different
Why then
toughness
found
complexities
most
shelves.
some rising
would
cannot
Diagram
Test)
for
fixture.
However,
other
This
The actual
NDT and briefest
The FAD presents
no matter
previously,
If
be used.
NDT.
NDT + 45”F).
except
to be used
acceptance
the
used
fracture
structures.
use,
for
NDTs different
be measurable
using
use.
due to the
As emphasized
brittle
its
be expected
mechanics
slopes,
material
industry,
with
curves.
to
offshore
steels,
purposes
reasonably
lower
lowest
is,
Analysis
NRL Drop-Weight
materials
appear
the
in the
NDT (e.g.,
Partly for
the
Fracture
sizes the
of the
different
energy
absorbed
sampled
that
above
result.
have different
the
behavior,
the
who disagree
fracture
of
probably
temperature.
Regarding
those
piece
behavior
would
be their
might
ductile
ductile
curves
curves
The best most
distinct
as might
transition
showed the
stress
several
and grade,
precedents for
the
the bounds
for
deviation
BSI PD6493:1980.
its of
Even with properties
the
are
The loading
of
enough,
Most of
a tubular
joint
experts
or
material’s
if
and thus
susceptible
to
toughness
of the
steels, in
and
some
reluctant
true
to give
should
is
up the
steels.
tubular
joint
tests,
disagreement
on weldments
in
same criteria
as the
impossible
prevent
initiation
grown out
of the
weldment
For
experts,
the
these
that is
of the
Needless process
the
welding
changes
the
toughness
or
is
puts
of the
HAZ must
say, heat
arrest
the
a
a crack
or
material
more
rate,
a
the
of low strength crack
dynamic,
many
as
should
to
extension
experts
are
how much testing
be met.
Should
Some people and that
the
plate
parent
weldment
heat-affected
loading
and Drop-Weight.
plate?
into
static. measure
strain
critical
industry
others
the
true
in a weld,
of the
to
are
criteria
parent
around
increased
what
a disagreement
dynamic
makes
Since
the
up to
toughness
plate.
that
material
it
flow
is
toughness
Under
especially
Charpy
and what
crack
is
is
a static
constraint
strength
dynamic
There
with
about
COD testing
needed.
This
questions
with
plastic
This
ment meet the to
also
Therefore,
a
of
also
be done
is
preventing
of high
still
on whether
decreases.
loadings
There
measure
fracture.
material
are
experience
constraint.
brittle
and less
the
industry
increases,
adding
there
can be more dynamic.
in the
a dynamic
strength
notch,
of COD testing
needed.
among materials is
use
arrest
is much less
plate
(HAZ).
the
is has
crack.
important
than
arguments
and substantially
In their
dynamically-growing
it
crack
One of their
parent
zone
that
once the
to
disagree. the
feel
a weld-
view the
crack
and
weld
should
be
tested. Potentially, eliminate
the
some of the
plastic
fracture
authors
and
Failure uncertainties
tests
their use
its
structural
fitness-for-purpose
of
offshore
long-term vs.
number
strongly
note,
research, cycles .
above
brute-force employed
a supplement
(FASD) with
procedure
individual
to
FASD Procedure other
elasticThe
empiricism. the
could
methods
present
often
and
of
defect
and
for
the
evaluations.
there in the to
have as
while
structures,
of
its
colleagues
Diagram
described
through
recommend
One final
Assessment
fatigue has U.S.
failure)
is not
an important
criteria
been much apparent
offshore curves
industry and
crack
with
concern, material
growth
rates,
design
outside
of
S-N (stress “da/dt
or
da/dN”
(for
stress Most
respectively). from
environmental
will
crack
cracking
the
or
published
the
growth
on material
subcritical
da/dN
data
Materials
UK.
rate
environmentally-assisted
are
behavior
are
offshore
not
The reasons
behavior. crack
for
fatig~~e, steels
selected
for
this
2.
Without loading material.
fatigue of the
It
is
result
in crack
Principal
more
that
increased
interest
growth
and
properties
and under a strong
into
expansion effort
in
a given function
more hostile
determining structural
of offshore
low
apparently: problem
believed
their
currently
Fatigue has not been demonstrated to be a critical contributor in Gulf of, Mexico applications. environmental assistance da/dN is not condition,
comes
for
1.
subcritical
3.5
of
especially
Europe,
emphasis
corrosion
environments
and
modeling
the
materials.
References
Methods and ASTM Standard
Definitions A370-77,
for ASTM,
1.
American Society for Testing, “Standard Mechanical Testing of Steel -”Products,” Philadelphia, Pa., 1981 .
2.
Pellini, Design
3.
American Society “Standard Method for Conducting Drop-Weight for Testing, Determine Nil-Ductility Transition Temperature of Ferritic Test to Steels,” ASTMStandard E208-69, ASTM, Philadelphia, Pa., 1981.
4.
“Methods BS 5762,
5.
American Petroleum Institute, “API Recommended Practice Designing, and Constructing Fixed Offshore Platforms,” “Material, ” API RP-2A, Thirteenth Edition, Dallas, Texas,
6.
American Petroleum Institute, “API Steel Plate for Offshore Platform Edition, American Petroleum Institute,
7.
Carter, R.M., Marshall, P.W., Swanson, T.M., Problems in Offshore Platforms,” Proceedings Conference. . OTC 1043. Houston. Texas. 1969.
W.S., of Steel
“Evolution Structures,”
of Engineering Naval Research
for Crack Opening Displacement British Standards Institution,
Principles Laboratory
(COD) Testing,” London, England,
British 1979.
Standard
for Planning, Section 2.9, January 1982.
Specification for Carbon Manganese Tubular Joints,” API 2H, Second Dallas, Texas, 1978. and Thomas, P.D., “Material of the Offshore Technology
32
486-334
for Fracture-Safe report 6957, 1969.
0‘$+
‘
8.
Marshall, P.W., Design of Fixed
9.
Dieter, G.E., “Brittle Mechanical Metallurgy,
10.
11.
“Steel Selection for Offshore Structures. Fracture pp. 490-527,
British Standards Institution, Derivation of Acceptance Levels PD 6493:1980, London, 1980.
for
Fracture
Control,”
Chapter
19 in The
and Impact Testing,” Chapter McGraw-Hill, New York, 1976. “Guidance Defects
on Some Methods for in Fusion Welded Joints,”
14
in the BSI
Harrison, R.P., Loosemore, K., and Milnay, I., “Assessment of the Integrity of Structures Containing Defects,” CEGB Report R/H/R6-Revison 1 and Supplement I to this report, 1977.
4.0
CURRENTPRACTICES: Unlike
practices
the
used industry.
practice,
each
provisions
with
its
understood 4.1
Gulf
section
the
specifications,
Mexico
are
office
or
relatively
design
analysis
the
problems
today
work that
is
the
and
discussed
fracture
of the API
programs. respect
control are
the
the
with
to
considered,
used?
What are
correct?
affect
backbone
and computer
procedures
design
throughout
executing
is fracture
What design
design
form
are
process
nominal
uniform
methods,
questions
design
implicitly?
the
implementing
following
Where in the
know if
the
How can less
well-
control?
Philosophy
structure’s tional
possible
ship
typical
mate
design
elastic
loads.
or
With
the
only
considered
along
accelerated
buckling of
defined to
or
higher
or
subcritical
fatigue,
or
rapid
crack
stress
(i.e.,
plastic
steels
hinge)
loads;
compression
higher
operational
extension--either
failure
growth
or ulti-
under
under
crack
func-
mentioned
deformation,
instabilities
strength
complete
previously
a
modes were considered
plastic
other
against
a near-optimum
of the
of failure
bending
as unstable
partial
with
types
or
guarding
as producing
excessive
tensile
for
occurrence
two general
generally
plastic--leading
the
yielding,
advent
fracture,
stresses,
Before
design.
plastic
responsible
modes as well
efforts:
under
is
designer
failure
failures,
failure
and
the
and economic
Liberty
or
of
design
Traditionally,
in
material
own procedures,
explicitly operator
in
The API RP-2A recommendations
control,
either the
the
major
In this fracture
variations
in
offshore
DESIGN
of
under
the
elastic
member,
is
often
fatigue,
environmentally of
corrosion,
leading
to
loss
modes
as
corrosion
section
and
fracture. There
are
complex
load-related
residual,
thermal,
discussed
by Rolfe
often
considered.
now started
to
other
failure
facets and
of all
the
The fracture receive
of the
dynamically
and Barsom,
attention
such
above
induced first
four
failure
and subcritical
.—
6
and
modes such fields.
creep
that
growth
and
as complex
However,
modes mentioned
crack
approximating
(9
failure stress
34
486”334
bulk
are
as most
modes have only
accorded
the
general
plastic
deformation
severity
failure
concentration is
due
modes
only
to
the
in
loads
of a sound,
and
tubular
control
controls
joints.
On the
global
fracture,
either
to
presence
its
to-find
are
crack
cracks.
then,
again,
by
level,
extension
ultimate
at
all
stage,
within
against
survey
these and
in
design
high
and the
this
tensile
local
failure
compressive
techniques
for
design
of
toughness The most
is usually
the
or overloading.
the
process:
initiation.
in design
with
is
and sufficient
fracture
is
the
level
strength
preventing
concern
levels
the
for
control
the
and other
two main
without
be discussed
Second, the both
design
First,
in the
fatigue
structure’s
tolerance
to
Some qualitative
attention
is
failure
in
avoiding
trade-offs
if
do not if
the
inspection
have
to
structure
catastrophic
section
joint
inspection
of the
can tolerate
degrading
its
be designed
to
can tolerate
procedures
in this
with
a tubular
significantly
procedures
find
design
designing
on two
design
of fracture
operation.
inspection
the
that
of a fracture.
There during
against
concern
due to fatigue
redundancy
within
of
considered
fracture
of the
self-evident
structure.
detailed
consideration
design
designing
is
crack
difficulty
is
Their
It
be considered
One major
global.
implicitly
the
with
modes.
and critical
untracked
joints.
explicit
should
relative
comparison
Fracture local
buckling
upon subcritical
modes
paid
and
a large,
locate
small,
a seriously
section
easy-
then
performance,
can be more relaxed.
and in the
structure
the
harder-to-
damaged joint, These
on operation
points
will
and inspec-
tion. 4.2
Current
Practices
Structural
design
concerns
on the
structure
by the
environment
of
the
structure
to
fracture
control
extreme
environmental
forces
due to
is
itself
these
loadings.
implicit
in
installation
(e.g.,
two aspects:
and by its On the
the
conditions
with
design (e. g.,
load
loading
for
0
side
of
the
operating
earthquake,
placed
resistance equation, conditions,
and
and launch).
—.
Y7
loadings
and the
normal
transport,
35
486-334 . .
operation,
storm,
out,
the
ice),
and
Fracture
control
is
fixed
explicitly
offshore
considered
structures
in the
are
design
usually
for
fatigue
dominated
loadings,
by the
which
for
many repetitions
of
wave loadings. On the tubular
joint
fracture such
is
in that
they
These
factors sion
focuses
Tubular
design
The
design
consideration
of
modern
tubular
other,
welded is
the
to the
often
Figure can
chord
at
to the design
account joint
for in the
and the
other
joint,
design
for
failure
modes,
do not think
requirements,
joint
designs
the i.e., has
of it
and con-
implicit
safety discus-
brace,
is
joints,
and the
is
called
to
fit
brace
is
The chord
or stiffeners.
what
by Most
cut
a
by one of three
“can”
(see
modes.
with
tension
of the
joint
and subsequent
design
is
stress
along
over
between
several
For example,
the
concept
by pull-out
evolved
the
punching the
in
the
The brace,
plastic
shear,
years,
joint
a “K.”
36
@
brace of the
connection concept.
is
to at
attached
a to
in tension
from one brace
situation
the
provisions
are
When one brace the
from
simultaneously
two braces
load
shear.
was a useful
acting
transfers with
the
and now has
in a K-joint
forming
of
of punching
perimeter
braces
Compare this
486-$34
loads.
loads;
failed
the
controlled
may be due to fracture.
the
thus
is
design
the
the
is usually
limiting
compression, action.
forming
joint
average
the
diaphrams,
yielding
for
joints
one tube,
brace
failure
tubular
through
joints
basis
same side,
in a shearing
joint
The following
against
continuous
gross
transfer
in
codes,
failures.
of
any gussets,
tension
same plane.
on the
in
to loadings.
i.e.,
compressive
technique
a joint
implicit
structure’s
many designers
strength
of these
due to
chord,
load
tubular
is
upon to have enough
majority
is
the
under
limiting
the
serious
“simple,”
without
or the
Thus,
depended
ultimate
which
can.
in
design
resistance
vast
chord,
The simplest
normal
the
are
tubular
are
control
Design
their
Some early
meeting
to preclude
The failure
instability;
other
rules
of
may occur
however,
simply
joints
may buckle
and explicit
goal;
structure’s
thickened
4).
failure
This
main
are
Joint
fracture
equation,
part , when considering
the
on the
of the
strength,
and “forgiveness”
4.2.1
the
for
most
control
straints.
side
design
For the
fatigue.
as
resistance
in a cross
to the or X-
G
Joint
c
Figure 4. Sjmple tubular joint. Note: The unshaded area is a node.
FaAA-83-6-2
486-334
(Q)
joint,
in which
both
braces
fers
the
load
Thus
the
X-joint
is
shear
design
punching account well
for
from one brace
in
are
different
pendicular
and
may be
transferred,
other
brace.
One possible
stress
analysis
through
regression Note
For
at
execution
punching
well
local
that
plastic
designed
so
exist.
For
entire
brace times
safety
“hot
the
is
factors the
which
buckling
is
braced
as
load
the
through of
able
one
in
the
effects
extensive (e.g.,
to designers.
stress
over
of the
principal
many square
in tubular
inches
stresses
distribution
to withstand
in
results
is
peak elastically-calculated implicit
in
in two per-
directly
stresses
this
loads
bending
dissemination
practice
Thus,
material
is
of
out-of-plane
spot”
and direction
level.
leg
is to consider
an average
spots,”
joints
that
reasonable
a
example, load
analysis
might
more
joints so
that
The gaps
magnitude
the
vary
not
calcu-
stresses joint
and contain
rings
are
design large
joints,
care large, between
shell
is
taken highly
members
be
is
the
amounts
of
ring each
designed in
element with
38
of
the
carrying carry
using
stresses carry
a portion
half a
to
the
simple
are
load closed
analyses
are
the
most common.
provide
smooth
controlled
stress to
concentrations minimize
a
ring
a finite
to
the
times
joints,
details
the
of
critical
constrained are
diaphrams
may be designed
each
or
or
distribution
stiffeners
two rings,
may
Thin
stiffeners, and
chord,
(e.g.,
complex
be used.
gussets,
ring
to
factor The
all
using
internal
normal
a safety In
transitions,
include control
or nomography)
ordinary
complex
analysis.
avoided.
the
factor).
In
can.
deformation.
More
load
at
in
above yield
assumption
the
The current
A compressive
to
effective
shear
and
often
trans-
ovalizes
interaction
one.
on “hot
tables,
around
joint,
approach
and
equations,
In fact,
vely
joint
K-joint.
RP-2A
a corner
partly,
transfer
The magnitude
lated.
that
the
the
a diagonal least
of cross-section. the
API
the
example,
future
and load
that
of
is
investigating
sometimes
brace
geometries
than
and effect”
currently
planes.
planes,
of such
in an action
buckling
interactions
Then the
modes of failure.
Researchers braces
to
in compression.
other
provisions
load
pull-out
say,
to the
more prone
these
as the
are,
element
flowing may be
constraint
of
ductile
behavior
chances
for
attention
and avoid
restraint
is
paid
and
several
joints
resultant
inadvertent
supposedly
4.2.2
“hard
cracks
Allowable A very
simple
template-type
about
mental
fatigue
loading
ksi
to
+5 ksi).
ksi
for
K-joints,
if
the
hot
stress
For 7 ksi
spot
stress
concentration
factor
Therefore, need
be analyzed.
throughout for
using
this
by the about
the is
that
analysis 400 ft.
of the
(this
is
important
to
use
of
example,
“redundant”
case
analyze and
limit
plates.
it
is
limited
of 3.0
is
assumed
for
only
the
spectrum
of
need
for
than
depths
the
structure
to
shear
brace
effects
environrange
range
ksi,
or +25
allowable
are
10
Alternatively, range.
end, of
Thus
a
etc.
the
design
wave
occurring
The justification
of wave heights in the
less
stress
wave heights
be analyzed. found
of
minus the
-5
a 60 ksi
the
three
maximum stress
X-joints.
to
distribution
distribution
less extreme
+15 ksi
different
not
API RP-2A for
The allowable
punching
and 5 ksi
of
design the
(e.g.,
known,
the
the
through
can,
procedure,
by the
in water
values.
20 ksi
T-joints,
allowed
mode periods the
to
joint
long-term
being
is
wave passes
structure
a certain
is
if
to
current)
the
this
rings
example,
and can welds
allowable
The entire
life
is
stress
is
For
For
generally
as the
for
concentra-
Design
and
sizing
stress
effects.
is
brace-end
weld.
negative
have
this
to predetermined
of the
elements.
the
special
excessive
dynamic
minimum stress)
necessary,
to avoid
of the
increase
or
first
wave,
might
effect
inappropriate
with
encountered
nominal
The
procedure
(maximum stress
the
placement
The procedure
●
(wind,
for
the
design
templates
400 ft
“redundant”
in brace
Fatigue
platforms For
seconds. than
Stress
notch
must be taken
can
have occurred
the
to
spots.”
elements
redundant
unexpected
used,
which
When judged
lessen
care due
are
proximity
cracks. to
special
rings
close
shrinkage
cracking
internal
avoid
and
in
to weld profiles
In complex tions
welds
is
implied
Gulf
of Mexico
in
generic
analyses
of
of water).
The 60 ksi typical
structures
curve.
That
is,
hot
spot
stress
in these complete
range
environmental
fatigue
analyses
39
is
based
on prior
conditions
and on a particular
have been made,
developing
the
S-N full
distributions
of
stress
distributions
case.
And the
life
and
life
occurrences.
a particular
hot
spot
times
as 60 ksi
The fatigue
S-N curve
stress
a factor
range
of
has
that
safety)
damage due to
been
gives
has
calculated the
been
these
in
desired
each
fatigue
back-calculated
and
in API RP-2A.
The following .
cycle
design
(service
specified
stress
factors
Hot spot
are thus
stress
K-, T-,
concentration
and X-joint
●
Template-type,
●
Gulf
●
Long-term
assumed:
factors
for
the
brace
end and
cans:
static
response
of Mexico wave climate
stress
cycle
this
method
to wave loads;
in about
400 foot
distribution
water
depth;
resulting
from
the
above;
S-N curve.
●
quite
Experience
with
conservative
in
approaches it
reasonable
these this
400 feet.
procedure.
4.2.3
for Fatigue For
dynamic
analysis fundamental
is
water
In fact,
conduct
conditions,
attractive
the
to
shallow
but
on at
a full
other
hand,
the
for which is
design
of Mexico
has
conservative one occasion
fatigue have
Analysis
response
least
would
even preliminary
Gulf
and less
which
On the
structures
in the
analysis been
for
too
procedure
shown that
as the
in deeper
is
depth
has
a structure
is so easy
of structures
water
a designer
conservatively
it
found
which
met
designed
hy
to use that
it
is
water.
Wave Loadings do not
thought
performed.
All
principles,
S-N curves
fatigue
meet
to
the
be an
above important
analysis and Miner’s
40
conditions,
techniques rule.
factor, are
especially a full based
if
fatigue
on the
same
The S-N curves tubular
joints
are
joints
in
cracks
initiate
inch),
grow at
occurs
at
ness,
and,
is
or
assumptions
size)
thereby
does
may produce Applying
stress,
which
damage due to cycles amplitude
Thus
it
when
the
cycles
is
of stress
of “damage”
This
to accumulated
N cycles
would
of various
stress
reaches
1 and
stress
there
that
(not
the
one
a wide
bounds
the
deviations
is
is wide.
one stress
cycle
a measure
conditions
and
damage due to n cycles defined
is the
as
sum of the
= nl/N1 that
1/2
of stiff-
necessarily
is
is
failure
data
life.
rule,
until
either
of fatigue
~ ni/Ni
that
is
failure,
Miner’s
which
actual
amplitudes
as
covering
reflect
damage,
cause
e.g.,
n/N.
The
damage due
+ n21N2 + n3/N3 + . . . fatigue
failure
no interaction
occurs
between
the
amplitudes.
between
usually
may not
or under-estimate
of different
industry
response,
same amount
rule
The differences in the
in S-N curves
end.
(such
such that
tests
test
fatigue
loss
two standard
inherent
the
cycling,
of cracking
fatigue
for
on tubular
depth
for
minus
failure)
tests
substantial
plotted
joint
to
continues
an S-N curve
in tubular
when applying
summation
large
amount is
purposes,
independently,
assumed,
load
fracture,
mean curve
an over-
Miner’s
for
each
the
as one at
amplitude
The test
rates. either
statistical
One of the test
cycle
reasonably
The S-N curve
scatter
in the
a
cycles
load
or an arbitrary
For design the
constant
reaching
being
imminent.
is
of stress
amplitude
constant
The statistical
of crack
to
upon
much displacement,
data
early
Under
failure
N cycles,
number
on constant
apparently
of stresses.
used.
of
based
versus
laboratory.
above
range test
the
too
of the
(stress
the
involve
analysis,
various
the
stress
types
treatment cycle
of fatigue of loading
counting,
and
analysis history, damage
employed structural
accumulation
assumptions. 4.2.3.1 fatigue
Loading
calculations. There
considered. ment over
the
service
History. In rare
are
Wave loading cases,
two basic life
of the
the
ways to structure.
41
486-334
@
effects describe
is
the
fundamental
of current the
concern
and wind are
long-term
in also
wave environ-
The first of
various
heights
height).
For
feet,
description (an
example,
states,
by two
parameters,
the
third of the
waves
average
occurrences point. for
Typically, about
marked its
the
three
on a scatter
x- and y-axes.
of the
sea
seconds.
This
wave height scatter
diagram
of sea
Refinements
cated
descriptions,
about
an
the
state
calculated
water
density,
storm diagram.
the
description
That
is equal particles
heading,
scatter
4.5).
water
the
moving cylinder
the unit in
the
shape,
wave
each
or
wave times etc.),
plus
generated
42
(e.g.,
state
then
and mean period i.e.,
as
30 hours, of 10
than
the
from the
description
by using
More sophisti(in
combination
waves,
are
waves
possible
with
ABS Rules,
cylinder
times
a coefficient area
a sea) of
wave forces see
the
a different
directions
length
unit
a
state
is
be derived
unit
the
the at
same sea
wave heading.
wave climate,
volume of the
between (zero)
sea
either
the
per
one-
impossible.
including
force
time level
can
wave
equation
crossing
wave environment
of
locally
described
and a mean period
With
spreading
of the
Morison’s
is,
to the
with
for
different
is a measure
in the
can be considered
diagram
general
to
waves
9 feet.
of
occurrences,
possible.
than
period
height
is
3
highest
of each
count
reverse
are
of the
scatter
according
wave height
above
remain
of the
O to
of the
average
of 4 feet
description
from
mean zero
mean sea
may be ten height
the
crossing
to
wave
or blocks.
height
significant
but the
including
average
Whatever
cylinder
or
from a distant
sea
Section
on the
count
has
there
the
states,
or heading
wave height
arriving
fact,
and
above
each
is typically
is the
assumed
a significant
In
it
heights
occurrences
state
of occurrences
which
is a more detailed
count.
directionality
is
The number
having
of
average
rising
environment
of
with
groups
height
more precisely,
number of waves
and 1 greater
The mean zero
For example,
state
A sea
is the
diagram,
are
9 feet,
waves.
state.
expected
associated
600
number
wave height
surface
hours.
is
the
wave
water
the
wave-height
significant
in a sea
the
period
into
counts
wave period;
of
count
waves,
divided
individual
The significant of the
wave total
description than
to
99 from 6 to
is thus
rather
period.
1000
6 feet,
history
The second sea
assumed
of
300 from 3 to
The loading
i.s simply
are
especially
on a long the
slender
acceleration
(which of the
usually
accounts cylinder
of for times
the
square
of the
The procedure might
velocity
is
the
be different
many cases
occurs
mode period, platforms that
that
in the
dynamic
greater
the Gulf
at
in time
that
effects
of Mexico,
the
as the
wave is
is
only
instant
the
general
the
coefficients
the
greater
acting.
by the
is,
in
dynamic For
industry
is
period
is
400 feet.
is calculated
assuming
That
first
structure’s
than
response
through,
dynamic,
in an analysis. used
unless is
is
structure’s
guideline
depth
stepped
coefficient.
wave loading
to the
structure’s
load
except
may be neglected
water
the
another
waves and currents.
be considered
or the
analysis,
wave loadings,
compared
dynamic
3 seconds,
times
Even though
enough,
not
particles
fatigue
Response.
need
In a static
storm
smaller
slowly
effects
than
points
the
Structural
it
water
same as for
for
4.2.3.2
of the
that
the
the
at several load
inertial
applied
effects
are
neglected. There basic
is
grate
the
Another
are
to
pass
a series
state
results.
refinement
is
and the part
squared,
the
of
moving
it
through
time
until
a
the
wave
load
waves,
and
should
under
effect
of
on the
wave force
is
its
series
solve
for
the
theoretically
identical
non-identical,
waves
used
in design.
velocity
Since,
according
to
especially
reached.
Fourier
relative
proportional
and inteis
directly
give
loading.
can be substantial,
state
into
to
The most
structure
steady
rarely
the
analysis.
the
random,
has been only
the
structure
the
effects
in
structures
however,
dynamic
waves
identical
consider
of
identical
The two methods
response
to
performing
decompose
assuming
to compute;
equation,
to
response. The
possible
is
again
of
of
response
possibility
steady
methods
structure’s
components,
water
several
the for
Another
between
the
to Morison’s
(relative) tall
is
velocity
flexible
struc-
tures. It
is
makes Morison’s are
linear
used
in
appropriate equation
functions certain
to
point
non-linear.
(spectral)
fatigue
in wave height.
Therefore,
height
for
for
Cycle
analysis
analyses. great
reasons
that
particle
according
care
to the
43
A@
the
velocity
velocity linear
However,
squared
term
and acceleration (Airy)
the
must be taken
to be discussed
Counting.
486-334
here
Water
of wave height
linear
the
out
wave theory
wave force
is
in choosing
a wave
in Section
4.2.3.4:
not
Stress
4.2.3.3
Stress
calculation lyzed
of stresses
using
model .“ each
Analysis.
beam
elements
From this
analysis
brace
are
The described
is
Using
fairly
standard.
next
within
the
stresses
nominal
level
of
the
refinement
stress
is
around
plane
the
the
ratio
diameter
(d/D),
braces,
and
extremely
thickness
angle
of
the
the the
it
is
can
ratio to
gap between
chord
braces
stress
impossible
to
cracks
initiate
are
radius
placed
the
effects
analysis,
space
frame
the is
three-dimensional at
the
stresses
ends
of the
Also
principal
stress,
the
of the
another
inch
ana-
“stick
of
(or
along)
inches,
that
but
the
the
the
brace
to
can
the
to
other
diameter-to-
can thickness braces
stress
that
strain
i.e.,
would
gages
to the
can
(t/T),
and
acting
as K-
at the
member being
rapidly
in
in the
this
at
region
(weld-crack-driving)
Typically,
Note, due
to
may not
stress
the
however, the
measurements
measured
where
(R and t being
gaged).
by
size,
a point
weld toe.
experimental
being
be measured
have finite
stresses
41TlYof the weld toe
stress
depend-
or out-of-
diameter
For
As
varies
bending,
include
surface
joints,
of the
extremely
joint
stresses.
factor.
Since
to 0.1
of the
(6).
as the
So variability
note
thickness
them and measure
in
weld.
expected.
factors
brace
spot
through
Geometric ratio
hot
in-plane
transfers
situation.
0.25
vary
(axial,
load
defined
in tubular
within
compute
intersection
is
is
place
and thickness,
that
of the
brace
spot
(D/T),
gage in a laboratory
fatigue they
way
important.
of the
The hot a strain
a
perimeter
of loading
bending),
to
the
The type
the
The tubular
distributed
ing on many factors.
are
or a dynamic
found.
earlier,
braces,
a static
notch
would be
be the
maximum
perpendicular
to
the
weld toe. The reason were
derived
is
not the
to
the
this
this
next
crack, included
is
That
is,
way.
maximum principal crack
is
refinement itself, in the
is
the
is
similar
at the
enough
to
The notch
44
-334
that
~
0
tubular
strain)
crack,
finding
S-N curve.
4gfj
(or
the
joint
used stress
Thus,
(if
used the effects
to
produce
stress
S-N curve
perpendicular the
inservice
the
S-N curve)
at the
of the
S-N curves
in the
but the
away.
analysis,
necessary.
because
stress
distance
in stress not
used
stress
finite
a
structure/geometry the
stress
weld
weld toe, are
or
already
In light spot
stress
ment
over
of the
above,
in an experiment the
stresses.
analytical
There
and using
Finite
for
the
rather
complex
and are
placed
on the
element
models
need
join
and
and solid
being
incorporated The
parametric lated, numerous
to
sets
Kuang,
et
of the
results
tubular
accepted
parametric
equations,
and Smedley models material Kuang’s
and
(due to their Large
popular
were
equations
Wordsworth
or
and Smedley
discrepancies
gives
hot most
are
presently
based
element
use
calcuare
derived
(curve
analyses
finite
to
There
was
on regression
element
is
stress.
popular
from
fitting)
on various
results
joint
could
comparing
SCFS computed
analyses.
So Kuang’s
by
be fit by other
equations
benchmark. of parametric investigators They
in the
thick
The SCF so spot
for
able
to
have model
format
equations
had the
weld
of a series
to
hot
spot
by Wordsworth
strains
with
the
in acrylic
cheap
reinforcement.
of factors
regression exist
45
was derived
success
are more similar
been found
4$6-334@
equations measured
from exponential
have
the
often
used
two cylinders
analysis
equations.
the
finite
be
elements
isoparametric
stress
are
joints.
derivation
spot
(SCF)
are
the
shell
programs.
hot
One of
used
would
shell
capabilities)
or experimental
These
are
for
mesh
are
that
Thin
reason,
element
from thin
where
analytical
set
even
this
estab-
elements
stress
toe.
region
well
the
shell
“true”
weld
analysis
shell
(other
an industry
tubular
brace
elements,
fairly
Stresses
3-D elasticity
stress,
thin
equations
(1978).
of
finite
generating
the
the
For
equations
are
thin
thick
factor
equations.
These
at
joint
such
These
them is
get
extra
full
brace
as well).
to
technique
of numerous
Another
of hot
from nominal
using
cylinders.
weld.
nominal
(1977).
in a sense,
value
even more disagree-
stresses
joints
Generally,
the
concentration
and loadings
using
plate
by the
into
of
al.
spot
correct is
techniques:
of the
the
(with
geometries
are,
of
elements
the
the
There
tubular
be corrected
modeling
stress
times
with
mid-planes
surface
other
accepted
geometries.
reinforced
shell
of hot
for
difficulty
difficulty are
be expected.
techniques
The primary
have
over
equations.
lished.
also
to
two primary
element
on the
is
calculation
are
parametric
measured
some disagreement
acrylic
13ut,
raised
while
to exponents
equation
curve
fits),
to punching
shear
equations.
between
the
two for
certain
the con-
ditions.
Apparently
sets
of parametric
tion
of hot
in
stress
black
have
which
true
evolved
ideally
and presentation
for over
should
of
any pair the
of the
years.
numerous
The computa-
be a straightforward
results,
is
still
exercise
viewed
by many as a
art. Stress
there
known as
used
are the
when the
Cycle
range
discrete
of waves
which
in the
on the
the
due to
both
in the
number of stress from
abilistic
peaks
damage
a dynamic
The simple This
blocks.
analyzed.
effects
picked is
method
method
is
The stresses
The maximum stress S-N curve.
The
to be the
same as the
number
within
the
important
to use enough wave
in
the
the
range
known in advance,
spectral
relies
are
and dynamic
each
effects
block
should
also
be
only
of the
range
of
not
of wave periods,
known as the
waves.
cycles
response,
especially
if
a
block.
is
of
structure’s
loading
representative,
cycles
the
of the
represent
also
or
is
on the
to
truly
but also
it
and valleys
is
The method
frequency
it
pre-determined
number
tions.
are
or
method.
assumed
method
method,
the
is
wave method the
block,
occurs
The above
culated
block
calculate
geometric
so that
peak or valley
cycles.
by wave height
range
discrete
The wave
carefully
wave heights
stress
define
response.
chosen
to
stress
a static
deterministic
described
used
either
block.
to adequately occur
only
The second method.
estimated
heavily
deterministic the
using
on linear
stresses
method
III this
method
is
because
need to be calknown as the
method
the
stress
analysis
probranges
probability-theory-based
systems
the
equa-
techniques
in the
domain.
The sea and mean period, can be calculated, formula.
then
of that
When using blocks
is
the
wave from each
is
of cycles
With
of counting
wave or
wave climate
calculated
number
Counting.
two methods
due to a representative
is
currently
that
stresses,
analysis
analysis
and
same is
equations
spot
4.2.3.4
is
the
This
one way of
state is
description required. using
spectrum
the is
representing
of the Given
wave climate, these
Pierson-Moskowitz actually
a power
a random process
46
4$(5-334
(@J
i.e.,
parameters formula spectral such
the
significant sea
state
or some other density
height spectrum empirical
function,
as wave amplitude
which or wave-
induced the
stress.
The sea
trum.
response For
forces
fatigue
of
stress
tically
for
and the
fatigue
states. of
cycles
response
is
the
hot
range
of
at
the
considered,
then
analysis,
If
for
the
the
response,
analysis
is
amplitude
this
for
each
wave
eight
or
must
probabiliseach
sea state, for
sea
wave can occur,
the
generally
the
of
of unit
greater
is
wave.
analyses
response
than
if
In this of unit the
cost
case,
transfer
the over
function through
the
directions)
eight
frequencies
directions
needed
the
system
amplitude
amplitude
wave analyses the
compute of the
About twelve
are
be dynamic,
to
consideration,
(Thus,
sixteen
the
all
method
under
direction.
96
larger
velocity
to
waves
are
however,
non-linear
performed
chosen
technique
earlier, is
not
give then
with the
the
with unit
there
squared
is term
also
correct
are
for
the
fatigue
used
for
strength
of the
fatigue
and thus
the
height.
So the
analy-
waves,
but with
response.
back
unit
to
waves
The stresses
amplitude
hot
spot wave
of larger due to
wave responses.
linearization.
makes
the
47
‘ @
wave
average
one problem
486-334
to
amplitude
normalized
still
wave force,
respect
is known as equivalent
However, the
only
is
due to waves
if
spectrum.
skyrockets.)
stress
This
each
per
or
spectrum,
for
frequencies.
(and,
stress
wave analysis
As mentioned
the
spot
stress
stress
heights
wave method.
many waves
frequencies
actually
versus
design.
running
spec-
accumulated
Determination
frequencies.
hot
enough
amplitude
output
hot spot
is
describes of
system
be calculated
method
in
a range
stress
different
usually
over
requires
and calculating
sis
spot
theoretically
structure
are
loading
function
the
system,
the
wave
cycle
discrete
of
relates
repeated
S-N curve
effort
A transfer
is
one stress
in the
square
From the can
by this
computational
harmonic
relevant
thus
the
is the
calculation
calculated
to a linear
produces
function
output
by the
as assumed
functions.
a unit
the
of
function,
below.)
more than
input
by the
distribution
This
since
the
spectrum
the
and their
damage measured
number of waves,
transfer to
be discussed
state.
thus
transfer
will
sea
stress
Most
the thus,
Note that
number the
each
input
stresses;
cycles
is
or transfer
analysis
functions
number
the
function,
to hot spot
(Transfer
spectrum
Multiplying
structure.
frequency
state
with
this
wave force,
linearization. and thus
That the
hot
is, spot
stress
response,
non-linear
to wave height. structure,
the
monics
frequency.
are
the
In
American
practice
hot
spot
stress
riser
ity
of the
stress
is
at
riser
construction
the
(buttering) the
improved
weld
profile, care
specifying
that
are and
and cost
the
type
As with
AWS D1.1,
European
curves
DOE “Guidelines”
the
the
values
of the
“Structural are will
the
mostly have
must
for
riser
account
joints.
the
the
designers
by
weld
toes
and may also
weld
For joints
X’ curve
in
accomplished
to
near
reason
critical is
for
The sever-
smoother
used.
generic
the
stress stress
can
are
fatigue
toe.
For
without
an
is
used.
Due to
generally
try
to avoid
curve.
In American
curves,
nominal
used
design
concentration American
Code,”
similar, S-N curves
are
that
essentially
~-
the are
brace
instead method,
practice
of
stress
hot
spot
this
S-N curve
tubular
joints,
factors.
document
although
48
a
curve
of
X’ curves.
For this
transition is
types
be used.
in
486-334
for
been discussed
effects.
passes
To use these
Welding
harmonic
function
API X and
notch
material
is the
other
two basic
S-N curve
butter
X curve
profile
allowable for
the
stress
required,
range
are
the
(pessimistic)
of S-N curve
shear
severe
surface
API D’ and K curves.
punching
certain
the
har-
apparent
transfer
have already
weld’s
makes the
in welding
an “improved”
becoming
domain
there
to a smooth
adding
profile,
is
has higher
of the weld profile.
more stringent
The S-N curves the
of
also
usually
the
a less
This
higher
thus,
weld toe,
i.e.,
by
and at
stresses
are
ground
profile
the
The other
assumes
is
construction
an improved
extra
stresses.
weld
frequency
the
it.
spot
a function
microstructure
with
range
toe,
and by grinding.
improve
these
is
the
In offshore improving
weld
through
that
Finally,
these
respect
passing
time-frequency
on hot
with
frequency
non-linearity
away from the
the
at
to handle
based
the
the
of this
The curves
as in amplitude
response,
Damage Accumulation.
S-N curves. briefly.
is
The stress
developed
as well
of a single
and new hybrid
being
4.2.3.5
joints
waves
wave force
structures
techniques
bridge
for
The significance
water
Since
is,
resulting
of that
components. deep
That
in frequency
covering the
same as the
soon-to-be-released
much more stringent
API.
new UK for
thick
joints.
This
degrading
fatigue
in Section
change
fatigue
note,
all
in
air.
tests
performance
research
in
fatigue
curve,
have
occurring
the
the
fatigue to 2.0,
points
points
around
equations the
eighth
the
spots
bending, are
least
For
further
per the
joint
perimeter
of
chord
(0°,
usually
give
points.
hot
are the
. ..)
spot
the
to
the
add
and out-of-plane fatigue
bending) lives
stress
an S-N that
cycle
the range
+ n2/N2 + n3/N3 + . . .
its
expected
reciprocal,
fatigue
life
Due to
life.
factor
life
of
generally
For
intersection sides
are
the
(SF)
15 years
(0°,
to
of many the
and SF
can be calculated
‘@
180°,
point
(due to
axial
When finite wherever
critical.
are
points, parametric
locations, these
or
quarter
270°)
For sixteen
By analyzing
damage
‘“”j
90°,
eight
the
The standard
quarter
covered.
either
points,
of the weld.
the
fatigue are
eight
considered.
49
486-334
analysis
Recall
a safety
spots,
used.
SCFS for
most
the
damage using life.
service
Those must be interpolated.
susceptible
fatigue
one year,
a service
at hot
and brace
f15°,
protection
be 30 years.
computed
brace-can
the
with
should
are
fatigue
during
to
the
1.0.
intended
desirable
on tub-
some continuing
of
DT = nl/Nl than
based
in air.
due to each
Obviously,
example,
life
lives
of
structure,
the
is
of joints
damages
life.
it
design
points
available,
a
environment,
cathodic
variations
damage occuring
fatigue
water
damage as fatigue
sum of the of the
be at
on both
eighth
the
are
However,
proper
calculation
of that
average
life. the
sixteen
the
a sea
those
damage must be less
When fatigue
the
All
the
involved,
considered
Lives.
life
total
must
uncertainties
equal
approach
goal
expected
structure
design
the
DI is
I/Dl , is
their
damage is
the
If
indicates
be discussed
above
degrade.
with
joints
interpretation
during
Nominally,
the
as
in
to
that
suggests
Fatigue
fatigue
that
will
described
unprotected
be expected
of submerged
and the
total
research
This
plates.
S-N curves
Left
area
4.2.3.6 described
in thick
of the
would
this
lives
by European
Discussion.
4.4:
joint
motivated
performance
One last ular
was
but
not
locations,
load,
in-plane
element
results
The report of
fatigue
to the
into
North
that
experience joints
are
different different
are
designed
to
frequencies
of
various
fore,
fatigue
bottom)
their
fatigue
Lastly, lives
should
be best fatigue
with
a 100 year
fatigue
life
with
life. both
joints
4.2.4
ranked
in order
these
fatigue
in
the
allowable
parts
of
calculated
results
(say, spot
cycles
can
with
of industry
as relative
differ
two
and near
the
stress
range,
markedly
and,
experts
calculated
that
the
joints
That
susceptible
to
these
a joint
fatigue
the
fatigue
survive
that is,
of
method.
will
analysis
the
there-
a ranking stress
indicators.
more
if
an allowable
the
will
even
top
Thus,
mean that in
the
allowable
different.
do not
Thus,
near
stress
a structure
distributions.
hot
be quite
has been expressed
30 years,
the
above
should
structures
Self-floating
towers or exposed
For
year
life,
but
both
the
that
numbers with
failure
same by the
loadings
are
a 20
than
one
take
the
-334
Gulf
a
analysis
of a
a 30.1
the
year
same,
and
designer.
those
loads
occur
during
of the
barge,
especially
same motions,
~
effectively
life
design
one with
considered:
motions
Current
alone
the the
sometimes
due to the
50
486
are
refine
Shedding
Repeated
these
to
if
and Vortex
shedding.
experience
some designers
instance,
leave
numbers
Due to Transport
members.
that
may be a tendency
reasons,
fatigue vortex
by some experts literally.
there
be treated
Loadings
and to
too
29.9
template-type submerged
a design
life.
Two other transport
in
of fatigue-sensitive
incorporate
structure
fatigue
probably
a calculated
Fatigue
are
a list
different
many uncertainties
is
listed
joints
with
frequency
consensus
interpreted
is
Given
that
same 60 ksi
can
requirement
requirement joint
so
life
Concern
the
and 100 years
are
year
also
of the
lives
is the
of 20 years There
not
members is not possible it
long.
parts
their
sensitive
are
cycle
from
generally
programs.
Note
stress
supplied
known to
lives
method.
are
most critical
is
inspection fatigue
design
the
operator
operators
inservice
Note fatigue
Sea
calculated
Usually
Thus the
their
thus
operator.
severity.
parts.
lives
as well
of Mexico practices
due
transport
of
rolling.
as wave loads usually
to
on
do not
consider
these
tow from
fabrication
sometimes are
loadings
used
generally
design
be serious
yard
to
reduce
good.
condition
to
to
location
the
effects,
As with
stresses
fatigue is
relatively
fatigue,
numerous
This
short,
and weather
inservice
and the
problems.
is
extra
conditions
the
fatigue
because
the
cribbing
is
during
relative
magnitudes
stresses
play
the
tow
of the
an important
role. More attention and lately longer
in American
tows
forms
have
in tows been for
longer
from
The vortex
tends
to
These
vorticies
forces
form
that
properly
are
to this
resonance
and
members.
Often
structural their
as pipes
problem.
Vortex
caissons
and their
failures
Repeated
fall
mud, water,
can cause
fatigue
water
into
side
the
of
which
due
to
objects, water,
the
object.
exert
lateral
supports
vibrations
are
not
can lead
to
particularly
especially
does
not
shedding
structural
and impact
to see
critical
to vortex
affect
increases
for
suscep-
slender are
non-
integrity,
damage to
if
but
structural
structure. such
and the This
are
is now commonly investigated
members subject
loads,
as the
operation
filling
@
and emptying
of cranes
may be a problem
51
486-334
slender
object’s
These
and sewage,
problems,
cost
the
or oil, damage.
may occur.
nominally
operational
the
is
supports.
be
through
the
have
considered
considered
gradients If
Pacific
and Canada.
long
off
pressure
shedding
both
being
of a platform
eddies,
the
plat-
platforms
are
U.S.
sometimes
deck
to vibrate.
and other
loss
may cause
or
for
will
practice,
need for
Basin
across
tows
practice,
by the
of Pacific
relatively
the
vibration
such
members as they
holding
object
damage of the
fatigue
off
long
of the
is
and cause
resonant
fatigue
Caissons,
the
and
by
vorticies,
periodic
may cause
designed,
premature
shed)
In American
that
hanging
motivated
and transported
Coast
passing
is
in overseas
a number
East
East
loading
caissons
(and
Far
the
Water
the
example,
California,
for
fatigue
attention
one month. to
repeated
shedding.
particularly
than
platforms
other
For
in the
Singapore
prospective
This
weather.
been fabricated
lasting
to transport
practice.
in worse
towed
tible
has been paid
for
the
lifting deck
of tanks
heavy
loads,
and supporting
trusses.
However,
quent, the
compared
to the
Failure
their there
structure is
existence
are
is,
the
failure
used
as
too
small
in the
and/or
tubular
is that
once
infre-
members of
above tubular
into
the
members
deformation
and
forestall
catastrophic
remains.
Such
is
separation
of the
testing
cracking. helps
first
tributes deformation
structural part
a
reason detail
is
and/or
to
adjacent
displacement
chord;
Here, the
mere
structure. to survive
the
load
stressed
reaches parts
its
not
hq G
That
is,
load
failure
is
plastic enough
to
capacity
of a tubular
necessarily
mean complete
mean too much displacement of
joint
stiffness, of
uniformly.
ultimate
the
or
parts
on just
of
the
failure.
strength
or cross-sections,
control.
failure
reached,
carrying
and catastrophic
rarely
is
some
strength
reserve
part
ductile.
fatigue
safety
an average
rapidly
could
reserve
of
do not
the
cross-section
does
loss
normally Due to
load
in the
it
52
486-334
of
due to two related
reached.
therefore,
structural
or cross-section
some load
the
failure
immediate for
as
be able
in turn,
normally
significant
Thus ,
to prevent
The second offshore
or
system.
than
will
“failure”
example,
Thus,
from the
machine,
significant structure
brace
is
and, for
test.
of the
offshore
indeterminate
failure
throughout
rupture
in a laboratory
the
is
And when the
case,
strength
reserve
rather
is,
strength
formulas,
loads
the
the
members and connections
connections
distributes
ability
In
context
structure
design
load.
its
member.
individual
design
design
present
is
failure.
redundancy
in a statically
of the
their
defense:
fail-safety,
in the
structural
The first
this
and the
as
cracks
catastrophic
to
for
paths
has against
a
a failure,
structure
strength
built
without
load
The reserve
factors
the
usually
problems
a structure
a synonym
of a major
brittlely
joint
such
a redundant
reasons.
the
are
to cause
two components
of multiple
That
fail
effects
against
redundancy
of
load,
of defense
line
withstand
structures
well
total
variations
Modes
The last
the
load
jacket.
4.2.5
to
these
load
is that
a fixed
So usually and fails, again
through
steel before
it
redisplastic
A characteristic of the
“unzipping”
lateral
involves
both
critical
cross-section
Thus,
the
weakest
types
line
suppose
that
discussed
frames
at a time. above,
strength
is
fail-safe
the mode
within
and
beyond the
is
This
i.e.,
cross-sections
reserve
stress.
the
alternate
the
components.
capacity
load
Due structure
of its
paths loads
adequate
during
steel
environmental
to
difficult
structure
hardly
feels
measurements
will
have
detect
to
be
of
load
without
of
very
loading
brace’s
undue
or six
cracking
effect
be picked
collapse.
because the
the
five
causes
In a
normal
failure)
template,
loading
techniques
made
carry
ends.
could
Under
to
alternate
For example,
brace
a sudden
of Mexico-type
are
at one of its
frames.
struc-
stiffening
loads.
by that
other
are
there
applied
carried in
redundant
that
a fracture
of the
braces The use
damage the
in
a
structural
a missing
sensitive
brace,
to
detect
reliably.
to
reserve
can withstand
how to quantify
this
quantification that
strength
and
a certain
amount
resistance.
is
not
substantial
storms
implies
more
the
vibration cracks
is
with
externally
due to
monitoring
component If
the
redundancy
confused
rings)
normally
an extreme
vibration
be
possibly
Gulf
redundancy.
indicate
load
For a typical
particular
to
fail
braces,
before
the
as internal
dynamic
ambient
fatigue
(not
temporary
need to fail
ambient
defense
were to
other the
(including
and
reserve
attempted.
reserve
strength
in
fixed
designs
Gulf
exists
steel
offshore
The remaining of Mexico design
Platforms
and against
exists
a
of cracking.
In normal
even
earthquakes,
strength
redundancy,
studied
against
fatigue. following
overloads
So it the
in
is
general
problem practice the
past
due
assumed
to
that
patterns
of
most attention
in
platforms. The study
designs
of bracing
neighboring
of sustaining
structure,
conditions,
those
into
such
a brace
up by several
similar
and
Redundancy
capable
redundant
severe
redistribution
of
in a joint,
paths
such
load
has additional
system.
elements
is
one level
structure
parts.
tural
of
mode of template-type
bracing,
of
structure
The second
load
failure
for
“shakedown”
of failure
earthquake analyses
modes has probably
zones are
and arctic
routinely
regions.
performed
53
486”334
(jf G
received
to
the
For earthquake evaluate
structural
zone designs, ductility
in the
event
of arctic of
platforms
the
most
of a larger-than-design addressed
integrity
of
difficult
the
their
in
strength
of
involved
may make a thorough
tions
during
4.3
Design
must
modes
criteria
of
which
are
certain
procedures,
such
always,
see
is
the
maintenance probably
in
the
quantifying
tremendous
number
possible
the
of
failure
against
considered
to
in
joints
combina-
Conference
programs,
between
The
deterministic
programs
problems,
and
are
The differences
possible. implementations
fatigue
in the
proceedings
computer
simple
if
the
and
Design
industry.
in
been
methods
are well-documented
on
different
have
the
is,
damaging
locations
offshore
design That
probably
section,
journals.
existing
of
most
proceedings,
“debugged”
means
as accuracy.
and
this the
are
be
the
here,
professional
exist
as dynamic
of
described
Technology
as well not
beginning
as those
activity
components
similar
in
control
or
analyses,
of
are
similar
usually,
only
minor
significance
from
the
work
itself
is
by the
but
fracture
perspective. of the
design
representative
as the
number of waves to use in a fatigue
designer
and the
or New Orleans) California
the
the
scope,
design
visits
modes
fracture
whether
the
methods
to
Verification
the
and fracture,
of
structural
at
and
these
are
this
to
such
benchmarked
control
the
study
expensive.
relatively
in Offshore
implement
that
Also,
various
conferences,
usually
not
the
and methods,
other
joints.
must address
check
in
design
API RP-2A,
fatigue
failure
for
As mentioned
addressed. of
for
investigation
Verification
verifier
tools
modes and stressed
studying
prohibitively
assurance
verification.
failure
But
An industry-sponsored
Verification
Quality
the
tubular
design
failure
welds.
problem cracked
earthquake.
operator
operator
so contact
or
to the
, who is
abroad, design
is assured
the office, that
consulted are
usually
between
the
or is the
analysis.
For Gulf
same area is easy.
in-house work meets
54
all
such
of Mexico work,
(typically,
either office
operator’s
decisions,
For other
representative
design
@
design
parties
given
4$6-334
on significant in the
operator’s
conducted
work,
makes
space.
design
Houston
frequent
In this
criteria.
e.g., way,
For
the
major
(MMs) Platform over
Verification
400 foot
in U.S.
water
that
these
run
parallel
fatigue major
tasks
From this subject are
their
design,
studies the
the
for
scrutiny other
method,
4.4.1
aspect
to
have
new platforms
all
level
other
Thus,
platforms
program
party.
is
requires
The CVA will
member and
aspects.
in
of verification
of the
third
dynamics,
all
the
joint
parallel
strength,
efforts
attention
to
range that paid
most
be of
fatigue
in the
their to
shallow
tools
water
by the
allowable
is
should
conservative,
for
special
subject
Shallow applicability
water
are
plat-
used
with
same platforms
design
structures
Deepwater
analysis
designed its
deepwater
sophisticated
perspective.
likely
is
major
in design.
And the
a verification
shallower
the
spectral
effects.
more
that
attention
a dynamic
Many incompletely
understood
discussion
highlight
to
platforms, stress be quite the
least
structures.
will
design the
problems
most
have been mentioned.
important
problems
that
single
The
and trends
in
practices.
Tubular
Joint
It
is
of
fracture
materials, should
most
Service
Discussion
following current
another
(CVA) phase
control
And assuming
control
and for
can be seen
non-linear
the
of Mexico,
by an independent
design
it
are
for
Gulf)
loads,
Management
provided.
likely
from
in
conservative.
4.4
are
Minerals
particular,
Agent
most fracture
hand,
which
fracture
major
example,
to examine
most
on the
most
the
check
discussion
to the
forms
to
and other
analytical
Gulf
outside
be checked
analyses
by the
(in
Verification
platforms
lives,
in the
areas
The Certified
covered
program
depths
operational
used.
are
platforms
generally
is be
controversy
Design agreed
control
in
good detailing.
avoided;
this
surrounding
by designers design, Stress
is
beyond
detail:
55
486-334
an understanding
concentrations
universally
one important
the
0/$7
and close
accepted. the
most of
loads
proximity
However,
weld profile.
important
there
and welds is
The API RP-2A makes special ent
S-N curve,
This
available
profile
Shielded
is
Metal
to the
described
in
fatigue
design
designer
provisions,
who uses
i.e.,
an “improved” 4.1.3C,
API RP-2A paragraph
a differ-
weld profile.
“Joint
Details
for
Arc Welding:”
A special effort should be made to achieve an as-welded surface smoothly with the adjoining base metal, and which merges “T“ approximates the concave profiles shown in Figure 4.1.3. is the size of the diagrammatic weld exclusive of toe fillets added for this purpose.
Ideal
profiles
difficult
such
to
achieve
“Commentary”
to the
cap
and butter
the
API does
between
passes not
attempt
How does
weld
of
the
notch
effects
unimproved.
tional
benefits
weld
bead.
microstructure (e. g.,
deep scratches
loss
of area
the
designer
the
requirement?
removal along
and
of
cost
a point
structures.
Nominally stress
the
residual and the
and
be? notch
Recall
that
profiles, addi-
presumably
with
the worst
possibility
toe
the
“improved”
stresses.
of a fillet
they
may have three
last,
of material
of dispute
reduces riser.
X for
a profile
Are the
should
it
and grinding
post-weld
or misblending
in load-carrying
worst, defects,
Detrimental of surface
radius)
damage
and significant
cross-sections.
The effects
of
weld
plastic
deformation
is
currently
mental
fracture
mechanics.
removal
are the
been
S-N curves:
removal
when is
How high
surface
welding
probable
to answer:
of offshore
in the
with
diagram
apparently
the
properties,
of material
at all.
reduces
are
effects
profile
i.e.,
The additional
So the
the
work?
included
etc.
grinding,
However,
profile
are
5) are
what was intended.
improved it
Figure
demonstrate
has
another
(see
shows small
fabricators only
4.1.3
which
This
the
associated
These
for
not?
passes,
weld,
to
left
it
Figure without
provides
to quantify
and the
the
X’ for
section
is then
butter
in API’s
construction
on a weld
designers
the
effect
offshore
and when is
the
fillets,
in
depicted
fatigue
The question “improved”
as those
profiling
on cracks
an area While
of research
profiling
56
growing
is
in
regions
in analytical certain
to
of
large
and experiaffect
fatigue
RP2A:Planning, Designing,
andConstructing Fixed Offshore Platforms
57
ROOTOPENING,G 45° TO 90” 1/16 TO 3/16
SMOOTH TRANSITION BETWEEN DETAILS TYPICAL CONNECTION ANGLEa Is THEANGLE FORMED BY THEEXTERIOR SURFACES OFTHE BRACEANDCHORD ATANYPOINTON THEIRJOINTLINE.(LOcALDIHEDRAL ANGLE]
1.6 TO 4.6
.-.
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I SEE SEC. B-B
/A 9 6
BUILDOVTTOFUU.THICKNESS EXCEPT‘T“NEEDNOTEXCEED 1.75t, SECT ION “B’’-’iB”
SECT ION “A’’-%”
MIN. 1/16 IN (1.6mm) MAx.1/4 IN (6.4 mm)
BACK-UP WELDNOT
OPT
SUBJECT TO INSPECTION
9sre
x
?\
\\l
T SECT
10N
SECTION“C’’-’’C” (ALTERNATE)
“C’’-’’C”
FIG.4.1.3 WELDED TUBULAR SHIELDED METAL
Figure 5.
CONNECTIONS ARC WELDING
Taken from API RP-2A, Twelfth Edition.
FaAA-83-6-2
4&3j~
~ m
‘
initiation
and
growth
grows
away
from
occurs,
the
profile
4.4.2
Fatigue
the
There,
and
aspects
contributions
sea
with
surface,
of
state,
larly
is
severe
of
for
for
not
number)
in for
fati!
in
proper’
crack
the
joint
But
of appropriate
of hot
dominant
prediction
wave
spot
analysis
the
come from the
calculation
a fatigue
dynamic
analysis.
the
equat” on.
and
since
shipping
but
as
a
The equation
relatively
in
a random sea
of
forces
stresses
in for
most significant
for
North
tends
forecast
100 years
While
a
use
to
of
such
avoid
it at
of Mexico particu-
bad weather.
of waves
from two
of sea-state
extrapolation
Thus,
conditions
Gulf
uncer-
Atlantic,
extrapolation
analysis. state
the
in the
traffic
accuracy
sea
methods
vary
from
one
wave
passes;
of marine
growth
the
data
represents
seems
that
worst
With
on the
adjustments
structure
the
drag
to
another
on these
and regular
structural-member
the
a least
geographical
waves, where
interactions, (such
wave forces
and inertial (i.e.,
as
0 0
relies
force with
are
assumed
coefficients
are
difficult
for
instead
of
appropriate this overturning
a single for
for
approach
terms
Reynolds
constants
and developed
58
486-334
calculating
however,
is empirical flow
for
for
wave
itself
state.
forces
to
The coefficients
undisturbed
flow-related
data
location.
fatigue
given
predict. in
less
attempt
the
the
may be the The wave climate
is
state-of-the-art
constants
global
around
the
ue.
The effects
give
the
analysis
areas.
to
the
analysis.
other
structural
there
practice,
The current on Morison’s
but
a remote
exists
locations
about
distribution
frontier
may have
error
knowledge
are
global
state
storms,
an unusual
source
once
many uncertainties
questions
computation
sea
known,
of data,
not
are
are
distribution,
for
An oceanographer years
that
S-N curve.
well
for
probable
redistribution
in a fatigue
and the
especially
fairly
is
stress
There
the
state
The long-term is
it
effect.
course, of
sea
a given
tainty,
and
has little
to uncertainty
long-term
given
cracks,
Uncertainties.
analysis.
the
small
Analysis
4.4.2.1
other
of
for to
cylinder
a structure
shielding is
moment)
agreed which
and to are
acceptably
accurate
for
design
individual
members
which
are
thus
contribute That
other is spite of
the
has
not
large
been
of the various
plane authors the
where
braces
is
are
unaware
together
growth What is
paths
dual
could
the
taken
a host
all
and mean stresses?
lives
are
control
4.4.2.2 to
deeper
problem water joints. level
such
European
the
poorly
program
that
under minor
involves
does
only
gage
is.
the
properly
in
interaction out-of(The
accounts
for
wave loads.)
uncertainties,
but
analysis.
How valid
summation
fatigue
a sea water
effects
to
sense.
is
growth? of resi-
environment
easy
which
of crack
crack
when investigated
in a relative
And,
different
linear
is
often
documented).
What are the
it
each
spot
(especially)
predicting
prediction),
to
fourth
power,
inversely t-o.25*
of
the
other,
stressed
fatigue
test,
on S-N Curves.
operators
play? in terms
see
of
why fatigue
What is the
given
to
thicker
is, the
that
fatigue
proportional That
in the
to the for
North
impact
fatigue
life)
of
plate
will
and
have
a
@
have
a joint
thickness nominal lesser
moves
grappling
with
effects. larger,
strength
joints,
spot
practice
thickness Sea,
the
59
486-334
plate
two similar
same hot
joint
As American themselves
namely,
found
indicates a
may find
Europeans,
as those
research
corresponding
the
least,
(especially
Effects
before
structures,
role
and
with
hot
tools, and
cycles?
estimated,
the
strain
in-plane
for
on
plans?
American
currently
approximately
life
that
analysis
joint
which
poorly
do not agree
upon a fatigue
stress
by experts
Thickness
water,
rule,
count
upon
interpreted
on fracture
impact
uncertainties
effects
the
of comparatively
And what
these
where
the
transients)
way to
problem
at
wave forces
estimates.
the
both (or
are
life
stress
in
local
available
analysis
have major
by
Given all
but
through
an equivalent
proper
quantitative
loads
of any fatigue
contributed
is
is,
understood
are
(or
also
the
fatigue
SCF formulas
crack
of
poorly
load
rule
spot
of appropriate
combinations
Then there
Miner’s
the
to
to fatigue
There
existence
different
taken
hot
discussed.
defined
important
uncertainties
various
However,
purposes.
Deep thicker
(i.e.,
stress
configuration raised
to the
one a scaled-up stress allowable
a
levels stress
is onecopy in
a in
approximately
, where
(t#tl)-o”*5 thinner,
its
to have the
major
fit
roles.
approaches,
with
mechanics
approach
reliably,
with
geometries simple
that,
in
deeper
than,
or
say,
be predicted
around
cracks
a crack
than
of
uncertainty
fatigue
effect
of environment
ments
describing
or
environmental
Conference
Proceedings
Engineers)
and
a series
organized
point
their
crack shared
life
to
complex reduction
problem
a surface
is
crack
amplitude
as would
nor-
crack
model
mechanics
load
mechanics
of explaining
hoped
of the
accelerating
Fracture
aides
above.
in a constant
fracture
joints.
the
shedding
models
how stress
must
redistri-
acceleration. by the crack
of the
effects
fracture growth
reports
60
@
mechanics
analysis
structure.
upon
by Smith,
of progress
4$6=334
is
some displacement-controlled
subcritical
on useful
of
fracture
or full-scale
Another
rate
thickness
rather
grow in typical and inhibits
etc.
growth
the
described
and then
ratio,
play
design
analysis
joints,
the
of
mechanics
the
to
problem.
specimens
effects
One is
due
mechanics
or similar
fracture
as diameter
that
this
of
thickness
joint,
is to be hoped--to
any other
goal,
in tubular
constant,
suggests
thought
HAZ, may also
A goal
tests
cross-section’s
remain
a more
is
to fracture
resolve
by a load-controlled-based
This
A source with
to
now looking
S-N curves
approach.
such
25% of the
enjoy
notches,
of the
Approach.
as the
found
size
to help
of this
problems
brace-chord
appears
be refined--it
are
fatigue
parameters
large
relative
of
fields
notches
stresses
researchers
this
small
thinner’s.
residual
a minimum number
face
which
the
and
derive
such
84% of the
is twice
thicknesses
to
for
plate
such as degrading
and the
As a part
thicker
power,
effects,
Mechanics
to
for
one-fourth
Other
The is
the
the
phenomenon.
large
Fracture
if
self-similar
success,
a typical
as large
than
some apparent
problems
solution.
butes
on this
design
test
occurs
effect
and stress
into
mally
gradient
to
to about
be able
answers Major
load
stress
details.
provide
reduced
of the weld,
European
4.4.2.3
structural
is
and welding,
significant
thicknesses
For example,
effect
for
up,
their
stress
properties
rolling,
of > tl.
size
subsurface
material
t2
allowable
The notch rapid
ratio
the
et
crack al
techniques
approach
is
The most useful propagation
(20).,
by Burnside,
rates
(Institution et
al
the
docuare
of Civil (21).
These
references
report
function
a
of stress
frequencies,
large
amount
intensity
factor
environments,
materials.
The observed
range
negligible
from
cracks
under to
such
under
higher
as
This
an upper
growth
consensus
that
rate
(In
change
fact,
in the
of
at
as
near-threshold
testing
conditions
encountered
the
effect
threshold
of AK rather
stress
m/cycle. be
in
of environment
level
material,
rates
than
of 6 1Ow
not
be taken
near-threshold,
There
is
a general
for
the
relevant
done
offshore
be described
in terms
low
intensity
must
fixed
should
small
and
corroding
comes from the
must
for
by factors
growth
10-8
and
propagation
frequencies
alternating
data
than
crack
a
cycling
increased
freely
in crack
less
high
as
protection,
improvements)
were
lower
test
cathodic
cases,
that
and
of the
levels
frequency/environment joints.
rates
increase
little
more
some
combinations
as
of
results
(AK and R),
upon fatigue
tested
propagation
data
and mean values levels
in
temperatures,
bound
propagation
of environment
protection
factor-of-six
low crack
range
(including,
detrimental
frequencies, factors.
effects
crack
crack
temperatures,
cathodic
temperatures,
of
platform in terms
of crack
of
propagation
rates.) Of
course
stresses,
and fracture
impressive
list
and load models
mechanics
best
4.4.2.4
Fatigue
is the
service
usually
around
2.0
compared
to those
usually
Lives. life
in
used
of the
Gulf
much larger
than
of
in other
practices,
research,
structure
times
practice,
industries.
the
ratio
2.0.
61
486-334
D 73
will
impinge
in spite
of this
with
inservice
fracture
in the required
a rather
design
spot
mechanics
approach.
a safety
However,
mean life)
actual
with
uncertainties the
hot
of many geometrical
of this
by engineers,
(not
above
elimination
viability
Mexico
loads,
calibration
associated
Due to the
practice
discussed
and the
the
with
is to be hoped that
parameters
use of minimum-life
conservative
growth,
increase
conservative
It
further
crack
will
a joint
other
analysis.
distribution
at their
associated
distributions
of uncertainties,
and stress
the
uncertainties stress
of fatigue
as a standard
through
the
subsurface
upon the
observations
all
it
design
factor,
and life
of
SF.
SF is
low nominal
value
should
curves
analysis,
be noted
that,
in API RP-2A and
of mean and “minimum” lives
is
As with this
case
safety
developed
relationship certain
any
for
implies,
loading
structure be,
is
tures.
to
other
be able
Such
research
current
wave heights is
quantitative
if
such
as
the
conditions
are
should
criteria,
correct
presently
being
API and the
level
these
it
of offshore
conducted
by,
it
or the risk
To answer
and reliability
is
different,
the
earthquakes?
risk
is
struc-
for
example,
ASCE.
Platforms
API and ASCE committees a long-term
in the
offshore based
toward
reliability
the
two design
Design
fracture
with
design
control
for
these
factors
options
of
factors
criteria
have been developed of reliability
of off-
implementation
At least
on reliability.
magnitudes
between
trend
on the
industry.
development
trade-offs
design
This
and so on.
And, what
is
future
fatigue
in
Mexico.
are
and earthquake that
of
loading
API RP-2A are
possible
stress
Gulf
How appropriate
members of the
there
the
level,
SF relationship.
quantify
by the
in
risk
from this
efforts
technology
in the
It
to
an accepted
type,
of Offshore
structures,
reliability
allowable
redundant?
and committee
As evidenced
depths
to
structure
hazards,
The Reliability
shore
the
of SF elsewhere,
to
individual
4.4.3
arise
more or less
relative
necessary
do
conditions,
value
SF relates
medium water
as
Two questions to use this
factor,
storm
methods. will
and
make
practices
possible. There
are
tubular
joint
Fatigue
Design
approach; applies computes
the
at
least
against
two current
fatigue:
Criteria other,
for
under
analysis
probability
Reliability-based ferent
practices,
structural its
detail
non-redundant
options
might
design
such could
as
be in the
by the
Structures,” by CONOCO, Inc. (the
so-called
and safety
be assigned type
sponsored
the
reliability
of a
API,
“Probability-Based
uses
the
safety
and Det Norske Level
index Veritas,
II analysis),
which
of failure.
redundancy
counterpart,
to quantify
Offshore
development
a more sophisticated an approximate
one,
efforts
all of hot
factors
design a lower other spot
structures.
(more liberal) aspects
stress
62
486-334
in
would
@
equal.
analysis,
be assigned
to
dif-
Thus a redundant safety
factor
Another by parametric
than
pair
of
equa-
tions
or
by
element-based the
designer be
factors
or
finite
element
could
returns,”
i.e.,
equations,
for
a
this
perhaps
be
safety
factor
design
such
redundancy
trade-offs
variable
More current
is
to
structural current
immediate
of
the
identification
parts
are
not
identified
always
later
detailed
indicate
Most present the
rule
a .
fail-safety
1.
as
of a part
primary
A brace
Principal
American Designing, “Loading
in use load
at the
that
transfer
likely
be the
failure
by fraccritical One may be
process. inspections,
criticality
a lateral path
bottom
probably
more
accurate
develop,
The most
or
more
it.
fracture
is
of fail-
Fracture
design
with
for
the
be measured?
most
one whose
experience,
exists
for
Of course,
adequate?
structure.
of the
should
a problem
techniques will
part
basis
parts. as
the
SCF
Reliability-
redundancy
will
critical
of
parametric
contributions
questions
be defined
integrity
design
analyses
is
fracture-critical
4.5
can
of thumb currently
structure
criticality
the
the
these
of fracture
part
specific
And how much is
to
identified
in
analyses
A quick
answers
threatens
the
most
away.
How can
designs?
critical
seriously
allocations.
far
the
diminishing
costly.
resource are
with
SCF computations,
a rational
factors into
element
provide
reliability.
and improvement
A fracture
between
research
in
result
could
safety
the
How much exists
ture
much less
of
with
safety
design and
point
combined
but
variable
in
redundant
“the
finite
reliable,
present
at
Then
of analysis
reliable,
with
less
the
is
most
type
variation
of
structure
slightly make such
the
structure
only to
no
finite
of safety.
what
Given
obviously
redundant
factor
combination
combination
a
lower-uncertainty
about
joint.
exemplified is
the
decisions
critical
results
perhaps
case
a more liberal
informed
the
SCF
this
be awarded
or a non-redundant
decisions
safe
In
make more
“allowable,”
But
for
design
performed
costly.
based,
elements.
could
should
would
finite
and is
are
brace is
not.
intuitive.
at the
hence
most
As reliability
quantification
of
top
of
likely and part
result.
References
Petroleum Institute, “API Recommended Practice and Constructing Fixed Offshore Platforms:” Conditions;” Section 2.3, “Design Loads;”
486”33463
@
for Planning, Section 2.2, Section 2.5.3,
“Fatigue;” Section 2.5.5, “Connections;” Section Connections” and “Commentary on Allowable Stresses Section C2.5.3, “Fatigue,” API RP-2A, Thirteenth January 1982.
4.1.3, “Welded Tubular for Structural Steel;” Edition, Dallas, Texas,
2.
Marshall, P.W., “Basic Considerations for Tubular shore Construction,” WRCBulletin 193, April 1974.
3.
Marshall, P.W., Welding Journal,
4.
Marshall, P.W., “A Review of American Criteria And Proposed Revisions,” IIW Dec. XV-405-77.
5.
Marshall, Offshore Nostrand
6.
Kuang, Joints,”
7.
Wordsworth, A.C., and $medley, G.P., “Stress Concentrations at Proceedings of the European Offshore Steels Unstiffened Tubular Joints,” Research Seminar, Welding Institute, Abington, Cambridge, November 1978.
8.
Rodabaugh, E.C., for Use in Fixed
9.
Vugts, J.H., and Kinra, R.K., “Probabilistic Fatigue Analysis of Fixed Offshore Structures,” Proceedings of the Offshore Technology Conference, OTC 2608, Houston, Texas, 1976.
and Toprac, A.A., Research Supplement,
“Basis for May 1974.
P.W., “Tubular Joint Design,” Chapter Structures, ed. by B. McClelland, Relnhol d, New York, 1982. J.A., Potvin, SPE Journal,
A.B., August
et al., 1977.
“Stress
Joint
Design
Tubular
for
Joint
Tubular
“Review of Data Relevant to the Design Offshore Platforms,” WRCBulletin 256,
Off-
Design,”
Structures--
18 in The Design to be published
Concentration
in
in
of Fixed by Van
Tubular
of Tubular Joints January 1980.
10.
Kinra, R.K., and Marshall, Platform,” Proceedings of the Houston, Texas, 1979.
P.W., “Fatigue Analysis of Offshore Technolog y Conference,
11.
Kan, D.K.Y., and Petrauskas, C., “Hybrid Time-Frequency Domain Fatigue Analysis for Deepwater Platforms,” Proceedings of the Offshore Technology Conference, OTC 3965, Houston, Texas, 1981.
12.
Marshall, P.W., and Bea, R.G., “Failure Modes for Offshore Proceedings of the First Conference on the Behavior Structures (BOSS ‘76), Trondheim, Norway, 1976.
13.
Marshall, Methods .
P.W., “Failure of Structural
14.
Marshall, Offshore Offshore
P.W., “Strategy for Monitoring, Inspection Platforms,” Proceedings of the Conference Structures (BOSS ‘79), London, 1979.
Modes for Analysis,
Offshore Structures ASCE Conference,
64
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m
the Cognac OTC 3378,
Structures,” of Offshore
Part II--Fatigue,” Madison, Wisconsin,
and Repair for Fixed on the Behavior of
Geological Survey, Conservation Department of the Interior,
Division, Washington,
OCS Platform D.C., 19/9.
Verification
15.
U.S. -,
16.
Design Rules for Tomkins, B., “Fatigue Proceedings of the Offshore Structures,” Houston, Texas, 1982.
17.
Dover, W.D., and Dharmavasan, Proceedings T and Y Joints,” 4404, Houston, Texas, 1982.
18.
Bristoll, Problems Offshore Research
19.
Marshall, P.W., “Fracture Structures in the Ocean,” June 1982.
20.
Institution of Civil Engineers, ed. J. R. Smith, H. G. Crisp, Telford Ltd., London, 1981.
21.
Burnside, O.H., et al., “Long-term Corrosion Steels -- First Year Assessment,” U.S. Coast Southwest Research Institute, August 1981.
22.
American Bureau of Shipping, Rules Installations, ABS Special Committee (Draft) 1982.
23.
Englesvick, Knut M., Analysis of Uncertainties in the Fatigue Welded Joints, University of Trondheim, Division of Marine Report No. UP-82-17, The Norwegian Institute of Technology, December 1981.
24.
Wirsching, P.H., “Probability-Based Fatigue Design Criteria for Offshore Structures,” prepared for the American Petroleum Institute, Dallas, Texas, Final Project Report, API-PRAC Project #81-15, January 1983.
25.
Rolfe, S.T. and Barsom, J.M., Fracture and Fatigue Control in Structures, Chapters 14 and 15, Prentice-Hall, Inc., New Jersey, 1978.’
Steel Welded Joints in Offshore OTC 4403, Technology Conference,
Fracture Mechanics Analysis S., “Fatigue Conference, of the Offshore Technology
of OTC
“A Review of the Fracture Mechanics Approach to the P., of Design, Quality Assurance, Maintenance, and Repair of of the European Offshore Steels Structures,” Proceedings Seminar, Welding Institute, Abington, Cambridge, November 1978. Control presented
and Reliability for Welded Tubular at the 9th NATCAM,Ithaca, New York, Fatigue N. A.
in Offshore Structural Randall, and F. Sterry,
of Welded Marine Interim Report by
for Building and Classing on Offshore Installations,
65
4$6-334
Fatigue Guard
0
77
Steels, Thomas
Offshore New York,
Capacity of Structures, Trondheim,
5.0
CURRENT PRACTICES:
Although such
as
there
different
control
are
mented
by the
CONSTRUCTION
are
installation
quite
similar
American
wide
range
in the
Gulf
of Mexico fabrication This
fracture
are
control,
in
Gulf
of Mexico.
scheme
practices
of
construction,
related
These
(AWS), as well
of construction,
only
discusses
those
the
related
What kinds
trends
overall
the
Society
examines and
the
to
practices
are
as by the
especially
fabrication,
construction
practices
fracture docuYet a
API. exists
in
related
to
yards.
practices
involved?
current
Welding
in
methods,
in the
quality
section
construction
variations
to
of
following
fracture
are
practices
And,
performed?
the
how are
What specific
control?
inspections
construction
In general,
aspects:
subject
much
of
practices
why are
some
concern
and
discussion?
5.1
Philosophy
In terms built-in
stresses,
specified
material
bly
the
which the
size
during
might
parts
of
of the
main goal
initial
are are
designed
of
and
large
to
minimize
to
of course,
correctly,
so that
to
avoid
is
behaves
assemresidual Care
defects.
accidental
care
from
end, and
critical
great it
variations
restraint
eliminate
structure
is to limit
To this
dimensions.
And,
cracking. structure
the
in construction
defects,
performed
construction
cause
the
and structural
procedures
and inspections
taken
control,
properties
and welding
stresses, is
of fracture
taken
as it
overloads to assemble
was designed
to
behave. In
choosing
fabrication
related
to
method,
relates
defined
and discussed
tion
method,
example,
if
legs,
rather
ture
(versus
fracture
control directly
is
than
are to
to
decided in a skirt
template-type)
installation
cost
of
The second a large to
degree,
install around
is
likely
methods,
The first,
made.
the
later).
controls, it
and
the the
the
two
66
the
the
design
piles
in
result
choice
methods
trade-off,
perimeter, to
the
two main trade-offs
choice
groups
(see
(frame
of the
then
of fabricatiori
the
Figure
or
of installa-
structure.
around
the
tower-type 1).
node, For corner struc-
For a tower
platform,
larger
problems
related
offs
related
report,
member and joint to size
to the
although
effects
5.2
Current
material
and the
welds
made,
the
defects, ture
welding
to
repair
is
defects
control The trade-
addressed
recognized
by this
as important
section.
are
discussed.
In
There
preventing
and of
of tubular
joints
or
Inspections
will
and fabri-
acceptance
installation for
of
weld frac-
damage to of
fixed
treatment
be covered
are
phase,
construction and the
that
on how welds
inspecting
the
incoming
agreement
designed
controls
phase,
of
general
of well
In the
into
fabrication
assurance
rejection
welds.
overview
the
is
involves
the
can be divided
quality
importance
welds,
a brief welding
with
Welding
with
structures
the
steel
of fabrica-
in Section
5.3.
Overview
in
four
lateral
Gulf
Viewed
bracing These
position
four
and joined In what
a brace
is
a saddle in
to
fit
thicker
at
the
node method,
will
frames
are
is
plate
brace
above,
visible,
formed
chord.
the
the
is
completed
center
fabricated
fabricated
structure
two frames
horizontally,
The brace
a segment
frames,
the is
on its
usually
being
then
the
tilted
into to
its the
brace
has launch
up into
a “can”. joint
Special fabrication
in Section
(ZD
5.4.
joint in the
between shape
of
position
in the
struc-
chord.
Special
steel
may be placed
67
486-334
a tubular
end of the
put
material)
called
later,
method,
and welded
An alternate
be discussed
frame
by cutting
or different
ends.
structure
by more braces.
or between
by inserting
from
commonly known as the the
a frame
template-type
frames
together
and a chord
(either
of Mexico
a shipyard.
trusses.
used
4.4.2.2).
of this
together.
of defective
Following
side
joint
are
offshore
the
these
concerned
A typical
ture,
fracture
main concerns
installation.
control. of
structures,
5.2.1
Section
as part
concerned
overemphasize
inspection
control
and
of parts
fracture
and the
offshore
the
and the
installation
steel
primarily
to
structure.
tion
of fixed
is
difficult
cated
(see
are
some length
fabrication
phases,
control
is
likely,
Practices
fracture
it
and
at
The construction main
method
transportation
and are addressed
are
may result
fabrication
issues
two
sizes
in the steel method,
is
chord not
at the usually
known as the
Once the
installation
tection, If
the
is
called
drilling
site
ture
will
its
legs,
to
on its with
the
on location
are
installed,
If
the
tion
methods
load
out launch
platform
replaced is
until
it
and landed
is
not
is
the
launched set
of
a large
on the
bottom.
on top
the
Also,
off
for
the
barge,
barge.
The strucflooding
Once upright,
crane.
an
is towed to the
by selective
The piles
of the
pro-
be installed.
onto
the
of it
and well
is
conduc-
And, finally,
the
platform.
structure,
self--floaters
dry
cathodic
to
work deck.
a template-type
by flooding
ready
structure
upright
a temporary
placed
is
with
is jacked
carrying
For example,
necessary.
it
is
with
are
may be used.
is
it
assistance
trusses
it
on a barge,
where
sometimes
deck and supporting
no
side
and outfitted
so forth,
The barge
out”.
(“transport”),
and/or
and
be carried
“load
float
positioned
has been fabricated equipment,
structure
operation
tors
structure
alternate do not
dock and floating
gravity-type
installa-
require
the
a barge;
structure,
structures,
no
and
piles
are
needed.
5.2.2
Welding
of Tubular
Joints
Welding
procedures
for
American
Welding
detail
in
the
AWSD1.l; the
“Guide
“Rules
for
Building
“Rules
for
Building
joint
welds
are
core”
faster
wire
covered
the
choice
by the
The base special Mexico
Hull
metal ovens
is
Offshore
Installations,”
with
hand-held
years
above
Standard
made for
preheated
yards).
if
voltage, the
weld
necessary,
important The edge
rods
for
to
assure
of
68
low hydrogen the
brace
and
Metal
example, thus
using
allowing
gas shielded
of these
procedures
and welding
for
All
11,
(Shielding
applications,
current,
Steel”,
Most tubular
30.
FCAW), have been used,
documentation.
Code:
in
API RP-2A; and ABS
welding,
And in special
described
Section
welding
GblAW)has been used.
polarity,
the
Section
semiautomatic
rates.
Arc Welding,
(especially
fabrication
AWSD3.5;
Vessels,”
are
Welding
Welding”,
Cored Arc Welding,
deposition
structures
“Structural
Steel
manually,
are
offshore
Society’s,
Classing
of electrodes,
Preparations
steel
Classing
and
(Flux
(Gas Metal
are
and
done
weld metal
welding
Steel
SMAW). In recent
Arc Welding, “flux
for
fixed
are
techniques
available
for
etc. weldability rods
may be kept
rods end
and
is
in
humid
usually
fusion. dry in Gulf
of
prepared
without
a root
only. the
landing,
And the root
fit-up
opening. In the
outside
however,
material,
large
weld
stress
is
extra
butter
passes
are to
(e.g.,
it
at
inspected. not
are
usually
the
to
been
weld to
each
to
outside control
substantially
The root
bring
it
are
capable
of filling
is then
thickness,
decrease
this
notch
degree
among
weld
is to
Idhile
a convex
not
common,
improves
the
fatigue
is
usually
least
profile.
although,
API does be detersmall
extra
Grinding
given
a
weld
one
Sometimes
for.
the
the can
at
base
causing
joints
that
called
rapidly;
of a notch
possibly
“improvement”
fabricators
toes
weld
adjoining
sensitive
of
FCAWI)pro-
effect
effect.
the
to control.
the
The weld has the plate
the
with
required
(or the
weld-
affect
more difficult
smoothly
the
its
parameters
Semiautomatic
may thus
skilled
to
the
welding
pass
made by less
weld.
weld
from only
single-sided
These
fatigue
is
this
and
In
form the
profile
passes
order.
etc.
the
accessible
backing.
toe.
of
is for
weld
merge
so that
help
the
they
accepted
at
cap
of the
a nonaggressive
performance
of weld
toes
A).
weld
common in not
especially
the
of
through
is
Post-welding
are
also
the
and
undercut,
profile
see Appendix Once
closely,
required
the
completed
excessive
pass
control
be made from the
can
weld
and other
should
generally
used
environment,
has
profile
the
it
without
because
may be “improved”
mined,
welds
size,
of the
riser
quantify
is
complete
distribution
stress
profile
weld
used
without
on the
to
of the
skill
welder,
number,
profile
The
weld
is watched
root
and distortion
often
the
the
groove
in
stresses are
root
parts
Special
passes,
vary
cedures
not
joint.
Subsequent
residual
method,
by a special
thickness,
the
~
penetration
be performed ers.
of the
frame
of the
of complete
so that
Gulf
large done
complete, treatments,
such
of Mexico
practice.
enough with
it
in American
grinding
or
69
left
as post-weld Joint practice
peening
alone
heat
treatment
sizes,
i.e.,
to warrant the
weld
until
to
it
is
(PwHT),
thicknesses,
PWHT. Some work reduce
residual
5.2.3
Fabrication
Defects
In terms concern:
of fracture
welding
macroscopic braces
such
a joint. cracking,
structural
engineer
principal
inadequate
root
porosity,
crack.
of
by
or
to
consulted
welding
discontinuities
gross
gaps
welds,
overlapping cases
are
between
undue
of
of
errors
insufficient
congested
in these
of defects
residual
braces.
to assess
The
the
these
(e.g.,
severity
significance
of
cracks
containing
on the
placed
on the
depth
x 1 inch
absolute on the
trend
Current models
of the
checked
failure,
can withstand in
size
of
fatigue
to
brittle
fracture,
exist
in
or
are
found
concern
found
and
joint
not
is
a fatigue
such not
to
some
defects
be
problem
determining
compromise
welds
all
may
the
inclusions,
initiate
tubular
limitations,
that
a weld with
are
accurately
of defining
the
defects,
length.
and toward of the
the
disposition
There
to
the
known as “fitness
strict
as,
of
for
limits
a
for
on the which
purpose,”
it,
is
have
been
1/8
inch
away from
such
than
a trend
based
purpose
reject
no larger
currently
criteria
serve
defect’s it
impact
was intended;
“fitness
for
service,”
Assessment.”
fitness-for-purpose behavior
evaluations
of crack-like
brittle
tests
is
or to
past,
such
acceptance
joint
Critical
the
discontinuities, In the
defect.
acceptable
surface
against under
could
are
of the
is popularly
or “Engineering
and cracks
Of equal
to accept
severity
standards fitness
defects
are:
such a crack.
The decision based
crack-like
those
that
joints
slag
system
sized).
welded
undercut,
inspection and
in
fusion,
discontinuities
of
found
incomplete lead
weld,
inspections,
characterized
flaw
lead
types
The
errors.
inadvertent
These
the
Because
found
is
can
even
penetration,
of
All
extent.
this
gross
usually
and cracking.
strength
weld
two primary
member misalignment
and is
are
problem. The
the
as
and
Such errors
stresses,
of the
there
discontinuities
errors
in
control
for
(multiple
defects
fracture
design
and
failure
is
welded
single
extreme
loads,
also
considered
to
0
2-
mechanics The defect
joint. possible If
loadings.
70
486-334
on fracture
and other
critical
under
based
in the
initiation,
other
loading)
are
the the
see
if
modes
modeled growth the
of
weld of the
critical
crack does
size not
could
be reached
adversely
affect
Otherwise,
the
weld is
As an example, chord.
strain)
through
The stresses
weld. analysis,
formulas
with
fracture
toughness
of
(COD) of the steels
for
values,
actually
determined then
crack
around
the
is
Welded Joints”.
checked
crack
is
cycle
history,
for
fatigue
considered
assumptions
instance,
it
is
such the
the
from the
model as used to
simplify
known that
the
the
crack
growth
displacement-limited,
occurring
around
growth
However,
experiments
show that,
the
rate
goes little
through-thickness additional
fatigue
the
crack
growth life
of
the
*By the terms “two- or three-dimensional” the stress analysis used to estimate crack tip stress parameters.
is
for
Defects
tests,
it of
in
with
in many cases,
once
increases,
leaving
Assuming
the
the
The conser-
analysis.
slowing
is
stress
analysis.
possibly or
or
fracture
expected
fatigue
model
beneficial
For stress down the
the
crack only
a
two-dimensional*
we refer to the theoretical scope of stress intensity factor and other near-
71
486-334
tests,
three-dimensional*
rapidly joint.
generic
propagation
and the
design
from
BSI PD6493:1980,
loading
above would be used,
redistribution rate.
laws,
in ductile
Levels
The
The
fracture
brittle
by the
loadings.
parame-
weld).
Charpy
for
of Acceptance
element
displacement
be assumed
single
and the
to the
with
the
slice
stress
toughness
presented
growth
in
brace
opening
criteria
applicable
crack
may be available
tip
correlations
multiple
simple
respect
COD might
Derivation
toe
of the
of fracture
as those
passes under
using
which
of
spot
flaw
acceptable.
by finite
effect
crack
measure
value
the
hot
with
The acceptance
for
crack
same two-dimensional* vative
popular
the
is
a two-dimensional
(e.g., notch
If the
weld
including
checked
from conservative
in Fusion
then
then
the
be determined
the
Currently,
on methods
example
chord,
might
for
at
consider
analyses
is
by testing.
found
to
of the
modified
The
derived
likely
generic
the
on Some Methods
the
it
crack
crack
detail
“Guidance
If
is
thickness
material.
be based
fitness-for-purpose,
a small
purpose.
material
structure.
consider
of this
this
of the
and repaired.
the
material
life
rejected
stresses
the
design
joint’s
or by conservative
brittle
might
the
The model of this
(plane
tric
in the
a
3“-’
solution
calculates
conservatively
reach
been implemented In
faster the
to
may be considered
joints,
yielding
Even arbitrary thickness, than
since
or
consists
of
material the
purpose
remaining
crack
size.
cross
crack
crack
is
often
of
plastic
be considered.
fraction
far
modes
section,
might
have
In tubular
of the
joint
more costly
to repair
either
unfit,
by a fitness-for-purpose
eval-
it
must the
be
A weld
repaired.
from
ground
out
under
the
close
assure
that
the
defect
“indication”
of
defect
to
to
is
removed at
the
a very
weld,
This
especially
size
tedious
and then
serves if
actual
slow
one time,
defect.
process,
the
possible
defect
is
corresponds
predicted
tion
the
removal
is
critical
would
programs
other
such as a certain
standard,
usually
Their
defect
be chosen,
Computer
the
a through-thickness
crack
rewelding
the
repair
joint,
and
work. is
inspection
Thus,
initiation,
be judged
absolute
the
inspectors. the
weld
an
A defect
sooner.
fracture
of
modeled
crack.
removing
reinspecting
the
automatically.
a through-thickness
the
by
size
determine
from
rates,
problems
overloading
might
a part-through
uation
to
leakage limits
Should
these
due to
or
crack
brittle
failure
instability,
growth
critical
to solve
addition
crack
the
process;
the
exposed
as an important
embedded
scrutiny
flaws
only
of a team
in
the
a thin
surface
is
layer
of the
indicated
by
in
weld.
inspected
calibration
are
found
of
of for
inspec-
ultrasonic
testing. After the
steps
ing
the
resulting attention, welds
the
weld defect
described correct
for
procedures,
weld should might
is described
the
even
has been original such
removed, weld.
as
However,
preheating,
be as good as an original be above
average.
72
~fl
proceeds
according
more attention
etc.,
can
weld and,
Inspection
below.
486-33#
rewelding
of
to follow-
be expected.
because this
to
of the
weld
and
The extra other
5.3
Inspection
In the scheme find
is
to
them
required
prevent
after for
complex Also,
fabrication
the
quality
The that
are
details
following
to
which
a welder
of
above,
tested.
in
are in the
is
needed.
investigate
the
fracture
control
are
attention
part
to
of
fracture
describes
the
details
inspection
of
procedures
“how-to-do-it”.
Such
listed.
the
full
welders procedures
(SMAW), and fill
in the
chemistries,
designed
place.
Since
groove
skill
vertical, necessary
to
may be designated
to
body of the result
and microstructure,
The API RP-2A requires
the
Charpy
toughness
~
impact
welding
in
pipeline
For example,
hand
held
heat
inputs,
of the
weld
test
of
position
the
a semiautomatic
different
V-notch
73
486-33+
using
using
to
welds.
be qualified.
weld
in
As
root
type
the
and
job.
to do the
root weld
etc.),
this
levels
by the
etc.),
the
must also
make the
procedures
fillet,
is
with
skill set
large
skill
welders
are
overhead,
prevent
special
different
levels
butt,
to
welds,
describes
groove,
themselves
propose
are
penetration
These
horizontal,
different
first
The AWSD1.1
might
Since
care
to
Special
qualifications
penetration,
special
fabricator
rates,
these
(SMAW, FCAW, GMAW, etc.)
(FCAW).
for
the
references
may be qualified.
The welding
rods
important
construction.
go into
occurring
(flat,
construction,
the
in the
or partial
welding
inspections
briefly
not
be identified.
(full
process
of
make one-sided
must
weld
does
from
skill
is
tests
of welds
and special
to
Qualifications
As stated
required
phase,
discussion
but
used,
defects
it
than
qualification Inspection
is difficult, found,
rather
may be necessary.
Welding
weld
end extensive
a weld,
inspection
error.
control
can be found
5.3:1
are
most effective
into
procedures.
joints
welds
installation
aspects
To this
of tubular
the
of defects
and welding
of systematic
overall
related
occur.
defective
In the
of construction
introduction
welders
geometries
possibility
the
they
both
when
phase
welding procedure cooling should
specimens
to
be be
removed
from
energy)
the
qualification.
of 20 ft-lbs
welds
in
actual
plate
5.3.2
Ins~ection
at
tubular
are
tion
of
qualified
and diameters
kinds:
different
that
excessive this
merges
its
examination
welds,
some sort
of NDE is
Non-destructive
ditions, sible.
meaningful
and magnetic allowed
for
electronically results
is
is
of
the
above
difficult,
issued
to
check
base
that
its
profile
material
a
without
may be useful
surface
only
to
see
cracks,
as cratering.
joints
a tubular
joint
the
outside
for
such
For
as
fillet
For
inspection.
technique
test,
other
very
and to
the
difficult. the
joint. or
fact
that
Under
these
x-ray)
are
upon two techniques, using
This
dye penetrants
the con-
impos-
ultrasonic is
sometimes
be performed.
the
most
important
ultrasonic
echoes
technicians. for
Since tubular
the joints,
74
486-334
is
(gamma ray
falls
become the
especially
of
inspections
cannot
has
In this
by skilled
to
glasses
be the
are
and
carefully.
adjoining
such
welders
easy
visually
small
examina-
weld.
hats
it
are watched
of tubular of
A third
testing
joints.
to
hard
inspec-
visual
qualified
make
magnifying
of NDE inspection
particle.
that
to
may detect
likely
process,
colored
defects
Field
likely.
from
when one of the
tubular
for
structure.
important.
assure
the
and
larger
radiographic
burden
Ultrasonic
or
geometry
accessible
So the
representative
(NDE) of the
inspected
with
examination
complex
only
is
gauges
toe,
vitally
welds
examination
a visual
is
weld
Pocket
at
to
Critical
smoothly
welds,
weld
observed
levels,
A visual
the
V-notch
weld material
in the
examination
qualification
weld
due to
be
of the welding
different
in the
is
as-deposited
should
is
Sometimes,
the
undercut.
or
is
is working.
it
purpose.
welds
observation
process
used.
welder
see
A CVE (Charpy
to be welded
and non-destructive
Upon completion, and
of the pieces
of production
The welding are
tested.
of Welds
of three
procedures
and
required
The test
thicknesses
of the weld,
welders
O“F is
joints.
The inspection tions
welds
D6
inspection of flaws
interpretation the
API has
technique are measured of the published
test a
“Recommended Practice
for
Ultrasonic
cation
for
Qualification
and Guidelines
The test angle
results
are
best
brace-to-chord
conclusive
tubular
inspection.
of Ultrasonic
plates,
ultrasonic
and least
testing
is
Structural
Technicians,”
Because
often
used
FabriAPI RP-2X.
conclusive
in K- and Y-joints.
field.
magnetic
field;
surface.
for
detecting
inspection
Flaws this
flaws
tested In
some
equipment.
deeper
for
the
no single
in
acutetest
conjunction
this cannot
Besides
tubular Here
of the
fort hese
welds welds
is with
Treatment
possibility
inherent welding
weld in
the
process
on other
is
around
the
washed
cracks
would
and
distort
particles becomes
in
the
welds
a the
dusted
less
on
effective
plate.
Typical
to be both
ultra-
in
lieu
of
inaccessible
over
the
to
metal
can be detected
one of the
and
the
testing
penetrates
visually.
However,
defects. pipe
seam
can
and
girth
be used
Typical and/or
welds
(usually
inspection
must
also
be
gamma radiation requirements
ultrasonically
tested.
call Miscel-
of methods.
would
joint
For
may be at
creating
Welds
error
joint.
joints
is
when a defective
of systematic
entire
weld
by a variety
In common practice,
it
may be used
portability).
of Defective
is that
particle
inspected.
methods
inspected
magnetic
joint
to be 100% radiographed
are
in
100% of tubular
welds,
radiographic
equipment’s
flow
found
subsurface
joint
current
are
in the
detect
the
flaws
a dye
metal,
the
be seen
when the
method,
magnetic
through
penetrants
example,
is
passed
alter
particle
dye
technique
technique the
for
Dye left
dye penetrants
inspected.
call
is
can
to this
instances,
cracks.
5.3.3
the
major
metal
distortion
For In
surface
the
and magnetic
methods:
laneous
second current
in
requirements
sonically
because
the
One drawback
the
above
joints
Here an electric
magnetic
work
of Offshore
NDE technique. For
the
flat
weld areas
by itself,
another
for
Examination
be checked
would
example,
fault.
weld
the
fit
The welder
0 (?
If to
see
up may have himself
as well.
75
found
first.
be inspected
be examined
486-334
is
and rejected, not already if
the
been
may be at The quality
the done,
problem
poor, fault,
is
or the so his
of a welder’s
work
remains
other
defects
welder’s type
nearly
constant,
are
work,
likely
so if to
he would
a defect
exist.
be disqual
If if- ed
is
too
(at
found
in
one of
his
welds,
are
found
in
many defects
least
until
retraining)
for
a
that
of welding. When checking
useful
to
should
be identified
water
have
Gulf
each
weld
since
construction
occur
due
fabrication
to
example
to the
jacket.
through
cause sion. trations
the
the the
out,
more
during
are
not always
available.
complete,
inspection
of the
to
“How” shallow
records,
as
indicated
subtle
opportunities
like
knife-edge
can
for
cracks
to
to a welding
between
the
is
a sacrificial
anode,
resulting
into
the
making metal,
local
jacket
composition This
them. in
deck
located
the
chemical
up between
weld,
and
A
the
machine
barge
set
the
initiate.
as welding
has a different
potential
obviously
processes.
ground
weld metal
slices
initiation
such
flowing
of
is to prevent
operations,
or inadequate
the
crack
procedures.
inspections
and launch
welding
structure’s
of installation
of these
causing
may be pitting
or
execution
main goal
an electric
behave
The damage
proper
a current
Since
metal,
is
transport,
offshore
create
water.
worse,
weld
and welds
Overloads load
also
can
weld
to
operators/fabricators
the
An improper
base
refer
of
on the
accidents.
is
barge
can
is
remember who made
a number
phase
control,
sensitive
prime
was done.
can usually
they
it
On typical
specifications.
or
joints,
of Installation
There are
than
of welders
different
and how each
inspectors
small
with
among
weld
or
the
are
concentrates
in the
on a
crews
of fracture
damage
drawings
platforms,
records
Once the
In terms
on the
interviews
Inspection
errors
systematic
of who made which
the
Our
detailed
!5.3.4
records
of Mexico
available. that
for
rapid
stress
effectively
can
corroconcen-
initiating
cracks. Therefore, trol, related
even
if
result
all a direct is
not
inspections correlation apparent.
of the
construction
between
the
Inspection
regulations of welds
76
486-334
(3 ff
may affect
in
fracture
and the tubular
con-
fracturejoints
for
defects
is
the
most
those
which
tions
are equally
5.4
call
for
Offshore
state
inspection
grounding
Procedures
of offshore
such
welding
as
opera-
not as obvious.
technology
and ultrasonic
Yet the
art.
for
measure.
Topics
welding
of the
control
of the
but
construction
cored
practice
fracture
important,
Two Special
of flux
obvious
ultrasonic
acceptance
criteria.
method
of joint
fabrication.
A second
topic
changing
testing
introduction
testing
defect
is
rapidly.
have
significantly
in 1980 of the
stirred
a major
Closely
related
advanced
the
API RP-2X recommended
controversy
to
The following
The introduction
this
over
criteria
is
examines
this
discussion
weld
root
the
frame contro-
versy.
during yard
transport. to
the
concern. 5.4.1
toward
Fatigue
site. of this
is
problem
is
Acceptance
Criteria
Before
RP-2X was
first
the
size
of defects
from the In
fatigue
smooth
to
bridges,
and
as
buildings.
fatigue
is
expressed,
fatigue,
the
and
design
less
Recognizing
loosely
weld
in tubular
are
fatigue Thus, are
usually
in
somewhere
between
bridges
and
specific
welds
criteria, for
the
to
criteria fill
the
on the
joint
ground
smooth
to
assume
reject
criteria
in their
the
The criteria
apply
in
77
(m
to are,
regards
to
operator.
acceptable are
are
structures
buildings
a
as-welded
criteria
offshore
by
improve
calculations
left
welds
specified
gap in specifications
I
486-334
become a
guidance are
up to the
and quantifies operator.
thus
criteria
strict
joints
the
1980,
So more relaxed
important.
fabrication
tubular
life
tubular
and building joint
in
that
AWS131.1 leaves
has
control
discussed.
surfaces
welds
The API RP-2X was introduced bridge
transport
of reject
defects.
fracture
from the
of full-penetration
the
In buildings,
to
tows
published
root
minimum weld
longer
also
Two levels
AWSD1.1.
bridges.
condition,
in the
performance,
surface
applied
API
attention
during
Weld Root Defect
AWSD1.1.
the
increased
recently trend
installation
came mainly
their
the
A current
The handling
acceptable
the
is
based
between defect
sizes
on experience
with
Gulf
of
template-type and
notch
assumed,
Mexico
type
platforms
are
tough i.e.,
quantified
the
applied
acceptable
defect
in the
root
area
versus
1/8”
x 2“).
is
How is
are
less
detrimental
the
weld.
the
root
at
actually
to
have
significantly
rather
than
root
So what criticism root
that
defect
is
experience
backed
up by further
effect
mentioned
braces. protect the
art
Second, the in
overall
alone criteria
testing
above
could
there
is
status fracture
~
in
toe,
weld fatigue
about
these
for and
not
and
practices
Specifically,
78
in
weld
is
not
of tubular
shell
passes
there
the
more critical
criteria
are
as
defects,
has
relaxing
been the
should shell
in
be
bending
compression
were designed advance
or
so gross
criteria
example,
to
weld
joints.
for
these
these
to
toe
First,
than
the
in
And third,
stresses,
criteria
effect
is pulled
passes.
justification
For
these
stresses
due
root
defect
criteria?
defects
that
fabrication
root
situations;
make root
in
to be compressive
weld
strength
enough
all
the
AWS
elsewhere
The notch
low tension,
since
area
analysis.
a feeling
control.
as the
defects
and then
tend
x 1/2”
root
effect),
also
but,
the
that defects
a
all,
brace
applied
the
is
of
When the
very
root
the
as long
controversial
acceptance
at
1/8”
key crack-driving
toe.
root
long;
(i.e.,
weld.
cri-
of the
and 1/2”
First
in the
or
at the
control
wide
than
(Poisson
by subsequently
the
defects,
all
elsewhere
shrinks
shown that, affect
that
also
relaxed
outside
2“ long
repair)
to the
stresses
relieved
to
is
what have been
greatly
API RP-2X was released,
than
than
the
1/8”
profiles
of Mexico practices.
justified?
compression,
lower
stress tests
criteria
restricted
in
residual
substantially
effectively
than
redundant
surface
Thus,
For example,
less
is
radius
placed
Second,
least
fatigue
mean brace
is
is,
fabrication
Gulf
may be up to
explanation
is often
joint
typical
root.
the
less
of
criteria
is
of
are typically
the
these
defect
as-welded
from one side.
(and more difficult
A probable
bending.
wide
method
for
it
even before
of a weld profile
is
if
That
typical
are
weld
relaxation
recognized,
root
in the
a 1/8”
has long
area
of
acceptable
this
frame
criteria
aspect
to defects
planar
tension,
The
construction.
with
and inspection
A controversial teria
and
assumed,
materials.
welding
are
design
the are
more to state
of
based
on
typical lent
quality
levels
achievable
Gulf
of Mexico
(rather
in
benign
wave load
fatigue
To understand tubular
joint
ing.
This
occur
rather
weld
welds
is
very
of root
structures
still
have
(3) the
repair
of of
qualitative to
be
extreme
and
defects
root root
defects
of comparable still
very
costly,
is
involved
and a high
In short,
they
there
no
not
unusual
that
correctly
uncertainty complex
in geome-
all
structure by
right);
and
considering
the
the
experience, criteria
stricter
are
were missed
be all
that
extensive
the
that
especially
for
they
repaired, size
num-
in installed
that
probably
need
in
must exist
has shown these
is
is
(1) significant
probability
say that
explanations, so
that,
were
would
the
completely.
indicates
detected
to
should
factor
large
size
structures
that,
(due to the
API RP-2X argue
these
no back-
be expected
difficulty
defects
method,
with
might
is
of weld
root
frame
Another
There
type
structure
fine,
defects
API RP-2X acceptable
stress-related
practices
preva-
practice
outside,
is so great
have
defect.
of the
defects
welds
to be redone.
Sea)
in the
the
difficulty
of missing
the
root
root
outside
with
the
so weld
of this
of the
all
side,
that,
may be no better.
root
tests
(and
number
need
a
numerous
inspection
large
of
proponents
if
of fabrication
or North
one
the
the
probability
(2) even
do,
weld
from
and experience
right;
the
to
repair
defects
method
recall
criticism,
In fact,
ultrasonic
Therefore,
would
be made from
size
and a high
hers
must
the
the
interpreting try),
second
conducted
estimating
frame
new frontier
this
frequently.
inspections
than
the
damage spectra.
difficult
be repaired,
using
repair
will
with
sound
and current
standards
to
be
applied. Actually in
other
defects
fields. often
The question these have the past,
this
root
will
be found
of the
been
something there
defects
new,
all
experience
refined
where
significance
detected
they
perhaps
to
in fracture
of inspection
previously
were
defects
related
to
The position
by ultrasonic
testing
defects
by the
have
exist.
raised.
Are
practice?
Or,
API RP-2X is
always particle
and
introduced,
known to
in
to be acceptable.
~ @
not
a change
control is
immediately
or magnetic
79
486-334
is
taken
by radiographic
has shown these
find
method
of these
along?
but were not detectable
and that
is
When a new or
defects they
situation
existed
that
in
inspection,
the
So, possible with
what
to
high
is
fabricate
hot-spot
technique The response
node
to (i.e.,
was to
fabricate
of the
oven for are
welded
splicing
the
braces
to
Complicated
can imagine
the
fabricated
locations
may be
chosen
on the
Second,
special
steel
Third,
a higher
forth,
when
jigs,
than
the
location
the
when the of
in
easily
of
control is
of a cylinder, this
from
both
brace
welds
are
stub
With the large root
sides only
for
from the
and
practice,
strict
drawback,
however;
unambiguous standards it
is
on the
instead
accessible
apply costlier
to than
these
spot
the
80
Those
the
girth One
may be
locations
skill,
etc.
as the
can.
welding,
and
or
special
in
ground. at
welds
a are
stresses.
inspection.
Fourth, cumbersome
made around
Fifth,
and most
weld
(The
so
less
may
be
critical
outside.) from two sides
are
root
to
nodes
(brace-to-can)
fabrication
inspection
the
ground,
longer
reduced
“Tinker-Toys.”
up,
no
from the
root
fit
or
simple
as well
PWHT is
and
weld
defects
over
ends,
eliminated.
yard.
off
welding
brace
then
First,
feet
critical
brace-to-can
with
a hundred
most
accessible
node into
can with
stubs,
of PWHT
to put the
fabrication
the
in
from the
are
assembly
away from hot the
context,
to the
method.
in
surfaces;
The
tubular
problem
is
preparations
fabricated
without
paths
separate
assembly
possible
is working
welds
and then
availability,
is
not
practice, large
of a can and the
final
be used
assembly
the
the
Sea
as a “node” 4),
this
steel
can
welder
accessible
the
of
of two cylindrical
important
of
than
basis
level
circumference
eliminate
other
North
as building
from
root
weld
to the
and edge
result
in the
The solution
chords
structure
load
(PWHT) of
Field
and the
ups
of the
a smaller
intersection
fit
advantages
in
can.
stubs
field
in
connections
the
in critical
treatment
(Figure
“t
node method.
a node consists
to
the
assembly
Several
heat
and chord
PWHT. Thus,
which
welds.
weld
be moot were
defects
originated
brace-to-chord
stubs ,
large
thicknesses).
braces
likely
those
is known as the
post
plate
the
such
fabrication
for
would
especially
, without
of
need
large
main lengths
joints,
possible
method
the
joints
tubular
this
The point
argument?
stresses
making
a large
the
since
welding
possible.
So,
defects. frame
it
The node method.
is
possible
from the in
North
method
has
to back Sea one
Thus, of
weld
stage
is
and
defects,
eliminate the
the
the
largest
set
for
knowing
that
The
argue
rationale
alternate
with
root
defect
rejection
experience the
is
Mexico?
made.
about
Thus,
damage imposed
5.4.2
to
can
opponents
of
against
criteria
to
back
and forth.
part
Its
it.
be all
right. So have
which must be answered
criteria
are
outside
design
operation,
of
and its
of the
Gulf
of
have
been
regarding
acceptable
the
of
of fracture
Gulf
in
certain
environment
assumptions
aspects
of the
and elsewhere,
fitness-for-purpose,
and other
outside
here,
fatigue
criteria
years
rate control
fatigue
which
may
The investigation
of Mexico.
probably
In the lation
for
loads,
such
conditions
past,
design,
loads , causing
attitude for
aspects
has the
the
and A full
as simply
another
in
in some
transport trend
upon
areas,
pitch,
was
in
con-
dismantling,
thereby
transport
is
design
treatment
including
fatigue
@)
part
as other
loadings,
and heave), With the
81
486-334
perhaps
failures found
a current
frontier
design
were not.
structure,
been during
same treatment
individual roll,
fatigue,
structure.
of the
is,
barge
changed,
is
(and,
and atten-
exposing
risk.
roughly
That
(e.g.,
for
was considered
receiving
as launch. expected
fracture
to
have
initiation
there
tows
concern
Occasional
Cracks
important,
especially
transport
concern.
fracture.
longer
site,
this
of increased
and crack
most
toward
to greater
an area
motivated
involving
installation
structure
been
installation,
practices the
Transport has
have
after
And,
struction
this
occurred,
soon
suspected.
During
factors
have
structures
mission
these
areas
Control
recent
transport
port
for
Several
this
which
continue.
In
repeated
argue
argued
dislike
exists
probability,
criteria
been
a natural
fabrication
high
relaxed
its
by wave loads,
Fracture
tion.
these
respect
“purpose”
be suitable
and debate
the
very
to be asked
structure, into
the
are
with
the
built
levels,
from)
of
shown these
has
question
how suitable
As discussed
assumptions
not
Due to
criteria.
future,
risk
has
criteria
The most important the
a method
defects
that
behind
controversy.
weld
API RP-2X relaxed
proponents
the
trend
recognized
the
construction most extreme
were considered, toward as
may be given analysis.
of instal-
longer an
but tows,
additional
to the
trans-
On the and
after
directed
rather
than
toward
the the
more attention
has
such
been
for
plete.
is
out,
can visual
the
to see
An awareness ture’s
Iife
has
to
long
subject for ’short developing
case
It
is
a long fatigue
form
combined,
structure
growing
will
that
performed
tow
spots
plus
route.
the
sea
as
diagram
aboard
the
And after
it that
is
severe
com-
loadings,
for
fracture
control
to
be only
a quick
this
phase
particularly
true
areas,
but
of
a struc-
of structures it
A sophisticated
fastenings
addition,
the
analysis,
to
be
is also
example
North
Pacific
the
dynamic the
the
response
states
fatigue
true of the
damage
at
and added
is
expected is
each to the
limited hot
spot
West
in which
would be necessary. barge
to
and plathistory
performed
for analysis
to a Miner’s is
carried
damage expected
is
be encountered
a deterministic
fatigue
U.S.
The loading
analysis
(alternatively,
the
in Japan,
of the
design.
sea
on
be built
the
of
installed
might
across
A spectral
In
the
in
to Frontier
platform
of
or
fatigue
is
some degree.
part
Damage due to transport
inservice
likely
problems This
the
be, used). less.
carried
gener-
intact.
platform
including
the
launched),
time
now be described.
analysis,
along
inspection is
the
installation
the
rough water
one month,
by a scatter
until
of potentially
next
barge,
is so important
a series
lately.
over
described
“impossible”
control
hypothetical
is
is
of installation
is
to
it
has
the
(between
equipment
an inspection
fracture
anticipated
tow,
A full
the
of Mexico tows
a
is
that
tows across
Consider Coast.
control
to
and instrumentation.
rest
and launch,
of
practice
valves,
such
time
during
attention
structure
transport
installation
after
been
Gulf
and the
the
control
the
site
well
if
tying After
of the
And then
fracture
itself.
lines,
speed
not until
place.
check
to
fracture
transport,
take
paid
has been low in priority
the
fastenings
intended
as hydraulic
an inspection
load
is
launched,
So it
sea
its
control
transport,
structure
reaches
structure,
fracture
During
toward
structure
ally
end,
transport.
been
the
inspection
over
all
hot might
sum of 1.0 over
to
the
the
life
of
structure. Such fatigue
important important
general to conduct
analyses
have
conclusions such
been
performed
from
those
an analysis
for
analyses a long
82
4$6-334
for
0
%’
tow.
various
platforms.
are
follows.
as
Knowing the
Some It
character-
is
istics
of
the
temporary
the
particularly transport
design
of
those
near
likely
but
fatigue
structure
comparison further
in
optional
the
platform
Verifying
should
tie
the
number
downs
probably
and
the.
barge
to
of
the
joints
little
in
barge.
wave loadings is
flexibility
Transport
be considered.
significant
tows
area special
sensitive
future next
are
and
are
and
fatigue the
may
structure,
However, resisted
interaction
involved, inspected
attention joints.
may be made to with
vital,
since
the
by different
between
transport
fatigue.
protected
transport
is
inservice
when long
visual,
used
the
there
and inservice
intermediate
a
and the
systems,
Finally,
be
supports
loadings
framing
the
to
structural
control
fatigue
barge
is
its
and
inspections
is
Structural
paid
to
important
installation,
integrity
a more detailed given
in
Integrity
of
Section Outer
may be taken
The inspections major
and provide
most
joints
and
survey
a baseline
surveys
description 9 of the
to an
are
a complete
Post-installation
inspections.
section
structure
there.
After
check
the
are
of
of for
treated
required
and
“Requirements
for
Continental
Shelf
(OCS)
Platforms.”
5.5
Principal
References
1.
American Petroleum Institute, “API Recommended Practice Designing, and Constructing Fixed Offshore Platforms:” “Installation Forces;” Section 3, “Welding;” Section 4, “Installation;” “Inspection,” Section 5, and Section 6, Thirteenth Edition, Dallas, Texas, January 1982.
2.
American Petroleum Institute, “API Recommended Practice for Examination of Offshore Structural Fabrication and Guidelines fication of Ultrasonic Technicians,” API RP-2X, Dallas, Texas,
3.
American Welding Society, “Structural Third Edition, Miami, Florida, 1979.
4.
American Welding Society, Miami, Florida, 1976.
5.
Marshall, P.W., H.F. Crick, and P.T. Marks, “Welding and Inspection,” Chapter 27 in The Design of Fixed Offshore Structures, edited by B. McClelland, to be published by Van Nostrand Reinhold, New York, 1982.
“Guide
for
Welding Steel
Code - Steel,” Hull
Welding,”
for Planning, Section 2.4, “Fabrication;” API RP-2A,
Ultrasonic for Quali1980. AWSD1.1-79, AWSD3.5-76,
6.
Marshall, P.W., “Experienced-Based, Criteria for Tubular Structures,” May 1982.
7.
British Standards Institution, tion of Acceptance Levels for 6493, London, 1980.
8.
American Bureau Installations,” (Draft ) 1982.
9.
Us.
Fitness-for-Purpose AWS-WI Conference,
Ultrasonic Reject Atlanta, Georgia,
“Guidance on Some Methods for the Defects in Fusion Welded Joints,”
of Shipping, “Rules for Building and Classing Special Committee on Offshore Installations,
Geological Survey, Integrity of OCS Platforms, Conservation Division-OCS October 1979.
Offshore New York,
Requirements for Verifying the Structural U.S. Geological Survey especially Section Platform Verificatio~ Program-Order No. 8,
10.
Institution of Civil Engineers, Fatigue in Offshore Structural ed. by J. R. Smith, H. O. Crisp, N. A. Randall, and F. Sterry, Telford Ltd., London, 1981.
11.
Burnside, O. H., et al., “Long-Term Corrosion Year Assessment,” U.S. Coast Steels -- First Southwest Research Institute, August 1981.
84
486-334
DerivaBSI PD
Q ~L
Fatigue Guard
Steels, Thomas
of Welded Marine Interim Report by
6.0
CURRENTPRACTICES: The fourth
major
offshore
structures
aspects
of operation
received
the is
practices
described of
is
structure
considered here
practice.
followed
varies
an
cracks
from
questions
important
repaired?
and
however,
principal
by the
the
degree
from
operator
advances
addressed
during
When they
occurring?
during
are
activity
How are
control
cracks
operation
steel
the
many
repair
have
the
subject
elements
industry to
of
the
and form the
which to
of
the
standard
operator
and
from
found? are
in this
operation? do
occur,
And what
being
section: What
is
how are
aspects
reconsidered
Why is fracture done
cracks
to
assessed
of conventional
due to
prevent
recent
and
fracture
experience
and
in technology?
Philosophy
The primary risk*
ditions
of of
structure (or
control
is
accidents
the
other
as
In with
and
necessary,
*Risk equals summed over
their repairs
it
control
structure must
was
be
during
in the
designed
to
protecting
collisions)
damaged
the
exist,
preserving
the
significance
structure
of
integrity evaluated
the of
before
when the
design
conditions,
conditions,
from damage, operating
are
when fracture
either
conditions fracture
structure: they
minimize
conditions,
structure, the
to
Two con-
and damaged
Under
is
environment.
routine
or due to normal
conditions
operation
offshore
considered:
may be present.
with
boat
fracture
the
damage)
concerned
corrosion).
of
structure
(e.g.,
concerned
goal
operating
exists
cracks
found
The
accepted
considerably
fixed Of
inspection Here,
sense.
However,
for
to structure.
control
the
control,
uniformly
control
installation.
literature.
a wider
are
fracture
after
fracture
in the in
to
operation to
attention
The following
6.1
related
their
related
a standard
practict?
activity
is
most
operation
basis
OPERATION
due to (e.g.,
control
cracks
must
be
dangerous,
and
if
must be made.
probability all possible
of failure times failure events.
85
the
loss
is
as a consequence
of failure
To these the
ends
inspection
high
more likely
cracks
the
structure.
ation,
be found cracks
are
an operator
has,
and the
of
that
able,
the
depending
6.2
might
on his
Current
the
6.2.1
the
the
necessary 6.3,
other
initiate
crack’s
is
doing
“fail-
inspections
to
risk is
risk
different
the
repair
short-term
of
riskiest
by changing between
of oper-
option
the
routine
several
types
procedures
and
crack
operation
the
to be the
effec-
on the
routine
minimize
the
between
a repair
the
may prove
during
operation
of cracks,
impact
find
on the
steps.
Inspection
avail-
long-term
fixes
can
and Monitoring.
into
the
any cracks
that
may
structure,
and take
and monitor
integrity
The task
be divided
of
of the
finding
The other
cracks
tasks
are
is
discussed
discussed
in
below.
O~eration
structure
control
aspects
are mostly
damage during
from
and
falling
initiating
cracks.
procedures
written
protection
system
due
These
can
or inadequate
concerned
pieces
for
of the
occurring.
operation
collisions
Improper
redundant
and low level
changing
the
chose
formation
The fracture
and
(1)
trade-off
Besides
practices
remedial
Routine
offshore
goal
The third
often
control
assess
Section
the
and the
Practices
minimize
start,
But for
cost
needs.
Fracture tasks:
can
but
will
may be
members may be more cost
of avoiding
be done.
operator
crack,
First,
inspections
more it
small.
between
that
choice
mode may be best.
the
ruptured
circumstances,
operating repair
still
to consider.
periodic
may not be dangerous,
on the
in certain
for
level,
or even
Remembering
trade-offs
chosen
while
trade-offs
no repair
a repair,
important
inspection
cracks
there
doing
the
will
large
Second, (2)
and amount)
small
aimed at finding
and
(type
The higher
structures,
tive.
are three
level
or low.
safe”
there
the lead
can
with
routine
operation
minimizing
As discussed to
of
accidents
or
equipment
are
be avoided
in
of a fixed
high Section
improper
common examples
by closely
observing
Improper
maintenance
to
damage,
including
maintenance
of appurtenances,
86
486-334
D (?
2,
such
cracks, cracks
maintenance.
structure. corrosion
stresses,
steel
of crack
can Boat
of
accidents
the
operating
the
cathodic
initiations.
as cathodic
protec-
tion
sleeves,
can lead
through
the
avoided
by closely
structure,
Another manding”
of
forms
to
risk
important
hurricanes
associated
of
with
the
they
have
aspect
large
control
well
In the
Gulf
enough
in
falling
damage can be
measures
routine
evacuation
advance
the
most
are also
operation
or “de-
of Mexico the for
plat-
lowers serious
the
conse-
important. are
close
the
substantially
by limiting
during
such
is the
a platform
avoiding
Not all
specifically
for
of cracks.
of routine
initiated.
Again
operation
failure
risk
supports,
procedures.
De-manning
Other
from their
damage.
storms.
be predicted
structural
aspect
impact
of routine
for
development
The last
detaching
operating
evacuated.
fracture
minimizing
the
can
of failure.
aspects
objects
causing
structure
be safely
quences
and
following
of the
passing
to these
This
is
operation
discussed
is
the
below
monitoring
in Section
of cracks
6.3,
after
Inspection
and
has been found
in
Monitoring. Suppose the
for
structure.
integrity
of Cracks
There
four
are
integrity.
effect
of the effect
is
steps
on the
the
damaged
action
must
in
evaluations,
the
joint
element
to
that
of
of the
crack’s
crack
impact
impact
on struc-
must be determined.
structure.
A fifth the
a crack’s
member must be studied,
on the
determine
of the
measures.
evaluation
upon.
a crack
evaluation
or structural
be decided is
the
remedial
and location
crack
of
discussion
now with
in
The size
remedial such
of this
and with
Evaluation
tural
the
rest
The concern
on structural
6.2.2
the
Finally,
step,
which
cause
of
cracking,
are
of
primary
with
a
is
The as must
a course often if
not
of
included already
known. The length
of
however, evaluation construction
size a crack
and
location
of
surface
crack
can
depth
cannot
(NDE) techniques inspections,
be have
ultrasonic
the be
crack
determined
measured been
from
applied testing
photographic
photographs.
underwater. has
gained
importance.
survey;
Non-destructive As is
in
The
the
popularity.
case
in
With
ultrasonic
methods
the
in
the
of plastic
NDE is
certain both
use
crack
circumstances, surface
The assessment
sis.
The analysis
tion
Defects.
of
The a fracture
by a fracture might
a tubular
joint
fatigue of
life
analysis
rapidly
at
mainly
the
of
function
the
of
crack of the
Due to fatigue
the
life with
of
crack
size
difficulties a crack,
assumption
that
strength
of the
“damaged”
structure
offshore
structures
are
one or more braces
(beyond
overloading
Then
collapse. redundant
without the
structure
as-designed cumstances
is
ultimate a
the
load
new fatigue
So the
critical
if
the
growth
crack,
the
while
the
life
analyFabricamay
may also
that
the
be be
critical
fracture; to
fatigue
and,
strength
crack
rate
for
repair
ultimate
of
size
for
for
the
purpose
has
accelerated
life,
a
comprised
may be a very
small,
critical
effect
or the
is then
evaluated. they
member,
can
So the that
of
the
the
weak
analysis
for
usually
usually The
most fixed stand
structure
is the
diminished
the
loss
of for
structure strength
to of
the
to
the
In some cir-
measure. “damaged”
steel
analyzed
strength
ultimate
the
and the
has separated.
Since
relative
size
structure
push
diminished
some other
crack
on the
joint,
levels)
of or
structural
life
or conjecture
of the
measure
strength,
note
on the
predicting
ratio
fatigue
of a crack
much difficulty.
relevant
The
costly
redundant,
design
the
fracture
very
growth
the
highly
brittle
may be judged
analysis
the
crack,
definition.
in
the
for
a definition
slow
the
5.2.3,
brittle
a critical
and
in Section
than
However,
initiation
or
other
effect
of
“open”
joint
However,
understood.
size
critical
crack
analysis.
may be merely
Under
crack.
impression.
on the
by criteria
analysis.
helps
from the
analysis.
the
of the
innovation
known as a fitness-for-purpose
the
crack
yet
defined
stress
same as described
mechanics
not
tensile
effect
mechanics
Also, is
performing
begins
the
a through-thickness critical.
an impression
is also
be determined
therefore,
get
crack’s
are
A recent
be estimated.
measurable
potential
estimated
example,
are
the
steps
by
size
to if
has initiated
evaluated
crack
molds
and depth
it
can also
especially
length
member in which
depth
structure
is
in
order. Once the effect must
of the be made.
effect
of the
damaged element Should
the
crack on the
crack
on the
joint
structure
be repaired?
or structural have
been
If yes,
member and the
assessed, then
a decision
how should
it
be
It
repai red? repairs
may add
understood
choice
which
of
cle
risk
the
must
risk
the
tion not
already
this
the
is,
that
only
the
risk
to
cost
however,
neering
without should
repairs
The decision not
analytical--’’engi
structure That
obvious
to quantify.
sometimes
predict
the
included
a determination
Such an analysis
be
may also
repair
and the
of each
choice,
this
judgement”
if
and
could
locations
the
crack
identify of other
appears
of a crack
calibrates
as a fifth
is made of the
the
appearance
performance
decision
being
the
is vehi-
cause
generic possible
in the
crack,
if
problems
with
the
This
cracks.
as an indicator
fracture
potential
evalua-
of the
in an unexpected
serves
the
step
for
step
spot.
So,
of the
struc-
other
in
spots
in
structure.
Repair
6.2.3
The
of Cracks
repair
of
a crack
expensive,
potentially
comparable
repair
done
avoid
such
operator
to
from starting ing
the
tive
is
rigs,
(i.e.,
operation greater
e.g.,
docked,
least
First,
enough,
they
radius,
without be,
size
can
is
operators the
of the
by
be repaired
486-334
than
the for
the
rigs
grinding
the
for
the
cracks
or by justifyThis
operators
can
for
than
the
cracks.
incen-
of mobile
be regularly
dry-
repairs. chosen
or large?
definition
extremely
incentive
etc.)
procedure
small
is
by preventing
water,
by grinding.”
89
great
selection,
mobile
simply
A functional
thus
either
repair
crack
joint
more expensive
repairing
in calm shallow
repaired
rewelding.
without
the
underwater
of magnitude
material
because
must be made, often
“one that
design,
platform
elevated
an
when possible,
structure
fixed
is the are
in
There
land.
repairs
semisubmersibles,
or at
factors.
on dry
through
for
offshore
one or two orders
of the
When a repair
thus
less
consider
the
structure.
As commonly practiced,
is
important
context,
ture’s
that
and thus
especially
is
in
analysis.
known.
structure,
It
therefore
analysis
of cracks;
crack
of the
analyses.
than
decision
the
operation
of each.
more intuitive for
the
leaving
is more difficult
A failure
is
to
that
above
repairs
also
often
obvious
from the
add risk,.
but
is
If cracks
them of
A large
depends
out
a small crack
with
on many are
small
a
large
crack is
might
generally
considered
to
be
anything
through-thickness part-through
cracks
work,
the
repair
factors
the
of the
Thus,
apply
less
removed
always
underwater stress
from
repaired underwater to
highly
stressed
is
material
back
tubular
region
so as
toughness
not
offshore
only
the
structure
joint
in.
repair
operation,
the
A recent clamps.
These of
in terms
often
is
it
joint
are
is
such
entire of
and
problem water.
involved,
and
price
are
than
nodal
is
paid
in
a more
in
part
proportional
far
joint
produce
(This is
not
the
than
for
higher
the also
The are
away from the
node
in risk.
variability
The
which
duration:
the
material,
otherwise
degree
weather their
dry.
repair.
would
position
not
However,
the
rewelds
is
a normal
welds, enough
of
(or node)
while
precarious
to
opportunity
themselves.
that
but
to
the
a way as to
the
be a problem
of
to do,
low quality
circumferential
lower
difficult
prevent
to
to complete
to
repair
surrounding
structure
not
favorable
by the
in
is
weld,
are
quenching
the
joint
important.
is
into
due
be
of confi-
ldhile
the
node
than
with
the
the the
risk
longer
of the
is.
development
a cracked
out
are
that
rewelding
and inspected
left
operations
clamps
raised
node
confidence
do
stress.
the
joint
to
precautions
less
rewelded
control
Also,
riskier
weld
of money and time, is
a
then
a high
Of course,
cracked
small,
required
special
is
is
more
but
time
available
that
a leg
itself
the
perhaps
required.
out,
Large
than
especially
required
precautions
and
is
is
rapid are
requires
if
point
and residual
structure
placed
do than
necessitating
chord
repair
and the
this
of
welds to reconnect the
easier
is
repair
window,
unusual,
or wet-backed
especially
node
dence,
to
the
Underwater
special
wet
The crack
operation.)
not
options
concentration
the
feasible,
brace
crack.
repair,
last
precautions
Among the
geometric
is
possibility
special
A more reliable be
This
other
the
unless
the
weather
difficulty.
of
and brittleness. to
given
of the
the
rewelding
increased
because
water.
through-thickness more costly
cost
be made properly.
However,
part,
the
conditions,
underwater
required.
in
a
significantly
include
diving
can
For example,
because
are
than
cracks.
Other the
bigger
in
tubular
joint
prefabricated with
repair
onshore
good clearance
to
is fit
between
the
90
486-334
C-9 o ‘“
the around inside
use
of
the
grouted
brace
of the
and clamp
and the which
outside are
the
bolted
clamp
the
tubulars.
together
This
is
poor
of the
repair
of a joint
clamps,
of
with
joint.
etc.
joint’s
stiffness,
the in
does
allowing
the
load
live it
cracking high
must also
mitigate
is
higher
over
the
use
fit
the
the
repairs,
risk
making of this
than
node
or
of
stress
good.
into
crack
the can
the
example,
immediate
temporary
season
rough
braces
repair
weather,
is to
4$6-334
high
design
of the con-
affects
a
It
may
example,
reveal
a year
this For
example,
brace
without
to
which
any problem,
temporary
a severe
and perma-
fatigue
A permanent
Thus,
the
permanent
reduce
the
cyclic
to
the
structure
get
means
path.
solution.
adopted
grow
by removing
loads.
be made between
to
crack
a redundant
load
be added
doesn’t
and may now be seen
fail-safe
91
problem
fixed.
Typically,
be removed
solution.
be fabri-
research
as it
the
might
be followed
The
of the
is
as long
“repair”
cracking.
often
itself
or
analysis
of the
can
how a clamp
extension.
be a temporary
cause that
to
original
should
failure
only
crack
Another
a joint
they
new and
crack
concentration in the
without
can be inspected. the
crack
tubulars
geometry
joints,
a small
appropriate
friction
friction.
that
relatively
joint
is
of
up due to out-of-roundness
clamped
with
is
to the
still
to flow down another
Should
the
of
Sea
The immediate
by steel-on-steel
fit
mean that
some distinction
rewelding
for
North
confidence
clamps
to
of
always
much load
repairs.
require,
complete
short,
snugly
respect
clamped
more harm than
the
with
may have beerr used
too
friction
difficult
can
to
in place
of clamps
not
of
Finally, nent
are
Then
sources
may bring
over
and how the
of
do relatively
to
between
window is
an advancement
held
performance
fatigue.
plates
weather
and the
fabricated
tolerances
structure
cause
for
important.
annulus
grout
developed
lower,
is
are
clamps
long-term
“Repair”
gusset
clamps
The use
on
removing
but
clamps
tinues
the
similar,
much looser
tubulars,
larger
repair
grouted
Friction
be that
consequently
clamp
These
advantage
most
Then the
strength
and the
in two pieces,
and replacement.
are
an annulus.
cated
is
grouted
which
operation
underwater
joint.
high
are
speed
The
with originally
conditions
repair,
filled
cracked
was
diving
removal,
the
procedure
the
repair
The clamp is taken
around
and tubulars
operation. where
of the
or more later
problem, solution repair
stresses.
by the
may An
through permanent
a
6.2.4
Non-Routine Crack
repairs
ator.
Non-routine
chosen
instead
associated
are
might
is
removed ciated
offshore
work
and
the
platform
for
The hazard
must be a part
is to inspect
crack,
rather
especially is
introduced
detail
in
Platforms
the
nothing
would
pollution
be
be
cracked
storage
operating
might
strucmight
hazard
case
by the
be
asso-
personnel
de-manned
the
platforms
otherwise
more closely
of
the
failure
above
is
more
an fre-
during
routine
cracked
platform
result
tolerable,
another
and monitor
the
actions.
analysis
is
construction.
or whether
In this
of the
rest
growth
of the
to determine it
way,
of the can
Examining
inconclusive. help
remedial
approach
This
can also
of fatigue
performance
is a stable
the
crack
be the
whether defect
may serve
as
structure.
Monitoring
of inspection
National
procedures
Research
and Risers”
No. 8,
discussion
during
and
The subject
any the
of the
Inspection
than
or
structure
as the
sometime
risks such
Even doing
oil
the
the
platform
and fractographically
growing
measure
6.3
rather,
if
metallurgically
high
divers),
of the
example,
to
to ariy nearby
small
take
valuable it
is the
than
may be
considerations.
crack
measure
the
to
risk.
reducing
danger
storms)
posed
of these
When the
Order
smaller
(e.g.,
operation
For
thereby
storms,
the
risk.
If
of the
In ligh’
repair
oper-
in some cases.
with
the
failure.
during
repair.
to the
operations,
routine
way to minimize
associated
platform,
threat
another
of action
hazard
a cracked
(i.e.,
or not
course
from
operations.
crack
best
reduce
increased quently
best
to
underwater
be the
ava+ lable
measures
changes
sup~ lement
a good way to
with
is,
or to
be the
Removing
that
repair
of
may occasionally
at all
on” y remedial
not the
operation,
with
options
ture
Operation
(1979)
especially
and the
Section
is,
therefore,
on the
integration
Council
not
and requirements report,
“Inspection
OCS Platform
Verification
9 under
“Requirements”.
on
“how to”
the
of inspection
with
fracture
C3 /d y
dealt of
Oil
of
control
with
inspection, plans.
in
and Gas
Program,
The emphasis
aspects
92
486-334
is
(ICS
of this but
The
task
of
structure
can
inspection
program
find
cracks.
tion
of the
special
is the
Second
6.3.1
types
most
there
Mexico
are
have
if
The critical
structure
the
to condi-
and (2) on
might
be contin-
uncertainty
connected
several
independent
each other.
for
the
operator
and joints
inspection
schedule
fracture
these
merits
and,
is
each
implied
other
above,
condition
accomplished every
year. joint
or more years). followed
One philosophy the
were
critical
five
philosophies
while
as
the
not inspected (e.g.,
on
inspections
review
such that
control
concentrated
inspection
intervals
comb,”
to
are
inspections.
a “fine-toothed
above These
some underwater
two different
comb” approach
before the
order must
with
means
appropriate
for
mentioned
structures.
at
they
cracks
by grinding
inspected usually
inspected
“fine-toothed
made to catch
In
check
installation
combine
an
by Gulf
approaches
uses
a “broad
may provide
the
brush.”
beneficial
combined.
joints
repaired
to
of
specifically
to
Given the
components
a rotated
independent
synergism,
the
installed
backbone
structure
soon after
report
offshore
some cases,
operators with
Council
of
critical
is is
inspections Both
check
important
In
but all
There of
can be used to
the
inspections
health. policy
Research
a structure.
or component
general good
inspection the
Instead,
of the
the
Inspections
periodic
annually,
inspection
(1)
in
First,
And fourth,
is
started
sub-tasks.
more general
its it
which
The National
of
may have
may be conducted
inspections,
considered
four
periodic
to check
Periodic
the
that
when warranted.
monitored
inspection
in
and third,
structure
many
cracks
be considered
occasions,
uously with
finding
early
or other
to
find
first
be
a water
can
become
in their minor
“small” cleaned
jet
is
and wire
designed
development.
the
marine
cracks
at
least
100 mm (-4”)
long
on the
cracks
at
least
30 mm (-l”)
long
can
is,
These
surface
growth Then,
surface
be detected
93
small
cracks
in
attempt
is
can then
be
the
cracks
methods.
cracks,
brush.
find
That
dangerous.
repair
of
to
of
the
(down to according
bright to
can be detected reliably
joint
being metal),
some experts, visually,
by non-destructive
and
examination Sea).
(NDE) (magnetic
Other
size
that
time
practitioners
is
detected
consuming;
examined
are
each
with
eventually which
joints,
their
inspection
more
high
optimistic
a
annual
joints
for
to
fatigue
inspect lives,
or
twenty
inspection.
the
this
pessimistic
to
five
time
on
of
their
convenience
North
on the
crack
extremely
joints
can
joints all
years.
b sed
in the
are
thirty,
t< me so that
every
each
for
The particular
each
example,
used
Such examinations
few,
may be rotated
examined,
is
probability.
only
a typical
inspection
decide
are
therefore,
during
during
particle
the
examined
critical The
be
joints
operator
importance
may of
inspection,
or
the other
criteria. The “fine” ators
to
It
has
the
But,
it
and regulators.
repairs joints
less
expensive.
to inspect The
designed
to
and are
and when.
second
examination
has
approach
generally
a
great
These
cracks
more costly
severely
cracked
quickly,
as
compared
checked
during
every
a
other
that
uation
of historic
Gulf
of Mexico
practices.
water
NDE methods
were
developed,
typical
is
based
typical
on sounder Gulf
cracks where examine
so it
possible. they the
are
cracked
joints
be through-thickness
cracks,
assumes
structure
examination the
can
entire
the
than
be considered
that
the
is
safe
be
done
joint with
structure
Before practice
braces.”
just
to merely the
a
fairly can
be
However,
to avoid
be absolutely
necessary
experience so the
at the
has
shown
“fine-toothed
right
time,
94
if
have the
expensive
may not
that
be a contin-
sophisticated
might
can tolerate
486-334
is
by visual
structures
expected,
It
approach.
easily
redundant
And two, not
wrongly
“count
reasoning
of Mexico
much difficulty,
might
to
knows which
period.
approach
down periodically
operator
makes the
detected
the
visual
The “broad”
divers
Sea oper-
which
brush”
are
NDE techniques,
inspection
the
The approach
and a
“broad
generally
repair.
Since to
by North
warning,
that
which
must
cracks
joint.
of
cracks
to
for
adopted
of early
assumes
more
big
tolerance
also
been
is a drawback.
is
relatively
alone.
has
advantage
This
approach
find
inspection
been
“broad”
to
big to
find
cracks
send
approach one,
inspections: cracks
without
the
smallest
usually
comb” approach at all.
under-
might
occur not
These sophies the
two
in Gulf
sparsely
for
clear, combined
optimally.
of
the
visual
6.3.2
both
fine
and broad is
particle
Ins~ections
Special
inspections when there
example,
accident
an
structure severe
storm,
by the
periodic
inspection), When a
procedure
used
6.3.3
should
be whatever
basically Gulf
positioning
at
unless
foregoing
inspection
is
construction for
recent
installation.
by Section
its
a survey
site
general is
trend
the
a
a full
subjected
occurred.
mudslide,
else
a
for
to
in the
the to
common sense
may
particularly may be
structure
(by the
structure.
inspection
find
For Or the
cracking
“similar”
warranted, appropriate
is
site,
9.2.7
the
of the
broad
the
measure,
complete
than
(see
OCS Section
it
and
that
helps
486-33;
the
method
kind and
and
of cracking is
structure during
final
Verification
commonly
inspections
9.2.7.5).) inspect
establish
(@)
Divers
as
soon
load
field has
described examine
some critical a baseline
as
out,
tie
erection
is
Program.
overstressing
the
and may closely
of
inspection
OCS Platform
indicate
because
survey
installation,
less
condition
useful
interval
inspections
may have
on another,
(A very
inspections far
elements
Survey
after
required
a
be
have occurred.
Or a potential
found
is
could
have
are
periodic
damage might
somewhere
is
joints
usual
earthquake,
is
programs
seem
of Mexico.
An important
down,
the
overloading.
inspection
does
and
inspection
collision
of cracks
It
techniques.
that
an
or by cracks
each
between
no comprehensive
disadvantages
some of the
to
boat
experienced
Post-Installation
possible
a
special
in the
only
indications as
detection
This
suspected. practiced
are
or some other
indicated
and
in addition
such
may have
in that
is
philo-
ground
be mixed.
Sea inspection
NDE inspection
and other
S~ecial
North
and
dominant
middle
should
advantages
approach
the
moment there
two approaches
conducted
be warranted
At the
most
are
The potential
today.
have
Indeed,
inspection
magnetic
how the
that
approaches
populated.
deciding
however,
both
inspection
of Mexico practice
two appears
rationale
operational
However,
occurred, above the
this at
structure
joints. condition
the
Such of
the
structure
as-built.
is
when it
found,
installation crack
in
helps
Cracks
be identified during
while
in turn
initiated.
can
initiated
fatigue
This
as
transport
service;
the
answer
the
initiated
question
during
such.
As noted
should
be different
later
being
of,
when a crack
transport,
before,
the
from
a more
launch,
and
treatment
one
serious
of a
initiated
(i.e.,
by
ongoing)
problem. Thus, It
plan.
this
recent
provides The initial
reasonable
confidence.
inspections 6.3.4
the
a comprehensive
which
the
and growth
Of course, should
crack
into
against
the
periodic
examination
fracture
can be
can be monitored
techniques
with
control
inspections
of a crack
be compatible
indications
those
used
used
in the
with in
the
periodic
are to be compared.
Monitoring A number of structural
oped
with
the
structure, niques
goal
versus also
ambient measured structural in this measured the
of
the
have the
Of the
to
well
appearance
survey if
fits
a baseline
compared.
post-installation
trend
to
any
stiffness way is that
to
In these changes
small
as a crack
properties
tolerance,
arising
structural
redundancy.
equipment
operating
monitoring
is
structure,
the
until that
in the the
noise
potentially application
small and
a useful of this
the the
general
the
signal
to
vibration.
96
be
for
been done on
joint
while
checking
the
to small
crack
modes are
to
a loss
must be
difficulty
to
is
cracks, the
may be ambient general detection
of
cracks
stiffness
against
detected
So
technique
in development.
of the
tech-
detecting
This
structure
control,
tool
be due with
a
activities.
vibration
structure’s
separates.
of
Monitoring
most work has
might
devel-
“health”
diver
natural
which
being
the
today.
One problem
joint
protect
from displacement The
time
of
and risky
methods,
from cracks.
changes
grows
costly studied,
over
currently
review
available
reduce being
resulting
are
a continuous
techniques
methods.
techniques
“snapshots”
potential
detect
same
providing
periodic
various
vibration
monitoring
due
i.e.,
cracks
and
masked
by
vibration health is
of a still
A related individual
technique
braces.
and a change
its
end restraints,
not
been
early
as
that
redundancy;
have to be to cause A third grows,
it
sitive
listening
level
of
and the a
major
more
it
is
reliability
problems
moment,
this
one
well
also
as
sensitive
is
less
a crack
detectable
with the
type
the
ratio and
is
local
equipment
overcome
normal
and
of material
ambient
to
sen-
rate
low signal-to-noise
from
would
When a crack
the
interference
of
ringing.
including
struggles
(some
it
created,
As with
has
factor
signal
the
filtering.
diver
monitoring
activities
is
technique
the
use
directly
welded
to platform
coupons
may be
constructed
of
and ideally
should
monitored
region
up the
full
fatigue
performance
from ensure
load
of the
the
that
the
placement
pre-crack the
production
and
which pre-cracked
near
critical
same
material,
be attached environment
structure
such
These
including
welds,
is
not
specimen as
the
affecting
the
As can be seen
It
affect
6)
so as to pick
attached.
complex.
elim-
Figure
hot spots.
without
does
not
(see
spectrum it
if
specimens
structure
are
a coupon
reduce,
to the
to which
procedures
of
could
is
important
to
the
integrity
of
member.
is
difference
of
then
purposely
equipment use
the
connection
maximum weld
monitoring
of
structure
corrosive
7,
supporting
through
and
Figure
The major
than
the
which
method
emissions. are
of
noise.
inate,
the
of
stiffness
how large
which
the
a limiting
in the
acoustic
the
vibration
a time,
of
when struck,
new technique
still
change
measure
At the
degrade
uncertain
emissions)
after
as
at
on many factors,
growth.
An innovative
the
still
The quality
even
is
modes
“ring”
ambient
detectable to
depends
techniques,
global
one brace
checks
devices.
vibration
a relatively
it
(acoustic
vibration
is
reliability
also,
local
result when cracks
equipment
noises
of crack
the
own characteristic
This
is
obstacle,
drilling
its
a confidently
emissions, rate
has
will
technique
makes
measure
joints.
Because
structural
to
much as the
indicate
technique).
to
ringing the
studied
tests
this
Each brace
in
its
is
set
defects is
coupons
the
in the
problem
varying
which
crack
size,
97
486-334
coupon
so as to be larger expected
another of
between
0
0
and the
structure
is that
and much more likely structure. must
The reliability
be overcome.
a very
to grow
accurate
of
In theory, reading
of
/--3
T c .-o g m
Crack Weld
/8” PL
1
_
z a
● y
Crack
3
“m
L
metal \\
Detail (plan
A
Detail (edge
view)
J
/’ B
view)
Figure 6. Location of starter cracks in in-situ precracked panel specimens. From Failure Analysis Associates report, “Development, Design, and Installation of In-Situ Corrosion Fatigue Gage.”
,~ (
FaAA-83-6-2
486-334
~
a 3
.& t
diameter
pipe
Precracked
Reaction
Preload
plate
block
yoke
specimen
I
1
J
I
0000 a) 0000
I
0000 0000
I
0000 0000
)
-1
1
1
0000 a) 0000
-1 I
1
i
I
~
1 1
I
I
F gure 7. Schematic of precracked plate specimens. report, “Development, Design, and Installation of
i
al
I
n
n
H
I
From Fa lure Analysis Associates In-Situ Corrosion Fatigue Gage.”
J
n
damage accumulation
could be achieved.
It is our understanding
from informal
interviews, however, that when this technique was tried by a consortium of oil companies
on
cracking
Gulf
of
Mexico
in the coupons.
platforms,
the
initial
results
indicated
no
It is speculated that this lack of cracking is due
to a combination of a relatively benign wave load spectrum in the Gulf and/or the use of cracks in the coupons of less than optimumal severity.
Thus, the
idea may still be practical and good although additional information would be needed
to
separate
the
individual
effects
upon
fatigue
damage
of
loading
spectrum components, corrosive environment, and, unless several materials were used, of material properties. by clever placement load
component
environmental
(Even some of these variables could be separated
of the crack coupons so as, for example, to minimize one
while
maximizing
effect
would
another.
include
Possibilities
greater-than-normal
for estimating protection
the
against
corrosion for selected specimens or, even the use of a material unaffected by the environment for selected specimens, i.e., stainless steel.) Most other monitoring techniques being studied emphasize the limitation or
removal
of diver
remotely operated
submersibles
control
fracture important
activities;
the
instance,
divers
might
or remote sensing devices.
perspective,
aspects:
for
be replaced
However,
by
from the
all
monitoring
techniques
share
the
continuous
(or almost
continuous)
scrutiny
same
of the
structure and the capability to survey the entire structure.
6.4
Discussion As
considered
in this
steel offshore structure control
plan.
Recall
report,
the
operation
of the
installed
fixed
is the fourth and last major activity in a fracture that
the
other
three
activities
identified
are the
selection of material, the design of the structure, and its construction. four
activities
are
interrelated
in terms
of fracture
control
considered together in a comprehensive fracture control plan.
Al 1
and must
Therefore,
be
some
of the issues raised by recent reconsideration of operating practices bring in question some long-held tenets which formed the bases of many fracture control plans.
The following discussion attempts to tie together a few of the inter-
100
related
aspects
of the
four major
activities.
The discussion
will address
these aspects in terms of the evaluation and repair of a discovered crack and the inspection of the structure to find such cracks.
6.4.1
Evaluation and Repair of Cracks The
four
integrity
steps
in the
evaluation
(see Section
6.2.2)
are performed
the situation
and the operator.
of a crack’s
impact on structural
to varying degrees
depending
on
A survey of the crack must be performed
in
all instances, but the visible surface length is probably sufficient information for most operators.
The evaluation
of the crack’s impact on the joint
and on structural integrity is often based on experience, not analysis. most
notably
factors.
and rules of thumb,
And the decision to repair is influenced by “external” factors, liability
and
insurance,
not just
the
“internal”
engineering
So, in many cases, if not most, the analytical process described in
Section 6.2.2 is followed only in outline, at best.
Few cases will see full
analytical treatment. The newness of the fitness-for-purpose experience
with
experience
is
more
serious
interpretation
it are partly
responsible
analysis and industry’s lack of
gained, more of a trend toward its use is expected. drawback of the
is the
uncertainty
results difficult.
As
for its lack of popularity.
of the
calculations,
The technology
Perhaps a which
makes
is currently
rela-
tively unsuccessful at predicting the ductile failure and ultimate strength of a cracked tubular joint. specified
directly
or
The fracture toughness inspected
for,
adding
(COD) of the materials is not
to
the
uncertainty.
And
the
growth of cracks in tubular joints is not understood to a sufficient degree. So, the fracture potential joints are basically
and fatigue
engineering
difficult to interpret properly.
life calculations
judgments
for cracked tubular
at this time and are, therefore,
These judgments
can be improved with more
research.
In light of the above, the engineer would be likely to go immediately to the
structural
separation,
integrity
analysis:
the crack
is assumed to cause joint
and a strength analysis can be performed.
101
4$6-334
0/3
But then the question
is, what is the necessary strength of the structure? asked by the designer:
This echoes the question
HOW much redundancy is enough?
So again, the inter-
pretation of the results of an integrity analysis is uncertain.
The engineer
is likely to resort to engineering judgement and rules of thumb.
For example,
braces near the waterline of these
is probably
are primary shear transfer members; a crack in one
serious
and should be repaired.
mudline or at depth are fundamentally
But,
braces near the
redundant; cracks here are less serious
and could probably be tolerated. So, a crack is often repaired automatically ing judgement” factors
also
and a formal influence
ically, the
influence
risk analysis decision
the
In terms of liability and litigation,
These
liability,
from an unlikely,
likelihood.
environment
These
which
are, specif-
and litigation.
it seems likely that a court would find involving a structure with a known
but arguably
foreseeable,
both would have been judged a priori by state-of-the-art same
Rut “external”
the “act of God” accident is easier to defend than
crack, than one without; one stemming
insurance,
liable for an accident
the operator more
is not performed.
to repair a crack.
regulations,
of
because of “good engineer-
“external”
heavily
favors
factors the
contribute
automatic
fracture,
even
if
analyses to have the to a decision-making
repair
of
the
crack
as
a
remedial measure. This
attitude
favoring
automatic
decisions are often the result. actions
repair
is unfortunate
because
poor
Not only are repairs made when other remedial
(e.g., non-routine operation) might be more economical, but the total
risk picture
might
be worse
with
the
repair
than with
other
actions.
To
illustrate, consider an incident related to the authors by one of the survey’s interviewees.
A
crack
was
decided to repair the crack. consideration crack
discovered
in an
offshore
platform
No risk analysis was performed
and
it was
(or less forma 1
given) to compare the risks of operating the structure w+ th the
unrepaired
to the
risks of
repairing
the
crack.
The type of repair
chosen was to cut the node out of the framing, raise it to the surface with a crane working over the side of the platform, and reweld the crack on the deck. By luck or design, this turned out to be an unfortunate crane was
overloaded
and pulled
off the deck
102
486-334
0
W
choice, because the
into the water.
As the crane
fell, it hit the structure and knocked out at least one brace. the “repair” operation was a structure Either have
no action,
The result of
in much worse condition than before.
a less risky repair, or some other remedial action should
been taken.
But,
apparently,
the
influenced the operation negatively. but it illustrates the potential
rapid
reflex
Admittedly
reaction
to the
crack
this is an extreme example,
risk that must be considered in any decision
to repair a crack.
In terms of a comprehensive the problems of the evaluation are
nearly
the
What
same.
fatigue life is necessary? is enough?
is the
fracture
residual
strength
of a cracked
fatigue life of a tubular joint?
How much strength is enough?
These are all questions
comprehensive
control plan, the objectives
and
and repair of cracks and of the design process
What is the residual/total
joint?
fracture
tubular How much
How much redundancy
relevant to both evaluation and design.
control plan must address these questions
A
and provide
answers consistent with the fracture control philosophy.
6.4.2
Inspection A comprehensive
opment
fracture control plan should also integrate the devel-
of an inspection
especially
In
design.
scheme with developing
all other
an inspection
fracture
control
scheme,
activities,
it is important
to
determine what size cracks the inspection should be designed to find. As described
earlier,
there
are two periodic inspection
philosophies,
designated here as the “fine-toothed comb” and “broad brush” approaches. success
of the
correctly
approach
“fine”
predict where
depends
on the
ability
of the
operator
The to
cracks are likely to initiate at any given time and Recent experience
upon the reliability of the inspections.
has demonstrated
that operators are not very successful at making these predictions.
Further-
more, the authors have received comments from the American Bureau of Shipping to the effect that they do not generally accept the reliability of ultrasonic testing crack
for underwater
depth
magnetic
after
particle
a
crack detection, crack
inspection
has
been
but recognize its use in determining
located.
has performed
In
fact,
in the
North
Sea
better than ultrasonic techniques
for underwater crack detection, since the typical crack has a surface or near-
103
4$6-334
@
surface location and because of the difficulty to the level required by current ultrasonic were
told that
increase
the
improved
methods
applicability
of
devices.
are underway ultrasonic
in cleaning inspected surfaces During an interview we
(funded by the MtvIS)which
inspection
for
underwater
may
crack
detect on as opposed to crack sizing. The experience with the conductor bracing of North Sea platforms, mentioned in Section 2.3, is an example of the inability to predict where cracks
In this case, the fatigue loads were not predicted well enough
might occur.
and the anticipated
fatigue
lives were
In
over-optimistic.
retrospect,
the
cracking might have been predicted with the proper analysis had current design practices
However, using overly optimistic
been applied.
fatigue lives as a
guide for inspection schedules, the conductor bracing was not inspected soon enough
to
catch
the
fatigue
cracks
while
they
were
still
small,
thereby
defeating the purpose of the “fine” inspection. The general experience of offshore operators is that cracks often form where
they
are
illustrates,
not
often
anticipated.
catch
these
inspections,
“Fine”
unanticipated
cracks
Some operators are therefore
already large or not at all.
Instead of concentrating
use of such inspections.
only
as
the
after
example they
are
reconsidering the
the inspection
on only a
few joints each time, they reason, it would be better to look for the larger, easily detectable
cracks over the entire
structure.
If large cracks are at
least temporarily tolerable, then would not a “broad” inspection, which would find
the
unanticipated
cracks,
be preferable?
approaches would be appropriate:
Perhaps
a blend
of the two
“fine” inspections for locations identified
as “fracture critical,” and “broad” inspections for the rest of the structure. A comprehensive fracture control plan should thus identify where “fine” inspections
are necessary
design
phase
parts.
The
controlled.
is
the
material
and where
logical and
point
“broad” inspections would at which
construction
of
to
such
identify parts
suffice.
fracture
should
be
The
critical carefully
In designating a part “fracture critical,” both the likelihood of
fracture
and its consequences
fracture
control
activities
should must
be taken
then
into consideration.
be integrated
fracture control plan.
104
486-334
Iu 0
into the
All four
comprehensive
6.5
Principal
References
1.
American Petroleum Institute, “API Recommended Pratt’ice for Planning, Designing, and Constructing Fixed Offshore Platforms:” Section 7, “Surveys,” API RP-2A, Thirteenth Edition, Dallas, Texas, January 1982.
2.
National Research Council, “Inspection of Oil Risers,” Marine Board, Washington, D.C., 1979.
3*
Marshall, P. W., “Strategy for Monitoring, Inspection, and Repair for Fixed Offshore Platforms,” Proceedings of the Conference on the 13ehavior of Offshore Structures (BOSS ‘79), London, 1979.
4.
Marshall, P. W., Crick, H. F., and Marks, P. T., “Welding and Inspection,” Chapter 27 in The Design of Fixed Offshore Platforms, ed. by B. McClelland, to be published by Van Nostrand Reinhold, New York, 1982.
5.
Tebbett, I. E., and Robertson, D. A., “Novel Underwater Strengthening System for Tubular Joints,” Proceedings of the Offshore Technology Conference, OTC 4110, Houston, Texas, 1981.
6.
Marshall, P. W., “Fracture Control and Reliability for Welded Tubular Structures in the Ocean,” presented at the 9th NATCAM, Ithaca, New York, June 1982.
7.
U*S. Geological Survey, Requirements for Verifying the Structural Integrity of OCS Platforms, especially $ectl on . Geological Survey Conservation Division-OCS Platform Verification’ P~ogram-Order No. 8, October 1979.
8.
Wade, B. G., J. L. Grover, and G. R. Egan, “Development, Design, and Installation of In-Situ Corrosion Fatigue Gage,” Failure Analysis 1979. Associates for Shell Oil Company, January
105
486-334
0 //7
and
Gas
Platforms
and
7.0
COMPARISONOF CURRENT PRACTICES The development of offshore petroleum resources in the North Sea became
an
important
influence
on worldwide
offshore
practices
during
the
1970’s.
Since fatigue was such a major problem during the early development, there has been
considerable
interest
in
fracture
Much of the full-scale fracture-related European
laboratories.
Through
control
among
North
Sea
operators.
research in the world is conducted by
documentation,
codification,
and
research
publications, the North Sea standard of practice has become established as one of two main approaches to fracture control of fixed steel offshore structures, the other being the Gulf of Mexico standard of practice. International
offshore practices tend to fall then into two types:
Gulf of Mexico type and a North Sea type.
a
The type of practice followed in a
particular location is affected by the type of offshore environment
(Gulf type
versus North Sea type), who the operators are (American versus European), and the time at which offshore development began (before versus after the 1970’s). For instance,
in the Persian
Gulf, the Gulf of Mexico type standard
expected:
the
development
began before the 1970’s.
environment
is mild,
the
operators
the major operators are European, and development agencies
ties,
and
While in the Tasman Sea (New Zealand),
the North Sea type of standard can be expected:
ties to classifying
have American
can be
the environment
is severe,
began in the 1970’s.
Local
(e.g., DNV, Lloyds, ABS) also influence the type
of practice. This Gulf
final
of Mexico
aspects
that
section
of the
type of standard
affect
fracture
summary
of current
practices
compares
the
practice with the North Sea type for those
control.
Other
relevant
aspects
of worldwide
practice not typical of the Gulf of Mexico or the North Sea will be mentioned where appropriate.
The section is divided into three major parts.
deals with differences in the offshore and operating environments. treats the current
practices
in the four major
fracture
control
The first The second activities,
and the third discusses the selection of fracture control plans for frontier areas.
106
486-334
(@j
Offshore
7.1
and Operating
Environments
As summarized in Figure 8, the environments, both the physical offshore environment
and the financial
and regulatory
(philosophical) operating
ronment, have played the major
roles in establishing
control practices.
fatigue
include the great water
loading of the North Sea has already
depths,
the cold temperatures,
tances from land, and the short weather window
the large dis-
for offshore construction and
In other offshore environments, other hazards, such as
inspection activities. earthquakes,
fracture
Other contributing factors in the North Sea offshore environ-
been mentioned. ments
The severe
the different
envi-
mudslides,
and
ice
may
loadings,
affect
the
fracture
control
plans. aside, there are two aspects
The physical environment environment first
that
aspect
have
affected
North
is related to the
Sea
fracture
risk of operations
to the operating
control
practices.
The
and the
second to regu-
lations. First, the risk aspects: the thousands
of offshore
Compared to the Gulf of Mexico where most of
structures
are in relatively
than 300 feet), many North Sea structures These
structures
are expensive
shallow water
are major deepwater
to build and to replace.
(less
installations.
Thus, the economic
consequences of a single platform failure, or even of a major repair, col,~ldbe devastating
to a North Sea operator.
the
of experience
length
operating
Also, compared to the Gulf of Mexico, in the North Sea
is much
shorter.
The
uncertainties are therefore higher, increasing the probability of failure at a given
level
of
loading.
Thus,
the
total
risk
(cost
of
failure
times
probability of failure) could be much greater were not the North Sea operating companies extra conservative in their designs. Second,
the
regulations:
pounded by the conservatism the Federal
government
the
conservatism
of the operators
of the North Sea regulators.
requires verification
is com-
In American waters,
of a platform’s
design,
fabri-
cation, and installation through the OCS Platform Verification Program. program
requires
standards
the
platform
of practice;
to be designed
for ingtance,
tubular
107
486-334
0
/(9
and constructed
This
to acceptable
joints are to be designed
by a
OPERATING
ENVIRORIMENT
I I
I NORTH ●
*
SEA
GULF
European Offshore
Severe fatigue loading from wave spectra
●
Deep
●
Far
.
Short
●
water
o Cold temperatures from
●
land
weather
●
post
1970
●
Deepwater installations are expensive to build and to replace. (Can’t afford loss). Shorter length of experience = higher UnCf3rtaintY= greater conservatism
Relatively benign environment and fatigue damage from wave load spectra. Shaliow
design
window ●
American Offshore
Clperators Development:
Specifications (DNV Rules and U.K. DOE Guidance) are required by government and insurers
Warm
water temperature
Close to land (for demanning) Operation and inspection is possible all year
1970
PHILOSOPHICAL
●
Cheaper frame construction is possible. (Loss is more affordable).
9 Years of operating experience= less uncertainty = less design conservatism ●
API, MMS, ABS rules, etc., are not blndlng regulations. (However, OCS piatform verlflcatlon program does conta!n blncflng regulations.)
Certification and periodic recertification is required for insurance
Figure 8. Differences between the operating environments in the North Sea and the Gulf of Mexico.
pre (
/ PHYSICAL
PHILOSOPHICAL
PHYSICAL
●
Operators Development:
OF MEXICO
of fixed offshore platform
procedure
comparable
to that
set forth
in the API RP-2A recommendations.
third party reviews the design, verifying the design process.
A
The operator is
required to submit general information to the government, such as a summary’ of the
fatigue
Service,
analysis,
and
the
issues a drilling
government,
through
The MMS
permit.
the
Minerals
requires periodic
Management
inspection
of
the structure during its life.
In
comparison,
norske Veritas’s required.
in the
North
Sea
adherence
“Rules” and/or the UK Department
Acceptable by
to the much of Energy’s
stricter “Guidance”
Det is
design practices for these waters are specified in much
greater
detail
these
Further,
a certificate
documents
of fitness
than
by
their
American
counterparts.
is issued by the government
(i.e., either
the UK or Norway) after the platform has been installed, which must be renewed periodically are
(every five years,
at present
made
for the
Rigorous inspection requirements
or sooner).
recertification,
although this
practice may be
changing in favor of one more like that required by the MMS.
Thus, government
regulators building
and their
and
agents
operation
currently
of North
Sea
play a much more platforms
than
active
in the
role in the
Gulf
of Mexico
platforms. Thus, potential develop
the extreme
conditions
risk, and the strict a
discussion
of the offshore environment,
regulatory
cautious
approach
continues
next with the major differences
Mexico practices
to
environment
fracture
control
the higher
have combined
in
the
North
to help
Sea.
The
between current Gulf of
and current North Sea practices and describes
how the envi-
ronment has helped create these differences.
7.2
Major
Differences
Figure
in
9 summarizes
Current
Fracture
the subject
Control
differences
Practices in practices
between the
North Sea and the Gulf of Mexico platforms.
7.2.1
Material Selection
It
was recognized early in North Sea development that the toughness of
the materials used is important in controlling transition
temperature
(NDT) is especially
fracture.
important because of the low ser-
109
486-334
A low nil-ductility
n
~LI
FRACTURE
CONTROL
1
1 NORTH .
●
Emphasis {selected
on
material
GULF
SEA toughness
●
Less experience with COD testing (and material fabrication), since material toughness is not as important in the Gulf environment.
●
Lighter, template-style platforms - 8-legged pile foundations (Long predictable weather window; no need for speedy installation)
for COD properties)
Heavy, large-diameter legged towers - 4-legged pile type foundation (can be installed quickly within short weather window) - 4-legged
tower
is less redundant
- More ●
Tower has larger, more fatigue-sensitive - PWHT necessary - Node
method
legs
offer
more
redundancy
joints - Cheaper
to fabricate
of construction ●
●
OF MEXICO
‘Fine-Toothed Comb” Inspections - Platform is permanently manned - Inspections are limited by environment (weather, water depth, sea conditions).
Joints are smaller, tess - PWHT not necessary - Allows
●
frame
method
of construction
‘Broad Brush” inspections - Platform can be demanned - Environment rarely or inhibits repair.
Figure
fatigue-susceptible
limits
9. Differences between current fracture control practices in the North Sea and the Gulf of Mexico.
,..
inspection
vice temperatures
experienced
It is significant that the most severe
there.
storms occur in the North Sea when the water temperatures are the coldest, but in the Gulf of Mexico the hurricane
season occurs during warm months.
Thus,
the most demand for toughness, (for the North Sea) is placed on a material when its supply is lowest, while the reverse is true in the Gulf of Mexico. European ductile
research
materials
has
opening displacement
and
development
resulted
in the
selection
(COD) properties.
Sea have seen COD testing
of fracture
toughness
of materials
measures
in
for crack tip
Recent platform designs for the North
explicitly
required by specifications
and regula-
tions. To
obtain
cooperation
these
material
properties
of the steel making
to satisfy
this
when
assurance
contracting of
toughness
basis,
required.
as have the Japanese.
in this, but are gaining.
U.S. operators have reported difficulty tests
regular
with
steel
then
must
the
close
European
steel
have the necessary experience with COD
requirement,
makers have had less experience
a
industry has been
makers supplying North Sea fabricators testing
on
American
steel
In contrast, some
in getting even simple Charpy impact in
makers be
some
attempted
developing
through
other
countries; means--for
example, through correlations with strength testing. The wide spread use of COD testing is one of the most explicit fracture control measures used in North Sea practice.
7.2.2
Design Fixed
than
their
operating
steel structures Gulf
of
companies
Mexico
designed
for the North Sea tend to be heavier
counterparts.
For
platform
designs,
North
Sea
seem to favor large diameter legged towers over the tem-
plates found in the Gulf
region.
These
differences
affect fracture control
through the redundancy of the structure and the size of the joints. Foundation
design
North Sea environments. limited.
is one of the main
reasons towers
are favored
for
Due to the short weather window, installation time is
Pile groups clustered
around the four
111
486-334
a
23
mrner
legs can be installed
quickly, because the driving equipment does not have to be relocated for each pile.
This
leads to a four-legged Other
self-floaters.
tower-type
considerations,
such
design and, in some cases, to
as
fabrication
costs,
are
also
important. The framing of a four-legged tower tends to be less redundant, as there are fewer
braces, than
an eight-legged
Thus, the
Gulf of Mexico template.
loss of a single brace in the structure has a more severe effect on the tower’s structural integrity.
The tower is therefore inherently less tolerant of
cracks. The large diameters of a tower’s legs necessitate large tubular joints. These
joints
are thickened
further
of dynamics
and fatigue.
The resulting joints are both large in diameter and in thickness.
Detrimental
size effects’ then come into play: stress)
of tubular
joints
by the effects
the fatigue strength
decreases
(measured in units of
as their size increases.
They are also
more difficult to fabricate. Fracture
control
Sea platforms. in combination
is therefore
affected by the typical
design of North
Tower-type designs result in lower structural
redundancy and,
with more fatigue content
fatigue-sensitive
joints.
in wave
These structures
load spectra, larger, more
are less tolerant
of cracks than
are the template type structures typical of the Gulf region.
7.2.3
Construction The
heat
large joint sizes used in North Sea structures
treatment
development
(PWHT).
of the
As
discussed
node method
of
earlier,
this
fabrication.
require post-weld
requirement
From the
led to the
fracture
control
viewpoint, this has several advantages.
First, the node can be fabricated in
a more controlled
a higher quality
environment,
the critical joints. welding
and
defects
at the
brace-end
for
of fabrication
for
Second, access to both sides of a tubular joint weld for
inspection root.
assuring
is
Third,
possible, special
stub when extra fracture
allowing
material
resistance
better
control
can easily
is necessary.
be used
4gfj”334
0
p
for the
And fourth, the
node method shifts the weld stress away from the node intersections.
112
over weld
Thus, the predominant use of the node method of fabrication, especially for critical joints, has several advantages in limiting initial defect size.
7.2.4
Operation The
structures
inspection
philosophy
is to find small
“fine-toothed
comb”
for the
cracks
approach
to
periodic
before
they
inspections
inspections
can become
is
of North Sea The
dangerous.
followed,
in
and,
fact,
required by regulation. There are two major operational reasons for this inspection philosophy. One
is
that
the
platforms
are
permanently
contingents than those in the Gulf of Mexico. large storms occurs
and
structural
is often impossible the
distances
failure are much
reason is that limits
large
the
because
inspections
accessibility
to
are limited
of
critical
and
with
much
larger
Evacuation of the platforms for
of the short notice before a storm
land
higher than
manned,
bases.
So
the
consequences
The second
in the Gulf of Mexico. by the environment.
joints
to
diver
of
The water depth
inspections,
and
the
short weather window for diver activities limits the duration of those inspections.
So the operator must make use of very limited resources. Under these circumstances,
emphasizing
a careful
it is logical that an inspection philosophy
inspection of the joints most likely to develop cracks
is used in the North Sea.
7.3
Discussion North Sea
severe
demands
practices
have thus
on materials
(crack
ities and precision not
particularly
consequences, Gulf
a
of Mexico
environment,
evolved
resistance),
and necessitates
tolerant very
the
In the
attitude
limits inspection
towards
face
of devastating
fracture
on the other hand, have evolved design
cracks in a region that produces
of
that
places
opportun-
the use of structural designs that are
cracks.
cautious
practices,
enabling
of
in an environment
structures
that
are
control
failure prevails.
in a more benign very
tolerant
of
little stress and low failure consequences.
113
Both
practices
remember
that
have
advantages
each
is
a
and
disadvantages,
product
of
its
but
it
is
environment,
important physical
to and
philosophical. Perhaps the most exciting question is the
best practice
international
for new
frontiers?
As mentioned
before,
is, what
the
current
practice is to adopt one of the two main approaches, either in
whole or with small modifications. choice
in fracture control today
is not made
and
wisely,
This can clearly be inappropriate
could
in fact
be dangerous,
if the
especially
if
fracture control provisions are taken out of important context. For
example,
consideration. tempting
to
hurricanes,
consider
If the
follow
the
a
fatigue Gulf
region
where
environment
of Mexico
occur without warning,
earthquakes
are
is relatively Rut
practice.
so evacuations
an
benign,
important it may be
earthquakes,
are not possible.
unlike There-
fore, a risk analysis would show the risk of platform failure to be more like that
of
the
North
Sea
situation.
The
control plans depends on the individual
effect
of
platform’s
earthquakes
on
fracture
location and response and
is an area which requires further study. The choice of which fracture control practice to follow, or the design of
a comprehensive
fracture
fracture control mentioned
control
plan,
in this report.
must
address
all the aspects
of
Starting with the basics of frac-
ture and fatigue, the fracture control plan should integrate the practices in material selection, design, construction, control must be considered
and operation.
as a part of the entire
Ultimately,
fracture
risk of an offshore pro-
ject; the risk of fracture must be weighed against all other risks.
7.4
Principal
References
1.
Det norske Veritas, “Rules for the Design, Construction Fixed Offshore Structures,” Oslo, Norway, 1974.
2.
United Kingdom Department of Energy, Petroleum Engineering Division, “Offshore Installations: Guidance on Design and Construction,” Her Majesty’s Stationery Office, London, 1977.
114
486-33#
I 2L D
and Inspection of
3.
U*S. Geological Survey, Requirements for Verifying the Structural Integrity of CICS Platforms, especially Section 9, U.S. Geological Survey Conservation Division-OCS Platform Verification Program-Order No. 8, October 1979.
115
4$6-334
Q
X7
DISCUSSION OF AND RECOMMENDATIONS
III.
IMPROVEMENTS AND FUTURE
1.0
FOR
RESEARCH
INTRODUCTION Most of the recommendations
research and technological current
practices
made in this section for new and follow-on
development
performed
for the
derived from the literature Summary
II).
(Section
survey of
In stating
the
current state of affairs or reviewing present technology, many of the authors interjected critical comments and suggested new work and directions. the
project
these
“recommendations”
and
identified
trends
were
Early in
marked
and
filed for future reference. Added to these, tions during
made the
by
the
telephone
as the project
designers
and
fracture
interviews.
opportunity
to
discuss,
often
behind many
of the recommendations
progressed,
Here
“off
control
the
the
were comments
authors
record,”
prevalent
and sugges-
“generalists” of this
the
contacted
report
reasons
in the literature.
viewees directed the authors to other literature and specialists
and
had
an
concerns
The interin the field
for further verification and discussion, and out of this process and their own experience
the
authors
have
come
up with
Sections 3 through 8, following.
116
the
recommendations
presented
in
2.0
ELENENTS
2.1
Scope
AND RATIONALE
and Method
of
FOR A FRACTURE
CONTROL PLAN
Documentation
In this section a summary of Survey Tasks 2, 3, and 4 identified in the Introduction
to
the
report
(Section
I) is presented.
For
convenience
we
restate these tasks below:
Task
2: Identify the essential elements and rationale of a fracture control plan to provide a framework which could eventually evolve to a fracture control plan for fixed offshore structures.
Task 3:
Identify areas where existing technology would suggest cost-effective improvements in current practices.
Task
Identify promising areas of technical research which would provide a sounder basis for fracture control of fixed offshore structures.
4:
Based Subcommittee
upon discussions and
the
with
Committee
and suggestions
on Marine
from the Ship Structures
Structures,
documentation
of the
above three tasks will comprise the following outline form.
2.2
Outline The
described,
fracture and,
specialties,
control
if necessary,
such as materials
plan
element
will
be
Elements
justified.
identified,
will
briefly
be technical
sub-
and design, rather than functional categories,
as detailed, for example, by Rolfe and Barsom in their extensive discussion of fracture
control.
Those
fracture control as: (2) establishnmt tiveness
identify
(1) identification
the
basic
design of
methods
specific
functional
elements
of
of factors contributing to fracture;
of their relative contribution;
of various
(4) recommendation
authors
to minimize
design
(3) determination
the chance of fracture;
considerations
safety and reliability against fracture.
of effec-
to
ensure
and
structural
We might employ such a breakdown of
functional elements for a specific application but have found it more convenient to
define
elements
as technical
subspecialties
survey.
117
for the
current
broad
A.
Summary of Status
A short
statement
of the current
element
is given with
status
of the
fracture
control
reference to the Task 1 summary, as appro-
priate.
B.
Relevant Documentation
Reference is made to codes, specifications, concerning
the
identified
element
and other documentation
in general
usage by designers,
builders and operators of U.S. fixed steel offshore structures.
c.
Recommendations
The
for Use of Existing Technology
authors’
recommendations
for
cost-effective
improvements
in
current practice related to the identified fracture control element are given.
Emphasis will be placed on those areas where existing
technology
has
evolved
from
efforts
to
control
and
prevent
failures.
D.
Recommendations
Technical
for Future Research
research
areas which
may
provide
a sounder
basis
or a
more effective or less expensive procedure for fracture control of offshore
structures
recommendations be
on
will
be
identified.
As
opposed
to
the
section, emphasis in this future work section will
development
technology
of
rather
than
application
of
existing technology.
Distinction this report, operating
those
company
between
C
items
that
and
D
could
is
not
be
or could be implemented
always
invoked
easy.
unilaterally
by consensus
Those items requiring further development
is practical are considered
“Future Research”.
118
4$6-334
(@J
purposes
of
by a platform
(e.g., API RP-2A) or
regulation change (e.g., USGS “Requirements”) are considered Technology”.
For
“Use of Existing
before implementation
With these distinctions, those
recommendations ti ons
(e.g.,
that Ship
for sponsorship
seem appropriate Structures
Committee)
fall
by research organiza-
into
the
future
research
category. The above outline cusses
the
itself.
integration
of
all
fracture
control
material
selection,
design,
construction,
Within these sections, the recommendations
categories: testing;
1) data
3)
specialists; impact. dation
elements
as
Section 3 disan element
in
Sections 4 through 8 break down fracture control into its technical
elements: tion.
is applied in Sections 3 through 8.
2) experimental
evaluation/development;
5) education,
training;
and opera-
fall into one or more of six
base development/evaluation;
analytical
inspection,
4)
research,
procedures/guidelines,
and 6) detailed inspection/analysis
for
These categories sometimes overlap, and in these cases the recommenis placed
bined.
in the
Of course,
control element.
most
general
category
not all of the categories
Each recommendation
or the
categories
are
are needed for every
com-
fracture
is first stated simply and then elabor-
In many cases the elaboration results in a set of closely related
ated upon.
recommendations that support the main. For ease in future reference, a three-character been
devised
technical tion,
for the
element
(F for integrated
1.
first
character
identifies
.
●
the
selec-
I for inspection, and O for opera-
The second character identifies whether the recommendation
of existing technology
2.3
The
scheme has
fracture control, M for materials
D for design, C for construction,
tion).
(1,2,
recommendations.
cataloging
is for use
(E) or for future research (R), and the third character
.) identifies the specific recommendation.
Reference
Rolfe, S. T. and J. M. Barsom, Fracture and Fatigue Control in Structures:Applications of Fracture Mechanics, Chapters 14 and 15, especially the list of four basic elements of fracture control on page 415, PrenticeHall, Inc., Englewood Cliffs, New Jersey, 1978.
119
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(@
3.0
INTEGRATEDFRACTURE CONTROL The
appears
definition
and
method
to be an issue almost
of
documenting
as large
fracture
control
plan
as any one of the single technical
elements within the plan and has, therefore, itself.
a
been isolated as an element unto
Our best short definition of fracture control has already been given
in Section 1.2 and is repeated here for convenience. rigorous turing,
application
of those
and operations
branches
technology
Fracture control is the
of engineering,
management,
dealing with the understanding
manufac-
and preven-
tion of crack initiation and propagation and member failures leading to catastrophic failure of the structure.
3.1
Sunmary
Status
of
.
The above definition of fracture control was accepted or “tolerated” by a consensus
of those
interviewed.
One level down from a global definition,
the five technical elements (material selection, design, construction,
inspec-
tion, and operation) in Section 2.2 are identified as being part of a fracture control plan. been
Further, an overall goal or philosophy of fracture control has
identified
in
terms
of
three
lines
of
defense
against
catastrophic
failure:
1.
Prevent cracks when possible.
2.
Contain or tolerate growth of those cracks not prevented.
3.
Contain a fracture within a part or tolerate the part if a crack should grow critically.
Explicit clear-cut
documentation
status.
Aside
of fracture
from some
feeling
portions of the OCS Platform Verification references provide an excellent fracture
control,
only
explicit
written
fracture
control,
control
loss of
as defined
that API RP-2A,
above,
has no
the ABS Rules,
Program, and other more specialized
start to documenting
one entity,
the
a major plan.
some of the elements of
oil company, However,
appears to have an
the
elements
of
an
integrated fracture control plan exist implicitly within the offshore industry through two media:
120
486-334
@=’~’
1.
Use of experienced, competent engineers who may be called fracture control generalists. Such engineers, who may or may not also specialize in one of the elements of fracture control, take it upon themselves to ensure that any aspect of one of the elements of fracture control is examined for its impact upon other fracture control elements.
2.
Team efforts of engineers in the various fracture control subspecialties responsible for ensuring that all aspects of their specialties are considered in the context of the other technical elements of the overall fracture control plan.
Although designer,
the participants
or fabricator,
pursued appears to
be the
in these media may come from the operator,
a key factor
in how vigorously
attitude of the operator
fracture control
(who eventually
is
bears the
cost).
3.2
Relevant
Documentation
The following U.S. documents have been cited as containing either rules or recommendations that encompass several fracture control elements:
“Rules for Building and Classing Offshore American Bureau of Shipping, Installations,” Special Committee on Offshore Installations, New York, 1983 (The 1982 Draft report was used in preparation of this report). American Petroleum Institute, “API Recommended Practice for Planning, Designing, and Constructing Fixed Offshore Platforms,” API RP-2A, Thirteenth Edition, Dallas, Texas, January 1982. Geological Survey, Conservation Division, “OCS Platform VerifiUnited States cation Program,” Department of the Interior, Washington, D.C., 1979.
Publications which make specific recommendations for the development of industry-wide fracture control planning (or plans) include the following:
Bristoll, P., “A Review of the Fracture Mechanics A~~roach to the Problems of Design, Qualit.v Assurance, Maintenance. and ‘Re~air of Offshore Structures,” Proceedings of the European O~fshore Steels Research Seminar, Welding Institute, Abington, Cambridge, November 1918.
121
4$(3-334
[33
o
McClelland, B. (cd.), The Design of Fixed Offshore Structures, with specific reference to Chapters 18 and 19 “Tubular Joint Desiqn” and “Steel Selection for Fracture Control”) by Peter Marshall, ~nd Chapter 27 (“Welding and Inspection”) by P. W. Marshall, H. F. Cricks, and P. T. Marks, to be published by van Nostrand Reinhold, New York, 1982. Pellini, W. S., “Criteria for Fracture Control Plans,” Naval Research Laboratory, NRL Report 7405, May 1972. Rolfe,
S. T. and J. M. Barsom, Fracture and Fatigue Control in Structures, New Jersey, 1977. Chapters 14 and 15, Prentice-Hall, Inc.,
McHenry, H. I., and S. T. Rolfe, Fracture Control Practices for Metal Structures, National Bureau of Standards IRi’9-1623, January 1980.
3.3
Recommendations
for
Only a minority ately Many
create in the
document
in the
form
of
Existing
integrated are of
averse a
Technology
in the offshore industry want to immedi -
of engineers
a written industry
Use
fracture to
control
any sort
regulation
plan
for general
of official
or widely-used
fracture
recommended
usage. control
practice.
This aversion is due to their fear that the document would not contain sufficient generality and freedom for competent structural engineers to optimize all aspects of their structures
including
safety and fracture control, and there
is a voiced concern that the recommendations would begin to take on the “cook book aspects
now seen in the nuclear power industry”.
Rolfe and Barsom,
in
the broadest of the recent surveys of fracture control plans we have reviewed, do not advocate
an industry-wide
document
for a generic class of structures.
Rather, they advocate a document for each critical structure. In spite of this reluctance
and aversion
to
a fracture
control
“Bible”, there
is a general
feeling in the offshore engineering community that some additional integration of the subspecialties
In comments
of fracture control is necessary and desirable.
Rolfe and Barsom’s of
Dr. George
turn, we would
book, several pages are devoted to the general
Irwin on the
subject
like to repeat two quotations
of fracture
control
plans.
that most influenced our recom-
mendations in this section, and which stand as recommendations on their own:
●
“. . . termed
●
*
It
is
necessary
comprehensive
to point out that the concept
fracture
control plan is quite recent
122
486-334
0
/Jw
In
and
cannot
be
yet
illustrations.
supported
with
completely
developed
We know in a general way how to establish
plans for fracture control in advance of extensive trials. control
plans
study,
until
However, have
detailed
a number
been
recommendations
plans
..*
“ [Emphasis is OUrS. ]
.
.
for
selected
.
One
comprehensive on
see
and
to
critical
can
comprehensive
formulated
such
m“..
of
guide
are
that
fracture
available
the
structures
for
development cannot
efficient
service
of
be given
operation
of
a
fracture control plan requires a large amount
inter-group
If a
coordination.
fracture failure
complete
avoidance
of
is the goal of the plan, this goal cannot
be assured if the elements of the fracture control plan are supplied by different divisions or groups in a voluntary or independent special
fracture
[Emphasis develop
It
way.
is
would
control ours.]
Such
suitable
study
for their
for
a
group
justifications,
to
coordination might
procedures
be
establish
a
purposes. expected
to
for the purpose of
of the plans are conducted in a
purpose.
and improve the fracture
suitable
suitable
group
and operate checking
assuring that all elements way
appear
Other
tests
might
be to
control plan and to supply
where
necessary,
of the adequacy
of the plan.”
Since fracture
control
as such, are reasonably well recommendations
practices, established
can be expressed
lowing recommendations
though not always explicitly within
the offshore
stated
industry,
as changes to an existing system.
most
The fol-
then, some of which are repeated and discussed in more
detail within the technical subspecialties,
exemplify such changes.
123
486-334
0
~3c
Recommendation
Encourage
FE1
Fracture
Control
Encourage under and
the
Integration.
two
areas
of
the
use
Status--namely of
engineering
sufficient
(early
group
as
in the
the
of
integration
fracture
inherently
integration.
appointment small
teams
fracture
identified
control
capable
generalists
of
providing
Encouragement
could
come
design
of
individual
fracture
stage)
control
an
focal
point
through or
for
each
decision
or
installation.
Recommendation
FE2
Docunmt
the
upon
fracture
all
As
anticipated
a
effects
control
informal teams)
minimum tion.
every
between
what
is
and what might documented
every
structure’s
structural
major
fracture
done
fracture
change
(generalists
plans,
introduce
to improve and ensure or
and
or done in future,
control
decision
control,
now
be feasible
level of documentation For
major
elements.
compromise
comprehensive,
of
change
a
integra-
involving
a
ask the most cognizant engineers
to identify the reason for the change and the fracture control element
most
impact
on
could
turn
other
calculations example
involved
of
fracture
into or the
and
a
to
control
major
could latter
list
take
any
elements.
report a
with
short,
is included
many
in Appendix
change
in a tubular
joint,
improve
stress
concentration
at the
described qualitatively
Such
A
involving toe
or
probable
a document quantitative
qualitative
hypothetical the
possible
form.
An
in which
a
grinding to
of the weld,
is
in terms of its effects upon the other
elements of an integrated fracture control effort.
An effort spent
by
key
like this
engineering
could be expensive, talent.
documentation may only be justifiable
Therefore,
at
present
@
and in time
this
for truly fracture-critical 124
486-334
both in dollars
kind
parts.
of
Recommendations that
of working
rather than
to
1
and
2
optimally
to tear
exemplify nmdify
a
the
it down and build
strongly
existing
recommended
fracture
control
Recommendations
anew.
approach: system
3 through
5,
which could be listed in the following sections within technical subspecialty elements, are included below to exemplify more specific ways of optimizing a sound existing system.
Recommendation
FE3
Modify punching shear criteria to prevent other failure modes.
For
design
against
within
the
impact
of
example,
overload,
punching failure
shear modes
see the changes
American
Criteria
for
attempt
to
criteria other
that
than
proposed Tubular
make
modest
will
punching
by Marshall
minimize
the
shear.
For
in “A Review of --
Structures
changes
and
Proposed
Revisions.”
Recommendation
Use
a
fracture
FE4
fracture
calibrated
wchanics
nmdel
to
address
tough
fatigue
problems.
For
fatigue
non-routine
evaluations, mechanics typical
start
model
defect
with
and
the
consistent sizes
only to extrapolate
S-N with
in weld
subcritical approach, all
toes,
build
known
etc.,
in the structure.
the
and
of
mechanics
with
tradition the
procedure
growth fracture such
as
use this model
from the original S-N curve to account for
or detriments
combined
a
facts,
and
improvements comfort
crack
special to
the
S-N
curve
capabilities
account
for
the
distribution and notch and part size effects.
125
In this manner, approach of
the complex
can
be
fracture stress
and
Recommendation
Apply
FE5
structural
reliability
nmdels
to
optimize
design
and
operational
decisions.
The
probability
of
fracture
other
risks
by
to
design
can
applying
structural
reliability
models
to choose an optimum combination
of design,
selection,
redundancy.
inspection
Marshall
the
with
acronym
triangle.”
minimized
related
structural
material
be
and
specifications,
DIRT,
Optimization
reliability
uncertain
dependencies
upon
can be
simply a minimization
can
analysis,
and robust. input
of
-
“design-inspection-redundancy
for
estimates
input to the
still be accurate
structural
refers to the study of these trade-offs
subject to meeting or exceeding traditional While
and
be
safety constraints.
strong
functions
the optimization
Such accuracy historically
of cost
process
of can
is due to weaker
calibrated
relative
costs and probabilities which influence tradeoff decisions more than do absolute probabilistic estimates.
3.4
Recommendations
Recommendation
for
Future
Research
FR1
Survey industries using more formal fracture control documents than does the offshore industry.
It
is
control
not
clear
how
comprehensive
is
feasible
document
an
for
various
applications within the offshore industry. survey
(mostly interviews)
and McHenry’s notably
be conducted,
as a starting
aerospace
formulated such
work
and
nuclear
point, power,
and used such documents.
representatives
as
the
486-334
fracture
projects
and
We recommend that a using Rolfe, for those which
Barsom,
industries,
have
already
The survey should include
document’s
126
integrated
drafters,
regulators,
If, after such a survey, the idea of an integrated
and users.
plan is dropped, be
encouraged
methods
continuous for
already
engineering
the
Prove that
generalists,
and improvements
fracture namely,
control the
use
teams, and case-by-case
should
integration of
competent
“effect state-
in Appendix A.
FR2
Charpy
Starting Charpy
three
identified:
ments” as exemplified
Recommendation
discussion
tests
are
maningful
for
new situations.
with
generic
fracture
tests,
do only
enough quantitative
learn whether
the
Charpy-based
toughness
material
grades
and
fracture testing
correlations
will
stand
to
up to
some new situations, such as the increase of brace and can wall thicknesses.
Recommendation
Calibrate
FR3
structural
reliability
Recognizing
that
reliability
are
sensitivity baseline
nmdels
accurate not
to
field
fundamental
yet
possible,
estimates strive
studies and extrapolations
condition
experience.
inspection
to
estimate
procedures,
the
effects
redundancy,
worthwhile
probabilistic
These models should contain enough deterministic relationships
for
to be made from a known
calibrated
using
of structural
of
and
models.
and stochastic
changes
material
in
design,
properties.
That is, calibrate a probabilistic model to field experience.
Recommendation
FR4
Build
a data
base
those
who influence
of
failure, fracture
accident,
and
control.
127
success
experience
for
feedback
to
Since
many
inservice
of the
experience,
system,
as
influence
The
Examples
the operator who such abnormal
no
in
Carlsen, spectrum
are
given
is expected
calibration
failure/accident et
of
al.,
people
and accidents would in Carlsen,
who
benefit
personal who
strongly
might
have
from such a
et al., ranging from
to act “normally” in the face of structural
(in spite of the fact that he probably
experience may
against feedback
is
events as a fire, blow-out, impending
or evacuation
researcher
involve
formalized
entire
on failures
system.
has
a
described
recommended.
failure,
recommendations
need
to
with be
these kept
rare in
events),
touch
with
to
the
reality
through feedback of inservice experience.
This would provide a
definition
of
and,
throughout
these
important
research
recommendations,
areas a
means
as for
called “real
for life
calibration of theoretical models.”
Examples
of
documentated
feedback
given
in Carlsen,
et
al.,
include the detailed reports of the Alexander Kielland incident and
the
broad
data
associated
with
Lloyd’s List and other similar sources.
128
accidents
reported
in
4.0
MATERIAL
SELECTION
AND SPECIFICATION
The first step in designing against fracture is to select and specify a material
for
each
location/application
within
the
structure
to
meet
its
intended purpose without suffering one of the five modes of failure covered in (Section 5.2)*
our summary of design practices
4.1
$urmnary of Status Materials
selection
for various
applications
in the steel jackets
of
fixed offshore platforms is typically a matter of choosing from among the few dozen
steel
alloy/grade
design envelope.
combinations
Standard
levels, carbon-equivalent, ties.
or
Notch-
to
mill tests strength,
crack-toughness
fit
the
studies
affected
of material,
zones.
Other
ductility, is most
initial
material
studies
are
qualification,
NRL Drop-Weight
alloy to arrest
materials
plane-strain
brittle
even
defect
tests,
and other
often
toughness
by
the
defect
Charpy
for any extensive quality conincluding
tests
focus
weld
used
metal
and heat-
for such purposes
and
as
fitness-for-purpose
on the
ability
of the
crack; crack-tip
opening
displacement
and, most often, reliance on generic toughness (e.g., per API
and
“standard” proper-
determined
evaluation,
which
a rapidly propagating
(COD) specifications; qualified
occasionally
fracture
application
are used to determine material
V-Notch test, which is used almost exclusively trol
intended
RP-2A
fracture toughness
Classes
tests,
A,
B,
and
as for example
base
of preStandard
C).
in ASTM E399,
are only rarely applicable since, for most critical jacket locations, standard practice is to ensure toughness levels such that even full thickness specimens Will
fail by brittle (Kapplied > KIC) fracture.
nOt
One of the most important tradeoffs to be considered in material selection is the relationship between strength, fracture toughness, and cost of the
In U.S.
steel. critical crack
crack behavior.
propagation,
corrosion fatigue
practice, there is currently a low emphasis on material sub-
cracking. crack
structural
with The
propagation
unreliability
By subcritical and
without
apparent has
not
in Gulf
environmental
reasons been
a
of Mexico
129
486-334
crack behavior we refer to fatigue
for
the
critical
assistance, low
emphasis
contributor
applications,
and
stress
are
that
to
overall
and under
typical
Gulf conditions
(including cathodic protection) crack propagation in steels is
not a strong function of the material/environment.
4.2
Relevant Documentation Relevant
source materials,
standards,
specifications,
and industry or
university studies are listed at the end of the Material Selection section of
the
Section 1.
II Summary.
Pellini, W.S., Criteria for Fracture Control Plans, Naval Research Laboratory, NRL Report 7405, Washington, D.C., May
11,
4.3
Another key reference is:
1972.
Reconmtendations
for
Use
of
Existing
Technology
Analysis/Procedures
Recommendation
Make
routine
ME1
and
extensive
use
of
simplified
analysis
procedures,
such
as the
FASD .
Simplified analysis procedures for quantitative fracture analysis which
connect
fracture
both the mechanical
should continue
FASD procedure force empirical improving fracture
and metallurgical
to be developed.
(which has the twin procedure
theoretical criteria
and
basis)
can
be
of being
a reasonable, both
of
We believe that the
benefits
having
aspects
a
a brute-
and rapidly
valid
and
safe
and a handy repository for fracture data base
information of various kinds as mentioned above. use of the FASD procedure
Examples of the
are given by both the CEGB and Chell
(Volume 2).
130
Recommendation ME2
Designate which
a
materials
fracture
a correlation
properties
has
The
between
not
been
burden
control standard
is on the materials
for the
making
material
statistical
valid
specifications,
specialist for
established, to
ensure
each
and
quantitative
application
for fracture
control specialist
to
of toughness
in
variability This
typically
designated
which
this
that
this
for
Future
for
We recommend that
each
aspect
fixed-platform has
correlation
and that guidelines
standard
grades or Charpy
fracture properties.
be
data
involves
between
correlation
by means of generic toughness
tests, and quantitative
application
fracture
specifications.
statistically
a
specifications
for
established.
account
such
specialist
been
not
be identified for his/her use
of
fracture
mechanics
is
not
overlooked.
4.4
Recommendations
Data
Base
Research
Development/Evaluation
Recommendation MR1
Start a materials data base for all important masured
fracture parameters of
offshore structural steels.
A materials
data base for offshore structural
started
(published
fracture
parameters.
should be described where
s
material
an
to
enough such
Mainly, in terms
for the
brute-force,
basic
standard
variations
information
all
important fracture
of mean and design
appropriate
property
Ideally, points
is
literature)
steels should be
in
could
be
empirical,
131
4869334
~’%t 0
reflecting
structure)
collected general
properties
(e.g., “-2s”;
deviation
the
measured
to
values. add
data
quantitative
for
techniques diagram
defect
procedure
standards
evaluation
as
(Section 11:3.3).
by which all contributions Data
qualified.
not
meeting
the
the
failure
assessment
A committee could propose to the data base could be standards,
but
potentially
useful for fracture control spec” alists, could be included and identified as such.
Recommendation MR2
Use a data “data
base
such
as
suggested
in
MRl
to
identify
programs
for
closing
state
of
gap.”
Comments
made
by
W.
S.
Pellini,
regarding
the
engineering fracture data in 1972 are nearly as applicable now. To quote:
Unfortunately, the literature on fracture research is highly specialized and the notable agreements which exist on fracture-state criteria are obscured by masses of detai 1. Moreover, the processes of metal property surveys and the sources of this information are not considered in adequate detail. Serious concern must be expressed for the paucity of statistically reliable engineering fracture data for standard grade metals that Developing this is provided by the existing literature. information is of foremost consequence at this time.
We agree with the need to collect, organize, and unambiguously express data.
valid
and
Then, what
statistically is available
reliable engineering
can be matched up with what
required for offshore
applications
tests
can
to
first
recommendation
be
devised
should be expressed measuring
close
fracture
the
under existing
and cost-effective “data
gap.”
technology
is
fracture
As with
our
above, the data
in terms of properties the investigator was
and in terms of some global brute-force quantitative
fracture criterion, if possible, such as the FASD procedure, as exemplified by Figures 2 and 3 (in Section 11:3.3.1), or COD.
132
486-334
(f@
the
Experinmtal
Fund cal
Research,
quantitative
Testing
toughness
characteristics
at
TheJ most problem to
testing
the
crack
serious
border
source
in a complex
of
from
differences
in
near-crack-tip
especially problems
thickness.
can
including
arise
in addressing
constraint
the
steel-jacket
constraint
with
problem areas are
between
specimen
in section
severe
the
is
laboratory
and
size,
interpretation
flow at crack tips,
associated
fracture
structure
simple
differences
Specifically,
if plastic
the
structure.
encountered
related matter,
simulate
which
comparatively
The two most widely
and, a
the
of error
specimens.
structure
specimens
fixed offshore
data
interpret
using
especially
developed
triaxial
stress fields and shear lips, are not similar between specimen and
This
structure.
problem
current
practices
by the
tests,
sometimes
to
Pellini
specimens. exhibiting pounds
superb
CVNE)
intended,
very
imperative
crack
These
out
properties
inadequate
quantitative simulate
supplementary
offshore
of inexpensive the
use
examples
larger
where
steels
toughness
toughness
Charpy
of
than
application.
50
foot-
for
their it
Thus,
testing
be
along with
tests
could
be
Charpy added
is
done,
the physical characteristics
structure,
toughness
in the
(greater
fracture
some
of the
of
notable
thickness,
border
use
exclusion
points
using specimens which the
the
large
that
compounded
extensive
Charpy
had
is
at
tests. to
any
extensive Charpy testing program or, more likely, could be conducted
in industry-wide
classes.
efforts
for generic material
The final step would be the demonstrated
between
Charpy
fracture
toughness,
provide
research
assurance
impact at
data least
and in
of some minimum
for the structural application.
133
quantitative
a
bounding
correlation measures
sense,
of
so as to
value of fracture toughness
physi-
Recommendation MR4
Collect
existing
evaluate crack
data
constant
and
define
amplitude
growth.
Ensure
and
execute
fatigue,
that
an
an and
overload,
adequate
experimental
range
of
program
underload
crack
effects
propagation
to upon
rates
is
covered.
Collect weld
existing
cracking
Insofar should
and
generate
as is practical, be
enveloped
money, tests focus
fatigue crack growth data applicable to joint
upon
conducting
previous
the
boundaries large
experimental
Key test
loops
of the
matrix
experimental
feedback
the
of
rather
than
necessary.
should
To
save
should be developed to rather than upon
combinations be adjusted
by designing blind
parameters
program.
“envelope” many
variables results
as
range of inservice
and reliable extrapolation
a
variables.
a wide
in
data
additional
test
execution
of to
test
reflect
programs
with
tests
fully
of
specified before the first specimen was tested.
Included in the envelope of data collected and generated should be
(1) frequencies
associated
excitation
of the structures;
submerged
and
without
near-surface
various
detrimental
levels
of
residual
stresses
pressive
loads or compressive by
to
the
beneficial
wave
loading
(2) environments (wet/dry) cathodic
“overprotection”);
from high R values associated
caused
with
and
reflecting
conditions, protection
other both
with
and
(including
(3) a full range of R*-ratios-with the presence
lowest
R
values
of full-yield
arising
from
com-
residual stresses that might
overload
effects;
and
be
(4) specimen
geometries simple enough to model the in-service interaction of material
and environment
but varied enough to test whether
or
*For a given cyclic stress intensit factor versus time K(t) excursion from a minimum value, Kmin, to a maximum fevel, Kmax > 0, R is defined as Kmin/Kmax.
134
d86
-334@
not
the
fracture
mechanics
model
analytically, as it is supposed to.
●
Define
the
overload
experimental
and
subsequent
crack
account
geometry
to
evaluate
In addition:
necessary
program
(compressive
growth
for
overload)
in offshore
effects
upon
Bounding
structures.
can be made to determine where load interaction
experiments effects
underload
can
can
be
ignored
and where
proof-test
logic
can
be
rates
is
used for screening of flaws.
●
Ensure
an
range
adequate
of
crack
propagation
We recommend covered in both of the above recommendations. -4 inches per cycle at least, a range between 10-7 and 10 and, more ideally, an expanded per
cycle
and
similar
stress-corrosion
●
Statistically experiments
da/dt
range of 10-8 to 10-3 inches ranges,
as
practical,
for
cracking.
analyze to
all
variability
input
provide
to
generated
probabilistic
in
the
fracture
mechanics approaches discussed in Section 4.2.3, fo” lowing.
●
Compare these distributions with those in Engesvik
and other
sources and try to resolve or explain differences.
●
Design experiments
reflecting worst-case
such expensive-to-simulate can be eliminated the
potential
test variables as the environment
or greatly simplified.
for
quote from Carlsen, environmental
scenarios to see if
simplification,
As an example of
consider
the
following
et al., which gives the impression that
acceleration
of
subcritical
crack
growth
be well controlled using existing technology:
respect
With
to
corrosion/fatigue,
considerable
research for the last years has been carried From this work it appears that although out offshore structures under free corrosion will have fatigue strength lower than in air, cathodic pro●
●
●
can
tecti on with a normal potential close to -800 mV will restore the in-air fatigue strength.
Recommendation MR5
Set
up a “devil’s
There
advocate”
is
testing
always
going
programto
be
to
evaluate
much
fracture
controversy
criteria.
between
the
pragmatists who wish to use inexpensive screening tests such as the Charpy test for providing and the be
lower bound toughness
properties
“purists” who hold out for use of specimens which can
used
for
precisely
quantitative
the
crack-like
dynamics
structural
recommendation controversy
to
defect and
evaluation
triaxiality
defect.
reduce
As
the
is to mix the two
and
of
can
simulate
stresses
mentioned
detrimental
the
above,
impact
kinds of tests
at
of
one this
and investigate
data correlations.
A second idea, admittedly in an engineering
untried except as it occurs naturally
or scientific
field, is to set up “a devil’s
advocate” testing program in which a noted fracture-test
purist
could be funded to design and demonstrate a worst-case scenario in
which
the
inservice
(to the
technical
simulates be
If
would
a simplified
satisfaction
reviewer)
or
oversimplified
for
an
important
fracture criterion can
of the
with
discreditation, credibility. or
improved.
criterion
by knowledgeable
funder
or perhaps
an experimental
Similarly,
resists
the attempts
set-up
a
that
it
(the
criterion)
deserves
A few good experimental inability
convincing
literature
if
a
seemingly
to discredit
people who have something to gain through
of
a
to
the
simplified
containing
technical detailed
fracture
respect
its and
criterion
to
level would probably be
community theoretical
arguments. 136
4~GnSS~
more
it
examples which showed the
guarantee an adequate minimum toughness more
fail
in-service conditions, that criterion probably should
discarded
ability
criterion
application.
be defeated neutral
Charpy
0 $&
than
volumes
of
and
qualitative
5.0
DESIGN The
designer
responsible
is
for
guarding
against
the
possible failure modes as well as for producing a near-optimum
structure’s
functional and
Five relevant failure modes are listed below in order of
economic structure.
decreasing emphasis given by the authors in the fracture control survey:
1.
Fracture, defined as unstable rapid crack extension--either elastic or plastic-- leading to partial or complete failure of the member.
2.
Subcritical crack growth, under fatigue, environmentally accelerated fatigue, or stress corrosion cracking, leading to loss of section and fracture.
3.
plastic deformation; or ultimate Yielding, excessive failure under tensile or bending (plastic hinge) loads.
4.
Elastic or plastic compression loads.
5.
Bulk corrosion, leading to loss of cross section.
Structural
design
buckling
against
or other
fracture
instabilities
concerns
under
itself with two aspects:
The loadings placed on the structure by the environment
and by its operation,
and the resistance of the structure to these loadings.
5.1
$umnary
of
Status
The nominal design pract” ces used in the Gulf of Mexico (primarily from API RP-2A) are fairly uniform.
for
fatigue
in
extreme
environmental conditions and forces due to installation and transport.
Due to
ture,
fracture
loadings
and
reserve
control
To account for loadings placed on the struc-
is
strength
withstand
is exp” icitly
implicit
and
a certain
work
consulted Program
on
is
the
redundancy,
amount
practice, quantification design
in
design
a
for
typical
of cracking.
In
in the
normal
steel normal
design
operation
by
the
design
requires that the design
operator’s
decisions. of platforms
structure
jacket Gulf
of this resistance is not attempted.
conducted
significant
considered
of Mexico
MMS
design
Verification of
representative, The
can
Platform
for US operations
who
is
also
Verification outside the
Gulf, or new Gulf platforms which will be located in over 400 feet of water (among other qualifications),
be checked by an independent party--resulting
in
additional verification of the major analytical design tasks.
5.2
Relevant
Documentation
Relevant
source materials,
standards,
specifications,
and industry or
university studies are listed at the end of the Design section of the Section
II Summary.
Other key references include:
1.
Pellini, W.S., Criteria for Fracture Control Plans, Naval Research Laboratory, NRL Report 7405, Washington, D.C., May 11, 1972.
2.
Carlsen, C.A., T. Kvalstad, H. Moseby, E.M.Q. R6rn, T. Wiik, “Lessons Learned from Failure and Damage of Offshore Structures,” Eighth International Ship Structures Congress, Gdansk, 1982.
3.
P.J., “Summary of Current Design and Fatigue Fisher, Correlation,” Fatigue in Offshore Structures, Institute of Civil Engineers, Conference Proceedings, London, February 24-25, 1981.
4.
Rolfe, S.T. and J.M. Barsom, Fracture and Fatigue Control New Jersey, 1977. in Structures, Prentice Hall, Inc.,
5.
Telephone interview conducted with Robert G. Bea and Ashok Vaish, PMB Systems Engineering, Inc., San Francisco, California, on September 16 and 23, 1982.
6.
Gurney, T.R., “Revised Fatigue Design Rules,” Metal Construction, revision of UK DOE Guidance, January 1983.
7.
Failure
Analysis
Associates,
“BIGIF--Fracture
Code for Structures,” Electric EPRI NP-1830, April 1981.
5.3
Recommendations Analytical
for
fracture
nation of the significance the task growth.
of designing
Use
of
Existing
mechanics
fatigue
Many of the recommendations
Research
Mechanics Institute,
Technology
methods
of fatigue
against
Power
cracks
assist
a quantitative
in offshore structures
determiand aid
and other forms of subcritical
crack
listed under “Analysis” are suitable for
138
both applications
and, therefore,
cracks found in fabricated transport,
and
launching
for many fitness-for-purpose
structures,
evaluations
of
either in service, or during loadout,
operations.
The
remaining
recommendations
cover
other aspects of the design element of fracture control.
Analytical
Evaluation/Development
Recommendation DE1
Use
first-order
effects
of
bounding
toughness
It
is
of
the
fracture
and
mechanics
fatigue-design
models
low
determine
fatigue
control
damage
effort,
content
of
loading spectra and water depths, has concentrated monotonic-loading toughness
and
failure
arrest
of
As frontier
“pop-in”.
relative
control.
believed that the U.S. fracture relatively
to
modes
associated
critical,
rapid
areas which,
because
Gulf
wave
more on the
with
crack
fracture
growth
in combination
with
and new,
innovative structural designs, approach and perhaps surpass the fatigue is
damage potential
recommended
models, the
which
useful
rather
than
relative design
that take
life
first-order
advantage of
welded
initiating
effects
of the North Sea are encountered,
a
Many
fracture toughness
fracture
mechanics
of the fact the the majority structures be
crack,
of toughness
control.
bounding
used
control
situations
is
spent to
it
of
propagating
ascertain
the
as compared to fatigue-
may
arise
in
which
the
of the steel would have a negligible effect
on the useful life of the structural joint.
Recommendation
DE2
Supplement
curves
S-N
There
with
deterministic
is a general consensus
fracture
nwchanics
approaches.
that the use of S-N failure data
will remain the main design tool against fatigue.
139
Crack growth
.-
486-334
/ @
data
and
developed
fracture mainly
evaluation
and
to
mechanics
methods
should
provide
support
for
extrapolation
of
S-N
continue
to
be
fitness-for-purpose
data
to
situations
that
cannot be or have not been simulated in experimental work.
Recommendation
Model
DE3
size/shape
of
crack
and transition
behavior
in
brace
or
leg.
Where necessary, model the transition from partial- to throughthickness
cracks
in the brace or leg.
efforts in flaw size/shape
Concentrate
analytical
ranges for which most of the joint’s
fatigue life will be expended.
Education,
Training
Recommendation DE4
Teach
engineers
dangerous
to
review
their
design
assumptions
quote
Pellini
thinking which
may
on
safety
actually
factors reduce
in
the
order
to
structure’s
ability.
To
(Criteria
for
Fracture
Control
Plans,
page
45):
In developing fracture control plans, the engineer must develop completely new thinking with respect to factors Adherence to past conventions is dangerous of safety. ..*. [For example,] . . . the added “safety” assumptions [for “beefing up the structures” by] increasing wall thickness are not”[always] justified.
pellini goes on to give an example, based on data for a 100 ksi yield
strength
pressure
increase actually weakened susceptibility
vessel
steel ,
the welded
to moderately-sized
140
where
a
thickness
structure because of its
weld defects.
Such examples
avoid reli-
should be useful for educating true” it
fixes as beefing
in
the
presence
fracture
and
weaknesses defect-prone
the
of
defect-related crack
resulting
welds,
that such
“tried and
up the structure might actually weaken
subcritical
of
designers
failure
modes
lower toughness
as
Potential
propagation.
thicker
such
cross-sections
are
more
joints, and reduced member
and joint flexibility.
Recommendation DE5
Reduce
the
human
MMS Platform
error
factor
Verification
through
education
and
direct
impact of human errors
research
is
inspection,
needed
desired
error
modification
of
MMS
the
education
recommendations)
to
nmdification
of
control Platform
problem
of the
The best vehicle
techniques
may
Verification
(one
improve the
the
(as well as in fabri-
etc.).
programs
to
reduce
in design
maintenance,
implementing
through
minor
Program.
Effort
cation,
and
of
Peter
designer’s
be
for
minor
Program
and
Marshall’s
application
of API
RP-2A.
Detailed
Inspection/Analysis
for
Impact
Recommendation DE6
Survey
the
entire
structure
As emphasized
stress
of the
critical
elements.
by T. R. Gurney
UK DOE Guidance): member
for
(“Revised Fatigue Design Rules,”
“It should be noted that in any element or
structure,
concentration
every welded
is
potentially
joint a
or other form of
source
of
fatigue
cracking and each should be considered.”
Gurney then makes the
point that the Guidance
is to consider
Notes’
intention
joint and that the most insignificant-appearing
141
486-334
~
every
attachments may
the
be vital in the context of fatigue and structural It should be possible to avoid quantitative
reliability.
evaluation of every
weld in a structure by adopting standard details that have been generically evaluated in detail.
5.4
Recommendations
Data
Base
for
Future
Research
Development
Recommendation DR1
Develop a data base of service
and
Develop an industry-wide
repair
histories.
data repository for service experience
on fixed structures and for repair histories on same.
●
The
use
of
introduces critical
repair
welding
complexities
and
into
and subcritical
other the
procedures of
estimation
crack growth.
a need for data on the toughness
repair
both
There is therefore
and fatigue performance of
repaired joints.
●
Catalog
observations
important
of
high-severity These
cracking.
cracks,
failures,
situations
observations
which
would
and
did
both
nearly
not
as
lead to
challenge
and
calibrate improved design developments.
Analytical
Evaluation/Development
Recommendation DR2
As
a
long-term
goal,
intensity
factor
concern.
This
mst
tubular
it
would
solutions would
joint
for
include crack
be all
desirable stress
evaluation
of
geometries.
to fields
and
displacement
As a starting
142
4g(j -334
develop
@
accurate
crack
geometries
of
exhibited
by
control point,
stress
develop
a handbook
or
software
geometries
The
library and
of
load
state
stress
intensity
factor
solutions
for
often
occurring
cases.
of the art may be such as to allow some numerical,
accurate,
full
solutions
to
three-dimensional
be developed
stress
as a practical
factor
intensity matter.
However,
a
more cost effective approach based on existing technology would be
the
u,se of
dimension?l
variable
solutions
(cross-sectional)
and
approximations
thickness
for
many
two-
of
these
geometries.
Implicit in a recommendation
to develop stress intensity factor
solutions for all stress fields and crack geometries of concern is an evaluation tubular
joint
should
have
crack the
redistribution growth
of the displacement-control
of
geometries.
full
capacity
crack
through
intersection;
(2) effect
growth
the
structure=
While
for
such
load
into
or
regions stress
at
of
the
brace-can
subcritical lower
crack
stress,
directions;
and
and,
(3) load
rigorous models of these three types of load be
neither
accurate
state of the art.
and
currently
approximations
feasible are
well
nor
cost
within
the
The accuracies can be evaluated through both
studies
experimental
wall
critical
principal
may
some
sensitivity
account
developments
along the brace leg and to other members in the
redistribution effective,
to
the
of
brace
possibly, differing redistribution
The theoretical
by most
effects as (1) unbending of a brace wall due to
a
around
exhibited
and
comparisons
inservice
crack
with
observations
of
Additional
propagation.
suggesti on$ include:
a
Run full parametric studies for developing stress intensity factor
solutions
which
include
simultaneously
such
local
factors as the ability to handle any stress gradient in the untracked variations
structure,
the
of weld
profile
ability
143
to
geometry,
handle and
all
the
credible
ability
to
characterize generality, useful
residual the
tool
stress
weight
for
or
the
To
fields.
influence
function
analytical
obtain
such
method
is
for
developments
a
most
geometries.
*
Verify
all
using
an
program
solutions
for
independent,
to solve the
general
stress
distributions
state-of-the-art,
crack problems
finite
by
element
for certain
specified
“benchmark” stress distributions.
Create
●
a
intensity cases
that
handbook factor
(K)
occur
would
such
K
library
for
in design
This of
software
solutions
often
calculations. specialization
and/or
geometries
and
be
an
handbooks
of
stress
and
load
fitness-for-purpose extension
as those
and/or
of Tada,
et
al.; Sib; and Rooke, et al.
Recommendation
Develop analysis
DR3
deterministic of
design
significance
and
of
fitness-for-purpose
defects
and
for
approaches development
of
for
examples
guidelines.
Develop
both methods and software
fitness-for-purpose analysis
systems
of the significance
in
a
for deterministic form
suitable
design and for
routine
of cracks in offshore structures.
As noted by Carlsen, et al., in “Lessons Learned”:
Due to uncertainties on fatigue life predictions, . . . it is very important to calibrate design requirements against in-service experience, and to develop tailormade inspection in-service to follow up programs critical parts in service.
144
486-334
B STfj 1
routine and
From this:
●
Exercise
the
system
problems
which
environmental
for
envelop
both
hypothetical
geometric,
combinations
to
be
and
stress,
practical
material,
expected
the
in
and
field.
The fifteen to fifty analyses created can serve as modeling and
examples
for
guidelines
the
engineering
offshore
community.
Present in graphical and tabular form the effects of varia-
●
tions of key input parameters, tion,
upon the dependent
strength. develop
These a
individually and in combina-
variables,
will
provide
probabilistic
fatigue life or static
guidelines
and
mechanics
fracture
input
to
system
as
described below.
Recommendation DR4
Develop
a
fracture
probabilistic
mechanics
approach
for
routine
crack
analysis.
Develop
a
software The
probabilistic
for routine
software
tionally
should
practical
fracture
analysis
numerical
procedures
covariance
statistical
of cracked
probably
Monte
employ
Carlo
can
mechanics
be
devised
method
offshore
realistic
methods
approximations
(PFM)
which
structures.
and
although
and
computa-
less
employ
for computing
general
variancethe impact
of independent variable scatter on dependent variable scatter.
●
Develop should
a procedure and
should
deterministic implementation used with and
when
for not
be
employed.
when
a
Stress
PFM the
analysis use
of
(DFM) bounding calculations to avoid needless of PFM.
reliability DFM
determining
is
Specifically analyses,
inconclusive
145
PFM should normally be
cost optimization (i.e., when
studies,
worst-case
DFM
assumptions
predict
structural
failure
but
nominal
DFM
assumptions predict success).
●
Implicit
in
probability as
the
setting
up
distributions
coefficients
propagation
input
recommended
relate
to
material
crack
distribution
Each
should be qualified by a
of its range of validity and indication of what
might
that
be
as a function of stress intensity factor.
description changes
system will
for certain input variables, such used
suggested probability
Note
a PFM
be
high
expected
accuracy
probability
distributions reliability
estimates,
analysis.
deterministic
to
significant
the
use
potential
worst-case
will
simultaneously
of
needed
if
for these
different
input
differences
conservative
be
at
gain arises models their
while worst-case
this unrealistic assumption. literature
usually
conditions.
in
bounds
of
may still permit much to be gained from a PFM
This
variables
is not
worst-case
distributions--even
lead
distributions
under
on PFM-related
from the
assume most
that
fact that all
unfavorable
input values
PFM models can easily avoid
Note also the rapidly growing
data and methods,
as exemplified
by Engesvik.
●
Execute and
the
PFM
practical
mechanics
analysis
cases
(DFM).
for
analyzed
several with
of the
hypothetical
deterministic
fracture
Pick some examples which will benefit and
some of which will
fail to benefit
from PFM to illustrate
the optional character of this analysis technique.
146
4g6-334 0
E?
Recommendation DR5
Establish
design
explosions,
loads
for
and
protection
against
impact
damage
(collisions,
etc.)
More research is required to establish design loads and methods for protecting dropped
the structure
objects,
fires,
against damage due to collisions, Among
and explosions.
areas to be explored are the establishment forces,
energy
for offshore
dissipation
structures.
methods
the technical
of realistic impact
and other
protection
Some of the elastic-plastic
work being carried out should help significantly by establishing, a compressive
means
buckling
in this effort
for example, the residual buckling strength of
member
as a function
of the size of its impact
dent.
Recommendation
Define stress
DR6
applications methods
For
suitable
for
some
life
for
use
of
higher-order-average
prediction.
variable-amplitude
stress
histories,
the history on the basis of a root-mean-square order-average
alternating-
replacement
(RMS) or higher-
(e.g., cube root of mean cube) alternating stress
appears to offer a reasonable method for life prediction variable
of
amplitude
define which
Further
loading.
applications
will
work
methods
necessary
not suffer inaccuracies
this higher order averaging approximation cycle-counting
is
from stress
under to
due to
and to define optimal The effects
histories.
of
severe overloads or underloads require particular attention.
Recommendation DR7
Formalize subcritical
the
statistical crack
growth
approach data
to for
defining any
the
given
combination. 147
486-334 n
/m
variability
material
of or
S-N
environmental
and
There
is
approach crack stress
a
need
to
to
provide
defining
growth
data
intensity
the
in
simple mathematical
more
variability
the
factor
a
presence
S-N
routinely.
Two steps are required and recommended:
a
and
Relatively
respectively.
(and
Analysis
limits
endurance
evaluation
●
subcritical
and
exist to handle this statistical
techniques
similar
of of
thresholds,
statistical
formalized
ones
involving
fracture
properties)
of variance to determine which quantities maintain
constant
curve.
variance
over
For example,
the
entire
S-N
at the endurance
or
limit,
da/dN
(K)
it would
no
longer be proper to consider the standard deviation of “log cycles”
for
each
lab-specimen
appropriate
parameter
stress
log
or
standard
might
stress
deviation
of
data
be the
or,
of
points from the mean-trend
standard
greater
perpendicular
A
point.
more
deviation
complexity,
distance
of the
of the
data
S-N curve in a specified type of
plot (e.g., log S versus log N).
●
Having
established
variance
(and
the
under
parameter
what
which
situations
exhibits
this
is
constant
true),
use
standard regression methods to analyze it and calculate the “-2s or -3s line” to be recommended for design use.
Recommendation DR8
Estimate
the
effect
More work
of
nmltiple
crack-site
origins
at
the
weld
toe.
is needed to estimate the effect of multiple defect-
and crack-site
origins
linking characteristics
at weld toes and their
interaction
and
as they grow.
148
4$6-334
/0 (z?
.,.”
Recommendation DR9
Define
stress
gradients
and
other
characteristics
of the subsurface
distribution.
The
need
to
define
the
subsurface
fracture mechanics
applications
joints
shear
with
large
transfer would
to
work
stress
may
have
A
common
analysis
transfer
stress
the
Develop
use
of three-dimensional
techniques
or,
at the
between
versatile
the
analyzing
brace
crack
finite both
tubular joints, of
fracture in general
very
and
can
least,
other Some of
numerical analytical
assumption that all
walls
occurs
at
the
versatile
for
surfaces.
DR1O
accurate,
While
loading.
and
gradients
the
intersection of their mid-thickness
Recommendation
of such
detailed
that more work
models which do not employ the simplifying load
example
of the subsurface stress distribution. require
for
braces of which one
developed
is consensus
define
characteristics this
that
models there
needed
is
transfer.
and the other in high compressive
researchers
mechanics
distribution
is amplified by the presence of
be a K-joint with adjacent
is in high tensile Among
stress
analysis
element
untracked
methods.
method and,
is
to
the
a
most
lesser
extent,
cracked
it can be quite expensive, time consuming,
questionable
for
accuracy
the
most
difficult
and
problems.
Thus, there is a real need to come up with accurate, nearly-asversatile, system
to
developers
advanced the
methods
fracture
basis
for
if
be
used
effective,
mechanics
the
by
the
researcher
everyday
is to be realized.
recommendations
above
to
and
design
application This use
the
of
need is weight
function method and to formulate handbooks of useful stress and stress
intensity
factor solutions
of tubular joints subject to
cracking. 149
486-334
/~/ 0
stress
Recommendation DR1l
the effect of
Consider
greater-than-predicted
wave
loadings
upon
the
fatigue
life of a structure.
The
unpredictability
cause
of
for uncertainty
environmental
in fatigue
loadings
is
a
primary This
stress predictions.
is
especially true for frontier locations where long-term data are scarce or non-existant. be constructed
Sea state scatter diagrams must often
with only two years
of wave
recordings.
Thus,
we recommend that the impact of departures from the design wave environment
upon
the
fatigue
life
of
a
structure
should
be
considered.
Recommendation DR12
Study
methods
outside
the
for
determining
standard
Tubular
hot
spot
stresses
tubular
joint
behavior
case.
joints
have
been
classified
as T,
standard load paths through the joints. to
and
investigate
the
proper
way
to
K,
or X based
on
Studies should be done
interpolate
between
the
standard cases to find the hot spot stresses.
Also, the behavior mostly
in
of tubular
two-dimensions.
interaction
of braces
joints has to date been studied The
effect
out of plane
needs
of
three-dimensional
further
study.
The
effects of joint flexibility and eccentricity also need further study.
Recommendation
Study
wave
lines
for
DR13
loading
and
associated
phenomena
nmre
application.
150
4$6-334 0
62
closely
and
develop
guide-
It
is
recognized
by the industry that the calculation
loads on a structure recognized
that
miscalculated, structure
while
is
sponsored
force
wave the
adequately
studies
In particular,
is very imprecise.
local
effects
may
effect
integrated calculated
of wave
for
be
on
it is
seriously
the
entire
Industry-
design.
of the wave loading and associated
phenomena
need to be continued.
●
Guidelines
should be developed
to aid the designer
in the
choice of wave periods corresponding to chosen wave heights for the discrete wave fatigue analysis approach.
It is not
sufficient for fatigue analysis purposes to just choose the average wave fatigue
period
or other
for a given wave
life computation
height,
because in a
the stresses
above some
threshold
are raised to a large power and those below the
threshold
are
period
is
ignored
and thus
non-conservative.
choosing For
the average
each
wave
wave
height,
the
analyst should consider either a spectrum of periods or the averageperiod
*
Guidelines
corresponding to the high-stress waves.
should
computing
be
developed
the effective
damping
to
of the
as input for a dynamic analysis. for longer has
period
structures
a significant
effect
aid
the
designer
structure
in
required
This is particularly true
in deep water, where
on the
dynamic
damping
amplification
of
member stresses.
*
Guidelines loading loads
for
in fatigue
are
stresses
the
often
inclusion
analysis
only
in structure
added
of
simultaneous
current
should be developed.
Current
when
maximum
calculating
in combination
It should be noted that superposition changes ‘L- ‘–-’’L’‘= ‘L- -A---Lllf?
mean.
This
dlll~l
lLUU~
occurs
UI
L.[Id
because
equation includes a nonlinear
151
SLrf?SS
the
with
the
a design wave.
of current _.._1-
--
..-11
LYL15
d>
W~l
drag
term
in
and wave I
--
LL_
dS
Lrl~
Morison’s
(velocity-squared) term.
o
With the proliferation for
analyzing
offshore
microcomputers,
Typical term
not
structures,
designers
need to be warned which, do
of framed structures
on
now
available
certain
properly
handle
Morison’s
the
include
equation
nonlinear
wave
linearization and
using
on
applications
against the use of oversimplified
simplifications
in
working
some
computer codes
codes loads.
of the
water
drag
particle
velocity rather than relative water particle velocity.
Recommendation
Determine
DR14
the
gradients
in
feasibility
of
estimating
Investigate
determining
elastic-plastic
the
crack
feasibility
approximately
(in the
in estimating
elastic-plastic
of the
extensive-data
of
untracked
as COD and the J-integral. the state
art
approximate tip
stress
of
gradients
at the crack locus)
crack tip stress parameters
If
such
analytical solutions are beyond
(which may
be true
in many
cases),
the
base of COD and J experiments
that is evolving
For example, the experimental
data now used to
set lower bound design and failure assessment
curves in BSI PD
could be used.
6493:1980 gradients
can be employed to approximate due
to
the
many
stress
parameters.
stress
handling
structure
effects
tested
the effects of stress
combinations
of
axial,
bending, and residual stress levels.
Recommendation DR15
Develop
algorithms
Develop and
for
the
problem
of
and apply algorithms,
solutions
as employed
contained
plasticity
in
notches.
such as key Neuber-based methods
in BIGIF
(EPRI NP-1830),
to
handle
the frequent problem of contained plasticity in notches and its effects
on subsequent
subcritical
fatigue and stress corrosion
crack propagation.
152 ..,.,.
486-334
/6y 0
Recommendation
Develop
DR16
additional
design
data
for
We recommend development design
components a
of
capacity
“shell”
components.
of additional design data for ultimate
the
stocky
“shell”
steel
structural
of fixed offshore platforms which often fail due to
combination
buckling. already
steel
of
elastic
Comprehensive led
to
and
plastic
research
design
method
shell
and
column
in this area has apparently improvements,
according
to
Carlsen, et al., page 23.
Recommendation
Assess
DR17
effects
of
uncertain
hot
spot
stresses
and
incorporate
into
a
PFt4
model.
The effects of uncertain hot spot stresses due to uncertainties in the joint
wave
environment,
behavior
wave
loading,
should be assessed
dynamic
response,
and incorporated
and
into a PFM
model.
Recommendation DR18
Determine
the
The
accuracy
of
“identical
dynamic, amplification
computed
from a dynamic
wave” DAFs in variable
factor
analysis
(DAF)
for
assuming
a
wave spectrums.
given
steady-state
tions (i.e., a continuous series of identical waves). to be determined a variable-wave types
how accurate this spectrum.
of work.
histories
for typical
perform a dynamic with
First,
loads
wave
is
condi-
It needs
“identical wave” DAF is for
This determination would require two one would
sequences
need to obtain
of waves.
actual
Second,
time
one would
analysis with a step-by-step time integration
calculated
at
each
153
step
from
the
actual
time
r._ ---
The stress cycles for this time history could then be
history.
compared with those computed
by the usual techniques
involving
DAFs.
Recommendation
Evaluate
the
DR19
neglect
Waves
of
with
fundamental stress
larger
wave
periods period in
cycles
periods
in
two
approximately
three
times
cause two
This
structure.
analyses.
or
often
of the structure the
fatigue
effect
the
or three
is
usually
Research should be conducted to
neglected in fatigue analyses.
evaluate this nominally non-conservative
assumption.
Recommendation DR20
Study
effects
foundation
of
soil-structure
interaction
and,
particularly,
long-term
degradation.
The
effects
long-term
of
soil-structure
foundation
particularly
true
interaction
degradation
for many
and,
particularly, This
need further study.
areas
of the Gulf
is
of Mexico where
platforms stand on deep deposits of weak soils (for example, on deposits
from
the
Mississippi
River).
This
include both analytical work and instrumentation
effect
should
of appropriate
existing and future structures.
Develop structures
a
more using
accurate both
finite joint
element
and mmber
With the proliferation
program
for
the
global
elements.
of studies on tubular joints,
possible to better understand
analysis
joint flexibilities
deformation and member ovalization.
it is now
due to shear
The feasibility and neces-
154
486-334 0
66 ‘
of
sity of developing
joint elements which could account for this
flexibility should be investigated. accurate
finite
element
program
The result would be a more for
the
global
analysis
of
structures using both joint and member elements.
Recommendation
DR22
Employ
load
crack
natural arrest
shedding
and
displacement
control
features
to
help
features.
The natural complex
load shedding
tubular
help
design
and
joints
better understood crack
and displacement fixed
and analytically arrest
control features of
offshore modeled,
features.
Such
platforms,
when
can be employed to features
could
add
substantially to the fail safety and resistance to catastrophic fracture
at
supplements
the the
welded
joint
brace-level
“level”.
fail-safe
This
improvement
redundancy
discussed
elsewhere.
Experimental
Research,
Testing
Recommendation DR23
Expand
the
range
of
tubular
joints
Since the S-N approach
tested
and
used
to
set
S-N
curves.
is still the major tool used to prevent
fatigue failures, we recommend that the range of welded tubular joints that have been tested and used to set the S-N curves be significantly
expanded
namely, almost entirely
beyond
what
simple T-joints.
to obtain ,data on a wider
data
base
mechanics ,,models
upon for
been
done
to
date;
Aside from the needs
range of joint types and to adjust
the design curve when appropriate, ideal
has
which
to
designing
tubular joints.
155
such tests would provide an benchmark against
candidate fatigue
fracture in
welded
design
6.0
CONSTRUCTION
In terms of fracture control, the main goal in construction is to limit built-in
stresses,
variations
the
size
of initial
from specified material
defects,
properties
and
(most basically)
and structural
dimensions.
choosing fabrication and installation methods, two main trade-offs fracture
control
are
made.
The
trade-offs
are the
large
choice
of
In
related to fabrication
method (frame or node) and the choice of installation method.
6.1
Swmnary The
of
Status
construction
of
fixed
steel
off-shore
structures
In the fabrication phase,
into two main phases, fabrication and installation. fracture
control
is primarily
concerned
with
material and the welding of parts together.
can be divided
quality
assurance
of incoming
Both of the two primary fabrica-
tion methods, frame and node, involve the welding of tubular joints. the node method welding
under
although
offers
favorable
more
semiautomatic
have specific
advantages
conditions.
welding,
applications.
of potential
post-weld
Most
welds
However,
heat treating are
made
and
manually,
such as Flux Core and Gas Metal Arc Welding
Emphasis
is placed on producing
a weld profile
that merges smoothly with the base material to reduce the stress concentration effect. Fabrication misalignment,
defects
and weld discontinuities,
Nondestructive
purpose”
defect.
defects
evaluation,
A decision
potential
Disposition including
such as member
which all welds have to some extent.
a
of the defects is based on a “fitness fracture
systematic error in the welding
are prevented
nondestructive Fracture
mechanics
analysis
of
to use as is or repair is made and an investigation
by employing
by closely observing the welding
process is conducted.
qualif” ed welders
and we’ding
process, and by performing
the of
Fabrication procedures,
visual and other
inspection of the welds. control
of the installation tion
into gross errors,
inspection techniques are used to detect these cracks or crack-
like defects in the welds. for
can be divided
of some types
during
installation
concentrates
on proper execution
procedures to prevent damage due to accidents. of
platforms
includes
156
“load-out”
Installa-
(loading on a barge),
transport
to site,
launch
off the barge, erection,
and final assembly.
In
other cases, the platform is floated to the site, which modifies the installation
The trend
conditions.
toward
longer tows
has made
evaluation
of the
transport process more critical to sound fracture control.
6.2
Relevant
Documentation
Relevant
source materials,
university
studies
Section 11 Summary.
are listed
standards,
specifications,
at the end of the Construction
and industry or section of the
Other key references include:
1.
Haagensen, P.J., “Improving the Welded Joints,” Offshore Welded Trondheimy Norway, 1982.
2.
P.J., “Summary of Current Design and Fatigue Fisher, Fatigue in Offshore Structures, Institute of Correlation,” Civil Engineers, Conference Proceedings, London, February 24-15, 1981.
3.
Telephone interview conducted with Robert G. Bea and Ashok Vaish, PMB Systems Engineering, Inc., San Francisco, California, on September 16 and 23, 1982.
4.
Marshdll, Peter W., “Fracture Control and Reliability for Welded Tubular Structures in the Ocean,” Shell Oil Company, presented at the Ninth NATCAM, Ithaca, New York, June 1982.
5.
National Research Council, “Inspection Gas Platforms and Risers,” Marine Engineering, Washington, D.C., 1979.
6.
Telephone interview with Peter Marshall, Houston, Texas, on August 17, 1982.
7.
“Revised Gurney, T.R., Fatigue Design Rules,” Metal Construction, revision of UK DOE Guidance, January 1983.
Fatigue Performance of Structures Conference,
157
486-334
i6 @
of Offshore Oil and Board Assembly of
Shell
Oil
Co.,
6.3
Reconmnendations for Use of
Existing
Technology
Procedures/Guidelines
Recommendation
Establish welded
CE1
the level of cathodic protection
necessary
at all fatigue-critical
joints.
It
is
important to establish
protection because times
at
all
crack
fatigue-critical
growth
faster
control of the level of cathodic welded
joints.
This
is
rates of the order of 6, and even more,
than
in
air
are
possible
given
significant
deviations
either well below or well above the -800 mV optimum
level
cathodic
of
protection.
Variables
to
be
considered
include the material, weld, and environment.
Detailed
Inspection/Analysis
Recommendation
Determine
the
for
Improvement
and Repair
of
Welded
Joints
CE2
condition
of
the
weld
root
before
improving
the
weld
toe
mance.
The
improvement techniques recommended are effective mainly .through the reduction of weld toe defect size. Care must be taken
to
determine
whether
or
not
fatigue
life
improvement
might be negated by the mode of failure involving crack growth from
the
weld
dramatically was
s,ite
distributions
would
clearly
be
improve the weld toe performance
“scheduled”
failure
It
root.
to
fail
depends of
the
soon on
after
the
governing
penetration) defects.
158
the
and
if the weld
unmodified
relative toe
unfruitful
sizes root
to root
toe. and
(e.g.,
The stress
lack
of
perfor-
Recommendation CE3
Apply the principles tion
of
repair
of
integrated
fracture
control
to
the
design
and
execu-
procedures.
An especially
fruitful
field
of integrated
fracture
control
repair
procedures.
of application
is the design and execution
The need for repair usually
sort of problem has arisen and therefore cation
decisions
stakes.
When
of the principles
associated
this
is
with
combined
repair the
have
typically
atmosphere in arriving at a decision and implementing structure,
fracture
Therefore,
the use of a formal
documenting fracture
problems
control
the
impact
control,
of
repair
as exemp” ified
important than in the more
can
or partially
the
upon
be
higher hurried
it on the
compounded.
formal method of all
in Appendix A,
elements
of
is even more
eisurely design stage and decreases
that the fi ]al repair strategy will
the probability
some
the design and fabri-
the
with
connotes
of
impair the
function of the platform or cause any added risk.
6.4
for
Recommendations
Experimental
Research,
Recommendation
Undertake
CR1
of
ways
As examples peening
to
to
testing
improve
weld
of fatigue
large-scale
to
toe
detemnine
fatigue
performance
the
effectiveness
performance.
improvement
schemes, hammer
owe their effectiveness,
of residual stresses
determine
will
component
and shot peening
reduction that
Research
-
large-scale
applications
Future
at the weld toe.
in part, to We recommend
component tests and other tasks be undertaken
whether
last in certain
or
not the
situations
residual
improvements
(e.g., high compressive
159
$g (j-334
stress
@
7/
loads
and
can shake down the compressive
residual stresses introduced and
even produce detrimental tensile residual stresses), the effect of
cathodic
protection
on
the
fatigue
strength
of
improved
joints in sea water must be established,
and the efficiency of
improvement
structures
techniques
on
large
verified.
Large scale tests
the
of decreasing
effect
defects
in
welds
Recommendation
scale
should
be
are also needed to estimate both
the
severity
through
and frequency
techniques
the
of micro
discussed
in
CR2.
Procedures/Guidelines
Recommendation
Develop joints
CR2
guidelines- for the use of available where
fatigue
There
are several effective
the
performance
fatigue
tures. fatigue
is
performance
These
costly
performance
inadequate~
These
Haagensen,which
nmginal
or
techniques been
include
inadequate.
joints are
demonstrated many
offshore only
strucwhere
be marginal
or
investigated
by
improved the weld primarily by decreasing both and micro-defects
for
“improved
welds,”
in
to
techniques,
residual stresses
(measured
in
for improving
recommended
the
be expected
for improving welded
and reliable methods
of welded
has
techniques
profile
to
improve
units
of
the
at weld toes.
all techniques
fatigue
stress
at
a
strength given
Except
listed below can by
at least 30%
fatigue
lifetime).
Guidelines for their use should be developed, based on research already performed or additional testing, as appropriate.
●
Grinding profiles,
these
perpendicular material
Provided
Techniques. avoid
deep
to the highest
of a depth
they
scores
or
result
in
misblends
stress components,
smooth oriented
and remove
of at least 0.5 to 1 mm in order to
remove the deepest and most harmful defects associated with “typical” weld toes. 160
!.
●
Weld-Toe
Remelting
Techniques.
(tungsten-inert-gas) can
significantly
fatigue
cracks
Such
techniques
as
TIG
and plasma dressing, properly applied, improve
of
depths
fatigue up
to
lifetime 3
by
removing
millimeters
and
by
lowering the hardness and improving the properties of heataffected zones.
●
Improved
Profile
improvement
due
profile
a
and
inconclusive discourages
profile
Use
of
Quantitative
specifying
smooth
an
transition
evaluation
overall
at
the
of
concave
toe
the latest modification
the use of non-profile
is
the weld
somewhat
to API RP-2A
welds through
use of a
Of all the improvements listed, improving
appears to be the least sure to substantially
improve fatigue
●
to
although
lower S-N curve. the
Welds.
Special
performance.
Electrodes.
Designed
mainly
for
higher
strength steels, these electrodes produce a final weld pass at the toe of joints with good wetting, characteristics
and
therefore
flow, and material
a smooth transition
profile
that normally avoids costly weld improvement treatments.
161
4g6-334
/73 o
7.0
INSPECTION
AND MONITORING
The fracture control-related and
in the
otherwise
installed
structure
define the extent
task of finding cracks during fabrication
requires
the capability
to
find,
size,
and
This task can be broken down into
of cracking.
four sub-tasks: (1) monitor the condition of the structure during fabrication; (2) check the (4)
the
condition
condition if
of
of
the
appropriate,
the
structure
structure
on
continually
soon
special
monitor
after
installation;
occasions,
the
(3)
when
structure
to
check
warranted;
check
its
and general
condition.
7.1
Sumnary Given
tasks tion
of the
defined types
ments:
and a
Platforms
uncertainty
above,
which
procedures
Status
it
can
is
good policy
requirements
combine
is dealt with
Research
and Risers”
to
several
to check each other.
be used
National
with the many inspections
connected
Council
some
most
(1979) and the OCS Platform
of the effort
level:
welds,
observation
in field
or NDE of the welds.
both
ultrasonic
these
methods,
inspection
focuses
penetrants on the
in primarily of
Verification
particle are
proper
testing.
sometimes execution
and
Gas
Program,
OCS
inspection
visual examination
For welds
used. of
goal being to prevent damage due to accidents
Following
installation
at
of the requires
inaccessible
to
fabrication,
procedures--the
and avoid overloads during the
sensitive load out, transport, launch, and dismantlement processes.
162
two docu-
Given the criticality of
involves weld
process,
sub-
inspec-
Oil
NDE of welded tubular joints typically
and magnetic dye
inspections
of the welding
independent
“Inspection
Order #8, especially Section 9 under “Requirements”. welds,
the
The subject of inspection
in detail
report,
in
7.2
Relevant Documentation Relevant
source materials,
studies
university
are
Section 11 Summary.
7.3
specifications,
at the end of the
and industry or
Inspection
section
Marshall, Peter W., “Fracture Control and Reliability for Welded Tubular Structures in the Ocean,” Shell Oil Company, presented at the.Ninth NATCAM, Ithaca, New York, June 1982.
2.
National Research Council. “Inspection of Offshore Oil and Gas Platforms and Rise~s,” Marine of Board, Assembly Engineering, Washington, D.C., 1979.
Recommendations
for
Testing,
1~1’
Determine
suitability
known
Of
to
have
all
most
fluids
able,
other
taking
the
those
in
near
we
joints
in
design
techniques,
A
lacking
good
proof from
control
reason
testing
testing the
offshore
of
for this
in the design advocate
will
be by
selected
effectively the
process
careful
have
lack of
insurmountjoints
load
proof
out
and
While we are not necessarily
seen
for this
inspection stresses
163
486-334
is
general
complex steel jackets.
phase
to
procedure
envelope.
and in some cases,
proof
operational
known
inspection
is leak testing of members which store
be the extreme,
do
an
their
fracture
buoyancy).
operations.
as
or
with
certain- joints
credit
testing,
or
large and structurally
hand,
transport
testing at
(one exception
difficulties
before
Technologies
“inspection”
concerned
would
extremely
proof
deernphasized
or;provide
emphasis
Existing
stresses
common
notably
platforms
of
of
seen
the
literature
Use
Research
Recommendation
the
of the
Other key references include:
1.
Experimental
joints
listed
standards,
(j~~
at
or
On the tested other
advocating
effective and
in
proof
analysis near
of
their
for
design
envelope
before
placing
the
structure
into
operation.
In general, if only because proof testing is such a big part of fracture
control
complexity more
than
and
a fixed platform’s
consideration
tool, whether
components
of
should
be
of
lesser
steel jacket, we feel that
given
or not such proof
structures
this
valuable
inspection
loads are ever to be applied
independently of the normal transport of the platform.
Guidelines
and Detailed
Recommendation
Inspection/Analysis
for
Impact
IE2
Combine independent inspection mthods.
The varying strengths and weaknesses 1) global/visual,
inspection: to
find
measure
and any
opportunity
size
cracks;
degradation to
simultaneously
use
between the three types of
“to count the braces”;
and
3)
operational
in the structure
several
different
and synergistically
2) NDE,
monitors,
in time inspection
to dramatically
to
provides
an
procedures improve the
inspection resolution and false alarm rate over that associated with
a single type of inspection.
inspection procedures recommended,
tailored
Recommendation
Recommendation
Tailor
the
to
the
three needs
of independent
categories of
each
is strongly
platform
(see
IE3).
IE3
inspection
procedures
Given
the redundancy
fixed
offshore
tailor
from these
Combination
the
to
inspection
characteristics
ally,
if
“counting
intended
application.
and use of forgiving,
platforms,
alarm
the
we
believe
discrimination; to the
the
intended
braces”
164
4$6-334 0
/76
every
tough materials
in
it is very important
to
sensitivity, application. year
is
and
false
Specific-
sufficient
to
provide adequate structural
reliability of a platform, it makes
no sense to demand supersensitive small uncertainty used to monitor tion,
the
key
tions
which,
the highest
of, say, an underwater the joints.
is the in
be a mistake
increase
the
provide
strong
inspec-
protection
platform
against
low level of applications,
to spend large research funds to, say,
resolution
the
inspection
use of independent
For most fixed offshore
it would
half
ultrasonic
As stated in a prior recommenda-
simultaneous
aggregate,
or sizing or
risk failure modes with an acceptably
false alarms.
find
crack detection
cracks
of an inspection
of
depth
0.25 mm
procedure rather
so it can
than
half
the
cracks of depth 0.5 mm (1Ierhaps at the cost of additional false This
alarms).
improving ,local
money
would
monitor ng
probably
devices
be
which
better
can
spent
reliably
in
detect
significant loss of load carrying capacity of a brace.
Recommendation
Make
IE4
inspection
One
plans
of the
site-specific.
recommendations
(,1979) report
Counci 1 specific.
Each type
is
stressed that
inspection
of structure,
process, each operating environment
by the National plans
Research be
site-
each tow and installation has its own characteristics
and each involves separate inspection considerations.
Education,
Training
Recommendation
Ensure
that
quately
trained.
I&5
divers,
Ensure personnel
platform
that
divers,
are
operators,
platform
adequately
trained.
165
486-334
~
and
inspection
operators,
personnel
and
Monitoring
are
inspection
equipment
is
ade-
often
only
as
reliable
people
should
career
expectations
commitment
demands.
acceptance upgrading
by the
engineer. the
have
as
people
the
associated
that
such
This
the
the
a
education,
level
of
it.
These
attitude,
and
responsibility
and
is an idea that has recently gained
nuclear room
power
industry,
a technician
now
The attendant .aggregate increase in understanding
of
to
from
is
an
is expected
operator
which
to
system
control
operating
result in increased
reliability
and
safety for the industry.
7.4
Recmmnendations
Analytical
Future
Research
Evaluation/Development
Recommendation
Further
develop
the
of
use
for
IR1
nmnitoring
divers,
Several signal
and
systems have
structural the
catastrophic
these
monitoring
distress events
of
of
systems
the
structure
can occur.
false
have
minimize
alarms.
the
well
potential
before
This distress
fatigue damage,
monitoring systems
the
rate
be discriminating,
the
to
most
could involve a
large cracks, or a failed
Emphasis should be given to the further development of
use of divers, with
be portable,
an acceptable
higher-than-expected member.
to
produce
structure’s
to
make
them
portable,
a level of discrimination redundancy,
rate of false alarms.
166
486-334
@>
and
have
an
minimize
the
commensurate
acceptably
low
8.0
OPERATION The primary
goal of fracture
control during operation
the risk of operating the structure tions
of
the
structure cracks
structure
exists
must
as it was
in the offshore environment.
be considered:
designed
routine
control is concerned with protecting
Two condi-
conditions,
to exist, and damaged
(or other damage) may be present.
normal operating conditions
is to minimize
when
conditions,
Under design conditions,
the when
fracture
the structure from damage due either to
or such accidents as boat collisions.
One tech-
nique for lowering the risk associated with structural failure during routine operation
is to evacuate
conditions,
fracture
a platform
control
is
Under damaged
prior to a large storm.
concerned
with
preserving
the
structural
integrity.
8.1
Swmnary
of
Fracture tasks:
Status control
practices
during operation
can be divided
into four
minimize the formation of cracks, monitor any cracks that may start,
assess
the
crack’s
impact
necessary remedial steps.
on the
integrity
of the structure,
and take the
The fracture control aspects of the routine opera-
tion of a fixed steel offshore structure are mostly concerned with minimizing the occurrence of high stresses, cracks, and other damage.
Cracks can initi-
ate during operation due to accidents, such as falling pieces of equipment, or improper maintenance, tem.
This
procedures
damage
especially
can
be avoided
for the structure.
tions and structu,nal monitoring. are determined
using
maintenance
of the cathodic protection
by closely
Crack detection
following
specified
sys-
operating
is based on scheduled inspec-
Once a crack is found, its size and location
various nondestructive
examinations.
An evaluation
is
made of the effect, of the crack on the joint or member and the effect of the damaged
element
whether
to repair the crack and, if so, a suitable
ted.
on the structure.
A
risk analysis
is performed
to decide
repair method
is selec-
Alternatives to repair include non-routine operations to reduce the risk
inherent in the presence of the crack.
167
8.2
Relevant Documentation Relevant
source materials,
university
studies
Section II
Summary.
8.3
are
Recommendations
listed
standards,
at the
end
specifications, of the
and industry or
Operation
section
of the
for Use of Existing Technology
Recommendation OE1
Spell
out
available
crack
repair
schemes
in
operation,
emphasizing
their
effects on fracture control and safety risk.
In
order
to
treatment spelled schemes, risk
make
of
out
rational
cracks
and
action.
be
a
the
in turn,
structure,
primary
factor
Recommendation
(See
concerning
different
considered
de-rating
shou~d
the
decisions
alternatives including
and in
under
the
“do the
remedial
should
various
nothing.” choice
“Education,
of
be
repair Safety
remedial
Training”
in
Section 5.3).
8.4
Recmnendations
for
Future
Research
Recommendation OR1
Consider
redundancy
more
quantitatively
in
undertaking
risk-based
remedial
actions.
A
study
of
structural
redundancy
and the
qualification
of
a
structure’s tolerance to cracks should be performed for typical designs
so that
a measure
of
risk can be computed
based remedial action decisions.
168
for risk-
9.0
OVERVIEW
9.1
General
AND PRIORITIZATION
Conclusions
As stated plan already
in Section 3, the elements of an integrated fracture control
exist within
the offshore
industry,
from
rules and guidelines
such as API RP-2A and the ABS Rules to the use of fracture control generalists and
engineering
construction,
teams
consider
who
and operation
has an explicit, written
fracture
through
To our knowledge,
phases.
fracture
control
control plan.
the
design,
one major oil company
While such a detailed plan
may not be the answer, and, on an industry-wide basis, could not be expected to
cover
every
case,
a more
uniform
application
of existing
technology
is
report
most
expressed
as
recommended. The
recommendations
modifications ai ready
to
or
reasonably
recommendations existing
can
system
communication
this
applications well be
(rather
among
in
the
of
fracture
than
as
follows:
trying
various
to
(1)
start
disciplines
often
control
The
established.
stated
are
practices
major
themes
improve anew),
and and
and the many
that
these
in
optimize (2)
are
the
encourage
organizations
and
individuals impacting fracture control.
9.2
Prioritization Because
of
Recommendations
of the large number of recommendations,
all can be implemented at once.
it is clear that not
Therefore, it is desirable
of the relative importance of the recommendations.
to have a ranking
Such a ranking is given in
this subsection, based solely on the opinions of Failure Analysis Associates’ staff,
in view of the survey results.
It is expected that others will have
different opinions. The recommendations naturally fall into two groups: (1) those for which existing technology by designers,
is sufficiently developed so that they can be implemented
builders, and operators
(in some cases via regulations,
or codes), and (2) those directed toward future research. tions,
engineers
interested
in
upgrading
169
4$6-334
DPI
fracture
rules,
With these distinc-
control
on
immediate
projects
would
refer
to
the
first
group,
and
organizations
interested
in
research to advance the state of the art would emphasize the second group in their planning. The
most
important
features
of
recommendations
for use
of
existing
technology
are believed to be (in order) effect on control of fractures, ease
of
probability
(or
implementation. three-point
A
of
successful)
ranking
based
system
is
rating
implementation,
on these
used
for
factors
each
cost
and
is given in Table
feature,
with
higher
of
1.
A
rating
corresponding to greater motivation to implement the recommendation.
In
rating each recommendation
for its effect on fracture control, the
tendency was to give higher
rating to those that draw attention to the need
for fracture
control.
reasoning
ensure
the
that
responsible
control issues. involvement
The
engineers
The midlevel
with
specific
is that the most are
actively
critical focusing
step is to on
fracture
rating was given to those activities of direct fracture
problems,
and
the
lower
rating
to
activities which primarily supply more or better general information.
In
rating
ease
requiring
management
requiring
quantitative
of
decision
implementation,
those
or education were
application
of
existing
midrange, and one requiring a multidisciplinary
recommendations
simply
given highest
rating, those
methods
rated
were
in the
approach was rated difficult.
Cost of implementation was visualized in terms of engineering manpower required
for
a
platform
multidisciplinary Creation viewed
of
teams
permanent
as moderately
over over
its
life
extended
periods
or semi-permanent expensive.
cycle.
small
Activities
Thus, were groups
activities
regarded or
of specific
as
focal
requiring expensive.
points
problem
were
evaluation
were thought to be less expensive. After
each
recommendation
was
assigned
a
rating
for each
desirable
feature, an overall ranking was established to a first order by “point count” and to a second order by giving highest weight to effect on fracture control, second highest to ease of implementation,
170
and lowest weight to cost.
Table Ranking
of
Recommendations
for
1 Use
of
Recommendation
Existing
Technology
Effect on Control of Fractures
Ease of Implementation
cost of Implementation
l-Smal 1 2-Moderate 3-Large
l-Difficult 2-Moderate 3-Easy
l-High 2-Moderate 3-Low
1.
ME1
Make routine and extensive use of simplified analysis procedures, such as the FASD.
3
3
3
2.
FE1
Encourage Fracture Control Integration.
3
3
2
3.
DE6
Survey the entire-structure elements.
3
3
2
4.
FE3
Modify punching shear criteria to prevent other failure modes.
2
3
3
5.
IE2
Combine independent inspection methods.
3
2
2
6.
IE3
Tailor the inspection procedures to the intended application.
2
3
2
7.
FE2
Document the effects of every major structural decision or change upon all fracture control elements.
2
3
2
8.
DE1
Use first-order abounding fracture mechanics models to determine relative effects of toughness and fatigue-design control.
2
2
3
9.
DE2
Supplement S-N curves with deterministic fracture mechanics approaches.
2
2
10.
FE4
Use a calibratedfracture mechanics model to address tough fatigue and fracture problems.
2
2
11.
DE4
Teach engineers to review their thinking on safety factors in order to avoid dangerous design assumptions which may actually reduce the structure’s reliability.
2
2
12
DE5
Reduce the human error factor through education and minor modification of the MMS Platform Verification Program.
2
2
●
for critical
171
486-334
_
-
Table
1 (Cent’d)
Recommendation
Effect on Control of Fractures
Ease of Implementation
cost of Implementation
l-Smal 1 2-Moderate 3-Large
l-Difficult 2-Modeiate 3-Easy
l-High 2-Moderate 3-Low
13.
CE3
Apply the principles of integrated fracture control to the design and execution of repair procedures.
2
2
2
14.
IE4
Make inspection plans site-specific.
2
2
2
15.
IE5
Ensure that divers; ”platform operators, and inspection personnel are adequately trained.
2
2
2
16.
CE2
Determine the condition of the weld root before improving the weld toe performance.
1
2
3
17.
FE5
Apply structural reliability models to optimize design and operational decisions.
3
1
1
18.
ME2 Designate a materials fracture control
1
2
2
specialist for each application for which a correlation between standard specifications and quantitative fracture properties has not been established. 19.
DE3
Model size/shape of crack and transition behavior in brace or leg.
1
2
2
20.
CE1
Establish the level of cathodic protection necessary at all fatigue-critical welded joints.
1
2
2
21.
IE1
Determine the suitability of proof testing as an inspection procedure for joints known to have seen stresses at or near their design envelope.
1
2
2
22.
OE1
Spell out available crack repair schemes in operation , emphasizing their effects on fracture control and safety risk.
1
2
2
172
486”334
Q@
For future research recommendations, the desirable features were deemed to
be the
effect
successful), performing system
the
on control probability
the research.
and the third
of
fractures
of
success
(assuming
of the
the
research
research,
and the
would
be
cost
of
The first two features were rated on a three-point
on the
basis
of estimated
personhours
to perform
the
(Personhours are escalated to account for
research as indicated in Table 2.
additional costs in computer or test intensive research.)
In rating research recommendations for effect on control of fractures, the authors considered the likelihood that the results would be actively used as well as the more obvious consideration of what effect the method would have if used.
An
important
aspect was consideration
of what
is presently
that is, what
increment
over present technology
would the
used;
research provide?
There was also a tendency to give higher rating to research which would likely result in specific improved design or construction also made to rate these control
of
fractures
recommendations
research
only)
in Table 1.
since the correspondence
on
recommendations the
same
scale
features.
An attempt was
(with respect to effect on as
the
existing
technology
No such attempt was made for the other features,
between other features in the two groups is not one-
for-one. Ratings for the probability of success feature were influenced for most items
by
influenced
our
judgement
of
the
by how definitively
could be visualized.
technical
feasibility.
the final work
This
was
in
turn
product of the recommendation
The only low probability of success rating was given to
failure data base development, but because of our knowledge
not because of technical feasibility concerns, of the reluctance of people who have access to
such information to record, collect, and supply the data. even in the nuclear power industry where motivations
This has been true
for such activities are
considerably greater. Our estimates
of personhours
would need to be refined believe
the
opportunities.
estimates The
for
research
are tentative
at best and
if serious intentions to fund a project develop. are
estimates
reasonable generally
for
comparison
represent the smallest
of
173
‘
4$6-334
[f60
research
increment
work that has a reasonable chance of satisfying the recommendation.
We
of
Table Ranking
of
2
Recommendations
for
Recommendation
,!
Future
Research
Effect on Control of Fractures
Probability of Success
l-Smal 1
l-Low
Z-Moderate 3-Large
2-Moderate 3-Hi gh
cost Personhours of Research
1.
CR2
Develop guidelines for the use of available techniques for improving welded joints where fatigue performance is marginal or inadequate.
3
3
5000
2.
IR1
Further develop monitoring systems to be portable, be discriminating, minimize the use of divers, and have an acceptable rate of false alarms.
3
2
6000
3*
CR1
Undertake large-scale component testing the effectiveness and applications of ways to improve weld toe fatigue performance.
3
2
8000
4.
MR2
Collect reliable fracture data to be matched up with offshore needs in order to produce recommended programs for for closing the “data gap.”
2
3
2000
5.
DR15
Develop algorithms for the problem of contained plasticity in notches.
2
3
2000
6.
DR3
Develop deterministic design and fitnessfor-purpose approaches for routine analysis of significance of defects and for development of examples and guidelines.
2
3
3000
7.
DR9
Define stress, gradients and other characteristics of the subsurface stress distribution.
2
3
3000
8.
DR1O
Develop accurate, versatile crack analysis methods.
2
3
3000
9.
MR1
Start a materials data base for all important measured fracture parameters of offshore structural steels.
2
3
4000
4$(5-334
Table2
(Cent’d)
Recommendation
..
Effect on Control of Fractures
Probability of success
l-Smal 1 2-Floderate 3-Large
2-Moderate 3-High
l-Low
cost Personhours of Research
10.
DR2
Develop accurate stress intensity factor solutions for all stress fields and crack geometries of concern. Include evaluation of displacement control exhibited by most In additubular joint crack geometries. tion, develops handbook or software library of stress intensity factor solutions for often occurring geometries and load cases.
2
3
5000
11.
DR22
Employ natural load shedding and displacement control features to help design crack arrest features.
2
3
5000
12.
MR5
Set up a “devil’s advocate” testing program to evaluate fracture criteria.
2
2
1000
13.
DR14
Determine the feasibility of determining approximate effects of stress gradients in estimating elastic-plastic crack tip stress parameters.
2
2
1500
14.
DR1
Develop a data base of service and repair histories.
2
2
2000
15.
DR4
Develop a probabilistic fracture mechanics approach for routine crack analysis.
2
2
2000
16.
DR12
Study methods .for determining hot spot stresses and tubular joint behavior outside the standard case.
2
2
2000
17.
DR16
Develop additional design data for steel “shell” components.
2
2
2000
18.
DR13
Study wave loading and associated phenomena more closely and develop guidelines for application.
2
2
2000
19.
OR1
Consider redundancy more quantitatively in undertaking risk-based remedial actions.
2
2
3000
4$6-334
i
!,
Table
2 (Cent’d)
Recommendation
Effect on Control of Fractures
Probability of success
l-Smal 1 2-Moderate 3-Large
Z-Moderate 3-Hi gh
l-Low
cost Personhours of Research
20.
MR3
Fund quantitative toughness testing using specimens which simulate the physical characteristics at the crack border of the structure.
2
2
4000
21.
DR20
Study effectsof soil-structure interaction and, particularly, long-term foundation degradation.
2
2
4000
22.
DR21
Develop a more accurate finite element program for the global analysis of structures using both joint and member elements.
2
2
4000
23.
FR3
Build improved structural reliability models capable of calibrating to and extrapolating from field experience.
2
2
5000
24.
DR23
Expand the range of tubular joints tested and used to set S-N curves.
2
2
8000
25.
FR1
Survey industries using more formal fracture control documents than does the offshore industry.
1
3
1500
26.
DR19
Evaluate the neglect of larger wave periods in fatigue analyses.
1
3
1500
27.
FR2
Prove that Charpy tests are meaningful for new situations.
1
3
2000
28.
DR6
Define applications suitable for use of higher-order-average alternating-stress methods for life prediction.
1
3
2000
29.
DR18
Determine the accuracy of “identical wave” DAFs in variable wave spectrums.
1
3
2000
30.
MR4
Define and execute an experimental program to evaluate constant amplitude fatigue, overload, and underload effects upon crack growth. Ensure that an adequate range of crack propagation rates is covered.
1
3
6000
I
176
486”334
m
Table
2 (Cent’d) Effect on Control of Fractures
Recommendation
31.
FR4
Build a data base of failure, and success experience for those who influence fracture
32.
DR1l
Consider predicted life of
the effect of wave loadings a structure. the statistical the variability crack growth
33.
DR7
Formalize defining critical
34.
DR8
Estimate origins
35.
DR5
Establish design loads against impact damage sions, etc.)
36.
DR17
Assess stresses
the ~ffect at the weld
effects and
!’i.
,
Personhours of Research 8000
1
2
1000
1
2
1500
crack-site
1
2
1500
protection explo-
1
2
2000
1
2
3000
fatigue
approach to of S-N and subdata.
of uncertain hot spot incorporate into a PFM model.
,’
l-Low Z-Moderate 3-Hi gh 1
greater-thanupon the
for and (collisions,
l-smal 1 Z-Moderate 3-Large
cost
2
accident, feedback to control.
of mltiple toe.
Probability of Success
In combining the three features to obtain an overall ranking, cost was used only as a second order consideration.
This is justified
on the basis
that none of the costs are significant compared to the enormous potential cost avoidances
associated
avoidances
include
repairs,
and
with
a
successful
reduction
failsafe
completion.
in the
component
number
failures,
These
of premature
as
well
as
potential service
the
of fractures
was
given
slightly
more weight
cracks,
possibility
reducing the already low rate of more serious platform failure modes. on control
cost
of
Effect
than probability
of
success, partly for the same reasons.
In
Table
according
3,
the
to their
recommendations
subject
order of decreasing
matter.
cost.
The
fracture
(18%).
If
eleven
research
categories
the
and component testing
recommended
programs
control or the lowest probability
number of programs
future
with
the
lowest
and
fracture
intended
to
mechanics.
is different
control provide
effect
on
of success are deleted, then the and the cost The result is
As in Table 3, a majority of the total cost is expended
and fracture
terms of cost,
in
(28%) and fracture
is reduced from 36 to 24 (a 33% reduction)
presented in Table 4.
listed
are presented
decreases from 118,500 hours to 86,500 hours (a 27% reduction).
in testing
are
As shown in the table, almost half of the program
costs are in the areas of materials mechanics
for
the order of the programs,
from that of Table 3 and two categories,
systems, the
However,
do
reader
n&
appear
with
a
at
better
programs in terms of their application and cost.
178
d86-334
/90 O
all. picture
Tables of
the
3
in
impact
and 4
are
recommended
Table Reconnendations
Subject
by Subject
Recommendation
Material & Component Testing
Fracture Mechanics
3
‘-
CR1 MR2 MR3 MR5 DR16 DR23 1FR2 1NR4
Rankingz 3-2 2-3 2-2 2-2
Cost-1000 hrs 8.0 2.0 4.0 1.0 2.0 8.0 2.0 6.0
::; 1-3 1-3
33.0 (28%) DR2 DR3 DR1O DR15 DR4 DR14 1DR8 1DR17
5.0 3.0 3.0 2.0 2.0 1.5 ::: 21.0 (18%)
Data Base Enhancement
Fatigue
Stress Analysis
MR1 DR1 1FR4
2-3 2-2 2-1
4.0 2.0 8.0 14.0 (12%)
DR13 lDR7 lDR1l 1DR6 1DR18 1DR19
2-2 1-2 1-2 1-3 1-3 1-3
2.0 1.5 1.0 2.0 2.0 1.5 10.0
DR9 DR22 DR12 DR21
(9%)
3.0 5.0 2.0 4.0
::; 2-2 2-2
14.0 (12%) 1 Recommendations having either probability of success.
a low effect
on fracture
z Numbers refer to effect on fracture control--probability
control
or a low
of success.
Table
Subject
3 (Continued)
Recommendation 1
Risk Analysis & Reliability
OR1 FR3
Nondestructive Examination
3-2
Foundations
DR20
3 Cost of all recommendations percentage of the total cost.
in each
subject
expressed
(4%)
4.0
(3%)
2.0
(2%)
1.5
(1%)3
118.5(100%)
on fracture
z Numbers refer to effect on fracture control--probability
5.0
1.5
1-3
a low effect
(5%)
2.0
TOTAL
1 Recommendations having either probability of success.
6.0
4.0
1-2
lFR1
Fracture Control Systems
(7%)
5.0
2-2
1DR5
Impact
8.0 6.0
3-3
CR2
Cost-1000 hrs
3.0 5.0
2-2 2-2
IR1
Welding Improvement
Rankingz
control
or a low
of success. in hours and as a
Table Most
Significant
Subject
Material & Component Testing
4
Recommendations
By Subject
Recommendations
Rankingl
cost
CR1 MR2 MR3 MR5 DR16 DR23
3-2 2-3 2-2 2-2 2-2 2-2
DR2 DR3 DR1O DR15 DR4 DR14
2-3 2-3 2-3 ;:; 2-2
DR9 DR22 DR12 DR21
2-3 2-3 2-2 2-2
4.0
Risk Analysis & Reliability
OR1 FR3
2-2 2-2
3.0 5.0
Nondestructive Examination
IR1
3-2
6.0
Data Base Enhancement
MR1 DR1
Welding Improvement
CR2
3-3
5.0
Foundations
DR20
2-2
4.0
Fatigue
DR13
2-2
2.0
8.0 2.0 4.0 ;:: 8.0 25.0 (29%)
Fracture Mechanics
5.0 ;:: 2.0 2.0
1.5 16.5 (19%)
Stress Analysis
3.0 5.0 2.0 14.0 (16%)
1 Numbers refer to effect on fracture control--probability in each
(9%)
6.0
(7%)
6.0
(7%)
5.0
(6%)
4.0
(5%)
2.0
(2%)
4.0 2.0
TOTAL
z Cost of all recommendations percentage of the total cost.
8.0
subject expressed
86.5(100%)
of success. in hours
and as a
10.0
REFERENCES
1.
Pellini, W.S., Criteria for Fracture Control Plans, Naval Laboratory, NRL Report 7405, Washington, D.C., May 11, 1972.
2.
Marshall. P.W.. “A Review of American Criteria for Tubular Structures 1978. and Proposed Revisions,” IIW DOC. XV-405-77,
3.
Marshall, Peter W., “Fracture Control and Reliability for Welded Tubular Structures in the Ocean,” Shell Oil Company, presented at the Ninth NATCAM, Ithica, New York, June 1982.
4.
Haagensen, P.J., “Improving the Fatigue Performance of Welded Offshore Welded Structures Conference, Trondheim, Norway, 1982.
5*
Carlsen, C.A., T. Kvalstad, H. Moseby, E.M.Q. R~ren, T. Wiik, “Lessons Learned from Failure and Damage of Offshore Structures,” Eighth International Ship Structures Congress, Gdansk, 1982.
6.
Fisher, P.J., “Summary of Current Design and Fatigue Correlation,” Fatigue in Offshore Structures, Institute of Civil Engineers, Conference Proceedings, London, February 24-25, 1981.
7.
Marshall, P.W., “Fixed-Bottom, Pile-Supported, Steel Offshore Platforms,” ASCE Convention, Task Committee on Sea-Based Structures, Atlanta, October 1979.
8.
“Revised Fatigue Design Gurney, T.R., revision of UK DOE Guidance, January 1983.
9.
Hulbert, L.E., “Three-Dimensional Fracture Research Report, November 30, 1976.
10.
Rules,”
Metal
Analysis,”
Research
--
Joints,”
Construction,
Battelle-Columbus
Engesvik, Knut, M., ~ Technology Marine Norwegian Institute Welded Joints, Center, Technology, Report No. UR-82-17, Trondheim, Norway, December 1981.
of
11.
National Research Council, “Inspection of Offshore Oil and Gas Platforms and Risers,” Marine Broad, Assembly of Engineering, Washington, D.C., 1979.
12.
National Research Council, “Safety and Assembly of Engineering, Washington, D.C.,
Offshore
Oil,”
Marine
Board,
1981.
13.
Marshall, P.W., “Experience-Based, Fitness-for-Purpose Ultrasonic Reject Criteria for. Tubular Structures,” American Welding Society -- Welding Institute Conference, Atlanta, May 1982.
14.
Carter, R.M., P.W. Marshall, P.D. Thomas, and T.M. Swanson, Problems in Offshore Platforms,” OTC paper 1043, 1969.
182
+86-334
“Materials
15.
Failure Analysis Associates, “BIGIF -- Fracture Mechanics Code for Structures, ” Electric Power Research Institute, EPRI NP-1830, April 1981.
16.
Rolfe, S.T. and J.M. Prentice Hall, Inc.,
Barsom, Fracture and Fatigue Control in Structures,
New Jersey,
1977.
17.
Telephone interview conducted with Robert G. Bea and Ashok Vaish, PMB San Francisco, California, on September 16 and Systems Engineering, Inc., 23, 1982.
18.
and S.T. Rolfe, Fracture Control Practices for McHenry, H.I. Structures, National Bureau of Standards, IR79-1623, January 1980.
19.
Telephone Interview with Peter Marshall, Shell Oil Co., Houston, Texas, on August 17, 1982.
486-334
Metal
IV.
CONCLUSIONS
AND GENERAL RECOMMENDATIONS
A survey of experts and the open literature was conducted to determine the current essential
fracture
control
practices
for fixed offshore
elemen,ts of fracture control were identified operation,
construction,
and inspection.
structures.
as materials,
The
design,
Each of these areas was summarized
and evaluated to identify areas where use of existing technology would suggest cost-effective
improvements
and
to
identify
promising
areas
of
technical
research. This used other best
survey
revealed
that the more advanced
in the United States high-technology methods
used
offshore
industries
considerably
codes used in several other best methods they
should
industry
such
closely
surpasses
industries
Thus,
control methods
parallel
those used by
Sophistication
as aerospace. analysis
requirements
(e.g., nuclear power).
do not appear to be as uniformly be.
fracture
of
of the typical
However, the
applied and/or documented
it is recommended that emphasis
as
for the foreseeable
future be placed first on more widespread and uniform use of the best existing methods and second on steady improvements could be accomplished
in the methods.
voluntarily under leadership of the platform operators.
revealed that typical ly, designers
(The survey
The first emphasis
operators for guidance.)
and constructors
look to the
The second emphasis should be addressed by research
organizations. An
operator
who
wishes
to
take
positive
steps
toward
more
uniform
fracture control should do so by using the fracture control checklist provided in
Appendix
B
and
documenting
efforts
explicitly
pointed
control in a format such as recommended in Appendix A. tation,
used
technology
in
conjunction
in Section
III
of
with
this
the
forefront of fracture control in the industry.
fracture
This list and documen-
recommendations
report, would
toward
place
for an
use
of
operator
existing at the
Since instantaneous changes in
several areas at once are not usually feasible, the operator should make use of the priority ranking provided in Table 1.
184
4$6”334
It is believed that the economic
benefit
gained in
mentation.
The
problem
avoidance
particular
would
gain would
greatly
depend
exceed
the
on how much
cost
of
this
imple-
recommended
approach differs from the operator’s present approach. A need for substantial
future research at
to $12 million was identified. about
three
years)
Unless sources for this order of funding (over
are already
identifying such sources.
a minimum cost of about $8-
available,
the first
step should
be toward
Although not specifically addressed in this report,
it is believed that this research cost is quite small compared to the economic benefits
that would
problems.
Our
accrue
basically
from avoidance
positive
of cracking,
assessment
repair,
throughout
this
state of the art of fracture control of offshore structures to fracture
control practices
that cost-effective
here.
of
other
that technology
industries
Most of the
problems
report of the
(e.g., as compared
certainly
does not imply
improvements are not obtainable.
We do not believe support
in other industries)
and failure
will
greatly
recommendations
encountered
in
offshore
transfer
from ongoing
influence
the
research
needs
in
identified
are too closely associated with specific structures
to
gain
other
than
a
general
benefit from other research. To the
extent
used to establish
that
full funds cannot
research priorities.
list numerically,
but more certain
from
and
construction
strong contenders
inspection
be obtained, Table
Design
recommendations
and effective advances.
for high priority
funding.
Some
immediate materials
2 should be dominate
the
payoffs may come tasks
are
also
In the spirit of an integrated
fracture control improvement, a balanced approach is recommended, where one or more tasks are selected from at least the design, materials, construction, and inspection fracture control elements.
185
486-334
Appendix FRACTURE
CONTROL:
A
STATEMENT
OF EFFECTS
The following exemplifies a recommended compromise between the development and use of a comprehensive
fracture control plan “Bible” and no fracture
control
plan
all.
example
has been stated qualitatively;
documentation
at
For
Description
of
form which
illustration,
the
as will be indicated, in an
information could be included.
data for a frontier location indicates a more severe
than anticipated
in the design of a fixed offshore plat-
is already in an advanced
phase of fabrication.
audit reveals that with the important exception tubular-joint
of
Problem
New oceanographic fatigue environment
purpose
although,
actual application extensive quantitative
A.1
the
A careful design
of eight nominally
identical
brace-can intersection hot spot weld-toe locations, the original
design will be adequate for the harsher loading/environmental fatigue
analysis
of the eight hot spot locations
ginally
unacceptable
due to the
greater
fatigue
conditions.
A
reveals that they are mardamage
associated
with
the
more severe wave load spectrum. After careful consideration in
some
cases
have
measure),
it has
increase
adequately
tion.
included
been
a fracture
decided
the fatigue
(Assume that the weld
of several alternative measures
to
control
grind the
performance
effect
critical
(which may
statement
weld-toe
for
each
locations
to
of the eight hot spots in ques-
root performance
is al ready adequate. )
As part
of the effort to both select and justify this fatigue performance improvement method, the following il lustration of a fracture control plan statement might have been offered.
A.2
Primary Both
Impact
on Fracture
Control
relevant S-N literature
fatigue analysis
of
Held-Toe
Grinding
Improvement
data and a fracture mechanics-based
reveal that weld-toe
weld
grinding will improve the fatigue life
under a given spectrum by at least a factor of three.
A-1
(The most recent U.K.
DOE
fracture
design
rules
permit
the
equivalent
assumption
factor of 2.2 life increase without a supporting analysis.) is accomplished by reduction of the weld-toe tration
and
grinding
inherent
should
not
crack-like be taken
at
least
a
This improvement
(notch) geometry’s stress concen-
defects.
without
of
Life
cathodic
improvement
protection
credit
or the
from
corrosive
action may pit the surface and negate most of the benefits of grinding.
This
improvement has been calculated to be adequate to handle the more hostile wave loading
spectrum
associated
tainties
and/or
controversy
with the new platform engendered
location.
(If
by the above analysis
the uncer-
and literature
search were large , a structural simulation experiment, with and without weldtoe grinding could be performed to “prove” the fix, if time permitted.) This primary effect on fracture control is nothing more than the reason for the design impact
of
followed
change.
this
What
change
that
in some form.
The
follows good
is a more comprehensive
engineering
point
practice
of our illustration
survey
dictates
of the
should
be
is to show how this
“common-sense, good engineering practice” could be documented in the form of a fracture control effect statement.
A.3
Secondary Control
Influences Plan
Material: microstructure grinding
The and
of
last
hardest
operation.
properties,
welding weld
the
Grinding
pass,
metal,
Furthermore,
decrease due to grinding. material
Held-Toe
the
on an Integrated
which
will
be
effective
contains
Fracture
the
removed
by
initial
most the
crack
suspect
specified
size
should
With no other significant impact of grinding upon secondary
effect
of
this
change
on
the
material
element of fracture control can only be positive. The primary design effect of reducing the surface and relevant
m:
stress concentrations 111:4.2.
at the hot spot have already been addressed in Section
There are, however, several secondary effects on local geometry and
stress fields, and hence upon design, which might be detrimental. A worst-case tolerance analysis of the new weld profile indicates that the brace and can cross-sections Specifically,
it
is determined
themselves could be reduced by the grinding. that
the
local thickness
A-2
~86-334
~~(j D
of the
brace wall
could be reduced by 5% and of the can, 3%.
Rather than performing a detailed
finite element analysis to model this local thickness reduction, the brace and can wall bending and membrane
stresses
have been multiplied
by the following
tabulated factors.
Factors for Uorst-Case Of Section Properties
Reduction By Grinding
“Nominal” Maximum Principal Stress Component (perpendicular to Weld-Toe Defect)
Increase in Finite Element Stress Closest to Hot Spot Brace Can
Membrane [(l:i::!)-1]
[(1-:!0?+]
[(1-;!}:) -2]
[(1-;:;:) -2]
Bending
Assume for this example that these factors have been incorporated into and are discounted overall
within
the
primary
factor-of-three
secondary
impact
surface
to
mentioned
before which
revealed
of fatigue life performance.
with this
local thickness
reduction
the
Thus, the has already
bounded within the fatigue analysis.
Three geometrical considered.
improvements
associated
been conservatively
analysis
effects of grinding upon the weld profile have been
First, grind depth should be at least 40-mils below the original be
reasonably
sure
of
removing
weld-toe
fabrication
defects.
Second, it is standard practice that the grinding direction should be parallel to the maximum principle stress direction. to
can
rather than
grinding
would
not
fatigue cracking.
around be
the
(That is, most likely, from brace
brace.)
Thus,
in
direction
oriented
a
The third geometrical
any crack-like likely
to
scores due to
cause
premature
feature that has been accounted for
as a secondary
impact upon fracture control is the possibility of misblended
radius
a
due
to
difficult
weld
geometry
or poor workmanship.
(feasibility studies could be another possibility) inspection
Experience
has shown that with proper
(see below) and use of qualified technicians for the grinding oper-
A-3
486-334
a Ot
ation,
the worst
cas~
deviations
from
an ideally
cause nearly as much fatigue performance deep
crack.
fatigue
performance
worst-case words,
Since
weld-toe
grinding
such an initial of well-made
ground
surface would
not
reduction as introduction of a 5-roil
crack
depth
weld-toes
corresponds
to the
average
(ground or not) in general, this
profile error should have a tolerable
In other
impact.
can be used to help ensure that the welds behave as if they
had a 5-roil (rather than a much deeper) crack. The local residual stresses due to local weld shrinkage have been shown to be maximum tension grinding
fix
analytical
can
in the last one or two weld passes.
only
credit
improve
the
has been taken
plans to actually verify through
state
of
for such
residual
Thus, the subject
stress.
improvement
However,
because there
residual stress measurement
no
are no
1) that a stress
decrease has taken place due to grinding, and 2) that the maximum compressive stress excursions not
expected during launch and operation of the platform would
reintroduce
fatigue
yield-level
performance
due to grinding,
tensile
improvement
no specific
residual
might
arise
quantitative
stresses.
from
Thus,
while
residual stress
estimate
some
reduction,
or credit has been taken
for this potential positive effect upon fracture control. Construction: the
weld
profile
Other than the original feasibility
grinding
could
be
achieved
(given
study to show that
qualified
workmen
and
inspectors) as a primary effect, the grinding of the eight weld-toe locations has no known effect upon the fabrication of the as-built platform. The
Inspection:
weld
profile
has
been chosen to provide
clean surface which will only improve the crack-detection
a smooth,
and sizing inspec-
tions and measurement
of the profile itself to ensure that the ground geometry
is within tolerance.
Therefore, it has been decided to prepare the jacket for
load out
grinding
should
before the
apply
a
net
tension
operation. in the
hot
The
resulting
loads on the joints
spot
locations
and
allow
any
pre-
existing large weld-toe cracks to open up and reveal themselves as material is ground
away.
A
qualified
inspector
has
been
assigned
to
work
with
grinders to attempt to detect and size such cracks that might be revealed. repair weld/grinding
the A
procedure has been devised in case such cracks are found
beyond a certain fracture mechanics-based
A-4
maximum size.
Operation:
Beyond
the primary
aspect
of the
increased wave
loading
spectrum already discounted in the initial fatigue analysis, a special inspection schedule for these eight locations should be used to verify the benefits of the grinding.
Appendix FRACTURE
B
CONTROL CHECKLIST
Introduction
B.1
In order to execute a fracture control check and “semiformal” documentation, that
as outlined
fracture
list those
in Appendix
A, some guidelines
control specialists
items to emphasize
should be available
such
can select from a relatively comprehensive
in their fracture control checks.
An initial
version of such a checklist is outlined below:
B.2
Initial
Checklist
B.2.1
General Policv
Fracture
for
Control
Execution
A statement of policy should be made to minimize accidents due to fracture in the structure under study.
Then a more focused and specific statement
of policy could be made for the specific area under study.
B.2.2
Objective State
the
responsibilities,
criteria,
and procedures
to be used
for
fracture control project under study.
B.2.3
Scone
State
which
platforms
and
substructures
covered with the focused fracture control plan.
and
activities
are
to
be
State which activities are to
be emphasized among design, analysis, testing, material selection and control, fabrication,
nondestructive
evaluation,
operations,
maintenance
and inservice
inspection.
State whether the plan is for information only or is an internal
guideline or code.
B-l ,.-
4$6-334
~
B.2.4
Prerequisites
It
is
and Assumptions
often very difficult to define where basic engineering
practice
stops and where specialized fracture control calculations and checking begin. Any written
fracture
control document
should clearly delineate
prerequisites
and assumptions based on information outside the scope of that document. example,
a
rather
broad
set
of
prerequisites
and assumptions
might
For
be as
follows:
Assume that basic materials
●
and
tion,
comprehensive
data, loads and environmental
structural
analysis
are
defini-
available
as
a
matter of good engineering practice.
Assume
m
a comprehensive
test program to verify unique designs
and
conditions.
B.2.5
Organization and Responsibilities Each organization
under
study
specialist powers)
builder,
(designer, or committee
within
which
involved
in the specific offshore operator)
(possibly with are
could
advisory
represented
at
least
appoint
structural a fracture
detail control
rather than decision-making the
following
engineering
disciplines:
●
Structural design
*
Structural analysis
●
Materials engineering
●
Maintenance
●
Operation
●
Inspection
If ating
appropriate, each fracture control committee would develop an oper-
plan
(could be
informal written
document)
which
defines
the
specific
responsibilities of each specialist.
B-2
486-334 .—
0
206
.,..,
B.2.6
Fracture Critical Parts Selection Criteria A systematic
activities control
criteria
should be devised to determine which
are fracture critical and therefore
measures.
Certain
parts and
subject to stringent fracture
parts could be automatically
designated
fracture
critical parts.
6.3
Specific
Requirements,
B.3.1
Sources of Crack, Defect and Damage Initiation Cracks,
crack-like
crevice corrosion ion,
transport,
structure.
Procedures
defects,
and Documentation
or
other
damaging
imperfections
such
as
sources or impact dents, may initiate during the constructinstallation,
The following
operation,
defect types
or dismantling
of a fixed offshore
and causes should be considered
in a
fracture control effort.
●
Defect in the original material.
These include steel plate porous
regions,
or lamination.
nonmetallic
most significant
inclusions,
The largest and
of these defects should be detected and rejected
or repaired before the plate is used.
9
Cracks initiating from welding:
- Large crack-like weldability of technique
defects in the the materials
weld due or poor
to poor welding
- Cracks caused by overrestraint in the welds and associated high residual stresses (e.g., through improper pre-heat and other welding procedures) - Lamellar tearing near welded joints
●
Overload of a joint
●
Fatigue cracks
B-3
●
Impact
●
Corrosion
dents,
slices
B.3.2
design
cracks into
Material Assure
gouges,
the
Selection that
tears,
or
or complete
pits,
weld
or
or
crevice
other
joint
separations
corrosion
structural
(severe
knife-edge
detail).
and Specification
the
material
will
behave
at
least
as well
in
as assumed
calculations.
Limit
material
defects
Avoid
materials
too
fracture
Produce
at the
which
might
susceptible
service
near-optimum
initiate
to
subcritical
temperature
tradeoff
fracture.
of the
cracking
and brittle
structure.
among material
strength,
toughness
and cost.
Depending Section
upon the B.2.4,
properties
defined
scope
Prerequisites
as yield
of
the
fracture
and Assumptions),
strength,
ultimate
control assure
strength,
plan
(see
such general
ductility,
and
lamellar tearing resistance are up to defined standards.
Set
tolerance
levels
for
porosity,
inclusions,
laminations,
and
other rolled-plate defects.
Specify those aspects of material process that help meet the above tolerance levels.
Control
carbon-equivalence
and
any
other
important
chemical
composition properties to assure weldability.
Specify materials and processes for fracture-re” ated properties.
B-4
4$6-334
~ofi 0
●
-
Obtain adequate fracture toughness, crack arrest capacity, dynamic load resistance, crack growth rate resistance, threshold stress factor, intensity stress corrosion and environmental ly-accelerated fatigue susceptibility, effects of temperature and other environmental considerations.
-
Define, as far as possible, effects of processes, size effects (e.g., thickness), grain orientation, and geometric configuration.
Guard against such corrosion-related
failure modes as stress corro-
sion cracking, environmental ly-accelerated fatigue or fracture, and bulk corrosion thickness loss through a combination of paint, other coatings
such
as
metallic
wrap,
and,
most
likely,
cathodic
protection.
B.3.3
Design While producing a near-optimum functional and economic structure, guard
against the five failure modes:
fracture; subcritical crack growth caused by
a combination of cyclic and steady loads and environment; yielding,
including
excessive
plastic deformation modes such as ultimate failure or plastic-hinge
bending;
elastic
or
inelastic
buckling
or
other
instabilities
compression loads; and bulk corrosion leading to loss of cross-section.
●
●
General Design: -
Minimize stress concentrations.
-
Provide access for inspection and maintenance
-
Select stress levels so that life can be verified by analysis combined with nondestructive inspection.
-
Clearly identify fracture critical parts on drawings as appropriate.
Fatigue and Fracture Mechanics Analysis:
-
Account for impact of possible initial flaws on all relevant failure models.
B-5
A86-~3A
~O-. m
under
o
-
Perform nondestructive inspection proof test to screen flaws.
-
Evaluate residual stress effects.
Loading.
or, less
likely,
Account for the many sources of loading that might become
important to the five failure modes listed above.
●
These include:
-
Wave loads. Determination of wave loads includes specification and prediction of maximum wave heights and associated frequencies, characterization of wave spectra as functions of location and water depth, efforts analysis prediction statistical and necessary to specify and justify chosen parameters for wave sizes and spectra, proper computation and including forces, hydrodynamic application of vortices, treatment of wave spectra, and nonlinear combinations of wave and current.
-
Wind loads (involving proper computation of aerodynamic forces and vortices). Work is similar to that involved in specifying waves in that extreme oncein-a-lifetime winds and hurricane models must be specified.
-
Tides and currents
-
Submarine mudslides
-
Marine fouling
-
Where applicable, ice
-
Earthquake loads
-
during forces Installation launch, and dismantling
-
Operational loads associated with platform equipment supplies and lifting for and used drilling personnel.
loadout,
transport,
Design Analysis:
-
Perform standard space-frame analysis jacket and associated structure.
B-6
of
steel
●
-
Perform quasi static and dynamic analysis for design load cases.
-
Perform local analysis of tubular joints using an appropriate combination of stress analysis tools such as finite and shell theory, element experimental results, and regression (equational or nomographical) fits to analytical or experimental results.
Fatigue above;
Analysis. analysis
Including
of most
load
critical
spectrum
structural
analysis details
described
from global,
local, and crack propagation viewpoint; proper cycle counting using both deterministic
and spectral models; and appropriate cumulative
damage rule.
●
o
Standard design calculations:
-
Check for local and global buckling of tubular members including the effects of inelastic behavior and hydrostatic collapse. Check for static strength of tubular joints (1) using proven punching shear formulas appropriate for simpler with corrections classes of joints, and (2) designing appropriate experiments for more complex joints.
-
Compare fatigue analysis results described above with known experimental results in the fatigue of If appropriate, introduce fracture tubular joints. mechanics model. calibrated aaainst all known data. for extrapolation to differe~t section thicknesses and other situations.
Complex failure modes:
-
Investigate combined or common failure modes such as caused by progressive degradation due to corrosion and fatigue.
-
Investigate the redundancy role of fail-safe including appropriate dynamic effects, such as dynamic load amplification and dynamic effects on resistance (e.g., the structure might be fail-safe against static failure modes of a given member but not against sudden dynamic fracture).
B-7
4g6”334 \
\
~, 63
●
Pilings
and Foundation.
significant
tensile
Standard
design
load cases
loads in piling structures
often
so that foundation Some of
structures should be considered when controlling fracture. the more include analysis
important
fracture
determination
control
of axial
of the structure,
slides and earthquakes,
aspects
and shear
displacement
maximum
of foundation
loads
control
bending
cause
design
from space
frame
oads due to mud-
combined with compression
stresses at piling we ded details, and
to produce maximum tensile
for long slender pilings such nonlinear loading effects as the “PA“ effect,
and effects of overloads.
If deterministic design analyses, such as those in the checklist above, are inconclusive, consider a probabilistic approach to either perform a structural optimization or to more realistically model a “worst-case scenario”.
B.3.4
Construction
●
Understand and control the effects on fracture of processes such as material
removal,
forming,
joining
(especially
through
welding),
thermal treatment, and chemical cleaning.
●
Meet
specified
structural
assume worst-case
dimensions
effects
the control of fracture.
and
of variations
material
properties
from specifications
or
upon
Minimize residual stresses, especially in
welding.
*
Maintain tolerances and surface finishes.
o
Ensure qualifications
m
Ensure traceability
of personnel involved in fabrication.
of materials, personnel, and procedures used to
fabricate fracture-critical
m
Perform state-of-the-art
parts.
fitness-for-purpose
fabrication defects.
B-8
evaluations of unusual
●
Use
inspection
liberally
in
locating
weld
defects
and,
during
repair, calibrate the inspection by measuring the depths of defects repaired.
●
Learn from the occurrence
of weld and other defects.
Attempt to
determine the root cause of such defects and avoid the same during repair welding.
Inspect other joints which may have been subject
to the same root cause.
●
See
that
welds
merge
smoothly
with
the
adjoining
base
material
without excessive undercut.
●
Consider
the
advantages
and
disadvantages
of
use
of
extensive
prefabrication processes such as the nodal method of construction.
●
Concentrate
inspection upon those joints known to have been exposed
to the largest fabrication-induced
B.3.5
loads and residual stresses.
Operation and Inspection
●
Generally
protect
the
structure
from
damage
either
due to
such
accidents
as boat collisions or due to normal operating conditions
(e.g., corrosion).
●
Under
damage
concentrate $eri ous evaluated
conditions upon
of the
preserving
imperfections
must
structure,
the be
fracture
integrity found
and
of
control
the
their
must
structure. significance
before they are dangerous, and if necessary repairs must
be made.
●
Under nondestructive stand sensitivity
evaluation
practices, perform tests to under-
and reliability
of possible techniques.
Select
the most appropriate technique or, much more likely, combination of NDE techniques.
B-9
●
●
●
Review quality and report nonconformance.
-
Report incidence and characteristics
-
Report departures from prescribed properties and dimensions (presumably occurring during operation).
-
Record any actions.
failures,
their
causes
of defects.
and
corrective
Inspect structures periodically.
-
Define inspection scope, frequency, and intensity on basis of knowledge of design and operation. Consider stress levels, NDE capabilities, and level of fail-safe redundancy.
-
Require formal item-by-item checklist proper execution of inspection.
to
confirm
When combining inspection procedures, try to choose techniques that complement
each
other.
For
example,
combine a needed “fine-toothed-comb” a structural the
“general
Consider upon
monitoring health”
timing
of
an
scenarios
of
be
appropriate
to
crack detection technique with
capable of continuous
important
of inspections.
worst-case
growth.
technique
it may
section
of
review of
the
structure.
For example, use intervals based stress
corrosion
or
fatigue
Base certain inspections upon event-related
crack
phenomenon to
take advantage of field experience feedback.
●
Prior to actual damage, damage
could
decisions of
under
damage
order
of
members,
be
plan actions to repair whatever
found.
This
will
surprising
operational
should
considered,
that
importance,
be are
(1)
avoid
the
events. in
ruptured,
need
types of for
hasty
Among those types
approximate buckled,
decreasing and
missing
(2) large cracks anywhere in the structure but especially
at welded joints, cross-sectional
(3) severe bulk corrosion accompanied thickness,
B-10
(4) accelerated
local
by
10SS
Of
corrosion,
‘---—.
-..
especially affected
in the zones,
form
of
crevices
(5) dents,
or pits near welds
punctures,
and
abrasions,
or heat(6) small
cracks, and (7) corrosion in non-critical members.
●
Safety-risk
should be the prime consideration
in choosing a repair
procedure for operational abnormalities or unusual events.
The do-
nothing option should always be considered in order to balance the risks associated
with
various
repair schemes and to consider the
role of fail-safe redundancy and alternative
safety measures, such
as evacuation.
●
Assure a strong feedback loop to designers and other fracture controllers
in order to take
full advantage
of the
lessons
learned
from operational experience.
..,.,
B-n
/
