Feb 28, 2008 ... recognised codes and standards include API 579 and R6 [3,4], .... and for each
class there is a different S-N curve, e.g. in PD 5500, the class E ...
OPT 2008 Amsterdam, The Netherlands, 27-28 February 2008
ECAs: Are they fit-for-purpose? ANDREW COSHAM † Atkins Boreas, UK
ABSTRACT Engineering critical assessments (ECAs) have increasingly become a routine part of pipeline design to determine tolerable flaw sizes for weld defects. These assessments are now being applied to pipeline systems in deeper water with increased loadings arising from responses to thermal and pressure cycling. Often these are flowline systems in which fatigue damage is exacerbated by the presence of aggressive internal conditions. In these situations, ECAs can give 'alarming' results, indicating that only very small flaws would be acceptable. In some cases, applying the same methodology to in-service pipelines would suggest that the pipeline should have failed a long time ago, whereas in reality they have not. Therefore, a number of questions arise: are ECAs too conservative; are there situations where ECAs may be non-conservative; and do we fully understand what we are doing? In this paper, these issues are illustrated by means of several examples and an attempt is made to partly answer the above questions.
1
INTRODUCTION
Pipeline systems are being designed to operate in deep waters at high temperatures and high pressures, in aggressive internal environments. Design issues for such pipeline systems tend to arise in the flowlines rather than the risers or ‘platforms’. Fatigue can be a significant design constraint. The fatigue design of risers is typically governed by ‘hot spots’ at the top and bottom of the riser due to loads arising from wind, wave and current loading. Wave loading is typically 7 8 predictable and of a low frequency, with of the order of 10 or 10 cycles over the design life of the system. Current loading is more unpredictable. Factors of safety of five for flowlines and ten for risers are typically applied in fatigue design. Deep water flowlines operating at high temperatures and pressures need to be designed to accommodate issues such as significant end expansion, walking and lateral buckling. Insulation or direct electrical heating may be required for flow assurance. Shut-downs result in significant pressure and temperature cycles. The fatigue loading is characterised by a small number of large cycles, less than 10 3 cycles over the design life. The fatigue loading is in a completely different regime to that in a riser , giving rise to a different set of design challenges. The fatigue loading is also, in principle, under the direct control of the pipeline operator because it is driven by variations in pressure and temperature, unlike environmental loading. Factors of safety in fatigue design, and the associated ECAs, tend to be lower, in part because there is perceived to be a higher level of confidence in the fatigue loading, but also because higher factors of safety cannot be accommodated. The fatigue design is
†
Atkins Boreas, Churchill House, 12 Mosley Street, Newcastle upon Tyne, NE1 1DE, UK. Tel: +44 (0)191 230 8098, Fax: +44 (0)191 261 0200, e-mail:
[email protected]
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complicated by issues such as: corrosion-fatigue, frequency and strain rate, in addition to the effect of the internal environment on toughness. Fatigue design is normally based on S-N curves (e.g. the class D and E design curves). The ECA is a secondary consideration. S-N curves are only appropriate if the weld is free from significant defects. An ECA is conducted to determine the tolerable flaw size, i.e. the size of a significant defect. With the application of ECAs to pipeline systems in deeper water subject to high thermal and pressure cycling and aggressive internal environments, it is important to understand if ECAs can be overly conservative, or perhaps even non-conservative It is therefore informative to examine a number of the issues surrounding ECAs, and their relationship with S-N curves, and pipeline welding codes and standards. Although spoken in a different context, the words of former secretary of defense are relevant: “As we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns -- the ones we don't know we don't know.” [1].
2
ECAs IN CODES AND STANDARDS
An engineering critical assessment (ECA) is a method for assessing the acceptability of a flaw in a structure, i.e. to demonstrate fitness-for-purpose. Fitness-for-purpose is defined in BS 7910 : 2005 [2] as follows: “By this principle, a particular fabrication is considered to be adequate for its purpose, provided the conditions to cause failure are not reached.” BS 7910 describes in detail how to conduct an engineering critical assessment. Other widely recognised codes and standards include API 579 and R6 [3,4], although these are less commonly used in the pipeline industry. These generic codes and standards are supplemented by additional guidance in pipeline design codes and standards. The introduction to BS 7910 makes a number of statements regarding ECAs that are both informative and sometimes not remembered. “… a proliferation of flaws, even if shown to be acceptable by an ECA, is regarded as indicating that quality is in need of improvement. The use of an ECA can in no circumstances be viewed as an alternative to good workmanship. … [ECAs] are complementary to, and not a replacement for, good quality workmanship.” Appendix A of DNV-OS-F101 2007 [5] gives additional guidance on conducting ECAs of girth welds in offshore pipelines, intended to supplement that given in BS 7910. DNV-RP-F108 [6] give specific guidance for installation methods that introduce cyclic plastic strain, i.e. reeling. The pipeline welding codes BS 4515 and API 1104 also give relevant guidance [7,8]. In addition, although developed for onshore pipelines, the EPRG guidelines for defects in transmission pipeline girth welds are informative [9]. It is often instructive to compare the results of ECAs with pipeline specific guidance. ECAs are based on the application of the science of fracture mechanics. The conventional, applied mechanics, approach to design is to consider the loads acting on the structure and the tensile properties (yield strength, tensile strength, etc.) of the material. This approach is not valid if the structure contains stress concentrations (e.g. notches) or defects (e.g. cracks). Fracture mechanics considers the effect of a defect. In addition to the loads and the tensile properties, fracture mechanics considers the toughness of the material and the defect geometry. Fracture mechanics is a relatively mature discipline, but there are new developments, through joint industry projects, project-specific studies and academic research, and these are gradually being incorporated into codes and standards. Issues such as constraint, the ‘localapproach’, high strains and bi-axial loading are addressed to varying degrees.
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3
FATIGUE
Fatigue is a damage process whereby a crack can form and grow under the action of fluctuating (cyclic) loads. The fatigue life of a welded joint is lower than that of a plain plate because of the presence of stress concentrations and crack-like discontinuities. In a welded joint, fatigue crack initiation may occupy only a very small proportion of the fatigue life; it is dominated by fatigue crack propagation (growth) [10]. The environment has a significant effect on fatigue. The fatigue life in a corrosive environment is lower than that in a non-corrosive environment. Corrosion-fatigue can be more severe than either corrosion or fatigue. The rate of crack propagation is higher and the endurance limit (or threshold for the initiation of fatigue crack growth) is lower or nonexistent. A corrosive environment can be created simply by the presence of pre-existing cracks or crevices. Capillary condensation may cause there to be a corrosive environment in a crack, even though the bulk environment is non-corrosive. This issue can be of particular concern in sour environments, where corrosion-fatigue rates can be very high. Frequency is another issue that needs to be considered. A number of fatigue tests, both published and unpublished, have shown that in a corrosive environment, such as sea-water or a sour fluid, the fatigue life is reduced as the loading frequency is reduced. These tests tend also to show that there is a plateau in the frequency response. This is fortunate, because loads associated with, say, lateral buckling have a frequency of the order of 10 -6 Hz, or lower, which is too low for testing to be practical. Fatigue is an important consideration in an ECA. The two methods used to assess fatigue are: S-N curves; and fracture mechanics. S-N curves are derived from endurance testing. In fracture mechanics based fatigue, fatigue crack growth laws, also derived from testing, are used. It is important that the endurance and fatigue crack growth rates tests are conducted in conditions (e.g. material, geometry, frequency, temperature and environment) that are representative of the actual conditions.
4
PIPELINE WELDING CODES AND STANDARDS
Pipeline welding codes and standards, e.g. BS 4515-1, API 1104 : 2005 and DNV-OS-F101, specify workmanship acceptance levels for welding defects in pipeline girth welds. These acceptance levels represent what a ‘good’ welder should be able to achieve. They are not fitness-for-purpose defect limits. An ECA is required determine if workmanship acceptance levels are fit-for-purpose (see section 12). High static loads and/or high cyclic loads may necessitate smaller acceptance criteria. Welding defects can be categorised as planar or non-planar (these are sometimes referred to as linear and volumetric, respectively). Non-planar flaws include: slag, inclusions and porosity. Planar flaws include: incomplete (inadequate) penetration, lack of (incomplete) fusion, undercut, and cracks. Planar flaws are more significant than non-planar flaws, although non-planar flaws can be indicative of poor workmanship and may mask the presence of more severe planar flaws. There are differences between the workmanship acceptance levels in the various pipeline welding standards, but broadly speaking, the workmanship acceptance levels for planar flaws can be summarised as follows:
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the length of individual surface flaws should not exceed 25 mm (1 in.), and the total length of such flaws in any 300 mm (12 in.) should not exceed 25 mm; and the length of individual embedded (also referred to as buried) flaws should not exceed 50 mm (2 in.), and the total length of such flaws in any 300 mm (12 in.) should not exceed 50 mm. Workmanship acceptance levels were developed at a time when welding was manual and the completed welds were inspected using radiography. The type of flaw can be identified, and the length of the flaw can be measured using radiography, but not the height (depth). Consequently, workmanship acceptance levels were originally expressed in terms of the type of flaw and the length of the flaw. Semi-automatic and automatic welding systems have been developed, and automatic ultrasonic (AUT) inspection has been introduced. The profile of manual and automatic (or semi-automatic) welds is different, and some types of weld defect are more, or less, common depending upon the welding method. AUT inspection is more effective at identifying planar defects than radiography, and is capable of measuring the height, as well as length, of flaws. Inspection methods have improved over time. Consequently, the size and types of welding defect that can be found has been extended as inspection methods have improved. The workmanship acceptance levels in BS 4515-1 can only be applied if a minimum Charpy V-notch (CVN) impact energy requirement is satisfied. The average CVN impact energy at the minimum design temperature should be at least 40 J, and the minimum at least 30 J. Welding codes and standards also make reference to defect acceptance limits (or defect limits) based on fitness-for-purpose. Appendix A of API 1104 gives alternative acceptance criteria for girth welds, based on fitness-for-purpose methods. Appendix A is applicable if: the maximum applied axial strain does not exceed 0.5%, and the CTOD at the minimum design temperature is at least 0.127 mm (0.005 in.). Appendix A is not applicable to a pipeline subject to fatigue loading in excess of a prescribed limit; the “spectrum severity” limit in §A.2.2.1 is equivalent to a usage factor of 0.013 with respect to the class E design curve (see section 9). BS 4515-1 also allows acceptance criteria to be based on fitness-for-purpose, and refers to the guidance in BS 7910 for conducting ECAs.
5
THE EPRG GUIDELINES
The EPRG guidelines for the assessment of defects in transmission pipeline girth welds give simple defect limits, based on extensive small and full-scale testing, and fracture mechanics analysis [9]. The EPRG reviewed workmanship acceptance levels and fitness-for-purpose defect limits in various pipeline welding codes and standards, noted the differences and inconsistencies, and subsequently developed guidelines incorporating workmanship acceptance levels (Tier 1) and defect limits (Tiers 2 and 3). The EPRG guidelines are not applicable to a pipeline subject to “onerous fatigue duty”, but they can, nevertheless, be informative (see section 10). Tier 2 of the EPRG guidelines is applicable if: the maximum applied axial strain does not exceed 0.5%, the weld overmatches the pipe body, the yield to tensile ratio does not exceed 0.90, and the average Charpy V-notch impact energy at the minimum design temperature, from a set of three tests, is at least 40 J, and the minimum is at least 30 J. The CVN impact energy requirements in BS 4515-1 are taken from the EPRG guidelines. BS 4515-1 draws the readers attention to the publications of the EPRG. The Tier 2 defect limits for surface and embedded planar flaws are: flaw height less than or equal to 3 mm (based on the typical height of a weld run), and flaw length less than or equal to 7 times the wall thickness. 4/17
The Tier 3 defect limits are larger than the Tier 2 limits, and there are additional requirements. Tier 3 of the EPRG guidelines is applicable if: the maximum applied axial stress does not exceed the yield strength (similar to the Tier 2 requirement, in that API 5L defines the yield strength as the stress at a total strain of 0.5% [11]), and, in addition to the CVN requirement, the average CTOD (measured using a single edge notch bend specimen) at the minimum design temperature, from a set of three tests, must be at least 0.15 mm, and the minimum is at least 0.10 mm. The Tier 3 defect limits are larger than the Tier 2 limits, so it is reasonable (and conservative) to consider that 40/30 J is equivalent to 0.15/0.10 mm. The EPRG guidelines are partly based on wide plate, and full scale, testing, which means that the implications of constraint are considered implicitly. Bi-axial is also addressed, in that the guidelines are applied to transmission pipelines operating at hoop stresses up to 72% SMYS (but the axial strain is limited to 0.5%).
6
S-N CURVES
S-N curves can be used to estimate the fatigue life of a welded joint. An S -N curve presents the fatigue life, N, as a function of the applied stress range, S. S-N curves of welded joints are based on endurance tests of workmanship quality welds. Welded joints are classified, and for each class there is a different S-N curve, e.g. in PD 5500, the class E curve is for a full penetration butt weld made from both sides, and the class F2 curve is for a full penetration butt weld made from one side, without backing [12]. The class of an S-N curve refers to a particular mode of fatigue failure, e.g. initiation at the weld toe, or the weld root, so more than one class may apply to a particular welded joint. The effect of stress concentrations due to the weld shape and type are included in the S-N curves of welded joints. Design S-N curves, such as those given in BS 7608, PD 5500 and DNV-RP-C203 [13], are mean minus two standard deviation curves to the experimental data. In addition to classifying the welded joint, to select the appropriate S-N curve, the effect of plate thickness, the environment, and gross structural discontinuities and deviations from the intended design shape, need to be taken into account. Some codes also indicate an effect of material, if other than a ferritic steel at ambient temperature. The endurance test specimens used in the fatigue tests upon which the S-N curves will have included some degree of misalignment, and weld defects, but there is rarely sufficient information in the published data for these effects to be quantified [14]. Also, it is common practice in analysing endurance test data to plot the local stress range, corrected for misalignment (from strain gauge measurements), not the nominal stress range. There is a view that the class E design S-N curve includes an allowance for misalignment, corresponding to an SCF equal to 1.3, but this is not universally accepted in design codes [10]. BS 7608 states that the classifications for transverse butt welds allow for some degree of misalignment, but only if the root sides of joints with single-sided preparations are backgouged [15]. Macdonald et al. (2000), in a review of S-N curves for girth welds, concluded that the class E S-N curve could be used for full penetration girth welds made from one side, in conjunction with a thickness correction and SCFs [14]. The guidance in DNV-RP-C203 is based on the Macdonald et al. (2000) review. PD 5500 does not require a factor of safety (or usage factor) to be applied to the fatigue life estimated using the design S-N curves. DNV-RP-C203 and DNV-OS-F101 specify a factor of safety, which depends on the safety class. For a ‘normal’ safety class, a factor of safety of five must be applied to the fatigue life estimated using the design S-N curves. For a ‘high’ safety class, a factor of ten is applied. The fatigue limits in IGE/TD/1, a design code for onshore transmission pipelines, incorporate a factor of safety of ten on fatigue life [16]. PD 8010-1 and -2 indicate that factors of safety in pipeline design typically range from 1 for nonhazardous, non-critical pipelines to 3 to 10 for hazardous pipelines [17,18]. It further states 5/17
that if full penetration high quality welds are assured, then the relevant S-N curves from BS 7608 can be used without a factor of safety. There are a number of reasons for the different approach es, including redundancy, ease of inspection and weld quality. PD 5500 specifies acceptance levels that are more severe than the typical workmanship acceptance levels in pipeline welding codes.
7
S-N CURVES AND ‘SIGNIFICANT DEFECTS’
S-N curves are only appropriate if the weld is free from significant defects. The question is: what is a significant defect? It is commonly interpreted as workmanship acceptance levels, but some care is required because different welding codes specify different workmanship acceptance levels. The fatigue strength of a weld will be reduced by the presence of defects that are too small to detect with commonly available inspection methods. The improvement of inspection methods over time, therefore has implications for the definition and sizing of ‘significant defects’; the size of defect that can be detected and measured using modern non-destructive inspection methods may be smaller than could have been detected in the ‘workmanship’ quality welds used in the development of S-N curves. BS 7910 notes that the S-N curves for the various joint classes were derived from test data for nominally sound welds made under laboratory conditions, and further states that it is almost certain that none of the test welds had flaws of normally detectable size at the weld toe [2]. Macdonald et al. (2000) states that design S-N curves for girth welds do not cover fatigue failure from identifiable welding defects [14]. Reference is made to several tests of girth welds with root defects in which relatively low fatigue lives were recorded. It is stated that surface crack-like flaws are unlikely to be acceptable if a fatigue strength represented by any of the design S-N curves for girth welds is required. Embedded volumetric flaws and shallow weld toe undercut may be acceptable, and reference is made to the acceptance levels for the ‘quality categories’ given in BS 7910 (which are the same as those in PD 5500). It is further noted that these are known to be conservative [14]. BS 7608 : 1993 states that welding workmanship should be in accordance with BS 5135 : 1984 (now superseded) [15,19]. Quality category A of BS 5135 does not permit cracks, lack of fusion, lack of penetration, i.e. planar defects are not permitted. Annex C of PD 5500, a design code for unfired fusion welded pressure vessels, gives requirements for the fatigue design of pressure vessels. The guidance in this annex is informative for understanding the applicability of S-N curves. The S-N curves in PD 5500 have a common background to those in BS 7608, Macdonald et al. (2000), DNV-RP-C203, and UK HSE Offshore Guidance Notes [20]. PD 5500 states that the design S-N curves have been derived from fatigue test data obtained from welded specimens, fabricated to normal standards of workmanship. The material must have sufficient toughness to avoid brittle fracture. The welds must be proved to be free from significant defects by non-destructive testing. PD 5500 defines three construction categories, which differ with respect to the level of inspection (non-destructive testing), permitted material, maximum nominal thickness, and temperature limits [Table 3.4-1]. The requirement to ensure that welds are free from significant defects necessitates full (100 percent) inspection, implying construction category 1. Acceptance criteria for weld defects revealed by visual examination and non-destructive testing are given in §5.7 of PD 5500. Summarising the radiographic inspection levels for category 1 [Table 5.7-1]: planar defects (e.g. cracks and lamellar tears, lack of root, side or inter-run fusion, and lack of root penetration) are not permitted, and the limits for non-planar defects are
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similar to BS 4515-1. The acceptance levels for non-planar defects are summarised in Table 1, below. The important point to note is that PD 5500 does not permit planar defects. In addition to these acceptance criteria, the criteria in §C.3.4.2 of PD 5500 apply to vessels assessed to Annex C, i.e. vessels subject to fatigue loading. Acceptance levels for nonplanar embedded defects (i.e. inclusions and porosity) are given [Table C.4], with respect to the weld classes, and are summarised in Table 2, below. The only significant difference between the acceptance criteria in §5.7 and §C.3.4.2 are that the latter are more restrictive on the length of slag inclusions; the criteria are compared in Table 1 and Table 2, below. The weld defect acceptance criteria in §C.3.4.2 of PD 5500 correspond to the limits for nonplanar flaws for the ‘quality categories’ given in BS 7910. The ‘quality categories’ provide an alternative method of assessing the fatigue life of a flaw in a weld, compared to a fracture mechanics based fatigue calculation. PD 5500 refers to BS 7910 for assessing the fatigue lives of defects, or determining the tolerable defects for a given fatigue life. The common thread in the above codes and standards is that ‘free from significant defects’ means no planar defects. This is significant when it is noted that BS 4515-1 does permit planar defect.
code
maximum length of slag inclusion ,mm
maximum percentage area of porosity on radiograph
PD 5500 [Table 5.7-1]
[parent metal thickness]
2
BS 4515-1
50
2
API 1104
50
[the limit is expressed in a different form that is not readily translatable]
Table 1
Acceptance levels for non-planar defects, given in PD 5500, for pressure vessels not subject to fatigue loading
class required
maximum length of slag inclusion ,mm
maximum percentage area of porosity on radiograph
D (Q1)
2.5
3
E (Q2)
4
3
F (Q3)
10
5
F2 (Q4)
35
5
G (Q5)
no limit
5
W (Q6) and lower
no limit
5
NOTE 1 Tungsten inclusions in aluminium alloy welds do not affect fatigue behaviour and need not be considered as defects from the fatigue viewpoint. NOTE 2 For assessing porosity, the area of radiograph used should be the length of the weld affected by porosity multiplied by the maximum width of weld. NOTE 3 Individual pores are limited to a diameter of e/4 or 6 mm, whichever is the lesser. NOTE 4 The above levels can be relaxed in the case of steel welds which have been thermally stress relieved, as described in BS 7910.
Table 2
Acceptance levels for non-planar embedded defects, given in PD 5500 (and BS 7910), for pressure vessels subject to fatigue loading 7/17
8
WHAT IS AN ECA?
An ECA, in general terms, considers all of the modes of final failure of a flaw (e.g. fracture or plastic collapse) and all of the possible material damage mechanisms that may lead to the growth of a sub-critical flaw (e.g. fatigue) or deterioration in the material properties (e.g. embrittlement in a hydrogen charged environment). The maximum operating pressure of the typical pipeline is well below the creep regime. Environmental crack mechanisms, such as stress corrosion cracking, tend to exhibit high rates of crack propagation, so normal practice is to avoid the initiation of such mechanisms. Therefore, an ECA in a typical pipeline will be concerned with two issues: failure by fracture or plastic collapse (hereafter, referred to simply as fracture), and the growth of sub-critical flaws by fatigue. The ECA is therefore concerned with static loads and cyclic loads. The growth of sub-critical flaws by ductile tearing may be an issue if the static loads are high (e.g. reeling). Corrosion-fatigue, embrittlement, and the initiation of environmental crack mechanisms may be of concern in some environments. In simple terms, however, the former is addressed by an increase in the rate of fatigue crack growth, whilst the latter two are addressed by reducing the fracture toughness. The steps in an ECA are summarised in Figure 1. The simplest case (and perhaps the case that was originally envisaged when the concept of ECAs and fitness-for-purpose were first developed) is the ECA of a known flaw, see Figure 1 a). A structure (such as a pipeline) contains a known flaw (), detected by means of some inspection technique (e.g. radiography, ultrasonics, or an intelligent pig). The limiting flaw size () in the structure is then calculated, based upon the failure mode(s), applied loads and the material properties. Whether the known flaw in the structure is ‘fit-for-purpose’ depends on the difference between the known flaw size and the limiting flaw size (), taking into account material damage mechanisms and the capabilities of the inspection technique. This may include calculation of the remaining life of the known flaw, and even re-inspection at some regular interval into the future. It is a natural extension of this simple case to consider an ECA to determine a flaw acceptance criteria, see Figure 1 b). The limiting flaw size () in the structure is calculated, based upon the failure mode(s), applied loads and the material properties. In a design case, this would be the flaw size at the end of the design life. The relevant material damage mechanisms over the time period under consideration are identified, and their effect on the limiting flaw size is taken into account (). In a design case, the time period would be the design life of the structure. The result is a calculated flaw size (); if the structure contains a flaw that is greater than or equal to this calculated flaw size, then the structure will fail before the end of the time period under consideration. In a design case, the maximum tolerable flaw size is equal to this calculated flaw size. Then the flaw acceptance criteria () is determined, with reference to the tolerable flaw size, the capabilities of the inspection technique(s), workmanship considerations, and other factors. One approach would simply be to subtract the inspection tolerances from the tolerable flaw size. Another approach, assuming that workmanship acceptance levels are less than the subtracting the inspection tolerances from the tolerable flaw size, would be to apply workmanship acceptance levels, with the results from the ECA used for concessions. Figure 1 b) is a relatively simple illustration of the steps required to determine a flaw acceptance criteria. In practice, it can be significantly more complicated, as illustrated in Figure 1 c). Consider an offshore pipeline that is designed to accommodate lateral buckling. Firstly, it may be necessary to consider both installation and operation. Secondly, it may not be immediately obvious as to what is the limiting condition, necessitating a number of different calculations. The limiting flaw size at the end-of-life (e-o-l) is calculated, based on the end-of-life loads (). The corresponding flaw size at the start-of-life is then calculated by
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limiting flaw size
2 3 1
?
known flaw
flaw size (a & 2c) a) ECA of a known flaw
4
limiting flaw size
acceptance criteria
1 2
? 3 calculated flaw
flaw size (a & 2c)
limiting flaw size
flaw size after installation
b) ‘simple’ ECA to determine a flaw acceptance criteria
6
flaw size (a & 2c)
3
2
5
?
7
1
4
calculated flaw
limiting flaw size, e-o-l
acceptance criteria
8
OPERATION
s-o-l
INSTALLATION
flaw size (a & 2c) c) ‘complex’ ECA to determine a flaw acceptance criteria (e.g. offshore pipeline)
Figure 1
The steps in an ECA
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taking into account the relevant material damage mechanisms over the design life (). The structure may experience higher loads at the start-of-life than at the end-of-life, e.g. the stresses and strains in a lateral buckle tend to be highest when the buckle first forms. The limiting flaw size at the start-of-life (s-o-l) is calculated, based on the start-of-life loads (). The tolerable flaw size, with respect to operation, at the start -of-life () is the lower of that determined from & , and . It is then necessary to consider the effect of the relevant material damage mechanisms during installation (). In simple terms, this might only be fatigue loading during installation. In general terms, there may be a requirement for a further ECA to determine the tolerable flaw size for installation () and the flaw size after installation, taking into account factors such as fatigue and ductile tearing (and even the effects of installation on the material properties). The tolerable flaw size, with respect to installation, at the start-of-life () is the lower of that determined from & , and . Then the flaw acceptance criteria () is determined, as above.
9
ECAs AND S-N CURVES
Fatigue design is normally based on S-N curves. The ECA is a secondary consideration. A pipeline girth weld will be a class D or E weld (cap and root, respectively), with respect to PD 5500 (or similar curves in DNV-RP-C203). A factor of safety of say five (or ten) would be applied to this design curve. The allowable fatigue damage would then be split between installation, as laid and operation; DNV-OS-F101 indicates that a split of 10%, 10% and 80% is common. An ECA is then conducted. It is not uncommon for this to indicate small tolerable flaw sizes. The question that then arises is the relationship between ECAs and S -N curves. S-N curves are only applicable if the weld is free from sig nificant defects; this effectively means no planar flaws (see section 7). Fatigue crack initiation represents a small proportion of the fatigue life. In an ECA, the initial flaw is assumed to be a fatigue crack. There is no initiation. S-N curves are directly based on experimental data. An ECA is based on mathematical models, with an indirect relationship with experimental data though the fatigue crack growth law and its experimentally derived constants. Therefore, a direc t comparison of S-N curves and ECA is not straightforward. However, it is instructive to calculate the initial size of flaw that corresponds to a given design S-N curve, taking into account the safety factors that would be applied in design to the S-N curve, as would be done in an ECA. Figure 2 is a plot of the initial size of a surface flaw, in terms of flaw height and flaw length, that corresponds to a given fraction of the class E design curve. The calculations have been conducted in accordance with BS 7910, using the flat plate stress intensity factor solutions (§M3.2), two-dimensional weld toe stress concentration factors, and the simplified (one stage) in-air fatigue crack growth law. No stress concentration factors due to misalignment have been applied to either the ECA or the S-N curve. No factor of safety has been applied to the results of the ECA. To simply the calculations, the final flaw size is taken to be either 50 percent of the wall thickness, or 95 percent of the wall thickness. The difference between the calculated initial flaws sizes is small compared to the final flaw sizes, and decreases as the fatigue loading increases – in fatigue, the initial flaw size is more important than the final flaw size. Figure 2 a) is for a 15 mm thick pipe and Figure 2 b) is for a 25 mm thick pipe. The calculated initial flaw size is larger for the thicker pipe – the cyclic stress intensity is related to a/B. A set of curves are shown for fatigue lives ranging from 0.5 times the class E curve (i.e. a factor of safety of two) to 0.05 times the class E curve (a factor of safety of twenty). The initial flaw size increases as the factor of safety increases. The figures show that a factor of safety of between 5 and 10 on the fatigue life predicted using the S-N curve, depending on thickness, is required for the initial flaw size to exceed the typical workmanship acceptance levels in pipeline welding codes. 10/17
6
B = 15 mm
class E
flaw height, a (mm)
5 0.05 = f, 0.95xB = a f
4
0.05 = f, 0.5xB = af
3 0.1
0.1
2 0.2
1
0.2 0.3 0.4
0 0
10
20
30
40
50
60
flaw length, 2c (mm)
a) 6
B = 25 mm
class E
0.05 0.05
flaw height, a (mm)
5 0.1 = f, 0.95xB = a f
4
0.1 = f, 0.5xB = af
3
2
0.2 0.2
1
0.3 0.3 0.4 0.4 0.5 0.5
0 0
b) Figure 2
10
20
30
40
50
60
flaw length, 2c (mm) Initial flaw sizes corresponding to the class E S-N curve
This simple comparison between the results on an ECA and an S-N curve illustrates the following points: initial flaw size is more important than final flaw size; the influence of final flaw size decreases as the fatigue loading increases, indicating that in a design subject to high fatigue loading factors that influence fatigue are more important than those that affect the final flaw size; and
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if the fatigue design uses a high proportion of the allowable fatigue damage, then small initial flaw sizes are inevitable. It is also apparent that the factor of safety applied to S-N curves is, implicitly, also a ‘defect’ factor.
10 ARE ECAs TOO CONSERVATIVE? An ECA should be based on conservative data and assumptions. The failure assessment diagram (FAD) in BS 7910 is based on failure avoidance. Therefore, it is to be expected that an ECA is conservative. All design is conservative. The question is whether ECAs are too conservative. In part, this can be answered by comparing the results of an ECA conducted in accordance with BS 7910, with other guidance in documents such as the EPRG guidelines for the assessment of defects in transmission pipeline girth welds and Appendix A of API 1104. Figure 3 compares the tolerable flaw sizes calculated in accordance with BS 7910 (using the flat plate stress intensity factor solutions (§M3.2), two-dimensional weld toe stress concentration factors, and the reference stress solution for a circumferential flaw in a curved shell (§P.4.3.2)) with the Tier 2 and Tier 3 defect limits in the EPRG guidelines. Two wall thicknesses: 15 and 25 mm, and three diameters: 8.625, 12.75 and 16 in., are considered. The line pipe grade is X65. A fracture toughness of 0.15 mm is assumed. No stress concentration factors due to misalignment are applied. The applied load (a primary membrane stress) is equal to the specified minimum yield strength. Residual stresses due to welding are assumed to be uniform, and nominally equal to the yield strength, but reduced in accordance with the value of the reference stress. The tolerable flaw height decreases as the length increases, and is less than the Tier 2 and Tier 3 limits (and less than the equivalent API 1104 limits). Aside from the residual stresses, the main reason for the conservative results is that constraint has not been considered. The single edge notch bend test specimen for measuring the fracture toughness has a high level of constraint and will give a conservative value of the fracture toughness, if the structure has a lower level of constraint than the test specimen. Figure 4 shows the required toughness to give a tolerable flaw size equal to the Tier 2 and Tier 3 limits. It varies between approximately 1.5 and 2.25 times higher than 0.15 mm. This is perhaps slightly higher than the difference in constraint between a single edge notch bend test specimen and a girth weld in a pipeline. A more thorough (and complicated) calculation than the simple calculation described here would further reduce the conservatism. Tables A-2 to A-4 of Appendix A of DNV-OS-F101 give the required J-integral for a range of tolerable flaw sizes for a “generic ECA”. This “generic ECA” is applicable if the total longitudinal strain is less than 0.4 percent. Figure 5 compares the tolerable flaw sizes and toughness requirements from this “generic ECA” with the Tier 2 and Tier 3 limits. The required values of the J-integral have been converted to equivalent values of crack tip opening displacement using the conservative expression given in Appendix A / E106 of DNVOS-F101. The required toughness is significantly higher than the 0.15 mm of Tier 3, and the difference increases as the flaw length increases. The “generic ECA” is intended to be conservative.
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5
5
B = 15 mm, = 0.15 mm
3
2 D = 8.625, 12.75 & 16 in.
EPRG, Tier 2
2 D = 8.625, 12.75 & 16 in.
1
0
API 11 04
Tier 3
1
3
50
100
150
200
0
50
flaw length, 2c (mm)
Figure 3
150
200
EPRG, API 1104 and BS 7910 defect limits 0.35
X65
0.30
X = 2.0
0.25
B = 15 mm
X = 1.5
B = 25 mm
0.20
fracture tough ness, d (mm)
X65
B = 15 mm
X = 2.0
0.30
B = 25 mm
0.25 X = 1.5
0.20
EPRG, Tier 3
EPRG, Tier 2 0.15
0.15 0
200
400
600
800
0
1000
Figure 4
200
400
600
800
1000
diameter (mm)
diameter (mm)
Fracture toughness corresponding to EPRG Tier 2 and 3 limits
1.0
1.0
B = 15 mm, a = 3 mm
8.625 in. 12.75 in. 16 in.
0.6
0.4
EPRG, Tier 2
0.6
0.4
0.2
Tier 3
0.2
0.8
8.625 in. 12.75 in. 16 in.
EPRG, Tier 2
0.0
Tier 3
fracture toughne ss, d (mm)
B = 25 mm, a = 3 mm
0.8
0.0
0
50
100
150
flaw length, 2c (mm)
Figure 5
100
flaw length, 2c (mm)
0.35
fracture to ughn ess, d (mm)
EPRG, Tier 2
0
0
fracture toughness, d (mm )
Tier 3
flaw height, a (mm)
4
API 1 104
flaw height, a (mm)
4
B = 25 mm, = 0.15 mm
200
0
50
100
150
200
flaw length, 2c (mm)
Tolerable flaw sizes and toughness requirements in DNV-OS-F101 and the EPRG Tier 2 limits
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11 ARE ECAs UNCONSERVATIVE? ECAs are intended to be conservative. This conservatism is intentional. Leaving aside conservative data and assumptions, the conservatism can be attributed to: constraint, overmatching, tearing resistance, residual stresses, the FAD, and the various stress intensity factor and reference stress solutions. To address over-conservatism, the sources of conservatism are addressed. It is possible that hidden, or unrecognised sources of non-conservatism can be unintentionally unearthed in the pursuit of non-conservatism. A potential example of this is the influence of bi-axial loading. Also of concern are assumptions made without sufficient justification, on the grounds that the ECA is over-conservative. A potential example is the arbitrary selection of constraint factors.
12 ECAs AS A DESIGN TOOL ECAs are used in design to determine the tolerable flaw size at the start-of-life, prior to installation, commissioning and operation, and hence to inform the determination of the acceptable flaw sizes. The steps in this process are summarised in Figure 1. Normally, the ECA is concerned with the girth weld, although the same principles could be applied to the seam weld (but noting that thick-walled, small diameter flowlines are seamless) or even the pipe body. An ECA requires a large amount of detailed information, e.g. geometry, material properties (including fracture toughness), installation and operational loads, welding procedures, environmental effects, etc. This requirement is not conducive to conducting ECAs early in the design cycle. However, because the results of an ECA can have significant implications, it is desirable to conduct the ECA early in the design cycle. Fatigue design using S-N curves requires less information, but similar issues can arise. Detailed welding procedures, and the results of welding procedure q ualification/production tests are not available early in the design cycle. It is difficult to measure the fracture toughness of the girth welds if representative welds are not available. Installation loads vary significantly with installation method. Operational loads are refined as the design is developed. Experimental testing to determine the effects of the environment on endurance (i.e. S-N curves) and fatigue crack growth rates, toughness, and susceptibility to environmental cracking is both complicated and time consuming. Test programmes can easily take nine to twelve months to complete. Modern pipeline girth welds should easily meet current workmanship standards. Therefore, the important question to be answered early in design is: are workmanship acceptance levels fit-for-purpose? This is essentially the approach recommended by SAFEBUCK. It is possible to fabricate girth welds that are effectively ‘defect-free’, e.g. steel catenary risers, but this can have significant cost implications, particularly if the requirement is not identified early in the design cycle. Similarly, changing the S-N curve that the girth welds are required to meet can have significant cost implications. The commentary on the EPRG guidelines on the assessment of defects in transmission pipeline girth welds offer some informative comments on how defect limits based on fitness for-purpose (i.e. ECAs) should be used during construction/installation: special applications where longer defects are anticipated (e.g. new processes); as a concession by the pipeline operator (in conjunction with penalty clauses) to avoid unnecessary repairs; and
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as an insurance policy for cases where a defect is detected during post-construction audit or during in-service inspection. Limiting the objectives of the ECA to determining whether or not typical workmanship acceptance levels (e.g. surface planar flaws limited to 3 mm deep by 25 mm long) are fit-forpurpose, rather than precisely defining flaw acceptance criteria, means that the calculations can be framed in terms of reasonable and conservative (but not overly conservative) assumptions, and sensitivity studies. If workmanship acceptance levels are fit-for-purpose, then the information that becomes available as the design progress is simply used to verify the initial assumptions. Larger flaw acceptance criteria could be developed, but the question is then: is this of benefit to the long term integrity of the pipeline? If the ECA indicates that workmanship acceptance levels are not fit-for-purpose then there is a problem. Identifying the problem early on in the design means that it is more likely that it can be addressed without large cost implications, by re-design and/or raising the priority of project-specific testing or other studies. In addition, identify those types of design that are likely to be challenging in this regard (e.g. lateral buckling in sour environments, hydrogen embrittlement in sour environments) means that a proactive approach to solving the problems can be adopted. Factors of safety are another problematic area. Factors of safety are lower on ECAs than on the design S-N curves. BS 7910 does not require additional factors of safety provided that the data and assumptions are conservative. It is not always clear how conservative the data and assumptions are.
13 WHY THINGS GET COMPLICATED… The design of pipeline systems in deeper water with increased loadings arising from responses to thermal and pressure cycling, in the presence of aggressive internal conditions is not straightforward. ECAs are further complicated. A number of issues can cause problems. A sour environment implies higher rates of fatigue crack growth (due to corrosion-fatigue) and a susceptibility to environmental cracking (if the environment is not dry). The design premise is normally the avoidance of the initiation of environmental cracking, specifically stress corrosion cracking. The temperature in the pipeline system changes significantly during a thermal cycle associated with a shut-down. The loads reduce as the temperature falls, but so does the toughness. It does not follow that the worst case is at the maximum design temperature. The avoidance of the initiation of stress corrosion cracking may not be limiting condition at lower temperatures. Hydrogen embrittlement may be a concern (the toughness may be reduced by an order of magnitude or more). If the toughness is very low then assuming lower bound tensile properties may be non-conservative. Constraint is less relevant if the toughness is low. Operating at high temperatures and high pressures is associated with significant end expansion, pipeline walking and lateral buckling. Fatigue loading associated with thermal cycling is high, but low frequency. Frequency effects in a corrosion-fatigue environment can significantly increase the rate of fatigue crack growth. High cyclic strains may cause a combination of ductile tearing and fatigue crack growth. High strains reduce the toughness. Bi-axial loading issues increase as the strains increase. Strain localisations due to changes in coating type or thickness, weak joints and counter boring (to reduce misalignment) may have a significant effect. Over-matching may not be achieved over the stress-strain response of concern, because of the limitations in the ways that over-matching is specified. The operating and flow regime can give rise to unexpected sources of cyclic loading such as the effect of changes in contents density due to slugging flow. The effect of what happens during installation, both in terms of fatigue and high strains, on the subsequent response during operation may become more important.
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We stray into the murky waters of the known unknowns and the unknown unknowns…
14 CONCLUSIONS The pipeline systems that are likely to be developed in the future will involve a variety of challenges, from high pressures and high temperatures, to high fatigue damage in aggressive internal conditions. It is important that ECAs are part of the solution, and not part of the problem. Work is ongoing through various joint industry projects and project specific studies. More work will be required. Returning to the three questions posed at the start of this paper, the answers would appear to be: sometimes, possibly and no.
NOMENCLATURE a
flaw height
2c
flaw length
B
wall thickness
CTOD
crack tip opening displacement
CVN
Charpy V-notch
ECA
engineering critical assessment
REFERENCES 1.
Department of defense news briefing, Donald H. Rumsfeld, February 12, 2002. http://www.defenselink.mil/transcripts/transcript.aspx?transcriptid=2636
2.
ANON.; Guide to Methods for Assessing the Acceptability of Flaws in Metallic Structures, BS 7910 : 2005, British Standards Institution, London, UK, July 2005. 3. ANON.; Fitness-For-Service, API Recommended Practice 579, First Edition, American Petroleum Institute, January 2000. 4. ANON.; Assessment of the Integrity of Structures containing Defects, R6-Revision 4, British Energy, June 2005. 5. ANON.; Submarine Pipeline Systems, Offshore Standard DNV-OS-F101, Det Norske Veritas, October 2007. 6. ANON.; Fracture Control for Pipeline Installation Methods Introducing Cyclic Plastic Strain, Recommended Practice DNV-RP-F108, Det Norske Veritas, January 2006. 7. ANON.; Specification for Welding of Steel Pipelines on Land and Offshore. Carbon and Carbon Manganese Steel Pipelines, BS 4515-1 : 2004, British Standards Institution, London, UK, November 2004. 8. ANON.; Welding of Pipelines and Related Facilities, API Standard 1104, Twentieth Edition, American Petroleum Institute, Washington, USA, November 2005. 9. KNAUF,G., HOPKINS,P.; The EPRG Guidelines on the Assessment of Defects in Transmission Pipeline Girth Welds, 3R International, 35, Jahrgang, Heft, 10-11/1996, p. 620-624. 10. MADDOX,S.J.; Fatigue Strength of Welded Structures, Abington Publishing, Second Edition, 1991.
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11. ANON.; Specification for Line Pipe, Exploration and Production Department, API Specification 5L, American Petroleum Institute, Forty Second Edition, 2000. 12. ANON.; Specification for unfired fusion welded pressure vessels, Published Document PD 5500 : 2006, British Standards Institution, Third Edition, January 2006. 13. ANON.; Fatigue Design of Offshore Steel Structures, Recommended Practice DNV-RPC203, Det Norske Veritas, August 2005. 14. MACDONALD,K.A., MADDOX,S.J., and HAAGENSEN,P.J.; Guidance for Fatigue Design and Assessment of Pipeline Girth Welds, Health and Safety Executive, Offshore Technology Report OTO 2000 043, May 2000. 15. ANON.; Code of Practice for: Fatigue design and assessment of steel structures, BS 7608 : 1993, Incorporating Amendment No. 1, British Standards Institution, London, UK, 1993. 16. ANON.; Steel Pipelines for High Pressure Gas Transmission, IGE/TD/1 Edition 4 : 2000, Recommendations on Transmission and Distribution Practice, Institute of Gas Engineers, Communication 1670, 2001. 17. ANON.; Code of practice for pipelines - Part 1: Steel pipelines on land, PD 8010-1 : 2004, British Standards Institution, London, UK, 2004. 18. ANON.; Code of practice for pipelines - Part 2: Subsea pipelines, PD 8010-2 : 2004, British Standards Institution, London, UK, 2004. 19. ANON.; Specification for arc welding of carbon and carbon manganese steels, BS 5135 : 1984, British Standards Institution, London, UK, 1984. 20. ANON.; Background to New Fatigue Guidance for Steel Joints and Connections in Offshore Structures, Prepared by Failure Control Ltd. and MaTSU for the Health and Safety Executive, Health and Safety Executive, Offshore Technology Report OTH 92 390, December 1999.
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