The Royal Dutch/Shell Group. Corrosion Prevention Manual. 1989. 1400 CP of Offshore. Structures. Corrosion Protection. 2002. Offshore Technology Report.
Maritime Industry, Ocean Engineering and Coastal Resources – Guedes Soares & Kolev (eds) © 2008 Taylor & Francis Group, London, ISBN 978-0-415-45523-7
Cathodic Protection design procedure for steel offshore structures H. Leheta Department of Marine Engineering and Naval Architecture, Alexandria University, Egypt
R. Abd El-Ghany, A. Amin & S. El-Deen Department of Marine Engineering and Naval Architecture, Suez Canal University, Egypt
ABSTRACT: All offshore steel structures associated with the drilling and production of offshore gas and oil are provided with Cathodic Protection (CP). Without effective CP, these structures will suffer general corrosion loss resulting in structural weakness and possibly perforation of members. The purpose of this paper is to focus on the procedure of the galvanic (sacrificial) anode, as well as the impressed current cathodic protection system applied in the field; guided by specifications, standards and recommended practices for corrosion control. Also the structural analysis of a KT-joint model is carried out to find the effect of uniform corrosion (material loss) on the structure strength over the structure service life.
1
INTRODUCTION
2
Offshore steel structures are usually designed according to two critical design criteria, strength capacity and integrity. The first depends upon the amount of material loss due to surface or general corrosion (although it may be affected by localized corrosion). The second criterion is affected by pitting corrosion. Therefore, it is essential to have a reliable, powerful design for a corrosion control system. The external zone of fixed offshore steel structures is divided into several zones as described below according to the surrounding environment. Therefore, the method of protection may vary to comply with such environment. Atmospheric zone is the part of the structure that is exposed to air, it is normally dry and above the splash zone. It should be protected by suitable coatings. Splash zone is the part of the structure between the crest level of the 50-year (average) wave superimposed on the highest astronomical tide, and 3 meters below the lowest astronomical tide. It should be protected by one of these methods: extra steel in excess of that needed for strength, suitable claddings or wraps and suitable coatings. Submerged zone is the part of the structure below the splash zone. It is permanently located under water and sea floor. It should be protected by cathodic protection, HSE (2002).
CATHODIC PROTECTION (CP) SYSTEMS
The principle of CP is to make the potential of the whole surface of the steel structure sufficiently negative with respect to the surrounding medium to ensure that no current flows from the metal into the medium. This is done by forcing an electric current to flow through the electrolyte (i.e. seawater) towards the surface of the metal to be protected, thereby eliminating the anodic areas. The current may be obtained from any convenient external source, such as a battery and rectifier, or by galvanic action. A potential of (−0.80) V relative to the silver/silver chloride (Ag/AgCl/seawater) reference electrode is generally accepted as the design protective potential for carbon and low-alloy steels. It has been argued that a design protective potential of (−0.90) V should be applied in anaerobic environments, including typical seawater sediments as given in DNV-RP-B401 (2006). 2.1
Sacrificial anode cathodic protection (SACP)
It depends on the basis of connecting two dissimilar metals in an electrolyte, and then an electric current tends to flow from the less noble metal (anode) to the more noble (cathode). Any such current flow will increase the corrosion of the anode and reduce that of the cathode. So when the steel structure is connected with anodes (aluminum alloy or zinc), which are less
211
where ρ = environmental resistivity; L = length of anode; and r = radius of anode, DNV-RP-B401 (2006). 6. The anode current output (Ia ) is calculated from Ohm’s law, DNV-RP-B401 (2006) Figure 1. Major types of sacrificial anode.
noble than steel, it will corrode instead of steel, DEP 30.10.73 (1983). 2.1.1 Sacrificial anode material and types Sacrificial anodes for offshore applications are based on either aluminum (Al) or zinc (Zn). Al-base anode materials are most preferred due to their higher current out-put/weight ratio than zinc, Corrosion Prevention Manual (1989). The major types of sacrificial anodes designed for offshore applications are shown in Figure1.
where Ea = design closed circuit potential of the anode; Ec = design protective potential; and E = Ec − Ea = design driving potential. It should be noted that the anode current output is calculated for the initial and final life of the CP system. In the latter case, the anodes have been assumed to be consumed to their utilization factor. 7. To calculate the final anode resistance, the final geometry of the anode should be predicted. The final anode (mass, length) is calculated from:
2.1.2 Sacrificial anode design procedure Sacrificial anode system requires careful design, because the anode out- put and life are not adjustable after the installation. The following steps explain the design procedure: 1. The surface area for the submerged structure (water, mud) zones is calculated separately. 2. The current demand (Ic ) is the current required to achieve the initial and final polarization as well as to maintain the CP through the design life, and is calculated from
For long flush-mounted anode assume that the final shape is semi-cylindrical. 8. The anode current capacity (ca ) is to be calculated from 9. Total anode current capacity (Ca )
where Ac = structural surface area; fc = coating breakdown factor; and ic = current density. A current drain of 5A/m2 is included for each well when casing is a part of the object to be protected. 3. The total net anode mass (M ) required to maintain CP through the design life is to be calculated from Ic (average) including any current drain
10. These calculations shall be carried-out to meet the following requirements
11. The anode life is calculated from
where ε = electrochemical efficiency; t = design life; and u = utilization factor. 4. The number of anodes (n) is calculated from
where m = is the anode net mass. 5. The anode to electrolyte resistance (Ra ) is an important parameter to predict the anode current output. The resistance for slender stand-off anodes is calculated from
212
where Ia (initial) = initial anode current output; Ia (final) = final anode current output. Maintenance and repair of sacrificial anode cathodic protection systems for fixed offshore structures are generally costly and often
impractical. It is therefore practical to apply the same anode design life as for the object to be protected, DNV-RP-B401 (2006).
2.2
CP system driving potential + -
Anode-to structure potential + -
Impressed current cathodic protection (ICCP)
In an impressed current system, the current flows according to the following precedence 1. from DC rectifier (power supply), to the anodes through insulated cables, 2. from the anode to the steel structure through water, 3. back to the rectifier through the steel and insulated cables .The negative side of the rectifier is connected to the steel structure, and the positive side is connected to the anode, Corrosion Prevention Manual (1989).
Anode-to Structure-to External circiut electrolyte resistance electrolyte resistance resistance
Figure 2. Equivalent cathodic protection circuit.
2.2.1
Impressed current anode material and geometry Over the years the following anode materials have been used: Scrap Steel, High Silicon (14%) Cast Iron, Graphite, Lead Silver, Magnetite (Fe3 O4 ) and Mixed Metal Oxide, Britton (2001). Offshore systems will normally be required to operate at high current density, thus even distribution is required. Cylindrical shapes fulfill this purpose. 2.2.2 Impressed current design procedure 1. The surface area and current demand will be calculated as given for sacrificial anode. 2. After the selection of suitable anode, the minimum anode mass required to maintain cathodic protection through the design life is calculated from
where S = consumption rate; and η = anode efficiency 3. The required number of anodes is calculated from
4. The major factor in the determination of the total circuit resistance is the anode-to-electrolyte resistance; the equivalent resistance circuit from UFC-3-570-02N (2004) is shown in Figure 2. 5. The anode-to-electrolyte resistance (Rv , Rh ) for a single vertical and horizontal cylindrical anode respectively are calculated according to the formulae given in UFC-3-570-02N (2004); where the constant is modified due to the application of SI units.
where ρ = electrolyte resistance; L = anode length or backfill column length; D = effective diameter of anode or backfill column; and s = twice the depth of anode. The formulae have been simplified to the following form, UFC-3-570-02N (2004).
where K = shape function. 6. Common practice to reduce anode bed resistance is to connect several anodes in parallel in a group. If the vertical anodes are arranged in a parallel row, equally spaced, the resistance of a group of anodes (Rn ) can be approximated by the following formula: where n = number of anodes; ρs = electrolyte resistivity with pin spacing equal to s; and F = paralleling factor. 7. If multiple rows of anodes are used where the spacing between rows is more than 4 times the spacing between the anodes in each row, the total parallel resistance is calculated from where R = total anodes resistance. 8. The connecting cable resistance (Rc ) is determined by the size and length of cables used.
213
Table 1.
Jacket platform technical data.
Technical data Design life Location Water depth Seawater resistivity Mud resistivity Submerged surface area Mud surface area
20Yrs Mediterranean Sea 40.2 m 0.2 Ohm.m 0.6 Ohm.m 2188.09 m2 335.15 m2
where Lh = header cable length ; and Ln = negative cable length. 9. The total circuit resistance (Rt ) Figure 3. Anode life, weight for different anode types, SACP.
10. Rectifier DC Output
11. The anode life is calculated from
3
CATHODIC PROTECTION DESIGN PROGRAMS
Two CP design programs ( SACP & ICCP ) were created using C++ to carry out the design procedure and calculate the number, weight and life of anodes that are required to achieve the protective current for the structure surface area. These programs are provided with the modified values of the current densities which depend on the water temperature, depth and location. Moreover, many other types of anodes with different dimensions and technical specifications are provided to facilitate the choice of the best anode. 4
Figure 4. Anode life, weight for different anode types, SACP.
CASE STUDIES
The two programs (SACP& ICCP) were applied for two steel offshore structures used for drilling and production, a fixed jacket platform and a jack-up rig. 4.1 Case 1 fixed jacket platform
Figure 5. Anode life, weight for different anode types, ICCP.
The technical data for the specified jacket platform are in Table 1. 4.1.1 Programs output The results of the SACP and ICCP programs are plotted in Figures 3, 4, 5 and 6 for immersed and mud area, respectively.
According to the previous figures, the criteria of selecting the best anode are to achieve the design life as well as the minimum weight required to protect the surface area. The result for each system is summarized in Tables 2 and 3.
214
Figure 6. Anode life, weight for different anode types, ICCP. Table 2.
Figure 7. Anode life, weight for different anode types, SACP.
Sacrificial anodes.
Zone
Anode type
Anode life Yrs
Weight Kg
Immersed Mud Total weight
Al-base9 Al-base14
20.6 21.3
18675.96 3325.01 22000.97
Table 3.
Impressed current anodes.
Zone
Anode type
Anode life Yrs
Weight Kg
Immersed Mud Total weight
TAM TACD
20.19 20.28
2856 290 6470.87
Table 4.
Figure 8. Anode life, weight for different anode types, SACP.
Jack-up rig technical data.
Technical data Design life Location Water depth Seawater resistivity Mud resistivity Submerged leg surface area Mud leg surface area
4.2
10 Yrs Gulf of Suez 98.2 m 0.3 Ohm.m 1.5 Ohm.m 5103 m2 832 m2
Case 2 jack-up rig
The technical data for the specified jack-up rig are in Table 4. 4.2.1 Programs output The results of the SACP and ICCP programs are plotted in Figures 7, 8, 9 and10 for immersed and mud area respectively. The result for each system is summarized in Tables 5 and 6.
Figure 9. Anode life, weight for different anode types, ICCP.
5
STRUCTURAL ANALYSIS
A model KT-joint of a jack-up rig is idealized to explore the effect of uniform corrosion (material loss) of steel and welding material on the structure capacity over the structure service life.
215
Table 8.
Mechanical properties of materials.
Properties
Steel
Welding material
Yield strength Modulus of elasticity Poisson’s ratio
600 MPa 210 GPa 0.29
486 MPa 210 GPa 0.2
Figure 10. Anode life, weight for different anode types, ICCP. Table 5.
Sacrificial anodes.
Zone
Anode type
Anode life Yrs
Weight Kg
Immersed Mud Total weight
Al-base32 Al-base30
11.85 11.28
20808.1 1201.68 22009.8
Figure 11. KT-joints model. 33
Impressed current anodes.
32.5
Zone
Anode type
Anode life Yrs
Weight Kg
Immersed Mud Total weight
TACD TACD
10.46 11.17
4872 174 6473.4
Capacity (10-5Kg)
Table 6.
32 31.5 31 30.5 30 29.5 29 0
Table 7. Item
Length m
Diameter m
Thickness m
Vertical member Inclined chord Bracing
4 2.82 2
0.762 0.324 0.356
0.06 0.024 0.024
5.1
2
Joint dimensions.
4
6 Design life
8
10
Figure 12. Joint capacity change due to welding metal corrosion.
5.2 Analysis of results The analysis was performed based on above mentioned data, and the results summarized in Figures 12 and 13. From the previous analysis, it is found that the capacity of the joint due to the corrosion of welding material is less than that due to the corrosion of steel.
Model characteristics
The main dimensions and the material mechanical properties are given in Tables 7 and 8. The model is shown in Figure 11 and analyzed using ANSYS program. It is supposed to be subjected to a uniform load acting at the center line of the vertical member. The corrosion rate for steel and welding material is assumed to be the same (0.3 mm/yr). The service life varies from zero to 10 yrs.
6
CONCLUSIONS
From the study, the following are concluded:
216
•
The sacrificial anode system can not be modified after being installed, its design should be carried out carefully.
•
33 32.5
The whole structure should be designed at a potential that satisfies the overall corrosion protection for both the steel structure and the welding material.
Capacity(10-5Kg)
32 31.5
REFERENCES
31 30.5 30 29.5 29
0
2
4 6 Design life
8
10
Figure 13. Joint capacity change due to steel corrosion. •
The impressed current system can be modified simply by altering the input current. • The application of both SACP & ICCP programs enables a preliminary assessment of different anode types to facilitate the choice of the best anode. • The choice of the best CP system depends on many criteria, such as the economy, reliability and weight. • The welding material should be tested against corrosion in order to determine its potential at which no corrosion occurs.
Britton J. 2001. Impressed Current Retrofits on Offshore Platforms – The Good, the Bad and the Ugly. NACE – CORROSION01. Paper 01505. Cathodic Protection Design. 2006. Recommended Practice DNV-RP-B401. Cathodic Protection Manual. 1983. DEP 30.10.73.10-Gen. The Royal Dutch/Shell Group. Corrosion Prevention Manual. 1989. 1400 CP of Offshore Structures. Corrosion Protection. 2002. Offshore Technology Report 2001/011. HSE. Electrical Engineering Cathodic Protection. 2004. UFC 3570-02N.Unified Facilities Criteria, US Department of Defense.
217
