Preventing Failure of High-strength Fasteners Used in Offshore and Subsea Applications
Copper-based alloys are known for their susceptibility to SCC when exposed to ammonia or amines. An SCC failure of aluminum bronze screws was traced to ammonia, which was believed to be present due to decay of marine growth.20
caused by using curing epoxy paints was reported.18
In another case, SCC Failures have
also been reported for precipitation-hardened Cu–Ni–Al–Mn alloy (Hiduron 191) bolts.19
The US Navy has used alloy K-500 extensively and has experienced many failures of this alloy due to HE and galvanic corrosion problems in addition to corrosion resulting from galvanic interaction with more noble materials.17
Also, several HE failures of alloy K-500 in the
offshore industry have been reported in the presence of CP.18,20 Some failures were originally associated with highly susceptible microstructure caused by unsuitable heat treatment; however, failures could not be eliminated even with the proper heat treatment, suggesting inherent lack of resistance to HE.
Consequently, alloy K-500 has fallen out of favour and alternatives have been pursued. Alloy X-750 bolts used for subsea wellheads failed after four years in service as a result of HE that was accelerated by high applied stress due to over-torque of the bolts.21
Bolts made of alloy 718 have been used for BOP bolting in two-drill ship operations with combined anode and impressed current cathodic protection for over seven years with no failures. Alloy 718 has shown good performance in seawater under CP provided that its microstructure is free from a continuous network of delta phase along grain boundaries, which was found to be detrimental to HE.23
Alloy steels, mainly AISI 4140 and 4340, remain the most common material for subsea fasteners. The authors are familiar with successful applications of high-strength alloy steel fasteners for drilling risers.15
Another successful experience has been
reported for Ti-6-4 ELI, where over 850 bolts have been used on the Heidrun drilling riser20
for over 10 years.
High-strength Material Options
For applications requiring high yield strength and EAC resistance, the options are limited. Table 3 lists the various commercial alloys available for yield strength >150ksi. For large-diameter (>2.5-inch)
1. Atlas of Stress Corrosion Cracking Data, ASM International, 1984.
2. Bickford JH, An Introduction to the Design and Behavior of Bolted
Joints, 2nd Edn., Marcel Deckker, Inc., 1990.
3. Humphries TS, Nelson EE, Evaluation of the Stress Corrosion Cracking Resistance of Several High Strength Low Alloy Steels,
NASA TM-78276, May 1980, Marshall Space Flight Center, Alabama.
4. ASTM A 320/A 320M Standard Specification for Alloy- Steel and Stainless Steel Bolting Materials for Low- Temperature Service.
5. ASTM A193/A193M Standard Specification for Alloy-Steel and Stainless Steel Bolting Materials for High Temperature or High Pressure Service and Other Special Purpose Applications.
6. International Standard NACE MR0175/ISO15156 – Petroleum and Natural Gas Industries – Materials for use in H2S- containing Environments in Oil and Gas Production.
7. Guidelines for Materials Selection and Corrosion Control for Subsea Oil and Gas Production Equipment, Publication No. 194,
1999, The Engineering Equipment and Materials Users Association, London, UK.
8. ASME/ANSI B16 – Standards of Pipes and Fittings.
80
Acknowledgements
The authors express their sincere thanks to BP and IONIK Consulting/ JP Kenny for permission to publish this paper.
9. API Spec 6A, Specification for Wellhead and Christmas Tree Equipment, American Petroleum Institute, Washington DC, 1996.
10. Aylor DM, Bowles CA, Tregoning RL, Gaudett MA, High Strength Fasteners for Seawater Immersion Service,
Proceedings of Offshore Pipeline Workshop, Galveston,
Texas 1999.
11. Ross RW, Tuthill AH, Practical guide to using marine
fasteners, Materials Performance, 1989;29(4):65–9.
12. Wolfe LH, Burnette CC, Joosten MW, Hydrogen embrittlement of cathodically protected subsea bolting alloys, Corrosion 93, paper No. 288. NACE International, Houston, Texas.
13. Johnson R, Corrosion problems in the oil industry, 11th Scandinavian Corrosion Congress, Stavanger, Norway 1989. (Avesta Corrosion Management No. 3-1989.)
14. Gaudett MA, 150 KSI Yield Strength Fastener Material
Certification Plan, Naval Surface Warfare Center Carderock
Division Report, TR-61 – 98/26, November 1998.
15. Esaklul K, Martin J, High Strength Fasteners for Subsea Applications, Corrosion 2004, Paper No. 04151. NACE International, Houston Texas.
16. Esaklul KA, Unpublished BP Work. 17. Hibner EL, Shoemaker LE, High strength corrosion resistant Alloy 686 for seawater fastener srvice, Corrosion 2002, Paper No. 02195, NACE International, Houston Texas.
18. Natishan ME, Porr Jr WC, Issues surrounding the use of nickel-copper alloy K-500 fasteners in seawater, Corrosion 94, Paper No. 483, NACE International, Houston, Texas.
19. Andersen O, Joosten MW, et. al., Evaluation of high strength Cu-Ni-Mn-Al bolting used in oil and gas service, Corrosion 96, Paper 78, NACE International, Houston, Texas.
20. Bolting and Fastener Application in the Offshore and Process Industry State of the Art, DNV Report No. 2002-3301. Det Norske Veritas.
21. Toffolo G, Fassina P, Marangoni M, Msallem A, Nickel alloy stud bolts failure – a case history, 1996 OMAE – Volume III
Materials Engineering ASME, 1996;165–72.
22. Slind T, Eggen TG, Bardal E, Haagensen PJ, Fatigue performance of nine bolt materials in air and in seawater with cathodic protection, Avesta Sheffield Paper ACOM 1-1993.
23. API SPEC 6A718 Specification of Nickel Base Alloy 718 (UNS N07718) for Oil and Gas Drilling and Production Equipment.
EXPLORATION & PRODUCTION – VOLUME 8 ISSUE 1
fasteners with yield strength of 150ksi, the options are limited to the following alloys: alloy steels (AISI 4340), alloy 718, alloy 725, MP 159 and Ti alloys. Alloys MP 35N, K-500, 17-4PH H1100, alloy A286, alloy 925, Rene 41, alloy 625 and Be-Cu alloys do not meet the strength and size requirements.
Nickel alloys 718 and 725 offer excellent resistances to EAC in seawater with CP and are among the best candidates for deepwater subsea bolting. Alloy 725 has the advantage of better HE resistance and greater crevice and pitting corrosion resistance than alloy 718. This makes alloy 725 a better candidate for large- diameter bolts.
Nickel alloys, nickel cobalt and titanium alloys have higher toughness and EAC resistance and inherently higher defect tolerance compared with high-strength steels in subsea applications. Future applications may consider custom 455, PH13-8, MP159, Ti-15V-3Al-3Cr-3Sn and Ti-10V-2Fe-3Al in addition to alloy 718 and 725 for high-strength, large-diameter bolts. Further qualifications will be required to determine their EAC resistance limits.
Conclusions
Field experience and EAC test data confirm the following. Cathodic protection provides sufficient corrosion protection for bolts in subsea applications even in dissimilar material joints, and high-strength steels with a hardness below 34 HRC are resistant to EAC for cathodically protected components. High-strength steels with hardness above 34 HRC are highly susceptible to cracking due to HE induced by CP, while high-strength nickel alloys have high resistance to HE and are ideal candidates for high-strength subsea bolts. High- strength materials must adhere to known strength limits and specified maximum hardness, e.g. 34 HRC for alloy steel, to ensure resistance to HE and compatibility with CP. Several other materials such as custom 455, PH13-8, MP159, Ti-15V-3Al-3Cr-3Sn and Ti- 10V-2Fe-3Al have the EAC resistance and are good candidates for consideration for subsea applications. n
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