Fatigue design of deepwater riser system in a sour environment is a challenging task in a project cycle. More often, the challenge is in identifying an appropriate project specific fatigue curve using accurate environmental representation, and establishing a crack growth curve to confirm the acceptance criteria of the welds. Conventional practice in industry is to use a knock down factor (KDF) to capture the effect of corrosion fatigue, rather than taking an effort in executing detailed test/qualification program. A KDF approach can turn out to be very unreliable, specifically, for mild sour environment where significant test data are not available. As a result, there can be several impacts, including delivery of an under-designed system with potential operational risks.
This paper presents comprehensive findings from a sour service qualification program executed in a project cycle. The sour environment is mild, and is slightly above NACE limit (0.1psi ppH2S, pH ∼ 6). The program included a detailed scope of work consisting of in-air and in-environment fatigue test as well as limited scope in understanding fatigue crack growth rate in the same environment. The material used is API 5L X65 seamless pipe with mechanized GMAW girth weld. The results show the unpredictable side of corrosion fatigue, especially in this kind of mild sour environment, which is not well understood.