Offshore pipelines operating in sour environment require that the line pipe material have sound fatigue and fracture properties. The fatigue and fracture of X65 steel in sour environment is a hydrogen assisted cracking mechanism. Hydrogen is generated as a result of the corrosion of the steel in sour environments and its uptake in steel is facilitated in the presence of sulfide ion as the hydrogen recombination poison agent. It is essential to understand the interaction of hydrogen with the steel to help understand the fatigue and fracture performance of X65 steel in sour environments. This is typically done by performing hydrogen flux measurement on line pipe steel samples in sour environment. In this paper, hydrogen flux tests were performed on as manufactured X65 pipe (parent and weld) and strained and aged pipe in a sour environment. The analysis of the measured hydrogen flux in sour environment to determine the effective diffusivity was demonstrated. The hydrogen flux results were analyzed to characterize the trapping of hydrogen in the steel. The obtained results were compared to those obtained with thermal desorption analysis.
Offshore pipeline operations require understanding the fatigue crack growth rate and fracture toughness property of the pipe steel in sour or sweet environment. Previous work has been performed to understand the fatigue and fracture toughness performance of C-Mn steels in dynamic applications like risers and flowlines in sour environments (Buitrago, Weir, Kan, Hudak and McMaster, 2004; Thodla, Gui, Robin and Xia, 2010). Generally, the cracking of carbon steel in sour environments is a hydrogen driven mechanism. The presence of H2S typically will enhance the crack growth by promoting hydrogen generation and ingress into the steel (McMaster, Bowman, Thompson, Zhang and Kinyon, 2008; Gangloff, 2009). It has been found that in moderate to severely sour environments, bulk charged hydrogen from the corrosion processes as opposed to the hydrogen generated at the crack tip during plastic straining plays a dominant role in enhancing the FCGR of line pipe steels in sour environments (Gui, Ramgopal and Muller, 2012).
