Establish fundamental parameters controlling the influence of sour environments on fatigue crack growth and stress corrosion cracking in pipeline steels.
A series of critical experiments will be designed to examine the effect of sour environments on fatigue crack growth rates (FCGR) in pipeline steel and girth welds. Concentrating on the near threshold regime, tests will be conducted in aqueous environments containing controlled levels of hydrogen sulphide, under increasing/decreasing ΔK conditions, and with constant ΔK at a range of test frequencies. The effect of the environment in building up diffusing hydrogen in the steel will be explored by testing samples in which the aqueous environment is in contact with the steel, leading to hydrogen charging conditions, but is excluded from the crack by means of a protective coating. This simulates the growth of a flaw which is not directly open to the sour environment, for example an embedded defect.
Hydrogen charging conditions in the absence of the corrosive environment will also be examined by testing in high pressure gaseous hydrogen using TWIs recently commissioned test facility. Preliminary tests have shown that acceleration of FCGR and the variation of FCGR with frequency in high pressure hydrogen are similar to those in sour environments. Tests in gaseous hydrogen therefore provide a useful additional tool for exploring the fundamental mechanisms of crack growth acceleration.
Previous work conducted under the CRP has examined factors affecting stress corrosion cracking (SCC) in pipeline steels in sour environments. In order to fully exploit the findings of this study further work will be conducted to establish and validate a generally applicable fracture assessment method for sour conditions. The planned method will use a failure assessment diagram (FAD) approach, similar to that in the current fracture assessment procedures in BS 7910, tailored to suit sour service conditions
Relevant Industry Sectors
Oil and gas pipelines and risers, Gaseous hydrogen pipelines and storage facilities for the transport sector.
Technical and Economic Benefits
Improvement in the understanding of fundamental mechanisms of accelerated fatigue crack growth and stress corrosion cracking in sour environments.
Improved flaw acceptance criteria for pipelines in sour service.
Reduced dependence on project specific environmental tests.
Increased safety and reliability of pipelines, risers and hydrogen storage facilities.
Industrial Member Report
Access the Industrial Member Report resulting from this programme: Shallow Fatigue Crack Growth Behaviour in Sour Pipeline Girth Welds