The Behaviour of Shallow Cracks in a Pipeline Steel Operating in a Sour Environment

Setting conditions for the avoidance of in-service crack growth in aggressive corroding environments has long been a major challenge due to the number of variables that have a significant effect on material behaviour. One area where both experimental data and a validated assessment methodology are lacking is the behaviour of shallow cracks. This paper describes the early results of an ongoing research programme aimed at addressing the shortfall in experimental data to characterise material behaviour in the shallow crack regime, with the long-term aim of improving the understanding and assessment of the early stages of environment assisted cracking (EAC). There is an industry need for a better understanding of material behaviour under these conditions, and for the development of a more robust assessment methodology. API 5L X65 pipeline steel parent material was tested in a sour environment with initial flaw sizes in the range 1–2 mm. Fatigue crack growth rate (FCGR) tests have been performed to investigate the influence of crack depth on crack growth rate (da/dN). Initial results suggest that crack growth rates for deep flaws can increase by a factor of 5–100 compared to air depending on the applied stress intensity range (ΔK). Shallow cracks have been shown to grow up to 130 times faster in a sour environment than in air and up to an order of magnitude faster than deep cracks in a sour environment at the same value of ΔK. Constant load tests have also been performed to investigate the influence of crack depth on the threshold stress intensity factor for stress corrosion cracking (K_ISCC). Preliminary results suggest that in this case there is no crack depth dependence in the range of flaw sizes tested. While further experimental work is required, the results obtained to date highlight the potential non-conservatism associated with extrapolating deep-crack data. Guidance is therefore provided on how to generate appropriate experimental data to ensure that subsequent fitness for service assessments are conservative.