Fatigue crack growth rate parametric study on subsea X65 pipeline steel girth welds in H2S/CO2 environments

Abstract

Steel catenary risers, subsea flowlines and pipelines that are used to transport oil and gas products are susceptible to corrosion fatigue in environments containing CO2 or H2S, termed sweet and sour respectively. Fatigue crack growth rates (FCGRs) in such environments are generally enhanced compared to that in air due to crack tip dissolution or the embrittling effect of ingressed hydrogen generated from the corrosion process. Several factors influence the FCGR including partial pressure of CO2 or H2S, pH value, microstructure, temperature and loading frequency. Although there are reports in the literature on the effect of each of the parameters on FCGRs, data on the combined effects, for example, frequency and temperature are not well documented. The work conducted in this study focussed on the effects of temperature and loading frequency on FCGR of X65 pipeline steel welds in sweet and mildly sour environments. The results demonstrate the importance of the selection of the appropriate test temperature and loading frequency in FCGR laboratory tests in order to avoid use of non-conservative or unduly conservative results in engineering critical assessment (ECA). Furthermore, the influence of low concentrations of H2S and the presence of corrosion inhibitor on FCGRs of carbon steels was investigated. The dominant corrosion processes under specific testing conditions are proposed.