Weldable Martensitic 13% Chromium Stainless Steels (WMSS) are used for mildly sour welded flow-lines in the oil and gas industry, as an alternative to inhibited carbon steel or lined pipe.
For most material selection and qualification programs for sour applications the material is tested in accordance with NACE MR0175 / ISO 15156 at the maximum design temperature and at ambient temperature (i.e. 24 ±3°C). However WMSS may be more susceptible to Sulfide Stress Cracking (SSC) below ambient temperature and current information in the literature is limited. Consequently qualification to the requirements of NACE MR0175 / ISO 15156 may show acceptable results, whereas in-service cracking could result due to exposure to temperatures below ambient. This may not be applicable for some applications, however for sub-sea pipelines the typical seabed temperature is in the region of 5°C, which may be experienced during `shut-in' conditions.
SSC testing has been undertaken on WMSS at ambient and seabed temperatures using four-point bend (FPB) specimens in simulated condensed water (CW) and produced water (PW) environments at two partial pressures of H2S for parent line-pipe in the as-received and fully-machined conditions. In addition a test method has been developed to closely control the test parameters, with particular regard to dissolved oxygen and pH in order to ensure consistency of results.