Abstract
Full size SSC tests, which simulated pipeline exposed to wet sour gas environment, were carried out under the condition of about 80% of SMYS (Specified Minimum Yield Strength) using NACE-type solution to observe crack behaviour. Six pipes made of ordinary desulfurized steels or super-low sulfur Ca-treated steels were tested in this study. Four types of cracks, namely straight crack, stepwise crack, SSC, and blister generate in base metal. Initiation of HIC such as straight crack has almost finished by 2 weeks. Stepwise crack is propagating continuously as test period becomes longer, but its propagation is limited within the narrow area. SSC starts from a corrosion pit and propagates perpendicularly to hoop stress. This crack is likely to fork and then stop propagating. SSC also generates at the center of blister. These cracks generate very often in ordinary desulfurized steels and few crack in super-low sulfur Ca-treated steels for sour gas service. Some types of laboratory HIC test were carried out to investigate the correlation between the laboratory test and the full size SSC test. It is important to simulate hydrogen distribution of actual pipe because HIC initiation depends on hydrogen concentration of a sample. Single surface HIC test is most suitable for evaluating the HIC susceptibility of a pipe under the actual field. The test pipe exposed to wet sour gas environment does not fracture even under the stress condition which is equal to the threshold stress evaluated by NACE-type SSC test. These results suggest that pipeline exposed to wet sour gas environment scarecely fracture at the base metal.
However, some cracks associated with HIC(Stress Accerelated HIC) at the welded portion will be discussed in the next report.
