Comparison between Full Scale Tests and Small Scale Tests in Evaluating the Cracking Susceptibility of Line Pipe in Sour Environment

Abstract

Line pipes have possibility to encounter three types of crack morphology: SSC, HIBC/SWC, SOHIC (Stress Oriented HIC) in the sour environment. For modern line pipes, SOHIC is concerned under applied stress, particularly in softened heat-affected zone. In this study, an X65 UOE line pipe was evaluated as a CAPCIS type full ring test and a full pipe test were compared with tensile tests conforming to NACE standard TM0177-90 method A on the susceptibility to SOHIC in weld.
A rectangular tensile specimen 6.4 mm in thickness provided the same result as the round bar specimen specified in the NACE standard in terms of threshold stress. Ruptured specimens showed SOHIC preferably induced in the softened HAZ. In contrast, no defect indication was obtained after the completion of the full ring test performed at the applied stress of 90% SMYS above the threshold stress. With regard to the full pipe test, no defect was observed when loaded with 90% SMYS with an exception of bead toe cracking caused by rather high applied stress due to stress concentration. The rectangular tensile test in which a single wide side was exposed to the environment was also conducted. No internal SOHIC was observed at the applied stress of 100% SMYS. And the single side exposure test with weld reinforcement exposed to the sour environment also reproduced the bead toe cracking.
This study concludes that the NACE standard is more severe than CAPCIS type full ring test which can be substituted with the one-side exposed rectangular specimen and more severe than the full pipe test from the viewpoint of evaluating the susceptibility to SOHIC. This study proposes the single side exposure rectangular test as a realistic substitute for full scale tests.