Method for assessing girth weld failure of X80 gas pipelines in service

A large number of domestic X80 spiral welded pipes were used in the construction of the Second West-East Gas Pipeline and the Third West-East Gas Pipeline, which was the first time that X80 spiral welded pipes were put into service in large scale at home and abroad, and few studies were conducted for it. Hence, all-weld tensile test was conducted with the girth weld specimen of in-service domestic X80 gas pipelines, and accordingly, a girth weld failure assessment curve (FAC) was plotted. In addition, the abscissa cutoff line of the girth weld FAC is shorter than that of the general FAC, indicating that the plasticity of girth weld is lower than the conventional materials, and thus brittle failure may be easy to occur. Moreover, the girth weld FAC with the stress close to the yield strength is steeper than the general FAC, without any transition, which shows that X80 girth weld is more likely to suffer a sudden failure. As shown in the girth weld fracture toughness test of true pipe wall thickness in the circumferential and the wall thickness directions, the measured value of the fracture toughness of the true pipe wall thickness is 70.12 MPa·m^0.5, and comparing with the conversion formulas of Charpy impact energy and the fracture toughness with the test results, it is found that the conversion formula specified in BS 7910-2013 Guide to methods for assessing the acceptability of flaws in metalls structures is more accurate. The research results could provide technical reference for the safety assessment and steady operation of girth welds of large-diameter and high-grade steel gas pipelines.