Experimental evaluation of the effect of the internal pressure and flaw size on the tensile strain capacity of welded X42 vintage pipelines

Pipelines exposed to geo-hazards such as moving slopes, discontinuous permafrost, and other ground movements may be subjected to displacement-controlled loading, which may lead to significant plastic strains. This can potentially impact pipeline structural capacity and their leak-tight integrity. Traditional stress-based design methods may become uneconomical in the design of pipelines subjected to large plastic strain. In view of the extensive use of such pipelines, reliable calibration of the tensile strain capacity (TSC) plays a critical role in strain-based design (SBD) methodology. Recent studies were focused mostly on high toughness modern pipelines with limited research performed on lower-grade vintage pipelines. However, the effects of biaxial loading due to internal pressure and bending stresses imposed on pipelines as a result of interactions with geo-hazards were not properly addressed in previous studies. This paper investigates the effect of internal pressure and flaw size on the TSC of X42 welded vintage pipelines using experimental testing. Eight full-scale pressurized four-point bending tests were conducted on 22 inches (558.8 mm) diameter X42 grade welded vintage pipes with D/t ratio of 44 to examine the effect of the influential parameters on the TSC. Biaxial strain gauges and a digital image correlation (DIC) system were major instrumentations used for measuring strain. The results were used to examine the crack growth rate by estimating the crack mouth opening displacement (CMOD) at failure and the effects of internal pressure and flaw size on the TSC. The TSC obtained from the tests were compared with those predicted using TSC predictive models proposed by ExxonMobil and Pipeline Research Council International (PRCI).