Blending hydrogen in existing natural gas pipelines: Impact of reduced J_R-Curve slope on the pipeline integrity

Exposure to gaseous hydrogen decreases the fracture toughness, ductility and fatigue life of pipeline steels. Additionally, it reduces the material tearing resistance, or slope of the J-integral vs. crack growth resistance (J_R) curve. It is known that the reduction in fracture toughness decreases the critical crack size and the estimated failure pressure of hydrogen pipelines, compared to inert environments, like natural gas. The objective of this paper is to explore and quantify, for the first time, the effect of the J_R-Curve Slope on failure pressure and critical crack size, by conducting Level 3 crack assessments according to API 579-1/ASME FFS-1 international code. Upper and lower bound values for the tearing resistance in inert environment and in gaseous hydrogen were obtained from a literature review. The literature was reviewed to obtain true-stress–strain data of pipelines steels, required for constructing material specific failure assessment lines (FALs). The use of the generic FAL proposed in API 579-1/ASME FFS-1 international code resulted in an error less than 10 % in the estimated failure pressure, when compared to material specific FAL. The tearing resistance of the material had a significant effect on the estimated failure pressure, up to 26 % larger than the value obtained with a Level 2 assessment, for the set of pipe and crack geometry parameters chosen in this paper. Such larger level 3 failure pressure cannot be credited in hydrogen service, due to the flatter J_R curve and the risk of subcritical cracking. Hence, there is a double penalty of hydrogen blending in pipeline failure pressure, it decreases because K_IH < K_IC and the reserve safety factor is lost. The effect of tearing resistance on critical crack size and in crack detection limits during hydrotests were discussed and quantified in this paper. It is shown that the tearing resistance decreases the crack detection efficiency during hydrotests, so that it is possible that a pipeline that just passed a hydrotest at a stress level of 100 % of the specified minimum yield strength (SMYS) could have defects with a size larger than critical in hydrogen service at 72 % SMYS.