Blending hydrogen in the natural gas transmission and distribution systems is an option for hydrogen transportation. Integrity management of the existing infrastructure requires a risk evaluation for the planned hydrogen introduction and the challenge of hydrogen embrittlement needs to be considered. The aim of the current study was to evaluate the effect of blending different percentages of hydrogen in methane on the fatigue crack growth rate (FCGR) of X70 line pipe steel. Microstructures associated with base metal, weld center line (WCL) and heat affected zone (HAZ) of girth welds, as well as WCL and HAZ of SAWL seam weld were studied using compact tension specimens based on fracture mechanics. Hydrogen concentrations of 1, 5 and 10 mole% blended in methane were studied. The data was compared to baseline Paris curves in 100% methane as well as the BS7910 in-air mean curve for steel to quantify the increase in fatigue crack growth rate relative to inert environments. In addition, the data was also compared to the ASME B31 curve for carbon steel in hydrogen, to understand the degree of conservatism associated with the ASME curves for the concentrations of H2 evaluated. An increase in FCGR relative to methane baseline was observed for all the microstructures at all concentrations of hydrogen. However, the increase in FCGR relative to the in-air values varied with ΔK. Of the microstructures studied, the base metal showed the highest susceptibility to hydrogen embrittlement for all the hydrogen blends, followed by the girth weld – HAZ location. These two microstructures also had higher hardness compared to the other three locations. A crack growth assessment was also performed for circumferential and axial flaws utilizing the FCGR generated data coupled with pressure cycling data obtained from pipeline operations.

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