Hydrogen Degradation and Integrity Assessment of Cracked X80 Pipelines Under Hydrogen-Blended Natural Gas Conditions

Hydrogen-blended natural gas transportation offers a cost-effective pathway for large-scale hydrogen utilization by leveraging existing natural gas pipeline infrastructure. However, hydrogen-induced degradation of pipeline steel mechanical properties and accelerated crack growth increase the failure risk of defective pipelines. Establishing the relationship between hydrogen-induced material degradation and structural integrity is therefore essential for ensuring safe pipeline operation. In this study, a hydrogen concentration-dependent degradation model was developed through experimental calibration. The evolution of X80 steel mechanical properties was incorporated into finite element models of cracked pipelines to assess structural integrity under hydrogen-blended natural gas conditions. The effects of hydrogen concentration and crack geometry on plastic limit load, crack-front J-integral distribution, and fatigue crack growth behavior were systematically investigated. The results demonstrate that hydrogen concentration is the primary factor governing plastic load-carrying capacity by reducing material strength and ductility, whereas crack geometry, particularly crack depth, dominates the fracture driving force. Compared with static fracture behavior, hydrogen concentration has a limited effect on fracture driving force but significantly accelerates fatigue crack growth and reduces fatigue life, with the most pronounced degradation occurring during the increase in low hydrogen concentrations. These findings reveal the differentiated roles of hydrogen-induced degradation in plastic collapse, static fracture, and fatigue damage mechanisms. The proposed framework establishes a direct linkage between material degradation and pipeline-scale integrity assessment, providing a practical approach for safety evaluation and integrity management of defective hydrogen-blended natural gas pipelines.