Serviceability assessment of existing gas pipelines under hydrogen transportation

Transportation of hydrogen or its admixture with natural gas via existing gas pipelines increases the risk of failure due to hydrogen embrittlement and hydrogen-induced cracking of steels. The natural gas transmission pipelines in Ukraine have a long operation time, which leads to a deterioration of the mechanical properties of steels. Evaluation of the degradation degree of pipeline steels and determination of the current mechanical properties are essential to predict the serviceability of long-term operated pipes and to investigate maintenance and repairs for the pipeline system. Therefore, the sensitivity of pipeline steels to hydrogen embrittlement should be assessed, taking into account the degree of operational degradation. In this work, the effect of hydrogen on the susceptibility to hydrogen embrittlement and fracture mechanism of low-alloyed pipeline steel (API 5L X52 strength grade) widely used in the Ukrainian gas transit pipelines was investigated considering the operational degradation, anisotropy, and different hydrogen charging intensities. Pipeline steel in different states (reserved and after 38 years of operation on the transit gas pipeline) was studied. The electrolytic hydrogen pre-charging of different intensities was applied to tensile specimens to analyze the hydrogen effect on their mechanical behavior. A higher sensitivity of the operated steel to hydrogen embrittlement was observed than that of the reserved one. The important role of non-metallic inclusions in the hydrogen embrittlement of the steel, depending on the degradation degree and hydrogen charging conditions, was revealed. Moderate hydrogen charging led to intergranular embrittlement of the steel, while intensive – to transgranular one, indicating different mechanisms of diffusion of hydrogen absorbed by the metal.