Assessment of the Strength of Welded Joints in a Gas Pipeline Using FEM Modeling

Repurposing existing natural gas pipelines for hydrogen service requires an assessment of their current technical condition and remaining lifetime, especially those that may have reached or exceeded their design life. In this study, the strength of welded joints in the X52 steel gas pipeline after 36 years of operation and in the as-received state was investigated, focusing on circumferential welded joints. Hardness distributions in the welded joint zones (the weld metal, the heat-affected zone and the base metal) were determined, and the values of the material's strength and plasticity characteristics were estimated based on the correlation equations. This enables assessment of the stress-strain relationships for the metal from different zones of welded joints, which were compared with the experimental results determined from uniaxial tensile testing. Finite element models of welded joints were developed, in which the relevant material zones were defined in accordance with established stress-strain relationships, and the stress-strain distributions in the welded joints subjected to simulated tensile loading were determined, identifying the areas under the greatest stress. For the welded joint after 36 years of operation, the normalized graphs show higher relative extreme values of σ₁₁/σ_ys = 1.25 compared to the as-received state, where σ₁₁/σ_ys = 1.1. In the as-received welded joint, strain concentration was observed for a stress load about 5% lower than the σ_ys of the base metal, while in the operated one, a local strain increase was observed for a load even 11% lower than the σ_ys of the base metal.