PSI - Issue 77

Niels Grigat et al. / Procedia Structural Integrity 77 (2026) 365–375 Niels Grigat / Structural Integrity Procedia 00 (2026) 000 – 000

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2.1. Existing Steel Pipelines For over a century, steel pipelines have constituted the fundamental infrastructure of global energy distribution networks. The extensive utilisation of these materials in the transportation of natural gas and oil has led to the establishment of a robust technological foundation encompassing design standards, welding procedures, and inspection methodologies. The inherent robustness and pressure resistance of steel, in conjunction with its well documented failure behaviour, have led to its predominance in the domain of large-scale transport systems. In the case of conventional gas applications, steel pipelines generally operate at pressures of up to 100 bar. The selection of wall thickness and material grade is determined by the specific operating conditions and safety requirements. For over a century, steel pipelines have constituted the fundamental infrastructure of global energy distribution networks. The extensive utilisation of these materials in the transportation of natural gas and oil has led to the establishment of a robust technological foundation encompassing design standards, welding procedures, and inspection methodologies. The inherent robustness and pressure resistance of steel, in conjunction with its well documented failure behaviour, have led to its predominance in the domain of large-scale transport systems. In the case of conventional gas applications, steel pipelines generally operate at pressures of up to 100 bar. The selection of wall thickness and material grade is determined by the specific operating conditions and safety requirements. However, within the domain of hydrogen transport, the conventional design methodology of merely augmenting wall thickness to guarantee safety becomes challenging. Whilst it is evident that thicker steel walls can delay crack propagation and improve burst strength, it must be noted that they do not fundamentally eliminate the risk of hydrogen induced degradation. The diffusion of hydrogen atoms into the steel lattice can result in embrittlement and localised loss of ductility, particularly in areas exhibiting high stress concentrations. Welded joints, an unavoidable feature of long-distance pipeline networks, represent critical weak points due to the residual stresses and microstructural heterogeneity they exhibit. It is posited that, over time, this may result in a reduction in the fatigue life and an increase in the maintenance demands. Consequently, while existing steel pipelines can be repurposed or newly designed for hydrogen transport, their long-term performance and economic viability remain constrained by these inherent vulnerabilities. Mitigation strategies, including the application of internal coatings, the use of specialised steel alloys, and the blending of hydrogen with natural gas, are currently being explored. However, these approaches only provide partial solutions and do not fundamentally overcome the intrinsic susceptibility of metals to hydrogen embrittlement. This has led to the exploration of alternative materials, such as fibre-reinforced plastics (FRP), which possess the inherent property of resisting corrosion and hydrogen diffusion. [8,9] 2.2. Steel Pipelines for Hydrogen transport The present study explores the potential of repurposing existing steel pipeline infrastructure for the transportation of hydrogen as a pragmatic, near-term solution to facilitate the development of a hydrogen economy. In several European projects, portions of the natural gas grid are being repurposed for hydrogen blends or pure hydrogen. This allows the reuse of established assets and reduces investment costs. These endeavours predominantly depend on high strength carbon steels with augmented wall thicknesses, which are engineered to withstand the internal hydrogen pressures and preserve mechanical stability under cyclic loading conditions. The design of such pipelines must adhere to stringent safety factors, accounting for both the thermodynamic properties of hydrogen and the material's degradation behaviour over time. Notwithstanding these precautions, the interaction between hydrogen and steel remains a fundamental challenge. In the presence of pressurised hydrogen gas, atomic hydrogen has been observed to diffuse into the metal lattice and accumulate at microstructural defects, such as inclusions, grain boundaries, or dislocations. This process has been shown to promote hydrogen embrittlement (HE), which in turn leads to a significant reduction in fracture toughness and elongation at break. Weld seams are of particular concern, as they frequently contain residual tensile stresses and microstructural inhomogeneities resulting from the thermal cycles inherent to the welding process. Consequently, they