PSI - Issue 68

Oleksii Milenin et al. / Procedia Structural Integrity 68 (2025) 1010–1016 Oleksii Milenin et al./ Structural Integrity Procedia 00 (2025) 000–000

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1. Introduction The use of hydrogen and other renewable gases for energy transfer from remote sources is rapidly expanding, driven by the European Union's Hydrogen Strategy to 2050. This shift is motivated by economic, ecological, and technological factors, particularly the growth of green energy. Renewable energy sources, like solar stations and wind farms, are often located far from end users, making energy transport a critical challenge. Building new system of hydrogen transmission pipelines is costly and can raise the price of green energy. As a solution, utilizing existing gas pipelines to transport "green" hydrogen offers a more efficient and cost-effective alternative. This approach helps increase the share of renewable energy and reduces dependence on fossil fuels. However, the direct use of existent gas transportation systems for hydrogen introduces safety concerns, as these pipelines were originally designed for natural gas. Hydrogen's properties, such as its small molecular size and high diffusivity, increase the risk of material degradation, which can lead to pipeline failures (Dmytrakh et al., 2022, Meng et al., 2017). To address this, blending hydrogen with natural gas has emerged as a practical solution. In gas-hydrogen blends, the specific hydrogen concentration could be chosen to minimize its adverse effects on pipeline materials. Nonetheless, the presence of hydrogen in any form still impacts pipeline integrity, especially during maintenance and repair activities. In-service welding techniques are the common methods for repairing pipelines while they remain operational, allowing for repairs without interrupting gas flow (Kiefner et al., 1994). These methods, which adhere to various industry standards, help restore the structural integrity of pipelines and reduce the environmental and operational costs associated with shutting down for repairs. Despite established protocols, the presence of hydrogen in the transported product increase the hazards during welding, specifically the risk of cold cracking. Cold cracking in welding occurs under three key conditions: the presence of a martensitic microstructure, diffusible hydrogen, and tensile stresses (Lobanov et al., 2013). To prevent this, standard repair practices involve thorough preparation of the welding site, using low-hydrogen filler materials, and preheating the repair area. Preheating, typically between 100°C and 150°C, helps reduce the risk of cracking effectively. The transportation of gas-hydrogen blends, however, accelerates pipeline material degradation over time, making the metal more susceptible to cold cracking during welding. Existing repair protocols may not be sufficient for pipelines carrying such blends, necessitating stricter guidelines to ensure pipeline safety during and after repairs. This study investigates the susceptibility of pipeline steels to cold cracking during in-service repair welding under conditions of gas-hydrogen blend transportation. It aims to numerically evaluate the combined effects of hydrogen embrittlement, material degradation, and welding stresses to improve safety protocols for pipelines used in hydrogen transport. 2. Research methodology and case study 2.1. Mathematical model of temperature field kinetics, phase transformations, diffusion of hydrogen and state of stresses and strains development The multiphysical nature of cold cracking presents significant challenges in predicting the susceptibility to defect nucleation during repair welding. Therefore, the application of advanced mathematical modeling and computer simulation techniques is highly appropriate. Such models should account for the nonstationary kinetics of temperature fields and the associated phase transformations, which influence the evolution of material properties and, consequently, the susceptibility to cold cracking. Another critical aspect to address is the prediction of hydrogen diffusion, which depends on its concentration in the base material and filler metal, the diffusivity of hydrogen across various metal phases, and the temperature dependence of diffusion coefficients. Finally, the model must incorporate the development of stresses and strains within the framework of thermal elastic-plastic deformation theory for continuum media. The mathematical approach to the combined problem of the temperature field kinetics, phase transformations, diffusion of the hydrogen and stress and strain development is formulated using a finite element method framework. The temperature field during welding is primarily influenced by the thermal impact of the moving welding heat source. Typically, the distribution of welding heat energy follows a circular normal distribution. The heat transfer within the welded structure is assumed to be dominated by conduction processes (Akhonin et al., 2005), which can