Safety and Integrity of Hydrogen Transport Pipelines

The subsea pipeline network on the Norwegian Continental Shelf (NCS) is increasingly being recognized as an opportunity for the large-scale transport of hydrogen from Norway to Europe. Utilizing existing natural gas pipelines for transportation of hydrogen gas brings several challenges, especially the risk of hydrogen embrittlement. Ensuring the safety of hydrogen transport relies on understanding its impact on pipeline materials. Recent findings from the RCN project HyLINE (2019-2023) highlight that while hydrogen adversely affects the mechanical properties of investigated pipeline steels, they may still meet existing design standards. However, the heat-affected zone of welds appears to be more vulnerable. To address this gap, HyLINE II will focus on the material integrity of welded joints in subsea hydrogen pipelines, examining their impact on overall pipeline integrity. The first year was devoted to characterizing the relevant pipeline materials and establishing new methods for measuring hydrogen diffusion and performing nano-, micro-, and small-scale mechanical testing in hydrogen environment. Numerical models for predicting hydrogen-induced fracture and fatigue are under development. A well-functioning collaboration with Kyushu University in Japan is continuing, particularly focused on mechanical testing and diffusion in hydrogen environments. All three PhD candidates and the postdoctoral researcher are well progressing with their research activities. New experimental methods have been developed to study hydrogen uptake and diffusion, including the HYSAT method, which links electrochemical charging to an equivalent hydrogen pressure. This enables direct comparison of results obtained from different exposure methods and conditions. Gas permeation experiments under mechanical loading are ongoing, and work on samples from the heat-affected zone (HAZ) of welds are under planning. Further efforts have also been directed toward understanding how oxide layers on the steel surface influence hydrogen uptake. The results show that the condition and composition of the oxide film play a significant role in determining the rate of hydrogen uptake into the steel, observed for different surface treatments. Advanced micro- and nanoscale characterization has provided new insight into how hydrogen interacts with dislocations and various microstructural features in both base material and HAZ. In-situ micro- and nano-mechanical tests reveal clear differences in hardness, crack growth, and local plastic deformation between hydrogen-exposed and reference specimens, helping to link observed properties to underlying material mechanisms. Extensive testing of fracture mechanical properties and fatigue behavior in hydrogen gas is also ongoing. Baseline data for crack growth in 200 bar hydrogen have been established, and tests investigating the effects of plastic pre-straining and load cycling (FCGR and overload tests) are in progress. These experiments provide new knowledge on how hydrogen affects crack growth rate and ductility, forming the basis for improved test procedures and safety assessments for hydrogen transport in pipelines. The experimental results are being used in parallel to develop numerical models of hydrogen distribution, stress effects, and residual stresses. Models such as H-CGM+ and phase-field approaches are being further developed to simulate how hydrogen alters fracture mechanisms and material behavior in pipelines. The combination of experimental testing in hydrogen gas, electrochemical and micromechanical methods, and numerical modelling provides a comprehensive understanding of how hydrogen affects material integrity.

The subsea pipeline network on the Norwegian Continental Shelf is gaining traction as an opportunity for large scale transport of hydrogen from Norway to Europe, supporting a long-term and sustainable development of the energy system and contributing to the transition to a zero-emission society. The safety and integrity of the pipelines exposed to internal pressurized hydrogen gas must however be ensured. Pipeline steel welded joints are generally of higher strength than the adjacent base metal, featuring complex microstructures, potential flaws and residual stresses which in sum renders them more susceptible to being embrittled by hydrogen, so called hydrogen embrittlement. The existing Norwegian pipeline infrastructure consists of approximately 740,000 girth welded joints, all requiring special attention to ensure structural integrity in a hydrogen gas environment. HyLINE II will therefore address the follow key research questions: •How different microstructures, surface oxides and charging conditions affect uptake, diffusion and trapping of hydrogen globally and locally in the welded area? •How hydrogen influence the local mechanical properties of microstructures in the HAZ and what are the critical local damage and fracture mechanisms? •What is the susceptibility of hydrogen induced fracture in welded joints under static and cyclic loading in hydrogen conditions? •How to best represent and simulate hydrogen diffusion and trapping and the interplay between hydrogen, material and mechanical response on a local and global scale using numerical tools for fracture assessment? The project consortium includes the pipeline operator Gassco, the energy companies Equinor, Total E&P and Norske Shell and the technology provider Technip FMC. The research will be performed by SINTEF, NTNU and Kyushu University (JP) in collaboration with Imperial College (UK), Fukuoka University (JP) and Max Planck Institute for Iron Research (GER).