Influence of Gaseous Hydrogen on the Fatigue Life in Offshore Pipelines: Small Scale Fatigue Testing in Saipem Girth Welds

Abstract As of today, hydrogen can play a significant role in decarbonizing the world’s energy supply to mitigate climate change and can be transported via pipeline. Hence, it is becoming more and more important in many companies’ strategies to build a clean-energy business, exploring ways of transporting hydrogen as a gas replacement for natural gas or as an addition in blended configuration using existing pipeline networks, or new designed pipelines fit for full hydrogen transport. However, for steel pipelines, it is well recognized that hydrogen presents some peculiarities to be considered during pipeline system design as may promote hydrogen embrittlement phenomena which potentially could have an adverse effect on the pipeline integrity properties such as ductility, fracture toughness and fatigue performance. Fatigue is considered a major failure mode for Offshore pipeline girth welds where the longitudinal stresses can be as high as the actual yield of the line (sometimes even more) and subject to fluctuations that are significant during lifetime. Same conclusions can be observed for longitudinal flaws in the seam weld/base material in case the trunkline is used as a hydrogen storage tank. In that case high stress cycles contribute to a consistent fatigue crack growth with respect to natural gas scenario where the pressure and temperature fluctuations induce low stress cycles which have negligible effect on fatigue growth. The combination of embrittlement with the severe loads occurring to an offshore pipeline calls for a comprehensive awareness of material performance under such conditions. To achieve that, the first step has been the classification of failure modes by type of installation condition and selection of the tests required to characterize materials against them. Consequently, Saipem, as pipeline construction contractor, funded a test campaign to start filling the gap. The purpose of the testing campaign is to assess the behavior of base material, seam weld and Girth Welds under different hydrogen environments, to establish the limiting criteria for their resistance to H2 embrittlement, to contribute to the reduction of any over-conservatism in ASME B31.12 and bring solutions for the development of an achievable and affordable hydrogen pipeline network. This paper provides the experimental results in terms of fatigue endurance (S-N curves) and fatigue crack growth (FCGR curves) properties of testing performed in hydrogen environment at pressure of 200bar. Tests were also performed in air to compare the influence of hydrogen gas on the fatigue life of Saipem offshore girth welds. Additionally, a comparison of fatigue properties in severe H2S environment was conducted. The conclusions and the next steps of the campaign will be summarized in the paper, written for the sake of sharing knowledge and experience for a faster and efficient transition toward a sustainable and safe economy in offshore hydrogen transportation systems.