Fracture and fatigue testing was performed on the parent and weld material of a vintage offshore API 5L X60 pipeline to assess the impact of various pressures of H2. Fracture toughness values of samples exposed to H2 environment were lower than in air. Fatigue performance was consistent with published literature and ASME B31.12 for ΔK above 5 MPa√m. At ΔK below 5 MPa√m the fatigue crack growth is higher than predicted, possibly driven by a time dependent sustained cracking mechanism in certain microstructures. The practical relevance of these observations is presented in the context of the integrity assessment required to repurpose a pipeline for hydrogen, highlighting the importance of material performance in a H2 environment.
Hydrogen is becoming increasingly important as part of the energy transition and net zero initiatives. Pipelines are considered a cost-effective method for transporting larger volumes of gaseous hydrogen over longer distances, replicating the current natural gas supply chain (International Energy Agency, 2019; Wang et al., 2021). Reusing existing carbon steel natural gas transmission and/or other pipelines for hydrogen service is an attractive option. Provided existing line pipe materials and integrity management methodologies can be safely and economically adapted for transporting hydrogen, repurposed pipelines are a way to enable expansion of the hydrogen value chain at a much lower cost. A recent publication by Wang, et al. (2021) suggests that transmission by repurposing existing pipelines would be a cheaper option for distances below 5,000km.
There are about 4,500 km of hydrogen pipelines in operation today, which are mainly operated at stable pressures below 70 bar, by a few industrial gas companies (ASME, 2005; HyArc, 2017). However, existing H2 pipeline designs in the US tend to use low-strength grades of carbon steel, smaller diameters under 20 inches and conservative design factors that result in high wall thicknesses (in-house analysis of data from www.phmsa.dot.gov, accessed June 2020). Within these limitations it is likely to be difficult to economically achieve the high throughputs anticipated for the future hydrogen value chain in the long term without expanding the operating envelop outside current design practices, as summarised by Topolski et al. (2022). Larger diameter natural gas transmission pipelines designed with larger design factors for higher pressures are attractive to consider for reuse in hydrogen – the principles discussed by Fekete, Sowards, and Amaro (2015) can also be applied to reusing exiting pipelines. However, operators need to be cautious when stepping outside the current operating window and ensure that any assumptions made will not introduce unexpected failure mechanisms.