Over the coming decade many projects will be initiated to convert existing natural gas pipelines to hydrogen service, often with ambitious schedules. However, there is currently little experience globally in successfully converting pipelines to hydrogen service and operating them safely. In North America, the gas transmission infrastructure represents construction from the 1920s up to the present day. This infrastructure contains a wide spectrum of different pipeline materials with large variations in properties, and an array of different resident and time-dependent integrity threats.

Prior to embarking on changes to hydrogen service, it is imperative to understand and mitigate the effects associated with the change in risk profile, driven by the known effects of hydrogen gas on the toughness and fatigue resistance of steel pipelines. These effects are complicated by the significant variation and uncertainty in the extent to which different pipeline materials will be affected.

Many papers and industry projects have examined the effects of hydrogen on material properties. However, few have assessed the scale of the challenge posed to safe operation and integrity management involved with repurposing an entire infrastructure. This paper uses a novel approach to explore how the current natural gas transmission network might stand up to a hypothetical switch to 100% hydrogen.

Available data gathered through inspections of gas transmission pipelines in North America will be utilised to create a virtual pipeline network. This virtual system is built from a range of different pipeline sizes, attributes and material properties to most closely represent a ‘typical’ proportion of the gas transmission network in North America. An array of cracks and crack-like integrity threats with sizes and morphologies that reflect typical frequencies and severities observed from real projects will be introduced into this virtual network.

This virtual network is a construct that allows the impact of hydrogen conversion on integrity management to be explored in a way that is representative of what a gas transmission pipeline operator may expect to encounter across a range of assets. The impacts will be explored through different scenarios, representing different extents of reductions in toughness and increases in fatigue crack growth rates, based on available material test data. This approach will provide an indication of the number of features that may be currently stable in natural gas but that may fail in hydrogen service. This hypothetical exercise will draw insights into the practicalities of safe operation of pipelines being contemplated for hydrogen service and the scale of the task that would be necessary to navigate this transition.

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