Decarbonisation of the energy supply has become a priority to help address climate change concerns. One method of decarbonisation is the use of hydrogen in place of or supplementing natural gas. The predominant method of natural gas transmission in the world is by pipeline. Many pipeline operators wish to re-use existing infrastructure that was not originally designed for hydrogen service. Such a re-purposing is a significant change to the operation of the asset that must be assessed.

The conveyance of natural gas by pipeline has a long history and is demonstrably safer, cheaper, and more efficient than alternative methods. That said, pipelines still pose a risk to their environment, which must be managed and demonstrated to be acceptable. A common method employed to demonstrate this is Quantitative Risk Assessment (QRA), which is based on mathematical modelling of consequence effects, historical statistical data, experience, and expert judgement. The methodology for natural gas has been validated through comparison with actual failures.

QRA is complex and it is not readily apparent that existing methodologies for natural gas pipelines are fit for purpose to assess the risk of pipelines conveying alternative fluids. This paper reviews the existing QRA methodology utilised for pipelines conveying natural gas to assess its suitability for use on pipelines conveying hydrogen and hydrogen natural gas mixtures. The outcome of the review highlights where knowledge gaps are present, potential resolutions to such gaps and proposed modifications to the existing methodology based on theoretical considerations and published literature, where appropriate, for use on pipelines conveying such fluids. To demonstrate the impact that conveyance of alternative fluids has on the risk posed, a comparative case study is presented. Furthermore, sensitivity studies are considered to highlight the effect that current knowledge gaps may have on the results.