This paper is concerned with prediction of delayed failure frequency due to third party damage for pipelines carrying natural gas and hydrogen mixtures. The study reported in this paper is part of European NATURALHY project, whose aim is to investigate the possibility of using the existing natural gas transmission pipelines to convey natural gas/hydrogen mixtures. According to the EGIG database, the most common cause of failure for the existing natural gas pipelines is third party damage, which mainly refers to a gouge, a dent/gouge combination of known geometry. Among all third part damages, 90% of them will lead to immediate failure i.e. leakage or rupture of the pipeline and only 10% of them will cause delayed failure. Some of the damage is associated with the dent only rather than a dent-gouge, which is a more severe situation. As Europe is gradually moving towards hydrogen based society it has been acknowledged that adding certain amount of hydrogen, as a clean energy carrier, to the existing natural gas pipeline will help reduce the CO2 emissions which contribute to the greenhouse effect. On the other hand, hydrogen has been demonstrated to be able to change the behaviour of pipeline steel such as a lower toughness and faster crack growth due to hydrogen embrittlement. Therefore, it is necessary that the risks associated with the delayed failure arising from third party damage of the pipeline carrying mixtures of natural gas and hydrogen be assessed. A software tool has been developed to help fulfil such requirements.

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