This paper is concerned with predicting the probability of failure (POF) of pipelines containing cracks and corrosion defects. The current work was conducted within the framework of the European Union NATURALHY project.

The aim of the NATURALHY project is to investigate the possibility of using the existing natural gas transmission pipelines to deliver hydrogen or natural gas/hydrogen mixtures. Hydrogen has been demonstrated to accelerate defect growth which may affect the safety of pipeline and make it more expensive to operate and maintain. Failure of a structural member such as a pipeline may occur when a crack becomes unstable or metal loss corrosion leads to leakage or rupture of the pipeline. Based on fatigue reliability analysis, statistical methods are applied to assess the probability of failure of pipelines containing cracks like defects and corrosion defects.

A software package has been developed based on the stochastic approach to assess the POF of the gas pipeline due to the existence of cracks and corrosion defects. The inspection and repair procedures are also included in the tool. With various parameters such as defect sizes, material properties, internal pressure modelled as uncertainties, a reliability analysis based on BS 7910 level-2 failure assessment diagram is conducted through stratified Monte Carlo simulation. Inspection and repair programmes and realistic pipeline maintenance scenarios can be simulated. In the data preparation process, the accuracy of the probabilistic definition of the uncertainties is crucial as the results show that the POF is very sensitive to certain variables such as the defect sizes and the defect growth rate. The POF for a single defect with known dimensions and the POF for a system containing multiple defects can be computed separately. Various inspection and repair criteria are available in the Monte Carlo simulation whereby an optimal maintenance strategy can be achieved by comparing different combinations of repair  procedures. In addition, the hazard function can be obtained based on the POF results, so that the POF calculation tool can be used to satisfy certain target reliability requirement. Several examples are presented in the paper comparing the POF for a natural gas pipeline with the POF for a pipeline containing natural gas/hydrogen mixtures.

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