The Health and Safety Executive (HSE) uses a fracture mechanics model, PIPIN (PIPeline INtegrity model), to predict the likelihood of failure if a buried pipeline is struck by machinery (known as third party activity or TPA). The existing model uses a FORM/SORM (First/Second Order Reliability Method) to solve the equations, but the model fails to produce results for some scenarios. HSE asked the Health and Safety Laboratory (HSL) to rewrite PIPIN replacing the FORM/SORM methodology with a Monte Carlo solution method, with the aim of reproducing the results from the existing model as closely as possible. This report details the fracture mechanics within PIPIN, the Monte Carlo method and the process used to derive failure frequencies by specified hole sizes. Results are given for two sets of tests and these are compared against the existing model. In general, good agreement is seen between PIPIN and the new Monte Carlo version of PIPIN, with just 15 pipelines (approximately 2.5% of the dataset) showing significant changes. The effect on the land-use planning (LUP) distances of the revised failure rates has also been assessed. It was found that two pipelines saw a change to the inner zone, 39 to the middle zone and 21 to the outer zone.

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