Long-distance transmission pipelines with 3PE coating undergo a two-stage failure process, including the damage process of three-layer polyethylene coating and the failure process of metal pipelines. The two stages are random and independent of each other. A reliability model of transmission pipelines under periodic inspection is presented with the assumptions of delay-time concepts. Functions of maximum likelihood estimations and the goodness of model fit are given. The effect of the inspection interval on reliability is investigated. A case study of Xinqin natural gas transmission pipelines is presented. Data of the pitting depth are obtained at 43 excavation locations. Best estimations of distribution parameters are obtained in a genetic algorithm search. Results show that the average time of coating damage is around 1.04 years. The failure probability of a pipe segment after 36 years’ operation is 1.1E−3 if no inspections are carried out. The failure probability is around 5.5 times the failure probability of a pipe segment with a 6-year periodic inspection. The sensitivity of variance of distribution parameters to failure probability is presented.

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