This paper investigates the optimal timing of the first inspection for newly-built onshore underground natural gas pipelines with respect to external metal-loss corrosion by considering the generation of corrosion defects over time and time-dependent growth of individual defects. The non-homogeneous Poisson process is used to model the generation of new defects and the homogeneous gamma process is used to model the growth of individual defects. A realistic maintenance strategy that is consistent with the industry practice and accounts for the probability of detection (PoD) and sizing errors of the inspection tool is incorporated in the investigation. Both the direct and indirect costs of failure are considered. A simulation-based approach is developed to numerically evaluate the expected cost rate at a given inspection interval. The optimal inspection interval is determined based on either the cost criterion or the safety criterion. An example gas pipeline is used to examine the impact of the cost of failure, PoD, and the excavation and repair criteria on the optimal inspection interval through parametric analyses. The results of investigation will assist engineers in making the optimal maintenance decision for corroding natural gas pipelines and facilitate the reliability-based corrosion management.

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