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With the method of characteristic (MOC) and analysis for the dynamic state of trapped air, this study could be performed on the basic theory of gas-liquid two phase unsteady flow, and cavitation and bubble dynamics, to obtain the overpressure value during pigging. For a long slope pipeline, a severe rupture occurred in the vicinity of a drainage pipe during a segmental pigging process. In contrast to earlier accidents induced by hydraulic transients, the unique combination of topography and geometry of the drainage pipe was a key factor. Generally, a pipeline profile complexity has a significant effect on both the liquid-fill flow behavior and the state of trapped air, both of which are factors contributing to cavitation and water hammer. Pigging is a common technological process that has been studied for many years, but has rarely been analyzed by hydraulic transient methods with consideration for pig motion in a pipeline. From local data, it is shown that transient pig motion has a huge influence on operating conditions, especially outlet pressure. The purpose of this study was to identify the damage mechanism. Following extensive study, it was determined that cavitation and vapor cavity collapse cause instantaneous overpressure due to the interaction of topography, geometry of the drainage pipe, and pig motion. It would be rather difficult to simulate the pig motion by a conventional MOC and the simple water hammer equation for the collapse of large vapor cavities at a high point. Therefore, a new approach which describes the moving boundary with a dual-grid model has been implemented, along with an explicit solution procedure.

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