Premothballing, Mothballing, and Recommissioning Method for 762-mm and 102-km Offshore Gas Pipelines

Abstract

In the coming years, gas production in Indonesia is projected to decrease so that the option to abandon a plant and temporary shut down will increase. The 762-mm and 102-km offshore gas pipeline is planned to be shut down for 4 years. Therefore, the comprehensive method from the premothballing, mothballing, and recommissioning stages should be developed. In this study, these methods have been developed and the pipeline can be preserved for 4 years with some remarks where the coating repairs, freespan rectification, and analysis of the sacrificial anode remaining life must be performed before premothballing stage. In the premothballing stage, 4 different pigs are used to clean the pipeline until the deposit released is less than 20 L. In mothballing, the internal pipeline is filled with nitrogen gas to prevent the corrosion. The injection of nitrogen gas will be conducted by the train pig method, where each train pig is pushed by nitrogen gas. The internal pressure of the pipeline must be maintained at 1.1 MPa and should not be less than 0.9 MPa to avoid buckle propagation. Continuous maintenance and monitoring should be conducted on the pipeline. The corrosion coupon monitoring should be performed every six months, pressure and right of way (ROW) monitoring should be carried out every two weeks, and regular monitoring and maintenance reports should be conducted at least once a year. Recommissioning begins with evaluating data and conducting remotely operated vehicle (ROV) inspections. Nitrogen gas displacement is performed using a train pig driven by fresh nitrogen gas on two pigs and the production gas should be streamed to push the third pig. The inline inspection (ILI) must be performed to determine the percentage of actual metal loss. The ILI results can be used to perform remaining life analysis and as requirements for pipeline recertification so that the pipeline can be reoperated.