Risk Assessment on Subsea Gas Pipelines Located at Water Basin of Jetty Area due to Dredging and Operation After Dredging

Abstract

This study addresses risk assessment of 14″ gas pipelines buried closed to jetty area due to the plan to dredge the water basin at jetty area. The dredging plan is aimed to enable a more spacious maneuvering basin at the jetty area to enable larger vessel to be served and easier control of tug boat operation during berthing process. Before dredging, the gas pipeline is located 133 m from the slope. This condition provides enough protection to the gas pipeline since large vessel will not reach the pipeline due to vessel’s draft restriction. After dredging, however, gas pipeline will be located only 49 m away from the slope. The water area after dredging allows larger vessel will be in the vicinity of the pipeline and hence impose risk to the existence of the gas pipeline. Risk to the pipelines due to dredging activities (drop/drag anchor, drop clamshell/object, ship sinking, and ground instability) and due to various vessels operation after dredging (drop/drag anchor, ship sinking, and ship grounding) are assessed by implementing quantitative risk assessment. Hence, this study is focused on the assessment of risk to the 14″ gas pipeline due to dredging activity including risk assessment during operation of the jetty after dredging. Based on pipeline and environmental data, all possible hazards are identified. Some hazards are screened out using ‘Failure Modes and Effects Analysis’ (FMEA) to obtain the list of potential hazards. In order to evaluate the acceptance criteria of all potential risks, the risk profiles are composed according to DNV-RP-F107 “Risk Assessment of Pipeline Protection”. As part of the assessment, geotechnical assessments of submarine landslide due to dredging are also considered. The effective stress approach is implemented to the assessment and submarine slope stability is analyzed using Bishop’s and Janbu’s methods of analysis. The risk profiles for all potential hazards are reported, and simulation results for different slope ratios are given to illustrate the stability of slope configuration during dredging.