Subsea pipeline leakage can result in significant operational downtime and lead to severe environmental damage, along with considerable economic loss. This paper studies the flow behaviours and the potential of hydrate formation risks due to pipeline leakage in subsea natural gas (NG) pipelines with water depths in excess of 2,000 meters. Depending on the operating conditions at a given location, pipeline leakage can cause water ingress into pipeline and/or gas egress at both gas transportation and pipeline settle-out conditions. Water entered into the pipelines can form hydrates which may be carried away by the NG transported, or could aggregate and subsequently deposit and form partial or full blockage inside the pipeline.

In this study, OLGA simulations were performed to investigate the different flow behaviours with water ingress and gas egress at representative locations where the pipeline internal pressure was similar to the external hydrostatic head or significantly different. A detailed Computation Fluid Dynamics (CFD) study was used to comprehend the complex flow behaviour of gas and water in the vicinity of the leak, which contributes to the understanding of hydrate formation, aggregation and transportation in the system. The risk of hydrate formation was assessed based on the hydrate dissociation curve and the system operating conditions. OLGA hydrate kinetics model (CSMHyK) (originally developed to predict the kinetics of hydrate formation inside an oil-dominated flowline) has been used to assess the hydrate particle growth kinetics in this paper. Hydrate particle agglomeration and transportation inside the gas pipeline based on the system thermal hydraulic behaviours and the liquid distribution and accumulation in the vicinity of the leak derived from OLGA and CFD simulations. Sensitivity studies of the impact of heat and mass transfer on the rate of hydrate formation were carried out. Finally, studies on the system responses under a partial or full pipeline blockage scenario due to hydrate plugging were mimicked by varying the valve opening.

This study sheds light on the system behaviour if a pipeline leakage occurs. The different flowing characteristics observed for water ingress and gas egress at various locations helped to locate and diagnose a pipeline leakage. These predictions can help the development of pipeline integrity monitoring systems and emergency response plans. Furthermore, this paper also provides good insight into the capabilities and limitations of the hydrate kinetics and transportation models available in the industry for a gas dominated system.

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