Understanding the Impacts of Backpressure & Risk Analysis of Different Gas Hydrate Blockage Scenarios on the Integrity of Subsea Flowlines

Abstract

Offshore oil and gas pipelines are subjected to high pressure and high temperature (HP/HT) from the inner hydrocarbon content during operation. Both the rise in temperature and internal pressure may cause longitudinal expansion of the pipeline. This expansion is restrained or semi-restrained by the pipe end devices and the soil which results in build-up of compression stresses in the pipe wall. These pipelines are also exposed to so many familiar and unfamiliar forces related to static, dynamic and environmental forces. This study presents a thorough review of various sources from literature on the integrity challenges of subsea flowlines and pipelines amid challenging operating conditions especially with regards to flow assurance. This paper evaluates the impact of hydrate deposition and agitation on the overall integrity of the subsea flowlines, riser-base and fitting e.g. elbows, valves e.t.c. A bow tie model was developed to determine the threats, causes, consequences, the top event and the impending hydrates that are to be designed and cause blockage and failure. Stress analysis were done with finite element tools which are ANSYS and Autodesk INVENTOR with only the hoop, Von Mises stress and the corresponding back pressure that occurred with the scenario of 0, 10,30,50,70,90 and 100% blockage of flowlines being analyzed and taking the 0% or null blockage as the pilot case with no hydrate formation. The result gotten from both results were validated with hand calculation with excel and the initial design values for the stress values before the initial operation of the wells after the first commissioning. In addition, HAZOP was done to understand the inherent risk involved in all the cases under study and results gotten would serve as a tool of precautions to operators and stakeholders in period of adversity when facing similar scenario.