An improved model for severe slugging stability criteria in offshore pipeline-riser systems

Abstract

Flow assurance has been a major challenge in petroleum production systems especially in offshore operations where severe slugging is a dominant aching issue. This is characterized by pulsating flow behavior which impacts the reservoir productivity, pipeline-riser systems, and surface facilities (separator). The only means so far, of delivering reservoir fluids from offshore wells to the separator at the surface is by pipeline-riser systems where slugging is most prominent and oftentimes unavoidable. Though passive and active measures have been deployed over time to mitigate the associated problem, there is the need to develop a predictive tool for mitigating offshore slugging in the riser system. This study involves the use of a model and analysis approach to eliminate severe slugging based on the balance of forces acting on the pipeline-riser system. Modified stability criteria which consider accumulation and frictional pressure drops have been developed. The current approach is solely physicsdriven, to predict the corresponding liquid and gas velocities at which the fluctuations would be severe when producing multiphase fluids through pipeline-riser systems. By balancing the forces acting on a riser-pipeline system and accounting for frictional and accumulation effects, two hydraulic flow modeling approaches have been used in this study to improve the criteria for predicting the condition of severe slugging. The developed criteria were computed using MATLAB/Simulink and a severe slugging flow map was generated. The “new study 1” and “new study 2” showed a 16% and 13% increase respectively in predicted unstable cases than the Malekzadeh criteria. The criteria were tested at different separator pressures. The Taitel and Jansen criteria could not predict severe slugging at elevated separator pressure. Notwithstanding, the developed flow maps using the “new study 1” are more reliable than the “new study 2”. This is because the “new study 2” approach over defines the physics of severe slugging. The criteria were also tested at varying accumulation times. This was done by step increments in the order of magnitude of time. The separator pressure affects the optimal time for modeling the accumulation at the riser-base. The criteria are therefore useful for predicting optimal operating conditions for flow assurance for the pipeline-riser system in consideration.