Numerical Study Of The Influence Of Continuous Riser Base Gas Lift On The Characteristics Of Severe Slugging

Severe slugging,as an unstable flow process occurs in offshore multiphase pipeline-riser system,often results in periodic violent pressure and flow rate fluctuations and therefore,it poses a detrimental effect on production.To simulate this phenomenon,some mathematical models have been built.However,most of these models are limited to classic severe slugging and cannot handle other unstable flow processes in pipeline-riser system.When it comes to modelling the elimination methods for severe slugging,compared with choking at the top of the riser,there are few numerical studies on riser base gas lift.In addition,researchers often adopt some stability criteria as a tool to analyze the stability of two-phase flow in pipeline-riser system.As a prerequisite,it is necessary to verify the accuracy and applicability of these criteria.To address these issues mentioned above,theoretical studies were conducted in this paper.Based on experimental phenomena and physical mechanism,a one-dimensional quasi-equilibrium hydraulic model was built to simulate severe slugging in pipeline-riser system.A drift-flow correlation was used as closure law and the method of characteristics was employed to solve the partial differential equations.The developed model can handle discontinuities of two phase-single phase interface in both the pipeline and the riser.This model is capable of coping with different flow types,namely cyclic flow with fallback,cyclic flow without fallback,unstable oscillations and steady flow and predicting the flow parameters,such as liquid penetration into the pipeline,pressure fluctuations and cycle time.The model is tested against a broad range of data in literature showing excellent agreement.So far,most of investigations of riser base gas lift to eliminate severe slugging are limited to experimental studies and few theoretical studies had been carried out.In this paper,a one-dimensional quasi-equilibrium hydraulic model was developed to simulate the flow in the pipeline-riser system under the condition when the gas was injected at the bottom of the riser continuously.This model is able to predict the flow parameters in different flow types,namely cyclic flow with fallback,cyclic flow without fallback,unstable oscillations and steady flow.In general,a good agreement between the experimental data and the numerical results was found.Combined with the experimental data in literature and the simulation results,the influence of continuous gas lift on the flow parameters,such as liquid penetration into the pipeline,pressure fluctuations,cycle time and the flow type was assessed.There are many stability criteria for the flow in pipeline-riser system,for instance,Taitel-Dukler stability criterion of stratified flow,B?e criterion,modified B?e criterion,Taitel criteria and Jansen criteria.To validate and assess the accuracy and applicability of some of these commonly used criteria,the curves generated by these criteria in the flow patterns were tested against the experimental data reported in literature.Besides,the influence of continuous riser base gas lift on the flow behavior and flow stability was further investigated in the flow patterns with assistance of some stability criterion curves.