Numerical Simulation Study Of Severe Slug Flow And Optimization Of Control Measures In Combined Pipe

During the exploitation of offshore oil and gas resources,multiphase transportation is carried out through the downdip pipe laid on the seabed and the riser connected to the offshore platform,which can not only greatly improve the transportation efficiency,but also save the cost of pipeline construction for single-phase transportation to a certain extent,and shorten the time required for pipeline laying.The flow of oil and gas two-phase flow in the pipeline is relatively complex.When the gas and liquid delivery volume changes or is small,serious slug flow is likely to occur in the pipeline,resulting in large fluctuations in the pressure,flow and other parameters in the pipeline,causing severe vibration of the pipeline and downstream equipment,and even causing damage to the pipeline or equipment in serious cases.In order to better understand the flow characteristics of severe slug flow in the pipeline system and the change rule of characteristic parameters,and to find effective methods to suppress or eliminate the severe slug flow in the downdip pipe-vertical riser system,it is of great significance to stabilize the production of deep sea oil fields,optimize the transmission process of offshore oil and gas resources,and ensure the safe operation of oil and gas mixed transmission pipelines and processing equipment.In this paper,based on the research object of the combination pipe commonly used in offshore mining,i.e.downdip pipe-vertical riser,the characteristics of severe slug flow in the combination pipe system are analyzed by using theoretical research and numerical simulation methods,and according to the actual oil field background,different methods are used to prevent and control the severe slug flow,and the following conclusions are obtained:(1)Based on the mass conservation equation and dynamic conservation equation,a one-dimensional transient model of four flow stages in the process of severe slug flow in the combined pipe system is established.The model can accurately predict the relevant characteristic parameters of severe slug flow.(2)Based on the principle of geometric similarity,the three-dimensional flow model is equivalent to the two-dimensional flow principle,and the two-dimensional geometric model of the composite pipe is established.Through the verification of grid independence,the number of grids is determined to be 85652 for numerical calculation.The Fluent simulation results are compared with the experimental results,and the error is found to be within 15%,thus verifying the accuracy of the two-dimensional numerical simulation.(3)According to the difference of liquid flow rate in the downcomer,four flow types were successfully simulated by Fluent: severe slug flow type I(SSI),severe slug flow type II(SSII),severe slug flow type III(SSIII),and stable flow.SSI type has strict periodicity,and the flow stage includes liquid plug formation,liquid plug outflow,liquid gas eruption,and liquid reflux.Type SSII lacks liquid plug outflow stage due to high gas flow rate;Type SSIII is lack of liquid reflux stage due to high liquid flow rate;Stable flow has no severe slug flow at any characteristic flow stage,and the flow process has always been gas-liquid alternating flow without periodic characteristics.(4)In severe slug flow SSI type,the amplitude of pressure fluctuation increases with the increase of gas phase converted velocity,and in severe slug flow SSII type,the amplitude of pressure fluctuation decreases with the increase of gas phase converted velocity;The increase of gas-liquid equivalent velocity will speed up the time of gasliquid ejection stage,and the increase of liquid equivalent velocity will speed up the time of liquid slug formation stage,that is,the increase of gas-liquid equivalent velocity will shorten the period of severe slug flow.(5)Based on the simulation analysis of severe slug flow phenomenon by using the combined action of gas injection method,throttling method,and gas injection method plus throttling method in OLGA,it is found that gas injection method,throttling method,and gas injection method plus throttling method can eliminate severe slug flow.Compared with single gas injection and single throttling,the combined action of gas injection method and throttling method is the best method,and its optimal parameter is gas injection volume 16.2kg/s + valve opening 0.2.