| Severe slugging is a kind of special harmful flow pattern frequently occurring in offshore pipelines,which can cause extremely serious pressure and gas-liquid flow fluctuations,and seriously threaten the safety of offshore oil and gas production.During pigging operation in the offshore pipelines,severe slugging can block the pig or cause sudden speed change of the pig,resulting in problems,such as increased pressure fluctuation in the riser,and the excessive load of the gas-liquid separation device over a short time.The bypass pig has the function of bypass fluid,which can reduce the pig velocity and riser pressure fluctuation,and decrease the liquid hold-up fraction at the front of the pig,thus reducing the capacity requirements of gas-liquid separation equipment on offshore platforms.Focusing on these problems,this paper mainly studied flow characteristics and pressure fluctuations control method of the severe slugging in the bypass pigging conditions,and aimed to overcome severe slugging and alleviate pressure fluctuations by using the bypass pig the riser system,combined with theoretical analysis,model derivation,numerical simulation and experimental research.The influence of the pig bypass fractions as well as the gas and liquid flow rates on severe slugging characteristics during the pigging operation was studied,and the slug dispersion process during the bypass pigging process was also studied.In addition,on the basis of the research of the fixed bypass pig(the bypass fraction is fixed),the method of controlling the severe slug flow by using the adjustable bypass pig based on PID control was proposed.Specific work and results are as follows:(1)Taking the declination-riser system as the research object,the bypass pigging process in severe slugging conditions was simulated by OLGA software.The influence of different pig bypass fractions and inlet gas-liquid flow rates on the pressure of the riser bottom,pig velocity and pigging operation time were analyzed.The results of OLGA simulation show that pigging operation without bypass hole in severe slugging conditions can greatly increase the riser pressure fluctuation and cause large pig velocity fluctuation,while using bypass pig can alleviate the riser pressure and pig velocity fluctuations.The larger the pig bypass fraction,the smaller the riser pressure fluctuation amplitude,while the inlet mass flow rate(with constant components)has little influence on the pressure fluctuation amplitude of the riser during pigging operation.(2)The OLGA simulation results were taken as the boundary conditions of the CFD model,and the CFD simulation of the dispersion slug process bypass pig was carried out by combining Overset and UDF(User Defined Function).The effects of different pig bypass fractions and inlet gas-liquid flow rates on the dispersion capacity of the liquid slug,gas-liquid flow distribution,liquid holdup fraction and mass flow rate at the riser outlet were analyzed.In addition,the CFD results were compared with the OLGA results.The results show that the highly speed gas flow through the bypass hole can disperse the liquid slug at the front of the pig,thus inhibiting the formation of severe slugging.Increasing the pig bypass fraction and inlet flow rate can enhance the dispersion ability of the bypass pig to the liquid slug,but the inlet flow rate has less influence in mitigating severe slugging than the pig bypass fraction.Increasing the inlet flow rate can lead to the increase of the outlet mass flow rate,which may increase the capacity requirements of gas-liquid separation equipment.(3)An indoor experimental system for pressure fluctuation characteristics during bypass pigging process in severe slugging conditions was set up.The length of declination pipe is 10 m,the height of riser is 4m,and the inner diameter is 51 mm.Water and air were used as the fluid material to carry out experimental research.The variations of riser pressure fluctuation and pigging operation time in different pigging bypass fractions,inlet gas flow rates and inlet liquid flow rates were analyzed.In addition,the dimensionless parameter is defined to compare the OLGA simulation results with the indoor experimental results.The experimental results show that the larger the pig bypass fraction,the smaller the riser pressure maximum and fluctuation amplitude.The riser pressure fluctuation amplitude during pigging process first increases and then decreases with the increase of the inlet gas mass rate when the inlet liquid mass flow rate is constant.When the inlet gas mass flow rate is constant,the inlet liquid mass flow rate has little effect on the pressure fluctuation amplitude of the riser system during pigging process.(4)The CFD model of the slug dispersion process by the adjustable bypass pig was established.The fluid force on the pig was obtained through the macro(Compute＿force＿and＿moment)in UDF,the pig velocity was obtained by Runge-Kutta method,and the pig bypass fraction was adjusted by PID control method.Thus,the pig can maintain a relatively stable speed in the riser system,reduce the flow fluctuation of the riser outlet,and make the pig have a certain ability to disperse the liquid slug.Through the CFD simulation of the slug dispersion process of the adjustable bypass pig based on PID,the gas-liquid flow distribution characteristics,and the pig velocity and the variations of the outlet mass flow rate in different inlet flow rates conditions were mainly analyzed.In addition,the coupling model of severe slugging and bypass pig is solved to analyze the control effect of adjustable bypass pig on the pressure fluctuation of severe slugging.The results show that compared with the fixed bypass pig,the velocity of the adjustable bypass pig is more stable,and the pressure as well as flow fluctuation of the riser is smaller during the process of dispersing the severe slugging.The study on the flow characteristics of severe slugging and the control method of pressure fluctuations in bypass pigging conditions in this paper can contribute to providing theoretical basis for the pigging operation of offshore pipelines,providing a new method for the control of parameters such as pressure and flow in offshore riser system,determining the pigging parameters,and preventing the occurrence of pigging accidents in different field conditions. |
