Research On By-pass Pigging Process Of Submarine Natural Gas Pipeline

In recent years,the exploration of oil and gas resources has gradually shifted from land to sea.In order to ensure the safe transportation of submarine pipelines,reduce the corrosion of pipelines and improve the efficiency of pipeline transportation,the submarine pipeline needs regular pigging operation.When the traditional pigging device is used in the pigging of the natural gas pipeline in the sea,the backpressure is raised and the pigging movement is carried out by means of pressure difference during pigging operation.This kind of traditional pigging mode causes the pig to be too fast and unstable,especially in the sea pipe riser,the pipe has a large difference in height,and the amount of fluid in the tube is larger when running,and the corresponding slug in the pigging operation will be more serious.In order to reduce the influence of the plug on the downstream facilities and to meet the design of downstream process flow during the pigging period,a suitable pigging process must be selected to ensure the safe operation of the pipeline.And the bypass pig applies to the submarine pipeline can effectively adjust the speed of the pig,reduce the pressure fluctuations and the terminal fluid flow.First of all,this paper has summed up the domestic and foreign literatures,then studies the relevant theoretical knowledge of pigging,and completes the theoretical analysis of the mathematical model of the bypass pigging process,the force analysis of the bypass pig,the analysis of the jet hole throttling analysis and the analysis of the starting and blocking process of the pigging machine based on the previous work.A typical simple subsea pipeline is used to analyze the main influencing factors of pigging processes.With two real condensate gas submarine pipeline in our country as the research background,used OLGA software to simulate the process of bypass pig pigging,completed the optimization of pig bypass ratio and hydrate formation and forecasting work.The summary is as follows:It is widely investigated that structure and sealing material of the pig,and the leather bowl steel axis pig is selected as the basis of bypass pig desigin by comparison.Through the simulation,the bypass design of the pig is selected through the single hole,and the opening position is set in the center of the pig.We analyzed the force of the pigging and studied the effect of frictional resistance on the bypass pig,and analyzed the friction resistance due to gravity of the pig itself,the pig pressure,and the elastic compressive force due to the pressure difference of the pig upstream and downstream.The research has analyzed the physical model of the physics,and set up the basic differential equations,and set up the steady-state model of the gas line,and the transient model,which provides a theoretical basis f-or the simulation of the process of the pigging.The influence of the pipeline diameter,the excess volume of the pig and the tilt Angle of the riser on the pigging process was analyzed by OLGA simulation.We used the pig with bypass rate of 0%-18%to simulate the pipeline pigging process of an instance.Simulation results show that,compared with the ordinary pig,bypass pig used will slow down the running speed of pig,prolong the time needed for pigging,reduces the pipeline pressure fluctuations and to reduce the pipe plug flow.It is preferable that that bypass rate of the pig is selectivity from the pipeline pig run speed,pipeline pressure fluctuation,the flow rate of the liquid plug and the flow rate of the vertical pipe,and finally it is determined that the optimum bypass ratio of the bypass pig is 12%?15%.The principle of hydrate formation in pigging process was analyzed,and the control measures of hydrate were selected.The temperature pressure curve of hydrate was obtained by using PVTsim software.Based on the actual sea pipe data,the formation of hydrate in the pipeline is running normally and pigging condition was simulated.To establish a valve flow model in a place where the pipeline is jammed,the valve opens to the next to the pig's bypass,and it simulates the formation of the hydrate in the simulation.Under the three conditions,the minimum MEG injection quantity was 0.31,0.44 and 0.45,and the final recommended hydrate inhibitor MEG was 0.45.