| Pigging is one of the important methods for improving the transfer efficiency of pipelines and guaranteeing its safe operations.For the undulating natural gas pipeline,due to the complicated parameters of the terrain along the pipeline,the liquid phase in the pipeline gathers in the low-lying area,which increases the difficulty in pigging pipeline.In contrast,for the submarine mixed pipeline,its high flow rate of the inlet liquid flow and its riser structure lead to the result that these pig movement parameters and the flow parameters of the fluid in the tube show more substantial fluctuations in the process of pigging.Therefore,through the numerical simulation of the transient pigging of both land-based undulating pipeline and submarine mixed pipeline,the variation of the relevant parameters during pigging can be mastered,which will be useful to improve the efficiency of pipeline pigging and provide guidance for an efficient and safe on-site pigging operation.In order to solve these problems above,based on the ACE algorithm,fluid mechanics,heat and mass transfer theory,and numerical solution method,the model of gas-liquid two-phase flow pattern and the calculation model of the liquid holdup and pressure drop are established by combining experiment,theory and numerical simulation.At the same time,the corresponding transient pigging model is established for the land undulating pipelines and the submarine mixed pipelines.On such basis,an in-depth research on the variation of the pig operating parameters and the flow parameters of fluid is conducted.The main research contents and results are shown as follows:(1)Based on the already enunciated gas-liquid two phases pattern,the basic flow patterns are divided into four types:stratified flow,bubble flow,slug flow and annular flow.Meanwhile,in combination with the experimental data of flow pattern,these flow patterns are evaluated by the method of programming calculation.On the basis of the evaluation result,,a combination model of two-phase flow pattern is established for different inclinations.(2)Based on the experimental data of the liquid holdup and the theory of ACE algorithm,the calculation model of the two-phase's liquid holdup is established by taking into consideration the factors such as diameter,inclination,superficial gas velocity,superficial liquid velocity,pressure,temperature and viscosity.Through the comparison and verification,it is concluded that the calculation result of the liquid holdup calculation model based on ACE algorithm has a higher accuracy.Besides,the Spearman correlation coefficient is used to comprehensively rank the influencing factors of the liquid holdup.In the end,the original Beggs-Brill pressure drop model is improved by introducing a new liquid holdup calculation model.The results show that the improved BB pressure drop model has higher accuracy(3)The motion characteristics and the influencing factors of the operating parameters of the pig in the horizontal pipeline,the inclined pipeline and the vertical pipeline are analyzed respectively,and the movement model of pig is established by considering the influence of the gravity of the pig itself.The stress condition of the pig under different inclination conditions is studied,and the basic forces in the model are analyzed and calculated in detail,which provides foundation for the transient numerical simulation of the pigging process of the pipeline.(4)As to the inclined pipeline's pigging process,a transient pigging model considering the thermodynamic parameters is established.According to the distribution of fluid and pig in the pipeline,the pipeline is subdivided into four regions,and corresponding transient mathematical models are established for each region.For the liquid slug region,because the liquid slug flow pattern will change along with the inclination of pipeline,a different transient flow model is established respectively for different inclination angles;for the two-phase region,the transient flow model of the two-phase flow is established,based on the continuity equation,the momentum equation and the energy equation of the two-fluid model,and the change behavior of the thermodynamic parameters in the process of pigging is also taken into account.At the same time,the mathematical model of mass,momentum and energy transfer between gas-liquid phase under the condition of thermodynamic equilibrium and non-equilibrium is also established.(5)The numerical solution method for the transient flow model of two-phase flow,and the coupling algorithm for the pig region,the liquid slug region,the upstream and downstream two-phase flow are studied.With the help ofthe transient pigging simulation program which is developed by the Matlab,the pigging process under different operation conditions and of the on-site pipeline is compared and analyzed.The results show that:1)the presented pigging model can describe the operation parameters of the pig and the change of the parameters of each node in the pipeline during the process of pigging;2)for the gas pipeline with the liquid loading,the accuracy of the average speed of pig and the pigging time,that are calculated according to the presented model,is higher than that of the theoretical value calculated on site;3)the liquid flow rate has a great influence on the operating parameters,pressure,and the temperature of each node in the tube.(6)The transient pigging model for submarine gas-liquid mixed pipelines is built.The pigging process of the submarine mixed pipelines is different,to a certain degree,from that of the undulate pipelines,due to the influence of the conveying medium,the structure of the submarine pipeline and the surrounding environment of the riser.The Lebizon formula is introduced into the transient flow model of the original liquid slug region,and through coupling with the movement mode of pig and the transient flow model of the two-phase flow,the simulation of the relevant parameters of the submarine pipeline during pigging is realized.Finally,the accuracy of the model is verified by comparing with the field pigging parameters of one submarine pipeline in the Bohai Sea.Meanwhile,some simulation study on the pigging process of different working conditions is carried out,such as the pipeline with or without riser or the extremely cold/hot condition,and the corresponding pig optimization programs,put forth. |
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