Research On The Law Of Liquid Loading In Natural Gas Transportation Line In The Undulate Terrain

In the process of wet natural gas pipeline transportation,pipelines must be laid from different angles due to the undulating terrain.The change of pipeline angle will be accompanied by the change of pipeline liquid holdup,which makes the flow situation more complex,and the accumulation of fluid will also be formed in the pipeline,affecting the transportation efficiency.Under this condition,the gas pipeline in operation will bear impulse stress impact,which will cause strong vibration,produce relatively large fatigue damage,increase the pressure along the pipeline wall,accelerate the corrosion of pipeline,destroy the oil-gas mixed pipeline and equipment,so that the downstream instrumentation and equipment can not operate normally.In addition,the accumulation of accumulated fluid will also reduce the effective flow area of the pipeline,affecting the transportation efficiency.At a certain low temperature,hydrate will be formed,resulting in ice jam accidents.Therefore,it is particularly important to study the liquid loading in undulating pipelines for ensuring the safety of pipeline transportation and improving the efficiency of pipeline transportation.In order to explore the influence of liquid loading on the formation of hydrops,FLUENT software was used to simulate pipelines with different fluctuations.The results show that the liquid holding rate of the upward inclined section is higher than that of the horizontal section and the downward inclined section,and slug flow is more likely to occur,which causes pressure fluctuation.Liquid loading generally begins to form at the inflection point from downward tilt to upward tilt,then gradually diffuses to upward tilt section,and eventually fills the whole upward tilt section.In order to get some guidance for production practice,the D405-D404 pipeline and the Changzhi fan fourth gas gathering station-processing center pipeline in Dawan block of Puguang gas field are simulated by OLGA software.The pressure drop along the pipeline,total liquid loading and liquid holdup are analyzed,and some guidance is given for the optimization of pigging period.The results show that when the flow rate is small and the length of the pipeline is long,the liquid loading will continue to increase for a long time,and the accumulated fluid and the pressure difference of the pipelinesection will be the main factors affecting the pigging cycle.However,when the flow rate is large and the length of the pipeline is not very long,the pressure drop of the whole pipeline will continue to increase.Therefore,it is speculated that there are other factors besides the accumulation of liquid which will continue to affect the efficiency of gas transmission.At this time,the liquid loading will not be the direct factor determining the pigging cycle.At the same time,referring to the Tuner model for calculating the critical flow rate of wellbore fluid accumulation,the formula for calculating the critical flow rate of undulating pipeline is deduced,and the three-section undulation model with different inclination angle is established and verified by OLGA software.