The Research On Bypass Pig Movement And Liquid Loading Characteristics

Natural gas condensate pipelines often face with the problems like serious liquid loading and increased frictional resistance after long run,which seriously affect the normal production of oil and gas fields.Regular pigging operations are an important guarantee for pipeline flow safety.Due to the problems of fast pig velocity and large pig-generated slug volume caused by conventional pig,bypass pig has the advantage of lower pig velocity and pig-generated volume,making it of broadly prospects when applied to gas field pipelines.On account of the limitations of unclear mechanism,obscure movement rules,and insufficient theoretical framework for engineering applications in current bypass pigging technology,the paper combines theoretical research,numerical simulations,and experimental analyses to develop a wide range of in-depth studies on bypass pig movement and liquid loading characteristics,aiming at comprehensively elucidating the internal movement mechanism of bypass pigging and laying a theoretical foundation for promoting the applications of bypass pigging technology.In this paper,a dynamic pigging simulation on a deep-water condensate gas field was conducted by software OLGA and LedaFlow.The effects of pig structural parameters: pressure drop coefficient,bypass fraction and friction force,on the pig velocity and piggenerated slug volume were analyzed.The dissipative characteristics of pig-generated slug volume along the pipeline were investigated and the differences of the movement laws between natural gas condensate and liquid water during the bypass pigging process were compared as well.The study shows that the increase of the bypass fraction can significantly reduce the condensate slug,but the elimination effect of the water slug is weak.The presence of liquid water will weaken the effect of bypass pigging to some extent.When optimizing the bypass fraction,it is necessary to ensure that the volume of pig-generated slug is within the processing capacity of the terminal slug catcher,and meanwhile the pig velocity is appropriate and fluctuates stably.Secondly,the paper established the bypass pig equivalent pressure drop coefficient calculation model,of which the calculation methods with complex and variable pig structures are analyzed.When compared with the numerical software FLUENT calculation results,the deviations between them are less than 7%,suggesting the accuracy of the model.Then the FLUENT was applied to study the distribution of the liquid loading in front of bypass pigs,and a mathematical model of the liquid surge volume at pigging conditions was established.The factors affecting the liquid surge volume were analyzed,and meanwhile the slug controlling methods were proposed.The research shows: the morphological distribution of liquid loading depends on the balance between the push of bypass pigs and the carrying of driving gases.The presence of bypass fractions enhances both beneficial effects.The elimination effect of the liquid surge volume is more intense with the increase of the terminal liquid drain rates at the condition of a smaller liquid phase fraction in the pig-generated slug.Combined with the opening controlling method of the outlet valve in the slug catcher,the bypass pigging technology can effectively reduce the liquid surge volume,realizing the aim of liquid loading under control.Finally,a visualized horizontal pipeline experimental system was established equipped with a brand new self-adaptive bypass pig prototype.The studies carried out the bypass pigging experiments both in single gas phase and gas-liquid two-phase conditions with the intention of comprehensively clarifing the movement rules of average pig velocities,sectional pig velocities,pressure fluctuations along the pipe and terminal slug outflow characteristics.The study shows that the average pig velocity in the single gas phase condition is linearly related to the rear driving gas velocity.When the bypass fraction is maintained,the difference between them is constant;and with the increase of the bypass fraction,the difference increases.The existence of the liquid phase increases the randomness of the pig's movement,and the increase of the bypass fraction can smooth the fluctuation of the pig.The single gas bypass pig steady-state model calculation results are in good agreement with the experimental data,which means the model can be used to analyze the internal relationship between the pig velocity,driving gas velocity,bypass fraction,friction force,and pressure drop coefficient.The appropriate bypass fractions make the pressure PDF curve more prominent and concentrated distributed with a single-peak feature,meaning it can ease the pig velocity fluctuation,pressure pulsation,and reduce the stick-slip characteristic.The optimization of bypass fractions can eliminate the pig-generated slug volume,reduce the periodic pressure violent oscillation under the gas-liquid phase conditions,facilitating the pig of a better adaptability to the liquid pipeline.Through the summary and refinement of the full-text research contents,the design criteria of an adaptive bypass pig applied to gas pipelines are proposed at the end of the paper.