Study On The Pig Motion In The Waxy Crude Oil Pipeline

Wax deposits are normally to be seen on pipe wall during the transportation of waxy crude oil,which results in a reduction of the flow section or even a blockage of the pipeline.Pigs are commonly used to scrape the wax deposits.However,pigging operation in field depends on experience largely.It causes a high risk of a block.Studying on the pig motion to predict parameters,like pressure and flow rate,is helpful for making decisions about pigging.Firstly,the mechanical characteristics of the original wax deposits in field were systematically studied.An apparatus based on vane was designed and used to test the yield stress of wax deposits at different temperatures and wax deposits at different positions in the pipeline.The results show that the yield stress of original wax deposits is exponential to the temperature.The yield stress of inner-layer wax deposits is about twice as much as the surface-layer wax deposits.The structure of the original wax deposits changed greatly after it was melted and cooled,and its yield stress increased by 5¹3times.So it's necessary to take wax deposits in field as experiment materials.At the same time,DSC and Microscopic observation experiments were taken to explain the results.The average size of the wax crystals and the aspect ratio of the wax crystals in original wax deposits are 20⁴0?m² and 2³ respectively.This provides important references for preparing simulation wax deposits in room.Secondly,the data obtained above was used as material parameters in the FEM simulation.The scraping process of wax deposits while pigging was simulated.The results show that the pressure acting on wax deposits by the pig and the maximum stress in wax deposits are linearly related:?P=k?_max.And the slope of this linearly relationship decreases as the strength of wax deposits increases or the thickness of the wax deposits increases.However,it has nothing to do with the pipeline diameter.The forces between the pig with different oversize of the scraping disc and the pipe wall were also obtained A logarithmic relationship between the friction and the oversize was found:F_f=F₁ln???+F₂.The thickness of the sealing disc,the diameter of the separating disc and the friction coefficient do have great effects on this relationship and they are ranked as their degree of influence:the thickness of the sealing disc,the diameter of the separating disc and the friction coefficient.Finally,based the results above,a model to simulate the pig motion and the increasing slurry was established.The pig velocity,the pressure distribution along the pipeline,the pipeline inlet pressure and the outlet flow rate were obtained.The sensitivity analysis was performed on the important parameters in the calculation,like the friction force between the pig and the pipe wall,the thickness of the wax deposits and the viscosity of the slurry.The influence of these parameters on the calculation results were compared,thus providing theoretical support for operation in the field.Finally,the model was used to calculate the Northeast crude oil pipeline to validate its accuracy.