Tribological Research On Mechanical Wax Deposits Removal Process

The pigging operation is an important maintenance process for the flow assurance of oil and gas pipelines.Most of the oil produced in China is heavy oil,and the problem of wax deposition during pipeline transportation is particularly prominent.There are many common ways to remove wax deposition,of which mechanical pigging is the most common method.Due to the lack of research on mechanical wax deposition process,there are no reliable theroy to guide pigging process and current field operations rely mainly on the experience of operators.This will bring a great operational risk for pipeline cleaning.Therefore,it is necessary to carry out related research on mechanical wax deposition removal process to provide reliable theoretical guidance for pigging operations.In this paper,the tribological characteristics of mechanical wax deposition removal process are studied by experiments and numerical simulation methods.According to the actual working conditions of the wax deposition pigging process,some test devices had been designed like the mechanical wax removal device in the multi-phase flow pipeline;the friction force in the breaking process had been investigated;triboligical behavior of the waxy oil gel had been studied;and rubber friction in the waxy oil gel had been investigated.There are some disadvantages like uncontrollable friction force when bi-directional pigs remove wax deposition,so the design and research of the pig with controllable split discs is proposed.The main research results are as follows:(1)Mechanical cleaning wax deposits of multiphase flow piplines simulation test device and a crescent-shaped wax deposit mold had been designed and made;these devices were used to simulate the process of cleaning the crescent-shaped wax deposit of the oil-gas two-phase flow by bi-direction pigs.The in-situ observation of cleaning wax deposits test device had been designed and made to observe the wax deposit breaking process.(2)The migration characteristics of wax deposits during wax removal had been studied by in situ observation method;the variation of growth rate of wax chip and shear angle under different conditions of speed,wax concentration and wax deposit thickness had been analyzed;the variation of the wax breaking force under different conditions of speed,wax concentration and wax deposit thickness had been analyzed;combined with the cutting principle,the mechanical model of wax deposition removal process using bi-direction pigs had been established and the prediction method of breaking force of wax deposit had been proposed.(3)Tribological behavior of wax-oil gels aganist wax-oil gels or steel under different speed,load and wax concentrations had been studied by waxy oil gel test device;the friction mechanism model of waxy oil gel had been established and the influence of factors on waxy oil gel had been analyzed;the variation of wax plug resistant force during wax removal had been analyzed;and the prediction method of wax plug resistant force had been proposed.(4)Aiming at the frictional force between the disc and pipe wall during the wax removal process,tribological behavior of rubber discs under the waxy oil gel were studied,and conditions of different wax concentration,speed,load and material characteristics were revealed.The variation of the friction coefficient of rubber discs material provides a prerequisite for the finite element numerical method to evaluate the frictional force between the rubber discs and pipe wall during the wax removal process by pigs.(5)Aiming at the shortcomings of bi-direction pigs on cleaning wax deposition,a new type of pig with controllable split discs based on friction force had been designed.The friction force of the controllable split discs had been studied by numerical simulation and experimental methods under the conditions of angle,thickness,clamped rate,oversize rate and flaps.It pointed out that the controllable split discs were most affected by the thickness,the clamped rate secondly,and the interference least.It showed the feasibility of the working principle of the controllable split discs.