Study On Wax Deposition Prediction And Bypass Pigging Of LH16-2 Crude Oil Pipeline

Most crude oil produced in China belongs to high viscosity waxy crude oil which is easy to coagulate.At present,the research on wax deposition characteristics of onshore hot oil pipeline is relatively complete,while the research on wax deposition characteristics of offshore light waxy crude oil gathering and transportation pipeline is less.At the same time,pigging,as a common measure to remove wax in pipelines,has the risk of wax plugging.In this paper,LH16-2 crude oil pipeline is taken as the research object,and the wax crystallization simulation of waxy crude oil and the prediction of wax deposition on the pipe wall are carried out,and the wax removal process of bypass pig is studied,so as to provide a basis for the formulation of paraffin removal scheme of LH16-2 crude oil pipeline.The main research contents and conclusions are as follows:(1)Molecular simulation of wax evolution in crude oil system is carried out,and the process of producing ordered wax crystal structure in disordered crude oil system is divided into three stages:carbon chain extension,wax crystal nucleation and wax crystal growth.It is found that asphaltene accumulates in the process of wax extraction through parallel or vertical accumulation.LH16-2 crude oil has good properties,low asphaltene content and compact wax crystal structure.Due to the lack of space between asphaltene aggregate and wax crystal in the deposition process,the content of liquid oil in the sediment is less but the wax content is higher,and the yield stress of wax layer may increase,which makes the pigging operation more difficult.(2)According to the reported results of the loop test,the wax deposition in the LH16-2crude oil pipeline is analyzed.The wax deposition in the pipeline is more serious under the lower seabed temperature,and the deposition rate is faster and the hardness of the wax layer is higher.The thickness of wax deposition along the LH16-2 crude oil pipeline is predicted,and it is found that there is a temperature region with severe wax deposition between high temperature and low temperature when operating under the condition of large temperature difference.After comprehensive consideration,the maximum wax deposition thickness of 2mm and the amount of wax removed during pigging process should not exceed 10m~3 are taken as the basis for determining the wax removal cycle of oil pipeline.(3)The wax removal principle of the bypass pig is analyzed.The finite element method is used to simulate the resistance of the wax deposition layer to the pig and the calculation formula of the friction between the pig and the pipe wall.The increase of the thickness of the straight plate and the diameter of the isolation plate will increase the friction force.The bypass pigging model of crude oil pipeline is established,the flow field characteristics and the migration trajectory of wax debris are studied,and the bypass structure is optimized,and a straight plate bypass pig for wax removal is designed,which can adjust the bypass rate.(4)Numerical simulation of operation characteristics of bypass pig,loop test and onsite wax removal operation are carried out.The results show that the straight plate pig can effectively scrape the wax deposits on the inner wall of the crude oil pipeline,and under the action of the bypass hole jet,the scraped wax fragments cannot accumulate in front of the pig to form wax plugs,which are blown away and flushed into the downstream process.Bypass pigging can realize online wax removal of deepwater light waxy crude oil pipeline and ensure the efficient operation of LH16-2 oilfield pipeline.