A Study On Temperature Field Of Buried Hot Oil Pipeline After Shutdown With Wax Precipitation Characteristics

Buried pipelines are widely used in the field of oil and gas pipeline transportation in China because of less influential factors,security seal and long-time operation.At present,most of the crude oil produced in china are waxy crude oil,which has characteristics of high viscosity and high condensation point.After pipelines shutdown,the temperature of crude oil will gradually decrease,viscosity will increase,wax crystal will gradually form into reticular space structure as the shutdown time increase,finally congelation accident will occur and make restart process difficult.This dissertation aiming at the temperature field of buried hot oil pipelines after shutdown,it is of great significance to determine the shutdown time of pipeline,to formulate the restart scheme and to optimize the operation of pipelines.Therefore,the temperature field of crude oil inside pipelines and soil around pipelines during the unsteady process are studied in this dissertation,main contents are as follows:1.The change of heat transfer mode,the process of temperature drop and the influential factors of crude oil after shutdown of buried hot oil pipeline are researched,the process of wax precipitation and the formation mechanism of solid wax are provided.The wax point is detected by Differential Scanning Calorimetry,the latent heat of wax precipitation and the influence of the latent heat on the specific heat capacity are analyzed in this dissertation.2.The coupled physical model is established,simplified according to the soil constant temperature layer theory.The mathematical models of pipeline after shutdown are established by enthalpy method,respectively zoning and heat capacity method respectively,then capacity method is used by comparison which is more suitable for phase change problems.3.The shutdown process of buried pipelines is simulated by numerical method,the change law of crude oil inside pipelines,the influence of wax precipitation latent heat,initial oil temperature,ambient temperature,heat exchange coefficient between surface and atmosphere on temperature drop and law of temperature field distribution are researched in this dissertation,change process of wax precipitation boundary is also simulated.Beside these research,the effects of seasonal variation,soil thermal conductivity and buried depth on the distribution of soil temperature field around hot oil pipeline are studied.