Study On Thermodynamic Transient Process Of Emergency Plug Removal Of Subsea Waxy Crude Oil Pipeline By Direct Electrical Heating

In China’s offshore oilfields,most of the crude oil is waxy crude oil which can make subsea pipeline face high risk of wax plug during shutdown or pigging.Compared with traditional methods such as cutting pipeline and chemical injection,Direct Electrical Heating(DEH)technology can greatly save time and cost of wax plug removal.So the DEH method is expected to be an efficient and convenient method to melt plug in subsea waxy crude oil pipeline.In this paper,the process of phase change and heat transfer of plug removal by DEH method for subsea pipeline are studied comprehensively.The feasibility,efficiency and economy of DEH method in wax plug accident of subsea waxy crude oil pipeline are confirmed,which can provide theoretical reference for implementation and application of emergency rescue of wax plug accident in China’s subsea pipeline.An experimental setup for DEH plug removal of subsea waxy crude oil pipeline is designed and built.Six sets of experiment are carried out with current of 1300 A as well as wax content of 20%,35%,50% respectively and with wax content of 100% and current of 1300 A,1100A and 900 A respectively.The results show that the rate of plug melting is approximately inversely to wax content and proportional to heating current.Meanwhile,due to the move of the melt front,the thermocouple cannot capture the temperature invariance during the absorption of latent heat,but has a sudden jump instead.Solidification time when cooling is longer than melting time when heating,which makes DEH technology feasible for multi-stage plug removal of subsea pipeline.All the six sets of experiment have good regularity,which confirm the scientificity of the experimental setup and DEH plug removal method.A physical model and a mathematical model of subsea waxy crude oil pipeline under DEH method are developed.The melting process of wax plug in pipeline is simulated by FLUENT.According to the experimental data,the numerical model is proved validated from two aspects: the change of liquid fraction and the shape of the wax plug.Based on the limitation of the maximum safety temperature of steel tube and aimed to be more efficient and more economical,several influence factors such as the maximum effective power,effective liquid fraction,effective time and minimum cost of plug removal are investigated.Based on the above studies,the correlations are established between the influence factors and common pipe diameter(152.4mm~813mm).An engineering feasibility study of DEH plug removal technology is carried out based on the wax plug accident of subsea pipelines in Wenchang 8-3 oilfield.The numerical results show that under the total power of 2000 kW,the effective heating time of one single 1.1 km pipe is about 3.33 hours,the cost of plug removal is about 479.3 thousand RMB,and the total cost of for the whole pipeline is about 2.4 million RMB.