Study On Heat Transfer Characteristics Of Direct Electrical Heating In Subsea Long-distance Pipeline

With the development of oil and gas exploration into the deep sea,the pipeline connection distance is further increased.The deep-sea high-pressure and low-temperature environment makes the flow guarantee problems such as hydrate and wax blockage easy to appear in the long-distance pipeline.Because the formation of solid matter is mainly affected by temperature,this paper proposes to use direct electric heating to actively heat the pipeline.In order to reduce the power loss of riser cable,a design scheme of electric heating seabed is proposed.Based on the Joule law,the optimal output power of the power supply and the length of the heating pipe are obtained.Based on fluent,the thermal fluid structure coupling numerical model of electric heating pipeline is established,the grid independence is analyzed,the number of grids is optimized,and the effects of heating power and fluid velocity on pipeline temperature distribution are studied.Based on the principle of heat balance and heat conduction differential equation,the theoretical models of axial and radial steady-state temperature distribution of electric heating pipeline are established.The accuracy of the steady-state model is verified by comparing the numerical results with the theoretical results.By introducing the definition of average temperature and combining the two-point Hermite approximation,an improved lumped parameter model for the transient heat transfer process of direct electric heating pipeline is established.The improved lumped parameter model of each layer of pipeline and the transient energy equation of internal fluid are solved by finite difference method.The influence of the number of grids on the calculation results is studied.The results show that the average temperature difference of the fluid and the average temperature difference of each layer of the pipeline are small under different grid numbers,which proves that the convergence of the model is good.The accuracy of the transient model is verified by comparing the theoretical value with the simulated value.The model is applied to the start-up and shutdown conditions,the transient temperature response of the electric heating pipeline is obtained,and the sensitivity analysis of heating power,fluid flow rate and heating time to the temperature distribution of the pipeline is carried out.Through a large number of calculation data,the prediction formula of fluid average temperature under normal transportation and shutdown conditions is obtained.Based on the law of temperature distribution,the inter zone heating of long-distance pipeline is analyzed,the calculation model of starting heating position and heating power is established,and the effects of initial temperature,fluid velocity and pipeline length on the size of heating interval and energy-saving efficiency are studied.The results show that for pipelines with a length of more than 15 km,the energy consumption can be saved by more than 20% by using inter zone heating.