Study On Efficient Numerical Methods For The Shutdown And Restart Processes Of A Waxy Crude Oil Pipeline

The shutdown and restart problem is one of the core issues for the safe operation of a waxy crude oil pipeline,which has been seriously concerned by pipeline operation departments and domestic & foreign researchers for a long time.In order to assess the safety of a waxy crude oil pipeline during the shutdown and restart,the numerical simulation is generally needed.However,the long-distance waxy crude oil pipeline is hundreds of kilometers and even more one thousand kilometers in general,and thus it usually needs to take a long time to numerically simulate shutdown and restart processes.In order to shorten the safety assessment period of the shutdown and restart,efficient numerical methods for the the shutdown and restart are studied from four aspects: mathematical model,discretization & solution,adaptive method and GPU parallel computing.Based on three-dimensional shutdown & restart mathematical model,a pseudo-two-dimensional shutdown and restart mathematical model is deduced through reasonable simplifications.This model can describe the shutdown and restart processes of a waxy crude oil pipeline with multi-fluid type and multi-flow state coexistence.It successfully overcomes the shortcoming of the previous pseudo-two-dimensional hydraulic model which can only be applied to laminar flow or simple thixotropic models.In addition,it overcomes the shortcoming of the previous pseudo-two-dimensional thermal model in which the empirical formula of friction coefficient needs to be involved.The thixotropic model is an important part of restart model.The difficulty of the parameter determination of restart model is the parametric regression of thixotropic model.In view of the existing problems of parametric regression at present,a new single-objective regression model,a new multi-objective regression model and corresponding regression algorithms are proposed in this paper,which successfully achieve two regression objectives,i.e.,the small fitting deviation and the obtained parameters satisfying physical meaning.Combined with the change characteristics of physical quantities during the shutdown and restart,multiple grid systems are adopted to discretize computational domain.Based on the generated grids,the finite difference method and the finite volume method are adopted to discretize the governning equations of shutdown and restart,and then the hydraulic-thermal coupled solution algorithm is proposed for the shutdown and restart.In addition,in order to investigate the effect of solvers on the solution efficiency of shutdown and restart,the comparative study on several kinds of solvers for the non-linear and linear equations is conducted,which could provide important reference for the choice of solvers of numerical simulation.The hydraulic-thermal coupled solution algorithm for the shutdown and restart is combined with the adaptive wavelet collocation method,and then another shutdown and restart algorithm is proposed.This algorithm could dynamically adjust axial grid density to reduce the axial grid points and the corresponding cross-sections of generalized soil.In order to efficiently simulate the shutdown and restart processes of a waxy crude oil pipeline,the threshold,the coarsest level of resolution and the adaptive strategy affecting the algorithm are discussed,and the relevant suggestions suitable for engineering calculation are provided.In view of the huge number of GPU threads,parallel computing methods are designed for the hydraulic and thermal solution processes of shutdown and restart,and the GPU parallel solution of shutdown and restart processes is realized successfully.Compared with the CPU serial solution,the GPU parallel solution possesses higher efficiency.For the numerical example designed in this paper,the hydraulic solution and the thermal solution are accelerated by 6.2 and 4.1 times,respectively.When the GPU parallel computing is combined with the wavelet collocation method,the thermal solution could be further accelerated,and the speedup ratio could be up to 19.0 times.