Study On The Mechanical Response Characteristics Of Gelled Crude Oil During Pipeline Restart

By virtue of laboratory experimental loop, some researches were carried out on themechanical responses of gelled crude oil during start-up. The purpose is to provide scientificanalysis for the safe and economic operation of pipeline transporting waxy crude oil. Themain points of this paper include: the measure and forecast of the start-up yield stress, theproperties and solution of the initial start-up wave velocity in gelled crude oil pipeline, thecalculation of start-up pressure and start-up time for gelled crude oil pipeline based on thetheory of start-up wave velocity and the research on pressure diffusion during start-up ofpipeline transporting viscoplastic fluid based on the theory of transient flow.Due to the finite propagation speed of start-up pressure in gelled crude oil pipeline, themeasured value of start-up yield stress decreases with the pipe length. Put forward a dataprocessing method based on least squares fitting principle and obtain the start-up yield stresswhen the pipe length is infinitesimal. Thus the evaluation criterion of structural strength forgelled crude oil at particular condition is unified. Finally the start-up yield stress is regressedand forecasted with LS-SVM (least squares support vector machine) effectively.The propagation speed of pressure during start-up of pipeline with gelled crude oil isanalyzed and discussed, the properties of start-up wave speed were pointed out, and theformula of start-up wave speed or numerical solution method for pipeline with differentviscoplastic fluids were deduced. The propagation speed of start-up pressure in gelled crudeoil pipeline is mainly dominated by start-up wave speed instead of the transient pressure wavespeed. Generally, start-up wave speed is far slower than transient pressure wave speed,decreases with the increase of propagation distance of pressure and the strength of gelstructure, and increases with the increase of start-up flow rate and volume elastic/plasticcoefficient of gelled crude oil.A new start-up model of pipeline with gelled crude oil was proposed based on thestart-up wave theory. This new model can be used to simulate the variation of pressure andflow rate during the initial start-up of pipeline with gelled crude oil when using centrifugal pump to provide restarting energy, and finally forecast the minimum pressure required for asuccessful start-up and the corresponding start-up time.When using centrifugal pumps to provide restarting energy during the initial start-up ofgelled crude oil pipeline, pressure will gradually increase along pipeline, yet the pressuregradient and flow rate of pipeline will gradually decrease. The minimum pressure required fora successful start-up is far lower than the calculation value based on the incompressibility ofoil and pipeline. In the research range of this paper, both the minimum pressure required for asuccessful start-up and the corresponding start-up time decrease with the increase of insidediameter of pipeline.The theory of transient flow is applied in solving the pressure diffusion phenomenonduring the start-up of pipeline with viscoplastic fluids. The calculation results show that:before the dissipation of initial transient pressure wave, the propagation speed of pressurefront is consistent with the transient pressure wave speed, yet after that, the propagation speedof pressure front is slower than the transient pressure wave speed, and decays with thepropagation distance of pressure front, yet the decay rate tends to decrease. The bigger thestart-up flow rate is, the faster the pressure front propagates. The bigger the structure strengthand the consistency coefficient of medium in pipeline are, the slower the pressure frontpropagates. The bigger the expansibility of pipeline is, the slower the pressure frontpropagates. With the increase of inside diameter of pipeline, the propagation speed of pressurefront firstly increases, and after the inside diameter increases to a critical value, thepropagation speed of pressure front begins to decrease.