Study On Sand Deposition Law And Critical Condition Of Fluid Carrying Sand In Inclined Gas Gathering Pipeline

During the operation of offshore sand-containing gas gathering pipelines,sand deposition will occur when the flow velocities are lower than the critical condition,which will easily lead to pipeline blockage and corrosion damage.The most feasible solution is to reasonably predict the critical velocity needed to maintain the sand migration and carry the sand particles to the terminal platform for treatment.This paper focuses on the experimental and numerical simulation of sand migration and deposition,the critical conditions of fluid-carrying sand,and prediction models in inclined gas gathering pipelines.The main contents and conclusions are as follows.An experimental system for sand deposition in inclined gas gathering pipelines was designed and built.According to the migration degree of sand particles,the sand flow regimes in inclined multiphase flow pipelines can be divided into three types: full migration,partial migration and total deposition.Combined with the flow regime diagrams of sand particles,it is found that the full migration flow regimes are located in the range of gas-liquid velocity corresponding to the stratified wave flow,and their formation and transformation depend on the turbulence intensity of the gas-liquid phase.The partial migration flow regimes are mainly distributed in the transition region between stratified wave flow and stratified smooth flow or intermittent flow.The gravity and fluid drag force play a leading role in the partial migration flow regimes which are affected by the angle of repose and other sand properties.The total deposition flow regimes mainly appear in the stratified smooth flow region of horizontal and downward inclined pipes.Taking the sand-carrying efficiency of fluid as the evaluation index,the sand-carrying capacities of pipeline fluid under different working conditions were tested.And the critical state of the inclined pipeline fluid-carrying sand was determined to be the transition boundary between Concentrated flow and Forward moving bed.The results show that the sand-carrying efficiency of fluid in upward inclined pipe changes nonlinearly with the superficial gas velocity.And there is a maximum point after the slug is formed.However,slug flow can not carry sand effectively because of the slip between liquid and solid phases.Based on the experimental data and simulation data from Multiphase flow commercial software,the influential factors of critical velocity were compared and analyzed.In the range of-1.5° to +1.5°,the experimental and simulation results show that the critical velocity increases monotonously with the increase of inclination angle and particle size.Through the analysis of main factors and the calculation of Sajeev’s critical concentration,it is proved that the effect of sand particles concentration on critical velocity should be considered in the range of 0.01% to 1%.However,the commercial software’s model can not reflect the effect of low sand particles concentration on critical velocity by modifying the viscosity of the sandcontaining fluid.In this paper,a new expression for the expansion of Archibong’s model is obtained by modifying the reference velocity term and drag coefficient.The new model emphatically considers the influence of the sand repose angle,gravity,sand particles concentration on the critical velocity.Taking the pipe diameter D as the characteristic length parameter and combining the new model with Fan’s double mechanics model calculating the liquid holdup,the critical velocities of sand carrying in inclined gas gathering pipeline are solved.The mean absolute relative error of the new model is 15.65%.Compared with Multiphase flow commercial software’s model,the deviation from the experimental value is smaller,and the predicted values are basically within the ± 30% error line.