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The solid particle impurities generated by pipe wall corrosion might deposit at the elbow of hilly pipelines during the production shutdown of oil pipelines. These solid particle impurities will seriously affect the safety of the pipeline operation and the quality of the petroleum products. Thus, it is necessary to study the methods of removing these trapped particles from pipelines. At present, the most common way to remove these solid particle impurities is pigging oil pipelines periodically by utilizing the mechanical pigging method, while the frequent pigging operation will increase the cost and risk of pipeline operation. It is very convenient and economical to remove the accumulated particles out from the pipeline by oil stream, which can be named Hydraulic Pigging Method (HPM). However, the behavior mechanism of particle in flowing oil is still unclear. This motivates the present research on the particles flushed out by the flowing oil. A numerical model governing the trapped particles displacement from the elbow of an inclined oil pipeline is established in the Euler-Lagrangian framework. The simulation is achieved via CFD coupling with DEM. The CFD method is employed to solving the continuous phase flow, while the discrete particle phase is tracked by the DEM. The numerical model is first validated by comparison with results taken from the published literature. From the simulation results, it is observed that the oil stream, carrier phase, can only flush out the solid particles in a certain diameter range under the given operation conditions, and the particles whose diameter beyond that diameter range will cannot be removed out from the pipeline. The influence of the pipe inclined angle, the oil bulk velocity and the particle diameter on the particle migration characteristics is examined in detail. Furthermore, in order to enhance the efficiency of HPM, an Enhanced Hydraulic Pigging Method based on Multi-Physical Field Collaboration (EHPM-MPFC) is also proposed in the present work. The EHPM-MPFC is validated for having high pigging efficiency via the comparison of the migration characteristics of particles during the EHPM-MPFC and HPM process. The present results can provide the guidance to the HPM operation of products pipelines.

a b s t r a c tThe high sand content of the crude oil produced from some oilfields influences the production efficiency. The fine sand deposits gradually in the three-phase separator during the treatment process of crude oil, which also seriously affects the dewatering treatment. To address the desanding and removal issue in high-sand-content oilfield, the large-size sedimentation tank is applied to enhance the treatment of the desanding process. Therefore, the numerical simulation for the separation process of suspended fine sand from oil and water emulsion in large-size sedimentation tank is implemented. The multiphase flow model of Eulerian and DPM model are chosen to simulate the depositing situation. The full flow field under different operating conditions for desanding is simulated and the applicable emulsion effluent standards under both static and dynamic sedimentation conditions are obtained. Comparing the simulated results to experimental results, a good agreement could be achieved. It was found that, the effluent-recycle process is not recommended if the viscosity-reduction process via heating is adopted to upgrade the desanding efficiency. The time required for desanding was 7~10 hours, and the volume of the large sedimentation tank which meets the current requirement was 3000 m 3 . The desanding efficiency can reach 90% when the sand size is larger than 80 μm, which provides guidance for whether to add the next level of desanding equipment or not. The formula for calculating the periodic time of cleaning deposited sand at tank bottom is given. It is also suggested that the sedimentation tank should be directly heated to reduce the liquid viscosity and prolong the sedimentation time, which is the optimum design for the whole desanding process.

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