A numerical study for assessing bypass pigging slippage and stoppage using adaptive mesh

Abstract

The bypass hole allows the pig moving at the velocity lower than the gas velocity in the pipeline, thereby reducing its impact on pipe accessory equipment, while also leads to the uneven pig motion. To illustrate the factors influencing pigging slippage and stoppage, this paper proposes an iterative numerical solution for bypass pigging using adaptive mesh, which adopted detailed meshes around the pig, with pig boundaries treated as discretized grids. The accuracy of proposed numerical solution is validated through the consistency analysis of pigging slippages and stoppages with experimental results. The numerical findings indicate that the uneven pig motion is characterized by an initial stoppage, followed by a repeated acceleration-deceleration process with gradually smoothed fluctuations. Initial stoppages, observed both numerically and experimentally, are attributed to pressure gathering over the pig, which can be mitigated by increasing the gas flow, reducing the friction, or lowering the bypass rate. While, sudden stoppages in abnormal pipe sections manifest either as a deceleration-stop process influenced by increased friction when entering high friction section, or as a fluctuation-stop process decided by gas compressibility when entering low friction section, which highlights the necessity for precise friction to accurately assess transient pig motion using this numerical method.