Abstract: For oil-after-water commissioning that involves a maximum drop-in-height of 1 432.64 m and uses a crude oil pipeline containing a U-shaped section with a large drop-in-height, the density difference between crude oil and water leads to stagnation issues in the oil head. To resolve this problem, we proposed to drain the pipeline and release pressure using a pressure relief valve installed in the valve chamber near the oil-water interfacial point. Based on the OLGA multiphase flow transient simulation method and taking the actual situation as an example, we simulated the gas-driven water drainage process in the pipe section with a large drop-in-height. Specifically, the simulation focused on analyzing the change in flow rate and pressure over time with varying diameters of the leakage aperture. A series of available sizes of the leakage apertures and corresponding changes in the displacement, flow, and pressure over time were obtained from the simulation. The results demonstrate that the maximum displacement in the selected pipe section (capacity≈6 367.98 m³) can reach 3 375 m³. From the perspectives of economy, safety and effectiveness, the best leakage aperture range was found to be 90～110 mm. This number was consistent with that in the actual situation. This study provides a practical basis for alleviating similar abnormal conditions in real production processes.

Key words: large drop-in-height, oil pipeline, U-shaped pipe section, water displacement, diameter of leakage aperture