Water displacing oil is one of the main emptying methods for mobile pipelines. It has the advantages of being a simple process and highly safe. At present, the determination of a water displacing oil scheme of mobile pipelines is based on the oil–oil alternating transport theory of product oil pipelines. However, the insolubility of the oil phase and the water phase results in a great difference between the flow characteristics of water displacing oil and the oil–oil alternating transport of a product oil pipeline. In addition, due to the effect of buoyancy, the oil phase gathers at the high point of the pipeline and forms a liquid accumulation, which is difficult to carry away by water flow, resulting in the low emptying efficiency of the mobile pipeline. The essence of water displacing oil in a mobile pipeline is an oil–water two-phase unsteady displacement flow, involving liquid–liquid displacement flow, oil–water two-phase flow and water carrying oil. Aiming at such problems, domestic and foreign scholars have carried out a large number of theoretical and experimental studies, established the oil–water mixing model of water displacing oil and the relationship between macroscopic quantity (flow pattern, pressure drop and water content) and microscopic quantity (local flow field and droplet dispersion pattern, etc.) under each flow type, and explored the influence of pipeline diameter, oil phase velocity, pipeline inclination angle and other parameters on the capacity of carrying liquid accumulation. On this basis, this paper analyzes the shortcomings of the current research on the oil–water flow characteristics of water displacing oil in a mobile pipeline from three aspects: the formation mechanism of the oil–water mixture, displacing flow characteristics of immiscible fluids and flow characteristics of water carrying oil. Five future research directions are proposed, including the interface morphology and flow field characteristics of oil–water two-phase layered flow, local mixing characteristics of an oil–water two-phase dual continuous flow interface, droplet distribution and flow characteristics of oil–water two-phase dispersed flow, unsteady flow characteristics of the oil–water mixture of water displacing oil and oil accumulation and flow characteristics in topographic relief pipes.

Mobile pipelines are the most efficient and reliable tools for transporting a large amount of oil over long distances in combat, emergency rescue and disaster relief situations. In the completion of oil transfer or pipeline maintenance, the oil in the pipe is usually displaced by air. In the process of evacuation, due to the influence of terrain, working conditions and gravity, the gradual deposition of oil from upward-sloping pipeline sections to low-lying parts of the pipeline occurs, resulting in the formation of fluid deposits, which results in the incomplete drainage of the pipeline, directly affecting the recovery efficiency of the pipeline. According to the theory of gas–liquid two-phase stratified flow, the tail flow of a liquid membrane in an upward-sloping pipeline is analyzed as part of the gas-carrying oil flow process in an upward-sloping pipeline in this paper. The gas superficial velocity, which is the minimum velocity that can carry the accumulated liquid in the pipeline, is called the critical gas velocity of the gas-carrying liquid. A gas-carrying oil critical gas velocity model in an upward-sloping pipeline was established, and the change law of the critical gas-phase velocity in the upward-sloping pipeline was analyzed under the conditions of different tile angles, initial oil layer thicknesses and pipeline diameters. It was found that the tile angle had the greatest influence on the gas-carrying oil flow in the upward-sloping pipe, and the initial retained oil thickness had less of an influence on the gas-carrying oil flow in the upward-sloping pipe. When the pipeline diameter was small, the flow of the gas-carrying oil in the upward-sloping pipe was greatly affected by the pipeline diameter, but when the pipeline diameter was larger, the influence of the diameter on the gas-carrying oil flow in the upward-sloping pipe decreased gradually. The model developed can provide a useful means for the analysis and containment of accumulated liquid.