Abstract

Mobile pipeline refers to a type of pipeline that can be laid and recovered at any time as required, mainly used for product oil transportation. After the completion of the transportation mission, the oil remaining inside the pipeline needs to be evacuated before recovering. Water-driven oil evacuation is commonly used for this purpose. In this paper, the water-driven oil evacuation in an inclined inverted U-shaped pipe was investigated. A numerical model coupling the VOF interface-capturing method, the CSF surface-tension model, and the SST k–ω turbulence model was established and validated against visualization experiments. The oil–water two-phase displacement characteristics in this special pipe configuration were systematically analyzed. The results show that a critical inlet superficial water velocity exists during the evacuation process. Complete oil evacuation can be achieved only when the inlet superficial water velocity exceeds this critical value. The critical velocity increases with increasing pipe inclination angle and pipe diameter. When the inlet superficial water velocity is lower than the critical value, vortical structures and backflow are readily generated at the oil front and within the oil phase, leading to oil accumulation near the end of the upper horizontal section and the formation of a stable retained oil layer in the downward-inclined section. Moreover, larger inclination angles and pipe diameters intensify oil retention and reduce the evacuation efficiency under subcritical conditions. All conclusions are drawn based on the No. 0 diesel–water system at 25 °C, and the applicable scope is limited to working conditions with similar fluid physical properties.

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