Shutdown–restart operation is a critical transient condition in oil–water pipelines, yet the relationship between flow-pattern evolution and pressure response during shutdown–restart remains insufficiently understood. In this study, experiments were conducted in a 30° inclined pipeline under different water cuts and pump frequencies to investigate transient flow evolution and pressure-drop behavior during shutdown and restart. Flow visualization and pressure measurements identified eight representative flow patterns and four successive stages: stable flow, shutdown transition, shutdown stable period, and restart transition. The results show pronounced hysteresis between shutdown and restart. During shutdown, weakened shear promotes phase segregation and the formation of intermittent or stratified structures, whereas restart involves delayed interfacial reconstruction and a distinct pressure overshoot. The pressure response depends strongly on water cut, showing monotonic decay at low water cut, stepwise decay and prolonged stabilization at intermediate water cut, and underdamped oscillations at high water cut. A lumped-parameter model was developed to interpret the roles of inertia, gravity, elasticity, and damping, and a reduced model was further used to reconstruct the dominant pressure-drop response. The reduced model reproduced the main dynamic features of the shutdown–restart process with coefficients of determination above 0.85, although its accuracy decreased in the phase-transition region and near local peak values. These results improve the understanding of transient pressure evolution during shutdown–restart in inclined oil–water pipelines and are of direct engineering relevance for shutdown–restart safety assessment, transient-risk evaluation, and operational optimization of inclined oil–water pipeline systems.
Xue et al. (Fri,) studied this question.