Flow cutoff characteristics of gas–liquid two-phase flow in the pipeline–riser system: Experimental studies and transient calculations

Periodic flow cutoff in offshore oil and gas transportation systems, often triggered at slug catchers, causes severe pressure and flow fluctuations that may damage downstream equipment and threaten industrial operations and energy security. This study experimentally investigates gas–liquid two-phase flows in a pipeline–riser system under steady-state, choking, and start-up conditions. The coupling relationship between flow cutoff at the riser top and blockage at the riser base under different working conditions is explored, together with the impact of flow cutoff on system hydrodynamics. Based on the experimental findings, a transient model using ordinary differential equations is developed to describe two-phase flows in pipeline–riser systems. The analysis of the results demonstrates that, for steady-state conditions, the model described in this article is able to predict flow cutoff time and liquid slug length in the riser with less than 20% relative error for almost all configurations. The prediction regarding flow period shows even relative errors below 10%. Additionally, the model systematically captures all qualitative trends regarding pressure, slug length, and flow velocity, offering a robust numerical tool for pipeline design and safe operation. The predictions can be obtained within minutes on a standard portable computer. These insights enhance the reliability of offshore energy systems and mitigate potential risks to critical infrastructure.