Severe slugging in a long pipeline–riser system: Experiments and predictions

Abstract At constant inflow conditions, large-amplitude pressure and flow rate fluctuations may occur in a pipeline–riser system operating at relatively low liquid and gas flow rates. This cyclic flow instability has been referred to as severe slugging. This study is an experimental, theoretical and numerical investigation of severe slugging in a relatively long pipeline–riser system. The experiments were carried out in a 65 m long, 50.8 mm diameter horizontal steel pipeline connected to a 35 m long, 50.8 mm diameter Perspex pipeline which is inclined to −2.54° from the horizontal, followed by a 15.5 m high, 45 mm vertical PVC riser operating at atmospheric end pressure. The experimental facility also included a 250 l gas buffer vessel, placed upstream of the pipeline, to obtain extra pipeline compressibility. Air and water were used as the experimental fluids. Five types of flow regimes were found and characterized based on visual observation and on the measured pressure drop over the riser. It was found that transient slugs were generated in the pipeline upstream of the riser base and they effectively contributed to the initial blockage of the riser base. An existing model for the prediction of the flow behaviour in the pipeline–riser system was modified. The modified model, which was tested against new experimental results obtained in this study, showed a better performance than previously published models. Numerical simulations were also performed using a one-dimensional two-fluid model. A good agreement between the numerical simulations and the experimental data was found.