Gas–liquid two-phase flow patterns in a pipeline–riser system with an S-shaped riser

• Flow regimes were observed in a long pipeline–riser system with an S-shaped riser.
• Severe slugging in an S-shaped riser exhibits some new behaviors.
• Gas penetration into the upper limb leads to fluid blowout.
• Stable region is dominated by bubbly flow, slug flow and annular flow.
• A stability criterion for steady operation in S-shaped riser is given.

In this work, experiments on flow patterns during air–water two-phase flow in a long pipeline–riser system has been carried out. The test loop with 50 mm i.d. consists of a horizontal pipeline with 114 m in length, followed by a 16 m downward inclined section, and ended at an S-shaped flexible riser. The inclination of the downward section varied from −2° to −5° from the horizontal. Liquid and gas superficial velocities ranged from 0.03 to 1.5 m/s, and 0.04 to 6.0 m/s, respectively. Based on the analysis of the pressure at the riser base and liquid holdup at the riser top, the flow regimes observed were classified into four categories. In contrast with severe slugging in a vertical or catenary riser where fluid blowout is initiated by gas penetration into the riser base, fluid blowout of severe slugging in an S-shaped riser is induced by gas intrusion into the upper limb of the riser. Flow pattern maps for each declination were generated based on the experimental data. The region of severe slugging was found to be sensitive to pipeline inclination. An existing model for the stability of steady state operation was modified. The modified model was tested against our experimental results. An excellent agreement between the experimental data and the predictions was obtained.