Vortex-induced vibration of S-shaped riser with intermediate buoyancy module and conveying gas–liquid slug flow

This study presents an experimental investigation of the flow-induced vibration of an S-shaped flexible riser conveying the gas–liquid slug flow. The tests are conducted in a recirculating water flume using a non-intrusive optical measurement with two high-speed cameras for the riser undergoing the slug flow-induced vibration (SIV) coupled with the vortex-induced vibration (VIV) due to an external sheared flow. New insights into the riser dynamic features are presented and discussed in terms of response amplitudes, frequencies, and trajectories. Slug flow characteristics including the translational velocities, lengths, recurrence frequencies, and pressure fluctuations are evaluated. The S-shaped flexible riser experiences different equilibrium positions in SIV-only, VIV-only vs coupled SIV–VIV cases. These entail the external space-varying incident normal flow velocities, variable vortex shedding frequencies along the riser span, and modified top tensions at different conditions. Root-mean-squared amplitudes and corresponding dominant frequencies along the riser span in three flow conditions are compared in relation to the associated internal slug frequency and the pressure fluctuation, highlighting the contribution of the internal multiphase flow and the external sheared flow to the riser vibrations. The influence of the internal mixture velocity on the multi-frequency and mono-frequency VIV is analyzed. Due to the existence of the buoyancy module, a spatial mode competition of the riser response dominated by the internal SIV, external VIV, and buoyancy module motion results in a multi-frequency response. Overall, experimental results highlight the combined effects of the internal–external flows and the buoyancy module admitting an abrupt curvature change on the dynamic responses of the S-shaped riser.