INFLUENCE OF INTERNAL SLUG FLOW ON VORTEX-INDUCED VIBRATION OF FLEXIBLE RISER WITH VARIABLE CURVATURE

Flexible risers carrying multiphase gas-liquid flows may experience the vortex-induced vibration ( VIV) due to the external current flow. Previous studies have mostly focused on either VIV of flexible risers or multiphase flows in static straight pipes, whereas understanding of combined external-internal (VIV-multiphase) flow effects on flexible risers with variable inclinations and curvatures is still limited. The present study aims to model and numerically investigate the dynamics of a slug flow-carrying catenary riser subject to VIV and to evaluate the internal nonuniform slug gas-liquid flow effects on the overall flow-induced vibration behaviours. A steady slug flow model is applied to approximate slug flow-induced vibration (SIV) caused by the internal flow weights, momenta, and pressure variations along the riser pipe. The cross-flow and in-line VIV of the flexible riser under a perpendicular uniform steady flow is modelled by using the distributed van der Pol wake oscillators leading to 3-D responses with out-of-plane and in-plane (horizontal and vertical) displacements. Validation of the VIV model is conducted by comparing with experimental results in the literature. Some experimentally observed VIV and SIV features of flexible cylinders are numerically predicted. Through comparing the cases under internal-external flow conditions, riser responses are assessed in terms of dynamic responses, resonant frequencies, and vibration modes for the VIV-only versus combined VIV-SIV scenarios. The slug flow results in greater complex dynamics owing to the multimode oscillations. This observation should be recognized in a practical analysis and design of production risers.