Vortex-Induced Vibration (VIV) of flexible riser conveying severe slugging and straight flow in steady and oscillatory flows

This paper aims to investigate dynamic characteristics of Vortex-Induced Vibration (VIV) of a flexible riser conveying severe slug flow and straight flow in steady and oscillatory flows. Firstly, a classical wake oscillator model is adopted to simulate the coupling effects between internal and external flows on dynamic characteristics of VIV and an improved wake oscillator model is developed to calculate responses of VIV in oscillatory flow. A two-dimensional (2D) Computational Fluid Dynamics (CFD) model is developed to predict the behavior of severe slug flow. Then, structural equations of riser are discretized by adopting central difference method, and the Runge-Kuta method is adopted to solve them and the wake oscillator equations. The results show that characteristics of VIV, including root mean square (RMS) of displacement, dominant modes and vibration frequencies of riser conveying severe slugging differ significantly from those of riser conveying straight flow. New mode responses are triggered and multi-frequency phenomenon is observed due to severe slug flow. In addition, responses of VIV of riser conveying severe slug flow are influenced by the variation of the velocity or oscillation intensity of external flow. Model transition is normally triggered with a high-velocity sheared flow or a high Keulegan-Carpenter (KC) number.