Vortex-induced vibration response of flexible riser subjected to platform surge motion

This paper numerically investigates the vortex-induced vibration of flexible risers subjected to surge motion of floating platforms. The dynamic boundary condition associated with this degree of freedom is transformed into a superimposed flow composed of shear flow and sheared oscillatory flow. The effects of oscillation frequency and velocity ratio of the equivalent superimposed flow on riser vibration characteristics are examined across different Keulegan–Carpenter number ranges. Numerical results show that increasing the oscillation frequency of the superimposed flow strengthens the dominance of the dominant vibration mode, whereas increasing the velocity ratio enhances the contribution of higher-order modes. The vortex shedding mode at each riser section varies periodically, resulting in five distinct vortex shedding modes. Based on their evolution, mechanistic conclusions are drawn, indicating that the oscillation frequency of the superimposed flow influences the critical velocity governing transitions between high-frequency and low-frequency vortex shedding modes, while the velocity ratio has little effect on the magnitude of the critical velocity but alters its interval.