With influence of current, the platform and the connected riser form relative motion in the flow. In this paper, A computational fluid dynamics (CFD) model is built by strip theory and Bernoulli–Euler bending beam model. Based on this numerical model, the platform motions and corresponding current velocities are employed to investigate the responses of the riser. The numerical results indicate that the dominant crossflow and inline vibration modes are increased with the current velocity. The amplitudes of crossflow and inline vibration modes show periodic variations with the increase in the amplitude of the surge motion and the current velocity. The similar vortex shedding mode in the uniform flow is observed during vortex–induced vibrations of riser under the low frequency platform motions.
Platform experiences six degrees of freedom (6–DoF) motions when it suffers from waves, winds and currents. Furthermore, the relative motions in in–line and cross–flow directions are formed between current and top–tensioned riser connected with platform, which produces vortices in wake of the riser. The released vortices generate the vortexinduced vibrations (VIV), and affect the stability of the riser.
Many scholars are attracted by the researches on VIV of slender flexible riser under platform motion. Park et al (2004) conducted experiments to investigate the VIV of a hanging riser with regular top–end excitation, the results showed that the crossflow vibrations were quite different along span of the riser and the structural wave propagation was observed. Senga and Koterayama (2005) experimentally studied the riser responses under the irregular excitation of platform, they concluded that resonance phenomenon of the riser was suppressed by the irregular top–end excitation and the crossflow motion was affected by irregular vortex shedding.
Wang et al (2014) experimentally investigated the VIV of catenary riser with regular two–end excitations, and they found three developing processes of VIV including: building–up, locking–in, dying–out. Domala and Sharma (2018) conducted experimental investigations on the transverse motion of stainless steel braided flexible hose catenary riser with and without scaled semi–submersible. The results indicated that the vortex–induced motion of semi–submersible caused the different crossflow responses of the riser.
