Vortex-induced vibrations (VIV) of semi-submersible floating structures and offshore risers have been an important issue in offshore and ocean engineering. The geometry of the semi-submersible platforms implies a more complex VIV phenomenon than that identified for single cylindrical structures such as spars and mono-columns offshore structures. The vortex shedding from each column and thus the wake interference increases the complexity of VIV of semi-submersible platforms for different wave and current headings in harsh ultra-deepwater environments. On the other hand, the aspect ratio of long flexible risers pose a challenge to capture coupled mechanics of local vortex-induced vibration, global motion of traveling waves, coupling with platform and seabed.
In the case of vortex-induced vibration, the frequency of vortex shedding approaches that of the oscillating bluff body that results into the oscillating lift force with increasing amplitude of motion through a resonance shift. For sufficiently large amplitude, the wake is significantly perturbed and forced to move through the inertial coupling at natural resonating frequency of the oscillating structure. In such self-excited bluff bodies in fluid flows, the amplitude of vibration grows until it becomes so large that nonlinear dynamical effects become relevant and achieve self-limiting amplitude. The extent of lock-in, in terms of reduced velocity, is known to be significantly affected by the mass ratio and damping between the structure and fluid systems. We review recent CFD developments in the computational modeling of flexible risers and freely vibrating offshore platform interacting with turbulent flows. We shed light on the effects of the mass ratio, Reynolds number, reduced velocity, and proximity effects with upstream body and stationary wall. Finally, we provide a review of challenging problems that keep intense interest on the topic from practical and fundamental standpoints.