Dynamic analysis of coupling between floating top-end heave and riser's vortex-induced vibration by using finite element simulations

• Dynamic response of integrate system including both moving top-end and riser undergoing VIV is examined based on our coupled lift force approach.
• Riser response may be several times larger than the case of no-top motion. Impact of top heave on riser's VIV gets larger as modal order number drops.
• An interesting phenomenon, called mode transition, is observed, particularly at lower vibration frequencies.

The dynamic coupling between floating top and submarine riser becomes more remarkable owing to larger fluctuation amplitude of floating platform in deeper water, compared to fixed platform in shallow water. In this study, the impacts of top-end vertical motion (heave) on riser's vortex-induced vibration (VIV) are explored by means of finite element simulations. A coupled hydrodynamic force approach, regarding vortex-induced lift force along with fluid drag force, is developed, which takes into account of the interaction between instantaneous structure motion and fluid dynamics. Then the dynamic responses of the integrate system including both floating top-end and a riser undergoing VIV are examined based on our numerical simulations. The influences of platform heave, in terms of heave frequencies and tension ratios, on riser's VIV are presented. Our numerical results show that the dynamic response displacement of riser becomes several times larger than the displacement for the case without top-end motion. The impact of top heave on riser's VIV gets larger as the modal order number drops. Moreover, an interesting phenomenon, called the mode transition, is observed particularly at lower vibration frequencies due to the natural dynamic characteristics of the slender riser. We suggest that, in practices of riser design, a combined excitation needs to be considered for the accurate dynamic analysis of slender marine structures subjected to a top-end motion and VIV.