Impacts of top-end vessel sway on vortex-induced vibration of the submarine riser for a floating platform in deep water

• A coupled model concerning interaction between fluid and structure is developed.
• Effects of amplitude and frequency of top-end vessel sway on riser VIV are examined.
• The displacement becomes several times larger than the case without top-end motion.
• The motion may be amplified as it propagates from top to bottom end along riser.

The dynamic coupling between moving top-end vessel and submarine riser becomes more remarkable for floating platform in deep water due to its larger amplitude of top-end motion, compared to fixed platform in shallow water. The impacts of top-end motion on the riser undergoing vortex-induced vibration (VIV) are explored in this study. A coupled hydrodynamic force approach, involving the vortex-induced lift force along with the fluid drag force, is developed, which takes into account the interaction between fluid dynamic force and instantaneous riser motion. Then the dynamic behaviors of the riser suffering both top-end motion and VIV are examined by means of finite element simulations. The effects of the amplitude and frequency of top-end vessel sway on riser׳s VIV are studied. During the riser׳s dynamic responses, an interesting phenomenon, called nonlinear response amplification, is observed, which demonstrates that top-end motion may be amplified as the motion propagates along riser length. Our numerical results show that the riser׳s displacement becomes several times larger than that of the case without top-end motion. Moreover, the nonlinear amplification gets more pronounced as the number of mode order drops, but the amplification factor just slightly changes with the increase of sway amplitude.