Experimental investigation on the dynamic responses of a free-hanging water intake riser under vessel motion

• Vessel motion-induced VIV is confirmed for a free-hanging water intake riser.
• Vessel motion-induced VIV for a free-hanging water intake riser is less time-varying owing to small KC number effects.
• Vessel motion-induced VIV frequency has an integral relationship with the vessel motion frequency.
• Vessel motion-induced VIV has a significant drag amplification effect.

A large-scale model test of a free-hanging water intake riser (WIR) is performed in an ocean basin to investigate the riser responses under vessel motion. Top end of the WIR is forced to oscillate at given vessel motion trajectories. Fiber Brag Grating (FBG) strain sensors are used to measure the WIR dynamic responses. Experimental results firstly confirms that the free-hanging WIR would experience out-of-plane vortex-induced vibrations (VIVs) under pure vessel motion even for the case with a KC number as low as 5. Meanwhile, comparison between numerical results and experimental measurements suggests a significant drag amplification by out-of-plane vessel motion-induced VIV. What’s more, further study on WIR response frequencies and cross section trajectories reveals a strong correlation between vessel motion-induced VIV and local KC number distribution, owing to the small KC number effect. The presented work provides useful references for gaining a better understanding on VIV induced by vessel motion, and for the development of future prediction models.