Vortex-induced vibration of spar platforms for floating offshore wind turbines

An experimental study of flow-induced oscillations of a floating model spar-type wind turbine platform was conducted. The model spar consisted of a floating uniform cylinder moored in a water tunnel test section and free to oscillate about its mooring attachment point near the center of mass. For the bare cylinder, periodic oscillations with figure-eight trajectories were observed over a range of reduced velocities, resembling a lock-in region. This was shown to be a vortex-induced vibration response since the oscillation frequencies and the shedding frequencies stayed equal to each other during the lock-in region. While the lower part of the cylinder was moving upstream at the 2 crossflow extremes of the trajectory, referred to as a counterclockwise (CCW) motion, its upper part experienced clockwise motion. It was hypothesized that the portion of the spar undergoing CCW figure eights is the portion where the flow excites the structure. This hypothesis was then validated by showing that adding helical strakes to the portion of the cylinder with CCW orbital motion suppresses vortex-induced vibration almost entirely, while adding strakes to the portion with clockwise orbital motion had a minimal influence on the amplitude of response.