Numerical Investigation on Dynamic Responses of a Semi-Submersible Wind Turbine with Different Types of Heave Plates

The heave plate plays a significant role in reducing the dynamic responses of floating wind turbines (FWTs) because the flow separation and vortex shedding around the sharp edges of heave plate provide viscous damping. This paper investigates the effect of different types of heave plates on the dynamic responses of a semi-submersible FWT. The single circular perforated heave plates and the double circular solid heave plates are compared to the original (single circular solid) heave plate of the OC4-DeepCwind semi-submersible FWT with the total structural properties of platform kept constant. OpenFAST is employed to conduct the time-domain simulations under five different combinations of wind and wave conditions. The heave and pitch natural period estimated from free decay tests decrease when the perforated heave plate is deployed. The double solid heave plate only increases the heave natural period. Compared to the single solid heave plate, the heave and pitch motions increase by up to 40% for the platforms with the perforated or double heave plates, except for the decreasing heave motion for the perforated heave plate under turbulent wind condition and a smaller pitch motion (up to 15%) under the turbulent wind or irregular wave and turbulent wind condition. The perforated heave plates result in the increasing tower base bending moment while the double heave plates have little influence on the tower base bending moment, only decrease by 8% under irregular wave and constant wind condition. The tensions of mooring line on the starboard side increase by up to 15% for the platforms with the perforated heave plates while the tensions for the double heave plates are quite close to those for the single solid heave plate. The surge motions and the upwind mooring line tensions are independent on the heave plate. Additionally, the mean pitch motions decrease by up to 7% for the double heave plates relative to the results for the single solid heave plate while the changes in the mean values of other dynamic responses is negligible.