A fully-coupled analysis of the spar-type floating offshore wind turbine with bionic fractal heave plate under wind-wave excitation conditions

As shallow coastal areas for the installation of wind turbines approach saturation, wind turbines will need to be installed in deeper areas, requiring floating rather than seabed-fixed substructures. Considering factors such as economy and safety, floating offshore wind turbines (FOWTs) have become the major focus for offshore wind research and development. In the case of spar-type FOWTs, stability in the heave direction poses a challenge. With this in view, in this work a heave plate with bionic fractal structures is mounted on the bottom of the spar- type FOWT. The bionic fractal heave plates are innovatively proposed to further improve the dynamic response of the FOWT. In this study, the aero-hydro-mooring dynamic method of the FOWT is established to develop a reliable numerical solution model through the DFBI module using computational fluid dynamics software STARCCM+. The results of fully-coupled simulations of the original FOWT, the FOWT with heave plate (HP-FOWT) and FOWTs with 3 5-layer bionic fractal heave plate (3 5BFHP-FOWTs) are presented. Increases in average thrust and power of 0.44 % and 0.99 %, respectively, prove the optimal aerodynamic responses of the 5 BFHPFOWT. As for the hydrodynamic responses, the average heave response amplitudes of the HP-FOWT and 3 5BFHP-FOWTs are significantly lower than the original FOWT. The maximum reduction (25.03 %) is obtained by the 5BFHP-FOWT. The bionic fractal heave plate will slightly reduce the stability of the pitch response. For the standard deviation of the heave, surge, mooring lines 1 and 2 responses, the 5BFHP-FOWT decreases by 17.97%, 11.44 %, 17.50 %, and 8.25 % respectively, showing the best stability improvement among the HP-FOWT and BFHP-FOWTs. Furthermore, the vortices in the bionic fractal heave plates are analyzed in detail at the Z = +/- 0.25 m section of the flow field. Only when the specific fractal layer number is 5, the number and curl of the vortices in the fractal structure increase significantly, showing excellent effect of the energy absorption.