The Effect of Substructure Flexibility on the Performance of a 15MW Concrete Spar FOWT

Until recently, the rigid hull model with flexible tower and RNA have been used as standard procedure in the global performance analysis of FOWTs. Since the FOWT size continues to increase beyond 20MW, there has been increasing concern of the effect of hull flexibility on its global performance. In this study, the impact of hydroelasticity on the global performance of a 15MW concrete spar FOWT, called "WindCrete", has been investigated. The WindCrete spar-FOWT has been developed as a part of EU horizon 2020 project COREWIND and its model specifics are open to public. For comparisons, two numerical models were developed; one is the conventional rigid substructure model, and the other is the DMB (discrete-module-beam) model including the substructure flexibility. The most important observation by adding hull flexibility is the shift of the 1st fore-aft bending-mode natural frequency to lower frequency compared to the rigid-hull case (from 0.52 Hz to 0.41 Hz), which has been identified through coupled free-vibration test in the wet-condition. Coupled aero-hydro-servo-elastic-mooring dynamic simulations were carried out with the rigid-hull and flexible-hull models under various environmental conditions. Two operational (DLC 1.3 and DLC 1.6) and one parked (DLC 6.1) condition for ULS (ultimate limit state) analyses have been conducted. It has been observed that the platform rigid-DOF motions and mooring tensions are little changed but tower-base bending moment and nacelle horizontal acceleration may be appreciably increased compared to the rigid-hull model.