Effect of floating substructure flexibility of large-volume 10 MW offshore wind turbine semi-submersible platforms on dynamic response

As the dimension of the floating substructures for ultra-large wind turbines increases, the flexibility of the largevolume floating substructures may increase to the extent that may significantly affect the dynamic responses of an ultra-large semi-submersible floating offshore wind turbine (FOWT), which introduces new challenges in capturing relevant physical effect in numerical simulation analysis. This paper describes a newly designed semisubmersible substructure for the Technical University of Denmark (DTU) 10 MW wind turbine, and the substructural flexibility is considered in aero-hydro-servo-elastic dynamic simulations by extending the simulation OpenFAST Code, including wave-structure interactions. A comprehensive comparison of flexible and rigid largevolume substructures models is presented to highlight the effect of substructural flexibility on the hydrodynamic loads and dynamic responses of the integrated FOWT system by implementing a fully coupled simulation analysis in the time domain. Additionally, the difference of flexible and rigid large-volume substructures models on the structural fatigue behavior of the novel semi-submersible FOWT is investigated and discussed by computing ultimate and damage-equivalent loads (DELs) for selected environmental conditions. The results show that the substructural flexibility has a significant impact on fatigue damage of the integrated FOWT system in operating sea states than that in extreme sea state.