Gradient-based optimization of a 15 MW wind turbine spar floater

We discuss the optimization-based conceptual design of support structures and mooring system for floating wind turbines. A four degree-of-freedom frequency-domain model is used for the dynamic response of a spar floating wind turbine subjected to wind and wave loads. The framework allows for design optimization involving the geometrical properties of the floater and the mooring system and inclusion of long realizations of multiple load cases in the analysis. The adopted optimization approach adopted relies on analytical design sensitivities of the governing frequency-domain equations and of the design requirements. This ensures that modern gradient-based optimization algorithms can effectively be used to solve the design problem at hand. The optimization approach is applied to the design of the spar-buoy floater and its mooring system for the IEA 15 MW reference wind turbine. A post-processing approach for identifying discrete designs from predefined catalogues is also presented. The post-processing allows to transform continuous design solutions into practical ones that can be used in subsequent analyses with time domain response models for the full validation of the design solutions obtained. The numerical results highlight the capability of the approach discussed herein to provide discrete optimized designs for given design constraints and loads in few minutes requiring modest computational resources and time.