Gradient-Based Design Optimization of Fully-Flexible Floating Wind Turbines Using Modal Analysis

Abstract

A variety of substructure concepts for floating offshore wind have been developed. Design optimization can be used to efficiently explore this design space and guide further work. Previous design optimization studies have been limited by model simplifications, including the assumption of rigid body motions, lack of substructure flexibility, and a focus on a single floating foundation concept. High computational costs of gradient-free optimization methods have limited the number of design variables considered. In this work, gradient-based optimization methods and a frequency-domain modal analysis model based on three-dimensional modeshapes make it possible to consider a more detailed structural model with reasonable computational cost. This work implements a linearized aero-hydro-servo-elastic model of a tension-leg platform wind turbine. The optimization varies sizing parameters and the model computes responses to wind and wave forcing in multiple environmental conditions. The model computes forcing and response in generic modes rather than assuming rigid body motions. Implementation of the model with analytical gradients in OpenMDAO allows for efficient optimization with dozens of design variables. The optimization results show reduction of the objective while satisfying constraints. Furthermore, there is potential for this approach to be adapted to other floating wind turbine substructure designs.