Control Co-Design optimization of floating offshore wind turbines with tuned liquid multi-column dampers

Abstract. The technical progress in the development and industrialization of Floating Offshore Wind Turbines (FOWTs) over the past decade is significant. Yet, the higher Levelized Cost of Energy of FOWT, compared to onshore wind turbines, is still limiting the market share. One of the reasons for this is the larger motions and loads caused by the rough environmental excitations. Many prototype projects tend to employ more conservative substructure designs to meet the requirements on motion dynamics and structural safety. Another challenge lies in the multidisciplinary nature of a FOWT system, which consists of several strongly coupled subsystems. If these subsystems cannot work in synergy with each other, the overall system performance may not be optimized. Previous research has shown that a well-designed blade pitch controller is able to reduce the motions and structural loads of FOWTs. Nevertheless, due to the negative aerodynamic damping effect, improving the performance by tuning the controller is limited. One of the solutions is adding Tuned Liquid Multi-Column Damper Dampers (TLMCDs), meaning a structural solution to mitigate this limiting factor for the controller performance. It has been found that the additional damping, provided by TLMCDs, is able to improve the platform pitch stability, which allows a larger blade pitch controller bandwidth and thus a better dynamic response. However, if a TLMCD is not designed by taking the whole FOWT system dynamics into account, it may even deteriorate the overall performance. Essentially, an integrated optimization of these subsystems is needed. This paper has developed a Control Co-Design optimization framework for FOWTs installed with TLMCDs. By using the multi-objective optimizer Non-Dominated Sorting Genetic Algorithm II, the objective is to optimize the platform, the blade pitch controller and the TLMCD simultaneously. Five free variables characterizing these subsystems are selected and the objective function includes the FOWT's volume of displaced water (displacement), several motion and load indicators. Instead of searching for a unique optimal design, an optimal Pareto surface of the defined objectives is determined. It has been found that the optimization is able to improve the dynamic performance of the FOWT, quantified by motions and loads, when the displacement remains similar. On the other hand, if motions and loads are constant, the displacement of the FOWT can be reduced, which is an important indication of lower manufacturing, transportation and installation costs. In conclusion, this work demonstrates the potential of advanced technologies such as TLMCDs to advance FOWTs for commercial competitiveness.