Simplified Modelling of Semi-Submersible Floating Offshore Wind Turbines for Time-Domain Dynamic Analyses

Abstract

Offshore wind has great potential for coming years, with the EU-27 countries targeting to nearly quadruple their current capacity of 16.1 GW (end of 2022) to 60 GW by 2030. Within this field, floating offshore wind turbines (FOWTs) have become an area of great interest due to their capability to operate in greater water depths with higher and steadier wind speeds, achieving significantly higher capacity factors than bottom-fixed turbines. To facilitate the further development of those floating structures, there is need for reliable numerical software capable of accurately predicting their dynamic behaviour. Furthermore, analysing these systems presents a significant challenge due to the high complexity of the aero-hydro-servo-elastic model, which establishes a coupling between the hydrodynamics of the floater, the structural dynamics and aerodynamics of the turbine, the dynamics of the mooring lines and the complex control system that combines all these aspects. The objective of this research is to identify which modelling parameters are of importance for an accurate evaluation of a floating wind turbine's dynamic behaviour in the basic design stage, with the ultimate goal of creating a simplified model that accurately reflects the turbine's dynamics, while reducing computational expenses. A 3D diffraction and radiation software is used to conduct frequency analyses during conceptual design and results are utilised as inputs for the basic design stage, where a time domain finite element dynamic analysis is performed. An existing model is employed as the reference, and different environmental load cases are utilised to form the design space in which modifications are tested. The semi-submersible floater concept is utilised for this study, as it offers benefits such as a low life cycle cost, a simple and cost-efficient float-out process, and a suitability for a wide range of water depths. Nacelle acceleration, platform pitch and tower base bending moment prove useful to determine the accuracy of the dynamic analysis, for which the \textit{OrcaFlex} software is chosen, while simulation duration is used as a metric for computational cost. A sensitivity analysis reveals that small variations in modelling parameters can result in significant changes in dynamic outputs. Additionally, the intricate interrelationship among diverse domains, encompassing aerodynamics, structural dynamics, and hydrodynamics, is emphasised. The simplified turbine model created in this study provides a good overview over the system dynamics while reducing computational costs significantly. It is well-suited for the comparison of different floater layouts, allowing for rapid, reasonably accurate results.