Review of scaling laws applied to floating offshore wind turbines

The wind energy industry is moving to offshore installations allowing for larger wind turbines to be deployed in deep-water regions with higher and steadier wind speeds. Floating offshore wind turbines consist of two main subsystems: a wind turbine itself and a floating substructure that supports it and provides stability. While the wind turbine technology is mature, the floating support structures for offshore wind turbines are still evolving and have not been deployed at a commercial scale. Due to a significant increase in the size of wind turbines over the last decade, it is important to understand how to design the floating platform to support larger wind turbines, and how the dynamics of the entire system change with increasing scale. Firstly, this article provides an overview of the trends in wind energy systems for offshore applications. Secondly, a review of existing semi-submersible platforms designed to support 5-15 MW wind turbines is provided. In addition, this article provides a comparative analysis of the techniques proposed to upscale floating support structures for larger wind energy systems with a particular focus on the system dynamics. The results demonstrate that the wind turbine mass, rated power and rotor thrust force scale with close to square rotor diameter. Towers designed for floating wind applications are usually significantly stiffer and heavier as compared to their fixed-bottom counterparts to place the tower's natural frequencies outside the wave excitation region. The analysis of semi-submersible platforms revealed a strong correlation between the wind turbine rotor diameter and the product of the distance to the offset columns and their diameter. Also, it has been found that design practices adapted by the platform developers roughly follow the theoretical square-cube (or 'mass') scaling law when designing platforms for larger wind turbines.