Critical review and recommendations for scaled model testing of floating offshore wind turbines

Model testing plays an important role in the design assessment of Floating Offshore Wind Turbines (FOWTs). As the offshore wind industry is moving from fixed to floating units that are integrated into commercial wind farms, it is essential to reduce uncertainties in the design of these complex structures. Wind tunnel testing of aircraft and turbine blade structures employs Reynolds scaling, where viscous and turbulence effects are captured satisfactorily. On the other hand, hydrodynamic model testing of ships and offshore structures uses Froude scaling where importance is laid on gravitational and inertial forces, with suitable corrections for viscous drag effects. These model testing practices, including scaling laws developed for extrapolation to full scale prototypes, are found to be limited in scope for floating offshore wind turbine structures.
Current trends use geometrically scaled models of a FOWT structure with suitable approximate approaches for including the aerodynamics of the turbine. These approaches fall under two categories – “complete” or “performance” scaling where the wind turbine blade is redesigned to approximate the mean force in a Froude scaled wind field; and “simplified” or “software-in-the-loop” method, where simulated wind forces are applied onto a scaled model during experiments through appropriate actuation mechanisms. Published work on these methods and related aspects are critically reviewed in this study from the standpoints of advantages and limitations, ability to capture the relevant physics of the problem, and ease of experimentation.
The future of the wind energy industry is trending towards large wind turbines (15 – 22 MW) seated on floating structures moored to the seafloor by chains and synthetic lines in deep waters. As the size of wind turbines deployed in an offshore wind farm continues to grow, effects such as tower and blade flexibility, dynamic wind and resultant thrust, coupling between moored platform and turbine motions, and effects of various control configurations and strategies become progressively important. The ability of current scaling and model testing methods to capture these emerging challenges is evaluated and new research directions are identified in this paper.