Technical challenges in floating offshore wind turbine upscaling: A critical analysis based on the NREL 5 MW and IEA 15 MW Reference Turbines

Upscaling is currently seen as one of the most promising techniques to lower the Levelized Cost of Energy of wind farms and is a trend that has been ongoing for many years. Floating wind turbines are still a quite novel technology. In this kind of application, the benefits of upscaling are potentially even greater than those that can be seen in land-based turbines, with a direct impact not only on the turbine cost per installed kW, but also on that of the floater, mooring lines and ancillaries. In this study, a critical analysis on the technical implications of upscaling is carried out, focusing on aero-hydro-servo-elastic design. The study is based on the NREL 5 MW and IEA 15 MW Reference wind turbines in floating configuration; while the two turbines benefit from different design choices and technical maturity, they are well-known, open-access test cases and present several similarities. Both turbines use the same controller, and both are placed on a semi-submersible type floater. The mooring line designs are also conceptually identical, with both turbines being anchored to the seabed trough three 120° apart slack catenary lines. The numerical tools used to simulate the wind turbines are also the same: Blade Element Momentum (BEM)-based aerodynamics in combination with second order potential flow derived hydrodynamics. Such approaches are found in the state-of-the-art code OpenFAST®, which is used in the present analysis. The two floating wind turbines are tested in a series of identical sea and inflow conditions (i.e., analogous to the wind farm design process for a given authorized sea region) with varying degrees of severity. Results show how overall performance and rotor loads are only marginally affected by floating installation. When looking at tower loads, however, it is shown who platform motions affect extreme power-production loads significantly. In this regard, the two machines are closely matched and despite the increased stability of the larger floating platform, some ultimate loads tend to increase more on the IEA 15 MW.