Floating offshore wind is a technology that promises to unlock a huge potential in deeper waters. However, it is more than a simple extension of the current offshore wind industry, constituting a new technology on its own right. Despite the many advances already made - and where Norway is driving in pole position - there is still an urgent need for in-depth research on floating wind technology in realistic offshore conditions. One of the main challenges in wind energy is related to the lack of knowledge regarding the wind loading and associated dynamic response of offshore wind turbines. The velocity spectra proposed in standards are known to differ from the turbulence spectra observed offshore. Improved atmospheric turbulence models specifically developed for offshore conditions as targeted by this project, will reduce the uncertainty in the design of foundations, moorings, towers and blades for large floating offshore wind turbines. In the near future bottom fixed offshore wind turbines rated at 12MW will have lower natural frequencies. This means that their sensitivity to changes in turbulent energy at low frequencies may be significantly higher. Therefore, the current project is relevant to both fixed and floating offshore wind turbines in the future. Wind models used in the design stage of wind turbines are only valid in the surface layer which can be as low as 45 m above the sea. This implies that a significant portion of the rotor of large offshore wind turbines may be above the surface layer during a significant portion of the year. Failure to account for the limits of the surface layer can lead to poor estimations of the wind loading. Therefore, the adequate parametrization of the marine atmospheric boundary layer at heights up to 200m is required. Although this topic is one of the greatest challenges in boundary-layer meteorology, it is now achievable thanks to recent progress in remote sensing and numerical weather prediction models.
This project will develop new knowledge and models to improve the design basis for large floating wind turbines (LOWT)(>12MW) in freewind and wake conditions. Observations from Hywind Scotland have shown the thermal stratification of the atmosphere can substantially affect the structural response of a wind turbine to the incoming turbulent flow. The first objective is to use wind data from several offshore sites to characterise the wind field in non-neutral atmospheric conditions. The project will use high-frequency wind data combined with a brand new remote sensing dataset (COTUR). In the COTUR campaign the incoming flow over the ocean was recorded, both within and above the surface layer, thus providing new insight on the applicability surface-layer scaling to model the turbulent wind loads on LOWT. This unique dataset will be analysed for the first time to the to indicate whether the turbulence models used in the standards, which mainly relies on surface-layer scaling, are appropriate or not. The final output will be to recommend suitable wind and
coherence model for in non-neutral conditions as input to free wind aeroelastic simulations and DWM models offshore. The second objective is to validate the simulated wind turbine response using full scale data from offshore wind farms (Alpha Ventus, Sheringham Shoal, and Zefyros/Hywind Demo). The validated simulation tools will then be used to quantify the effect of non-neutral atmospheric conditions on future LOWT (>12MW) to ensure safe and cost-effective design in the next generation of offshore wind farms in Norway and beyond. The final focus of the project is wake simulations of LOWT in non-neutral conditions using DWM model. High-fidelity CFD simulations will be used to include variable velocity shear in the DWM method and validate the wake meandering in non-neutral conditions. The non-neutral wind spectra and coherence from the data analysis work will be used as input for the DWM simulations.