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.

The size of offshore wind turbines has increased significantly since the installation of the world’s first commercial offshore wind turbine at Vindeby in 1991. The average offshore wind turbine capacity has increased from 500 kW in the 1990s to 3 MW by the end of 2010. Today, the largest offshore wind turbines have a rated power of 9 MW and a rotor diameter of 164 meters. Large, floating wind turbines pose new challenges compared to bottom-fixed turbines. In particular, the Hywind Scotland wind farm posed new challenges with respect to turbulence and coherence. There is a need for a better understanding of the horizontal coherence over large rotors offshore, but few measurements are available for such analyses. The main objective of the project is to improve the understanding of offshore wind coherence through measurements with lidars. The measurements will form a new, unique and highly relevant dataset for future offshore wind research. The collected data and corresponding meta-data will be stored in a database for later analysis.