Coupled vibrations of large optimized floating wind turbines

The cost of floating wind turbines is still significantly higher than that of e.g. bottom-fixed wind turbines. To reduce the cost of floating wind turbines, designers strive to optimize their floating substructures, and have created significantly lighter substructures over the years. At the same time, the power generated by wind turbines has been constantly increasing (up to well above 20 MW currently). These rotors generate a large amount of thrust and of dynamic loads that should be transferred to the floater, through the tower that supports the rotor. Lighter floaters and larger rotors means stronger couplings between floater, tower and turbine blade vibrations. Such vibrations are generating fatigue loads on the floater and tower, which may eventually cause failure. OptiFLEX is a 4-years competence-building project aiming at addressing this issue. The objective is to fully understand all mechanisms involved in coupled vibrations, improve analysis methods, experimental methods, and monitoring of vibrations. Together with SINTEF Ocean and NTNU, OptiFLEX involves a world-leading team of designers, as well as Equinor, who has a unique track record in operating floating wind turbines.

A massive upscaling of floating wind is planned for the coming years, including the installation of hundreds of floating wind turbines (FWT) per year. The objective is to significantly contribute to a net zero-emission society by 2050, and to transition to new energy industries in Norway and the world. However, the last years have shown that floating wind faces challenges related to costs, and there is a clear trend to (1) opt for larger wind turbine generators (beyond 20MW), and (2) to minimize the mass of the floater. This yields more efficient designs in terms of levelized cost of energy, but leads also to a more complex dynamic behaviour of the FWT. Stronger couplings between floater, tower and blade vibrations, are expected. They can lead to extreme stresses and fatigue damage of the structure, and are not well covered by today's design procedures. Using a multi-disciplinary approach, the OptiFLEX project aims at answering the following research questions. (1) Which physical phenomena drive/limit coupled vibrations of large and optimized FWTs? (2) Which simplifying assumptions can be made during design analysis, while still capturing extreme response and fatigue damage? (3) How can sensor packages be designed optimally to monitor loads and fatigue damage in FWT parks? The project will be structured into four work packages (WP1-4), WP1 focusing on first-principle-based numerical modelling of fully flexible FWTs, WP2 on experimental modelling, and WP3 on inverse numerical methods for load estimation and model calibration. WP4 will be a cross-cutting work package, aiming at coordinating the project, and converting scientific results into guidelines to be used by the design community.