In order to grow from its present nascent stage – several demonstration projects and one small commercial farm – the floating wind turbine (FWT) industry needs further cost reductions and design improvements. The hull and mooring system are the enablers of floating wind, and the parts that truly differentiate floating wind from traditional bottom-fixed wind turbines. Therefore, the potential for cost reduction lies in hull and mooring system optimisation, when trying to close the gap in CAPEX between floating and bottom-fixed wind turbines.
Designing cost-effective mooring systems for FWTs is challenging for shallow water – where hundreds of meters of chain on the seabed might be required in order to survive storms, or high stiffness might be needed to protect the power cable – but also for very deep water, where the cost of material becomes high. As recently highlighted by several industry stakeholders, understanding the design drivers for mooring systems is crucial. Assessing novel mooring designs, including new materials, innovative methods of sharing anchors, or even floater-to-floater moorings, requires improved analysis methods, considering complex material behavior, accurate wave load models and wind field models, and aerodynamic interactions between FWTs in a farm.
Several experimental campaigns examining the responses of floating wind turbines to wave loads have been carried out. Numerical codes (particularly for semi-submersible concepts) do not adequately predict the low-frequency responses in waves, and the wave load formulations should be examined in greater detail, considering improved wave kinematic models, nonlinear potential flow theory or computational fluid dynamics (CFD). Additional high-quality experimental data for validation is needed, including quantification of the repeatability of low-frequency responses, systematic investigations of the effect of changing the platform pitch angle, and simultaneous inclusion of realistic aerodynamic loads.
The numerical wind field models, which are applied during FWT simulations, are based on an assumption of neutral atmospheric conditions, and do not consider the effects of neighboring wind turbines in a farm. Unstable atmospheric conditions typically dominate offshore, which implies that different boundary layer models should be considered. Furthermore, dynamic wake meandering (DWM) effects may include low-frequency changes in the wind speed seen by a FWT, which can have significant consequences for the global motions and mooring system loads.
The primary objective of the project is to enable more efficient design of floating wind turbine farms by improving load analysis methods.
Hydrodynamic models, methods for assessing novel mooring systems for wind farms, and wind field models, will be examined. Experimental testing will be used to support numerical analysis, including high-fidelity models.
To achieve this goal, the following secondary objectives are set:
- Assess novel mooring systems for wind farms in shallow and deep water.
- Improve predictions of low-frequency responses due to nonlinear hydrodynamic loads and wind loads, including better atmospheric models and wake effects.
- Challenge existing design practices for mooring system with improved tools, additional high-quality experimental data, and new analysis approaches.
- Improved design tools for FWT mooring systems, including material models and multiple FWT in a park.
- Experimental data for validation of tools.
- Recommendations for design analysis.
WINDMOOR will generate new data for validation of existing theories and methods, implement new methods, and develop new practical solutions and recommendation.
The activities proposed here - the generation of new experimental data, application and validation of different hydrodynamic load models, and development of improved wind field models – will all aid in reducing the limitations of today’s analysis tools. Furthermore, the response studies, optimisation, and analysis of novel mooring system designs will provide better understanding of the design space for FWT mooring systems.