Improved design methods for floating offshore wind
Offshore wind is going to be a major contributor to the production of renewable energy. Large scale fixed offshore wind farms are being built all over the world. However, not all countries with ambitions in offshore wind have shallow water locations suited for fixed offshore wind turbines at their disposal. Floating offshore wind might play an important role in harvesting the large wind energy resources at locations where fixed offshore wind energy is not an economically feasible option.
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The first floating offshore wind turbines are being designed and tested, however, risks and uncertainties in the design of these floating structures have to be reduced before full scale floating offshore wind farms are going to be operational. As part of the Dutch research project “TO2 – Floating Wind”, with TO2-partners Deltares, MARIN, ECN and NLR, Deltares and TU Delft improved existing design methods for floating offshore wind structures with respect to the behaviour of such structures in extreme wave conditions.
The result of this project is a fully nonlinear numerical wave tank which accurately describes;
- Viscous effects
- Extreme wave impacts
- Forces in mooring lines
The open-source CFD library OpenFOAM® forms the basis for this fully nonlinear numerical wave tank. This software makes use of a fully nonlinear Navier-Stokes/VOF solver for the computations of the two-phase flow field and a 6-DOF motion solver to compute the motions of the floating structure. OpenFOAM® was extended with waves2Foam, a wave generation and absorption toolbox and a catenary mooring line implementation for the simulations of the mooring system of the floating structure. Finally, the fully nonlinear potential flow solver, OceanWave3D, was utilised in a fully nonlinear and fully parallelised domain decomposed solver for the efficient computations of realistic sea states.
Thorough validation of this new design tool was performed in a series of two- and three-dimensional cases of increasing complexity. Here, the numerical solutions were found to be in excellent correspondence with experimental results. The numerical model was shown to be capable of providing accurate solutions with respect to wave propagation, wave induced forcing on a structure and wave induced motion of a free and moored floating structure. An elaborate description of these validation cases can be found in a thesis by Niek Bruinsma.
The new design tool has its strength for extreme design cases, where existing design tools fall short. With the development of this new model the hydrodynamic design system is closed. The model is not limited to floating offshore wind and can be applied to any system where detailed modelling of fluid structure interaction is needed, such as floating aquaculture, installation of Gravity-Base-Structures or jackup platforms.