An approximate modelling of the efforts
The hydrodynamic loads induced by a breaking wave on a wind turbine mast can be modelled in two ways:
- The first is to add the impact load to the force induced by the non-breaking portion of the wave. This approach is based on the curling factor, which is poorly defined. The models commonly used by the industry fall into the first category.
- The second approach is based on the exact geometry of the wave. In this case, the mast is “cut” into different portions and the hydrodynamic load depends on the depth of immersion of these portions in the fluid. Existing models using this approach rely on different formulations to deal with partially immersed and fully immersed portions. The force is therefore modelled in a discontinuous manner.
Two semi-analytical models to take better account of the breaking waves
Paul Renaud and his co-authors have therefore proposed two semi-analytical models that consider the progressive immersion of the mast in the fluid and the loads that are continuously applied to the structure. The results of these models were compared with numerical simulations of breaking waves, and it was found that:
- The predicted loads for the upper part of the mast are overestimated, due to the strong deformation of the wave crest by the structure;
- In contrast, the loads obtained for the partially submerged portions of the mast are similar to those obtained numerically and show an underestimation in the commonly used formulations described by Nestegård et al. in 2004, and Hansen & Kofoed-Hansen in 2017.
Towards a simplified model for use by the floating wind industry
The two semi-analytical models allow, with different assumptions, the calculation of the extreme loads induced by a breaking wave in the submerged sections of the mast, without the need to define the curling factor. However, they require knowledge of the exact shape of the wave, which is hardly feasible in practice.
The next step is to develop a simplified model that can be used by technology developers to better account for wave breaking when designing their floating wind turbines. The necessary input data will be the wave height, speed, and curling factor. The latter will be defined more precisely thanks to the various works carried out in the framework of the collaborative project DIMPACT.
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