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Wind waves: Forces due to waves on offshore wind turbines
Gepubliceerd door: Publicatie datum:
ECN Windenergie 1-9-2003
ECN publicatienummer: Publicatie type:
ECN-C--03-097 ECN rapport
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Offshore wind energy is a high-priority research area in the Netherlands,since it is recognised that in 2020 a large part of the installed wind power in the Netherlands is located offshore. The offshore conditions are quite different from the conditions on land. Especially the loads on offshore wind turbines are quite different than on onshore wind turbines. The loads not only originate from the wind conditions, also hydrodynamic loads are present. For the calculations of the mechanical loads on wind turbines ECN applies the model PHATAS. Environmental conditions are input for this model. In the present project two computer programmes are developed for the description of the wave fields. The first tool (ROWS) generates a stochastic linear wave field. The second tool simulates single non-linear waves. The difference between these models is that the first simulates the irregular field of waves that under normal conditions impact the turbine. These wave fields are used in fatigue load calculations mainly. Very large waves cannot be simulated with this theory with high accuracy; therefore the second tool should be used. This tool simulates single large (non-linear) waves that impact the turbine. These large waves are used to investigate extreme loads on offshore wind turbines.

In order to include hydrodynamic loading in the calculations of fatigue loads on offshore wind turbines a time record must be generated that describes a wave field. Using linear wave theory, ROWS generates a time series of wave heights and the associated velocities and accelerations of the water particles. The validation of the stochastic linear wave generator is two-fold. Apart from the verification of the velocities and accelerations within individual simulated waves, the spectral density function of the generated wave field is of essential importance. Since this function is reproduced with sufficient accuracy, the wave field is considered to have the correct statistics.

Very large waves cannot accurately be simulated with linear theory. A first tool based on stream functions has been developed to simulate large waves. The stream function theory is described. The tool to calculate the waves uses Dean?s tables. Depending on the demands of the load calculations, several initialisations of the regular wave can be chosen. Breaking waves are not included in the model. Although the model is limited in its use, for water depths starting from 15m, like North Sea conditions, most encountered waves can be modelled. Since the stream function is solved up to a maximum of 10 modes, large waves in extremely shallow water can therefore not be simulated. This would require more modes. The present computer program cannot be extended. For the validation of the non-linear wave simulator, the results of the tool are compared with other simulation packages.

The waves around the tower of the turbine induce forces on its supporting structure. The wave-induced forces on the underwater construction of the wind turbine are described in the report. Both described wave models communicate with tools to simulate mechanical loads of offshore wind turbines (like PHATAS) through external data files. Wheeler stretching is included in both wave simulators in order to calculate water particle velocities and accelerations at the right positions along the supporting structure of the wind turbine. Through the Morison equation the velocities and accelerations are translated into forces on the offshore wind turbine.

While at the time of the definition of the project wave data were scarcely available, RIKZ made a lot of data available in the meantime. The tools of ECN to analyse measured wave data are useful for the analysis of North Sea wave conditions.

The recommendations are the following. The non-linear wave simulator is able to generate extreme waves in a large parameter space. However, large waves in extremely shallow water can not be simulated. The present computer program cannot be extended and as a result a different program should be developed.
In addition, the hydrodynamic forces should be validated with measurements on offshore wind turbines.

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