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Nonlinear Hydrodynamic Effects for Bottom-Fixed Wind Turbines
Norwegian University of Science and Technology, Faculty of Engineering Science and Technology, Department of Marine Technology.
2013 (English)MasteroppgaveStudent thesis
Abstract [en]

This master thesis investigates possible effects from nonlinear wave loading on bottom fixed offshore wind turbines. The resonant phenomena springing and ringing are reviewed with focus on their cause, and their possible occurrence in a bottom fixed wind turbine. To investigate the effect of a nonlinear wave load model on a wind turbine, the calculation scheme in the wind turbine design-tool FAST has been extended with a new wave load model. The proposed load model was chosen because it could be coupled with fully nonlinear incident waves, and it is also expected that it can predict some aspects of the loads leading to ringing in a structure. The load model was developed by Rainey, and is based on conservation of energy arguments. The new load model is compared with experiments on a cylinder in regular waves, and with the existing load model in FAST (Morison?s equation). The numeri- cal wave generation has been performed with a nonlinear Fourier approximation method, developed by Fenton. Results show similar prediction of the first har- monic load, which is slightly underpredicted by both. Good comparison is present for the second harmonic for low kr-values for both models. However, for larger kr-values significant overprediction is present, with Rainey predicting the largest values. Reasonably good agreement is found or the high kA-values for the third harmonic, when calculated by Rainey, while some underprediction is present when calculated by Morsion. The results also indicates that for slender structures, the nonlinearities in the incident waves, give a larger contribution to the loads than nonlinearities originating in the load formulation. Simulations performed on a full scale turbine, have shown small differences be- tween the two load models for turbine with a running rotor. Differences between results with a linear irregular sea and a fully nonlinear irregular sea are somewhat larger, but still the effect on improving the wave model is limited. This is due to the fact that the motion of the turbine is governed by the aerodynamic forces. When the rotor has been set in a parked condition, sever springing occurs for all sea states investigated. The springing in a parked condition sometimes has a burst-like increase in amplitude, and time evolution of the tower top deflection is close to the one observed for ringing. However, no clear cases of ringing have been observed in these simulations. Both load models predicts the resonant oscillations, when used with either linear or fully nonlinear waves. The amplitude of the resonant oscillations are however dependent on the incident wave model. In general the fully nonlinear incident wave model leads to larger amplitudes in the resonant oscillations. For the case of fully nonlinear waves, there is a difference between the two load models, which is not present when linear waves are used. Wave induced resonant oscillations have not been encountered when the turbine is running, which indicates that the reason for the resonant oscillations in a parked condition is the lack of aero-elastic damping. Some signs of a transient resonant phenomena is seen in strong winds, with additional indication that it is not triggered by waves, but the amplitude of the oscillations might be affected by the wave loading.

Place, publisher, year, edition, pages
Institutt for marin teknikk , 2013. , 183 p.
URN: urn:nbn:no:ntnu:diva-22294Local ID: ntnudaim:9142OAI: diva2:648749
Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2013-09-16Bibliographically approved

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