Doctor of Philosophy (PhD)
Civil and Environmental Engineering
The increasing demand for reliable and clean energy with a low running cost, has developed wind energy harvesting industry in a fast rate during the past two decades, and has led to wind farms’ spreading in various regions with larger and taller wind turbines. Consequently, it is more likely that wind turbines are installed or planned to be installed in areas with higher possibility of natural multihazard. Therefore, flexible modern wind turbines are subjected to severe dynamic environmental loads. Generated excessive vibrations in the wind turbine’s structure due to these loads can have detrimental effects on energy production, structural lifecycle and initial cost. In the current research, a reference 5-MW National Renewable Energy Laboratory (NREL) wind turbine is modeled to evaluate the performance of the system and identify the best method to suppress the vibrations under diverse conditions. To achieve this objective, the overall wind-induced loadings in a parked condition were evaluated numerically and experimentally. In addition, a post-test dynamic analysis framework was developed to assess the inertial loads analytically, which showed the influence of rigidity on the magnitude of the base dynamics. Following the load estimation study, a dissipativity analysis study was carried out to find whether wind turbine towers require damping enhancement or rigidity modifications for vibration suppression. The results showed that damping enhancement is a more effective solution; hence, damping improvement techniques for vibration mitigation were examined. It was shown that viscous dampers, semi-active dampers and tuned mass dampers are acceptable solutions to reduce the vibrations. However, the evaluation of semi-active controllers’ performance required significant computational effort for multi-degree-of-freedom (MDOF) systems. Therefore, an analytical probabilistic modified independent modal space control (P-IMSC) proposed as an alternative approach to the current simulation methods. Based on the P-IMSC method, a wide range of controllers can be evaluated in a fraction of a second and optimum control parameters can be adjusted according to the control objectives. Finally, the performance of onshore and fixed offshore wind turbines under multihazard including wind, wave, earthquake, mass and aerodynamic imbalances for both parked and operating conditions were evaluated. For that purpose, Lagrangian-based model has been developed to consider blade-tower coupling effects. Blade Element Method (BEM) and Morison equation has been applied to calculate the wind force on the rotor and the wave load on the monopile of the offshore wind turbine, respectively. To model the earthquake event, north-south component of the ground acceleration of the El-Centro 1940 earthquake has been used. Vibration control has been done via external dampers designed based on the proposed analytical probabilistic approach. The disturbance of the system that has been used in P-IMSC method was a white noise excitation filtered based on the results obtained from the experimental study. It was shown that the analytical approach could be applied effectively to evaluate a wide range of possible control objectives and finally design appropriate dampers. Moreover, P-IMSC approach effectively reduced the CPU-time required to model the structure employing semi-active controllers, and it eliminated the need for significant computational resources. Also, it was observed that the controllers could effectively mitigate the responses of the nacelle for various loading events. Findings of this study will enhance the responses of wind turbines and heal excessive vibrations. Hence, it can increase the operational lifetime and reliability of the wind turbines. At the same time, by removing or reducing vibratory motions, wind turbines can generate energy more effectively. Moreover, mitigating acceleration of the system prevents malfunctioning that can be happened in the mechanical acceleration sensitive parts, which increase wind turbine’s availability and reduce the maintenance efforts. These entire can lead less capital and maintenance costs for wind farms and therefore less expensive and more competitive generated electricity. Moreover, the methodology can be broadly used for other type of offshore structures that are needed to be controlled, high-rise structures and other type of civil engineering structures and mechanical systems employing semi-active controllers.
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Rezaee, Milad, "Vibration Control in Wind Turbines for Multi Hazards" (2017). LSU Doctoral Dissertations. 4210.