Doctor of Philosophy (PhD)


Civil and Environmental Engineering

Document Type



Offshore wind turbines (OWTs) are becoming more attractive than their onshore counterparts due to advantages such as high wind speed, less visual impact and less noise constraints in the marine area. However, the combined wind, wave and seismic loading and other environmental effects render the offshore wind turbines suffering from excessive vibration which adversely influences the system performance and structural integrity. In this regard, the present dissertation aims to develop effective structural control techniques to mitigate three-dimensional structural vibrations of offshore wind turbines. Analytical models of a fixed-bottom and floating offshore wind turbine are established using Euler-Lagrangian equation wherein the interaction between the blades and the tower is modeled. The aerodynamic loading, hydrodynamic loading, hydrostatic effect and mooring cables are also incorporated in the model. The turbulent wind field profile has been generated using Kaimal spectrum and Matlab codes have been developed to map the full wind field profile onto each span rotation of the rotating blades. The aerodynamic loading is calculated using the blade element momentum method where the Prandtl’s tip loss factor and the Glauert correction are considered. JONSWAP spectrum is used to generate wave time histories, and the hydrodynamic loading is estimated using Morison’s equation. The hydrostatic effect on the floating wind turbine is modeled based on the Archimedes principle. A dynamic linking library, MoorDyn, is used to model the mooring cables. To reduce the bi-directional vibrations of the fixed-bottom monopile OWT, a three dimensional pendulum tuned mass damper (3d-PTMD) and a three dimensional pounding pendulum tuned mass damper (3d-PPTMD) are proposed. Fatigue damage of the tower is calculated for the controlled and uncontrolled OWT using Miner’s rule and rain-flow cycle counting method. It is found that the proposed 3d-PTMD can reduce the fatigue damage under real metocean conditions. Also, research results indicate that the 3d-PPTMD can improve the performance of the 3d-PTMD facing off-tuning issues and reduce the stroke. Next, two novel controllers are proposed to reduce a spar type floating offshore wind turbine structural responses in roll, pitch and heave directions. Dual linear pounding TMDs (2PTMDs) and a three-dimensional nonlinear tuned mass damper(3d-NTMD) are proposed. The 2PTMDs mitigation effect is evaluated and compared with traditional TMDs and it is observed that the 2PTMDs can provide effective reduction in pitch and roll directions with a 50% smaller stroke compared to the traditional TMDs. To reduce the heave, pitch and roll responses, a 3d-NTMD is proposed and deployed inside the platform of the floating wind turbine. It is found that the 3d-NTMD is effective in reducing the three-dimensional vibrations of the FWT under parked and operational conditions. As a critical component of OWTs, blades are vulnerable to wind loading. In this dissertation, a novel two-dimensional nonlinear tuned mass damper inerter (2d-NTMDI) is proposed to reduce the vibrations of the blade in edgewise and flapwise directions as well as mitigating the fatigue damage of the blade. It is shown that the proposed 2d-NTMDI can effectively reduce the bi-directional response of the blades and experience relatively small stroke considering the limited available space inside the blade. Also, it is found that the proposed controller can effectively mitigate the fatigue damage of the wind turbine blades.



Committee Chair

Sun, Chao

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