Date of Award

8-2011

Document Type

Thesis

Degree Name

Master of Science (MS)

Legacy Department

Mechanical Engineering

Advisor

Zumbrunnen, David

Committee Member

Wagner , John

Committee Member

Testik , Firat

Abstract

Wind has good potential for contributing to the national energy supply. Offshore sites and deep sea locations can be especially attractive as the wind turbine market grows. In such places larger wind resources are available with reduced turbulence intensity and wind shear. In addition, visual impact along with noise aspects are reduced. Offshore siting requires greater attention to structural stability and endurance. Forces on drive train components, such as the bearing system, are not well understood.
This work presents the development a model that calculates dynamical forces in drive train components of off-shore wind turbines. The model of a 5MW off-shore wind turbine was developed based on site conditions for the nearby South Carolina coast. The model accounts for elastic deformation of the tower and distributed loads due to gravity, wind, and waves on the wind turbine elements and tower. A finite element computational model was implemented with external forces estimated from analytical models. The main elements of the turbine were based on actual 5MW wind turbine specifications. The tower was represented as a hollow, tapered steel cylinder with a foundation fixed rigidly to the sea floor. A mono-pile supporting structure was specifically represented, due to its applicability to the relatively shallow coastal waters of South Carolina.
The results from time-domain analysis were shown to agree with results generated from other studies. The dynamic response of mean values of loads on drive train components were found to be very similar to those for land-based wind turbines. It was also concluded that magnitude of axial force R_bx in the drive train components depend mostly on thrust force produced on the rotor by the three turbine blades. Its maximum value is determined by peak in thrust force and its periodicity is a result of changing thrust force, when blades rotate. To show the influence of thrust force and ocean wave force on force R_bx, results were presented also in frequency domain. It was shown that force R_bx has the dominant frequency of 0.2 Hz, which is the frequency of the thrust force. Additionally, eigenfrequency analysis was performed to show the lowest natural frequency of the system. It was found to be 1Hz, which corresponds to the fore-aft oscillation of the tower. This value is higher than frequencies of externally applied force that may guarantee that resonance will not occur in the system. Unlike axial forces, vertical forces in drive train components R_bz only determined by weight of components and any change in wind speed, ocean wave height and ocean wave period do not affect the tower deflection in vertical direction.

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