Date of Award

5-2014

Level of Access

Campus-Only Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

Advisor

Andrew J. Goupee

Second Committee Member

Richard W. Kimball

Third Committee Member

Vincent Caccese

Abstract

This thesis supports the development of cost effective methods for the design and evaluation of floating offshore wind turbines. Three main components are presented, including a cost of energy (COE) analysis, development of numerical analysis tools, and methods for performing scale model testing of floating offshore wind turbines. The COE analysis investigates the potential cost savings of using alternative wind turbine designs, namely vertical-axis wind turbines (VAWTs), as opposed to the more standard horizontal-axis wind turbine (HAWT). The unique arrangement of a VAWT allows the heavy generator and related components to be located at the base of the tower as opposed to the top, as is typical of a HAWT. This configuration lowers the topside center of gravity which reduces the platform stability requirements, leading to smaller and cheaper platforms. Results from the cost of energy study provide motivation to pursue VAWTs as a cost effective alternative to HAWTs for floating wind turbine applications and demonstrate the need to develop numerical analysis tools capable of modeling the floating VAWT system.

The second part of this work presents the Offshore Wind Energy Simulation (OWENS) toolkit, which is being developed in conjunction with Texas A&M as an open source, modular aero-elastic analysis code with the capability to analyze floating VAWTs. The OWENS toolkit aims to establish a robust and flexible finite element framework and VAWT mesh generation utility, coupled with a modular interface that allows users to integrate easily with existing codes, such as aerodynamic and hydrodynamic codes. This work describes the development of the WavEC2Wire hydrodynamics module, based on the WavEC2Wire analysis code developed by Marco Alves of the Wave Energy Center, and the coupling methods utilized to integrate with OWENS. A verification study is performed to validate the coupling methodology and provides the groundwork for pursuing more advanced analyses and supports the development of a wind/wave basin scale model test matrix.

The final section of this thesis builds upon scale model testing experience obtained by the University of Maine through 1/50th scale model tests performed at the Maritime Research Institute of the Netherlands (MARIN) on various platform types. These tests were able to capture the global dynamic behavior of commercial scale model floating wind turbine systems; however, due to the severe mismatch in Reynolds number between full scale and model scale, the strictly Froude-scaled, geometrically-similar wind turbine underperformed greatly. This work presents an alternate turbine design engineered to emulate the performance of the National Renewable Energy Laboratory (NREL) 5-MW turbine at model scale conditions. The design methodology for creating this wind turbine is presented as well as a comparison of the wind turbine performance under Reynolds numbers corresponding to model test Froude-scale conditions. The results of this work support the development of protocols for properly designing scale model wind turbines that emulate the full scale design for Froude-scale wind/wave basin tests of floating offshore wind turbines.

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