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Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Mechanical Engineering

Year Degree Awarded

2016

Month Degree Awarded

May

First Advisor

David P Schmidt

Second Advisor

Matthew A Lackner

Subject Categories

Ocean Engineering

Abstract

There is great potential for the growth of wind energy in offshore locations where the structures are exposed to a variety of loading from waves, current and wind. A variety of computer-aided engineering (CAE) tools, based largely on engineering models employing potential-flow theory and/or Morison's equation, are currently being used to evaluate hydrodynamic loading on floating offshore wind turbine platforms. While these models are computationally inexpensive, they include many assumptions and approximations. Alternatively, high-fidelity computational fluid dynamics models contain almost no assumptions, but at the cost of high computational expense. In this work, CFD simulations provide detailed insight into the complex fluid flow that has not been captured experimentally, nor can be attained with reduced-order models.

This work includes a thorough validation of the various CFD toolboxes necessary for simulating offshore floating wind turbine platforms in the ocean environment, from numerical wave propagation to fluid-structure interactions. The fundamental physics of flow around complex structures is examined through various studies to better understand the effects of a fluid interface, truncated ends, structure size, multi-member arrangements and environmental conditions. These factors are explored in terms of drag, lift and frequency of the loads. Additionally, motion of structures in free decay tests and waves are investigated. The work provides insight into the complex fluid flow around floating offshore structures of small draft in a variety of environmental conditions. CFD simulations are used to assess assumptions and approximations of reduced-order engineering models, and explain why, and in which conditions, these models perform inaccurately. Finally, the work provides suggestions for improvements to engineering tools often used for hydrodynamics modeling of floating offshore wind turbines.

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