Date of Award

Spring 2023

Project Type

Thesis

Program or Major

Civil Engineering

Degree Name

Master of Science

First Advisor

Yashar E Azam

Second Advisor

Erin S Bell

Third Advisor

Raymond Cook

Abstract

Non-renewable energy sources such as fossil fuels have been the primary source of power generation around the world for decades. With these non-renewable energy sources rapidly depleting, the focus has switched to renewable, more environmentally friendly sources of energy, including wind energy. According to the Offshore Wind Market Report 2022, the U.S. has approximately 42 MW of offshore wind energy in operation, with 932 MW under construction and 18,581 MW in the permitting phase (Musial et al., 2022). Much of this potential comes from floating offshore wind turbines (FOWT) in waters greater than 60m of depth, where the winds are more consistent, and the turbines are larger with greater power generation potential. While researchers and industry professionals have identified the potential that FOWT can generate, past research has mainly focused on the response and optimization of the mooring lines and platforms under different loading conditions. Few studies have focused on the response of the turbine tower, specifically under extreme operating conditions. This study analyzed the behavior of a FOWT tower under extreme operating conditions. To do this, a 15 MW semi-submersible turbine was simulated at a location in the Gulf of Maine. Climate data was obtained from NOAA buoy data and extrapolated to represent conditions that the turbine would face in extreme operating conditions for a 50-year storm. These extreme conditions were used in conjunction with the aero-hydro-servo-elastic code, Horizontal Axis Wind Turbine Simulation Code 2nd Generation (HAWC2) from the Technical University of Denmark (DTU). A coupled time domain wind-wave analysis was used with a combination of different wind and wave inputs to determine how the turbine tower reacts to extreme loading. The parked condition load cases with 50-year extreme climate condition inputs were used to determine the resulting moments and stresses on the turbine tower. Results show that tower base yielding is the controlling factor in the design of a FOWT tower when compared to shear and fatigue stresses. In addition, an uncoupled time domain analysis was compared to the coupled analysis and showed that the uncoupled analysis overpredicts stresses in the tower base. While an uncoupled analysis may be suitable for preliminary design, it is important for final design to perform a coupled analysis for most accurate results. Lastly a sensitivity study was completed to analyze how the tower base reacts to changes in wind speed and wave height. It was concluded that a linear relationship between wind speed and average stress and wave height and average stress. However, changes in wind speed and wave height do not appear to have a significant effect on changes in design normal stress.

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