Graduate Thesis Or Dissertation
 

Hydrodynamic characterization of floating offshore wind turbines : an experimental and numerical multi-degree of freedom analysis

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https://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/gb19ff48m

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  • Wind energy has become a crucial resource in sustainably meeting increasing global energy demands. Recently, offshore wind energy has been gaining traction due to its higher gross resource and larger unclaimed real-estate relative to its onshore counterpart. Floating offshore wind turbines (FOWTs) are increasingly popular, particularly designs with semisubmersible platforms. However, transitioning from bottom-mounted to floating platforms introduces large complexities, e.g., due to wind-wave-structure-mooring interactions, and more research is needed to correctly estimate FOWT behavior. While, intensive validation campaigns with mid-fidelity numerical models could estimate FOWT behavior in the linear wave frequency region, they have consistently underestimated large low-frequency excitation observed in physical experiments. This low-frequency excitation occurred near the system’s surge and pitch natural frequencies, and was determined to be hydrodynamic in nature. Further investigation has suggested that the numerical underprediction was due to a mischaracterization of viscous drag terms and the influence of nonlinear wave hydrodynamics. Several suggested correction methods Have represented the low-frequency surge and pitch excitation, with various levels of success. However, these methods require large amounts of a priori information or are not widely applicable to differing conditions. This study investigates the impact of rotational viscous damping terms on improving the low-frequency hydrodynamic behavior of mid-fidelity FOWT models. The goal was to determine a physically justifiable, low parameter model methodology for accurately capturing the low frequency response. The project included both experimental system identification and numerical validation components. Free decay tests in surge, heave, and pitch were performed on a 1:50 scale model of the DeepCwind semisubmersible platform-supported wind turbine in the O.H. Hinsdale Directional Wave Basin at Oregon State University. Linear and quadratic damping coefficients were extracted from these tests using the PQ methodology and incorporated into the mid-fidelity hydrodynamic simulation program WEC-Sim. The effectiveness of including the rotational damping terms to represent low-frequency surge and pitch response was assessed by comparing experimental wave excitation results against the WEC-Sim models with and without rotational damping terms. Other parameters of interest included heave response, up-wave mooring line tension, and down-wave mooring line tension in the linear wave frequency and low-frequency regions. An additional “Extreme” wave load case was conducted purely numerically to investigate the relative impact that the rotational damping coefficients have on capturing low-frequency excitation for high-intensity sea sates. Results showed that the inclusion of rotational damping coefficients had a negligible impact on platform position and mooring line tension in the linear wave frequency region. Simulations including rotational damping coefficients did show a slight improvement in capturing the low-frequency surge response; however, they were generally still underpredicted. Low-frequency pitch response was significantly overestimated by the three degree of freedom (Baseline) viscous damping model and underestimated by the six degree of freedom (Rotational) viscous damping model. These results suggest that rotational damping does impact low-frequency response but further parameter tuning was required for model predictions to better match experimental results. Additionally, WEC-Sim simulations implementing experimental sea surface elevation timeseries rather than numerically generated elevation timeseries for random wave load cases exhibited reduced error between Rotational viscous damping models and experimental results in the low frequency region. It was hypothesized that the error reduction occurred due increased low-frequency excitation from nonlinear incident wave kinematics inherently included in the real-world timeseries.
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file details GadasiJonahB2023.pdf 2023-04-06 Pubblico Scaricare