Abstract
Nonlinear hydrodynamics play a significant role in accurate prediction of the dynamic responses of floating wind turbines (FWTs), especially near the resonance frequencies. This study investigates the use of computational fluid dynamics (CFD) simulations to improve an engineering model (based on potential flow theory with Morison-type drag) by modifying the second-order difference-frequency quadratic transfer functions (QTFs) and frequency-dependent added mass and damping for a semi-submersible FWT. The results from the original and modified engineering models are compared to experimental data from decay tests and irregular wave tests. In general, the CFD results based on forced oscillation tests suggest increasing the frequency-depending added mass and damping at low frequencies compared to first order potential flow theory. The modified engineering model predicts natural periods close to the experimental results in decay tests (within 5%), and the underprediction of the damping is reduced compared to the original engineering model. The motions, mooring line tensions and tower-base loads in the low-frequency response to an irregular wave are underestimated using the original engineering model. The additional linear damping increases this underestimation, while the modified QTFs based on CFD simulations of a fixed floater in bichromatic waves result in larger difference-frequency wave loads. The combined modifications give improved agreement with experimental data in terms of damage equivalent loads for the mooring lines and tower base.
Highlights
One of the challenges facing the development of floating wind turbines (FWTs) is to accurately predict the global responses due to nonlinear hydrodynamic loads on the floater [1,2,3,4,5]
The variations in the SIMA model are used to separate the effects of the modifications of the differencefrequency quadratic transfer functions (QTFs) and added mass and damping
Through modifying the added mass and damping based on the forced oscillations in computational fluid dynamics (CFD) simulations, the engineering tools can capture similar damping as the CFD model in the free decay
Summary
One of the challenges facing the development of floating wind turbines (FWTs) is to accurately predict the global responses due to nonlinear hydrodynamic loads on the floater [1,2,3,4,5]. In addition to the nonlinear wave loads, hydrodynamic coefficients, such as added mass or damping coefficients, significantly affect the dynamic responses of a semi-submersible FWT when using engineering models. The difference-frequency surge force and pitch moment QTFs from second-order potential flow theory are modified based on the estimated difference-frequency wave loads on a restrained floater subjected to bichromatic waves in the CFD simulations. The frequency-dependent added mass and damping coefficients from the first-order potential flow theory are modified based on the calculated linearized added mass and damping coefficients from CFD simulations of forced oscillations around the surge, heave and pitch natural periods. The engineering tool with modified QTFs and added mass and damping coefficients is validated against the free decay motions in surge, heave, and pitch (from both CFD simulations and experiments), and responses in irregular waves (from experiments).
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