Abstract
Added mass and damping play a significant role in accurate prediction of floating wind turbine (FWT) motions, especially near the resonance frequencies. This paper investigates the still-water hydrodynamic characteristics of a semi-submersible FWT around the natural periods of surge, heave and pitch motion. A higher-fidelity tool (Computational Fluid Dynamics, CFD) based on OpenFOAM is employed in the numerical computations. The tool is validated against experimental measurements (decay tests and forced surge motions) and then applied to investigate the hydrodynamic characteristics of the whole floater and each column at different amplitudes of forced motions. The heave and pitch decay match well with the experimental measurements, whereas the CFD simulations underestimate the damping in the surge decay. However, better agreement is obtained between measured and numerically estimated surge force in the forced oscillations in surge. Furthermore, the added mass derived from the CFD simulation is around 12% larger than that estimated by the potential flow theory, except the estimated heave added mass under the largest heave motion (up to 35% larger). This additional added mass in the CFD simulations is due to the viscous effects. The damping shows a small dependence on the oscillation period and a larger dependence on the oscillation amplitude within the tested period range. At these frequencies, radiation damping is completely negligible compared to the viscous damping due to vortex shedding, and the accuracy of Morison's drag forces in capturing the viscous damping is sensitive to the drag coefficient.
Highlights
As a promising way of harnessing the energy from winds over deep water and farther offshore, floating wind turbines (FWTs) have gained more and more attentions in recent years, especially semi-submersible FWTs (Peiffer et al, 2011)
The objective of this paper is to provide improved knowledge about the hydrodynamic characteristics of a semi-submersible FWT that can be potentially applied in engineering tools
In order to investigate the still-water hydrodynamic characteristics of the floater for a semi-submersible FWT, a series of free decay and forced oscillation tests have been conducted in experiments and in a CFD
Summary
As a promising way of harnessing the energy from winds over deep water and farther offshore, floating wind turbines (FWTs) have gained more and more attentions in recent years, especially semi-submersible FWTs (Peiffer et al, 2011). In order to reduce the amplitude of the heave resonance and move the heave resonance periods outside the wave frequency range, heave plates are attached to the base of some columns These plates provide additional added mass and enhance the flow sep aration and vortex shedding processes that provide viscous damping (WEI et al, 2010). Lopez-Pavon et al (2015) computed, with a CFD commercial code (ANSYS CFX) and frequency domain panel method (WADAM), the hydrodynamic char acteristics on heave plates of a semi-submersible FWT, and found added mass and damping were largely dependent on the motion amplitude. Dunbar et al (2015) compared the discrepancy of heave and pitch decay of a semi-submersible FWT between CFD methods and engineering tools and found the discrepancy is associated with Reynolds-number-dependent viscous effects.
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