The performance of circular thin plates in enhancing hydrodynamic damping of lightly damped offshore structures such as Spar Platforms and Tension Leg Platforms is studied. These platforms can experience resonant oscillations in heave under first and more likely second-order wave forces. As such, drag-augmenting devices are desired to limit the response amplitude to a safe range. This work includes two parts. The first part focuses on the damping coefficients' parametric dependence (KC number) and geometric dependence (thickness-to-diameter ratio). The study exploits a series of forced oscillation experiments. The experiment spans a range of KC numbers from 0.01 to 1.1 and a range of thickness-to-diameter ratios from 1/87.5 to 1/25. The second part of this study focuses on the underlying flow physics utilizing flow visualization experiments. The results of KC number dependence indicate three KC regimes where the damping coefficient behaves differently. Further flow visualization experiments demonstrate four unique vortex formation modes in these three KC regimes. A comparison of the slopes of the damping curve indicates that the interaction of vortices generated from two half cycles increases the damping effectiveness. For plates with different thickness-to-diameter ratio, similar characteristics of KC dependence are observed. The transitional KC numbers are thickness-to-diameter ratio dependent with the transitions occurring at larger KC numbers for thicker plates. While the total force experienced by oscillating plates is linear with KC number and essentially independent of the thickness-to-diameter ratio, a significant reduction in damping with an increase in thickness-to-diameter ratio is observed.
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