Estimation Methods for the Roll Damping Ratio of Floating Structures

This study compares four estimation methods for determining the roll damping ratio using a series of free roll decay tests conducted in a two-dimensional wave tank. Numerous studies have been performed to estimate the roll damping ratio of floating structures while considering nonlinear effects. In this work, a floating structure was tested at four initial roll angles to investigate the nonlinear behavior at large roll motions. The roll natural periods were determined using the Fast Fourier Transform and the peak-to-peak method applied to the time series of the free roll decay test. At initial roll angles below 10°, the roll damping ratios derived from the four methods exhibited similar results. However, at initial roll angles exceeding 10°, the results varied significantly across the methods, likely due to differences in their treatment of nonlinear viscous effects. This study examines the applicability of these methods for various initial roll angles and recommends the minimum roll angle and the number of rolling period required for accurate calculations from free roll decay tests. Further studies are needed to evaluate roll damping ratios for motions exceeding 10° while accounting for nonlinear viscous damping effects.