Empirical calculation of roll damping for ships and barges

For a large floating structure in waves, the damping is computed by the lin ear diffraction/radiation theory. For most degrees of freedom, this radiati on damping is adequate for an accurate prediction of the rigid body motions of the structure at the wave frequencies. This is not particularly true fo r the roll motion of a long floating structure. For ships, barges and simil ar long offshore structures, the roll damping is highly nonlinear. In these cases the radiation damping is generally quite small compared to the total damping in the system. Moreover, the dynamic amplification in roll may be large for such structures since the roll natural period generally falls wit hin the frequency range of a typical wave energy spectrum experienced by th em. Therefore, it is of utmost importance that a good estimate of the roll damping is made for such structures. The actual prediction of roll damping is a difficult analytical task. The nonlinear components of roll damping ar e determined from model and full scale experiments. This paper examines the roll damping components and their empirical contributions. These empirical expressions should help the designer of such floating structures. The nume rical values of roll damping components of typical ships and barges in wave s and current (or forward speed) are presented. (C) 2001 Elsevier Science L td. All rights reserved.