The hydrodynamic interaction of two bodies floating in waves is studied. The two-body hydrodynamic coefficients of added mass, wave damping and exciting forces and moments are calculated using the irregular frequency free radiation-diffraction potential solution of the improved Green integral equation associated with the free surface Green function (Hong 1987) according to the conventional two-body analysis. It is well known that the conventional two-body potential solution with usual grid fineness largely overestimates the hydrodynamic coefficients at and near the resonance frequency of the free surface in the gap between two floating bodies moored side-by-side in close proximity (Huijsmans et al. 2001, Hong et al. 2005). The two-body diffraction problem has been solved by both the conventional two-body analysis without damped free surface condition and a boundary matching method with and without damped free surface condition. Numerical results of the wave exciting force coefficients of two identical caissons floating side by side obtained by the two methods have been presented and the discrepancies between them have been discussed. Particular attention is paid to the wave elevation in the gap at the resonance frequencies. Amplitudes and phases of the scattering wave elevations in the gap at the first three free surface resonance frequencies computed by the boundary matching method without damped free surface condition have been presented. It has also been shown that the unrealistic wave elevation due to the resonance of the free surface in the gap can be reduced by imposing the damped free surface condition upon the flow in the gap as used in the oscillating water column hydrodynamics (Hong et al. 2004).

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