One of the difficulties of solving the hydrodynamic interaction problem is related to the fluid resonance phenomenon taking place in the gap between the adjacent bodies. Due to the absence of viscous flow effects in the potential flow diffraction and radiation calculation, unrealistic resonant wave oscillation may occur. It is necessary to simulate the additional damping due to viscous and separation effects to suppress these unrealistic wave phenomena by the ordinary potential theory.

In Aqwa, a new damped free-surface boundary is implemented by extending the widely used wave-absorbing beach method used in fully nonlinear time domain hydrodynamic analysis and applied on the free-surface between adjacent structures. This condition is expressed as

(4–36)

where is a damping factor which should be mainly determined by model tests or trial measurements, is a function which relates to the gap size, , between adjacent structures:

It is recommended that damping factor be set between 0.0 and 0.2. The value of 0 will give no effect, while 0.2 may result in heavy damping of surface elevation at the external LID panels (Cheetham, et al, [9]).

The range of external LID panels on the mean free surface between adjacent structures or within a moon-pool should be defined manually. The modified free surface boundary condition given by Equation 4–36 will be applied on those panels.