The radiation damping coefficients are determined by the radiation waves due to the unit structure's motions. In the Hydrodynamic Diffraction analysis, the hydrodynamic damping is of 6x6 matrix form. In your model, there is only heave motion in the time domain analysis, the radiation damping force will be contributed by the heave motion only, all the other damping coefficient elements, except heave-heave term, will have no effect as the motions in other DOFs are zero (or nearly zero due to fenders).
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