A numerical procedure is described for predicting the motion and structural responses of the very large floating offshore structures supported by multiple 3-D floating bodies of arbitrary shape in multi-directional irregular waves. The developed numerical approach taking into account of the hydrodynamic interactions among the multiple floating bodies is based on a combination of the 3-D source distribution method, the wave interaction theory, the finite element method and the spectral analysis method to get the significant values of the dynamic responses in the multi-directional irregular waves. The effects of wave interactions and directionality on the dynamic responses of a very large offshore structure, which is semisubmersible ring type, are numerically examined.