many floating systems are shown and their characterised in the offshore magazine(2000).one of the units ,the deep draft submersible(DDS)shown in fig.2.88(halkyard,et al 2002)is designed not necessarily to minimise the excitation force acting on it.rather,the plate structure in DDS provides the unit with substantial added mass in the vertical direction so that the natural heave legs and a deepdraft hull.this configuration was developed to reduce both the vertical and lateral excitation forces on DDS so that heave motions of the unit were reduced and mooring line forces due to wave drift and current were minimised.