You can create constraints between structures in the Hydrodynamic Diffraction analysis by using one or more linear Connection Stiffness matrices. These can be defined between the ground and a structure, or between two structures. For example, if you create a Connection Stiffness matrix between the float and the central column, then define large values for X-X, Y-Y, RX-RX, RY-RY, RZ-RZ and leave all other terms as zero, the float will be constrained so that its relative motion with respect to the column is almost completely in the Z direction.
Connection Stiffness only affects the Hydrodynamic Diffraction analysis. It is assumed that the effect of the Connection Stiffness is replaced by actual modelled connections (e.g. an arrangement of fenders) in any downstream Hydrodynamic Response calculations.
I hope this helps!