In a hydrodynamic interaction structure group with moonpools, the total number of degrees of freedom includes:
the total number of the rigid body motions, which is 6xM, where M is the number of the structures in the group;
the total number of the moonpool pressure modes in the group, which is 
, where 
and 
are the global sequence numbers (within the whole model) of the starting
and finishing structures of the hydrodynamic interaction structure group, 
,
is the number of moonpools associated with the s-th
structure, 
is the number of the pressure modes of the m-th
moonpool associated with the s-th structure, 
is the number of the pressure modes of the moonpools associated with the
s-th structure.
The total unsteady fluid potential around the hydrodynamic interaction structure group is usually expressed as a superposition,
 | (4–151) |
where 
is the isolated space dependent incident wave potential and
is the diffraction wave potential, 
is the RAO of the j-th rigid body motion of the
s-th structure; 
is the radiation potential due to the unit j-th rigid
motion of the s-th structure while other structures remain stationary;
is the RAO of the k-th pressure mode of the
m-th moonpool associated with the s-th structure.
is the radiation potential due to the unit k-th
pressure mode of the m-th moonpool associated with the
s-th structure.
Corresponding to the definition of the radiation potential component sequences in Equation 4–151, the coefficients of the added mass and the hydrodynamic damping matrices are arranged in the form shown in Coefficient Locations in Added Mass and Hydrodynamic Damping Matrices.
