Dear Xin,
Actually, NREL did not use BModes to derive the tower mode shapes of the NREL 5-MW turbine atop the ITI Energy barge. At the time that system was analyzed, we used ADAMS to derive tower mode shapes because BModes was not yet capable (as described in other forum posts).
Assuming you can create your own BModes input file, I would expect the platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K) to be effected by the change to the structural density of the barge.
Best regards,