Dear Hao Nguyen,
I would think you’d need to include the off-diagonal coupling terms to predict the correct natural frequencies of the NREL 5-MW turbine atop the OC3-Hywind spar buoy.
The full-system mass, center of mass, and inertia of the OC3-Hywind system, from which the 6x6 mass matrix can be formed, can be found in the following forum topic: The frequency component of flapwise.
Best regards,