Dear Lei,
The “hydro_K” matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (4-3) of the OC3-Hywind report (perhaps with slight numerical rounding) except that the (6,6) element has been augmented with the “additional yaw spring” documented in Table 5-1 of the OC3-Hywind specifications report.
The “mooring_K” matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (5-3) of the OC3-Hywind report except that the (4,4) and (5,5) elements have been augmented with the contribution of system weight/gravity, which is not otherwise accounted for in BModes as explained in the following forum topic: BModes for blades submerged in water?. System weight is important for the pitch and roll restoring of deep-drafted floating platforms, such as spar buoys. The augmentation equals -mgz, where m is the total system mass (platform + ballast + tower + nacelle + rotor), g is gravity, and z is the vertical location of the center of gravity of the total system mass (z is negative in value for this case).
I hope that helps.
Best regards,