Dear Lain,
You are getting C66 = 1.099E8 Nm/rad because the OC3-Hywind model includes an additional yaw spring, i.e. C66 = 98,340,000 Nm/rad + 11,560,000 Nm/rad. The reason you are not getting the correct values for C33, C44, and C55 from your FAST v7 linearization is because of the influence of the hydrostatic stiffness and system weight (gravitational restoring) terms. The procedure for deriving the linear 6x6 stiffness matrix associated with the mooring system (without the hydrostatic and weight terms) from FAST v7 is explained in my Oct 30, 2012 post in the following forum topic: Questions about the HydroDyn Module.
BModes does not include the influence of system weight on the modal calculation. Body weight is important for the pitch and roll restoring of deep-drafted floating platforms, such as spar buoys. As such, The “OC3Hywind.bmi” model uses an adjustment to the (4,4) and (5,5) elements of the mooring system stiffness to augment the overall restoring in pitch and roll.
I hope that helps.
Best regards,