Dear Hao,
I haven’t checked all of your numbers, but just a few comments:
- All of the 6x6 matrices, for mass, added mass, and stiffness, must be expressed relative to the same origin. I would suggest using (0,0,0) or the full-system center of mass (0,0,-78 m). The transformation from one system to another has been discussed in the following forum topic: Time domain analysis by AQWA - #2 by Jason.Jonkman.
- The spar alone center of mass is (0,0,-89.9155 m).
- The body weight restoring term (-mgz_CG) should be based on the full-system mass and center of mass, not just the mass and center of mass of the spar. It looks like you are mixing the mass of the spar alone (7.466E6 kg) and the full-system center of mass (-78 m).
Best regards,