Dear Liu,
Until a recent pull request into the dev branch of OpenFAST, SubDyn was only applicable to fixed-bottom, not floating systems. However, the following pull request allows SubDyn to be enabled for floating offshore wind turbines, which is applicable to modeling floating substructure flexibility and the calculation of internal member-level loads: github.com/OpenFAST/r-test/issues/39. (The testing we have done has been for floaters that contain proprietary information that cannot be shared publicly.) If you wish to introduce a SubDyn model of the OC3-Hywind spar, you’ll have to switch to the dev branch of OpenFAST and you’ll likely have to make your own assumptions about the structural characteristics of the spar (as these details were never published).
Best regards,