Dear @Shengtao.Zhou,
I agree with your comments.
When the ability to model structural flexibility and member-level loads in floating platforms was introduced in OpenFAST v2.6 and newer, we introduced the functionality in HydroDyn to model multiple potential flow bodies, with optional hydrodynamic interaction. So, for the OC4-DeepCwind semi, you could model each offset column (and perhaps the center column) as distinct potential-flow bodies with the pontoons and braces represented via the strip-theory formulation. Then in SubDyn, you could model the offset columns (and perhaps the center column) as rigid links with the pontoons and braces represented as beam element. This set-up would allow you to capture the large-volume effects from potential flow, while still capturing the structural flexibility and member-level loads within the pontoons and braces.
The following OpenFAST r-tests for standalone HydroDyn demonstrates this approach for the OC4-DeepCwind semisubmersible: r-test/modules/hydrodyn/HydroDyn_NBodyMod_cases at main · OpenFAST/r-test · GitHub, but at this time, there is not a full OpenFAST model available in the public domain that supports floater structural flexibility per the following issue: Add Test case for flexible platforms · Issue #39 · OpenFAST/r-test · GitHub.
The following NREL technical report is likely of interest to you have if you have not read it already: https://www.nrel.gov/docs/fy20osti/76822.pdf.
Best regards,