Dear Dongzhe.Lu,
The ElastoDyn and SubDyn modules of FAST v8 and OpenFAST are coupled via structure-to-structure coupling where the motions in 6 DOFs (displacements/rotations, velocities, accelerations) are transferred from ElastoDyn to SubDyn and the reaction loads (forces, moments) are transferred from SubDyn to ElastoDyn. The 6-DOF motion is referred to as the platform DOFs in ElastoDyn and the interface joint in SubDyn.
In your case, you’ve located the platform in ElastoDyn and the interface joint in SubDyn at the top of the tower, meaning the full support structure (tower + jacket) are modelled in SubDyn, and the tower geometry is zeroed and the tower DOFs are disabled in ElastoDyn. That looks like the correct approach and all 6 platform DOFs should be enabled in ElastoDyn
More information is provided in the online SubDyn documentation: openfast.readthedocs.io/en/main … e-coupling.
Best regards,