Dear Nikos,
I’m sure it is possible to add additional DOFs and the generalized modes approach to OpenFAST, with work. If you intend to make such a change to the source code, I would recommend starting with the version where we added our capability for modeling floating substructure flexibility and member-level loads (github.com/OpenFAST/openfast/pull/537), rather than starting with an older version.
As you could probably tell based on my prior comments/questions, I’m not sure I’ve fully understood how you intend to make the changes you are proposing. I’ve summarized the changes I would expect to be required at a very high level below:
- SubDyn – This is where you should add the new structural DOFs you want (mass-spring damper elements?). This would require changing the input file/processing, adding DOFs and associated equations of motion, ensuring that these additions are compatible with the underlying reduction approach used by SubDyn (Guyan reduction, Craig-Bampton reduction, and Static Improvement Method), and modifying the input and output meshes (to ensure that the motions of the new DOFs are output and the hydrodynamic loads applied to the new DOFs are input).
- HydroDyn - This is where you should add the new hydrodynamic models you want (generalized modes approach). This would requiring changing the input file/processing, changing how the WAMIT data is processed to compute hydrodynamic loads, and modifying the input and output meshes (to ensure that the motions of the new DOFs are input and the hydrodynamic loads applied to the new DOFs are output).
- Glue Code - This is where the coupling between the new inputs and outputs in SubDyn and HydroDyn (fluid-structure interaction) would occur. Assuming you don’t add new meshes (instead, only modify existing meshes), you may not need to change anything here.
Best regards,