Dear Abhinav,
I’m not sure I understand this:
Regardless, while I have seen some uncoupled analysis studies where the interface is at the tower top, the more common point is the transition piece because this is commonly where the design responsibility is split between the turbine designer and substructure designer.
Coupling software always presents its challenges. Some of the challenges for software coupling in the support structure are discussed in the following forum topic: FAST: Tower Model.
Best regards,