Dear Florence,
Here are my answers to your questions:
Yes, you can use hydro_K or mooring_K in BModes_JJ to represent the lumped (coupled springs, CS) stiffness of the foundation of the tower + monopile. But please keep in mind that hydro_K and mooring_K are applied at the platform reference point (0,0,0), so, you’ll want to ensure that you don’t set draft > 0 m. For example, for the OC6 Phase II system (DTU 10-MW turbine + monopile), you’ll want to set radius = 145.63 m, draft = 0 m instead of radius = 115.63 m, draft = 30 m.
The distributed elastic stiffness per unit length feature available in BModes_JJ would be useful for modeling the foundation via a distributed springs (DS) representation. FAST v8 and OpenFAST do not currently support such a model. At one point we developed a customized version of FAST v7 that supported DS modeling of the foundation. This is discussed in the following forum topic: Turbine-soil interaction. Influence on the mode shapes., especially my post dated Feb 01, 2013, where I share an example UserTwrLd_DS.f90 file.
Best regards,