Dear @Yosuke.Matsumoto,
OpenFAST supports the modeling of hydro-elastics and member-level loads, but to get proper member-level loads, you must properly distribute the hydrodynamic loads across the member. This is possible through the strip-theory of solution of HydroDyn, but not through the potential-flow solution. In the potential-flow solution of HydroDyn, the potential-flow body is assumed to rigid whereby hydrodynamic loads are lumped at a point.
This is why in my forum post above I suggested that one typically matches potential-flow bodies in HydroDyn with rigid bodies (called rigid links) in SubDyn and strip-theory members in HydroDyn with flexible beam members in SubDyn. That said, HydroDyn, does support multiple potential-flow bodies, so, you can lump the loads at multiple points, which allows you to obtain proper loads on flexible bodies that interconnect the rigid bodies.
It looks like you are modeling a semisubmersible. For a semisubmersible, I would normally expect that you’d model the large-volume bodies (offset columns) as potential-flow bodies in HydroDyn and rigid bodies in SubDyn, and the thin members (pontoons, braces) as strip-theory bodies in HydroDyn and flexible beams in SubDyn. In this case, OpenFAST can calculate the member-level loads in the pontoons and braces, but not in the offset columns.
Best regards,