Dear David,
I’m not understanding how you are arriving at your conclusions.
There should be no problem to simulate the V-shaped semi in OpenFAST / HydroDyn. But without changing the the source code, the hydrodynamic reference point for the hydrodynamic coefficients and hydrostatic restoring, must be (0,0,0), which is equivalent with the undisplaced / undeflected tower centerline at the mean sea level (which I assume would be the vertex of the V). And the hydrostatic restoring should be defined without consideration of center of mass offsets, i.e., with the center of mass also at (0,0,0), so as to not double count the body-mass-based restoring. The center of buoyancy and center of mass can be specified in OpenFAST’s HydroDyn and ElastoDyn modules, respectively, with offsets relative to (0,0,0).
I hope that helps.
Best regards,