Dear SungWon,
In FAST v7, a couple minor customizations of the source code are required to build a model that follows the specifications of the OC3-Hywind system. You can read about these customizations in the following forum topic: Additional linear hydrodynamic damping in OC3 Hywind. However, from the error message you are getting, it sounds like you are using the wrong controller. The OC3-Hywind system uses the NREL 5-MW turbine, which does not rely on Kirk Pierce’s UserVSCont() routine. Instead, the NREL 5-MW turbine uses a torque and pitch controller implemented in GH Bladed-style DLL format. The DLL (i.e., DISCON.dll) is supplied with the NREL 5-MW models. These controllers require FAST’s interface to GH Bladed-style DLLs (BladedDLLInterface.f90), which is not by default linked in FAST v7. The OC3-Hywind system archive for FAST v7 includes a FAST v7 executable compiled with all of these customizations.
That said, FAST v8 is capable of modeling the OC3-Hywind system directly, without any customizations. In fact, the OC3-Hywind system is distributed as Test24 in the latest release of FAST v8. You may want to upgrade to FAST v8.
Best regards,