FAST is a wind turbine modelling program developed by the US National Renewable Energy Laboratory (NREL). FASTlink is an experimental package that can be used to run coupled OrcaFlex + FAST simulations of floating wind turbine systems. In such coupled simulations FAST models the turbine aerodynamic loads and response, and the turbine control and power take-off system, but OrcaFlex models the platform hydrodynamic loads and mooring loads and response. To use FASTlink you need to have OrcaFlex 9.8a (or later) installed.
Full details are available from NREL at: https://nwtc.nrel.gov/OrcaFlexInterface. In order to execute coupled OrcaFlex + FAST simulations you will need the software provided by NREL and the FASTlink DLL: