Dear Yingyi Liu,
Modeling of the guy wires will require a change to the FAST source code, as the current version of FAST does not support this feature. In FAST v8, you could use e.g. MAP or MoorDyn as the guy wire model, but you’ve have to modify the source code to allow one end of the cable to connect to the tower-top/nacelle instead of the inertia frame (anchor).
The modeling of tower tilt in FAST has been discussed briefly for bottom-fixed systems in the following forum topic: Simulation with a tilted tower - #5 by Jason.Jonkman. But in your case, you have a floating wind system, which presumably means that you want to enable the six degrees-of-freedom (DOFs) of the floating platform. Again, you’ll need to modify the source code to consider this effect as FAST assumes the undeflected tower attaches vertically to the floating platform. You would have to modify the source code to set the position vectors and orientation of the tower coordinates to tilt relative to the platform.
Neither MAP nor MoorDyn consider rotational degrees, so if you consider all catenary lines to connect to the platform at the same point (fairlead), then the platform will be allowed to freely yaw around that point without inducing loads on the moorings. This approach would permit you to model a turret mooring without stiffness or damping. If you need to consider turret stiffness or damping or wish to space the fairleads apart, you would need to introduce a turret DOF, which would again require a change to the FAST source code.
Sounds like a fun project!
Best regards,