Dear Jay,
As I said in my prior post, LUnstrLen is the stretched length corresponding to the Tension. If you expect a pretension of 6868 kN for the undisplaced platform, then this stretched length should be 105 m for your system; using math similar to what you’ve described, you could calculate the actual unstretched length required (based on a pretension of zero) and the stretch due to the pretension to achieve a stretched length of 105 m.
NREL has much more experience with MoorDyn and is the mooring module we typically use for mooring dynamics.
Indeed HydroDyn calculates buoyancy forces in addition to drag and inertia forces. See section 6.8.1 of the draft HydroDyn User’s Guide and Theory for guidance on how to set-up a floating system model to be equilibrium in the heave direction.
Best regards,