Dear @Maurizio.Collu,
I agree that due to SubDyn’s assumption of linearity, the change eigenmodes/eigenfrequencies associated with the change in pretension is not captured by SubDyn if you are applying active control of cable pretensioning.
I know that NREL has tested the capability to actively control cable pretensioning, but I’m not aware of a model available in the public domain that demonstrates that.
Regarding the simulation of mooring line or tendon loss in the stationkeeping system for simulating DLC 9.1 and 10.1, we discussed that recently in a different forum topic: How to simulate IEC 61400-3-2 DLC 9.1 (10.1) in OpenFAST. (The easiest approach is to make a small change to the source code.) That said, I know NREL has plans to add the ability to simulate line loss directly within MoorDyn in an upcoming release (but I’m not sure exactly when yet).
Best regards,