Dear Purushotham,
Input WtrDpth in HydroDyn is set to 200 m in the FAST model NREL has provided for the MIT/NREL TLP.
You’re understanding of how to set a free-decay response is correct.
If you are still experiencing an error, even under small initial platform displacements, your problem could be caused by the use of the FEAMooring mooring module, and the known sensitivity to FEAmooring inputs Tension and LUnstrLen. See the following forum topic for more information: For TLP in Still Conditions, Pretension not matching with output tension - #4 by Jason.Jonkman. As a result of this sensitivity, it may be difficult to set up FAST models using FEAMooring with initial platform displacements. The other mooring modules in FAST (MAP++ and MoorDyn) do not have similar problems, so, you may wish to switch to one of those.
Best regards,