Dear @Jason.Jonkman ,
I am simulating the free decay of an onshore wind turbine. And I model the tower in Subdyn. Only Elastodyn and Subdyn is enabled in .fst file. In Elasotdyn file, I turned off all tower DOF, and turned on all platform DOF:
True FlapDOF1  First flapwise blade mode DOF (flag)
True FlapDOF2  Second flapwise blade mode DOF (flag)
True EdgeDOF  First edgewise blade mode DOF (flag)
False TeetDOF  Rotorteeter DOF (flag) [unused for 3 blades]
True DrTrDOF  Drivetrain rotationalflexibility DOF (flag)
True GenDOF  Generator DOF (flag)
False YawDOF  Yaw DOF (flag)
False TwFADOF1  First foreaft tower bendingmode DOF (flag)
False TwFADOF2  Second foreaft tower bendingmode DOF (flag)
False TwSSDOF1  First sidetoside tower bendingmode DOF (flag)
False TwSSDOF2  Second sidetoside tower bendingmode DOF (flag)
True PtfmSgDOF  Platform horizontal surge translation DOF (flag)
True PtfmSwDOF  Platform horizontal sway translation DOF (flag)
True PtfmHvDOF  Platform vertical heave translation DOF (flag)
True PtfmRDOF  Platform roll tilt rotation DOF (flag)
True PtfmPDOF  Platform pitch tilt rotation DOF (flag)
True PtfmYDOF  Platform yaw rotation DOF (flag)
To simulate the free decay, I want to apply intial displacement at tower top in x axis. So I applied an initial platform displacement of 1 meter in x axis with all other intial condition keeping 0:
0 OoPDefl  Initial outofplane bladetip displacement (meters)
0 IPDefl  Initial inplane bladetip deflection (meters)
0 BlPitch(1)  Blade 1 initial pitch (degrees)
0 BlPitch(2)  Blade 2 initial pitch (degrees)
0 BlPitch(3)  Blade 3 initial pitch (degrees) [unused for 2 blades]
0 TeetDefl  Initial or fixed teeter angle (degrees) [unused for 3 blades]
0 Azimuth  Initial azimuth angle for blade 1 (degrees)
0 RotSpeed  Initial or fixed rotor speed (rpm)
0 NacYaw  Initial or fixed nacelleyaw angle (degrees)
0 TTDspFA  Initial foreaft towertop displacement (meters)
0 TTDspSS  Initial sidetoside towertop displacement (meters)
1 PtfmSurge  Initial or fixed horizontal surge translational displacement of platform (meters)
0 PtfmSway  Initial or fixed horizontal sway translational displacement of platform (meters)
0 PtfmHeave  Initial or fixed vertical heave translational displacement of platform (meters)
0 PtfmRoll  Initial or fixed roll tilt rotational displacement of platform (degrees)
0 PtfmPitch  Initial or fixed pitch tilt rotational displacement of platform (degrees)
0 PtfmYaw  Initial or fixed yaw rotational displacement of platform (degrees)
As the tower height is 87.6, and I need to keep tower zeroed in Elastodyn. So I only assigin 0.001 meter to tower height, by changing TowerBsHt and PtfmRefzt to 87.599:
87.6 TowerHt  Height of tower above ground level [onshore] or MSL [offshore] (meters)
87.599 TowerBsHt  Height of tower base above ground level [onshore] or MSL [offshore] (meters)
0 PtfmCMxt  Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmCMyt  Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmCMzt  Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
87.599 PtfmRefzt  Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters)
And platform has no mass and inertia:
0 PtfmMass  Platform mass (kg)
0 PtfmRIner  Platform inertia for roll tilt rotation about the platform CM (kg m^2)
0 PtfmPIner  Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
0 PtfmYIner  Platform inertia for yaw rotation about the platform CM (kg m^2)
In Subdyn file, I have located the interface point at (0, 0, 87.599).
But when I start simulation, it aborted with following warning:
Running ElastoDyn.
Nodal outputs section of ElastoDyn input file not found or improperly formatted.
Running SubDyn.
Fixed bottom case detected
Performing CraigBampton reduction 66 DOFs > 0 modes + 6 DOFs
Using static improvement method for gravity and ext. loads
Calculating Full System Modes for output files
Exporting Summary file
Time: 0 of 120 seconds.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:
Denominator is zero in GetSmllRotAngs().
OpenFAST encountered an error at simulation time 0.49375 of 120 seconds.
Simulation error level: FATAL ERROR
Aborting OpenFAST.
My OpenFAST version is v3.2.1.

Could you please give me some suggestion on this error?

For the damping in Subdyn, I am also confused. It has Damping Ratios for each retained mode and GuyanDampMod. Are there any posts explaining this point? As in my previous model, Elastodyn adopted 1% of critical damping for tower. I don’t know the equivalent configuration in Subdyn.
How about:
True CBMod  [T/F] If True perform CB reduction, else full FEM dofs will be retained. If True, select Nmodes to retain in CB reduced system.
4 Nmodes  Number of internal modes to retain (ignored if CBMod=False). If Nmodes=0 > Guyan Reduction.
1 JDampings  Damping Ratios for each retained mode (% of critical) If Nmodes>0, list Nmodes structural damping ratios for each retained mode (% of critical), or a single damping ratio to be applied to all retained modes. (last entered value will be used for all remaining modes).
0 GuyanDampMod  Guyan damping {0=none, 1=Rayleigh Damping, 2=user specified 6x6 matrix}
 Can Subdyn model tapered beam now? Actually I used different crosssection property at two joints connecting one element. But the Subdyn file said Tapered EB (unavailable) and 2node tapered Timoshenko (unavailable):
1 FEMMod  FEM switch: element model in the FEM. [1= EulerBernoulli(EB); 2=Tapered EB (unavailable); 3= 2node Timoshenko; 4= 2node tapered Timoshenko (unavailable)]
Regards,
Ran Tu