Dear Jason

Thank you for your consistent help on my way of research!

I am now using OpenFAST to simulate an OC3 Monopile model, everything seems going well when the Nmodes of CBMod in SubDyn is 0.

However, I want to enable the structural damping of monopile to be 2%, which is consistent with my damping specification for superstructures set in ElastoDyn Module. I want to ask several questions about the realization:

1. If I want to enable substructure damping, is my follwing settings are correct?

-------------------- FEA and CRAIG-BAMPTON PARAMETERS---------------------------

3 FEMMod - FEM switch: element model in the FEM. [1= Euler-Bernoulli(E-B); 2=Tapered E-B (unavailable); 3= 2-node Timoshenko; 4= 2-node tapered Timoshenko (unavailable)]

3 NDiv - Number of sub-elements per member

True CBMod - [T/F] If True perform C-B reduction, else full FEM dofs will be retained. If True, select Nmodes to retain in C-B reduced system.

3 Nmodes - Number of internal modes to retain (ignored if CBMod=False). If Nmodes=0 → Guyan Reduction.

2 2 2 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).

2. I encountered a series of errors as follow when I switched the Nmodes from 0 to 3 in OpenFAST.

FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:Running with torque

and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll as written by J.

Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:

Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large UL input angles. The

solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will

be suppressed.

Additional debugging message from SUBROUTINE SmllRotTrans():

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption

violated in SUBROUTINE SmllRotTrans() due to a large platform displacement (ElastoDyn

SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from

SmllRotTrans() will be suppressed.

Additional debugging message from SUBROUTINE SmllRotTrans(): 0.11 s

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput:

Angles in GetSmllRotAngs() are larger than 0.4 radians.

FullOpt1_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4

radians.

FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.

FullOpt1_InputOutputSolve:HydroDyn_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.

FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.

FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_TSF

FWind_CalcOutput [position=(-27.522, 21.214, 559.17) in wind-file coordinates]: FF wind array

boundaries violated. Grid too small in Z direction (Z=559.17 m is above the grid).

SolveOption2:AD_CalcOutput:SetInputs:TwrInfl:getLocalTowerProps:Tower strike.

CalcOutputs_And_SolveForInputs:SolveOption1:FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in

GetSmllRotAngs() are larger than 0.4 radians.

FullOpt1_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4

radians.

FullOpt1_InputOutputSolve:SD_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4 radians.

FullOpt1_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4

radians.

Could you please help me in fixing that problem caused by simple swith of one parameter?

Or, is there an alternative way to realize the monopile damping?

Many thanks in advance for your reply!

Yours Sincerely

George