# Subdyn with Full FE option

Hello,

does anybody have experience with full FE calculation using subdyn.

We have a model of a tripod/tripile structure which runs stable and with plausible reuslts using the modal reduction (CM) but crashes if we switch it to full FE.
The whole setup including aerodynamics, bladed DLL … runs as onshore turbine and with subdyn using CM.

Here the settings:
-------------------- FEA and CRAIG-BAMPTON PARAMETERS---------------------------
1 FEMMod - FEM switch: element model in the FEM. [1= Euler-Bernoulli(E-B) ; 2=Tapered E-B (unavailable); 3= 2-node Timoshenko; 4= 2-nod
5 NDiv - Number of sub-elements per member
False 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 sys
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 mo

Anyhow I tried many different settings. Also different solver options. I have no clue where to continue looking.

Regards,
Florian

Dear Florian,

With the full FE model based on NDiv = 5, there may be many DOFs and the required time step may be very small.

As discussed in the SubDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/SubDyn_Manual.pdf, we typically recommend to use CBMod = True, to retain all C-B models up to 10 Hz, and to use SttcSolve = True.

Best regards,

Dear Jason,
thank you for the response.
I tried also with smaller NDiv.
I wanted to run only one short time serie with FE and this particular offshore structure as it is not fully symmetric I wanted to see if there is some difference to the CB reduced model. Particularly the effect of oblique bending resulting from misaligned load to principal axis.

Best Regards,
Florian

Hi,
after reviewing the model I was able to run it in FEM mode with a step size of 1e-6 s (Giving us 4 hrs simulation for 3 s). On the first glance, the displacements of the structure do look similar do reduction to 5 (?) modes but additional higher frequency movements.
Now we decided to reduce to 12 modes which covers 4 sets of very similar mode shapes also giving us higher frequency displacements.

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():

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
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

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).
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
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

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?