Unstable modes of the tower

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Milad.Abbasi
Posts: 2
Joined: Thu Nov 16, 2017 1:37 am
Organization: Sharif University of Thechnology
Location: iran

Unstable modes of the tower

Postby Milad.Abbasi » Fri Feb 22, 2019 11:28 pm

Hello to all,
I'm doing a project on the NREL 5-MW Baseline wind turbine with TLP Configuration. I tried to extract linearized models with FAST v8 and I realized that whenever the first side-to-side tower bending-mode DOF is on, an RHP zero appears in the 2*2 transfer function matrix from inputs [generator torque; collective blade pitch] to outputs [High-speed shaft power; Platform pitch tilt rotation]. Moreover, with turning the second side-to-side tower bending-mode on, an RHP pole appears in the transfer function matrix.
By applying the GSPI controller (when second side-to-side tower bending-mode is on), FAST encounters an error:

Code: Select all

FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption
violated in SUBROUTINE SmllRotTrans() due to a large tower deflection (ElastoDyn SetCoordSy). The
solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will
be suppressed.
 Additional debugging message from SUBROUTINE SmllRotTrans(): 3.25 s
 
 
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_TSF
FWind_CalcOutput [position=(3.77776E+08, -2.79132E+09, 2.71944E+05) in wind-file coordinates]:
Error: FF wind array was exhausted at 3.35 seconds (trying to access data at -2.07569E+07
seconds).
CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in
GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4
radians.

FAST encountered an error at simulation time 3.3 of 300 seconds.
 Simulation error level: FATAL ERROR
 
 Aborting FAST.

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Jason.Jonkman
Posts: 3961
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Unstable modes of the tower

Postby Jason.Jonkman » Mon Feb 25, 2019 6:46 am

Dear Milad,

Clearly your model is going unstable, but without knowing more about your simulation set-up, it is difficult for me to guess why. Does your model run stably without the controller enabled? Are you using the NREL 5-MW baseline turbine atop the MIT/NREL TLP or a TLP you developed yourself?

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Ajay.Patil
Posts: 4
Joined: Thu Feb 28, 2019 3:10 am
Organization: National Institute Of Technology Karnataka
Location: India

Re: Unstable modes of the tower

Postby Ajay.Patil » Thu Mar 14, 2019 6:30 am

Respected sir,
i am new FAST learner ..I am doing spar-torus platform ,in that spar modelling and turbine is everything according to NREL designing . so while performing this error occurs, so please guide me how to counteract this error..The Code showing error as follows :


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. Includes modifications for the
Hywind spar.


FAST_Solution:FAST_AdvanceStates:ED_ABM4:SetCoordSy:Small angle assumption violated in SUBROUTINE
SmllRotTrans() due to a large tower deflection (ElastoDyn SetCoordSy). The solution may be
inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 0.675 s


FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDyn_CalcOutp
ut: Angles in GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4
radians.


FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_TSF
FWind_CalcOutput [position=(-153.2, 111.81, 5.541) in wind-file coordinates]: FF wind array
boundaries violated: Grid too small in Y direction. Y=111.81; Y boundaries = [-85, 85]
CalcOutputs_And_SolveForInputs:SolveOption1:ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in
GetSmllRotAngs() are larger than 0.4 radians.
ED_HD_InputOutputSolve:HydroDyn_CalcOutput: Angles in GetSmllRotAngs() are larger than 0.4
radians.

FAST encountered an error at simulation time 0.725 of 3600 seconds.
Simulation error level: FATAL ERROR

Aborting FAST.


G:\New folder\FAST-Prog STC\STC>

Jason.Jonkman
Posts: 3961
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Unstable modes of the tower

Postby Jason.Jonkman » Thu Mar 14, 2019 6:41 am

Dear Ajay,

We've discussed this problem a bit by e-mail, but you haven't provided any more information that would help me identify why your model is going unstable. I suggest searching here on the forum to see how others have solved similar problems in the past. I would also suggest simplifying the model to debug. E.g. does the model still go unstable with fixed generator speed and disabled control? Can you isolate which structural degree of freedom (DOF) in ElastoDyn results in the model going unstable (by disabling all DOFs but a few)?

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Ajay.Patil
Posts: 4
Joined: Thu Feb 28, 2019 3:10 am
Organization: National Institute Of Technology Karnataka
Location: India

Re: Unstable modes of the tower

Postby Ajay.Patil » Thu Mar 14, 2019 7:03 am

thanks for replying..sir ,i tried with disabling few DOF's but it still showing error..so sir i will post here the code of elastodyn
Last edited by Ajay.Patil on Sat Mar 16, 2019 1:11 am, edited 1 time in total.

Jason.Jonkman
Posts: 3961
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Unstable modes of the tower

Postby Jason.Jonkman » Thu Mar 14, 2019 7:09 am

Dear Ajay,

I would not expect the model to go unstable with all structural DOFs disabled. So, I would suggest disabling all DOFs and systematically enabling them to identify the problematic DOF.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Ajay.Patil
Posts: 4
Joined: Thu Feb 28, 2019 3:10 am
Organization: National Institute Of Technology Karnataka
Location: India

Re: Unstable modes of the tower

Postby Ajay.Patil » Thu Mar 14, 2019 10:22 am

The code is running for following DOF's :

True PtfmSgDOF - Platform horizontal surge translation DOF (flag)
True PtfmSwDOF - Platform horizontal sway translation DOF (flag)
False PtfmHvDOF - Platform vertical heave translation DOF (flag)
False PtfmRDOF - Platform roll tilt rotation DOF (flag)
False PtfmPDOF - Platform pitch tilt rotation DOF (flag)
True PtfmYDOF - Platform yaw rotation DOF (flag)

or else its showing error os radian...so what next i do to know the kind errors ??

Jason.Jonkman
Posts: 3961
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Unstable modes of the tower

Postby Jason.Jonkman » Thu Mar 14, 2019 4:23 pm

Dear Ajay,

Are you saying that the model runs without error if all DOFs except platform heave, roll, and pitch are enabled? And if you enable any of the platform heave, roll, and pitch DOFs, the model goes unstable? If so, this sounds like a problem with the hydrostatic restoring in the model, which has a strong effect on the heave, roll, and pitch DOFs.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Ajay.Patil
Posts: 4
Joined: Thu Feb 28, 2019 3:10 am
Organization: National Institute Of Technology Karnataka
Location: India

Re: Unstable modes of the tower

Postby Ajay.Patil » Fri Mar 15, 2019 2:01 am

thank u for reply ..so sir what can i do further to eliminate the problem ??

Jason.Jonkman
Posts: 3961
Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
Contact:

Re: Unstable modes of the tower

Postby Jason.Jonkman » Fri Mar 15, 2019 6:51 am

Dear Ajay,

Assume what I said is true, you should check how you've defined the platform hydrostatic restoring in HydroDyn i.e. your WAMIT *.hst file for the potential-flow solution or the AddCLin matrix for the strip-theory solution. See the draft HydroDyn User's Guide and Theory Manual for guidance: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov


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