NREL 5MW OC3-Hywind (Test 24) Questions

Discuss theory and modeling of wind-turbine structures.

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Ioannis.Bertsios
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Ioannis.Bertsios » Thu Sep 03, 2020 1:43 am

Hi all,

I have used the existed NREL 5MW mounted on OC3-Hywind spar buoy *.fts file from r-test repository. Although it runs with the initial time step of 0.0125s, I tried to change the timestep to 0.25s. I got the following error:


FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_RK4:ED_CalcContStateDeriv:SetCoordSy:Small
angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn
SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from
SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 0.375 s
FAST_AdvanceStates:SolveOption2c_Inp2AD_SrvD:InflowWind_CalcOutput:CalcOutput:IfW_TSFFWind_CalcOut
put [position=(2.2556, -77.357, 106.19) in wind-file coordinates]: FF wind array boundaries
violated: Grid too small in Y direction. Y=-77.357; Y boundaries = [-72.5, 72.5]
FAST_AdvanceStates:AD_UpdateStates:BEMT_UpdateStates(node 5, blade 1):BEMT_UnCoupledSolve:There
is no valid value of phi for these operating conditions: Vx = -136.59, Vy = -882.38, rlocal =
11.547, theta = 0.25279, geometric phi = -2.988. This warning will not be repeated though the
condition may persist. (See GeomPhi output channel.)

FAST encountered an error at simulation time 0.25 of 5400 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.


Is there any explanation about it?

Thank you in advance.

Best regards,
Ioannis.

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Thu Sep 03, 2020 8:48 am

Hi Ioannis,

As discussed many times on this forum, the required time step is intrinsically tied to the frequencies inherent in the model. You can't increase the time step without disabling functionality, e.g., structural degrees of freedom. If you simply increase the time step without disabling functionality, the solution can become numerically unstable.

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

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Sun Sep 27, 2020 10:40 am

Hello,
With respect which location are the platform DOFs (Surge, Sway, Heave, Roll, Pitch, Yaw) presented for OC3 Hywind Platform under forced vibration in OpenFAST? Is it the response at the COG or at the platform top?

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Mon Sep 28, 2020 10:41 am

Dear Akheel,

I'm not sure what you mean by "forced vibration", as none of the OpenFAST r-tests have that. But the platform DOFs for the OC3-Hywind model in OpenFAST are defined about the point that is the intersection of the undisplaced tower centerline and mean sea level. This is because PtfmRefzt = 0 in ElastoDyn for this model.

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

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Mon Sep 28, 2020 10:40 pm

Thank you sir
By forced vibration, I meant that the structure is subjected to external forces like wind and wave.

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Sat Nov 07, 2020 6:42 am

Hey,
1. Does the hub mass (56780) given in ElastoDyn includes the mass of 3 blades?
2. What all changes do I need to make if try to model RNA as a lumped mass?
3. Do I need to recompile OpenFAST if I want to change the inertial coordinates? Or Changing PtfmRefzt alone is sufficient?

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Mon Nov 09, 2020 7:55 am

Dear Akheel,

Here are my answers to your questions:

1. No. The hub mass is only the mass of the hub. The rotor mass would be the sum of the hub mass the and masses of each blade.
2. To model a rigid RNA, you should disable all structural DOFs of ElastoDyn in the RNA, including the blades, drivetrain, and generator.
3. I'm not sure I understand what you mean by "inertial coordinates".

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

Georgia.Tsoukleri
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Georgia.Tsoukleri » Tue Nov 10, 2020 9:01 am

dear Jason,
I'm using FAST v8.16 in order to run NREL 5MW OC3-Hywind (Test 24)
I created wind files using TurbSim v1.50 for wind speeds 8m/s and 24m/s
for wind speed 8m/s : FAST v8.16 terminates successfully . tmestep: 0.0125s
for wind speed 24m/s ,i got the following message

i reduced the timestep but the problem remains.


Thank you in advance,
Georgia
Attachments
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Tue Nov 10, 2020 9:08 am

Dear Georgia,

Have you set proper initial conditions (based on the mean wind speed at hub height) for rotor speed and blade pitch? These should be 12.1 rpm and 22.35 deg at 24 m/s.

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

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Wed Nov 11, 2020 6:43 pm

Dear Jonkman,
In my previous query, By 'inertial coordinates', I meant the inertial frame. And my query was that if I want to change the inrtial frame to platform COG, Do I need to recompile or setting PtfmRefzt = 89.9 alone is sufficient?

Thanks

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Thu Nov 12, 2020 6:03 am

Dear Akheel,

Yes, that is correct. Changing PtfmRefzt in ElastoDyn will change the origin of the inertial frame coordinate system (for the purposes of defining the reference point for platform motions).

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

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Fri Nov 13, 2020 10:48 pm

Dear Sir,
Thank you for the response. I have received some warnings during simulation at different load cases:

Case 1: Wind Speed = 18, Turbulence intensity = 0.15, Significant avg height = 4.0, Spectral Period = 15
Warning: FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:AD_CalcOutput:BEMT_CalcOutput(node 19, blade 2): Compute_UA_AirfoilCoefs: UA_CalcOutput:Mach number exceeds 0.3. Theory is invalid. This warning will not be repeated though the condition may persist.

Case 2: Wind Speed = 24, Turbulence intensity = 0.12, Significant ave height = 5.6, Spectral Period = 18
Warning: FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_CalcContStateDeriv:SetCoordSy:Small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection (ElastoDyn SetCoordSy). The solution may be inaccurate. Simulation continuing, but future warnings from SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 0.6125 s


FAST_Solution:FAST_AdvanceStates:AD_UpdateStates:SetInputs:TwrInfl:getLocalTowerProps:Tower
strike.

FAST encountered an error at simulation time 0.7625 of 1000 seconds.
Simulation error level: FATAL ERROR

Aborting OpenFAST.

1. What do I need to infer from these warnings ?
2. Can I consider the results of Case 2. If not, what modifications do I need to make?
3. Why did case 2 abort?

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Mon Nov 16, 2020 6:37 am

Dear Akheel,

Case 2 is failing because of a tower strike. I would not expect that you'd receive warnings about Mach number exceeding 0.3 for the NREL 5-MW wind turbine model (Case 1), nor would I expect that you'd receive a tower-strike error (Case 2). This suggests to me that something is wrong in your simulation set up in both cases.

What have you changed in the input files relative to Test24? Clearly you changed the turbulent wind inflow and wave conditions, but did you change anything else? When changing the wind conditions, did you change the initial conditions of rotor speed and blade-pitch angle to match their expected values for the given mean hub-height wind speed?

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

Akheel.Kodambiyakamkizhakkakam
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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Akheel.Kodambiyakamkizhakkakam » Mon Nov 16, 2020 9:39 am

Dear Sir,
1. What is meant by tower strike.
2. I set initial rotor speed to 12.1 rpm, blade pitch = 0 (default value). The rotor speed was changed as I have noticed Unsteady Aerodynamics got switched off when rotor speed = 0 (Default case)

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Re: NREL 5MW OC3-Hywind (Test 24) Questions

Postby Jason.Jonkman » Mon Nov 16, 2020 9:49 am

Dear Akheel,

A tower strike means that the blade hit the tower. OpenFAST will abort when this happens

12.1 rpm is a correct initial condition for above-rated wind speeds, but a blade-pitch of zero is not. This is likely the problem...poor initial conditions on rotor speed and blade-pitch can result in the controller producing unreliable results. From Figure 9-1 of the NREL 5-MW baseline wind turbine specifications report (https://www.nrel.gov/docs/fy09osti/38060.pdf), an 18-m/s mean wind speed should use an initial blade-pitch angle of about 15 deg and an 24-m/s mean wind speed should use an initial blade-pitch angle of about 22 deg.

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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