FAST

Dear Arash,

There are a few time-histories from the FAST v8 models of the OC3-monopile and OC4-jacket published in our recent AIAA SciTech 2015 paper focused on the verification of the new fixed-bottom offshore modeling capability in FAST v8. You can find that paper here: nrel.gov/docs/fy15osti/63067.pdf.

Best regards,

Dear Jonkman,

Thank you very much for your help.

I tried to validate my jacket 5MW result with the result in the paper you recommended. But the results are not the same. I downloaded the Test21.fst sample and for the 4.3b loadcase (Without wind load, with regular wave conditions)

The changes I made to the sample file:

  1. I changed air density to 0 in Aerodyn input file to make wind condition zero
  2. In Hydrodyn input file, I changed Wavemod to 1 (regular) with H=8 m and T=10s
    3)In Elastodyn input file, I changed Initial rotor speed to 0

and I ran FAST. However the results seem more fluctuating than that of the paper. The range seems OK but my results have more frequencies.

Could you please take a look at the graphs and give your opinion about the discrepancy.

Thank you very much for your time,

Best Regards,

Arash,

Dear Arash,

For load case 4.3b of the OC4-jacket simulation, I would suggest:

FAST

  • CompAero = 0 - Disable AeroDyn instead of setting the air density to zero
  • CompServo = 0 - Disable ServoDyn

ElastoDyn - Disable all tower-top DOFs

  • FlapDOF1 = False
  • FlapDOF1 = False
  • EdgeDOF = False
  • DrTrDOF = False
  • GenDOF = False
  • YawDOF = False

(I agree with setting WaveMod = 1 in HydroDyn and RotSpeed = 0 in ElastoDyn.)

Regardless, the basic problem I see with your solution is that you haven’t run it long enough for the start-up transients to die out. Simply run the simulation longer and the start-up transients will dissipate. If you remove the first 30-60 s (or whatever is needed) from your post-processing, your results should match those of the paper.

Best regards,

Dear Jason,

I am running the NREL 5MW landbased turbine in DLC6.2 of IEC61400-1 in FAST v.7 with a loss of connection to the electrical grid so that the yawing system and blade pitch system is not active. I assume the wind turbine to be class 1 and the EWM then yields a hub height wind speed of 1,4*50 m/s = 70 m/s. My intention is to examine wind directions in the range (0;180 deg). Whenever the yaw error in the wind file is larger than 0, I get an error of “small angle assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection. The solution may be inaccurate. Simulation continuing, but future warnings will be suppressed”.

I have read the other posts concerning the same problem and tried changing the integration time step to 0,003, but the problem persists. Then I have tried lowering the wind speed and the problem disappears if the wind speed is maximum 59 m/s and occurs at 60 m/s or more.

I have set YCmode to 0, pcmode to 0, vscontrl to 0 and pitched all 3 blades to 90 degrees. I am mainly interested in the tower loads. I have tried disabling the blade dofs, and then the problem disappears, but can I then trust the tower loads that I get?

Dear Jacob,

I’m aware of a potential blade-edgewise and tower side-to-side-mode instability (negative damping) that can occur when the NREL 5-MW turbine is parked with all blades feathered to 90 degrees in high winds with yaw errors between 20-40 degrees. You can read about this in Section 6.2.1 of my PhD thesis-turned NREL report: onlinelibrary.wiley.com/doi/10.1002/we.442/pdf.

I’ve been able to verify this with simpler models, such as with steady uniform wind or as a linearized FAST model. I have talked to other modelers and heard that this problem has been seen in many different turbines and with different aero-elastic software. The current belief is that the instability would likely not occur in the physical world and that the aero-elastic software only predict a problem due to simplifications in how the software treat the dynamics of deep stall. My understanding is that the industry’s current approach to dealing with this problem is to either (1) bypass it by choosing yaw errors that don’t result in the instability or (2) increase the structural damping in the blade edge / tower side-to-side mode until the instability goes away.

I hope that helps.

Best regards,

Dear Jonkman,

I am doing Simulation for the 10MW Spar-buoy Wind Turbine system.I am doing load case power production(DLC:1.3, IEC turbulence type-ETM).

Unfortunately at 9 m/s wind speed(Hs-0.95,Tp-3.33) I am getting error “small angle assumption” for that I have upload the picture as below.
I have read all the post regarding “small angle assumption”. so far this error is concern,the problem is numerical instability in my model.So I have seen in many post that solved only by decreasing the time steps but for my model I can not decrease the time step more than 0.00125. If I use 0.0000125 then I am getting another error I have uploaded the picture of that error as below.

I have tried with grid size from 88 till 3030 ( timestep 0.00125s) to run the simulation.(Most of the time simulation aborting between 200 to 300 s) Also I have tried with more than 0.00125(time step) but simulation aborting.My last simulation at 7m/s wind speed completed with grid 8*8 and time step was 0.00125 s.

My question is that these error is due to numerical in stability so I need to try with very small time steps Am I right?? If is there any other input parameters need to change in TurbSim input file then please let me know. and can you please tell me what should I do to use the 0.000001 s time steps.
I am using OpenfAST-V1.0.0(Compile info-architecture :64bit, precision: single)

I hope you can clearly understand my problem and guide me as soon as possible.

thank you so much for your help and support.

Best regards
Mitesh Ramani


Dear Mitesh,

There is no reason to set the TurbSim and FAST / OpenFAST time steps identically.

There is usually no need to run TurbSim with a time step smaller than 0.05 s, which will generate turbulence up to 10 Hz.

Your OpenFAST model is clearly going numerically unstable, but you haven’t provided me with any information to guess as to why. You could try a time step smaller than 0.00125 s, but that is already quite small.

Best regards,

Dear jason,

Thank you so much for you time and you support.

I can understand that There is no reason to set the TurbSim and OpenFAST time steps identically.That is why I am using OpenFAST time steps 0.0125(s) and only changing the time steps in my TurbSim input file.

Thank you for information that,there is usually no need to run TurbSim with a time step smaller than 0.05 s.Because it will generate turbulence up to 10 Hz.

As you told me to try time steps smaller than 0.00125 s, so I have tried with 0.000125 s and grid size is 88 but still my Simulation is aborting around 40 s.Also I have tried grid size till 2020. but Its aborting around 300 s most of them.

If I use smaller than 0.000125 s for example 0.00001 then I am getting error like a “Error allocation #some numerical values# MB for the turbulence PSD Array” As I uploaded picture in my last post.

I have uploaded my turbSim file ,HydroDyna and ElastoDyn file as below kindly find it.Also I would like to ask that there would be any other input parameters responsible for this numerical unstable model.

I hope you will help me out as soon as possible.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani
HydroDyn.rtf (18.3 KB)
ElastoDyn.rtf (27.1 KB)
TurbSim.rtf (6.14 KB)

Dear Mitesh,

You misunderstood me. The time step in TurbSim should be set to 0.05 s. You may need to drop the time step in OpenFAST less than 0.0125 s.

Best regards,

Dear jason,

Thank you for your support and correct me, Sorry for that misunderstanding.

AS you told me to set time steps 0.05 s in TurbSim and I need to drop the time step in OpenFAST less than 0.0125 s .I have tried to run simulation with time step 0.01 s,0.001 s and 0.0001 s(In OpenFAST) But my OpenFAST aborting around 29 s.I have attached the picture of that error and chart of the Rotor speed ,Blade pitch and wind velocity in the folder “DT_0.0001(Wind speed 09)”.

As my point of view it does not seems like a problem with model stability because I have tried with very smallest time steps.Also I have run the simulation without platform DOF and simulation is running completely but starting of that simulation I got "Additional debugging message from SUBROUTINE SmllRotTrans(): 0.2125 s " I have attached that picture in the folder "Disable Dof_Wind speed 09"Also I have attached the chart of the Rotor speed ,Blade pitch and wind velocity.

My model is 10 MW Spar-Buoy Wind turbine and I am using Bladed interface Controller in ServoDyn is “DTU10MWTripleSparController_Lib.dll” Because “dtu_we_controller.dll” is not working.IS it happened due to this controller ??

Can you please guide me what should be a problem or do I need to check for this kind of aborting error.

I hope you will help me out as soon as possible.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani
Disable Dof_Wind speed 09.zip (52.6 KB)
DT_0.0001(Wind speed 09).zip (218 KB)

Dear Mitesh,

Indeed, your solution may not be numerically unstable because of the use of a large time step, but clearly something is causing large platform displacements. What do the plots of platform displacement look like? Can you isolate the problem to a specific degree of freedom? I suggest simplifying the model and increasing complexity in steps to debug the problem.

Best regards,

Dear jason,

Thank you so much for you time and you support.

I can understand that problem due to large platform displacements.Also I have isolated the problem to a specific degree of freedom.That is happened due to Platform pitch tilt rotation degree of freedom If I disable(False) the platform pitch degree of freedom then simulation is running completely fine till the end.If I enable(True) platform pitch degree of freedom then it is showing the same error message.Picture of that error message I have attached below. Also I have cross check with other degree of freedom for example Platform pitch is enable (True) and any else is disable(False) then it’s showing the same error message.So as I think this problem is only due to Platform pitch tilt rotation degree of freedom.

As you told me debug the problem by simplifying the model and increasing complexity in steps Also I have tried that but only Problem happened when I enable the Platform pitch tilt rotation degree of freedom.I have taken the initial conditions of transnational and rotational displacement of platform in ElasoDYn file from one simulation in that I have set all the initial condition to zero then from WindChart I got the mean value of the transnational and rotational displacement of platform.

I have attached the plots of platform displacement as below.In that folder pitch motion only I got when I disable(False) the yaw rotational displacement of platform(In the middle of that simulation I got the small angle assumption error but simulation was completed).I would like to conclude with that if I disable(False) the pitch tilt rotation degree of freedom then I could not get that error from start to end of the simulation.

I hope you can clearly understand my problem and explanation so could you please guide how to deal with this problem or which parameter should be responsible for this.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani
Platform Motion.zip (1.39 MB)

Dear Mitesh,

From your platform time series, I don’t see any signs of an instability, but the platform-pitch motion is very large (over 24 degrees). Are you expecting such large platform-pitch motion from your floating system design? If not, I would guess that there is a problem related to the platform-pitch restoring in your simulation set-up e.g. the mooring stiffness or hydrostatic stiffness are too low. Is the platform-pitch natural frequency predicted by FAST lower than you are expecting?

Best regards,

Dear jason,

Thank you so much for you time and you support.

  1. Yes,the platform-pitch motion is very large (over 24 degrees).NO I am not expecting such large platform-pitch motion from my floating system design.Because NTNU thesis report which I am using(same 10MW spar-buoy) right now,In that platform-pitch motion mean value is 2.85 deg. and maximum value is 11.32 deg.So as per my point of view problem is in the platform-pitch restoring.I have calculated Hydrostatic restoring in pitch and roll as per given equation in the HydroDyn manual((c44/c55 = -8159587654 N/m(ρgA/4R^2+ρgvzb))

where,ρ(Density of water)=1025 kg/m^3, g(Gravity)=9.80655, A(Water plan area)= (π)/4*8.3^2(where 8.3 is the diameter of spar at the water plan), R(Radius at the water plan)=)8.3/2, V( Submerged Volume )=13085.41480(As per HydroDyn output file), Zb(Center of buoyancy)=62.05271(I have manually calculated for my system).

But in the NTNU thesis report they have used another equation to define the hydrostatic restoring in roll and pitch. I have attached the picture of that equation as below.The value of that non zero term as per NTNU report is c44/c55=1690150000 N/m.

If I am using platform Hydrostatic restoring in pitch value(C55) = 1690150000 N/m (AS per NTNU report) or C55=0 N/m in Additional linear stiffness matrix then my Simulation running completely but starting of that simulation I am getting error of “Small angle assumption”(Only getting this error equal or grater than 17 m/s wind speed ). Also I have tried to solve by using very small time steps in OpenFAST but it does not work.I have also attached the picture of that error as below.

I would like to mention that for the (C55) = 1690150000 N/m (AS per NTNU report) and C55=0 N/m my system is in the hydrostatic equilibrium condition wihtout wind and waves because from the platform motion chart.

Question: I have seen in many other floating system HydroDYn file that they have not used pitch and roll hydrostatics stiffness value so It could be possible to run the simulation without those values??? Can you please explain me a bit why It should be possible without those values ??

What could be the reason for the error “small angle assumption” at the starting of the simulation for only 17 to 25 m/s wind speed?

  1. I can understand that the mooring stiffness or hydrostatic stiffness are too low. Also I have tried to solved by increasing the value of the mooring stiffness but It could not work. Currently I have used the equivalent mooring line extensional stiffness = 384243000.I have attached MoorDyn file as below.

The platform-pitch natural frequency predicted by FAST is higher(It is around 60 s) than I am expecting while using the c44/c55 = -8159587654 N/m. But The platform-pitch natural frequency predicted by FAST is lower (It is around 25 s) than I am expecting while using the c44/c55=1690150000 N/m (AS per NTNU report) and C55=0 N/m.Because as per NTNU thesis report it should be around 35 s.

Question:Is should be okey to use the platform Hydrostatic restoring in pitch value(C55) = 0.?
From the error " small angle assumption " at the starting of the simulation, does it really effect my output data?

I hope you can clearly understand my explanation and will help me out as soon as possible.

I really appreciate for your valuable time and your support.

Best Regards
Mitesh Ramani



MoorDyn.rtf (2.84 KB)

Dear Mitesh,

How are you modeling the hydrodynamics of this floating system…using strip theory, using potential-flow-theory, or a hybrid combination of the two?

If you are using strip theory, the hydrostatic restoring is not included in the implementation. In this case, you should set AddCLin as described in section 6.8.3 of the draft HydroDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf and C44/C55 should be negative-valued for a deep-drafted floater.

If you using potential-flow theory, the hydrostatic restoring should already be included in the *.hst WAMIT output file that is used within HydroDyn, so, you may set AddCLin = 0.

If you using a hybrid combination of the two, presumably you would only be modeling the viscous effects in strip theory and the radiation, diffraction, and hydrostatic effects in potential-flow, so again, AddCLin = 0 (because it is already included in the *hst file).

The equations from the NTNU report you attached include the gravitational restoring associated with the physical mass of the wind turbine. You should not include this term in the HydroDyn module because this term is already accounted for in the ElastoDyn module.

Best regards,

Dear jason,

Thank you so much for your explanation and your time.

  1. I am using strip theory to model my hydrodynamic modelling system,For that I have to use C44/C55and I have readt hat in manual I need to use the MSL2SWL(Offset between still-water level and mean sea level)In the ENVIRONMENTAL CONDITIONS In HydroDyn file,Do you have any idea how can I choose this??, Because If I add 2 meters in MSL2SWL then simulation is running till the end with only one error(Small angle assumption) at the starting of the simulation.

2)IF I add the rotor speed ,or nacelle Yaw angle(25m/s wind speed) in the ElastoDyn input file in Initial condition then Simulation is aborting otherwise without any kind of Initial condition in ElastoDyn Simulation is running completely. What could be the reason for this?

I have notice that in the starting 5 second the rotor speed ,wind speed and power is suddenly increasing while simulating without any kind of initial condition.

I hope you can guide me and help me out from this problem.

Always appreciated your kind help and your support.

Thank you so much

Best regards
Mitesh Ramani

Dear Mitesh,

I’m not sure I really understand your questions.

Regarding MSL2SWL, normally this parameter is used to model water-depth variations as a result of tides or storm surges. Are you asking how to estimate these for a given site?

Setting initial conditions in FAST has been discussed many times on this forum e.g. see: http://forums.nrel.gov/t/initial-conditions-for-tower-and-rotor/609/2.

Best regards,

Dear Dr. jason.

Thank you for your explanation and sending me that link I really solve that problem from your favor.

I have one simple question regarding the hydrostatic in roll and pitch coefficient Additional linear stiffness(AddCLin) AS you before mentioned If I am using the strip theory then I have to add this term as per HydroDyn manual equation.I can undersatnd that its very important for the model.

As you said for the deep draft Wind turbine it(C44/C55) should be a negative but my question is that if I am using deep draft(120 m) with ballast (34.7m of concrete) then should I use the last term of that question " mfgzf"(Where mf = total mass of ballasting,g = Gravity,zf =coordinates of the center of mass of the undisplaced filled fluid )??

If I use that last term then it(C44/C55) would not be negative anymore.

I hope you will clarify my doubt as soon as possible.

Thank you so much

Best Regards
Mitesh Ramani

Dear Mitesh,

Where are you modeling the concrete ballast mass?

  • If you are modeling the concrete ballast in ElastoDyn, then the gravitational restoring in pitch and roll will automically be calculated by ElastoDyn, so, you should not include that term in HydroDyn (and so, the C44/C55 terms of AddCLin will be negative for a deep-drafted spar).
  • If you are modeling the concrete ballast in HydroDyn (via the filled member option), then you should include that term in HydroDyn (and so, the C44/C55 terms of AddCL may be positive for a deep-drafted spar).

Best regards,

can you share this link again to sort out the issue of GBRatio for oc3 5MW in FAST v7 ?