Instruction for the Seismic fast version

Dear Jason,

I am new on fast and I’m trying to use the seismic fast version in order to study near ground motions effects. I’ve downloaded the seismic archive, added the pitch and the Spd_trq files from vers 7 but when I launch the 1st example with the iwin32 executable I have the the following message:

. The seismic input file is in the right folder yet.

Do you know how to fix this ? I don’t know how to make it find the right file. I tried to modify the UserPtfmLd_seismic file in vain. I may have missed some steps…

Thank you for your answer,

Best regards,

Pierre-Yves

Dear Jean-Pierre,

I can’t see if you did it, but for the input file you need to add the extension file in the cmd.

Best regards,
Alexandre
ETIENNE

Dear Pierre-Yves,

I agree with Alexandre’s comment. Furthermore, the seismic models do not need the Pitch.ipt or Spd_Trq.dat files, as the seismic models use blade-pitch and generator-torque control logic from a Bladed-style DLL (DISCON.dll).

Best regards,

Dear Jason,

Thank you for you last answer. I’ve figured out how to make everything work. However, I have a few questions concerning the linearization and the tower base moment I get with the software.

  1. I want to reproduce the results from Seismic Loading for FAST May 2011 — August 2011, M.A. Asareh and I. Prowell, and try to obtain the tower bases moment. I have the same results for the x direction, but for the y direction, there is a bigger stating moment and I don’t find any explanation for this. Do you have one…?

  2. Then, I’m trying to get the modes shapes but I don’t find solution to make the linearization converge. When I raise to much the tolerances, it converges but the frequencies of the modes are wrong… Do you have some indications of did I miss something ? Here are joined the files i’m using.

Thank you very much for your advices,

Best regards,

Pierre-Yves
Linear and Adams files.rtf (5.43 KB)
Primaryinputfile.rtf (19 KB)
Capture d’écran 2017-07-16 à 23.07.28.png

Dear Pierre-Yves,

I’m sorry, but I don’t know enough about how your simulation set-up differs from that of Asareh and Prowell to comment on that.

Regarding the linearization, the natural frequencies obtained through the FAST linearization analysis should match those visible through the nonlinear simulation. Are you saying they don’t for your case? As always, I suggest you simplify the model to debug e.g. start be linearizing with only a single degree-of-freedom enabled.

Best regards,

Dear Jason,

Thank you for your answer. Regarding to the linearization, that what I wanted to do. I should find the same frequencies as you’ve found in your report with Scoot (2009), but I don’t with the linearization using a How can have get the modes from the nonlinear ? Is the only solution is to use “modes” software ?

Sorry to be a little bit lost trying to become familiar to the use of fast.

Thank you very much,

Best regards,

Pierre-Yves

Dear Pierre-Yves,

When you say you don’t find the same natural frequencies as reported in the NREL 5-MW specifications report from 2009, how different are the results?

Regarding deriving natural frequencies from the nonlinear solution, natural frequencies can by found through FAST linearization followed by eigenanalysis or by exciting the system with broadband excitation and examining the FFT/PSD of the time-series response, as has been discussed many times in this forum.

Modes (nwtc.nrel.gov/BModes) be used in place of Modes: http://forums.nrel.gov/t/tower-eigenfrequencies-of-nrel-5mw-turbine/517/1.

Best regards,

Dear Jason,

Thank you very much for your quick answer. I will first try to find the frequencies with the linearization and the eigenalalysis, then with the BModes software.
For the the linearization, I’m using the exemple 1 from the seismic archive. I have found a steady state solution (in region 2, with the rotor speed of 12,1 rad/s), and the linearization is made without control inputs and with only a horizontal hub-height wind speed disturbance. Then when I use the lin file created to get data to use the Campbell Diagram, the natural frequencies which are given are the following ones (hence my Campbell excel file is totally wrong) :

0
3,9512
3,0558
3,0979
2,1885
2,0494
1,7977
1,7083
1,2931
0,8876
0,9014
0,7366
0,5777
0,4133
0,3975
0,0076
0,0017
1,742
0,000
5,7839

I don’t understand why this values are not the same as some I should definitely find for the 5 MW wind turbine. I have tried to reduce the number of DOFs without success. Do you have a typical working file for this or any advices ? I probably miss something but I can’t find any solutions.

Thank you very much one more time for your help…

Pierre-Yves

Dear Pierre-Yves,

I’m not sure your results are totally wrong. I see frequencies that are close to those expected for the tower and blades. I do see two sets of rigid-body modes with pairs of zero (or near-zero) frequency; I would normally expect to only see one associated with the rigid-body rotation of the drivetrain, but perhaps the second rigid-body mode has something to do with a linearization using the Seismic version of FAST and platform DOFs that are enabled? I’m not sure I can comment further without knowing more about your simulation set-up.

Best regards,

Dear Jason,

Thank you for your last answer ! I have nearly solved the issues of the frequencies. And I’m taking every day more control of the software. However, some results are still surprising. I tried to compare the results with a FEM of the NREL 5 MW wind turbine made on Matlab (I wanted to reproduce the Matlab results with Fast, a simulation with a idling turbine (rot speed=0), without wind conditions and with only an X direction earthquake motion), and it seems that there is a difference in the damping. After the earthquake, the structure should evolve with free oscillations, with a damping of 1%. Thank to Fourier transform, we see that the main frequency is as expected 0,328 Hz . And for this frequency, if we select only the free oscillations after the earthquake, we can see on the joined figure that the damping is more around 0.4 than 1%. Do you know where does this difference can come ? It’s really hard to reproduce results from papers (or obtained with a reliable model made with Matlab) in order to start personal analysis on solid bases.

Thank you very much for your help !!
Best regards,

Pierre-Yves

Dear Pierre,

I’m sorry, but I’m not really sure I understand your question or how to interpret your plot (what is the red curve?). Why are you expecting to see 1% damping?

Best regards,

Dear Jason,
Sorry there is a mistake in my figure. The blue curve are the free oscillations of the tower after the earthquake, the green one the exponential decrease for a 0.4% damping, and the red one for a 1% damping (for f = 0,328 Hz). Shouldn’t I expect to see 1% damping, if a set the tower mode structural damping ratios to 1% ?

Thank you for your answer,

Pierre-Yves

Dear Pierre-Yves,

OK, now I understand. Regarding the drop in tower damping from 1% to 0.4%, this is discussed in the following forum topic: http://forums.nrel.gov/t/natural-frequency-and-damping-ratio-calculation/646/12. But basically, the 1% damping ratio is specified for the isolated cantilevered tower, but when the tower is coupled to the full system, the overall damping level will change.

Best regards,

Dear Jason,

Thank you for your answer, I understand better the damping computation and how I can change them. Sorry for all the help I ask your for, but I have one last thing which annoys me. I was trying to understand why my response tend to be different with my FEM on Matlab, so I’ve made a free vibration test of the 5MW turbine.

The wind load is not considered. Yaw is disabled, rotor speed is 0. The base is fixed, no platform motion.

First fore-aft motion

Only the first fore-aft mode is enabled, all other modes are disabled. The initial displacement at the tower top is 0.5m. Damping ratio for the first fore-aft tower mode is 2% of critical. The displacement response is shown in Figure 1. Only a section of the response is shown here. Towards the end, the displacement oscillates about -0.015m level. It would be expected for the displacement to oscillate about 0 level. If we consider the small displacement due to eccentric mass of the rotor, the displacement should be positive. It is not clear why there is this residual negative displacement. When running seismic analysis with 0 initial displacement, the simulated result has a small negative initial displacement, which is also unexpected. It is not clear if the simulation considers the displacement due to gravity loads as the initial condition for simulating seismic response.

Figure 1.

The Fourier Amplitude of displacement is shown in Figure 2. The peak of the spectrum is at 0.327 Hz which is close to the first fore-aft frequency of the tower, as expected.

Figure 2.
From the computed response, by using the logarithmic decay method, the damping ratio was computed to be 0.75%. This is lower than the uncoupled tower fore-aft first mode damping ratio, which is due to inertia of the nacelle and rotor.

Figure 3 compares the FAST results with a SDOF response with the damping and frequency estimated from the FAST results. The comparison shows that the FAST results matches SDOF response relatively well. However, it is strangely overshoots the negative peaks, but matches the positive peaks. This is also an indication of the negative residual motion at the end of simulation as discussed above.

Figure 3

Second fore-aft mode:
Same analysis as before but only the second fore-aft mode enabled.
The Fourier Amplitude of the response is shown in Figure 4. It shows multiple peaks, which means that the motion is not harmonic, but contains multiple frequencies, or the frequency is changing in time. The largest peak is at 2.278 Hz, which corresponds to the second fore-aft frequency of the tower. The damping ratio is estimated to be 1.5%. This is twice the damping ratio in the first mode, although same value of 2% was used in FAST for the uncoupled tower. It appears that the difference between the tower damping ratio and the system damping ratio is higher for the first mode, but lower for higher modes, which is expected because of lower mass participation factors of higher modes.

Figure 4
The comparison between the FAST result and an equivalent SDOF is shown in Figure 5. The SDOF frequency is set to 2.13 Hz (different from the second mode frequency of 2.28 Hz) to match the first few peaks of the FAST response. It appears that after a few cycles, the FAST response lags behind the SDOF solution, and at some time the lag is so large that the two responses are out of phase.

Figure 5
The time-frequency plot of the FAST response and equivalent SDOF is shown in Figure 6. The results indicate that as time passes the frequency of FAST response decreases, and the motion is not harmonic as expected. This was unlike the first mode response which was found to be harmonic.

(a) FAST
(b) Harmonic
Figure 6
Figure 3.png
Figure 2.png
Figure 1.png

The last figures. I think that solving this dephasing issue may be really useful !

One more time thank you very much for your help !

Best regards


Figure 5 .png
Figure 4.png

Dear Pierre,

Interesting results. Just two comments:

  1. The negative mean displacement is expected. The tower displacement in FAST is defined as positive in the nominally downwind direction, but the overhanging weight of an upwind rotor will cause the tower to displace slightly upwind in the absence of aerodynamic loads.
  2. For the simulation with only the second fore-aft bending mode enabled, a tower-top displacement of 0.5 m is very large (normally I would expect much less excitation/displacement of the second modes than the first), so, I would guess the nonlinearities in the response are likely the result of geometric nonlinear terms (e.g. the axial shortening) that are modeled in FAST. The geometric nonlinear terms will have a smaller effect for smaller displacements.
    I hope that helps.

Best regards,

Hi Jason,

are there any plans to include a/the seismic modul in the latest version of OpenFAST?

Best regards,
Simon

Dear Simon,

That would be great, but NREL has not yet had the funding to do that.

Best regards,

Dear Jason,

Is there a way I can add the already existing seismic load code in FASTv8 or open fast and recompile the same? I understand that the seismic forces required to achieve the user motions are calculated at every step. I believe it would reasonably be not so difficult. But, I need to know which module do I need to focus on changing? It would be great if you could give a brief idea of where I can supply these force calculations at the base node in the source code.
I eagerly want to stop using the seismic module of FAST v7 and utilize the upgraded capabilities in the higher version. Please help me. Thanks.

Dear @Kashyap.Subham,

It is not possible without modification of the source code to interface the Seismic routines from FAST v7 into OpenFAST. And again, this is not something NREL has been funded to do yet.

That said, if you know the time-series of seismic motion as computed by the Seismic routines from FAST v7, you can use these within OpenFAST by developing a simple ExtPtfm routine available with CompSub = 2, e.g., as discussed in the following OpenFAST issue: Develop Prescribed Platform Motion capability with aero-elasticity · Issue #617 · OpenFAST/openfast · GitHub. But this simple approach cannot be combined with the offshore functionality of OpenFAST (with HydroDyn, SubDyn ) for combined wind + wave + seismic loading because the CompSub = 2 option is not available when SubDyn is enabled (which is CompSub = 1).

Best regards,

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