# Eigenanalysis FAST

Dear Jason:
One more question which is very important to me. When building an AF model, it is important to determine the representative load applied at the mudline. In the Memorandum Derivation and Description of the Soil-Pile-Interaction Models written by Patrik Passon, It is the used loads from phase run 5.3 scaled by a factor of 1.5. And I notice that the load case of 5.3 is accompanied with a wind velocity of 18m/s, but in load case 5.2 the wind velocity is 11.4m/s. So is the load for the derivation of AF model too large?
And in Foundation Models for Offshore Wind Turbines written by Bush and Manuel, the AF model are “averaged based on all of the 50 F and M pairs used”. How is it done? What are averaged? The L and EI, or the load pairs?
And could I use the mean values of M and F in the time series directly to derive the AF model?
Best regards.

Dear Yuqi,

I’m not an expert on the questions you are asking. Ideally, you would set the AF model specifically for each loading condition. Practically, it may fine to use the same AF model for a range of loading conditions. In Bush et al, I know that she averaged the loads from many realizations in a given sea state and used different AF models for each sea state. My guess is you’ll need to do a bit of a sensitivity analysis for your own system yourself to see how sensitive the results are to various AF models across different loading conditions.

Best regards,

Dear Jason:
Thank you so much. The detail I want to know is whether the loads to be averaged is the mean loads in a certain realization or the maximum loads. Which is more reasonable?
Best regards.

Dear Yuqi,

I looked back at Erica Bush’s work, and realized that I misspoke in my prior post. From her M.S. thesis, I gather that for each sea state she
(1) took contemporaneous values of shear force and bending moment from the time series and binned them into a histogram;
(2) derived the AF model properties (L and EI) for each bin; and
(3) computed a weighted-average of the AF model properties (L and EI) based on the probability of each bin.

Best regards,

Dear Jason:
Best regards.

Dear Jason:

Thank you so much for your replay on 24. October. Sorry for answering one month later because was out of my Account. Finally i linearized with FAST 7 about 36 Azimut Steps (NAzimStep). Now it is working well.

Best regards

Dear Jason,

referring to your post on Tue Nov 15, 2016 and the following I wonder when you can publish a stepwise procedure for a FFT (or PSD) to get the Campbell Diagram of a floating wind turbine with FAST v8.xx.

Or even better:
Will you implement full system linearization functionality for (floating) offshore turbines in the near future in FAST v8.xx?

Best regards,
Simon Wiedemann

Dear Simon,

We at NREL are not currently funded to work on either, so I can’t comment on when these will become available.

Perhaps someone else on the forum can respond.

Best regards,

Dear Jason,

can you explain why the GenDOF splits in two modes when applying MBC?

In my case one of these is increasing little with Wind speed and a lot with increasing pitch angle while the other is staying near zero. I attach a pic of the curve.
It don’t understand why it behaves like this.

Best regards,

Simon

Dear Simon,

As described in this forum topic: http://forums.nrel.gov/t/learizing-baseline-5mw-wind-turbine-with-fast/494/1, rigid-body modes (i.e. modes without stiffness) show up in MBC3 as a pair of zero-valued (or near-zero-valued) frequencies with +/- inf damping (i.e., eigenvalues with real values only). That is, each rigid-body mode will introduce an additional mode beyond the number of enabled DOFs and the damping is unphysical. When aerodynamics are enabled, perhaps there is a small amount of aerodynamic-induced stiffness that causes the rigid-body generator mode to have a slightly positive frequency.

Best regards,

Dear Jason,

I want to know how to calculate the 3rd FA frequency and 3rd SS frequency of the tower, or even higher order frequency? If you know the method or the third order results, can you share them with me?

Best wishes!

Dear @Xu.Pengfei,

I’m not sure I really understand your question. What do you mean by 3rd order? Are you referring to the 3rd fore-aft or side-side bending modes of the tower? For which system are you analyzing? Are you using FAST / OpenFAST?

Best regards,

Dear Jason,

What I mean is the 3rd fore-aft or side-side bending modes of the 5MW onshore wind turbine tower like the Table 9-1 in “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. I am using FAST, but it seems that it is unable to calculate the eigenanalysis higher than 2nd fore-after or side-side bending modes of the tower.

Bset wishes!

Dear @Xu.Pengfei,

If you are modeling the tower via the ElastoDyn module of OpenFAST, you are limited to two bending modes in each direction. If you are modeling the tower via the SubDyn module of OpenFAST, you can enable as many modes as you want (including modes other than bending like shear, torsion, and axial).

That said, in many wind turbines, only the lowest two bending modes are important in loads analysis applications.

Best regards,

Dear Jason,
I tried to use the linearization function in FAST_v7 and processed the results with mbc, and then pasted the corresponding results into the CampbellDiagram-pengfei.xls file. It appears that most of the frequency results are similar to those described in “Definition of a 5-MW Reference Wind Turbine for Offshore System Development”. But I have a doubt. In CampbellDiagram_pengfei.xls, the frequency order of the Natural (Undamped) Frequencies (Hz) row does not seem to correspond to the frequency order of the States column, so if I determine the Natural (Undamped) Frequencies (Hz) Which State does a certain frequency of the row correspond to? In other words, which parts of the vibration correspond to the 14th and 15th order frequencies? Sorry, I suddenly noticed that the forum doesn’t allow me to upload files.

Natural (Undamped) Frequencies (Hz)：
0.3136585860 0.3242010842 0.6188439335 0.6662559338 0.6672781122 0.6991227166 1.0791658789 1.0899244483 1.9210241590 1.9334641427 2.0205247347 2.9185184660 2.9527926795 3.6759704970 6.1382215693

States
‘Row/column 1 = 1st tower fore-aft bending mode DOF (internal DOF index = DOF_TFA1)’
‘Row/column 2 = 1st tower side-to-side bending mode DOF (internal DOF index = DOF_TSS1)’
‘Row/column 3 = 2nd tower fore-aft bending mode DOF (internal DOF index = DOF_TFA2)’
‘Row/column 4 = 2nd tower side-to-side bending mode DOF (internal DOF index = DOF_TSS2)’
‘Row/column 5 = Nacelle yaw DOF (internal DOF index = DOF_Yaw)’
‘Row/column 6 = Drivetrain rotational-flexibility DOF (internal DOF index = DOF_DrTr)’
‘Row/column 7 = 1st flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,1))’
‘Row/column 8 = 1st flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,1))’
‘Row/column 9 = 1st flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,1))’
‘Row/column 10 = 1st edgewise bending-mode DOF of blade 1 (internal DOF index = DOF_BE(1,1))’
‘Row/column 11 = 1st edgewise bending-mode DOF of blade 2 (internal DOF index = DOF_BE(2,1))’
‘Row/column 12 = 1st edgewise bending-mode DOF of blade 3 (internal DOF index = DOF_BE(3,1))’
‘Row/column 13 = 2nd flapwise bending-mode DOF of blade 1 (internal DOF index = DOF_BF(1,2))’
‘Row/column 14 = 2nd flapwise bending-mode DOF of blade 2 (internal DOF index = DOF_BF(2,2))’
‘Row/column 15 = 2nd flapwise bending-mode DOF of blade 3 (internal DOF index = DOF_BF(3,2))’

Bset wishes!

Dear @Xu.Pengfei,

I’m sorry, but I’m not sure I understand your question. Perhaps the question would be more clear if you shared your spreadsheet some way?

Best regards,

Dear Jason,

Though I have reviewed the posts where you have helped others interpret the modes to get a sense of how it works… My confusion still lies in not knowing how these two sets of data correspond. The Mode Number and Natural (Undamped) Frequencies (Hz) corresponding to Green Background are the natural frequencies corresponding to this state. Is my understanding correct? But how to find the DOF that belongs to a certain natural frequency if there are many mode shape magnitudes with green background in the natural frequency colum? For example, how to identify modes 12 and 13 as the 2nd tower fore-aft and 2nd tower side-to-side bending modes? How to identify which DOF mode number 14 and 15 belong to?

Another question is what you mentioned last time, If I am modeling the tower via the SubDyn module of OpenFAST, how can I enable as many modes as I want?
Last qustion is maybe I can input the white noise excitation by inputting the seismic wave at the bottom, extract the acceleration time history result at the tower, do the spectrum analysis, find the frequency corresponding to the peak and obtain the higher-order mode?

Best wishes!

Dear @Xu.Pengfei,

Each mode generated through the linearization + MBC3 post-process is a full system mode involving a combination of all degrees of freedom in the model. The green and red highlighting do not identify specific modes, but they likely help you pick out the most important DOFs prominent to each mode.

In your case, modes 12 and 13 correspond to the 2nd fore-aft and side-side bending modes of the tower. Mode 15 corresponds to the nacelle-yaw mode and Mode 14 is an extraneous edgewise bending mode.

When SubDyn is enabled, the number of finite-element DOFs and number of retained Craig-Bampton modes is specified by the user and can be as high as you want. But please note that SubDyn is only available in FAST v8 and OpenFAST and linearization with SubDyn enabled was not introduced until OpenFAST v2.6 and newer.

I’m not sure I fully understand your last question, but you can input white noise excitation through the seismic functionality of FAST v7. However, SubDyn is not part of FAST v7.

Best regards,

Dear Jason,

Thank you for your detailed answer. I think only the mode shape can tell which degree of freedom a certain order frequency corresponds to. In fact, the mode shape may also be the result of the coupling of multiple degrees of freedom. But in CampbellDiagram_pengfei.xls, can the specific Mode14 and Mode15 only be determined by experience as an extraneous edgewise bending mode and the nacelle-yaw mode?
The last question I want to express is that by inputting white noise excitation at the bottom, extracting the response of the structure, and then performing Fourier transform, the mode of the structure can also be identified, although mainly the mode of the corresponding part of the response.

Best wishes!

Dear @Xu.Pengfei,

In mode 15, the nacelle-yaw DOF is highlighted green, which implies that DOF is most relevant to this mode; I also know that the nacelle-yaw actuator was designed with this frequency in mind. Mode 14 includes a strong blade-edgewise bending component (seen via the red highlighting) and two other edgewise modes appear in modes 7 and 8. So, mode 14 can be ascertained knowing that all blade modes in a three-bladed turbine come in sets of three, and with edgewise modes, one mode typically has a much higher frequency than the other two. But I agree, it takes some experience to interpret the eigensolution correctly.

Yes, white-noise excitation can be used to identify natural frequencies of the structure (assuming the excitation is applied in a way that will excite the modes), but it is often difficult to deduce which modes correspond to which frequencies with white-noise excitation alone.

Best regards,