Natural frequency DeepCWind and Random Seeds

Wind & Water Computer-Aided Engineering Software Tools
21

Jason:
There was an error with table number 2, please see updated below.

Table 2 revised DOFs for the NREL 100 m DeepCwind was found to be:

Platform motion Nat frequency Hz Mode of vibration
Surge 0.039231 3
Sway 0.039328 4
Heave 0.059451 8
Roll 0.039328 4
Pitch 0.039231 3
Yaw 0.024052 1

Therefore the only queries are:
1) Does the system natural frequency for the 100 m system appear satisfactory? This ties into the query after table 4.
2) For table 3 the heave motion query about HD RdtnPtfmHv8 and not ED Platform vertical heave translation.

Thank you.

Regards,
AOAW

22

Dear Andre,

Just a few comments:

  • I’m not sure what you mean when you present different eigenalysis results at different sea states. OpenFAST will not allow you to linearize when incident waves are enabled; OpenFAST requires that WaveMod = 0 when linearizing, otherwise it would not be possible to find a steady state before linearizing.
  • If I understand correctly, the differences in platform natural frequencies between 200 m and 100 m water depth only appear in the surge, sway, and yaw modes (the natural periods in heave, roll and pitch are nearly identical between the two depths). I would expect this for a catenary mooring system because the surge, sway, and yaw natural frequencies are most impacted by the mooring system stiffness and the heave, roll, and pitch natural frequencies are most impacted by the hydrostatic stiffness, which is unchanged by depth.
  • Again, the eigenvectors show the relative contribution of each DOF to each full-system mode. The heave mode (or the like) does not need to have the heave mode identified by TRUE in the eigensolution.

Best regards,

23

Jason:
Many thanks.

Regards,
AOAW

24

Hi Jason:
I see where WaveMod5 in HyrdoDyn allows one to enter a user defined time series.
1) Can this time series be a standard.dat (or .txt) file with the time and wave elevation data?
2) How would you get HydroDyn to read this file?

Thank you.

Regards,
AOAW

25

Dear Andre,

Yes, the user-specified wave-elevation data file required with WaveMod = 5 is a simple text-based file. An example is provided in Test_006 in the standalone HydroDyn CertTest from FAST v8. The draft HydroDyn User’s Guide and Theory Manual provides some more information on this file. The HydroDyn archive and documentation from FAST v8 are available on my Google drive: drive.google.com/drive/folders/ … sp=sharing.

Best regards,

26

Thanks Jason.

Regards,
AOAW

27

Hi Jason:
Three questions regarding the system natural frequency:
1) When performing linearization to find natural frequency should the initial positions of the platform for 6-DOFs be set to zero in ElastoDyn.
2) If one wants to plot the time series for the free decay test, should those DOFs in 1) now be given an initial value, for example 5 m for surge, and so on, in order to do it.
3) Is there a paper that shows the values of the system natural frequency for OC4-DeepCwind FOWT system?
I thought I had seen them in the paper that defined the DeepCwind semi-sub but have checked back said paper and they are not there. Unfortunately I can’t remember where I saw them.
Thank you.

Regards,
AOAW

28

Dear Andre,

Here are my answers to your questions:

  1. It is best to linearize a model about a steady state condition (or periodic steady state condition if the rotor is spinning). This is typically not the condition where all displacements are zero. Linearizing about the condition where all displacements are zero may be satisfactory, but the linearized model may not be fully representative of the system dynamics compared to one derived about a steady-state condition (e.g., the stiffness may be off a bit). You can ensure the model is in steady state before linearizing by enabling the CalcSteady option in OpenFAST.
  2. Yes.
  3. The OC4 Phase II results paper (from OMAE 2014) presents the natural frequencies of the OC4-DeepCwind semisubmersible: nrel.gov/docs/fy14osti/61154.pdf.

Best regards,

29

Many thanks Jason.

Regards,
AOAW

30

Dear Dr.Jonkman,

Greetings for the day! I am trying to obtain the yaw natural frequency value of the OC4 semi submersible platform as published in the Figure 3 of ‘Offshore Code Comparison Collaboration Continuation Within IEA Wind Task 30: Phase II Results Regarding a Floating Semisubmersible Wind System’ report.

From the report it could be inferred that the value is close to 0.013Hz.

In an effort to get this value, I took the mass of the platform with ballast from Table 3-3 and mooring line yaw stiffness from equation 5-15 of ‘Definition of the Semisubmersible Floating System for Phase II of OC4’. Please find the attachment.

However, I’m unable to get the correct value. If you could spot the mistake I made in assumptions/calculation it would be great.

Thanks
Krishnaveni

48263.jpg
48263.jpg2480×3507 532 KB

31

Dear Krishnaveni,

For this rotational motion, you would need to use the yaw inertia of the full system (including NREL 5-MW turbine + floater) in kgm^2 in the calculation.

Best regards.

32

Dear Jason:
I am trying to find out if there is any literature that explains why random seeds are used in wind turbine simulations. Do you know of any? Also is this the only paper nrel.gov/docs/fy14osti/61154.pdf that gives the natural frequencies for DeepCWind? I wanted to find table of results to know the precise values.

Regards,
AOAW

33

Hi @Jason.Jonkman
I have done a free decay test and the heave motion is not the same as shown in the paper for the surge decay test and the surge motion is not the same as in the paper for heave free decay test. See images for surge and have free decay test below.

HeaveVsTime_heavefreedecay
HeaveVsTime_heavefreedecay4667×3500 170 KB

HeaveVsTime_surgefreedecay
HeaveVsTime_surgefreedecay4667×3500 200 KB

pitchVsTime_heavefreedecay
pitchVsTime_heavefreedecay4667×3500 160 KB

pitchVsTime_surgefreedecay
pitchVsTime_surgefreedecay4667×3500 180 KB

SurgeVsTime_heavefreedecay
SurgeVsTime_heavefreedecay4667×3500 174 KB

SurgeVsTime_surgefreedecay
SurgeVsTime_surgefreedecay4667×3500 196 KB

Please let me have your feedback on this.
Thank you.
Regards,
AOAW

34

Hi @Jason.Jonkman,
I checked the natural frequency of the of the DeepCwind platform and the values agree well with the values in the paper, save for the roll and pitch motions. In the paper they are about 0.039 but in my check I get about 0.046. Is this satisfactory? I am not sure what I am doing wrong.
I used the models from github. In Elastodyn all the DOFs were set to true and the rated rotor speed was set to 12 rpm. I turned on ElastoDyn, HydroDyn (still water) and Moordyn only.

Regards,
AOAW

35

Dear @Andre.White,

Regarding random seeds, this is just a matter of aiming for statistical significance in the simulation results considering that inflow turbulence and irregular waves are stochastic processes. The IEC standards imply that you can fewer long simulations or more short simulations, with 10-minute-length simulations being standard for normal wind applications because that is a typical length that wind can be considered a stationary process. It is often computationally expensive to generate long turbulent inflow simulations at the spatial-temporal resolution required for convergence of the aero-elastic simulation, so, it is generally preferred to run multiple simulations (seeds), each of 10 minutes.

I don’t recall a better paper showing the natural frequencies of the NREL 5-MW baseline wind turbine top the OC4-DeepCwind semisubmersible.

I’m not seeing big differences in your free-decay results. Your results seem to fit within the “noise” of the various simulation results. Do you have a specific concern? It is certainly like that results from FAST/OpenFAST would change a bit as improvements are made to the software. You can add your results to the plots by accessing the OC4 participants results from the following Google Drive: Phase II Floating Semisubmersible - Google Drive.

Regarding the pitch natural frequency, I would expect a result closer to the ≈0.04 Hz. Can you clarify how you derived the higher frequency? Do you get the same result if you disable rotor rotation?

Best regards,

36

Hi @Jason.Jonkman,
Thanks for the feedback. I am not sure why pitch natural frequency is so high. It is for OpenFAST v3.4.1, the last time I did this was with v2.5.0 and the pitch natural frequency was approx. 0.4Hz. What are the implications of having 0.4Hz instead of 0.47Hz, does it mean that the structure is stiffer and my results in pitch may be a bit overpredicted?
By the way, even when I change the rotation to 0 rpm, (I had set it at rated of 12 rpm), results remain the same i.e. pitch is still 0.47 Hz. I looked through the blade files, and ElastoDyn to see if anything changed but everything is the same as per v2.5.0 but these values come out like that. I know in the paper, 2 of the participants got high values for pitch, one of them qualified it as finetuning that still needs to take place but the other seemed to have accepted it, so I am wondering if I can do the same.
Please advise. Thank you.
Regards,
AOAW

37

Dear @Andre.White,

I would expect that you’d get the same natural frequencies in both OpenFAST v2.5 and v3.4.1.

Are you using the r-test versions of the OC4-DeepCwind semisubmersible for both the OpenFAST v2.5 and OpenFAST 3.4.1 comparisons or your own version of the model(s)? If your own, what are the differences relative to the versions in the r-test?

Is HydroDyn input AddCLin nonzero in your v2.5 model and zero in your v3.4.1 model?

Perhaps the discussion in the following OpenFAST issue is relevant for you: Diffrences in free-decay response OpenFAST v2.3 - v3.0 ¡ Issue #1022 ¡ OpenFAST/openfast ¡ GitHub?

Best regards,

38

Hi @Jason.Jonkman,
That’s what I was expecting as well. I am using the r-test versions of the DeepCwind model. AddCLin is nonzero in both v2.5 and v3.4.1. However, there are two off diagonal terms, 1451298897 , that is in v2.5 but not in v3.4.1. I have included in v3.4.1 but no change to the pitch natural frequency. I also notice that the stiffness and damping matrix in v2.5.0 has less terms/rows that v3.4.1; 19 as compared to 24. Not sure if this could be part of the reason for the difference.
Do you know where I can find models for v3.0. I have the binaries but don’t know where to find the models?

Regards,
AOAW

39

Dear @Andre.White,

I’m not sure I understand. You say:

AddCLin is nonzero in both v2.5 and v3.4.1.

But you also say, which seems contradictory:

There are two off diagonal terms, 1451298897 , that is in v2.5 but not in v3.4.1.

Actually, the values of 1451298897 should be on the diagonal of AddCLin in v2.5. And these should be zero in v3.4.1. Can you confirm?

The “PLATFORM ADDITIONAL STIFFNESS AND DAMPING” section of the HydroDyn input file should have 19 rows in v2.5 and 24 rows in v2.6 and newer when NBody = 1. (NBody was added in v2.6 and newer to support multiple potential-flow bodies.) This is because the AddF0 vector is input as a row-vector in v2.5 and as a column vector in v2.6 and newer. This was changed because the size of the vectors and matrices in this section change when NBody > 1.

Best regards,

40

Hi @Jason.Jonkman:
Sorry for the mix-up. You are correct. AddCLin is in fact zero for v3.4.1.
I am still not understanding why I am getting the pitch natural frequency to be above 0.04 Hz though. I will have to look into it some more.

Regards,
AOAW