Modes of BModes

Dear Jason,
I have a problem about BModes.I am using the FASTv7 to build my model, so I use the BModes to calculte the tower modes and compared with the GH Bladed’s result like below:


I want to know why there is so big difference in two software when the frequency become bigger,and if I choose four modes to tower input file,which one I should choose? Usually I choose the mode1,mode2,mode6 and mode7 in the tower input file.
Can you give me some guide?
Best Regard
Ruiliang.Wang

Dear Ruiliang,

I’m not sure I understand the mode shape descriptions in your table. I would expect that you’d take the first two fore-aft and side-to-side bending modes of the tower from BModes for use as the tower mode shapes in FAST.

I don’t really know why your results show increasing error between BModes and GH Bladed with increasing frequency. Are the distributed tower properties (mass and stiffness), tower-base boundary conditions (e.g. fixed/cantilevered), and tower-top boundary conditions (rigid mass/inertia of the rotor-nacelle assembly) identical between the two models?

Best regards,

Dear Jason,
I have run BModes looking for OC4 jacket substructure’s mode shapes considering apparent fixity length method. Firstly, I implied -26 m as the fixity length of the piles and ran the SubDyn stand-alone. Then I replaced the Kbbt and Mbbt in the BModes’ input file with those yielded from SubDyn. Moreover, I have set the tower section properties as you have suggested in some forum’s topics here. The result are quite vague for me and I thought you may help me.
The first and second mode frequencies are 0.502529E-05 and 0.398955E-04 respectively !!!
In addition, I have changed the fixity length into -13 m but I have gotten the same mode frequency as above.
Can you tell me what would the problem(s) can be ?
Best regards,

Dear Arsalan,

It sounds like you are using BModes correctly, but I agree that those frequencies don’t seem to match the conditions you are simulating. Are you using what is called BModesJJ that is discussed in this forum topic: http://forums.nrel.gov/t/run-bmodes-with-hydrodynamic-effect/690/1?

Best regards,

Dear Jason,
Yes I have used that BModes you are addressing. I changed the bottom of tower boundary condition to cantilevered and got reasonable frequencies but given that polyfitting procedure I have two questions;
Do we have to use 6th order normalized coefficients and substitute them with the former in elastodyn tower text file? If not,the coefficients other than those normalized are considerably small such az 0.001, do they seems correct?
Best regards,

Dear Arsalan,

I’m sorry, but I’m not really sure I understand your question. The polynomial coefficients for a given mode shape should sum to one.

Best regards,

Dear Jason,
You mean for example different values for coefficient a2 (relative to direct method, normalized direct method, improved method, normalized improved method, projection method, and normalized projection method) should sum and the result is the corresponding value to be placed in elastodyn-tower input file??
Best regards,

Dear Arsalan,

The coefficients a_2 through a_6 should sum to one.

Best regards,

Dear Jason,

I am really thankfull for your help.
I have another question if you could help me;
I want to check the mode shapes for different apparent fixity length. Thus I have changed the coordinates of last four joints in SubDyn input file. I run the code and obtained the kbbt and mbbt matrices and substituted them with those are in BModes bmi file. Afterwards I run the BModes but I got the same mode shapes precisely !!! By the way, corresponding tower_secs file was edited as you recommended and is used without change for each attempt.
Do you know what is the problem ?

Best regards,

Dear Arsalan,

I would expect that changing hydro_K and hydro_M in BModes based on updated Kbbt and Mbbt matrices from SubDyn would yield different mode shapes from BModes (unless the changes were minuscule). I can’t really comment more without knowing more about the steps you are using.

Best regards,

Dear Jason,

I explain the steps I have done in more details in the following;
1.the interested structure is the OC4 jacket.
2.I used the SubDyn stand-alone version in order to find KBBT and MBBT matrices where 4 new joints are added to structure joints list aligned with corresponding fixity length in input fìle.
3. SubDyn driver file Test04 with changes such wtrdpth 50 and Nsteps=126000 (10.5 minutes) is used.
4. OC3Hywind_tower_secs is used and mass_den, flp_stff, and edge_stff are applied from distributed tower properties in OC4jacket_elastodyn_tower. In addition, sec_loc column is changed in order of HtFract.
5. The BModes bmi file for the NREL 5MW Tower is used with following changes;

  • radius 88.15
  • draft -20.15
  • cm_pform -18.15
  • mass_pform 666E3
    Platform mass inertia 3×3 matrix:
    6002880 0.0 0.0
    0.0 6002880 0.0
    0.0 0.0 10229760
  • ref_msl -20.15
    *** hydro_M and hydro_K matrices were substituted with MBBT and KBBT matrices respectively.
  1. Run the BModes.
    *** for each fixity length I only have changed the 4 last joints coordinates in the Subdyn input file.

Best regards,

Dear Arsalan,

Your overall approach sounds fine. (But perhaps there is a problem in some of the details you glossed over?)

In step (2), did you change the base reactions joints (RJointID) to the new joints you’ve added at the apparent fixity depth? How much are Kbbt and Mbbt changing?

Best

Dear Jason,

Yes I have changed the base reaction joints with the 4 new joints added at the bottom of the list. Furthermore, I have attached here the KBBT and MBBT matrices yielded from two different apparent fixity length with values 12 m and 26 m.

regards,
[attachment=1]KBBTMBBT12m.jpg[/attachment]
[attachment=0]KBBTMBBT26m.jpg[/attachment]

Dear Arsalan,

Yes, it definitely looks like KBBt and MBBt changed by a significant amount. Can you also share your BModes input files?

Best regards,

Dear Jason,

Here are my BModes and Tower properties input files.
[attachment=1]Jacket.pdf[/attachment]
[attachment=0]Tower.txt[/attachment]

best regards,

Dear Arsalan,

I didn’t try to run your model, but I see two problems in your BModes input files. First, you have hub_conn set to 1, which means the tower beam is cantilevered at the tower base, so, the platform properties are unused. You should set hub_conn=2 to enable a free-free condition (with hub_conn=2, the tower-bsae is not “free”, but constrained by the platform stiffness, hydro_K). Second, the tor_stff and axial_stff values in distributed tower properties file are many, many orders of magnitude higher than the flp_stff and edge_stff. This difference will likely cause numerical problems with the eigensolver. If you want the torsional and axial stiffness much higher than the bending stiffness, I suggest that you set tor_stff and axial_stff only few orders of magnitude higher than flp_stff and edge_stff.

I hope that helps solve the problem.

Best regards,

Dear Jason,

The problem is solved and I really appreciate you for that.

Sincerely,

Dear Jason,

I am sorry but it is still something wrong with the generated mode shapes. As the first and second yielded mode shapes have the values of 0.000018 and 0.000026 Hz which are too low!!! Despite
I have applied all the tips you recommended.
Would you please help me ?

Best regards,

Dear Arsalan,

Can you upload your files so that I see if I can mimic the problem?

Best regards,

Dear Jason,

Here are my SubDyn input and driver files and BModes input file .

subdyn.pdf (35.1 KB)
Driver.pdf (24.1 KB)
BModes.pdf (32.7 KB)

Sincerely,