BModes : Input parameters about tower support subsystem

Dear Ngoc Ha Tran,

The BModes documentation is a bit misleading because it defines the origin of the principal elastic axes of bending as the shear center. However, this should be the tension center (the neutral axis). In general, the shear center may be at a different location in the cross section. However, BModes contains simplifications relative to the most general 6x6 beam theory, and in essence, BModes assumes that the shear center and tension center are coincident (while the tension center is specified separately in BModes, the input is ignored).

I’m not an expert on sectional analysis, but there are a number sectional-analysis tools e.g. VABS, BECAS, NuMAD/BPE, PreComp, etc. that can be used to calculate the sectional properties (6x6 or reduced mass and stiffness matrices, including the various centers and orientations) for you.

Best regards,

Dear Jason,

Thank you very much for your quick and useful response.

In this case, I will make tension center coincide with mass center and the principal elastic axes of bending coincide with the principal inertia axes.

I hope it will be right.

One more problem is that as I run the example “Test01_nonunif_blade_bmi” in Bmodes, it appears an error.

I attach the image of error and the file of BModes I extracted in C driver.

I hope that you can help me out.

Once again, thank you very much for your help.

Kind regards,

Ngoc Ha Tran.



Dear Ngoc Ha Tran,

While this may be a suitable approximation for the blade you are modeling, you should be aware that this assumption is not valid in general. For a general composite cross section, the tension center differs from the mass center and the orientation of the principal axes of bending differs from the orientation of the principal axes of inertia.

In your simulation, BModes cannot find the beam section properties input file because you are not running BModes from the directory containing the primary input file. BModes will run if you do this:

C:\Bmodes_v1.03.01>cd CertTest
C:\Bmodes_v1.03.01\CertTest>…\BModes.exe Test01_nonunif_blade.bmi

Best regards,

Dear Jason,

Thank you very much for your answer.

I run Bmodes according to your guide and it works well.

I also tried some software to calculate blade cross-section properties. However, the blades I am using are solid. They don’t have any shear webs. The results I got didn’t include all parameters required in BModes.

So, can you suggest me some other trial/free software to calculate sufficient parameters of blade solid cross-section, including tension cente, flapwise stiffness and edgewise stiffness?

Thank you very much.

Kind regards,

Ngoc Ha Tran.

Dear Ngoc Ha Tran,

I gave you a list of sectional-analysis software, some of which are free, in my post dated Sep 13, 2016 above.

If the solid cross section is composed of an isotropic material, you may be able to calculate most sectional properties by hand.

Best regards,

Dear Dr.Jason,

You mentioned that you are using the linearization functionality of ADAMS model to get the tower modes. Would you mind to tell that under which menu in ADAMS I can find this functionality? (Our university has purchased the license). I would like to analyze the tower modes of a wires-guyed tower by using both ADAMS and BModes, and then compare the two results.

Thank you very much.
Yingyi Liu
Kyushu University

Der Yingyi.Liu,

The ADAMS linearizations were run using the *_ADAMS_LIN.acf files generated through the FAST-to-ADAMS preprocessor. These ADAMS command files invoke a linearization analysis through the LINEAR/EIGNSOL command. More information is available in the “ADAMS Preprocessor” chapter of the old FAST User’s Guide: nwtc.nrel.gov/system/files/FAST.pdf.

Best regards,

Dear Dr. Jason,

I’m running the ADAMS solver from the ADAMS command prompt to calculate the “NRELOffshrBsline5MW_OC3Hywind” example in you FTP folder wind.nrel.gov/public/jjonkman/NR … Bsline5MW/, taking advantage of the A2AD functionality and the ADAMS_FAST preprocessor (I use the file “NRELOffshrBsline5MW_ADAMSSpecific.dat” included in your folder). However, a problem occurs as follows:

Before doing this, I have set up the A2AD interface (i.e., ADAMS2015_1_x64.dll) correctly using the CompileLinkA2AD.bat according to the guidance in FAST user’s manual. I have tested the Onshore wind turbine benchmark that the calculated time history results between FAST and ADAMS are almost same.

However, for the offshore case it fails to work correctly. Do you know what may cause this problem, according to the screenshot? Have I missed some settings in the ADAMS_FAST preprocessor or the ADAMS software for this offshore wind turbine case?

Best Regards,
Yingyi LIU

Dear Yingyi.Liu,

It is been years since I’ve ran MSC.ADAMS, but I do recall receiving that error on occasion. If I recall correctly, I’ve had some success with suggestion (3) i.e. playing with the ERROR argument.

Best regards,

Dear Jason,
We have solved this problem. To get rid of the above errors, one need to change the “Tstart” value in the *.fst file into a small number, e.g., zero, and the “error” value in the *acf file into a larger number.

Best Regards,
Yingyi Liu

Dear all,

Is there anyone explain how the BModes impose boundary condition to calculate the tower mode shape of FOWT? I have two version of BModes. One does not contain the source code and there is hydro_M, hydro_K and mooring_K in the input file, while the other does not have capabilty for offshore system (or I couldn’t see it). I couldn’t figure it out by examining what I have.

I just wonder the calculation methodology of BModes for the tower mode shape of FOWT?

Best regards.

Dear Enes,

The version of BModes with input parameters hydro_M, hydro_K, and mooring_K is the one you want when calculating tower mode shapes for floating offshore wind turbines (FOWTs). In this verison, the tower is cantilevered at its base to a six degree of freedom (DOF) rigid body (the floating platform), with associated 6x6 added mass (hydro_M), hydrostatic stiffness (hydro_K), and mooring stiffness (mooring_K) matrices.

Best regards,

Dear Jason,

Where can I find the source code of aforementioned version of BModes? Is it published somewhere?

Best regards.

Dear Enes,

The floating version of BModes is called BModesJJ in the following forum topic: http://forums.nrel.gov/t/run-bmodes-with-hydrodynamic-effect/690/8. As mentioned on that forum topic, due to the departure of Gunjit Bir from the NREL and his use of a proprietary eigensolver in BModesJJ that we don’t have access to, NREL is also currently unable to recompile BModesJJ.

Best regards,

Dear Jason

 I have some questions about the use of BModes:
1) I run the BModes with the input file OC3Hywind.bmi which is downloaded from [url]http://wind.nrel.gov/public/jjonkman/BModes/[/url]. When I checked the results, I found that the frequencies of the first several modes are very small (see the attachment), why?  If I want to calculate the  coefficiens ( TwFAM1Sh(i)and TwSSM1Sh(i) ) used to decide the mode shapes in EalstoDyn tower input file, how to choose the modes from the results? 
2) When BModes is used to calculate the tower modes of a floating offshore wind turbine, how to set the value of [b]hub_conn[/b] ? 2 or1?
3) I want to calculate the tower modes of a floating offshore wind turbine, my tower and turbine is the same as those of  OC3Hywind, but my tower base is 15m above MSL, so in my input file, the [b]radius[/b] is 92.6, the [b]hub_rad[/b] is 0 and  the [b]draft[/b] is -15. Isn't it right?

 Best regards

eigenvalue( 1) = 0.237266D+08 mode 1 frequency = 0.008118 eigenvalue( 2) = 0.237267D+08 mode 2 frequency = 0.008118 eigenvalue( 3) = 0.378574D+09 mode 3 frequency = 0.032428 eigenvalue( 4) = 0.556341D+09 mode 4 frequency = 0.039311 eigenvalue( 5) = 0.556782D+09 mode 5 frequency = 0.039327 eigenvalue( 6) = 0.522816D+10 mode 6 frequency = 0.120510 eigenvalue( 7) = 0.834905D+11 mode 7 frequency = 0.481579 eigenvalue( 8) = 0.867022D+11 mode 8 frequency = 0.490754 eigenvalue( 9) = 0.887017D+12 mode 9 frequency = 1.569693 eigenvalue( 10) = 0.151650D+13 mode 10 frequency = 2.052434 eigenvalue( 11) = 0.234819D+13 mode 11 frequency = 2.553970 eigenvalue( 12) = 0.129696D+14 mode 12 frequency = 6.002227 eigenvalue( 13) = 0.144491D+14 mode 13 frequency = 6.335328 eigenvalue( 14) = 0.232303D+14 mode 14 frequency = 8.032979 eigenvalue( 15) = 0.817870D+14 mode 15 frequency = 15.072697 eigenvalue( 16) = 0.833302D+14 mode 16 frequency = 15.214225 eigenvalue( 17) = 0.149830D+15 mode 17 frequency = 20.400834 eigenvalue( 18) = 0.302480D+15 mode 18 frequency = 28.986582 eigenvalue( 19) = 0.304189D+15 mode 19 frequency = 29.068344 eigenvalue( 20) = 0.404764D+15 mode 20 frequency = 33.531256

Dear Yu.Lei,

Here are my answers to your questions:

  1. When you run BModes to derive tower mode shapes for towers atop floating platforms, not only does BModes compute tower mode shapes, but it also computes modes associated with the platform motion. For the results for the OC3Hywind.bmi model, the first two modes correspond to the platfom surge and sway motions; the first fore-aft and side-to-side modes of the tower show up as modes 7 and 8.
  2. Normally I’d expect that hub_conn would be set to 2 when analyzing towers atop floating platforms in BModes (to enable the full 6 degrees of freedom of the platform).
  3. Yes. that sounds correct.

Best regards,

Dear Jason

Thanks a lot for your kind help.

Best regards

Dear Jason,
I am going to generate new mode shapes of the OC4 offshore wind turbine supported by the jacket using BModes as the boundary conditions have been altered. I have not found any post relative to this issue. I would be pleased if you can help me on following questions;

  1. Is that correct to use BModes OC3hywind for the purpose above considering BModes user guide ?
  2. As the jacket is the interested structure, do we only have to replace the values of the hydro_M and hydro_K yielded from Subdyn ? and will the mooring_K matrix take 0 values for all elements?
    Best regards,

Dear Arsalan,

My answer is “yes” to both of your questions.

Bets regards,

Dear Jason,
As always, I really appreciate for your quick reply.
Best regards,