Blade Tip and Tower Top Mass Property

[Kaan.Ciloglu]

Hello everyone,
I want to get a information about tip and tower top mass. What is criteria to select and assign these values.
Thank you for your consideration.

Kaan Ciloglu
Aerospace Engineer

[Jason.Jonkman]

Dear Kaan,

Are you referring to the use of BModes or something else? Please clarify.

Best regards,

[Kaan.Ciloglu]

Depar Jason,
I meant BModes input parameters that I mentioned

Best Regards

[Jason.Jonkman]

Dear Kaan,

Normally there is no lumped mass at the blade tip, so, the blade-tip mass, center of mass, and inertias can normally be set to zero in BModes. However, the tower-top mass, center of mass, and inertias normally have a strong impact on the tower mode shapes, so, normally these values should be set appropriately when deriving tower mode shapes from BModes.

If that doesn’t answer your question; please clarify.

Best regards,

[Kaan.Ciloglu]

Dear Jason
Thank you so much for your clarification. So what is the criteria assign the tower-top mass (Does it means nacelle mass?)

Best regards
Kaan Ciloglu

[Jason.Jonkman]

Dear Kaan,

When deriving tower mode shapes from BModes, you should set the appropriate tower-top mass and inertia in BModes. By “tower-top”, I mean the rotor, drivetrain, and nacelle treated as a single rigid body.

Best regards,

[Kaan.Ciloglu]

Dear Jason
Thank you so much for your interest.

Kind regards
Kaan Ciloglu

[Zong.Linyang]

Hi Jason,

and what about the mass and inertia of the blades? Should these also be included in the calculation of the tower-top mass properties? To be specific, should the blade mass be accounted into the tip_mass and the blade moment of inertia be accounted into ixx_tip, iyy_tip and so on?

Best regards

[Jason.Jonkman]

Dear @Zong.Linyang,

When calculating the mode shapes of the tower in BModes, I agree that the blade mass, center of mass, and inertias should be included in the specification of the tip mass, center of mass, and inertias, along with the mass, center of mass, and inertias of the other tower-top components like the hub, drivetrain, and nacelle.

Best regards,

[Juan.LopezMuro]

Hi @Jason.Jonkman,

I am working with BModes_JJ. I am studying the example “OC3Hywind.bmi”, where the NREL 5MW Tower is considered. I am stuck at the tower-top mass properties section, and I think this is a good thread on which to place my doubts.

The data in the example file looks like this:

--------- Blade-tip or tower-top mass properties --------------------------------------------
3.500003109E+005   tip_mass    blade-tip or tower-top mass (kg)
-0.4137754432      cm_loc      tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.9669893542       cm_axial    tip-mass c.m. offset tower tip measures axially along the z axis (m)
4.370E7            ixx_tip     blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.353E7            iyy_tip     blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.542E7            izz_tip     torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0.                 ixy_tip     cross product of inertia about x and y reference axes(kg-m^2)
1.169E6            izx_tip     cross product of inertia about z and x reference axes(kg-m^2)
0.                 iyz_tip     cross product of inertia about y and z reference axes(kg-m^2)

However, when I compute the values based on the parameters given in:

• “NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat” (r-test/glue-codes/openfast/5MW_OC3Spar_DLL_WTurb_WavesIrr/NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat at f282c79af7152d20ec7becb7a7c9edbf65cca71c · OpenFAST/r-test · GitHub) and,
• “Table 2-2: Undistributed Blade Structural Properties” from the document “Definition of a 5-MW Reference Wind Turbine for Offshore System Development” (https://www.nrel.gov/docs/fy09osti/38060.pdf)

I obtain different results for the inertia matrix, computed with respect to the tip-mass c.m, as specified in https://forums.nrel.gov/t/tower-fore-aft-modes-shapes/193/7discussion. In particular, the ixx_tip is considerably off, and I also get a sign inversion for izx_tip.

--------- Blade-tip or tower-top mass properties --------------------------------------------
3.50E+05           tip_mass    blade-tip or tower-top mass (kg)
-0.4038582         cm_loc      tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.96612282         cm_axial    tip-mass c.m. offset tower tip measures axially along the z axis (m)
3.528E+07          ixx_tip     blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.177E+07          iyy_tip     blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.447E+07          izz_tip     torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0.                 ixy_tip     cross product of inertia about x and y reference axes(kg-m^2)
-1.157E+06         izx_tip     cross product of inertia about z and x reference axes(kg-m^2)
0.                 iyz_tip     cross product of inertia about y and z reference axes(kg-m^2)

In my computations, I:

1. Express the inertia tensors of the three Blades in the Nacelle frame, due to Azimuth, PreCone, and ShftTilt.
2. Express the inertia tensor of the Hub and Generator in the Nacelle frame due to ShftTilt
3. Apply the parallel axis theorem to compute the inertia of the Blades, Hub, Generator, and Nacelle with respect to the c.m.
4. Add the inertias together.

The values that I use in my computation are displayed here:

---------------------- TURBINE CONFIGURATION -----------------------------------
       -2.5   PreCone(1)  - Blade 1 cone angle (degrees)
       -2.5   PreCone(2)  - Blade 2 cone angle (degrees)
       -2.5   PreCone(3)  - Blade 3 cone angle (degrees) [unused for 2 blades]
          0   HubCM       - Distance from rotor apex to hub mass [positive downwind] (meters)
          0   AzimB1Up    - Azimuth value to use for I/O when blade 1 points up (degrees)
    -5.0191   OverHang    - Distance from yaw axis to rotor apex [3 blades] or teeter pin [2 blades] (meters)
         -5   ShftTilt    - Rotor shaft tilt angle (degrees)
        1.9   NacCMxn     - Downwind distance from the tower-top to the nacelle CM (meters)
          0   NacCMyn     - Lateral  distance from the tower-top to the nacelle CM (meters)
       1.75   NacCMzn     - Vertical distance from the tower-top to the nacelle CM (meters)
    1.96256   Twr2Shft    - Vertical distance from the tower-top to the rotor shaft (meters)
---------------------- MASS AND INERTIA ----------------------------------------
          0   TipMass(1)  - Tip-brake mass, blade 1 (kg)
          0   TipMass(2)  - Tip-brake mass, blade 2 (kg)
          0   TipMass(3)  - Tip-brake mass, blade 3 (kg) [unused for 2 blades]
      56780   HubMass     - Hub mass (kg)
     115926   HubIner     - Hub inertia about rotor axis [3 blades] or teeter axis [2 blades] (kg m^2)
    534.116   GenIner     - Generator inertia about HSS (kg m^2)
     240000   NacMass     - Nacelle mass (kg)
2.60789E+06   NacYIner    - Nacelle inertia about yaw axis (kg m^2)
          0   YawBrMass   - Yaw bearing mass (kg)
---------- Table 2-2.  Undistributed Blade Structural Properties ---------------
          0   BldCMx      - Downwind distance from the root along preconed axis to the blade CM (meters)
          0   BldCMy      - Lateral distance from the root along preconed axis to the blade CM (meters)
     20.475   BldCMz      - Vertical distance from the root along preconed axis to the blade CM (meters)
      17740   BldMass     - Blade mass (kg)
   11776047   BldRInerRt  - Blade inertia for roll tilt rotation about the blade root (kg m^2)
   11776047   BldPInerRt  - Blade inertia for pitch tilt rotation about the blade root (kg m^2)
          0   BldYInerRt  - Blade inertia for yaw rotation about the blade root (kg m^2)
!Definition of a 5-MW Reference Wind Turbine for Offshore System Development

I believe that the main contributions to the ixx_tip inertia are given by the blades, and I don’t understand how it can be as high as 4.370E7 kg-m^2

I have a few questions.

1. Are there any errors in my procedure? Are there any additional elements that I need to consider?
2. Is it possible that the computations have been made considering different (higher) inertia for the blades?
3. Is the cross product of inertia positive by convention in BModes_JJ, or is it possible that it should be negative in the example “OC3Hywind.bmi” file?
4. Is there any tool available with BModes_JJ to compute these parameters (I will probably be asking about the rest of the input parameters in the future as well)?

Many thanks in advance!

[Jason.Jonkman]

Dear @Juan.LopezMuro,

Your process sounds reasonable to me, but I’m not sure where the issue may lie.

The following forum topics are likely of use to you:

BModes about the tower-top mass properties

Dear jonkman, I have a Bladed model and I want to transform this model to Fast. So I hope to use the BModes to calculate tower frequence and compare the two software(Bladed and BModes) results. In Bladed Model about the Turbine Information column only have Mass of Nacelle and Rotor,Yaw Moment of Inertia parameters. My question is how to find or calculate the 'cm_loc', 'ixx_tip','ixx_tip','ixx_tip','izx_tip' parameters in the 'tower.bmi' file? Does it must be using the ADAMS to calculate thes…

Tower fore-aft modes shapes

Hello everyone, Tower fore-aft and side-side modes shapes are needed in FAST Tower input file. I used ModesV2.22 to calculate the modes shapes. The frequencies of the five shapes are increasing. Are the first two fore-aft modes and the second two side-side modes? In Blade calculation it was clearly expained. If the tower has also point mass at different height position, like flange, how could we consider the point mass for Distributed Tower Properties Section in the Modes or BModes input file? …

Tower Eigenfrequencies of NREL 5MW Turbine

Dear Daniel, We obtained the data presented in Table 1 using the aggregate mass tool of MSC.ADAMS – see my post dated Jun 07, 2013 in the following forum topic for more information: Using Aggregate Mass in ADAMS to Check NREL CS_Monopile.bmi - #4 by Lingling.Yin. You could also obtain these properties using the FAST v7 linearization if you set up your model to eliminate the platform and tower and locate the platform reference point at the yaw bearing. Best regards,

Using Aggregate Mass in ADAMS to Check NREL CS_Monopile.bmi

Dear Jason, I first used Aggregate Mass in ADAMS to calculate SNL 13.2MW mass moment of inertia and I used the outputs into .bmi file. The results of N.F are quite off from what I got from ADAMS (with only Twr DoFs on). I then downloaded wind.nrel.gov/public/jjonkman/BModes/ and wind.nrel.gov/public/jjonkman/NR … Bsline5MW/ offshore part to run the Aggregate Mass in ADAMS. I specified a rigid NRELOffshrBsline5MW_Monopile_RF with all DoFs turning off since I only want to see the mass moment …

The last in this list explains how to derive the 6x6 mass matrix of the RNA through an OpenFAST linearization analysis. I would suggest following this process to check your hand calculation.

Best regards,

[Juan.LopezMuro]

Thank you very much @Jason.Jonkman for your response and all the links!

As you suggested, I followed the process to derive the 6x6 mass matrix of the RNA through an OpenFAST linearization analysis to check my hand calculation.

I ran 2 cases that I think should have given a similar result, but I found interesting differences when I compared them, and I’d like to share and ask about them.

The only difference between the two cases is that I changed the values of the switch CompElast from 1=ElastoDyn to 2=ElastoDyn + BeamDyn for blades in the .fst input file.

To give a little bit of context:

• In the .fst file, I set: “ES10.3E2”=OutFmt, True=Linearize, True=CalcSteady, 2=LinInputs, 1=LinOutputs.

• In the ElastoDyn file, I only enabled the six Platform degrees of freedom. and I chose the plaform outputs in the inertial frame: “PtfmTAxi”, “PtfmTAyi”, “PtfmTAzi”, “PtfmRAxi”, “PtfmRAyi”, “PtfmRAzi”.

• In the ElastoDyn_Tower, I set the TMassDen to 0.1E-13 (kg/m).

The values that I computed from the results with 1=ElastoDyn are:

--------- Blade-tip or tower-top mass properties --------------------------------------------
349390.900         tip_mass    blade-tip or tower-top mass (kg)
-0.4052381         cm_loc      tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.98118111         cm_axial    tip-mass c.m. offset tower tip measures axially along the z axis (m)
3.857E+07          ixx_tip     blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.344E+07          iyy_tip     blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
2.529E+07          izz_tip     torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0.                 ixy_tip     cross product of inertia about x and y reference axes(kg-m^2)
-1.298E+06         izx_tip     cross product of inertia about z and x reference axes(kg-m^2)
0.                 iyz_tip     cross product of inertia about y and z reference axes(kg-m^2)

The Inertias now are closer but, on average, still 10% higher than the ones I computed “by hand”.

Q1: In my computations, I am considering that:

• the inertia of the generator and the hub is only about the shaft axis
• the inertia of the nacelle is only about the yaw axis
• the blades have no inertia about the pitch axis.

Is this correct?

Also, based on my results and after reading the Wikipedia section (and although I have never used Adams software before), I suspect that Adams uses a different convention for the products of inertia, where the minus sign is removed from the product of inertia formulas. That may explain the change in the sign of the izx_tip…?

The values that I computed from the results with 2=ElastoDyn + BeamDyn, are:

--------- Blade-tip or tower-top mass properties --------------------------------------------
299978.467         tip_mass    blade-tip or tower-top mass (kg)
0.51966755         cm_loc      tip-mass c.m. offset from the tower axis measured along x-tower axis (m)
1.87977266         cm_axial    tip-mass c.m. offset tower tip measures axially along the z axis (m)
1.555E+05          ixx_tip     blade lag mass moment of inertia about the tip-section x reference axis (kg-m^2)
2.321E+06          iyy_tip     blade flap mass moment of inertia about the tip-section y reference axis (kg-m^2)
4.039E+06          izz_tip     torsion mass moment of inertia about the tip-section z reference axis (kg-m^2)
0.                 ixy_tip     cross product of inertia about x and y reference axes(kg-m^2)
2.037E+05          izx_tip     cross product of inertia about z and x reference axes(kg-m^2)
0.                 iyz_tip     cross product of inertia about y and z reference axes(kg-m^2)

When I compare the mass results, the difference is three times 16470.811kg, almost the mass of 16844.752 kg reported in the .sum.yaml information file generated by BeamDyn.

The Inertia tensor reported in the .sum.yaml information file generated by BeamDyn is similar to that reported in Definition of a 5-MW . Still, a note mentions, “NOTE: from mass distribution only, missing some important inertial contributions (see PR#1337)”.

Q2: Is it possible that when BeamDyn is used in a Linearization, the mass and inertia of the blades are not appropriately evaluated or even not considered?

Thanks again!

[Jason.Jonkman]

Dear @Juan.LopezMuro,

Overall your approach sounds fine, except that normally I’d suggest including more precision in your output format when generating a linearized solution, such as OutFmt = “ES17.9E2”.

Here are my responses:

1. I agree with your bullet points. Good question about the products of inertia used by ADAMS; I’m not sure I know the answer.
2. When the blades are modeled in BeamDyn, ElastoDyn will not know the blade mass, and so, post-processing the .ED.lin output file will not consider the blade mass. That said, the full system linearization will know about the blade mass, but BeamDyn does not have an option to consider the blades rigidly, and so, the rigid-body equivalent mass matrix cannot be computed in this manner.

Best regards,

[Juan.LopezMuro]

Hi @Jason.Jonkman,

I actually did not write my sentence right; I meant to say that in the .fst file, I set: “ES20.12E3”=OutFmt instead of “ES10.3E2”, as I saw in one of your suggestions in the forum since the latter gave me the wrong results. Thanks for that!

1. I don’t think increasing the precision more than “ES20.12E3” will lead to any changes in the computations. On the other hand, I’ll also check the precision of my “hand” computations.
2. Thank you so much for clarifying that!

[Jason.Jonkman]

I agree that using OutFmt = “ES20.12E3” is more than enough precision.

Best regards,