BModes : Input parameters about tower support subsystem

Dear NREL Team:
I’m using New version(Not Publicly released) of BModes code which could consider the flexibility of the foundation. The code is downloaded from wind.nrel.gov/public/jjonkman/BModes/ released by Dr. Jonkman.
In the “OC3Hywind.bmi” input file, there are 3 matrix representing the foundation and mooring’s parameters:
Platform-reference-point-referred hydrodynamic 6X6 matrix (hydro_M);
Platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K);
Mooring-system 6X6 stiffness matrix (mooring_K).

I compared the parameters with the paper “Definition of the Floating System for Phase IV of OC3”. The “hydro_K” seems to be the linearized restoring matrix from all mooring lines, and the “hydro_K” is more like the linear hydrostatic-restoring matrix, but both have small differences with the value in the paper.

Could you tell me what’s the meaning of those matrix, and how to get them?
Thanks a lot,

Lei

Dear Lei,

The “hydro_K” matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (4-3) of the OC3-Hywind report (perhaps with slight numerical rounding) except that the (6,6) element has been augmented with the “additional yaw spring” documented in Table 5-1 of the OC3-Hywind specifications report.

The “mooring_K” matrix in the OC3Hywind.bmi model is identical to what is specified in Eq. (5-3) of the OC3-Hywind report except that the (4,4) and (5,5) elements have been augmented with the contribution of system weight/gravity, which is not otherwise accounted for in BModes as explained in the following forum topic: http://forums.nrel.gov/t/bmodes-for-blades-submerged-in-water/422/1. System weight is important for the pitch and roll restoring of deep-drafted floating platforms, such as spar buoys. The augmentation equals -mgz, where m is the total system mass (platform + ballast + tower + nacelle + rotor), g is gravity, and z is the vertical location of the center of gravity of the total system mass (z is negative in value for this case).

I hope that helps.

Best regards,

Dear Jason,

thank you for your quick response, that helps a lot!

Lei

Dear Jason,

I have another question about the input file for BModes.
If the added mass matrix varies with the frequency, then how to define the “Hydro_M”?

also,the following lines in the input file, i think they are talking about the same value for tower, the different is just the +/- symbol, am i right?
hub_rad: hub radius measured along coned blade axis OR tower rigid-base height (m)
draft : depth of tower base from the ground or the MSL (mean sea level) (m)

best regards,

Lei

Dear Lei,

You cannot define a frequency-dependent hydrodynamic added mass in BModes, so, you, must pick a frequency at which to characterize the added mass. Typically, the zero- or infinite-frequency limit of added mass is chosen, but you could also choose the added mass closest to the frequency of most interest to your structural response.

I’m not sure I understand your second question. Please clarify.

Best regards,

Dear Jason,

My second question means:
The following lines colored in red in the input file for OC3Hywind (wind.nrel.gov/public/jjonkman/BM … Hywind.bmi), i think they are referring to the same distance - from the MSL to Tower base, but the direction is opposite. That means the “tower rigid-base height” should be 10, and the “draft” should be -10.
But in the input file, the “tower rigid-base height” value is 0. So, I’m a little confused, am I misunderstanding the meaning of the two parameters?

Best regards,

Lei

====================== BModes v3.00 Main Input File ==================
NREL 5MW Tower

--------- General parameters ---------------------------------------------------------------------
true Echo Echo input file contents to *.echo file if true.
2 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed, automatically set to zero for tower modal analysis (rpm)

  1.    romg_mult:  rotor speed muliplicative factor (-)
    

87.6 radius: rotor tip radius measured along coned blade axis, OR tower height above ground level [onshore] or MSL offshore
0.0 hub_rad: hub radius measured along coned blade axis OR tower rigid-base height (m)
0. precone: built-in precone angle, automatically set to zero for a tower (deg)
0. bl_thp: blade pitch setting, automatically set to zero for a tower (deg)
2 hub_conn: hub-to-blade or tower-base boundary condition [1: cantilevered; 2: free-free; 3: only axial and torsion constraints] (-)
20 modepr: number of modes to be printed (-)
t TabDelim (true: tab-delimited output tables; false: space-delimited tables)
f mid_node_tw (true: output twist at mid-node of elements; false: no mid-node outputs)

--------- 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)

--------- Distributed-property identifiers --------------------------------------------------------
1 id_mat: material_type [1: isotropic; non-isotropic composites option not yet available]
‘OC3Hywind_tower_secs.dat’ : sec_props_file name of beam section properties file (-)

Property scaling factors…
1.0 sec_mass_mult: mass density multiplier (-)
1.0 flp_iner_mult: blade flap or tower f-a inertia multiplier (-)
1.0 lag_iner_mult: blade lag or tower s-s inertia multiplier (-)
1.0 flp_stff_mult: blade flap or tower f-a bending stiffness multiplier (-)
1.0 edge_stff_mult: blade lag or tower s-s bending stiffness multiplier (-)
1.0 tor_stff_mult: torsion stiffness multiplier (-)
1.0 axial_stff_mult: axial stiffness multiplier (-)
1.0 cg_offst_mult: cg offset multiplier (-)
1.0 sc_offst_mult: shear center multiplier (-)
1.0 tc_offst_mult: tension center multiplier (-)

--------- Finite element discretization --------------------------------------------------
50 nselt: no of blade or tower elements (-)
Distance of element boundary nodes from blade or flexible-tower root (normalized wrt blade or tower length), el_loc()
0.0000 0.0200 0.0400 0.0600 0.0800 0.1000 0.1200 0.1400 0.1600 0.1800 0.2000 0.2200 0.2400 0.2600 0.2800 0.3000 0.3200 0.3400 0.3600 0.3800 0.4000 0.4200 0.4400 0.4600 0.4800 0.5000 0.5200 0.5400 0.5600 0.5800 0.6000 0.6200 0.6400 0.6600 0.6800 0.7000 0.7200 0.7400 0.7600 0.7800 0.8000 0.8200 0.8400 0.8600 0.8800 0.9000 0.9200 0.9400 0.9600 0.9800 1.0000

--------- Properties of tower support subsystem (read only if beam_type is 2) ------------
1 tow_support: : aditional tower support [0: no additional support; 1: floating-platform or monopile with or without tension wires] (-)
-10.0 draft : depth of tower base from the ground or the MSL (mean sea level) (m)
89.9155 cm_pform : distance of platform c.m. below the MSL (m)
7466.33E3 mass_pform : platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
4229.23E6 0. 0.
0. 4229.23E6 0.
0. 0. 164.23E6

Dear Lei,

“hub_rad” and “draft” are similar, but not identical (although they do have the opposite sign as you noticed). “hub_rad” in BModes is the distance from the top of the platform along the tower to the beginning of the flexible part of the tower. “draft” is the distance from the MSL to the top of the platform, positive downwards. So, the base of the flexible tower is located a distance of “hub_rad - draft” above the MSL. The tower top is located a distance of “radius” above the MSL. Thus, the flexible length of the tower equals “radius - hub_rad + draft”.

In the OC3-Hywind system, the rigid platform extends 10-m above the MSL and the tower is modeled flexibly along the entire tower – from 10-m to 87.6-m above the MSL. So, draft = -10 m, hub_rad = 0 m, radius = 87.6 m and flexible length of the tower equals 77.6 m.

I hope that helps.

Best regards,

Dear Dr. Jonkman,

Thank you for your help.
In order to confirm if i’m using BModes right, i do a calculation of Tower Mode of OC3Hywind with the input file and Code in wind.nrel.gov/public/jjonkman/BModes/. After that, a polynominal fitting is performed using the Excel table(ModeShapePolyFitting.xls) in FAST archive.
Then, i compared the results with the tower input file “NRELOffshrBsline5MW_Tower_OC3Hywind.dat”. The first order mode shape agree with each other well, but the second order mode shape have remarkable difference. The coefficient and figure is shown below.
Could you give me some hints where I do wrong?

the second order s-s mode shape:
! ~~~~~~~~ FAST_Tower_Input # Bmodes calcultion
coefficient of x^2 term 60.2285 8.396705988
coefficient of x^3 term -27.5868 -1.440145029
coefficient of x^4 term -30.3887 -5.071404927
coefficient of x^5 term -33.6738 0.201117338
coefficient of x^6 term 32.4208 -1.08627337

compare.JPG

Dear Lei,

The approach you describe to derive mode shapes for FAST from BModes sounds correct.

I’m sure that the reason for the differences you are seeing between the BModes-generated mode shapes and the mode shapes derived by NREL for the OC3-Hywind tower contained in “NRELOffshrBsline5MW_Tower_OC3Hywind.dat” is because NREL did not use BModes to derive the tower mode shapes for the OC3-Hywind tower contained in “NRELOffshrBsline5MW_Tower_OC3Hywind.dat”. When NREL derived the tower mode shapes for the OC3-Hywind tower, the ability to derive tower mode shapes for systems with floating platforms was not available in BModes. Instead of BModes, we used the linearization functionality of a full-system ADAMS model to obtain the tower modes for the NREL 5-MW turbine models (both land-based and floating). That is, we built an ADAMS model of the wind turbine (using the FAST-to-ADAMS preprocessor), enabled all system DOFs, and linearized the model. Then we passed a best-fit polynomial through the resulting tower mode shapes to get the equivalent polynomial representations of the tower mode shapes needed by FAST. For the tower modes, there is not only an influence from the floating platform DOFs on the tower mode shapes, but there can also be a great deal of coupling with the drivetrain and rotor DOFs. While BModes can now account for the floating platform DOFs, it still treats the drivetrain and rotor rigidly, so, the BModes- and ADAMS-generated mode shapes will differ.

Best regards,

Dear Jason,

Thank you for your patience, I really appreciate your kind help.

Best regards,

Lei

Hi Jason,

I am using BModes for the purpose of gaining the coefficients of the polynomial equation used to model the fore-aft and side-to-side mode shape of the tower in FAST.

I have 2 questions for modelling the tower in FAST and BModes.

Firstly, in BModes, I am required to place the value of tower-tip mass c.m. I am wondering that the tower is usually designed symmetrically, so its centre of mass will be on the tower axis, and therefore, tip-mass c.m in the table should be 0. However, it is a non-zero value in the example file as well as in the descriptive figure. Can you explain me this parameter?

Secondly, the model I am using is put in a small lab. So, the tower I define here is a cylinder containing 2 ends. One end is located on the surface of nacelle, while another one is emerged out of the surface, instead of touching to the ground. Now, I wonder that as I model this tower in FAST and BModes, the results I acquire are accurate enough, isn’t it? Should I model it as a tower?

Thank you very much.

I am looking forward to hearing from you.

Kind regards,

Ngoc Ha Tran.

Dear Ngoc Ha Tran,

The tower-top mass, center of mass, and inertias in BModes refer to the lumped-mass representation of the rotor-nacelle assembly (RNA). Normally, the mass and inertia of the RNA are large enough to have a sizable affect on the natural frequencies and mode shapes of the tower.

I’m not sure I understand your second question.

Best regards,

Hi Jason,

I solved that problem due to your answer. Thank you very much.

Now, one more thing is as I run BModes with your example case “Test01_nonunif_blade.bmi”, I always encounter the following error:

"Running BModes (v1.03.01, 25-Sept-2007, compiled using double precision).

Linked with the NWTC Subroutine Library (v1.01.08, 26-Sept-2007).

The input file, “blade_sec_props.dat”, was not found.

Aborting Bmodes."

So, can you guide me how to solve this problem to run Bmodes successfully?

Thank you very much.

Kind regards,

Ngoc Ha Tran.

Dear Ngoc Ha Tran,

Do you have such a file?

Best regards,

Dear Jason,

Now, I can know how to run Bmodes.

However, in the blade section property file, I need to supply the geometrical properties of the blade. I can obtain most parameters, except “shear center”.

I am trying to use PreComp to obtain this parameter. However, the blade I use to experiment in the lab is solid. It means that it doesn’t have any webs and laminates. But in PreComp, I need to complete an auxiliary input file to describe the internal shape of the blade. I try to put “unused” under the column requiring the name of this auxiliary file in the main input file. However, immediately, the software can not run.

What should I do in my case?

Thank you very much.

Kind regards,

Ngoc Ha Tran.

Dear Ngoc Ha Tran,

PreComp makes use of thin-walled assumptions. For a solid cross section, you’ll need to derive the cross-sectional properties using a different tool/method. If the solid is composed of an isotropic material, the calculation may be straightforward, depending on the shape of the cross section.

Best regards,

Dear Jason,

I really appreciate your response. It helps me so much to simulate my model accurately.

Now, I am having a trouble in determining flapwise and edgewise stiffness of cross-sections of a tidal turbine blade.

I read in BModes 's guide. They said that flapwise stiffness is the section flap bending stiffness about the principal elastic axis (Ye) and edgewise stiffness is the section lag bending stiffness about Xe elastic axis.

As I read in some books, they said that the elastic axis is the locus of shear centers of all cross-sections of blades. It means that the elastic axis is determined for the whole beam, not for any certain cross-section.

However, in BModes 's guide, it seems that the elastic axis is on the cross-section of blade. It makes me confused.

So, can you show me the method to determine the principal elastic axis on each cross-section of blades? I have found the shear center of each cross-section already.

Thank you very much.

I am looking forward to hearing from you.

Kind regards,

Ngoc Ha Tran.

Dear Jason,

One more question is about blade-tip mass. In my mind, blade-tip mass is mentioned as a tip brake is used on the blade. Am I thinking correctly? Can you explain me the meaning of this parameter and when should I use this parameter? Thank you very much.

I am using Bmodes for the purpose of determining the coefficients of blade shape modes in FAST. However, I realize that natural frequencies I got from Bmodes are too large, even up to 400Hz. Does it seem to have some problems here? I checked the parameters in the auxiliary input file and maybe I used the principal inertia co-ordinate system to obtain flapwise and edgewise stiffness instead of the principal elastic axes. As I mentioned in the previous question, I searched elastic axis and found that elastic axis is the locus of shear centers and is used for the whole beam, not for a cross-section. Can you show me the definition and the method to determine the principal elastic axis location for each cross-section of blade?

The third problem is that as I account for blade-tip mass in BModes input file, the natural frequencies reduced from 400Hz to approximately 20 Hz. It means that blade-tip mass plays an important role to natural frequencies, doesn’t it?

These problems make me confused.

I would be really grateful if you can help me out.

Thank you very much.

Have a nice week.

Kind regards,

Ngoc Ha Tran.

Dear Ngoc Ha Tran,

See my post dated Jan 25, 2013 in the following forum topic for my definitions of the various centers: http://forums.nrel.gov/t/coordinate-system-in-fast/632/6. By referring to the elastic axis as the “locus of elastic centers”, this means that a line (or curvilinear line) is passed from the root to the tip through the elastic center of each cross section. Note that some authors refer to the “elastic center” as the “tension center” and other authors refer to the “elastic center” as the “shear center”.

The principal axes of bending are defined as the orthogonal axes in a cross section whereby the cross bending stiffness terms are zero. The origin of the principal axes of bending is the tension center (the neutral axis).

A tip brake is one example where there is a large lumped mass near the tip of the blade. In most blades, there is no large lumped mass near the tip, so, the tip-mass inputs in BModes or FAST can usually be set to zero.

I know nothing about the blade you analyzing, but I would be surprised if a tip mass was so large that it reduced a blade natural frequency from 400 to 20 Hz.

I hope that helps.

Best regards,

Dear Jason,

I am really sorry if I make some inconvenience for you, but I am still confused with the coordinate system to determine structural twist angle, flapwise stiffness and edgewise stiffness in BModes.

These following information I read in BModes’ s guide:

  • Str_tw: for a blade, it is the angle the chordwise principal elastic axis would make with the blade reference plane.

  • Flp_Stff: for a blade, it is the second flap bending stiffness about the Ye elastic axis.

  • Edge_stff: for a blade, it is the second lad bending stiffness about the Xe elastic axis.

On the page 12 of BModes’ s guide: Point E is the section shear center and is the origin for the principal elastic axes Xe-Ye.

As I understand, in order to determine Str_tw, flp_stff and edge_stff, I need to determine shear center and the principal elastic axes Xe-Ye for each cross-section. Am I thinking correctly?

If the answer is yes, can you show me how to determine the principal elastic axes for each cross-section? I am really grateful for this help.

I attach the figure in BModes’ s guide for the coordinate systems in BModes.

Thank you very much.

I am looking forward to hearing from you soon.

Kind regards,

Ngoc Ha Tran.