# Tower Eigenfrequencies of NREL 5MW Turbine

Hi all,
I’ve a question regarding placing an infinitely stiff Coupled Spring (CS) foundation matrix into FAST

I’m solving FAST for the 5MW monopile turbine sited in sallow water,

First I solved for a fixed base (FB) foundation and I use the linearization function and then an Eigenvalue analysis to get the natural frequencis of the turbine.
These are listed below. (All DOF are turned on)

I then place the CS stiffness matrix for the OC3 pile foundation (given by Memorandum: ‘‘Derivation and Description of the Soil-Pile-Interaction Models’’)
K_H = 2.58E9, K_M = 2.64E11 and K_HM = -2.26E10
into UserPtfmLd in UserSubs recompile get an new .exe FAST file and resolve my turbine.
This gives a new set of natural frequencies (the tower natural frequencies are reduced due to the softening effect of the foundation) this is all fine.
These are listed below:

Now to test that everything is working ok what I usually do is put in a very stiff foundation (infinitely stiff)
and check that the tower natural frequency are unaffected by the foundation (as the foundation should now have no effect because its so stiff)
With this in mind i changed the foundation stiff to very high values (ie inf)
K_H = 2.58E20, K_M = 2.64E22 and K_HM = -2.26E21

Again I put this inf stiff foundation into UserPtfmLd in UserSubs recompiled and get a .exe FAST
and then I used this to compute new natural frequencies, I expected these to match the fixed base results but the first tower natural frequencies are off
My question is why is this happening?? I find this a bit worrying

(note i also included an intermediately inf stiff foundation, OC3 foundation xx10000)
All the natural frequencies are listed below they are not sorted and simply arranged in ascending order

``````FB	OC3foundation	OC3foundationx10000	inf
0	0.00016268	9.62E-05	9.66E-05
0.27895	0.23066	0.26168	0.30512
0.28385	0.2546	0.27737	0.34903
0.74414	0.74307	0.74408	0.74428
0.95032	0.94145	0.95587	0.95587
0.95484	0.94593	0.9604	0.9604
1.1408	1.1355	1.1444	1.1444
1.1502	1.1443	1.1541	1.1541
1.6537	1.4673	1.6538	1.6526
2.0708	1.5103	2.0706	2.071
2.2868	1.8306	2.2838	2.2898
2.3706	2.0825	2.3897	2.3897
2.3749	2.3433	2.3944	2.3944
2.6736	2.3469	2.6749	2.666
3.9406	3.8395	3.9406	3.9398
5.8528	5.3148	5.8528	5.8527
-	     5.5238	214.04	1.07E+05
-	     5.8584	214.36	1.07E+05
-	     18.347	1619.9	3.23E+06
-	      18.61	1620.5	3.23E+06
``````

Also just to note when I am using a foundation I also recomputed the tower mode shapes using Bmodes,
Bmodes also gives an output of the tower natural frequencies these are listed below
When the foundation is infinitely stiff Bmodes gives useable results so i just used tower mode shapes from the fixed base case.

Interestingly Bmodes converges for a stiff foundation (stiffness matrix *10000) but breaks down for a very stiff foundation (inf).
(Just to note ive just listed the first 6 natural frequencies given by Bmodes for the inf stiff foundation)

Bmodes First two tower mode

``````

FB	 OC3foundation	OC3foundationx1000          inf
SS 1st	0.273744	0.242353	0.273744	0.001384
FA 1st	0.276006	0.243885	 0.276006	0.005458
SS 2nd	1.588980	1.317520	1.328890	0.871748
FA 2nd	1.867030	1.498520	 1.867030	0.923020
1.330810
2.220860``````

Thanks
Michael

Dear Michael,

I’m guessing the incorrect frequencies resulting from the use of an extremely large stiffness is simply the result of numerical errors in the solution process. In FAST, for example, the linearized stiffness matrix is derived from the nonlinear model through a central-difference numerical perturbation method. With such a large stiffness, the perturbation method will involve taking differences between large numbers, which may lead to numerical errors in the floating point math. Instead of setting very large stiffness, it is always better to disable DOFs, which you’ve done.

I hope that helps.

Best regards,

Ok, That makes sense.
Thanks for the response.

Regards
Michael

Jason,

Can you tell me what the difference between BModes hosted on your site at http://wind.nrel.gov/public/jjonkman/BModes/ (JJ) and on the design codes website http://wind.nrel.gov/designcodes/ (DC) is.

Having looked at the source code for the design codes website and testing that hosted on JJ, it seems there is a difference in the options for tow_support. JJ allows 0 or 1, where 1 enables the read in of the platform properties and, I presume, the tension wires. The DC site allows 0, 1 for tension wires and 2 for the platform properties.

When testing the NREL 5MW monopile file CS_Monopile.bmi also hosted on your site with tow-support is 1 for JJ and tow_support is 2 for DC but all other parameters unchanged, the displacements caused by the first two modes (SS1 and FS1) are practically the same, but there is a difference in the mode shapes for the next two which are torsion/s-s modes, from one of which you would need to take the second s-s mode.

I have included comparison plots in the attached file.

All the best, Rebecca

JJ version

[code] ******** modal analysis results **********

`````` eigenvalue(  1) =  0.211357D+11        mode  1 frequency =      0.242302
eigenvalue(  2) =  0.214035D+11        mode  2 frequency =      0.243832
eigenvalue(  3) =  0.628000D+12        mode  3 frequency =      1.320774
eigenvalue(  4) =  0.674696D+12        mode  4 frequency =      1.368998
eigenvalue(  5) =  0.835069D+12        mode  5 frequency =      1.523035
eigenvalue(  6) =  0.288383D+13        mode  6 frequency =      2.830309
eigenvalue(  7) =  0.375475D+13        mode  7 frequency =      3.229530
eigenvalue(  8) =  0.133352D+14        mode  8 frequency =      6.086242
eigenvalue(  9) =  0.143371D+14        mode  9 frequency =      6.310724
eigenvalue( 10) =  0.180019D+14        mode 10 frequency =      7.071443
eigenvalue( 11) =  0.506911D+14        mode 11 frequency =     11.866286
eigenvalue( 12) =  0.517176D+14        mode 12 frequency =     11.985830
eigenvalue( 13) =  0.917630D+14        mode 13 frequency =     15.965505
eigenvalue( 14) =  0.138548D+15        mode 14 frequency =     19.617754
eigenvalue( 15) =  0.139579D+15        mode 15 frequency =     19.690570
eigenvalue( 16) =  0.258782D+15        mode 16 frequency =     26.811162
eigenvalue( 17) =  0.287978D+15        mode 17 frequency =     28.283179
eigenvalue( 18) =  0.311704D+15        mode 18 frequency =     29.425219
eigenvalue( 19) =  0.312880D+15        mode 19 frequency =     29.480713
eigenvalue( 20) =  0.617526D+15        mode 20 frequency =     41.416786[/code]
``````

DC version

``` mode 1 frequency (hz) = 0.249 mode 2 frequency (hz) = 0.250 mode 3 frequency (hz) = 1.322 mode 4 frequency (hz) = 1.375 mode 5 frequency (hz) = 1.544 mode 6 frequency (hz) = 2.781 mode 7 frequency (hz) = 3.124 mode 8 frequency (hz) = 6.039 mode 9 frequency (hz) = 6.143 mode 10 frequency (hz) = 7.292 mode 11 frequency (hz) = 11.833 mode 12 frequency (hz) = 11.911 mode 13 frequency (hz) = 15.965 mode 14 frequency (hz) = 19.594 mode 15 frequency (hz) = 19.631 mode 16 frequency (hz) = 26.877 mode 17 frequency (hz) = 28.283 mode 18 frequency (hz) = 29.405 mode 19 frequency (hz) = 29.431 mode 20 frequency (hz) = 41.417```
BMODESwebvsBmodesJJ.xls (462 KB)

Dear Rebecca,

The version of BModes on wind.nrel.gov/designcodes/preprocessors/bmodes/ (DC).

In your comparison between the two, how were you able to simulate the CS_Monopile.bmi in DC? Inputs such as “Draft” and “ref_msl” are available in JJ but not in DC and will certainly impact the results.

Best regards,

You made me question myself but I downloaded a new copy of the DC version from the website wind.nrel.gov/designcodes/preprocessors/bmodes/ to be sure. The additional supports section including draft etc. is included in the design codes website version. Line 590 onwards in bmodes.f90 are:

[code] if ( tow_support == 2 ) then

! platform data

`````` CALL ReadVar ( UnIn, InFile, draft, 'draft', 'depth of tower base from the MSL (mean sea level) (m)'  )
CALL ReadVar ( UnIn, InFile, cm_pform, 'cm_pform', 'distance of platform c.m. below the MSL (m)'      )
CALL ReadVar ( UnIn, InFile, mass_pform, 'mass_pform', 'platform mass (kg)' )[/code]
``````

Here is the header from the .out file for CS_monopile.bmi:

```Results generated by BModes (v3.00.00, 20-Mar-2008, compiled using double precision) on 22-Jan-2013 at 16:17:20. NREL 5MW Tower ```

and here is the echo file:

[code] T Echo - Echo input file contents to *.echo file if true
2 beam_type - beam type, 1: blade; 2: tower
0.0000E+00 rot_rpm - rotor speed (rpm)
1.0000E+00 rpm_mult - rotor speed multiplicative factor
8.7600E+01 radius - rotor tip radius or tower height above ground (m)
0.0000E+00 rroot - hub radius or tower-base height (m)
0.0000E+00 btp - precone (deg)
0.0000E+00 bl_thp - blade pitch setting (deg)
3 hub_conn - hub-to-blade connectivity identifier
20 modepr - number of modes to be printed
T TabDelim - output format (t: std; f: tab-delimited)
F mid_node_tw - t: output twist at mid nodes; f: do otherwise

--------- Blade-tip or tower-top mass properties --------------------------------------------
3.5000E+05 tip_mass - tip mass
-4.1378E-01 cm_loc - tip-mass c.m. location wrt the reference axis
1.9670E+00 cm_axial - tip-mass c.m. axial offset wrt tip
4.3700E+07 ixx_tip - mass moment of inertia about x axis (wt-specific)
2.3530E+07 iyy_tip - mass moment of inertia about y axis
2.5420E+07 izz_tip - mass moment of inertia about z axis
0.0000E+00 ixy_tip - cross product of inertia
1.1690E+06 izx_tip - cross product of inertia
0.0000E+00 iyz_tip - cross product of inertia

--------- Distributed-property identifiers --------------------------------------------------------
1 id_mat - material isotropy identifier (not used; use later)
sec_props_file - name of beam section properties file (-)
“CS_monopile_tower_secs.dat”

Property scaling factors…
1.0000E+00 sec_mass_mult - mass density multiplier (-)
1.0000E+00 flp_iner_mult - blade flap or tower f-a inertia multiplier (-)
1.0000E+00 lag_iner_mult - blade lag or tower s-s inertia multiplier (-)
1.0000E+00 flp_stff_mult - blade flap or tower f-a bending stiffness multiplier (-)
1.0000E+00 edge_stff_mult - blade lag or tower s-s bending stiffness multiplier (-)
1.0000E+00 tor_stff_mult - torsion stiffness multiplier (-)
1.0000E+00 axial_stff_mul - axial stiffness multiplier (-)
1.0000E+00 cg_offst_mult - cg offset multiplier (-)
1.0000E+00 sc_offst_mult - shear center multiplier (-)
1.0000E+00 tc_offst_mult - tension center multiplier (-)

--------- Finite element discretization --------------------------------------------------
61 nselt - number of blade or tower elements (-)
Distance of element boundary nodes from blade or flexible-tower root (normalized wrt blade or tower length), el_loc()
0.0000E+00 el_loc - array of normalized element locations (-)
3.4819E-03 el_loc - array of normalized element locations (-)
1.0446E-02 el_loc - array of normalized element locations (-)
1.7409E-02 el_loc - array of normalized element locations (-)
2.4373E-02 el_loc - array of normalized element locations (-)
3.1337E-02 el_loc - array of normalized element locations (-)
3.8301E-02 el_loc - array of normalized element locations (-)
4.5265E-02 el_loc - array of normalized element locations (-)
5.2228E-02 el_loc - array of normalized element locations (-)
5.9192E-02 el_loc - array of normalized element locations (-)
6.6156E-02 el_loc - array of normalized element locations (-)
7.3120E-02 el_loc - array of normalized element locations (-)
8.0084E-02 el_loc - array of normalized element locations (-)
8.7047E-02 el_loc - array of normalized element locations (-)
9.4011E-02 el_loc - array of normalized element locations (-)
1.0097E-01 el_loc - array of normalized element locations (-)
1.0794E-01 el_loc - array of normalized element locations (-)
1.1490E-01 el_loc - array of normalized element locations (-)
1.2187E-01 el_loc - array of normalized element locations (-)
1.2883E-01 el_loc - array of normalized element locations (-)
1.3579E-01 el_loc - array of normalized element locations (-)
1.3990E-01 el_loc - array of normalized element locations (-)
1.4972E-01 el_loc - array of normalized element locations (-)
1.5669E-01 el_loc - array of normalized element locations (-)
1.6365E-01 el_loc - array of normalized element locations (-)
1.7061E-01 el_loc - array of normalized element locations (-)
1.7758E-01 el_loc - array of normalized element locations (-)
1.8454E-01 el_loc - array of normalized element locations (-)
1.9150E-01 el_loc - array of normalized element locations (-)
1.9847E-01 el_loc - array of normalized element locations (-)
2.0543E-01 el_loc - array of normalized element locations (-)
2.1240E-01 el_loc - array of normalized element locations (-)
2.1936E-01 el_loc - array of normalized element locations (-)
2.2632E-01 el_loc - array of normalized element locations (-)
2.3329E-01 el_loc - array of normalized element locations (-)
2.4025E-01 el_loc - array of normalized element locations (-)
2.4721E-01 el_loc - array of normalized element locations (-)
2.5070E-01 el_loc - array of normalized element locations (-)
3.2033E-01 el_loc - array of normalized element locations (-)
3.7971E-01 el_loc - array of normalized element locations (-)
4.2479E-01 el_loc - array of normalized element locations (-)
4.5961E-01 el_loc - array of normalized element locations (-)
4.8663E-01 el_loc - array of normalized element locations (-)
5.1366E-01 el_loc - array of normalized element locations (-)
5.4068E-01 el_loc - array of normalized element locations (-)
5.6770E-01 el_loc - array of normalized element locations (-)
5.9471E-01 el_loc - array of normalized element locations (-)
6.2173E-01 el_loc - array of normalized element locations (-)
6.4875E-01 el_loc - array of normalized element locations (-)
6.7577E-01 el_loc - array of normalized element locations (-)
7.0279E-01 el_loc - array of normalized element locations (-)
7.2981E-01 el_loc - array of normalized element locations (-)
7.5683E-01 el_loc - array of normalized element locations (-)
7.8385E-01 el_loc - array of normalized element locations (-)
8.1087E-01 el_loc - array of normalized element locations (-)
8.3789E-01 el_loc - array of normalized element locations (-)
8.6491E-01 el_loc - array of normalized element locations (-)
8.9192E-01 el_loc - array of normalized element locations (-)
9.1894E-01 el_loc - array of normalized element locations (-)
9.4596E-01 el_loc - array of normalized element locations (-)
9.7298E-01 el_loc - array of normalized element locations (-)
1.0000E+00 el_loc - array of normalized element locations (-)

--------- Properties of tower support subsystem (read only if beam_type is 2) ------------
2 tow_support - aditional tower support (-)
2.0000E+01 draft - depth of tower base from the MSL (mean sea level) (m)
0.0000E+00 cm_pform - distance of platform c.m. below the MSL (m)
0.0000E+00 mass_pform - platform mass (kg)
Platform mass inertia 3X3 matrix (i_matrix_pform):
0.0000E+00 i_matrix_pform - platform inertia matrix-row1
0.0000E+00 i_matrix_pform - platform inertia matrix-row1
0.0000E+00 i_matrix_pform - platform inertia matrix-row1
0.0000E+00 i_matrix_pform - platform inertia matrix-row2
0.0000E+00 i_matrix_pform - platform inertia matrix-row2
0.0000E+00 i_matrix_pform - platform inertia matrix-row2
0.0000E+00 i_matrix_pform - platform inertia matrix-row3
0.0000E+00 i_matrix_pform - platform inertia matrix-row3
0.0000E+00 i_matrix_pform - platform inertia matrix-row3
2.0000E+01 ref_msl - distance of platform reference point below the MSL (m)
Platform-reference-point-referred hydrodynamic 6X6 matrix (hydro_M):
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
0.0000E+00 hydro_M - platform added-mass inertia matrix
Platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K):
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
0.0000E+00 hydro_K - platform hydrodynamic stiffness matrix
Mooring-system 6X6 stiffness matrix (mooring_K):
2.5748E+09 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
-2.2532E+10 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
2.5748E+09 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
2.2532E+10 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
2.2532E+10 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
2.6291E+11 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
-2.2532E+10 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
2.6291E+11 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix
0.0000E+00 mooring_K - platform mooring-system stiffness matrix

Distributed (hydrodynamic) added-mass per unit length along a flexible portion of the tower length:
0 n_secs_m_distr - number of points at which added mass per unit length is specified (-)

Distributed elastic stiffness per unit length along a flexible portion of the tower length:
0 n_secs_k_distr - number of points at which distributed stiffness per unit length is specified (-)
Tower section properties
13 n_secs - number of blade or tower sections (-)

sec_loc str_tw tw_iner mass_den flp_iner edge_iner flp_stff edge_stff tor_stff axial_stff cg_offst sc_offst tc_offst
(-) (deg) (deg) (kg/m) (kg-m) (kg-m) (Nm^2) (Nm^2) (Nm^2) (N) (m) (m) (m)
0.00000 0.000 0.000 9517.140 41979.200 41979.200 1.04E+12 1.04E+12 7.98E+11 2.35E+11 0.000 0.000 0.000
0.27881 0.000 0.000 9517.140 41979.200 41979.200 1.04E+12 1.04E+12 7.98E+11 2.35E+11 0.000 0.000 0.000
0.27882 0.000 0.000 4306.510 19205.600 19205.600 4.74E+11 4.74E+11 3.65E+11 1.06E+11 0.000 0.000 0.000
0.35094 0.000 0.000 4030.440 16720.000 16720.000 4.13E+11 4.13E+11 3.18E+11 9.96E+10 0.000 0.000 0.000
0.42306 0.000 0.000 3763.450 14483.400 14483.400 3.58E+11 3.58E+11 2.75E+11 9.30E+10 0.000 0.000 0.000
0.49517 0.000 0.000 3505.520 12478.700 12478.700 3.08E+11 3.08E+11 2.37E+11 8.66E+10 0.000 0.000 0.000
0.56729 0.000 0.000 3256.660 10689.200 10689.200 2.64E+11 2.64E+11 2.03E+11 8.05E+10 0.000 0.000 0.000
0.63941 0.000 0.000 3016.860 9098.900 9098.900 2.25E+11 2.25E+11 1.73E+11 7.45E+10 0.000 0.000 0.000
0.71153 0.000 0.000 2786.130 7692.700 7692.700 1.90E+11 1.90E+11 1.46E+11 6.88E+10 0.000 0.000 0.000
0.78365 0.000 0.000 2564.460 6455.700 6455.700 1.59E+11 1.59E+11 1.23E+11 6.34E+10 0.000 0.000 0.000
0.85576 0.000 0.000 2351.870 5373.900 5373.900 1.33E+11 1.33E+11 1.02E+11 5.81E+10 0.000 0.000 0.000
0.92788 0.000 0.000 2148.340 4433.600 4433.600 1.10E+11 1.10E+11 8.43E+10 5.31E+10 0.000 0.000 0.000
1.00000 0.000 0.000 1953.870 3622.100 3622.100 8.95E+10 8.95E+10 6.89E+10 4.83E+10 0.000 0.000 0.000
[/code]

All the best,
Rebecca

Dear Rebecca,

OK, I was unaware that those inputs even existed in the DC version (v3.00.00) of BModes. None of the sample models in the BModes CertTest have those inputs. It is difficult to support BModes now that the developer has left NREL.

Best regards,

Hello,
My question regards BModes vs. Modes. I understand that BModes is the currently recommended pre-processor for obtaining FAST-input mode shapes. However, when I used Table 6-1 from the Definition of a 5MW Reference Turbine report for the inputs to BModes and Modes, my results indicated Modes was closer to the NREL distributed mode shapes/frequencies than the results from BModes. As an example, the fore-aft frequency result for Mode 2 from BModes was 2.2416 Hz vs. Modes 2.9405 Hz vs. NREL (from Table 9-1 in “Definition”) 2.8590.

Perhaps I’ve entered something incorrectly in my BModes input file to cause this discrepancy?
Here are my tower sections:

[code][size=85]Tower section properties (NREL 5MW Onshore Reference Turbine)
11 n_secs: number of blade or tower sections at which properties are specified (-)

sec_loc str_tw tw_iner mass_den flp_iner edge_iner flp_stff edge_stff tor_stff axial_stff cg_offst sc_offst tc_offst
(-) (deg) (deg) (kg/m) (kg-m) (kg-m) (Nm^2) (Nm^2) (Nm^2) (N) (m) (m) (m)
0 0 0 5590.87 24866.3 24866.3 6.14E+11 6.14E+11 4.73E+11 1.38E+11 0 0 0
0.1 0 0 5232.43 21647.5 21647.5 5.35E+11 5.35E+11 4.12E+11 1.29E+11 0 0 0
0.2 0 0 4885.76 18751.3 18751.3 4.63E+11 4.63E+11 3.56E+11 1.21E+11 0 0 0
0.3 0 0 4550.87 16155.3 16155.3 3.99E+11 3.99E+11 3.07E+11 1.12E+11 0 0 0
0.4 0 0 4227.75 13838.1 13838.1 3.42E+11 3.42E+11 2.63E+11 1.04E+11 0 0 0
0.5 0 0 3916.41 11779 11779 2.91E+11 2.91E+11 2.24E+11 9.68E+10 0 0 0
0.6 0 0 3616.83 9958.2 9958.2 2.46E+11 2.46E+11 1.89E+11 8.94E+10 0 0 0
0.7 0 0 3329.03 8356.6 8356.6 2.06E+11 2.06E+11 1.59E+11 8.22E+10 0 0 0
0.8 0 0 3053.01 6955.9 6955.9 1.72E+11 1.72E+11 1.32E+11 7.54E+10 0 0 0
0.9 0 0 2788.75 5738.6 5738.6 1.42E+11 1.42E+11 1.09E+11 6.89E+10 0 0 0
1 0 0 2536.27 4688 4688 1.16E+11 1.16E+11 8.91E+10 6.27E+10 0 0 0

**Note: If this file is for a TOWER, the following section properties are read but overwritten as follows:
str_tw is set to zero
tw_iner is set to zero
cg_offst is set to zero
sc_offst is set to zero
tc_offst is set to zero
edge_iner is set equal to flp_iner
edge_stff is set equal to flp_stff[/size][/code]

And here is my .bmi input file (I copied the tower top mass properties from the CS_Monopile .dat-file).

[code][size=85]====================== BModes v1.03 Main Input File ==================
Modes of ORT tower (89.6 m) with tip mass 350e3 kg

--------- General parameters ---------------------------------------------------------------------
False Echo Echo input file contents to *.echo file if true.
2 beam_type 1: blade, 2: tower (-)
0. romg: rotor speed (rpm), automatically set to zero for tower modal analysis
1.0 romg_mult: rotor speed muliplicative factor (-)
0. precone: built-in precone angle (deg), automatically set to zero for a tower
0. bl_thp: blade pitch setting (deg), automatically set to zero for a tower
1 hub_conn: hub-to-blade connection [1: cantilevered; other options not yet available]
7 modepr: number of modes to be printed (-)
f 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]
‘ORT_props.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 --------------------------------------------------
61 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 0.003481894 0.010445682 0.017409471 0.024373259 0.031337047 0.038300836 0.045264624 0.052228412 0.059192201 0.066155989 0.073119777 0.080083565 0.087047354 0.094011142 0.10097493 0.107938719 0.114902507 0.121866295 0.128830084 0.135793872 0.13990 0.149721448 0.156685237 0.163649025 0.170612813 0.177576602 0.18454039 0.191504178 0.198467967 0.205431755 0.212395543 0.219359331 0.22632312 0.233286908 0.240250696 0.247214485 0.250696379 0.320334262 0.37971 0.424791072 0.45961 0.486635 0.51366 0.54068 0.5677 0.594715 0.62173 0.64875 0.67577 0.70279 0.72981 0.75683 0.78385 0.81087 0.83789 0.864905 0.89192 0.91894 0.94596 0.97298 1.0

--------- Properties of additional tower support subsystem (read only if beam_type is 2) ------------
0 tow_support: : additional tower support [0: no additional support; 1: Tension guy wires for land-based tower; 2: offshore turbine support: floating platform or monopile] (-)
Tension-wires data
0 n_attachments: no of wire-attachment locations on tower, maxm allowable is 2; 0: no tension-wire support (-)
3 3 n_wires: no of wires attached at each location (must be 3 or higher) (-)
6 9 node_attach: node numbers of attacments location (node number must be more than 1 and less than nselt+2) (-)
1.e8 1.e8 wire_stfness: wire sifnness in each set (see users’ manual) (N/m)
45. 45. th_wire: angle of tension wires (wrt a horizontal plane) at each attachment point (deg)
[/size][/code]

Kind Regards,

Wystan Carswell

Dear Wystan,

Your BModes input files look fine to me. For an explanation on why your BModes results are not matching the results from Table 9-1 for the 2nd tower fore-aft mode, please see my Aug 28, 2012 post in this forum topic above. My guess is if Modes is matching the results from Table 9-1 better than BModes, that this is only coincidental. In general, BModes will be more accurate than Modes.

Best regards,

Hello everyone!

I’m new in this topic; I read most of posts of this topic and the others but I didn’t get to my answer;
I’m working on a NREL 5MW offshore TLP; and in my research the first mode and frequency of the structure is necessary; I searched for this frequency and how I can get to that;
for using BMode I need an input file for 5MW TLP but I didn’t find any! should I make one? or is there any?
and also I didn’t find the frequency or modes in the output table of User Guide as a result of the BMode!! how can I get to this structures frequency?
After BMode is there something else that I have to do to get to the result?

Sincerely
Hamid

Dear Hamid,

BModes can be used to derive mode shapes and frequencies of blades or towers. When used together with FAST, it is only the blade and tower mode shapes that are needed. For the NREL 5-MW turbine atop the MIT/NREL TLP, we’ve already created mode shapes for FAST using ADAMS in place of BModes. So, we don’t have the BModes input files for this system. You should be able to make them yourself, but there really is no need to. If all your interest is in obtaining the natural frequenices of the TLP, you can use the linearization feature of FAST to derive them. Otherwise, these frequencies are published in Table 6 of the following report: http://forums.nrel.gov/t/bladeddllinterface/773/1.

Best regards,

Dear Jason

I ran the linearization of FAST for the frequency;
I got the result; thanks for that; but now I need Eigenanalysis.m for MATLAB but there isn’t any file like that in CertTest folder! how can I get to that! is the file in somewhere else that I have to download it?

Regards
Hamid

Dear Hamid,

I’m fairly sure the Eignanalysis.m file you are looking for has been superceded by the mbc3 package, where I think cce.m gives the information you’re looking for. Take a look at this post to see if it helps you http://forums.nrel.gov/t/eigenanalysis-fast/362/1.

Regards,

Monika

Dear Monika;

Yeah, your right; I’ve read that post but I forgot that, thanks for reminding.

Regards
Hamid

Hello everyone;
I got the Natural Frequency (Rad/S and Hz) for 5MW TLP as following, USING CCE function in MATLAB:

AvgNaturalFrequency
1: 4.255
2: 4.513
3: 2.749
4: 1.300
5: 1.354
6: 0.617
7: 0.104
8: 0.104

and by Hz:
AvgNaturalFrequencyHz
1: 0.677
2: 0.718
3: 0.437
4: 0.206
5: 0.215
6: 0.098
7: 0.016
8: 0.016

Using CambellDiagram that Jason had been sent for the first mode I mean Surge it is 0.104 rad/s and 0.016Hz but I thinks it is not right!
what is the form of these outputs? is (number 1:) natural frequency of the first DOF (SURGE) or numbers are upside down I mean number 8 is for the first DOF (SURGE)? I have 8 DOF: 6 for structure and 2 first bending modes!
I need structures natural frequency in each DOFs mostly in Surge and Pitch.

I really confused!
Any suggestions would be so great!

Dear Hamid,

I’m not really sure I understand your question, but MATLAB’s eigensolver (used within CCE) does not sort the eigensolution in the order of increasing frequency. However, the CampbellDiagram.xls workbook, for aid in interpretation, sorts the eigenvalues and eigenvectors by frequency and highlights the dominant components of each mode.

If you are referring to the NREL 5-MW turbine atop the MIT/NREL TLP, the first surge natural frequency is 0.0165 Hz. You can find other natural frequencies for this system documented in Table 6 of Denis Matha’s MS thesis-turned NREL report: nrel.gov/docs/fy10osti/45891.pdf.

Best regards,

Dear all,
I am working with BMODES and I have a question about the input for the 5MW Turbine:
Hier
wind.nrel.gov/public/jjonkman/BM … nopile.bmi
in the tower-top mass properties I reda the following:

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)

But then in the article:
“Modal Dynamics of Large Wind Turbines with Different Support Structures” from G. Bir and J. Jonkman I read the following values

which referring to the BMODES tower axes systems must be transformed from the the cm coordinate system to the tip-section reference axis . Is that right?

In this case, the input data of the CS_Monopile.bmi are not correct, I guess.

Any idea?

Thanks

Dear Francesca,

The BModes inputs for tower-top inertia (ixx_tip, etc.) are specified about the center of mass (CM) of the tower top. The BModes input file you linked to and Table 1 from the paper you referenced are consistent.

Best regards,

Dear Dr. Jason.

Could you let me know how you obtained those tower-head mass moment of inertia in above table 1? I thought that those was calculated by using FAST linearization, but I could not make the same moment of inertia Ixx as values in above table 1 when I used linearization.

Sincerely,
Daniel Kim.

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: http://forums.nrel.gov/t/using-aggregate-mass-in-adams-to-check-nrel-cs-monopile-bmi/696/4.

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,