I want to modify the tower-top mass, the thickness of the tower (keeping the outer radii of the tower constant but increasing the internal radii), as well as the structural density of the barge (without changing its shape and size) of the NREL 5MW ITIBarge4 wind turbine. So I need to generate new tower mode shapes for FAST simulations using BModes. I think to do this I do not need to change any properties below the line ‘Platform-reference-point-referred hydrodynamic 6X6 matrix (hydro_M):’ in the main input file for BModes, right?

I note that the BModes_Alpha version does not provide the BModes input file for the NREL 5MW ITIBarge4 wind turbine. If it is possible, could I request this input file please? Thank you very much for your feedback.

Actually, NREL did not use BModes to derive the tower mode shapes of the NREL 5-MW turbine atop the ITI Energy barge. At the time that system was analyzed, we used ADAMS to derive tower mode shapes because BModes was not yet capable (as described in other forum posts).

Assuming you can create your own BModes input file, I would expect the platform-reference-point-referred hydrodynamic 6X6 stiffness matrix (hydro_K) to be effected by the change to the structural density of the barge.

I’m not sure that we’ve published this, but–as derived from FAST linearization analysis–the linearized mooring stiffness of the ITI Energy barge about the undisplaced position is (with units of N/m, N/rad, N-m/m, N-m/rad):

Thank you very much for your reply. According to the topic http://forums.nrel.gov/t/bmodes-input-parameters-about-tower-support-subsystem/604/13, to obtain the “mooring_K” matrix in the BModes input file, the (4,4) and (5,5) elements of the above linearized mooring stiffness matrix of the ITI Energy barge about the undisplaced position should be added by -mgz. Where can I find the value for the parameter z please? Thank you for your feedback.

Thank you for your reply. I finally got the augmented (4,4) and (5,5) elements of the “mooring_K” matrix, which is -2.9153e+08 because for the ITIBarge4 turbine z is positive. Is such a large negative value reasonable? Thank you for your feedback.

I haven’t confirmed your value, but I agree that if the full-system center of mass is above the still water level, that -mgz will be negative. I suspect this negative stiffness will be offset by the positive stiffness from the moorings and hydrostatics (the large water-plane area of the ITI Energy barge).

On calculating the -mgz augmentation discussed above, I find the values of NREL 5MW Turbine tower mass and its CM location are different in different NWTC publications, as shown below.

In “Definition of the Floating System for Phase IV of OC3” (nrel.gov/docs/fy10osti/47535.pdf), Page 3, Table 2-2, written: Overall (Integrated) Tower Mass 249,718 kg, and CM Location of Tower Above SWL Along Tower Centerline 43.4 m.

In “Definition of a 5-MW Reference Wind Turbine for Offshore System Development” (nrel.gov/docs/fy09osti/38060.pdf), Page 16, Table 6-2, written: Overall (Integrated) Mass 347,460 kg, and CM Location (w.r.t. Ground along Tower Centerline) 38.234 m.

In “A Quantitative Comparison of the Responses of Three Floating Platforms” (nrel.gov/docs/fy10osti/46726.pdf), Page 5, Table 2, written: Tower Mass 347,500 kg, and Coordinate location of overall center of mass (CM) 64.0 m.

I’m wondering which one are correct? And I was confused why the values of tower mass and CM location are so different (especially the latter). Could you kindly check it?

The tower masses and center of masses (CMs) reported in those publications are all correct. The tower in the OC3-Hywind floating offshore wind system is different than the tower for the land-based NREL 5-MW turbine, hence the differences between NREL reports 47535 and 38060. Also, the CM reported in NREL report 46726 is for the total land-based NREL 5-MW turbine (tower plus nacelle + drivetrain + rotor).

I am using fastv7 to derive the linearized matrix of ITI barge.
But the Stiffness matrix seems to be pretty different from the one you offered above.
Below is one I get from a previous run.

1.587E+04 3.374E-01 6.885E-02 -3.025E+00 2.185E+05 3.275E+00
-3.917E-02 1.587E+04 -4.230E+00 -2.185E+05 1.944E+00 4.238E+01
-1.146E+01 5.726E+00 1.611E+07 -5.743E+01 1.242E+04 -4.941E-03
2.865E+00 -2.079E+05 1.554E+02 1.627E+09 1.158E+01 -1.462E+06
2.079E+05 8.473E+00 -6.003E+01 -5.563E+01 1.627E+09 -1.064E+02
-2.686E-01 1.325E+01 0.000E+00 -7.181E+05 -4.163E+01 2.464E+07
I know this linearazed stiffness matrix should be subtracted with the K_h K_wt etc to get the k_moor,but the quantity here makes me a little confused.
So attach the input files below ,could you help me check the input files and tell me what mistakes I’ve made ?I really appreciate it .

I’ve tried many times , but the result I get still looks strange to me ,especially the (6,6)stiffness element.I thought it’s too huge compared to the data you offered above.
Could you help me out with this ?

I took a look at your files and nothing stands out to me as incorrect.

I’m questioning whether the linearized mooring system matrix that I gave you on Oct 11, 2016 above is correct. I found this in my notes, but I no longer have the original files that were used to generate it, and can’t now reproduce it. Moreover, the (5,5) element doesn’t seem to match the value of 26210000 Nm/rad that can be derived from your result, which matches other files I have. Perhaps the vales I reported above were for an older design?

Here is what I derive from your solution, which is likely more correct than the values I reported above:

(Some of the nonzero off-diagonal terms will likely go toward zero when the model is linearized about the undisplaced position instead of the static-equilibrium position.)