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 BModes : Input parameters about tower support subsystem - #13 by Ha.Tran, 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.)
I’d like to get the equivalent linear viscous damping of the mooring lines for the ITIBarge model through linearization. I noticed that the damping parameter CIntDamp in MAP file is not actually used and it gives a zero damping matrix. Therefore I turned to MoorDyn option with the damping ratio being set as the default value of 0.8. I used the most recent OpenFast version of 3.5.0, both mooring files are from the r-teat example folder, and both models are linearized about the static equilibrium position.
I was surprised to find that the stiffness matrices I got from the two options are very different. It seems to me that the stiffness I got from MAP is more realistic and it is pretty similar to your values above. I was wondering the reason for this large difference.
I was thinking about using K_moor_map and a stiffness-proportional damping matrix in my simulation, but not sure how large the damping could be in practice. Any suggestions would be very much appreciated!
Thanks for your reply! I derived K_moor_moordyn the same way as I got K_moor_map. It would be much appreciated if you could help check the procedure.
Linearize OpenFAST model with only 6 platform Dofs enabled in ElastoDyn, with still water and no radiation calculation set in HydroDyn, and with either MAP or MoorDyn selected.
Extract from the linearized A and B matrices the rows and columns corresponding to the platform states/inputs: A_plt and B_plt.
Derive total mass M, stiffness K, and damping C matrices from the extracted A_plt, B_plt matrices.
Subtract infinite-frequency added mass M_inf from M to get the rigid-body mass matrix of the platform M_plt;
Calculate the position of the center of gravity [xg, yg, zg] from M_plt and then get the gravity-restoring matrix K_g;
Subtract K_g and hydrostatic restoring stiffness K_hst from K to get K_moor;
C_moor equals C as moorings are the only damping source.
I suspect that the extracted A_plt matrix in step 2 is incorrect when MoorDyn is selected, as I ignored the states associated with the mooring lines nodes which can possibly greatly affect the K, C matrices. However, I am still surprised that it gives such large K and C values.
I decided to use K_moor_map and an associated damping matrix with the modal damping ratio of 10% for all 6 DoFs. I was hoping to know if this damping value makes sense to you.
Thank you in advance for any comments or suggestions!
Thanks for clarifying. Can you clarify in step 3 how you separate out the M, K, and C from A? A is given in first-order form, so, terms such as -M^-1K and -M^-1C appear, but you’ll need M to derive K and C.
I agree that simply neglecting the MoorDyn states would not give you the stifness you want. When you only perturb the platform displacements with the MoorDyn states fixed, then you’ll only see the stiffness associated with the last segment of each mooring line connecting to the fairlead.
Many thanks for your reply! I derived M from the bottom half of the B matrix: B=[zeros(6,6); M^(-1)]. Your reply about the MoorDyn linearization makes a lot of sense to me and I was wondering what would be the correct way to get C_moor.
OK, thanks for clarifying. It looks like you are using inputs associated with loads applied to the platform to derive the mass matrix. I’ve asked a colleague to respond regarding the expected damping level from a mooring system.
Many thanks for your help and look forward to the suggestions for the damping of moorings!
Can I ask if there are other methods to derive the rigid-body mass matrix, or if there are any options in OpenFAST to output the matrix? I thought deriving it from the B (or D) matrix was the only way. Thanks in advance!