Hydrodynamic analysis of OC3-Hywind spar buoy

Dear Sir,
I’m currently researching on the substructure part of OC3-Hywind spar buoy (under SWL) where i need WAMIT input and output files to import hydrodynamic data into OrcaFlex. However, I do not have access to WAMIT. Is it possible for NREL to provide me with these files so that I could use the data for OrcaFlex?
Thank you.

Dear Kongli.Goh,

The WAMIT output files for the OC3-Hywind spar – spar.1, spar.3, and spar.hst – are provided with the complete FAST v8 model in the archive of FAST, available from here: nwtc.nrel.gov/FAST8. See the CertTest\5MW_Baseline\HydroData directory. I don’t believe OrcaFlex should need the WAMIT input files, but I can provide them if needed.

Best regards,

Dear Sir,
Thanks for the reply. I have modelled the structure in MultiSurf and generated the input files. However, since I do not have access to WAMIT, I wish to compare my input files instead and see whether they are similar. Therefore it would be great if you could provide me with the WAMIT input files.

Dear KongLi.Goh,

Please find the WAMIT input files for the OC3-Hywind spar attached.

Best regards,
WAMITInputFiles_Spar.zip (36.3 KB)

Dear Sir,
You mentioned that it is important to specify the mass, CM, and radii of gyration of the complete system properly in the WAMIT model to compute RAO. However, I’m not using WAMIT as a pre-processor for FAST. May I know how do I calculate the radii of gyration to be inputted into WAMIT? And also, does this mean I have to substitute the VCG from 0 to 1 and the specify the mass as 7,466,330kg?

Dear KongLi.Goh,

The mass, CM, and inertias of the OC3-Hywind system are given in my Nov 16, 2012 post in the following forum topic: http://forums.nrel.gov/t/inertial-moments-of-oc3-hywind-components/610/1. The mass of 7,466,330 kg is only of the spar (including ballast); the total mass of the system pertinent to the RAO calculation is 8,066,000 kg, as given in the post linked above. I’m not sure what you mean be VCG = 1, but the VCG should be -78 m, as given in the post linked above.

Best regards,

Dear Dr. Jason,

Could you provide the WAMIT input files for the OC4 DeepCwindSemi floater?
I’m using another software to generate the hydrodynamic data of a semisubmersible, hence I want to validate it first by comparing with WAMIT result. Due to spar is a regular shape and many terms are zeroes, I want to check more cross terms using a more complex shape, eg, DeepCwind semisubmersible.

Thank you very much.
Yingyi Liu

Dear Yingyi Liu,

Test 25 in the CertTest of the FAST v8 archive is a model of the NREL 5-MW turbine atop the OC4-DeepCwind semisubmersible. The WAMIT output data (marin_semi.* files) are included in the CertTest\5MW_Baseline\HydroData directory. You may download the newest FAST v8 archive from here: nwtc.nrel.gov/FAST8.

Best regards,

Dear Dr. Jason,

I’m sorry for having not expressed my request clearly.
Actually what I need is the geometry WAMIT mesh file for the DeepCWind Semisubmersible.
Because I’m using another hydrodynamics software instead of WAMIT, I need to verify it first by comparing the data with WAMIT. And DeepCWind Semisubmersible is complex enough for me to do the comparison. I would be appreciated very much if you could provide me the mesh data.

Thanks a lot in advance.
Yingyi Liu

Dear Yingyi Liu,

Please find the WAMIT input files used to generate the 1st-order WAMIT solution described in the OC4-DeepCwind specifications report: nwtc.nrel.gov/FAST8. This WAMIT model is based on high-order panels through a MultiSurf surface file (.ms2); the WAMIT-generated low-order graphics data file (.gdf) file based on this high-order surface file is also attached. This WAMIT model accounts for all wetted surfaces, including the columns, braces and pontoons.

I hope that helps.

Best regards,
marin_semi_low.gdf.txt (297 KB)
marin_semi.pot.txt (1.69 KB)
marin_semi.gdf.txt (184 Bytes)
marin_semi.ms2.txt (11.1 KB)
marin_semi.cfg.txt (902 Bytes)
marin_semi.frc.txt (415 Bytes)
fnames.wam.txt (64 Bytes)

Dear Dr. Jason,

That really solves my problem.
I appreciate your help very much.

Best regards,
Yingyi Liu

Dear Dr. Jonkman,

In a part of my study I am comparing the response of 2nd order wave excitation modelled with the strip-theory and with the hybrid method implemented in FAST.
As far as I know there aren’t open-source code for the 2nd order potential wave modelling (Nemoh is a 1st order linear).
I am wondering if you have (and can share) the wamit output (like the ones of the OC4 Semisubmersible in FAST archive) for the Hywind Wind Turbine, or, if you know some way to obtain these files (e.g. from other open-source codes).

Thank you for you help.

Kind regards,
Gabriele

Dear Gabriele,

I don’t have the second-order WAMIT solution for the OC3-Hywind spar available, but previous work has shown that the second-order effects on this system are negligible–e.g. see: nrel.gov/docs/fy13osti/58718.pdf.

I’m not aware of any freely available software capable of solving the second-order hydrodynamics problem.

Best regards,

Dear Dr. Jonkman,

thank you very much for your fast reply.
I am trying to model the Hywind spar-buoy wind turbine using only the strip-theory.
To do this I have to add the AddClin matrix (posted in the Hydrodyn user guide) to FAST.
I am facing some difficulties because I am not sure of some values that I have to implement.
Since the Spar-Buoy presents a double symmetry in the plane and I am not considering the marine growth, some terms cancel out (please see the attached image).
I am not sure how to calculate two terms:

  • the undisplaced area A0; since the undisplaced volume is known (80291 m^3) it is correct to divide this value by the spar width (9.4m) to get this area?
  • the total mass of ballasting, mf; in a older post you wrote “some members of OC3 tried to reverse-engineer the ballast and steel parts from the overall mass/inertia/cg properties of the OC3-Hywind spar, but these were never “officially” prescribed.”(Further Details about the Floating Platform of OC3 - #2 by Jason.Jonkman). I tried to figure out how to calculate this mass following your instruction, but I come across into new unknown data;
    How can I obtain the “steel” mass?

Thank you for your help.

Best regards,
Gabriele

Dear Gabriele,

A0 in that equation is just the cross-sectional area at the waterplane i.e. based on a diameter at the waterplane of 6.5 m.

The ballasting in that equation refers to water ballasting, which was not implemented in the HydroDyn model of the OC3-Hywind spar, so, those terms are zero.

Regardless, the nondimensional hydrostatic stiffness matrix is used by the potential-flow solution and stored in the HydroData\spar.hst file. All you’ll have to do is dimensionalize this data by multiplying by WtrDens*Gravity.

Best regards,

Dear Dr. Jonkman,

i very appreciate your help. Thank you.
Now I am only doubtful about the input that I have to insert in AddClin.
You said that I can dimensionalize the spar.hst file to obtain that matrix, but the values in this file seems me different from what they should be.
For example the (3,3) entry in spar.hst is 3.312247E+01; this value should be the undisplaced area if I have to dimensionalize this data by multypling by WtrDens*Gravity. Do you agree?
Thus I will obtain a value of 3.312247E+01 * 1025 * 9.81 = 334962 N/m.
Obviously this value is very different from the value that (after what you said to me regarding A0) should be: 6.5 (A0) * 1025 (WtrDen) * 9.81 (g) = 65359 N/m.

I am certain that I have misanderstood something, so please correct me.

Thank you.

Best regards,
Gabriele

Dear Gabriele,

A0 is the cross-sectional area at the waterplane i.e. pi*6.5^2/4 = 33.183 m^2 – nearly identical to the value reported for the (3,3) entry in the spar.hst file.

Best regards,

Dear Dr. Jonkman,

thank you again for your support and your patience.
Now I have implemented the stifness terms in AddClin (I could have took these also from the matrix Cij in “Definition of the Flaoting System for Phase IV of OC3” right?).
I ran a simulation, but the simulation crushed. Please see the image attached for the error.
The simulation considers only an irregular sea, with no wind at all.
I ran this simulation with the hybrid method (i.e. considering also the potential); the simulation ended normally. I expect to obtain similar values (almost the same) for the Hywind concept between these two simulations.
I attached also the Hydrodyn input file.

Thank you very much.

Gabriele
Hywind_v-10.5_H-1.4_T-6.5_HydroDyn.txt (17.5 KB)
FAST error.PNG

Dear Dr. Jonkman,

I found an error in the Hydrodyn input file. Now the simulation run properly.
I am making a comparison between this simulation with the Strip-theory and that with the Hybrid method.
Except for the initial transient of the simulation, the results are quite similar, but I found some differences in some motions and sollecitations, like in the heave motion, in the mooring tensions and in the tower base moments.
It seems that with the strip-theory the response is “smoother”; especially for the TwrBsMyt the strip-theory smooths many peaks. For the TwrBsMxt the strip-theory miss the response.
Please see in the attachment some graphs about this comparison.
I expected less differences between the two simulations for the Spar-Buoy. Do you think this results sounds realistic?

Thank you for your support.

Best regards,
Gabriele



Dear Dr. Jonkman,

I am writing to you only to specify that the legend is inverted.
So the blue line is the model with the Strip-Theory, and red the Hybrid.
I checked this because I found strange the values of TwrBsMxt since the wave is allingned with the wind (0°), so the this moment should be very small.
Only this graph let me doubtful.

Thank you.

Regards,
Gabriele