OC4-DeepCwind semisubmersible - WAMIT Files

Dear All

I am working with the OC4-DeepCwind semisubmersible model. I am developing a model of the OC4-DeepCwind semisubmersible in a different software package (ProteusDS). I to compare the ProteusDS model to the FAST model for validation purposes.

I am trying to produce WAMIT output files (.1 added Mass and Damping, .3 frequency + heading dependant wave excitation and .hst hydrostatic coefficients) for the OC4- DeepCwind system. I have looked at the WAMIT input files provided by NREL – which I was able to download through this forum- and I notice that the centre of gravity VCG specified in the .frc file is set to zero. Reading up I understand that this is because the ElastoDyn module accounts for the pitch and roll restoring associated with platform weight. I am also aware that the water ballasting, and its effect on pitch and roll restoring, is modelled through Hydrodyn.

However, in generating the WAMIT files for ProteusDS where should VCG be set? Is it -13.46m. Also are you able to provide information on the Inertia Moments of the overall OC4-DeepCwind system (platform + ballast + tower), specifically the Ixy, Ixz, and Iyz products of inertia?

I believe the overall mass of the system tower + platform +ballast is 1,3722718 x10^7 kg. Is that correct?

1 Like

Dear Benjamin,

Your understanding is correct.

Just to clarify: are you interested in the mass, mass center, and inertias of the full OC4-DeepCwind system (rotor + drivetrain + nacelle + tower + substructure + ballast)? You mention the tower, platform, and ballast, but not the tower-top effects.

Best regards,


Yes, sorry I probably should have been more specific. It would be the mass of the entire system including the tower-top. If there is data on both i.e. full system (rotor + drivetrain + nacelle + tower + substructure + ballast) and just platform +ballast + tower that would be ideal. However, either would be useful.


Dear Benjamin,

I’m not the one who developed this, but the WAMIT input files that NREL used to generate the 2nd-order WAMIT solution for the OC4-DeepCwind semisubmersible are available in my Aug 21, 2018 post in the following forum topic: Hydrodynamic analysis of OC3-Hywind spar buoy - #44 by Jason.Jonkman. The 6x6 mass matrix of the full system is given in the marin_semi.frc file as:

1.407E+07 0.000E+00 0.000E+00 0.000E+00 -1.392E+08 0.000E+00
0.000E+00 1.407E+07 0.000E+00 1.392E+08 0.000E+00 -1.448E+05
0.000E+00 0.000E+00 1.407E+07 0.000E+00 1.448E+05 0.000E+00
0.000E+00 1.392E+08 0.000E+00 1.270E+10 -1.094E-01 1.167E+07
-1.392E+08 0.000E+00 1.448E+05 -1.094E-01 1.269E+10 -1.953E+00
0.000E+00 -1.448E+05 0.000E+00 1.167E+07 -1.953E+00 1.229E+10

from which you can derive the mass (upper-left quadrant), center of mass (upper-right and lower-left quadrants), and inertias (lower-right quadrant) about the WAMIT reference point (0,0,0).

Best regards,

marin_semi.1.txt (492 KB)Hi

Jason, Thank you for that. I can derive the info I need from the mass matrix. I just wanted to confirm something with you. The WAMIT output files contained in the HydroData folder (reg_tests\r-test\glue-codes\openfast\5MW_Baseline\HydroData) of OpenFast are attached. What format are the frequency ranges( column 1 of the .1 and .3 files) in? They seem to be periods in seconds (i.e. -1 s to 1.26168 s)?


Hi Benjamin,

Yes, column 1 is the period in seconds. The format of the WAMIT-generated output files (*.1, *.3, and *.hst for first order, etc.) needed by HydroDyn can be found in Chapter 4 of the WAMIT User’s Manual, available online: wamit.com/manual6.4/Chap4.pdf.

Best regards,

Dear Jonkman
I am confused about the setting of SWL in the frequency domain calculation of OC4-DeepCwind semi in WAMIT.
In report 60601, distance from the bottom of the platform to SWL is 20m, and I think it accounts for the mass of turbine, platform and mooring lines. But in time domain calculation, aerodynamics, platform hydrodynamics and mooring dynamics are separate.
So in frequency domain calculation, SWL is set accorrding to overall draft (20m) or just the platform draft (about 17.98m)? Could you provide the input file of OC4-DeepCwind semi in WAMIT :blush:

Dear @Lei.Xue,

I agree that in the absence of wind or wave excitation, the draft of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semisubmersible is 20 m, which is the draft that balances the system mass/weight, buoyancy, and vertical mooring pretension. The WAMIT solution should be derived about the mean position of the body, which includes this 20 m of draft.

Please find the WAMIT input files that we used to generate the 1st-order WAMIT solution for the OC4-DeepCwind semisubmersible here: WAMITInputFiles_OC4_Semi_1st.zip - Google Drive.

Best regards,

Dear Jonkman

I am not sure about the workflow about WAMIT. Do you mean that the modelling of turbine and mooring lines are represented in WAMIT, or just replace the platform mass (including steel and ballast) with overall mass (including turbine, mooring and ballast)? If the latter, how to determine the centre of gravity of the overall structures?

Beat regards,

Dear @TianYuan.Wang – It looks like you posted very similar questions to @Lei.Xue, which I’ve answered above.

Dear @Lei.Xue – If you are only generating the first-order solution from WAMIT for use by HydroDyn, WAMIT does not need to know the system mass, center of mass, or mooring properties. (That said, these properties would be needed for WAMIT to generate the second-order solution.) See the guidance for using WAMIT to generate potential flow solutions for use in HydroDyn for more information: Modeling Considerations β€” OpenFAST v3.4.1 documentation.

Best regards,