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?

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: http://forums.nrel.gov/t/hydrodynamic-analysis-of-oc3-hywind-spar-buoy/1103/44. 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,