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?
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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,
Hi,
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.
Thanks
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)?
Thanks
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 
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: 4.2.8.4. Modeling Considerations — OpenFAST v3.4.1 documentation.
Best regards,
I am working on a project that involves stabilization of the OC4 platform with nonlinear tuned mass-damper inerters and am using a simplified MATLAB model for processing.
I have previously been using a hydrodynamic file a peer produced in AQWA, but the pitch natural frequency is a bit off and the dataset is much smaller than the WAMIT files in the OpenFast folder so I have been trying to improve accuracy by using the NREL provided WAMIT files. I pulled data from the WAMIT files and converted from the normalized values such that I can reproduce figure 4-3 (A/B matrices) and the C hydrostatic matrix (equation 4-3) in the OC4 reference doc. However, these number vary significantly from the AQWA results in the following ways:
- WAMIT’s A51/A15 and B51/B15 are all negative while AQWA’s A51/A15 and B51/B15 are mixed positive and negative (also different magnitudes)
- WAMIT C55 is negative and AQWA’s C55 is positive
While running our model, we are noticing that the run with the WAMIT file is drifting to infinity, as seen below:
Do you have any idea what is causing these differences or any suggestions for how we need to modify the WAMIT outputs pre-processing to eliminate this drift? For reference, we are processing using ode45 on the equation:
(M+ME+A)Xddot+ (B+BE)Xdot+ (C+CE)X= 0
where M, A, B, and C are the mass/inertia matrix, added mass, radiation damping, and hydrostatic stiffness taken from the reference doc/.1/.hst files and the ME, BE, and CE are taken from the .frc file. My suspicion is that this has to do with CoG in WAMIT being at 0, and that we are not accounting for that. Any insight would be appreciated. Thanks!
Dear @Lauren.Hall,
Can you share your updated plot of Figure 4-3 based on the AQWA results?
Regarding the equations of motion you are solving, what is meant be ME, BE, and CE from the .frc file? What terms do these represent? Are you considering body weight in CE? Are you considering mooring stiffness in CE? Also, how are you considering the frequency dependence of added mass and radiation damping in your equations of motion?
The CoG in the first-order WAMIT solution used by OpenFAST is zero because the body weight terms are accounted for in the structural module(s) of OpenFAST. This is why the hydrostatic C44 and C55 terms in the WAMIT model are zero. When added with the body weight terms, the roll and pitch stiffness should, of course, be positive.
Best regards,
Thanks for the quick response.
Here is a side-by-side comparison of the WAMIT and AQWA files based on Figure 4-3’s layout:
. I believe that the AQWA may have been created from a 180 rotation (about z) which explains the positive/negative flip on the bottom row, but not the fact that there are mixed positive-negative values or overall very different trends (particularly on the A15/A24 graph).
The ME, BE, and CE are the external mass, external damping, and external stiffness from the .frc file and are all 0 for the AQWA runs.
Per the other questions:
We are considering the mass and inertia of the structure in the M term but not sure if this is what you mean by body weight terms - if not we may be missing that. Mooring is currently not considered for free decay, but it was not in the AQWA run either. As for frequency dependence, we are currently following a simplified model pulling data from a singular wave frequency (.1 or .2 Hz) for our regular wave runs. For the free decay, previously we have tried the same singular frequency method and an approximation with instantaneous (peak) frequency – we are aware neither are as accurate as convolution (which is more time consuming computationally), but we are currently focusing on the regular wave data where we believe this is a more valid simplification. Even though amplitudes may be inaccurate for the AQWA free decay, it is at least decaying while the current approach for processing the WAMIT data is resulting in the drift to infinity.
Dear @Lauren.Hall,
I agree that the positive/negative flip of A15/B15 and A24/B24 look odd.
Do you mean that ME, BE, and CE are all zero? Does this mean that you are not considering body weight in the stiffness? This likely explains your instability in pitch. And not including mooring stiffness would impact floater motions, especially surge/sway.
By choosing added mass and damping at a specific frequency, please note that your equations of motion would only be valid if the response is sinusoidal at that frequency.
Best regards,
ME, BE, and CE are 0 only for AQWA, which doesn’t seem to separate these terms. For the WAMIT processing we have added the matrices under external mass and external stiffness as ME and CE.
Dear @Lauren.Hall,
I’m still not understanding. What values of nonzero values of ME and CE are you using and what physically do they represent?
Best regards,
These are the two matrices that are being input for ME and CE:
Are there additional terms that need to be considered here that are usually processed by one of the other OpenFAST modules? If so, how would I go about getting those terms?
Dear @Lauren.Hall,
These values of ME=IMASS and CE=ISTIF are the rigid-body mass matrix (including full-system mass, center of mass, and inertias of the NREL 5-MW baseline wind turbine atop the OC4-DeepCwind semi) and mooring stiffness matrix, respectively. I would guess this ME would duplicate what you refer to as M in your equation of motion. And it doesn’t appear that you’ve included the body weight contribution to the stiffness, which is discussed in the following forum topic: Properties of NREL 5MW-OC4-DeepCwind SemiSubmersible wind turbine in FAST and reference.
Best regards,
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