I’m using FAST to do some simulation for 5MW OC3 Hywind model, I’ve known that the hydro data are derived from WAMIT. My question is when modeling in WAMIT, the mass, CM, Rx, Ry, Rz refer to the whole system including tower, RNA or just the platform?
Plus, since the OC3 report mentioned that there is need to add additional linear damping, can this be achieved in FAST and how? Or can I just add the additional damping when calculating in WAMIT?
Dear Yu Ma,
In WAMIT, the mass, center of mass, and radii of gyration are used to form the 6x6 body-mass matrix needed to compute the response amplitude operators (RAOs). If you want to use WAMIT to compute the proper RAOs, it would be important to specify the mass, center of mass, and radii of gyration of the complete system (platform + tower + nacelle + rotor) properly in the WAMIT model.
However, when WAMIT is used as a preprocess to FAST’s HydroDyn module, WAMIT is only needed to compute the hydrostatic restoring matrix (.hst), the frequency-domain added-mass and damping matrices (.1) and the frequency-domain wave-excitation force vector (*.3) (not the ROAs). All other terms are included in FAST, including the platform weight and inertias, so, it is not important to specify the mass, center of mass, and radii of gyration properly in the WAMIT model when using it as a preprocess for FAST.
One important thing to keep in mind is that the pitch and roll restoring of a floating body depends on the vertical distance between the center of buoyancy and center of mass of the body. In WAMIT, the vertical center of gravity (VCG) is also used to determine the pitch and roll restoring associated with platform weight and WAMIT will include these effects in the restoring matrix that it outputs (the *.hst file). However, FAST intrinsically accounts for the platform weight’s influence on the pitch and roll restoring if the platform weight and center-of-mass location (PtfmCM) are defined appropriately. To avoid double booking these terms, it is important to neglect the terms in WAMIT. This can be achieved by setting VCG to zero in WAMIT.
So, when I use WAMIT as a preprocess for FAST, I always set the center of mass, and radii of gyration to zero in WAMIT.
For the OC3-Hywind spar buoy, the “additional linear damping” cannot currently be specified within the FAST/HydroDyn input files or WAMIT. Instead, we modeled the “additional linear damping” with a slight customization to the HydroDyn module and a recompiled version of FAST. You can find this slight customization of the FAST executable, as well as the FAST model of the NREL 5-MW wind turbine atop the OC3-Hywind spar buoy here: wind.nrel.gov/public/jjonkman/NR … Hywind.zip.
I hope that helps.
Thanks for replying. It’s very helpful.
When I use Orcaflex with FASTv8, the WAMIT*.out file is imported to the Orcaflex model. I have two questions about this: 1) the WAMIT*.out file used for Orcaflex is the same as the WAMIT*.out file used for HydroDyn? I mean there is no difference between the input files of WAMIT. 2) when I use WAMIT for HydroDyn, how to define the position of the platform? I guess there are two positions maybe used: a) where rotor + drivetrain + nacelle + tower + platform balance with the hydrostatic buoyancy b) where rotor + drivetrain + nacelle + tower + platform+mooring weight balance with the hydrostatic buoyancy which position should I used? Best regards
Dear Yu Lei,
Here are my answers to your questions:
Actually, FAST’s HydroDyn module does not read WAMIT *.out files. Instead, HydroDyn makes use of distinct WAMIT files for distinct hydrodynamic quantities e.g. *.1 files containing the frequency-dependent added mass and damping from the radiation problem, *.3 files containing the frequency- and direction-dependent wave excitation loads from the diffraction problem, and *.hst files containing the restoring from the hydrostatics problem. WAMIT *.out files contain similar data, but stored in a different format.
For floating systems, you should define the WAMIT geometry at the equilibrium position of the floater in the heave (vertical) direction i.e. where the external buoyancy from displaced water balances with the weight of the system (including the weight of the rotor-nacelle assembly, tower and substructure) and mooring system pretension.
I hope that helps.
I'm sorry that I did't make my question clear. My first question is as below: When I use Orcaflex with FAST, the[i] WAMIT*.out[/i] file will be imported to the Orcaflex model. When I use HydroDyn with FAST, the WAMIT output files([i] .1,.3 and .hst file[/i]) will be used by the HydroDyn. The [i]WAMIT*.out[/i] file and the [i].1,.3 and .hst file[/i] are the results of the same WAMIT input files? In other words, how many times shoud I run WAMIT to get the [i] WAMIT*.out[/i] file for Orcaflex and the [i].1,.3 and .hst file[/i] for HydroDyn ? If two, what's the difference between the WAMIT input files?
The same WAMIT input file will generate both the *.out and *.1, *.3, and *.hst files. You can use the same WAMIT input file for both HydroDyn and OrcaFlex.
Thanks for your technical support! Best regards
I do not have access to WAMIT but require the OC3 spar *.out file for Orcaflex. The *.1, *.3, and *.hst files which Hydrodyn uses are availble online but not the *.out file. Is this file available and can I get it? Looking forward to your reply.
Please find attached the WAMIT-generated spar.out file for the OC3-Hywind spar that was created when the spar.1, spar.3, and spar.hst files were created.
spar.out.txt (1.85 MB)