Dear Jason,
Regarding the 5MW OC3 Hywind in FAST, can you comment on the HydroData folder? Did you generate the added mass, added damping, and hydrostatics based on the 5MW OC3 Hywind with blades, tower, spar, but without mooring lines?
In the hydrostatic data there is a comment:
6 6 9.783307E+03 !JASON:ADDITIONAL YAW SPRING TO AUGMENT MOORING SYSTEM RESTORING IN YAW:0.000000E+00
What is the reason for this additional yaw spring?
Best regards
Søren
Dear Søren,
I generated the added mass, damping, and hydrostatic matrices and the wave excitation vector using WAMIT. These properties only depend on the shape of the spar and the frequency/direction of the body motion/waves. They do not depend on the blades, tower, or mooring lines. The process is described in Section 4 of the OC3-Hywind specifications report: nrel.gov/docs/fy10osti/47535.pdf.
The additional yaw spring was used to augment the mooring system restoring because the FAST model did not consider the so-called “crowfoot” (delta connection) of the mooring lines. This is described in Section 5 of the OC3-Hywind specifications report.
I hope that helps.
Best regards,
Dear Jason,
Thank you for the answer.
Can your clarify on what the units are of the WAMIT output files spar.1 and spar.hst? I understand what the columns are and I assume the wave frequency in the first column of spar.1 is wave period time in seconds. It seems as if I have to multiply the hydrostatics in spar.hst with 10^5 to get N/m and Nm/rad, but the units of the added mass matrices and damping matrices I can not recongnize.
Regards Søren Christiansen
Dear Søren,
Other than the wave period (first column, in seconds) and the wave direction (2nd column in the .3 file, in degrees), the values in the WAMIT .1, .3, and .hst files are nondimensional. The added mass is normalized by density, rho, the damping is normalized by rhoomega, the wave excitation force is normalized by rhogravity(wave amplitude), and the hydrostatic restoring is normalized by rhogravity. These normalizations also include length scales; however, all of the WAMIT models we’ve created are based on a length scale of unity (L = 1). More details can be found in Chapter 4 of the WAMIT User’s Manual: wamit.com/manual6.4/Chap4.pdf.
I hope that helps.
Best regards,
Dear Jason,
Thank you for this important information and for taking the time to answer all my questions. Now it all makes sense.
Søren
Dear Jason,
Is the wave period signal available in FAST?
I’m trying to calculate it like this:
2piOutData(:,strmatch(‘WaveElev’,OutList))./OutData(:,strmatch(‘Wave1Vxi’,OutList))
But the result has large spikes due to zero crossings.
Best regards
Søren Christiansen
Dear Søren,
The wave period is an input to FAST’s HydroDyn module, not an output.
For a regular (periodic) wave-elevation time series, the wave period is easy to identify.
For an irregular (stochastic) wave, there is not really one period, but a distribution of periods at different energy levels (i.e., the wave energy spectrum). In the case of irregular waves, the input to FAST’s HydroDyn module is the peak spectral period (i.e., the period at which the peak energy occurs). For an irregular wave-elevation time series, the peak-spectral period (and distribution of energy levels across different periods) could be derived by computing the power spectral density of the time series.
I hope that helps.
Best regards,
Dear Jason,
I have a question about WAMIT HydroData (*. 1 and *.3) files.
Is the first column (" PER" column ) in the HydroData must be period in second? In the WAMIT input file, I specified the IPERIN = 2 (2: frequency in rad/sec). As a result my first column of WAMIT output file is also frequency not period. I didn’t see any switch in FAST input file to change this and I found that all the HydroDatas for OC4 are in period.
Thanks,
Jinsong
Dear Jinsong,
HydroDyn assumes that the first column in the WAMIT output *.1 and *.3 files is the period (PER) in seconds. This was the only format available in WAMIT v6.X. I understand that WAMIT v7.X now allows the user to identify whether the first column is the period, frequency, or wavenumber through the use of the IPEROUT configuration switch. However, because of HydroDyn’s requirement, I suggest that you run WAMIT with IPEROUT = 1 (resulting in the first column being the period in seconds).
Best regards,
Dear Jason.
I have a question regarding the periods used in the WAMIT-files in FAST 8.16. The lowest period is 1,25664 s and the highest period is 125,6 s. Are there som requirements in FAST regarding the minimum and maximum value of the periods?
Also, the normalisation in WAMIT is done using rhoL^k for A_ij and rhoL^k*omega for B_ij, but which value of L did you use? Is omega in rad/s?
Dear Jason.
I have an additional question regarding the .3-file from WAMIT. In the user manual, on p. 4-3 it reads that the excitation force is normalized by division with rhogA*L^m, where A is the amplitude of the incident-wave. Which amplitude A should be used for the .3-file when FAST should read it?
Dear Jacob,
Section 6.8.4 of the draft HydroDyn User’s Guide and Theory Manual provides some guidance on how to choose the proper frequency range and discretization of the WAMIT data: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.
In all of the WAMIT results provided by NREL, the WAMIT length scale was set to unity. If you have used a different length scale, you can redimensionalize the data properly using HydroDyn input parameter WAMITULEN.
Yes, “omega” in the WAMIT nondimensionalization is in rad/s.
The wave amplitude used in the redimensionalization of the WAMIT data is computed internally within HydroDyn based on the wave data prescribed.
Best regards,
Dear Jason,
Thank you very much for taking your time to respond. I just want to make sure what to set the wave amplitude A to as I am working with NEMOH, which gives exciting force outputs with units. The way I understand your last post, A is set to unity (1,0) in WAMIT and then the significant wave height is specified with the peak spectral period in HydroDyn, when using FAST?
Dear Jacob,
Yes, that’s correct. A is unity in WAMIT and the actual amplitude within HydroDyn is calculated based on the specified significant wave height (and other wave inputs).
Best regards,
Dear Jason,
I am running FAST with .1, .3 and .hst-files generated from data obtained with NEMOH. I have a question regarding the location of the centre of gravity used when generating these files. I have used the hywind-spar buoy support structure as my offset so that I can compare my data to that provided in the FAST-package. First, I generated data from NEMOH with the centre of gravity set to -89,9155 m as specified in the OC3-paper, but when I compared the added mass and damping to that of WAMIT, there was a large difference except for sway, surge and heave (see the attached pictures for roll and pitch and coupled sway and roll and coupled pitch and surge). Then, I changed the centre of gravity to 0 (at MSL), which then yields roughly the same coefficients as that of WAMIT (see attached pictures). My question is what the general rule is, when generating hydrodynamic data for FAST - because originally I would say that one should use the actual centre of gravity for the .1 and .3-files and Z_G=0 for the .hst-file, but looking at the data, it seems one should rather use z_g=0 for everything?
Added_mass_and_damping_z_G_89_9155_m.pdf (89.6 KB)
Added_mass_and_damping_z_G_0_m.pdf (94.6 KB)
Dear Jacob,
I’m a bit confused. The hydrodynamic radiation, diffraction, and hydrostatic coefficients should not depend on the center of gravity, as these only depend on the wetted surface geometry of the body and frequency/direction. (The hydrostatic input to FAST should only include the effect of buoyancy, not the weight of the body.)
Best regards,
Dear Jason,
OK, I am thinking that maybe WAMIT and NEMOH work in a different way. To me it seems that when specifying the centre of mass as the actual value in NEMOH, I am also specifying that the structure rotates about this point and thus gives different values of the added mass and damping compared to a situation, where (0,0,0) is used as reference point. Nevertheless, I will continue with (0,0,0) as the reference point for the NEMOH calculations when generating data for HydroDyn.
Dear Dr. Jason,
I have a question about the WAMIT files in the HydroData folder of FAST (V8).
As you may notice in the following screenshot for marine_semi.1 file, the Cross-Modes for added mass and damping are not exhausted, only modes from respectively in between (1,3,5) or (2,4,6) are presented. In the file barge.1, the terms are even less, where only modes from respectively in between (1,5) or (2,4) are presented.
Thanks for the answers.
Yingyi Liu
Dear Yingyi,
Yes, you can include additional cross terms in the WAMIT data for use in FAST/HydroDyn. I believe that WAMIT simply neglects writing small-valued data to the file. FAST/HydroDyn will assume that cross terms not present in the file are zero, but FAST/HydroDyn will use all of the nonzero terms.
Best regards,
Dear Jason,
I am simulating FAST version v8.1500a-bjj(12 april 2016), using the turbine model (test_24) "NREL 5.0 MW Baseline_OC3_hywind Wind Turbine for coupled dynamic analysis. therefore, the HydroDyn files (.1, .2 and .fst) are imported from the WAMIT. The output file (obtained from WAMIT) is showing very strange values for centre of buoyancy and volume. hereby i want to know that
1.) what is the reason behind these strange outcomes ? (i.e negative volumes, high centre of buoyancy )
2.) which one volume (e.g VOLX,VOLY ‘or’ VOLZ) is used as undisplaced volume in FAST to arrive at the condition mention in HydroDyn_Manual section 6.8.1 (Undisplaced Position for Floating Systems). please find WAMIT output file
New WinRAR ZIP archive.zip (331 KB)
