Good evening,
I am running HydroDyn as a standalone module to study the response of the OC4 semi submersible. I have performed tests in periodic waves. The wave elevations and forces seem reasonable with respect to the parameters WaveTp and WaveHs. I have added a current with the depth-dependent model but even with enabling McoefMod to be different from zero, the drag force remain null.
I also wanted to perform a test without wave, only with current but the simulation could not run. I attached to this post parts of the HydroDyn input file (sorry to send captures but I could not load the .txt) and a capture of the error. I may have missed something obvious and crucial. If you have any idea, I would be grateful.
Many thanks in adavance.
Best regards.
Florence Haudin.
Hmm. I just took Test_006.dvr and Test_006.dat from the HydroDyn v2.05 archive (nwtc.nrel.gov/HydroDyn) and modified it to be current only. HydroDyn runs without error and produced an output with a constant force in the x-direction (HydroFxi) as expected. Please find my input and output files attached. Does this model run for you?
Did you compile HydroDyn yourself or did you use the Windows executable provided in the HydroDyn v2.05 archive?
Good afternoon Mr Jonkman,
Thanks for your past answer on the PropPot flag that made things clearer for me.
I have another query to run simulations with the semi submersible : to generate a forced motion on the floater, what is the proper way to proceed; I would think to put WAMITInputsMod = 2 and to give the following time series informations :
Column 1 : Time step value (s)
Columns 2-4 : Translational displacements along X, Y, and Z (m)
Columns 5-7 : Rotational displacements about X, Y, and Z [small angle] (radians)
Columns 8-10: Translational velocities along X, Y, and Z (m/s)
Columns 11-13: Rotational velocities about X, Y, and Z (radians/s)
Columns 14-16: Translational accelerations along X, Y, and Z (m/s2)
Columns 17-19: Rotational accelerations about X, Y, and Z (radians/s2)
Is it correct ?
Thanks in advance for your answer.
Best regards.
Florence Haudin.
The text from the draft HydroDyn User’s Guide and Theory Manual that you are quoting is from the section titled, “Axial Coefficients”. So, this text only applies to the axial effects on member ends.
However, the PropPot flag applies to all member-level loads–both distributed along the member and member-end effects. When PropPot = TRUE, only viscous-drag loads, and ballasting loads will be computed for that member.
Just to be sure I fully understand, putting PotProp to True means the only loads are viscous drag and ballasting one. In other words, I disable the potential loads so I am not having a mix approach with potential flow + additionnal drag term?
If I am not mistaking, I have run the same simulation for the OC4 semi submersible with PotProb equal to True and then to False for every member, and I got the same HydroFxi force for the case of a floater translated with a constant velocity along x, without waves.
Is it normal ? I would have expected a different result. I am missing something important I guess.
The PropPot flag in HydroDyn only effects the strip-theory solution and has no effect on the potential-flow solution. Setting PropPot = TRUE for a member disables hydrodynamic terms of the strip-theory solution (keeping only the viscous-drag and ballasting terms for that member). Setting PropPot = FALSE for a member enables all hydrodynamic terms of the strip-theory solution (keeping the inertia, added mass, viscous, and buoyancy terms for that member). If the potential-flow solution is enabled, its calculations are not effected by the PropPot flag. The PropPot flag is useful in hybrid potential-flow plus strip-theory solutions to ensure that hydrodynamic terms like inertia and added mass are not double-counted in both solutions.
For steady flow past a fixed cylinder, the only transverse load is the viscous drag term, so, the transverse load should be unaffected by the PropPot flag and/or the enabling/disabling of the potential-flow solution.
I am working on semi submersible platform combined with a point absorber wave energy converter. I have obtained hydrodata for FAST simulation through WAMIT.
I’ve done FAST analysis of that platform for 8m/s windspeed and 2.5m waveheight and I’m obtaining all the platform heave values as zero.
Also, I have considered different windspeeds like 11.2m/s, 14m/s, 17m/s and waveheights 3m, 3.6m, 4.2m respectively. Even then the heave values are zero and all other responses are more or less same without much difference. Please help.
It looks like the platform heave and pitch are fixed during the simulation. Have you disabled the platform heave and pitch degrees of freedom in ElastoDyn? Or has the solution reached a steady state before outputting the data?
Thanks for your reply. I have done the corrections. Now I am getting the following error.
Generating incident wave kinematics and current time history.
Reading in WAMIT output with root name “.\5MW_Baseline/HydroData/SSB”.
Computing radiation impulse response functions and wave diffraction forces.
MAP++ environment properties (set externally)…
Gravity constant [m/s^2] : 9.81
Sea density [kg/m^3] : 1025.00
Water depth [m] : 200.00
Vessel reference position [m] : 0.00 , 0.00 , 0.00
Timestep: 0 of 3600 seconds.
FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:Running with torque
and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll as written by J.
Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes modifications for the
Hywind spar.
Timestep: 8 of 3600 seconds. Estimated final completion at 15:18:07.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_Uni
formWind_CalcOutput:GetWindSpeed:Height must not be negative.
IfW_UniformWind_CalcOutput: Error calculating the wind speed at position (-7.95252E-02, -22.145,
-8.80057E-02) in the wind-file coordinates
FAST encountered an error at simulation time 8.725 of 3600 seconds.
Simulation error level: FATAL ERROR
Aborting FAST.
I am unable to figure out which exact parameter should I change to increase the elevation of the first tower aerodynamic node in the AeroDyn file.
Please help.
I’ve got your mail. I am typing here, just to share with everyone.
AeroDyn.dat file contains a parameter “TwrElev”. TwrElev is the parameter that stores tower node elevations. InflowWind calculates the wind speed at TwrElev nodes.
In case of MIT NREL TLP, first tower node is defined at z=0 mt. When heave motion occurs, InflowWind can not calculate the wind speed in negative elevations so FAST aborts.
So Jason Jonkman advised me to slightly uplift the first tower node. And that worked with me. Here: Error calculating the wind speed at position
But with the Semi-submersible, Tower base node is defined at +10 m. (TwrElev =0, AeroDyn.dat file) So your case is different here. But your FAST abort message is the same with my case that indicates a submerged tower base. I think at time step 8, heave is more than 10 m so tower base is below the still water level where InflowWind alerts you and FAST aborts.
This discussion might be helpful where there is also a excessive heave motion.
I am also new to FAST and the community. I hope what I’ve experienced helps.
Thanks for your reply. I have tried changing the tower elevation which is 10m for semi-submersible and yet the FAST is aborting.
FAST_Solution0:CalcOutputs_And_SolveForInputs:SolveOption2:SrvD_CalcOutput:Running with torque
and pitch control of the NREL offshore 5MW baseline wind turbine from DISCON.dll as written by J.
Jonkman of NREL/NWTC for use in the IEA Annex XXIII OC3 studies. Includes modifications for the
Hywind spar.
Timestep: 17 of 3600 seconds. Estimated final completion at 20:37:56.
FAST_Solution:CalcOutputs_And_SolveForInputs:SolveOption2:InflowWind_CalcOutput:CalcOutput:IfW_Uni
formWind_CalcOutput:GetWindSpeed:Height must not be negative.
IfW_UniformWind_CalcOutput: Error calculating the wind speed at position (0.41846, -28.418,
-0.27097) in the wind-file coordinates
FAST encountered an error at simulation time 17.7 of 3600 seconds.
Simulation error level: FATAL ERROR
I am also new to the community and to the FAST code. So I am not an expert. I can only say that, your heave response is generating a problem. I repeat that our cases are different but the error message was the same. But in your case, I think that you should not change the tower base node elevation. Can you share the heave response of your system? At the time step of 17.7/3600 seconds, I am expecting a have response more than 10 meters and this sounds really weird.
I’ve also checked your AeroDyn file. I see that your WakeMod=0. Why did you do that?
Maybe you should better share your files in here and clarify the problem in details.
