Free decay tests

Good morning,
I would like to know if there is among the examples provided for openfast, an example of free decay tests for a semi submersible or another platform?
I guess that the user has to implement himself the AddCLin matrix taken into account the mooring configuration/parameters?

Thanks in advance.
Best regards.
Florence Haudin.

Dear Florence,

No, none of the example simulations provided with FAST are for a free-decay test, but such a simulation is quite simple to set up. Normally free-decay tests are performed without wind or wave excitation (CompInflow=0 and most likely CompAero=0 in the FAST primary input file, and WaveMod=0 in the HydroDyn input file). Enable whichever degrees of freedom you want (most certainly the platform DOFs) and set a nonzero initial platform displacement. You shouldn’t need to set AddCLin for the mooring system; instead you can enable one of the mooring modules.

Best regards,

Dear Jason,
Thanks for your answer.
Is it possible with Openfast to run a simulation with Hydrodyn, Moordyn and Elastodyn with nothing on the floater or only a rigid tower for instance but no turbine. I don’t understand how to “remove” the tower or the blades in Elastodyn.
Thanks in advance.
Best regards.

Dear Florence,

To “remove” the rotor, nacelle, and tower from an ElastoDyn model, disable the relevant degrees of freedom (blades, drivetrain/generator, nacelle, tower) and set the relevant geometry and mass properties to zero (or epsilon for the variables that do not permit a value of exactly zero).

Best regards,

Dear Jason,
Thanks for your answer. In fact that was what I thought I had to do and started to do : freeze unrelevant degrees of freedom, and set to zero mass and inertia.
Nervertheless, I still don’t really understand what I have to do with the geometric data of the section Turbine Configuration (copied hereunder). It seems that there are some strong relationships between some of them like and can’t be set to zero :

FAST_InitializeAll:ED_Init:ED_ValidateInput:ValidatePrimaryData:TowerHt must be greater than zero.
ValidatePrimaryData:TowerBsHt must be less than TowerHt.
ValidatePrimaryData:HubRad must be less than TipRad.
ValidatePrimaryData:TowerHt + Twr2Shft + OverHang*SIN(ShftTilt) must be greater than TipRad.

Also, I am wondering about the number of nodes for the blades and tower that can’t be put to 0 or 1, in a case where there is no structure on the floater (so no turbine).

Naively for me, I would set all dimensions to zero and set nodes to zero for unexistant structures.

Many thanks for any help.
Best regards.

---------------------- TURBINE CONFIGURATION -----------------------------------
3 NumBl - Number of blades (-)
63 TipRad - The distance from the rotor apex to the blade tip (meters)
1.5 HubRad - The distance from the rotor apex to the blade root (meters)
-2.5 PreCone(1) - Blade 1 cone angle (degrees)
-2.5 PreCone(2) - Blade 2 cone angle (degrees)
-2.5 PreCone(3) - Blade 3 cone angle (degrees) [unused for 2 blades]
0 HubCM - Distance from rotor apex to hub mass [positive downwind] (meters)
0 UndSling - Undersling length [distance from teeter pin to the rotor apex] (meters) [unused for 3 blades]
0 Delta3 - Delta-3 angle for teetering rotors (degrees) [unused for 3 blades]
0 AzimB1Up - Azimuth value to use for I/O when blade 1 points up (degrees)
-5.0191 OverHang - Distance from yaw axis to rotor apex [3 blades] or teeter pin [2 blades] (meters)
1.912 ShftGagL - Distance from rotor apex [3 blades] or teeter pin [2 blades] to shaft strain gages [positive for upwind rotors] (meters)
-5 ShftTilt - Rotor shaft tilt angle (degrees)
1.9 NacCMxn - Downwind distance from the tower-top to the nacelle CM (meters)
0 NacCMyn - Lateral distance from the tower-top to the nacelle CM (meters)
1.75 NacCMzn - Vertical distance from the tower-top to the nacelle CM (meters)
-3.09528 NcIMUxn - Downwind distance from the tower-top to the nacelle IMU (meters)
0 NcIMUyn - Lateral distance from the tower-top to the nacelle IMU (meters)
2.23336 NcIMUzn - Vertical distance from the tower-top to the nacelle IMU (meters)
1.96256 Twr2Shft - Vertical distance from the tower-top to the rotor shaft (meters)
87.6 TowerHt - Height of tower above ground level [onshore] or MSL [offshore] (meters)
10 TowerBsHt - Height of tower base above ground level [onshore] or MSL [offshore] (meters)
0 PtfmCMxt - Downwind distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmCMyt - Lateral distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
-8.6588 PtfmCMzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform CM (meters)
0 PtfmRefzt - Vertical distance from the ground level [onshore] or MSL [offshore] to the platform reference point (meters)

Dear Florence,

There are a few checks within ElastoDyn to check for physical correctness of the input settings. I would recommend setting input parameters TipRad through TowerBsHt to zero in the “Turbine Configuration” section, then make a few small adjustments (by setting values slightly nonzero) based on the errors received when trying to run the simulation. When doing so, make sure to disable all structural degrees of freedom (DOFs) in ElastoDyn except the platform DOFs.

Best regards,

Dear Jason,

as I am currently comparing the results of the free decay tests of our 1:50 scaled model of our TLP, some questions arose. I would be thankfully if you could share your thoughts on this.

  • In HydroDyn three approaches are available for computing hydrodynamic forces on the substructure: (a) Strip Theory, (b) Potential Flow Theory, and (c) Potential Flow theory with an augmentation by strip theory to account for the drag load effects on the members
  • For our structure whose buoyancy bodies have a diameter of 14m, strip theory might not be applicable for many load cases (waves) as radiation and diffraction would grow in importance compared to more slender members
  • This is why I calculated the frequency domain solution in AQWA, converted it to WAMIT format (.1 .3 and .hst) and used this as inputs for HydroDyn. Furthermore, for the decay tests I assume low Reynolds numbers and therefore chose CD=1.2 for the strip theory solution for the different members of our substructure to augment the potential flow solution with the drag term from the strip theory solution

My major questions now would be: Which solution you would prefer to calculate the natural frequencies by use of a free-decay test in OpenFAST? Do you think my approach is reasonable? I think strip theory would also work for the decay tests, as there are no waves included. But I am not sure how exactly HydroDyn would calculate the loads on the members, when there is no current nor waves but the structure itself is somehow “moved” through the water, due to the deflected initial position. I imagine, this works just as the members would be subjected to current. Am I right?
However, even if for the decay tests strip theory might be applicable, when considering serious see states, regular and irregular waves, and the geometry of our platform, potential flow solution + drag augmentation should be the better approach. Do you agree?
Of course, for other sea states with higher Reynolds numbers, respective CD should be chosen.

Thank you,

Dear Daniel,

Just a few comments:

  • You should be able to use any of the three methods (potential flow, strip theory, or hybrid) for both free-decay simulations and simulations with wave excitation.
  • In the strip-theory solution, HydroDyn accounts for the relative motion between fluid and structure, and so, includes hydrodynamic added-mass effects and viscous drag even in the absence of wave and current excitation.
  • Diffraction effects are likely most important for less severe sea states where the wave lengths are not much larger than the structure.
  • Radiation damping is likely important when viscous effects are less dominant, e.g. small amplitude oscillations.
  • Deriving appropriate drag coefficients is always difficult, as the appropriate drag coefficient likely differs between free-decay motions, current, and wave excitation situations.

Best regards,