# Equilibrium position in heave is too high

Dear Dr. Jason.

First I sincerely apologize for forgetting to thank you for solving my last problem.
Based on Test25, I change the file ElastoDyn.dat, HydroDyn.dat, MoorDyn.dat and HydroDatas.
In ElastoDyn.dat,based on Sesam, I change the following parameters
PtfmCMzt:-27.79m
PtfmMass: 2.65667E+06kg
PtfmRIner: 2.06914E+09kg m^2
PtfmPIner: 2.06914E+09kg m^2
PtfmYIner: 2.98733E+09kg m^2
About the PtfmRefzt, I am not sure what it means so I don’t change.
In HydroDyn.dat, the WtrDpth is 100m, PtfmVol0 is 7416m^3, the z coordinate of structure is from -44m to 5m.
About the PLATFORM ADDITIONAL STIFFNESS AND DAMPING, I don’t know how they are calculated, so I don’t change.
I change the Joints and Members for a new structure.
In time domain calculation, PtfmHeave is between 23m to 25m, I think the Gravity and buoyancy are not equal,
but with my calculation,
Rotor Mass: 110,000 kg
Nacelle Mass: 240,000 kg
Tower Mass: 249,718 kg
Platform Mass: 2,656,670 kg
Mooring Weight: 56,459kg, remaining is the ballast water.
So I don’t know why the equilibrium position is too far from 0.

Best Regards

Dear Yuanzhao,

When you say that the mooring weight is 56,459 kg, do you mean that the vertical mooring pretension divided by gravity equals 56,459 kg?

Summing your total rotor + nacelle + tower + platform mass and the mooring pretension equals 3,312,847 kg. Your total external buoyancy is about 7,601,400 kg. So the platform will heave upward unless you’ve added a large amount of water ballast to your HydroDyn model. Have you done this? The platform additional stiffness (AddCLin) should be set corresponding to the water ballast you have set – see Section 6.8.3 of the draft HydroDyn User’s Guide and Theory Manual for more information: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,

Dear Dr. Jason.

Thank you for your help ,with the balance of buoyancy and gravity, the equilibrium position is 0.
There is another problem that when the simulation time is more than 1000s,
a warning appears,that is:
FAST_CreateCheckpoint_Tary:FAST_CreateCheckpoint_T:FAST_PackTurbineType:FAST_PackHydroDyn_Data:Hyd
roDyn_PackParam:Morison_PackParam:Error allocating ReKiBuf.
WARNING: Checkpoint file could not be generated. Simulation continuing.

Every 1000s the warning appreas once, will the warning influence my result?

Best Regards.

Dear Yuanzhao,

No, that warning will not influence your results, but the check point files that you requested are not being created. If you have no need for checkpoint/restart capability and want to eliminate the warning, ensure that input ChkptTime in the primary FAST input (*.fst) file is set to a value larger than TMax.

Best regards,

Dear Dr. Jason,

After reading the above discussions between you and Mr. Yuanzhao, I would like to know that:
(1) How to know the mooring line weight in FAST, from its input files or from the FAST calculation result?
(2) How to know the mooring pretension, is it an input to FAST?
(3) If the total external buoyancy cannot provide enough buoyancy to balance the total system weight (no water ballast exist), i.e., the equilibrium position of heave is below mean sea level, how can we do to make the equilibrium position back to z=0? (because in this case, we cannot adjust it by reducing water ballast)

Thank you very much.

Best Regards,
Yingyi Liu

Dear Yingyi,

(1) The mooring system weight (in air, not considering buoyancy) is the product of the mooring system mass times gravity. The mooring line mass per unit length and mooring line lengths are specified within the mooring modules, so, you could calculate the mooring system weight using these values.
(2) The vertical mooring pretension can be derived by summing the vertical component of the mooring line tension at each fairlead from the mooring module outputs.
(3) If the total external buoyancy is not enough to balance the total system weight and mooring pretension, the system is not designed sufficiently. You’d have to redesign the system so as to increase the buoyancy or reduce the system weight or mooring pretension.

Best regards,

Dear Dr. Jason,

I would kindly to ask that, is it possible to output or check from FAST the following information at the initial time step (hydrostatic, without wave/current/wind)：

1. how much the real draft of the platform is, under the current settings of turbine, platform and mooring system
2. whether the platform has an inclination of roll or pitch or not (i.e., whether the platform keeps hydrostatically horizontal)

I ask this question because I sometimes met FAST stopping with errors like (in case of steady wind): “wind cannot be calculated at the position Z=-18.0m” (or sometimes even reporting errors far above water level, e.g, “wind cannot be calculated at the position Z=280m”). I’m wondering that whether they were caused by the initial inclination of platform (I can see from the FAST output file that the heave motion is always very small, showing that the platform gets to hydrostatic balance in heave all the time). Do you know what might be the possible reasons for this kind of errors?

Best Regards,
Yingyi Liu

Dear Yingyi,

The equilibrium displacement of the platform in all six DOFs without excitation from wind, waves, or current can be found by running a simple time-domain simulation in FAST without aerodynamics (CompInflow = CompAero = 0) and with still water hydrostatics (WaveMod = 0) and running the simulation long enough for all start-up transients to die out (or measuring the “mean response” as the response is slowly decaying). You will not get the error your reporting if you disable the InflowWind module (CompInflow = 0).

I hope that helps,

Best regards,

Dear Dr. Jason,

I followed your instruction to set CompInflow = CompAero = 0 and WaveMod = 0, and performed calculation on parked DeepCWind semisub wind turbine.

To my surprise, the tower base forces and moment are not zeroes (see attached output). Do you know possible reasons for that?

Best regards,
Yingyi Liu
Test25.out.txt (1.98 MB)

Dear Dr. Jason,

Later I also perform a still water calculation on a two-bladed floating wind turbine which I am working on (Currently the turbine is simply changed from the NREL 5MW by reducing the num of blades). To have a comparison, I perform calculation twice (CompMooring=0 and CompMooring=3). When using CompMooring=3 option, I use a single mooring line to simplify the problem (see below). In both cases, I set CompInflow =0, CompAero = 0 and WaveMod = 0, in order to check the hydrostatic stability of the floating turbine.

I got a surprising comparison result. When there are no mooring lines, all seem to be reasonable. However, when I use single line (even it is slack), the floater has significant motions in surge and yaw. I cannot figure out the reason, do you know the possible reasons for that? (For your reference, I also attach the other FAST input files)

--------------------- MoorDyn Input File ------------------------------------
Mooring system for Nezzy Semi
FALSE Echo - echo the input file data (flag)
----------------------- LINE TYPES ------------------------------------------
1 NTypes - number of LineTypes
Name Diam MassDen EA BA/-zeta Can Cat Cdn Cdt
(-) (m) (kg/m) (N) (N-s/-) (-) (-) (-) (-)
main 0.0899 57.6 7.18E8 -1.0 0.8 0.25 2.0 0.4
---------------------- CONNECTION PROPERTIES --------------------------------
2 NConnects - number of connections including anchors and fairleads
Node Type X Y Z M V FX FY FZ CdA CA
(-) (-) (m) (m) (m) (kg) (m^3) (kN) (kN) (kN) (m^2) (-)
1 Fixed 60.0 0.0 -100.0 0 0 0 0 0 0 0
2 Vessel 60.0 0.0 -14.75 0 0 0 0 0 0 0
---------------------- LINE PROPERTIES --------------------------------------
1 NLines - number of line objects
Line LineType UnstrLen NumSegs NodeAnch NodeFair Flags/Outputs
(-) (-) (m) (-) (-) (-) (-)
1 main 150.0 7 1 2 -
---------------------- SOLVER OPTIONS ---------------------------------------
0.001 dtM - time step to use in mooring integration (s)
3.0e6 kbot - bottom stiffness (Pa/m)
3.0e5 cbot - bottom damping (Pa-s/m)
2.0 dtIC - time interval for analyzing convergence during IC gen (s)
60.0 TmaxIC - max time for ic gen (s)
4.0 CdScaleIC - factor by which to scale drag coefficients during dynamic relaxation (-)
0.01 threshIC - threshold for IC convergence (-)
------------------------ OUTPUTS --------------------------------------------
FairTen1
AnchTen1
END
------------------------- need this line --------------------------------------
FAST_InputFiles.zip (1.94 MB)

Dear Yingyi,

I have not looked at your files, but a couple of comments:

• Even in the absence of hydrodynamic or aerodynamic forcing, the overhanging weight of the rotor/nacelle-assembly will cause the floating wind system to surge and pitch a bit, which will induce tower-base loads.
• With only one mooring line, the mooring loads are not balanced and the platform will move in the direction pulled by the mooring tension.

I hope that helps.

Best regards,

Dear Dr.Jason,

I would like to further confirm several points in order to check the hydrostatic stability of my model.

1. In MoorDyn Input File, there is an input parameter “UnstrLen”, is it the length of a portion of mooring line which is lying on the sea bed, or the length of an entire mooring line before pretension?

2. In MoorDyn input file, what is the parameter “BA/-zeta”? And in MAP input file, what is the parameter “CIntDamp”? Are they the same?

3. When doing WAMIT calculations for FAST, in WAMIT input file, where should the rotation-moment reference center/point be set? (which affects the values of roll, pitch, yaw hydrodynamic forces)

4. In FAST simulation, where is the origin of the inertial frame system, is it fixed by default or can be freely set by users?

5. In FAST simulation, can we know the location of the rotation center of whole FOWT system, is there any possibility to output it or calculate it?

Best regards,
Yingyi Liu

Dear Yingyi Liu,

1. UnstrLen in MoorDyn is the unstretched length of the mooring line (in the absence of tension). See the MoorDyn User’s Guide for more information: matt-hall.ca/files/MoorDyn-U … -08-16.pdf.

2. BA / -zeta refer to the structural damping of the mooring lines; again, see the MoorDyn User’s Guide for details. Furthermore, inputs CIntDamp, Ca, Cdn, and Cdt are unused by MAP++. More information on the MAP++ theory and usage are provided in the online documentation: map-plus-plus.readthedocs.io/en/ … index.html.

3. The WAMIT reference point should be the intersection of the undeflected tower centerline for the undisplaced platform and mean sea level (MSL), as described in the draft HydroDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

4. The origin of the inertial frame coordinate system in FAST is the intersection of the undeflected tower center for the undisplaced platform and ground level (for land-based wind turbines) or MSL (for offshore turbines). This cannot be changed.

5. The center of rotation of the floating wind system is not directly output by FAST, but can calculated through knowledge of the platform surge and pitch or platform sway and roll.

I hope that helps.

Best regards,