Platform motion appears strange for an onshore wind turbine

Dear Dr. Jason

Thank a lot for your kind reply. I really appreciate your answer.

Sorry for that,It could not open So I have attache the hydroDyn and MoorDyn file as below.

Also I have attached the platform surge, sway and heave motion picture without wave and wind (In the hydraustaics conditon)

Can you please check the system need any other calculation for the hydrostatic equilibrium.(I will upload the another files in the next question can you please have a look those too )

AS I am doing ptoject for 10MW DTU wind turbine so I am using the all the inout files from The DTU 10MW Reference Wind Turbine website.

I hope you will give me a positive answer.

thank a lot

Best Regards
Mitesh Ramani
HydroDyn.docx (7.61 KB)


MoorDyn.docx (3.02 KB)

Dear Mitesh,

I see that you uploaded a figure of the platform sway, roll, and pitch motions, but I don’t see the platform surge or heave motions. Platform heave seems to be most critical to respond to your question.

Best regards,

Dear Dr. Jason

AS you told me to check the the lowest aerodynamic node likely on the tower in the AeroDyn file. I have checked that and Also I have uploaded which AeroDyn file I am using for a simulation.

OpenFAST having 10MW floating Wind Turbine.Tower hight is 115.63m with 10m base. Hub height is around 129m.

I hope you can guide me for this problem from my attached files.

Thank a lot

Best Regards
Mitesh Ramani
ElastoDyn.rtf (27 KB)
AeroDyn.rtf (4.14 KB)
Hydrostatics Calculation .pdf (121 KB)

Dear Dr. jason

Really sorry for the inconvenient. I have uploaded the picture with the platfrom heave surge and sway motion.

Thank you so much

Best regards
Mitesh Ramani

Dear Mitesh,

Looking at your platform-heave plot, the model clearly does not have the proper hydrostatic restoring in heave, and the system is falling due to gravity without proper buoyancy.

I only took a brief look at your files, but I see the main problem. You’ve set up your HydroDyn input file to be a strip-theory only model. However, you have not defined any hydrostatic restoring for this strip-theory model. 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,

Thank you for your reply,

I have Defined the hydrostatic restoring matrix non-zero term and I would like to show the calculation its given as below,

Density ρ=1025 kg/m3
Gravity g=9.81 m/s2
Water plane area A=54.10 m2 (=piD2/4,D=8.3)
Displaced volume V= 13085.4 m3
Metacentric Height GM=12.4m
C33=544050.2 N (ρgA)
C35=C53=0
C55=C44=1649987792 N(ρgV
GM)

After the add my new value I am getting the error in the OpenFAST as below,

"FAST_Solution:FAST_AdvanceStates:ED_ABM4:ED_AB4:ED_CalcContStateDeriv:SetCoordSy
:Small angle
assumption violated in SUBROUTINE SmllRotTrans() due to a large blade deflection
(ElastoDyn
SetCoordSy). The solution may be inaccurate. Simulation continuing, but future w
arnings from
SmllRotTrans() will be suppressed.
Additional debugging message from SUBROUTINE SmllRotTrans(): 2.75000E-02 s

MD_UpdateStates: NaN state detected.

FAST_Solution:FAST_AdvanceStates: MD_UpdateStates: NaN state detected.

FAST encountered an error at simulation time 3.75000E-02 of 50 seconds.
Simulation error level: FATAL ERROR"

Also I would like to ask that can you share any HydroDyn file for 10MW with Spar-buoy if it is possible so I can compare some data too for stability,Or can you please help me out now where I am going wrong.

I have attached my HydroDyn file as below kindly find it.

I hope you will help me out from this problem as soon as possible.

Thank again to guide me.
HydroDyn_New Metrix.rtf (18.3 KB)

Dear Mitesh,

I see two problems:
*You have not changed the correct matrix. From your HydroDyn input file, I see that you changed the additional linear damping matrix, not the additional linear stiffness matrix. (And you have not changed the correct terms; you should only need to change AddCLin(3,3), AddCLin(4,4), and AddCLin(5,5).
*The body weight (gravity) term is intrinsically included in the ElastoDyn module. Only the buoyancy term should be included in the AddCLin matrix (and so the AddCLin(4,4) and AddCLin(5,5) terms should be negative valued for a deep-drafted spar). This topic is described in Section 6.8.3 of the draft HydroDyn User’s Guide and Theory Manual that I referenced earlier.

I have not modeled the DTU 10-MW turbine atop a floating platform, but I’m sure these are available in the research community.

Best regards,

Dear Dr. jason

Thank you very much for your continues support and help.

I am really sorry for that, I have corrected those data in that matrix and I have added the those value in the additional linear stiffness matrix.
Also I have corrected my calculation and it is give as below.

c33=54450 N\m
c44= -8846126298 N\m
c55= -8846126298 N\m

I was trying to do simulation with ElastoDyn,SeroDyn,HydroDyn and MoorDyn without wind and waves but I am getting the error like as below attached picture. Can you please help me out where I am going wrong.

I have attached the picture of the error, Input files and HydroDyn file as below kindly find it.

I hope you can guide me for my this problem as soon as possible.

I really appreciate your help.

Thank you

Best regards
Mitesh Ramani
HydroDyn v2.docx (30.7 KB)


Simulation17.7z (270 KB)

Dear Mitesh,

Your AddCLin matrix is still not specified correctly. You’ve defined the (2,3), (3,4), and (4,5) elements nonzero, when you should be specifying the (3,3), (4,4), and (5,5) elements nonzero.

Best regards,

Dear Dr. jason

Thank you very much for correcting me and your help.

I really solved that problem and I have changed that matrix values as you told me.Till 50 time-step I am getting value around zero of platform motion.But if I increase the value of Total run time then at around 78 timestep I am getting the same error I have tried to fix that error but I could really find the solution.

have calculated Vertical force at the fairlead by hand and it is around =2294812.291 N
In the Fast MoorDyn summary it is around =2550000 N

I hope you can help me and guide me to solve this problem.

I have attached the error picture and HydroDyn and MoorDyn please kindly find it below.

Thanks again for your continuous help and support me.

Best Regards
Mitesh Ramani
MoorDyn.rtf (2.84 KB)
HydroDyn.rtf (2.84 KB)

Dear Mitesh,

Are you saying that your simulation produces reasonable platform motions for a while and then suddenly diverges? (What do the platform motions look like as a function of time?) I would not expect this.

Your model is clearly going unstable (due to either numerical or physical reasons). If numerical, dropping the time step or adding corrections could help. If physical, there is likely still something specified nonphysically in one or more of your input files.

I can’t see if you’ve fixed your HydroDyn file because what you attached as a HydroDyn file is actually a MoorDyn file.

Best regards,

Dear Dr. Jason

Thank you for your reply and help.

I have really solved that problem from your help.It was problem in the ElastoDyn file in the PtfmCMzt parameter. I have corrected it.
Now I am getting the platform motion as below. do you agree it is in hydrostatically stable .It you are not agree then please tell me in which parameters I have to do change ?

for my system Vertical force at the fairlead is around 0.2061E+07 as per MoorDyn summary file. how should I account this force in my hydrostatic stability calculation.
should it be calculate like (vertical force+structure mass including ballast = buoyancy force ) this ??

Sorry for the last time I have attached the wrong HydroDyn file this time I have attached the correct as below. also I have attached the ElastoDyn file.

I hope you will reply me as soon as possible.

Thank you very much

Best Regards
Mitesh Ramani
ElastoDyn.rtf (27 KB)
HydroDyn.rtf (18.3 KB)

Dear Mitesh,

The platform motions you are showing are clearly better than before. I do, however, see a mean platform-heave displacement of around 8 m, which suggests that the heave is not in equilibrium the way it should be. The vertical equilibrium equation is explained in section 6.8.1 of the draft HydroDyn User’s Guide and Theory Manual: wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,

Dear Dr. Jason

I really appreciate your help and support.

I have done that calculation and changed the all related parameters in my input file as you suggest me from HydroDyn manual.I have proved that condition in hydrostatic equilibrium calculation. I have attached my platform Motion picture as below and still I am getting platform-Heave around 4m but it improved much better than before. DO you agree the system is in hydrostatic equilibrium.

Also I have tried to solve that but it could not work(it’s stay still like that) so can you please suggest me if there is still need to change in any other parameters.

I have noticed that in HydroDyn Summary file BuoyFzi is 1.369697E+08 and for my hand calculated it is around 131577109.9 so can you please guide me if is there any problem and how can I fix it? I should count total vertical mooring pretension =3*T RIGHT ??

Water density = 1025 kg/m3
Submerged Volume=13085.41 m3 (Hand calculated and HydroDynfile summary both are same)
Gravity= 9.81 m/s2

Buoyancy Force =water densitysubmerged volumeGravity=131577109.9

I can not understand that submerged volume of my hand calculated and HydroDyn summary file both are same then why I am not getting the same Buoyancy force in the HydroDyn file.

If I start the load cases for this system then there would be any problem in my result. Or do you agree that I can do that with this system ??

I have also attached my input file folder kindly find it below.

I hope you will reply me as soon as possible.

Thank you very much for your guidance.

Best Regards
Mitesh Ramani

Dear Mitesh,

I would encourage you to define the vertical equilibrium such that the mean heave displacement in the absence of wave excitation is zero.

The various terms in the vertical equilibrium equation are found in the various summary files (ElastoDyn, HydroDyn, MAP++). I can’t guess as to what the problem is. (And I don’t see that you’ve attached these files.)

Yes, the vertical mooring pretension is the vertical tension (not the tension along the line, unless each line is vertical) multiplied by the number of mooring lines you have.

I’m not aware of any discrepancy between the submerged volume and buoyancy force stated in the HydroDyn summary file. But there should be one small change to your hand calculation–you’ve defined gravity to be 9.80665 m/s^2 in your ElastoDyn input file (which you attached in your post above), whereas you use the rounded 9.81 m/s^2 in your most recent forum post. (But I can’t confirm your submerged volume without the HydroDyn summary file.)

Best regards,

Dear Dr. Jason

Thank you for your speedy reply and your help.

I have changed the value of gravity in my calculation. and also i have tried to improve my hydro-statics calculation but it still stay like this.

sorry for that I did not uploaded input files but this time I have attached my hydro statics calculation and Input files too.

can you please run if it is possible.I hope you will guide me as soon as possible.

Thank you very much

Best Regards
Mitesh Ramani
hydrostatic equilibrium calculation .xlsx (15.4 KB)
Simulation21.zip (4.76 MB)

Dear Mitesh,

Just a few comments:

  • I still see gravity set to 9.81 m/s^2 in your spreadsheet.
  • There is a bug in the vertical component of the buoyancy force (BuoyFzi) reported in the HydroDyn summary file; it is incorrectly reported based on the total structure volume rather than the submerged volume. It sounds like we were aware of this bug and offered a fix a couple years ago, but this fix doesn’t seem to have been implemented in the current OpenFAST code base. More information is available in the following topic on the forum: http://forums.nrel.gov/t/water-ballasting-buoyancy-loads/1380/3. I’ve now reported this issue in OpenFAST: github.com/OpenFAST/openfast/issues/202. Regardless, I agree with your hand calculation based on the submerged volume.
  • The total mass in your spreadsheet doesn’t match your ElastoDyn summary file. It should be 13162804 kg.
  • Running your model, it doesn’t appear that MoorDyn even generates a summary file (and I don’t see that you’ve attached one, so I’m not sure how you got your value). I converted your MoorDyn input file to MAP++ and when I ran that, the vertical mooring tension on each fairlead is around 1.6E6 N, so, quite a bit lower than you are reporting.
  • The above differences lead to an imbalance in the vertical equilibrium condition. But I’m not sure which term (displaced volume, mass, pretension) should be changed to resolve the problem.

Best regards,

Dear Dr. Jason,

Thank you for your speedy reply and help me out.

Sorry I had changed the gravity in the similar another file.But now I have changed,Thank you for this correction.

Thank you for your explanation about bug in HydroDyn External Buoyancy force and improving my knowledge about Hydro-static calculation.

This time I have changed Platform mass in ElastoDyn because as you said my Fair tension vertical force is around 1.6E6. Then I am getting Heave and other motions of platform very near to zero as you can you see the pictures in the Graph folder, it is attached as below.

Also I have attached my Fair Tension Picture in Graph folder,I have generated that from the Moor Out file , I can not understand why I am getting vertical force tension is around 2.6E6. you can seein picture and my moorDyn output file as well.

I think it is problem in MoorDyn input file,Can you please send me your MAP++ file if it is possible so I can try with that too.
My question is also that did you take a fair Tension force while ruining simulation in OpenFAST at t=0 second, because also I can see there and it is also around 1.6E6 or we have to take it from output file only?? Can you please explain me how did you get 1.6E6!!

I hope you will reply me as soon as possible and guide me for my this problem.

Thank you very much

Best Regards
Mitesh Ramani
Graphs.zip (50.4 KB)
hydrostatic equilibrium calculation .xlsx (15.7 KB)
Simulation25.zip (313 KB)

Dear Mitesh,

I agree that your floating system finally looks to be in a reasonable vertical equilibrium!

Please find my MAP++ input and summary files attached. I based this MAP++ input file your own MoorDyn input file. The summary file is automatically generated when enabling MAP++ within FAST and contains the vertical pretension on each line (FZ for nodes 4-6, which are the fairleads).

The tension you are reporting of 2.6E6 N is the tension along the line. This is not the same as the vertical tension because the mooring line is not oriented vertically. MAP++ predicts the same tension along the line as MoorDyn at initialization (i.e., SQRT(FX^2 + FY^2 + FZ^2) = 2.6E6 N).

DTU_10MW_MAP.dat.txt (1.39 KB)
DTU_10MW.MAP.sum.txt (5.01 KB)
I hope that helps.

Best regards,

Dear Dr. Jason,

I really appreciate your help to do vertical equilibrium of my floating system.

Thank a lot for sending me the MAp++ files and your important explanation about vertical pretension.

Now I am getting all the platform motions near by zero so the system is in static equilibrium.

Thanks again

Best Regards
Mitesh Ramani