Platform motion appears strange for an onshore wind turbine

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Jason.Jonkman
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Re: Platform motion appears strange for an onshore wind turbine

Postby Jason.Jonkman » Fri Nov 09, 2018 6:52 am

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mitesh.Ramani
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Re: Platform motion appears strange for an onshore wind turbine

Postby Mitesh.Ramani » Sun Nov 11, 2018 3:56 pm

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
Attachments
ElastoDyn.rtf
ElastoDyn
(27.05 KiB) Downloaded 32 times
HydroDyn.rtf
HydroDyn
(18.25 KiB) Downloaded 37 times
Platform motions. .PNG
Platfrom Motions
Platform motions. .PNG (32.66 KiB) Viewed 1290 times

Jason.Jonkman
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Re: Platform motion appears strange for an onshore wind turbine

Postby Jason.Jonkman » Mon Nov 12, 2018 10:18 am

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: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mitesh.Ramani
Posts: 29
Joined: Sat Oct 27, 2018 9:10 am
Organization: university of rostock
Location: Germany
Location: Rostock,Germany
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Re: Platform motion appears strange for an onshore wind turbine

Postby Mitesh.Ramani » Mon Nov 12, 2018 10:58 pm

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 density*submerged volume*Gravity=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
Attachments
Platform motion.PNG
Platform motion.PNG (30.91 KiB) Viewed 1253 times

Jason.Jonkman
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Re: Platform motion appears strange for an onshore wind turbine

Postby Jason.Jonkman » Tue Nov 13, 2018 6:49 am

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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mitesh.Ramani
Posts: 29
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Re: Platform motion appears strange for an onshore wind turbine

Postby Mitesh.Ramani » Wed Nov 14, 2018 5:32 am

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
Attachments
hydrostatic equilibrium calculation .xlsx
Hydrostatics calculation
(15.42 KiB) Downloaded 33 times
Simulation21.zip
Input files
(4.76 MiB) Downloaded 34 times

Jason.Jonkman
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Re: Platform motion appears strange for an onshore wind turbine

Postby Jason.Jonkman » Wed Nov 14, 2018 7:10 am

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: viewtopic.php?f=4&t=1592&p=7576. I've now reported this issue in OpenFAST: https://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,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mitesh.Ramani
Posts: 29
Joined: Sat Oct 27, 2018 9:10 am
Organization: university of rostock
Location: Germany
Location: Rostock,Germany
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Re: Platform motion appears strange for an onshore wind turbine

Postby Mitesh.Ramani » Wed Nov 14, 2018 11:49 am

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
Attachments
Graphs.zip
(50.42 KiB) Downloaded 32 times
hydrostatic equilibrium calculation .xlsx
(15.66 KiB) Downloaded 32 times
Simulation25.zip
(312.85 KiB) Downloaded 31 times

Jason.Jonkman
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Re: Platform motion appears strange for an onshore wind turbine

Postby Jason.Jonkman » Wed Nov 14, 2018 12:36 pm

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
MAP++ input file.
(1.39 KiB) Downloaded 36 times

DTU_10MW.MAP.sum.txt
MAP++ summary file.
(5.01 KiB) Downloaded 38 times

I hope that helps.

Best regards,
Jason Jonkman, Ph.D.
Senior Engineer | National Wind Technology Center (NWTC)

National Renewable Energy Laboratory (NREL)
15013 Denver West Parkway | Golden, CO 80401
+1 (303) 384 – 7026 | Fax: +1 (303) 384 – 6901
nwtc.nrel.gov

Mitesh.Ramani
Posts: 29
Joined: Sat Oct 27, 2018 9:10 am
Organization: university of rostock
Location: Germany
Location: Rostock,Germany
Contact:

Re: Platform motion appears strange for an onshore wind turbine

Postby Mitesh.Ramani » Wed Nov 14, 2018 7:11 pm

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


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