Inertia issue

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Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Thu Feb 09, 2017 9:17 am

Dear Simon,

I haven't looked at your MATLAB script, but your approach sounds OK. There are likely further design details (e.g. plates and stiffeners) in the original spar design that would lead to differences relative the simple hollow cylinder assumption. I'm not sure the details are really that critical as long as the total mass, center of mass, and inertias are correct and the stiffness is large enough to avoid low-frequency flexural modes, which would be unphysical.

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
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Qian.Yi
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Re: Inertia issue

Postby Qian.Yi » Fri Feb 10, 2017 6:45 am

Dear Jason,
Thanks for your help.
I tried using the zeros wamit file ( *.1 and *.2 ),and the linearized M matrix seems reasonable which proves you are right about the hydrodynamic effect.
Then when I ran a sanity check on the damping matrix ,it turns out that the linearized damping matrix I got isn't a zero matrix.

Code: Select all

   C - Damp                                                                      
      Matrix                                                                        
      0.000E+00    0.000E+00    0.000E+00   -1.311E-01   -1.802E+01    1.967E-01
      0.000E+00    0.000E+00    0.000E+00    8.997E+00   -1.302E-01    4.665E+04
      0.000E+00    0.000E+00    0.000E+00    2.531E-05   -9.328E+04    6.126E-06
      0.000E+00    0.000E+00    0.000E+00   -1.050E+03   -1.046E+01    5.029E+06
      0.000E+00    0.000E+00    0.000E+00    1.418E+01    9.773E+00   -2.127E+01
      0.000E+00    0.000E+00    0.000E+00    1.119E+03    4.538E-05    1.053E+03
      

But I don't know what causes this problem, I try to describe my configuration in this sanity check below.
1. I am using the original FASTv7 exe (without recompile) from https://nwtc.nrel.gov/FAST7
2. In the primary input file ,I set AnalMode=2,PCMode =0,RotSpeed =0 , Disabled all the DOFs and CompAero as well .
In the linearization control file ,I set CalStdy =True MdlOrder=2.
In the platform file ,I set the
PtfmLdMod =FltngPtfmLd ,
WAMITFile to zeroed *.1 and *.2 file and a correct *.hst file,
PtfmCD=0
wavemod =0
CurrentMode=0

Is there anything wrong with this configuration? If there is please tell me how to get it fixed ,I really appreciate your help .
I attached the major input file below in case I didn't make it clear to you.
Best regards.
Attachments
OC3SanityCheck-Linear.dat.txt
(2.09 KiB) Downloaded 24 times
OC3SanityCheck.fst.txt
(22.47 KiB) Downloaded 25 times
OC3SanityCheck-Platform.dat.txt
(7.91 KiB) Downloaded 26 times

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Fri Feb 10, 2017 10:03 am

Dear Qian,

Your files approach look reasonable to me.

My guess is that there is small residual motion of the platform that is leading to a small amount of damping from linearization of the nonlinear structural model. You've eliminated all aerodynamic and hydrodynamic damping from your model, which may make it difficult to find a static-equilibrium solution through a time-domain-based steady state calculation. If you initialize the platform displacement so that it is at (or very close) to the natural steady state solution, my guess is the damping terms will decrease.

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

Qian.Yi
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Re: Inertia issue

Postby Qian.Yi » Sat Feb 11, 2017 7:34 am

Dear Jason,
First of all ,thank you for your help as always.
I noticed there is a ptfmCD option in the platform input file to define the viscous drag coefficient .
So I tried two tests .
one with the ptfmCD=0 like the sanity test I mentioned in the former post,one with the ptfmCD=0.6.
And it turns out the linearized C,K matrix are different,I wonder what causes this difference since I didn't define any morison element in the input file(excatly the same with the attachment in my post above).
Hope you could help me figure this out ,many thanks.
Best regards.

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Mon Feb 13, 2017 6:53 am

Dear Qian,

I'm not sure why you say that you "didn't define any Morison element". In your OC3SanityCheck-Platform.dat.txt file, you have PtfmNodes, PtfmDraft, and PtfmDiam all set nonzero. This will lead to Morison-style viscous drag when PtfmCD is nonzero.

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

Qian.Yi
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Organization: Tsinghua University
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Re: Inertia issue

Postby Qian.Yi » Tue Feb 21, 2017 1:25 am

Dear Jason,

Yes ,you are right about the viscous drag issue,I forgot that I have defined that already,thanks for your indication.

I just ran some linearization process with OC3Spar model under different wind conditon.

one with CompAero off to get a non wind impact M,C,K.

Another one with CompAero True and GenDOF False to keep a fixd rotor speed at 12.1rpm, steady wind input is 11.4m/s with power shear law at 0.14,
and also try to get the M,C,K.

Then I find the Mass matrix generated from above tests are different from each other.
I understand that the damping matrix and stiffness matrix would be different from each other because they count for different aerodynamic influence,but would the mass matrix linearized by fast also be influenced by different aerodynamic condition input? If not ,how can I fix this ?

Hope you could help me out with this ,much appreciated.

Best regards.

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Tue Feb 21, 2017 6:16 am

Dear Qian,

The aerodynamics in FAST do not directly influence the mass matrix obtained through a FAST linearization analysis. However, the mass matrix in FAST does depend on structural displacements, so, if the aerodynamic loads influence the structural displacements, you would see this indirect effect on the mass matrix.

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

Yu.Lei
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Re: Inertia issue

Postby Yu.Lei » Wed Jan 10, 2018 5:21 am

Jason.Jonkman wrote:Dear Simon,

The platform mass, center of mass, and inertias specified in the ElastoDyn module of FAST v8 are for the platform itself, not including the tower, nacelle, drivetrain, rotor, or moorings.

It sounds like for Bladed you'll need to derive a wall thickness/density, and ballast so as to mimic the global platform mass, center of mass, and inertias, but I've not done this myself so I don't have suitable properties to give you.

Best regards,


Dear Jason,
From your answer, you mean that the platform mass, center of mass, and inertias specified in the ElastoDyn module of FAST v8 are for the platform itself, not including the tower, nacelle, drivetrain, rotor, or moorings. But in the file 'NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat' which is downloaded from NREL, the mass and inertia properties of OC3-Hywind are defined as below:

7.46633E+06 PtfmMass - Platform mass (kg)
4.22923E+09 PtfmRIner - Platform inertia for roll tilt rotation about the platform CM (kg m^2)
4.22923E+09 PtfmPIner - Platform inertia for pitch tilt rotation about the platform CM (kg m^2)
1.6423E+08 PtfmYIner - Platform inertia for yaw rotation about the platform CM (kg m^2)

But in the report 'Definition of the Floating System for Phase IV of OC3' , the mass is calculated as follow:

The mass, including ballast, of the floating platform is 7,466,330 kg. This mass was calculated
such that the combined weight of the rotor-nacelle assembly, tower, and platform, plus the
weight of the mooring system (not including the small portion resting on the seafloor) in water,
balances with the buoyancy (i.e. weight of the displaced fluid) of the undisplaced platform in still
water.

The total mass is the same, but it contains different items. Which one is right? I am confused about it. This is the first time to use FORUM, forgive me can't give you a figure.
Best regards

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Wed Jan 10, 2018 6:25 am

Dear Yu Lei,

Both of my statements are correct.

The OC3-Hywind spar was developed from specifications provided by Statoil, but the platform mass was not specified directly by Statoil. Instead. the full-system (rotor + nacelle + tower + platform) mass was provided. So, I back-calculated the platform mass from the full-system mass by subtracting out the known masses of the rotor, nacelle, and tower.

I hope that clarifies things.

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

Gabriele.Barbanti
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Re: Inertia issue

Postby Gabriele.Barbanti » Thu Jan 11, 2018 4:11 am

Dear Dr. Jonkaman,


following the last post I would add a question in this topic.
If I have understand correctly you have calculated the Hywind mass of the platform from the displaced volume of water of the Spar V0, then subtracting the rotor + nacelle + tower of the NREL 5MW wind turbine and then weight of the mooring system (not including the small portion resting on the seafloor). Is it right?
If so I would ask you how do you calculate the weigth of mooring lines resting on the seafloor?
If I do the calculation of the mooring line mass is should be 902m * 76.7066 kg/m = 69183 kg but if I do the reverse calculation starting from the displaced volume and subtracting all the terms (roter, nacelle, tower and plaftform) I obtain the mooring line mass of 54833 kg.
If I do the subtraction of this two terms I should obtain the mooring mass line resting on the sea floor, amounting in 69183 - 54833 = 14350 kg.

My question is how do you calculate this value in order to obtain the platform mass.


Thank you very much.

Best regards,
Gabriele

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Thu Jan 11, 2018 6:51 am

Dear Gabriele,

I haven't checked your numbers, but the mooring tension at the fairlead is calculated by whichever mooring module is enabled in FAST. The vertical tension at the fairlead equals the hanging weight of the mooring lines, not including the weight resting on the seabed. E.g. the portion of the mooring lines resting on the seabed is automatically calculated by MAP++ based on the anchor and fairlead positions.

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

Yu.Lei
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Re: Inertia issue

Postby Yu.Lei » Thu Jan 11, 2018 11:51 pm

Jason.Jonkman wrote:Dear Yu Lei,

Both of my statements are correct.

The OC3-Hywind spar was developed from specifications provided by Statoil, but the platform mass was not specified directly by Statoil. Instead. the full-system (rotor + nacelle + tower + platform) mass was provided. So, I back-calculated the platform mass from the full-system mass by subtracting out the known masses of the rotor, nacelle, and tower.

I hope that clarifies things.

Best regards,

Dean Jason

Maybe I didn't give my meaning clearly. I mean the total mass of the full-system (rotor + nacelle + tower + platform) is 7.46633E+06 kg, so the platform mass (not includs the tower, nacelle, drivetrain, rotor, or moorings) must be less than 7.46633E+06 kg. Thus, the mass of the platform in the ElastoDyn module shouldn't be 7.46633E+06 kg. But in he file 'NRELOffshrBsline5MW_OC3Hywind_ElastoDyn.dat', the mass of the platform is defined as 7.46633E+06 kg. Isn't this a contradiction?
Thank you very much.

Best regards

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Fri Jan 12, 2018 6:35 am

Dear Yu Lei,

No, the mass of the OC3-Hywind spar platform, including ballast, is 7.46633E+6 kg. The full-system mass of the OC3-Hywind floating wind system (rotor + nacelle + tower + platform/ballast) is 8.066E+6 kg.

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

Yu.Lei
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Organization: Tsinghua University
Location: China

Re: Inertia issue

Postby Yu.Lei » Sun Jan 14, 2018 5:35 am

Jason.Jonkman wrote:Dear Yu Lei,

No, the mass of the OC3-Hywind spar platform, including ballast, is 7.46633E+6 kg. The full-system mass of the OC3-Hywind floating wind system (rotor + nacelle + tower + platform/ballast) is 8.066E+6 kg.

Best regards,

Dear Jason
In the third chapter of the report 'Definition of the Floating System for Phase IV of OC3', the mass is defined as:

The mass, including ballast, of the floating platform is 7,466,330 kg. This mass was calculated such that the combined weight of the rotor-nacelle assembly, tower, and platform, plus the weight of the mooring system (not including the small portion resting on the seafloor) in water, balances with the buoyancy (i.e. weight of the displaced fluid) of the undisplaced platform in still water.

Can I think that the full-system mass (rotor + nacelle + tower + platform/ballast+mooring system) is 7.46633E+6 kg rather than 8.066E+6 kg from the definition? How do you get the number 8.066E+6 kg? Could you give me a reference? Thanks a lot!
Best regards

Jason.Jonkman
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Re: Inertia issue

Postby Jason.Jonkman » Sun Jan 14, 2018 12:02 pm

Dear Yu Lei,

As I said before, the 7.46633E+6 kg is the mass only of the floating spar platform (including ballast). The tower, nacelle, and rotor masses are separate. The total full-system mass of 8.066E6 kg is not specifically documented in the OC3-Hywind specifications report, but it can be calculated manually by summing the platform tower, nacelle, and rotor masses.

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


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