Free decay analysis and natural frequency

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Purushotham.Chinu
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Location: India

Re: Free decay analysis and natural frequency

Postby Purushotham.Chinu » Fri Mar 10, 2017 6:13 am

Dear jason,

When i mean i changed to 200 m i mean the hydrodyn input for water depth not the default value taken by the fea mooring file, also jason regarding the initial platform displacements i gave about 5m surge with all the rest degrees of freeddom initial displacement zero , also i have changed that to 1m and 2 m and tried with that as well,now is my approach correct because i believe the only two things you need to do is set wavemod to zero and have a constant or steady wind speed, but still seems to be a error, kindly tell me how should i change my input so that i can get the free decay responses.


regards

purushotham

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Fri Mar 10, 2017 6:29 am

Dear Purushotham,

Input WtrDpth in HydroDyn is set to 200 m in the FAST model NREL has provided for the MIT/NREL TLP.

You're understanding of how to set a free-decay response is correct.

If you are still experiencing an error, even under small initial platform displacements, your problem could be caused by the use of the FEAMooring mooring module, and the known sensitivity to FEAmooring inputs Tension and LUnstrLen. See the following forum topic for more information: viewtopic.php?f=13&t=1633&p=7598. As a result of this sensitivity, it may be difficult to set up FAST models using FEAMooring with initial platform displacements. The other mooring modules in FAST (MAP++ and MoorDyn) do not have similar problems, so, you may wish to switch to one of those.

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

Purushotham.Chinu
Posts: 39
Joined: Tue Feb 21, 2017 7:37 am
Organization: IITM
Location: India

Re: Free decay analysis and natural frequency

Postby Purushotham.Chinu » Wed Mar 15, 2017 9:57 am

Dear jason,

i have enclosed my heave free decay response , kindly go through it also jaosn what would be the expected natural frequency in heave period and also what would be the damping ratio, i am getting a response which is not qualitatively consistent with the other free decay results , that is the trend is not same kindly reply on this aspect.
Image

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Wed Mar 15, 2017 10:03 pm

Dear Purushotham,

The natural frequencies of the MIT/NREL TLP in the undisplaced position are documented in Table 6 of Denis Matha's MS thesis-turned NREL report: http://www.nrel.gov/docs/fy10osti/45891.pdf. Here it is reported that the platform-heave natural frequency is 0.4375 Hz, which matches the results you've shown. Your results show a very small level of damping, likely because only hydrodynamic radiation damping is included in the heave direction in this FAST model.

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

Purushotham.Chinu
Posts: 39
Joined: Tue Feb 21, 2017 7:37 am
Organization: IITM
Location: India

Re: Free decay analysis and natural frequency

Postby Purushotham.Chinu » Thu Mar 16, 2017 8:16 am

Dear jason,

How do i include the other forms of damping in this model in fast so that i get a more damped reponse, which is the case with all other degrees of freedom except with that of the heave, i want this information to calculate the damping ratio, which files need to be changed so that i can get a more damped output. kindly reply in this regard.


regards

Jason.Jonkman
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Thu Mar 16, 2017 11:08 am

Dear Purushotham,

For the heave direction, if radiation damping from potential-flow theory is not giving a satisfactory level of damping, you can either add additional linear damping or axial quadratic drag through the strip-theory formulation in HydroDyn. See the draft HydroDyn User's Guide and Theory Manual for more information: 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

hailey.seo
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Joined: Tue Jul 11, 2017 4:30 am
Organization: Newcastle University
Location: Newcastle

Re: Free decay analysis and natural frequency

Postby hailey.seo » Mon Jul 17, 2017 9:24 pm

Dear Jason,

Regarding free decay test for a TLP floating wind turbine, I got surge decay test like the attachment.
I'm wondering if the displacement value is too small because the previous test in the journal indicates the surge starts with -4m to 4m range.
I know it's not enough data and you can feel it's difficult to say what is the exact problem. but, with your relative knowledge, Could you explain the reason why I have very small transient value and it is okay to use for my research?

I would really appreciate if you answer my question.

Jason.Jonkman
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Location: Boulder, CO
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Tue Jul 18, 2017 6:29 am

Dear Hailey,

I'm sorry, but I'm not really sure I understand your question. You can set the initial surge displacement from within FAST in order to simulate a surge free-decay test.

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

hailey.seo
Posts: 4
Joined: Tue Jul 11, 2017 4:30 am
Organization: Newcastle University
Location: Newcastle

Re: Free decay analysis and natural frequency

Postby hailey.seo » Tue Jul 18, 2017 9:14 am

I'm so sorry I confused I put my initial displacement with 4m already. I'm done!
Thank you for your reply.

Mustafa.Vardaroglu
Posts: 18
Joined: Sat Jan 06, 2018 11:45 am
Organization: University of Campania
Location: Italy

Re: Free decay analysis and natural frequency

Postby Mustafa.Vardaroglu » Fri Oct 11, 2019 10:46 am

Dear Jason & Dear All,

1) Is there any reference value for the damping ratio values from free decay analysis of MIT NREL TLP? I've checked Matha (2009) but I can not see.
2) I am checking the hydrodynamic solution method (potential flow or strip theory or both) of the MIT/NREL TLP from the "NRELOffshrBsline5MW_MIT_NREL_TLP_HydroDyn.dat" file in the FAST v8 directory. Default value for PropPot=TRUE. Does this mean that only potential flow solution is considered in the solution?
3) In order to consider viscous drag forces in a free decay analysis, is it sufficient to add "Additional linear stiffness" & "Additional linear damping" values?

Damping ratio that I get from a surge free decay analysis on the MIT NREL TLP is 2.3% On the other hand, surge period is 63 seconds , which is quite close to the reference value, 60.6 sec.

Thanks in advance for any help.

Sincerely,
Mustafa

Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: Free decay analysis and natural frequency

Postby Jason.Jonkman » Fri Oct 11, 2019 11:35 am

Dear Mustafa,

Here are my answers to your questions:

1) I'm not aware of a reference values for the damping ratio of the MIT/NREL TLP in various modes of motion.
2) When PropPot is set to TRUE in HydroDyn (as for the MIT/NREL TLP model), that means that the fluid inertia, added mass, and buoyancy terms will not be calculated for the strip-theory member; instead, these terms will be calculated in the potential-flow solution. When PropPot = TRUE, only the viscous drag term (not accounted for in potential-flow theory) will be calculated for that strip-theory member.
3) You can use the additional linear stiffness and damping if needed, but the viscous terms can be directly calculated through the strip-theory solution.

See the draft HydroDyn User's Guide and Theory Manual for more information: 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


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