OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests

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Bertrand.Canetti
Posts: 9
Joined: Thu Oct 31, 2019 6:35 pm
Organization: University College of Cork
Location: Ireland

OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests

Postby Bertrand.Canetti » Sat Nov 02, 2019 5:34 pm

Hello everyone,

I am trying to reproduce with FAST v8 the tests from the OC4 DeepCwind semisubmersible floating wind turbine, as presented in the "Offshore Code Comparison Collaboration Continuation Within IEA Wind Task 30: Phase II Results Regarding a Floating Semisubmersible Wind System" here attached, load cases page 5.

As you will see, I am confused, even after reading the FAST user guide, and looking for answers on the forum. My questions may be trivial for you, sorry about that, but I hope it will be trivial for me soon as well.

I know the questions are numerous, maybe I should make different posts, but I know you cannot necessarily answer to all of them, as it takes some time. I am sorry for the inconvenience.

1. The sole purpose of Eigenanalysis test (load case 1.1) is to find all the natural frequencies (motions, structural) at once, is that right? Is there a simple example I could find to understand easily how to do it? If not, where in the simulation results can I read those natural frequencies? Do I have to use the "eianalysis.m" Matlab file found in the installation directory? Do i have to activate the Linearization (just Linearize = True, ?) ? Could we also do that with Free Decay tests (load cases 1.3.X): FD for surge will tell us the natural frequency for surge, heave for heave, etc. ? Should the system stabilize at a steady position in this load case that has no load but gravity and buoyancy?

2. The sole purpose of the Static Equilibrium test (load case 1.2) is to find the static equilibrium position of the system (just asking the obvious to be sure there is no other use)? Could we also do that with Free Decay tests (load cases 1.3.X): FD for surge will tell us the static equilibrium surge position when stabilized, heave for heave, etc. ?

3. Eigenanalysis (load case 1.1) and Static Equilibrium (load case 1.2) both work without wind (CompAero = 0, CompInflow = 0, right?) nor waves (just WaveMod = 0, right?). For those 2 load cases, are all DOFs really enabled as written in the table page 5 (platform, blade flapwise/edgewise modes, drivetrain, generator, yawing system, tower fore-aft/side-to-side modes)? What about the initial conditions? Should they all be equal to 0 for those 2 tests, since we can know the static equilibrium position only after the equilibrium load case 1.2?

4. Then, what is the difference between those 2 tests if they share the same simulation parameters?

5. Concerning the natural damping of the system, in which tests do you compute it (Eigenanaysis, Static Equilibrium, Free Decay?), and how to you obtain it exactly?

6. I have performed Free Decay tests for surge, heave, pitch and yaw, to find their natural frequencies and their equilibrium position. I have found:
- 0.021 PtfmSurge (inital displacement: 15m)
- 0.009 PtfmHeave (initial displacement: 5m)
- 0.08 PtfmPitch (initial displacement: 5°)
- 0 PtfmYaw (initial displacement: 5°)
Does it seem correct, or not at all?


Thank you, sorry again for the length, but it would help me a lot.

Have a nice day,

Bertrand
Attachments
NREL - Offshore Code Comparison (2014).pdf
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Jason.Jonkman
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Joined: Thu Nov 03, 2005 4:38 pm
Location: Boulder, CO
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Re: OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests

Postby Jason.Jonkman » Mon Nov 04, 2019 1:34 pm

Dear Bertrand,

Here are my answers to your questions:

1. A similar question was asked and answered in the following forum topic: viewtopic.php?f=4&t=1953&p=12075. The new linearization functionality for floating offshore wind turbines mentioned in the post dated Nov 02, 2018 has now been submitted as a pull request into OpenFAST--see: https://github.com/OpenFAST/openfast/pull/350.

2. Correct, but there is only one static-equilibrium position in the absence of external loading from wind and waves, so, only one simulation is required.

3. Correct. It is always best to perform an eigenanalysis about an equilibrium position, so, load case 1.2 is really a prerequisite of load case 1.1.

4. The difference is regarding which outputs are being reported for the code-to-code comparisons.

5. The linearized natural damping can be derived directly from the linearization analysis / eigenanalysis. Free-decay tests are often used to derive the linear and quadratic (viscous) damping (via the so-called p-q analysis of the free-decay time series response).

6. No, these natural frequencies do not look correct (assuming you are reporting values in Hz). The natural frequencies of the OC4-DeepCwind semisubmersible are reported in Figure 3 of the paper you attached.

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

Bertrand.Canetti
Posts: 9
Joined: Thu Oct 31, 2019 6:35 pm
Organization: University College of Cork
Location: Ireland

Re: OC4 DeepCWind Eigenanalysis/Static Equilibrium/Free Decay tests

Postby Bertrand.Canetti » Thu Nov 07, 2019 10:37 am

Dear Dr. Jonkman,

Than you very much for the accurate answers. Also, for the question 6, the results shown were not natural frequencies but the equilibrium position, though I have to say my question was quite unclear... :roll:

Thank you again!


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