Platform Additional Stiffness and Damping

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Tomas.Cortes
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Platform Additional Stiffness and Damping

Postby Tomas.Cortes » Thu Jun 06, 2019 9:02 am

Hello everybody,

I am trying to understand the use of the Hydrodyn module's section of the Platform Additional Stiffness and Damping (PASD). For example, I am modeling the OC4 semi-submersible platform using only potential flow theory. In this case, shouldn't all entries in the PASD be equal to zero?

On the other hand, the initial setup of this platform, as when downloaded, comes with two entries [AddCLin(4,4) and AddCLin(5,5)], with values different from zero. Why? Is this due to the ballasted volume? Meaning that only when I have no ballasted volume all entries in the PASD should be zero (as wamit doesn't account for ballasted volume, one would correct that in the hydrodyn file)?

In this case, if I change the ballasted volume I would need to change those two entries?

Best regards,

Tomás

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Re: Platform Additional Stiffness and Damping

Postby Jason.Jonkman » Thu Jun 06, 2019 12:53 pm

Dear Tomás,

Your understanding is correct. The nonzero (4,4) and (5,5) elements of AddCLin in the HydroDyn model of OC4-DeepCwind semisubmersible model are associated with the water ballast. These can be derived using the equations documented in section 6.8.3 of the draft HydroDyn User's Guide and Theory Manual: https://wind.nrel.gov/nwtc/docs/HydroDyn_Manual.pdf. Changing the water ballast should result in a change to AddCLin.

But if you plan to use the water ballasting feature of HydroDyn, please be aware of the recently discovered issue raised in OpenFAST, i.e.: https://github.com/OpenFAST/openfast/issues/245.

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

Tomas.Cortes
Posts: 29
Joined: Fri Mar 15, 2019 7:18 am
Organization: Mr
Location: Portugal

Re: Platform Additional Stiffness and Damping

Postby Tomas.Cortes » Thu Jun 06, 2019 4:53 pm

Dear Jason,

In case I follow the advice you provide in:https://github.com/OpenFAST/openfast/issues/245., I would use for the platform inertia (in elastodyn) the values provided in table 3.3 from https://www.nrel.gov/docs/fy14osti/60601.pdf right? Then, I would disable all ballasted members in section "filled members" of hydrodyn. But I would still need to keep the AddCLin(4,4) and AddCLin(5,5) with their "original/downloaded" values to account for the increased stiffness which is not accounted for in elastodyn, even if I increase the inertias there. Is this correct?

Best Regards,

Tomás

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Re: Platform Additional Stiffness and Damping

Postby Jason.Jonkman » Thu Jun 06, 2019 5:06 pm

Dear Tomás,

Yes, you would use the values specified in Table 3-3 in ElastoDyn and remove the water ballast from HydroDyn. But in this case, you don't need to specify nonzero values for AddCLin(4,4) and (5,5) because ElastoDyn intrinsically accounts for gravitational stiffness in roll and pitch.

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

Tomas.Cortes
Posts: 29
Joined: Fri Mar 15, 2019 7:18 am
Organization: Mr
Location: Portugal

Re: Platform Additional Stiffness and Damping

Postby Tomas.Cortes » Thu Jun 06, 2019 6:12 pm

Thank you very much!

Tomas.Cortes
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Re: Platform Additional Stiffness and Damping

Postby Tomas.Cortes » Thu Jun 06, 2019 6:55 pm

Dear Jason,

I'm sorry but I run three cases (pitch decay test) and they all look different.

1case (blue): semi-submersible as it comes when downloaded

2case (yellow): semi-submersible with no ballasted water (inertias and mass transferred to elastodyn) and all AddCLin entries equal to zero

3case (red): semi-submersible with no ballasted water (inertias and mass transferred to elastodyn) but AddCLin(4,4) and (5,5) entries equal to the value of 1case (1451298897)

Acording to your last post, cases 1) and 2) should be equal right? What's wrong here then?

Best regards,

Tomás
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Jason.Jonkman
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Re: Platform Additional Stiffness and Damping

Postby Jason.Jonkman » Thu Jun 06, 2019 8:35 pm

Dear Tomás,

Yes, I would expect cases (1) and (2) to match. Have you double-checked your inputs? For example, have you set PtfmCMzt = -13.46 m in case 2?

While there is no reason to run case (3), because you would be double counting the gravitational stiffening, I'm also surprised by these results. Why would adding pitch stiffness reduce the natural frequency? Are you sure the inputs are set properly?

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

Tomas.Cortes
Posts: 29
Joined: Fri Mar 15, 2019 7:18 am
Organization: Mr
Location: Portugal

Re: Platform Additional Stiffness and Damping

Postby Tomas.Cortes » Fri Jun 07, 2019 5:54 am

Dear Jason,

I've re-run the cases, correcting the PtfmCMzt = -13.46 m (which I had forgotten), and everything matches: case (1) and (2) are equal and case (3) has a higher natural frequency, as would be expected.

Thanks for the debugging,

Tomás

Jason.Jonkman
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Re: Platform Additional Stiffness and Damping

Postby Jason.Jonkman » Fri Jun 07, 2019 6:35 am

Great!
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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