Current-effect analysis: viscous-drag coefficients for strip theory members

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Deza.Nicolas
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Current-effect analysis: viscous-drag coefficients for strip theory members

Postby Deza.Nicolas » Mon Jun 08, 2020 7:57 am

Good afternoon,



I am analysing the effect of the current on a semisub, by adding strip-theory members with viscous-drag coefficients.



This semisub includes four vertical cylindrical columns and a flat horizontal pontoon consisting of four rectangular parallelepipeds joining the bases of the columns.



I'd like to know if there are any reference drag coefficients I can use for cylinders and parallelepipeds. On the other side, I am not sure how to model the pontoon:
- A vertical parallelepiped with a height equal to the thickness of the pontoon, or
- Four horizontal parallelepipeds
... or perhaps another model is more appropiate.



Finally, I am not sure if it would be wise to add any other coefficients to the members for the analysis. I think the viscous-drag coefficients are sufficient, but I may be wrong.



Many thanks in advance for any help,
Best regards,
Nicolás Deza

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Re: Current-effect analysis: viscous-drag coefficients for strip theory members

Postby Jason.Jonkman » Mon Jun 08, 2020 11:16 am

Dear Nicolás,

Here are my responses:
I'd like to know if there are any reference drag coefficients I can use for cylinders and parallelepipeds. On the other side, I am not sure how to model the pontoon:
- A vertical parallelepiped with a height equal to the thickness of the pontoon, or
- Four horizontal parallelepipeds
... or perhaps another model is more appropiate.

You can probably find general guidance on setting drag coefficients in the literature or floating platform guidelines, but from my experience, any given floating substructure likely needs substructure-specific drag coefficients derived from tank tasks or high-fidelity simulations.

Finally, I am not sure if it would be wise to add any other coefficients to the members for the analysis. I think the viscous-drag coefficients are sufficient, but I may be wrong.

Is this a hybrid model with potential-flow and viscous effects from strip theory, or are you modeling the whole semisubmersible with strip theory onlyh? If this is a hybrid model and the entire substructure is represented in the potential-flow solution, than the strip-theory model need only include the viscous effects. Otherwise, the strip-theory model should include fluid-inertia, added mass, and buoyancy terms, with appropriate added mass and dynamic pressure coefficients.

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

Deza.Nicolas
Posts: 8
Joined: Thu Jun 04, 2020 3:42 am
Organization: SENER Ingenieria y Sistemas
Location: Getxo, Bizkaia, Spain

Re: Current-effect analysis: viscous-drag coefficients for strip theory members

Postby Deza.Nicolas » Tue Jun 09, 2020 1:39 am

Thank you for your prompt reply Dr. Jonkman.

I use a hybrid model for the platform with potential flow and viscous-effects from strip-theory members, so I will only include the viscous-drag coefficients for the strip-theory members.

Best regards,

Deza.Nicolas
Posts: 8
Joined: Thu Jun 04, 2020 3:42 am
Organization: SENER Ingenieria y Sistemas
Location: Getxo, Bizkaia, Spain

Re: Current-effect analysis: viscous-drag coefficients for strip theory members

Postby Deza.Nicolas » Wed Jun 10, 2020 1:42 am

Following your advice, I will compare the OpenFAST results with drag coeffcients taken from DNV-RP-C205 Recomended Practice on Environmental Conditions and Environmental Loads, with results from CFD simulations. In case of differences, I will tune the drag coefficients accordingly.

Best regards


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