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

Dear Nicolás,

Here are my responses:

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.

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,

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,

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