Selection of appropriate drag coefficients in MoorDyn.

[darkstar71]

Hi,
I am running coupled simulations in OpenFAST v5 using available input file in https://github.com/OpenFAST/r-test/tree/main/glue-codes/openfast/5MW_OC4Semi_WSt_WavesWN. I was wondering someone helps me

1. to select proper drag coefficient for the mooring chain, as I noticed, line tensions varies substantially for different Cd values. What could be an ideal procedure to select Cd (both transverse and axial) values for any real project.

2. Is there any published report which could be used to verify the simulation results obtained from the OC4 model available through the link above? This could help to boost my confidence on OpenFAST simulations.

Best regards,

[jjonkman]

Dear @darkstar71,

Regarding recommend drag coefficients, see a similar question asked and answered on our forum: https://forums.nlr.gov/t/modeling-polyester-rope-in-moordyn/3636. Perhaps @RyanDavies19 or @mattEhall have other guidance.

The NLR-computed baselines for each r-test case are included, along with the input files, e.g., 5MW_OC4Semi_WSt_WavesWN.outb for the case in question.

Best regards,

[darkstar71]

Many thanks for your reply, Dr. @jjonkman.

What I find confusing is that Table 5-1 in this report https://docs.nlr.gov/docs/fy14osti/60601.pdf specifies Cd=1.1 and Ca=1.0, while the r-test MoorDyn file available here https://github.com/OpenFAST/r-test/tree/main/glue-codes/openfast/5MW_OC4Semi_WSt_WavesWN uses Cd=2.0 and Ca=0.8. DNV OS-E301, on the other hand, gives Cd=2.4 and Ca=1.15 for stud less chain.

I would appreciate any clarification on whether these differences are due to different definitions, assumptions, or modelling approaches.

If you or your colleagues have any insights from tank testing or other validation work regarding appropriate Cd values for use in MoorDyn, I would be very grateful.

Best regards,

[mattEhall]

Hi @darkstar71,

Here are a couple things to keep in mind for this topic:

• When it comes to comparing drag coefficients between models or standards, it’s important to convert between the equations and factors used by each (e.g. OrcaFlex uses a drag diameter that can be set separately by the user, MD always uses volume-equivalent diameter, and standards often use nominal diameter). These are generally stated or documented, so it's a matter of looking them up then doing a bit of math to convert to the equivalent form for what you're comparing.

• Drag coefficients are empirical and depend on the situation. A scaled basin test might have quite different values than something full scale because of the different Reynolds number. If in doubt, the standards are a good default. We also have pretty good defaults used in the file export function in MoorPy (see https://github.com/NatLabRockies/MoorPy/blob/main/moorpy/library/MoorProps_default.yaml).

The examples you cited have different purposes -- one is a reference design definition, another is a software consistency check -- so it's fine that they differ, and they may not all be realistic. If you're looking for quick answers rather than digging deeper into this topic yourself, my suggestion would be the MoorPy defaults that I linked above.

Cheers,
Matt