Question regarding moordyn coefficients

Wind & Water Computer-Aided Engineering Software Tools
1

Hello to all.

I have a very basic question about MoorDyn that I am embarrassed to ask in the Forum, but I have seen that it can sometimes cause confusion.

The DNV OS E301 standard states that the drag force per unit length is 1/2 rho Cd D v |v| where, for studless chain, Cd_longitudinal = 1.15 and Cd_transverse = 2.4.

What are the formulas for obtaining MoorDyn’s Cd, Ca, CdAx, and CaAx values from the standard values?

Considering that d is the chain link diameter, are these correct?

   Cd = 1.15*d

   Ca = 1

   CdAx = 2.4*d*PI

   CaAx = 0.5

Best regards
Iñaki

2

Hi @Inaki.Zabala ,

The coefficients you input to MoorDyn don’t need to be modified with any diameter or Pi factors, that is all handled internally (example here). The equation internal to MoorDyn is the same as the one you cite above. One thing to be careful of is MoorDyn uses the volumetric diameter, not the nominal diameter of chain

3

Thank you @Ryan.Davies

So from the user point of view, using

Cd = 1.15
Ca = 1
CdAx = 2.4
CaAx = 0.5

is a good start point for a studless chain?

Best regards
Iñaki

4

Hi @Inaki.Zabala ,

That would be a good start, but you need to account for volumetric diameter. We typically use the values in MoorPy as our starting points, with the coefficients have already been adjusted for volumetric diameter (instead of chain nominal diameter): MoorPy/moorpy/MoorProps_default.yaml at dev · NREL/MoorPy · GitHub

5

Hi @Ryan.Davies

So, if Diam is the volume-equivalent diameter of the line (the diameter of a cylinder having the same displacement per unit length), in meters, the equations would be like these?:

CdAx = 1.15*Diam
Cd = 2.4*Diam

Best regards
Iñaki

6

You need to multiply by the ratios of diameters, because MoorDyn internally multiplies the drag coefficient by the volumetric diameter when calculating hydrodynamics. So the inputs to MoorDyn would be like this:

Cd_vol = Cd_nom (D_nom/D_vol)

For studless chain, D_vol/D_nom is 1.8, so from DNV standards 2.4 / 1.8 = 1.33 (which is what we have in MoorPy).

7

Thank you Ryan. Now I understand.

Best regards
Iñaki

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