Modeling polyester rope in MoorDyn

Wind & Water Computer-Aided Engineering Software Tools
1

Hi everyone,

Thank you in advance for the guidance. I am trying to model a polyester rope in MoorDyn v1, and I am very confused about how to populate the Line Types section. The rope I need to model comes from the DeepRope-brand catalog, which provides the following properties:

  • Specific gravity = 1.38
    • uses rhoSeawater = 1050 kg/m^3
  • Diameter = 185 mm
  • Total weight = 21.8 kg/m
    • in air
    • this is the true mass-per-length of the line
  • Submerged weight = 5.22 kg/m
    • in seawater
    • these units are confusing to me, but I assume this is the submerged weight of the line divided by gravity. Therefore, this value takes into account buoyancy, but the line still has positive weight underwater, meaning it does not float.
  • Stiffness = 2.16e5 kN

I am most confused about the relationship between the volume-equivalent diameter (Diam) and the mass-per-length (MassDen) in the Line Types properties. I assume MassDen should be the dry weight of the line, 21.8 kg/m, based on this post. But what should Diam be?

If I use 185 mm from the catalog, then calculating the specific gravity of the line material yields 0.77 (this comes from s.g. = MassDen/CrossArea/rhoSeawater), meaning the line would float, which is not correct. Therefore, water must be infiltrating the line when it is submerged, and its actual cross-sectional area must be less than pi/4×(185mm)^2. To account for this, I could take the given s.g. and then back-calculate the area, yielding a Diam of 138 mm. Is that the correct method for deriving Diam and MassDen from these given properties of the polyester rope? Is it correct to use specific gravity as the driving property here and keep MassDen equal to the total weight value? Should I be using the catalog’s submerged weight value at all?

For EA, I can use the catalog value, and for BA/-zeta, I will assume -1, critical damping. For the last four properties, I am not sure what to do. How should I calculate the transverse and tangential added mass and drag coefficients? The MoorDyn v1 manual does not explain what these values are or how to find them from the information I have.

Thank you again,
Doron T Rose

2

Dear @Doron.Rose,

Regarding the mass density in MoorDyn, the buoyancy of the line is calculated as rhoSeawater*Gravity*pi*Diam^2/4. So the submerged weight in water is related to the weight in air by:

WeightInWater = WeightInAir - rhoSeawater*Gravity*pi*Diam^2/4

In your case, it sounds like MassDen = WeightInAir/Gravity = 21.8 kg/m and the WeightInWater/Gravity = 5.22 kg/m, so, you can derive Diam from that.

The added mass and drag coefficients specified in MoorDyn are documented in the MoorDyn User’s Guide (http://www.matt-hall.ca/files/MoorDyn%20Users%20Guide%202017-08-16.pdf), but I’m not sure what the recommended values are. Sounds like a better question for @Matthew.Hall.

Best regards,

3

Hi Dr. Jonkman,

This makes a submerged-ton of sense. Thank you! This is definitely a more logical and appropriate way to use the catalog information to calculate line properties.

And sounds good. I will keep researching the coefficients, and any guidance Dr. Hall can provide would be greatly appreciated.

Best,
Doron

4

Hi Doron,
The paper linked below describes how the hydrodynamic coefficients are used by MoorDyn to calcluate drag and added mass. For modeling a rope, the simplest approach would be to neglect the tangential/axial drag and added mass (i.e. set the coefficients to zero). The normal/transverse drag and added mass equations are what you would expect from the Morison equation: the added mass coefficient gets multiplied by the line’s volume and the drag coefficient gets multiplied by the line’s side area (i.e. diameter time length). Suggestions for these coefficients can be found in various places including some standards documents. Ca=1 and Cd=1.5 are reasonable examples, but of course a precise value depends on the material, the scale, etc.

Best,
Matt

1 Like