MoorDyn:A combination of steel cable and polyester rope, errors occur in the mooring tension results

[Mandy428]

Dear all,

I am currently modeling a taut mooring system in OpenFAST/MoorDyn using a combined steel wire–polyester rope configuration. However, I encountered a problem with the simulation results.

Under all environmental conditions I tested, the tensions of mooring lines 1, 2, and 3 are very similar, with only small differences between them. In my understanding, this does not seem physically reasonable. Since lines 2 and 3 are on the windward side while line 1 is on the leeward side, I would expect the tensions in lines 2 and 3 to be significantly larger than that in line 1.

I am wondering whether this issue could be related to:

• an error in my MoorDyn/OpenFAST setup,
• incorrect initial equilibrium or pretension,
• inappropriate stiffness settings for the polyester segment,
• or some other modeling mistake.

This issue has been troubling me for quite a long time, and I would sincerely appreciate any advice or suggestions. I am truly looking forward to your reply. Thank you very much for your time and help.

[Mandy428]

I would also like to add that “main” represents the steel line, while “pet” represents the polyester rope. Thank you very much for your time and assistance.

[jjonkman]

Dear @Mandy428,

For a taut mooring system, small displacements can results in large changes to mooring tension, all lines may be under high tension, and it may be hard to compare tensions along lines that are not aligned. Have you reviewed the individual vector components (X,Y,Z) of the tension and compared there average values across the 3 lines to the applied wind and wave loads (probably easiest to compare at the fairlead rather than the anchor)?

Best regards,