MoorDyn AnchTen vs FairTen problem for segmented lines

Hello,

I’m having a problem in MoorDyn when I try to make chain-poly-chain taut systems. For a 3-point mooring system, when I try to have more than 3 lines defined under LINE PROPERTIES, I’m getting normal fairlead tensions (around 660 kN) but extremely small anchor tensions (around 0.15 kN). When I do the same design but with just 3 mooring lines, the fairlead and anchor tensions are extremely similar, as would be expected. I’ve tried the following methods to remedy this problem, but nothing has worked so far:

  • using all the same mooring material (all poly, no chain)
  • having 2 line segments instead of 3
  • decreasing the UnstrLen of the largest line segment

I’ve attached a picture of my MoorDyn file for an all polyester system with 3 segments per line. Do you see any errors in the MoorDyn file or can you suggest what might be causing this problem?

Thanks,
Katherine Coughlan

Hi Katherine,

I’m happy to help. From my understanding, you have a 3-point mooring system, each with a chain-polyester rope-chain configuration, and the outputs for the fairlead tensions look good, but the outputs for the anchor tensions are very small.

The problem looks like it is due to your output channel list:

  • MoorDyn outputs for “FairTen#” and “AnchTen#” expect the number (#) to be a line number.
  • Your MoorDyn file is outputting the fairlead tensions for lines 1-3 and the anchor tensions for lines 1-3.
  • Lines 1-3 look to be the top portion of your “moorings” with a length of 33.2, which connect to the fairlead and an upper intermediate Connect point.
  • FairTen outputs the tension of the upper node of that line and AnchTen outputs the tension of the lower node of that line.
  • Therefore, your FairTen outputs are correct, as they are outputting the fairlead tension of the whole “mooring”, but by only using the data from Lines 1-3. Your AnchTen outputs are very small because MoorDyn does not recognize Lines 1-3 as lines that attach to an anchor (or “fixed”) point.
  • The following fix should be able to solve your problem:

---------OUTPUTS----------
FairTen1
FairTen2
FairTen3
AnchTen7
AnchTen8
AnchTen9

It then makes sense that your test of a 3-point mooring system with a single-line type configuration outputs correct fairlead and anchor tensions, because each line in the system (1-3) attaches to the fairlead and the anchor.

Hope that helps!

Stein

Ah that makes sense. Thanks so much for clarifying!