MoorDyn static tensions for 15 MW UMaine semisub


My team and I have been designing some chain-poly-chain mooring systems for the UMaine 15 MW semisubmersible platform, and we’re encountering some peculiar behavior in our static simulations. When we are designing mooring systems, we first do a static simulation to check the pretension in the line. We do this by setting CompInflow and CompAero to 0 and inputting a HydroDyn file with no waves. Some of the systems display an acceptable amount of variation, like the one pictured below that has a range of variation of about 20 kN (150 m water depth, taut angle of 8 degrees).

However, some of the simulations have a significant amount of variation which does not appear to fully decay, such as this system, which has a 100 m water depth, taut angle of 15 degrees, and an extremely large standard deviation:

We have also encountered a similar problem in deeper waters, such as this system designed for a water depth of 850 m with a taut angle of 36 degrees. The variation does not damp out.

Here are some of my general findings so far:

  • increasing the pretension by decreasing the unstretched line length causes an increase in the standard deviation
  • Decreasing the taut angle decreases the standard deviation

I have also tried increasing and decreasing line thickness and increasing the discretization (NumSegs), as well as adjusting the anchor point coordinates. None of these seemed to significantly affect the standard deviation of the line.

Do you have any idea what could be causing this variation? Does this behavior indicate a faulty mooring design, or is more a property intrinsic to static simulations? Let me know if you have any questions or would like more information.

Katherine Coughlan

Hi Katherine,
Is the platform oscillating? In other words, have you turned off the platform degrees of freedom in the ElastoDyn input file? If not, I’d expect these are are result of the platform bobbing around due to the system not being perfectly in equilibrium at the start of the simulation. I haven’t seen these sort of ongoing tension fluctuations when the platform is stationary (unless the hydrodynamic drag coefficients weren’t set properly in the MoorDyn).

Hi Matt,

I set all of the degrees of freedom in ElastoDyn to False and that seemed to fix the problem. Thanks for your help!