Taut mooring line slack

Wind & Water General
1

Dear all,
I am working on a TLP design and I have a question about taut mooring slack.
I am a bit confused about when to consider the mooring to be getting slack.

  1. Referring to reports NREL/CP-5000-57615 and NREL/CP-5000-58076, the mooring lines/tendons are considered slack when the tension is negative.

  2. From DNV-ST-0119 (Floating wind turbine structures):
    Tdesign = Gamma1Tmean + Gamma2Tdynamic >0, otherwise, the tendons are slack

  3. One of my colleagues is suggesting using 20% of the pretension as the limit for slack, for example when the tension drops below 20% of the pretension, the line is slack. He remembers reading it somewhere (probably in a DNV regulation) but he forgot which one it is.

Points 1 and 2 look similar, I wonder if there is any post-processing to do with OpenFast’s outputs (Moordyn, map++, FEAM), or if I can just check if my values are positive or not.

I am not sure about point 3, has anyone seen it before somewhere or it is just a safety limit? (or something else unrelated to Slack)

To clarify, the tensions in the TLP design remain positive and far away from 0 in Hs50, but two points in a 3-hour simulation are a bit less than 20% of the pretension. So I wonder if these cases are considered slack or if it is completely fine.

Thank you in advance for your help and all clarification.
Best Regards.
Mohamad Hmedi

2

Hi Mohamad,

I think there may be multiple definitions of what “slack” means. For designing a TLP, it is probably most useful to just determine a minimum tension that you want your design to always stay above. I have very little experience with TLPs, but I expect that the minimum tension (as a percentage of the pretension) should be chosen depending on the strength characteristics of the tendon material and/or the risk to the structure from snap loads. Someone with experience designing TLPs could give a much better answer.
Best,
Matt

3

Dear Matthew,
Thank you for your reply and clarifications.

I wonder if you know any documentation (or a colleague to tag him here) that can help me to define the minimum tension according to the tendon’s strength (or pretension). I couldn’t find a clear answer in the DNVs.

In the DeepCwind testing campaign, NREL used negative tensions to identify slack cases. Would that be roughly a general rule or it is obligatory to find out a minimum tension function of the material strength?

Thank you in advance for your help, I really appreciate it.
Best Regards.
Mohamad

4

Hi @Mohamad.Hmedi ,
not an expert in TLPs, but if you have extremely varying tensions in your tendons, wouldn’t that become a fatigue problem, before becoming a slack problem?
Maybe that 20% comes from fatigue considerations?