AQWA Tether

Hi everyone, I am really new to this forum. I am a M.Sc. student from Greece and I would like your help regarding the Tether module of the ANSYS AQWA Suite.

What I'm trying to model is a TLP, coupled with its tendons. I would like to include the mass of the tendons in my analysis and compare the results with an uncoupled time domain analysis I have already performed. In order to achieve this, I have to use AQWA Naut along with the AQWA Tether module.

However, I am encountering problems regarding the definition of the tether elements (TELM) for the TLP.

To begin with, I am modeling the tendons as installed tethers. I define several nodes (4-5 per tether) that are connected by the tether elements. Reading the AQWA Tether manual and the installed TLP tethers example, I understand that the first node (Anchor) and the last node (Vessel attachment) must not be included in the description of the tether elements. These nodes should be only included in the TETH card, which finalizes the definition of each Tether. Please verify the above.

So I start by defining the first tether element with its first node close to the seabed but slightly elevated from the seabed (i.e. if the depth is 450m, my first node is at 449.9m) and the last node very close to the hull of the TLP but again, not on the hull (if, for example, the hull's draft is 35m, the last tether node is defined at 35.1m depth). To finalize the description of the tether element, I insert the TETH option that requires the input of the anchor and vessel attachment points. In this, I define the two required nodes (that I have not previously included anywhere in the TELM records): the first one on the structure and the second one on the seabed.

Apart from the TELM records, I also define the TCAP option equal to zero as well as the TLOW option (that is, the lower stop of the tether below the anchor - I still haven't figured out what this is supposed to mean) with a nominal large value, similar to the relevant entry in the AQWA Tether manual example.

By running the simulation and examining the results, I have two problems:
1. The unstretched length of tether and the initial distance between anchor/attachment have a very big difference. By experimenting with the input of the tether elements, I have figured out that the unstreched length of the tether is calculated by adding all the lengths of the separate tether elements. However, by not including the anchor and vessel end of the tether nodes, this results in a "shorter" total tether, which produces the very big difference between the above mentioned two values. So, I would really appreciate your input regarding the proper definition of the tether elements.

2. Regarding the pretension of the tendons: When I input the mass of the structure and run the Diffraction analysis (LINE), I get a warning regarding a significant difference between mesh calculated mass (that is, the mass of the structure calculated according to its displacement - buoyancy equilibrium) with the mass I have manually input. That is natural because the static equilibrium of the TLP is described by the equation: Mass + Tendons' Pretension = Buoyancy. However, the AQWA Naut seems to fail to properly include the tendons' pretension in the analysis. Again, reading the manual, I understand that the pretension can only be modeled by the starting position of the structure, i.e. set the structure slightly higher in the vertical axis in order to cause a stretch in the tendons and the corresponding initial tension - pretension. Again, please verify the above or make some suggestions regarding points I might be omitting.

Thank you very much guys, I hope we will figure this out.

SUMMARY of the above post:

1) Do you include the first (anchor) and last (vessel) node in the TELM data record input?
2) How do you model pretension in installed tethers?

 

You really need to make a summary here.

 

Edited.