I m working on a TLP structure using FAST_v8.15.00a-bjj. My doubt goes as follows.

Could you Please Explain

Why the initial tether tension are not matching with the output tether tension when WaveMod=0 (still water), CompInflow=0 (no windflow), CompAero=0, CompServo=0, RotSpeed=0 (no rotation) ??

I'm getting the tensions three folds than what is given in inputs. I'm using FEAMooring. I just ran for 300 seconds.

How I worked out is given below

For a TLP, we know the (nascelle+hub+pltfm+tower)=pretension. in simple terms, for static equilibrium, downward force=upward force. I know the pretension of tethers (say 6868kN/tether), I have 4 tethers in total. From this I calculated the ptfm_mass. Modelling is carried out considering TLP in still water. By modelling in software and then using WAMIT, i know the submerged volume (from .out file or by geometry calcs, i prefer the earlier one) which i input for PtfmVol0 in hydrodyn. Similary, .1,.3,.hst are input in appropriate locations for FAST.

For axial stiffness of tethers, i have referred the following assuming a wire dia (say 8cm)

https://www.orcina.com/SoftwareProducts/OrcaFlex/Documentation/Help/Content/html/RopeWire,AxialandBendingStiffness.htm

The remaining inputs are calculated and changed appropriately in FEAMooring.dat

My TLP water depth is 150m with TLP draft 45m.

As my draft is 45m, LUnstrLen=105m with Tension=6868.0E3 (N)

I'm attaching the input files for your reference.

Could you please explain where I miss out in the understanding and why FAST is giving very high tether tension for still conditions.?

Many Thanks

Vijay