For TLP in Still Conditions, Pretension not matching with output tension

[Jay.Kaas]

Dear Jonkman,

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

[Jason.Jonkman]

Dear Vijay,

FEAMooring was developed by Texas A&M (led by Yoon Hyeok Bae), and there is no internal expertise at NREL, so, it is difficult to offer support for FEAMooring. I don't believe Yoon checks this forum either. I suggest reaching out to him directly; his contact information is on the FEAMooring website: https://nwtc.nrel.gov/FEAMooring.

From my discussions with Yoon, my understanding is that FEAMooring input parameters Tension and LUnstrLen are tightly coordinated, and that input parameter LUnstrLen should not actually be the unstretched length as implied by its name, but should be the stretched length associated with the given pretension defined by input parameter Tension. This would tell me that one could specify LUnstrLen equal to the unstretched length and Tension equal to zero, but we have found that FEAMooring does not run with Tension set to zero (although we have some success with LUnstrLen set equal to the unstretched length and Tension set equal to a value much smaller than the mean). I've never received a clear answer from Yoon as to why.

From my quick look at your FEAMooring input file (which I have not tried to run), it looks like you've specified LUnstrLen as the stretched length and Tension equal to the expected mean tension, which matches my understanding and is also how we set up Test23 from the FAST CertTest. You say that the initial tension output too high; but is the mean tension correct after the initial start-up transients die out?

Best regards,

[Jay.Kaas]

Dear Jason,

Thanks for the reply. I really appreciate your support.

How i worked out:

Draft= 45m
initial length (l) = waterdepth - draft = 105 m
F = 6868kN/tether
EA = 4.04x10^7xd^2 kN, d= tether dia
stiffness (k)=EA= F/defl = F/(dl/l)
dl=2.79 m

I have followed the write-up given in page 19 of https://nwtc.nrel.gov/system/files/2014-12-FEAM_Final_report.pdf

So i tried by varying, LUnstrLen=105+2.79=107.79 m with Tension=6868.0E3 N
I actually kinda tried different combinations to see, if the tension is coming down, It remained high for all combinations.

For still conditions, Why are we observing variations in results (for example, take heave response shown in attachment Figure 1)
The Fair1 Tension observed is shown in Figure 2.

The mean tension is not correct after the initial transients die out.

Doubt 1: Could you please suggest ways to mitigate this? Is there any other alternative that we can work on for better understanding? Should i try with other module? (say MoorDyn or OrcaFlex)
Doubt 2: How well can i rely on this output (be it motions) if the tension what we observe is of three folds?
Doubt 3: Does inclusion of any members in HydroDyn contributes to net buoyancy effects by adding to pretension value for net upward forces? or it is only to take the effect of drag forces and inertia forces.?

Could you please correct me if I'm wrong anywhere in understanding?

[Jason.Jonkman]

Dear Jay,

As I said in my prior post, LUnstrLen is the stretched length corresponding to the Tension. If you expect a pretension of 6868 kN for the undisplaced platform, then this stretched length should be 105 m for your system; using math similar to what you've described, you could calculate the actual unstretched length required (based on a pretension of zero) and the stretch due to the pretension to achieve a stretched length of 105 m.

NREL has much more experience with MoorDyn and is the mooring module we typically use for mooring dynamics.

Indeed HydroDyn calculates buoyancy forces in addition to drag and inertia forces. See section 6.8.1 of the draft HydroDyn User's Guide and Theory for guidance on how to set-up a floating system model to be equilibrium in the heave direction.

Best regards,

[Jay.Kaas]

Dear Jason,

Thanks for your replies. I made a mess in the understanding. My doubts are cleared. We really appreciate your support.

Thanks