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Mooring lines' pre-tension

Hello David,

I’m now focusing on the tower base shear force of the structure, when the code started to run, I found there was a sudden-rise shear force as Graph 1 shows ( the first peak of the curve).

I assumed that was because, at the beginning of the simulation, the mooring lines were loose, a few seconds later it was suddenly fastened, then caused the high and unusual shear force.

I want to add pre-tension to the mooring lines to validate my assumption, how can I do this to make the mooring lines tighten at the beginning?

BR,

Easey

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Hello Easey,

in dynamic simulations, it’s common to encounter transient effects initially, which may cause atypical loads. To address this, the initial period of the simulation, often marked by these transients, is generally excluded from data recording. For instance, recording might start only after 100 seconds, ensuring the transient effects don’t impact the simulation data.

Regarding mooring lines in the simulation: Initially, the length of a mooring line is set to the straight-line distance between its start and end points. During the ramp-up phase, this length is gradually modified—either increased or decreased—to match the specified length in the mooring line table. This approach typically allows mooring lines to quickly reach their equilibrium shape.

If you want to increase/decrease the tension in a mooring line you can do that by simply changing its length. Directly defining a pretension is not possible as the line tension is always a result of the mooring line dynamics, the seastate and the dynamics of the bodies attached to it.

Let me know if you have any further questions.

BR,

David