Subdyn - Cable properties - T0 (Pretension)

I wanted to better understand what happens when I specify a specific T0 in the CABLE PROPERTIES section.

My idea was that, setting T0 to the value X, will give an internal tension in the “pretension cable” member equal to X.
To monitor the internal tension in this member, I specificed in MEMBER OUTPUT LIST the Member ID of the pretension cable member 26, NOutCnt 2, and NodeCnt 1 2.
Then I have two questions:

1- When I output M1N1FKze, i.e. the static (elastic) component of the reaction force at M1N1 along the local coordinate system according to AppendixD , I would expect that (at least at time=0 s) this would be equal to the T0=X value that I imposed, since elastic component of the reaction force = pretension? What I obtain is a value 50% lower than T0. The cable is not vertical, but not sure this is important since it is local coordinates

2 - When outputting M1N1FKze and M1N2FKZe, I can see that they are identical, but I would expect them to be different, since one is at one height and the other at another height, so the weight force should be added to the internal tension, with a max value at the top and a min value at the bottom.

Thanks a lot for the help


Dear @Maurizio.Collu,

The joints and members of SubDyn are defined assuming the structure is unstrained in the absence of applied loading (including from the supported wind turbine, self weight, and hydrodynamics). Adding a nonzero cable pretension (T0) influences the cable stiffness and adds a preload (see the cable theory documented on OpenFAST readthedocs here: SubDyn Theory — OpenFAST v3.5.0 documentation), which combines with other applied loads. You are not necessarily going to get the specified pretension back directly because other members of the structure will likely be compressed by the pretensioned cable, and resist deformation of the structure caused by the pretension.

To answer your direct questions:

  1. If you have the static improvement method enabled (SttcSolve = True), then the static modes of the structure will deform instantaneously based on the applied loads, including from the pretensioned cable elements, and the dynamic (Craig-Bampton) modes will take some time to deform (influenced by inertia).
  2. I would expect the self weight to influence the nodal displacements (differently, depending on static and dynamic modes as mentioned in (1)), but the weight is not directly used in the nodal outputs ( SubDyn Theory — OpenFAST v3.5.0 documentation); rather, the nodal stiffness load outputs are calculated based on multiplication of the nodal displacements by the stiffness matrix.

Best regards,

Dear @Jason.Jonkman ,
thanks a lot for the quick and clear answer.

Below something building up on what has been discussed.

I tried to put SttcSolve = False, and the M5N1FKze is now much closer to the pretension value T0 imposed.

In here it is explained that:

To account for the effects of static gravity (member self-weight) and buoyancy forces, one would have to include all of the structural axial modes in the C-B reduction…

So, if SttcSolve = False, and CBMod = True, then it means that the simulation is not taking into account the buoyancy/self-weight, since with SttcSolve = False to take into account static gravity and buoyancy one would need all the modes (at least the axial ones),and therefore that is the reason why T0 is much closer to the value imposed?

This would also mean that having SttcSolve = False and CBMod = True is not a correct approach, since would not take into account self-weight and buoyancy?

Best regards

Dear @Maurizio.Collu,

Effectively, when CBMod = True, SubDyn computes the full eigensolution based on all of finite-element degrees of freedom of the model. You as the user choose how many of these modes you want to use as Craig-Bampton modes (with dynamics/inertia, typically those with low frequency that can get excited by external loading and through the supported wind turbine). The remaining modes are treated as static modes only when SttcSolve = True; otherwise, these remaining modes are ignored. The remaining modes tend to be what would have been high-frequency modes, such as axial modes, which is why ignoring them often eliminates the effect of gravity and buoyancy. Ignoring the modes will result in unphysical behavior. So, I would generally recommend always using SttcSolve = True when CBMod = True. Moreover, without the Craig-Bampton reduction, SubDyn models often have thousands of degrees of freedom with many high-frequencies that would reduce the necessary time step size, so, I would generally recommend always using CBMod = True, which then implies always using SttcSolve = True.

Best regards,

Dear @Jason.Jonkman ,

thanks for the crystal clear explanation!

Best regards,