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Number of discretization nodes

Dear David,

I’m a PhD student working on a project which has as goal to model the fatigue behaviour of a FOWT. To do this we will be using the QBlade software, which I’m now learning to use.

While working in the substructure file of the OC4 DeepCWind semi-submersible, I noticed that the the value of ElmDsc of the submember does not match the number of nodes visible in the GUI (see the attached pictures). From the documentation I thought that ElmDsc was the number of nodes that would be used to model the member, however when I increase the value of ElmDsc the number of nodes in the GUI decrease (and vice versa).

Am I interpreting the purpose of ElmDsc incorrectly or is there something else going on?

With kind regards,

Victor

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Hello Victor,

the MemDisc (ElmDisc) column indeed controls the discretization of the members. The documentation says:

The member can be subdivided into smaller elements for a more accurate structural and hydrodynamic evaluation. This is done in the MemDisc column; it gives the maximum allowed length of a discrete structural element of the member in [m].

This choice might be a bit unintuitive. However, the main purpose of the subdivision of substructure members is to have a higher reolution for the evaluation of hydrodynamic loads and wave kinematics. t is generally beneficial to have finer discretization near the water’s surface, where hydrodynamic effects are more pronounced, while a coarser discretization can be adequate further away from the surface. In terms of structural dynamics, a finer member discretization does not significantly increase accuracy, as the properties of the member are uniform throughout its length.

The approach of defining discretization as the “maximum allowable member length” offers a straightforward way for users to set a uniform discretization across the substructure. It also simplifies the process of refining discretization near the water surface, if needed.

On the other hand in the MOORMEMBERS table the discretization is indeed defined as the number of element per cable.

I hope this helps to clear things up.

BR,

David

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