General Mechanical

General Mechanical

Boundary conditions of a floating body (Static Structural)

    • floatingstones


      I'm doing the Static Structural analysis of a floating body. I performed load mapping from Aqwa to apply hydrodynamic pressures on a body and obtain stresses.

      From what I've learned, there is no explicit boundary condition for restraining the body. Rather, Weak Springs are turned on in the Analysis Settings.

      I understand that Ansys applies weak springs in strong spots of the body to ease rigid body motion. However, it does not fully prevent rigid body motion. For example, I get total deformations of 0.5 m up to 4 m, depending on the wave. Deformations are composed by the body deformation + rigid body motion. I see this motion from the animation of the results.

      My questions are:

      1) Does this present a problem? That is, should a proper static structural analysis of a floating body present ZERO rigid body motion, or a small amount is acceptable? If so, what is "acceptable"?

      2) Is there a way where I can subtract the rigid body motion from the total deformation in order to only know structural deformations?

      Any insights on this would be super helpful!

    • Erik Kostson
      Ansys Employee
      Never looked into this (generally though one would want to have small reactions on the springs ansys adds)
      Perhaps has some idea on how to do that.

    • Mike Pettit
      Ansys Employee

      In the pressure mapping process we calculate and apply acceleration components that should balance the resultant forces on a structure due to the mapped hydrodynamic pressures. (These are basically the acceleration RAOs, applied to the structural reference frame.) This is why it is important for the total mass/inertia/CoG to be consistent between the hydrodynamic and structural models.
      Ideally these accelerations would exactly balance the hydrodynamic pressures. However, naturally there is some numerical imprecision in the load mapping process, mostly because the hydrodynamic and structural meshes are not identical. Therefore we end up with some unbalanced rigid body acceleration which we need to remove using Weak Springs.
      The forces on the Weak Springs boundary condition can be checked by right-clicking on Solution, then Insert > Probe > Force Reaction. The magnitude of the total spring reaction should be 'small' (maybe a few tenths of a percent) relative to the hydrodynamic forces acting on the structure and/or the weight of the structure.
      Weak Springs should allow the Static Structural system to solve when it would otherwise complain about unconstrained motions. To cancel out the rigid body motion completely, you can also add Inertia Relief (under Analysis Settings). With this option you should find that the spring reactions drop to zero, but it is useful to try without Inertia Relief first to check that the spring reactions are not large.
      Hope this helps!
      Cheers, Mike
    • floatingstones
      ! You deserve an entire acknowledgment chapter on my thesis for all your help. Inertia Relief is really the solution for eliminating rigid body motions and thus, obtaining only the body deformations on the Total Deformation result. As you said, I'm first trying without Inertia Relief and checking reactions - if they are small, then I ensure that my modelling is okay.
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