Learning Forum

[Diego Molina]

Hi! I am trying to run a static analysis on an oil storage tank, the geometry was 3D scanned, and postprocesed to create a surface, the surface was splitted into 9 different bodies in order to assign different thickness to each shell. I'm using a fixed support in the base of the tank, a displacement support in the top, edge-edge automatic contacts, and hydrostatic presure in all the faces as showed in the following image:

If I run the analysis just like that in a perfect cylinder it works just fine:

But in the scanned surface everything fails:

I tried using large deflection on and it gets better but the thing is I don't know how it works, and if this problem need to use that setting. So mi first question would be. I need to use large deflection? when I have to use this setting?

Using Large deflection On:

Also, someone recommended me use non-linear structural steel, but again, I don't know if that is correct and I if I should use it in this kind of problems. When do you recommend using non-linear materials?

Is there anything else I could be missing in order to solve this problem accurately?

Thank you!

[peteroznewman]

You should turn on Large Deflection under Analysis Settings.  The scanned tank geometry has some waviness around the surface without any fluid pressure. When the hydrostatic pressure is applied, that load puts the tank skin into tension, and flattens out the waviness. Therefore the nodal positions need to be updated during the solution to get accurate stress values.

After it has solved with a linear material, you can examine the peak stress and see if it is larger than the yield strength of the steel. If it is, then you would want to rerun with a plasticity material model so that the material that exceeded the yield strength can plastically deform and redistribute some of the stress to adjacent material.

I don't understand why you have a displacement support on the upper edge of the tank. Is the upper edge of the tank attached to something?

[Diego Molina]

Hi Peteroznewman. Thank you so much for your reply. I turned on large deflection, and got some peak stresses larger than the yield strength of the steel (Structural steel 250MPa - Red Areas in the picture), so do you recommend me use NL Structural Steel in the whole tank, or just in some areas? Also, i use a displacement suport in the upper edge because it goes attached to the roof of the tank (also structural steel plates).

 

[peteroznewman]

Hi Diego,

You need to know the yield strength for the steel used to make the tank. Then you can add the Structural Steel NL from the General Non-linear Materials library and edit the value of Yield Strength in that generic example and use the specific value for the steel used to make the tank.

If the upper edge of the tank is connected to a roof structure, it would be more accurate to include the roof structure in the model because the roof has some amount of flexibility while the displacement support provides no flexibility. This may change the stress in the tank walls slightly.