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?
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