The last image helped me to understand that your model does include the cylindrical support barrel.
If the bottom of the tank is welded to the support barrel, then using Shared Topology in SpaceClaim and having no contact is an accurate model.
If the bottom of the tank is sitting on the support barrel, then frictional contact is appropriate. If you are getting stress concentrations, you might need more detailed geometry to more accurately represent the stress.
You could create a 2D Axisymmetric model and have elements through the thickness of the walls to see detailed stress concentrations if the geometry is axisymmetric.