Pressure vessel is pre-shaped with linear buckling (bifurcation) analysis, 2nd eigenshape. Deformation is for better illustration showed in right figure with higher ration. Critical external pressure for pre-shaped vessel is 0.32 MPa, instead of 0.90 MPa for ideal geometry vessel.

Video of pressure vessel nonlinear buckling: https://lnkd.in/dBzVUxV