Stability analysis of local thinned shallow spherical roof of storage tank

The nonlinear finite element method is adopted to calculate the global critical load of spherical roof of oil storage tank with axial symmetrical local corrosion region, and the local corrosion region is equivalent to a circular corrosion pit with uniform depth. The numerical calculation results show that the tank wall and inner pressure of the stored oil have slight effects on the stability of the roof. To build the formula of local critical load of the tank roof, the circular corrosion pit is separated from the whole roof and treated as a shallow spherical shell which is elastically supported on the rest part of the roof. The equivalent support stiffness is obtained by the deformation compatibility at the edge of the corrosion pit. And the nonlinear stability equation is solved with a modified iteration method to determine the local critical load. This proposed method is then applied to evaluate the local critical load of an in-service corroded oil tank roof, the results agree well with those obtained by conventional nonlinear finite element method. The results also show that the geometrical parameter of the corrosion region corresponding to the minimal critical load is 9.5.