Abstract

An aboveground oil storage tank with fixed roof explodes when flammable vapor inside is ignited for some reason. All fixed roof tanks are designed as a frangible roof joint in which the sidewall-to-roof joint is ruptured before the sidewall-to-bottom joint is ruptured under over pressurization. API Standard 650 is the code for the frangible roof joint design. However the large number of explosion incidents have showed that API design method does not work for small diameter tanks. Buckling of both joints of the tank with fixed roof may occur under internal pressure, and is a bifurcation buckling with many circumferential wave numbers. The sidewall-to-bottom joint may uplift from the foundation under internal pressure even when liquid pressure is applied, and stresses in this joint may reach elastic-plastic state. In this study, an elastic-plastic bifurcation buckling analysis under internal pressure is carried out for the small diameter fixed roof tanks using the axisymmetric shell finite element method. This study clarifies which joint buckles depending on the liquid level.