The 2003 Tokachi-Oki earthquake caused severe damage to oil storage tanks due to liquid sloshing. Six single-deck floating roofs had experienced structural problems as evidenced by sinking failure in large diameter tanks at the refinery at Tomakomai, Japan. The pontoon of floating roof might be buckled due to circumferential bending moment during the sloshing. The content in the tank was spilled on the floating roof from small failures which were caused in the rap welding joints of pontoon bottom plate by the buckling. Then the floating roof began to lose buoyancy and submerged into the content slowly. The failure of the roof expanded gradually in the sinking process. It is presumed that the initial small failures were caused by the elastic buckling of the pontoon due to circumferential bending moment. This paper presents the buckling strength of the pontoon using axisymmetric shell finite element analysis. Linear elastic bifurcation buckling analysis is carried out and the buckling characteristics of the pontoon are investigated. The result shows that the thickness of both pontoon roof and bottom plates have significantly affect the buckling strength.

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