Ring stiffened cylinder is generally used as main structural members of offshore and submergible structures. In designing of such structures, it is very important to estimate the buckling strength of ring stiffened cylinder under external pressure accurately. The buckling mode of ring stiffened cylinder under external pressure can be divided into the following four modes. The first is the general buckling mode which occurs over the entire length of the cylinder involving ring stiffeners. The second is the shell buckling mode which is the buckling of cylindrical shell between the ring stiffeners. The third is the frame tripping mode of ring stiffener. The fourth is the local buckling mode of ring stiffener’s web or face. The purpose of this research is to propose the simplified formula for predicting the elastic shell buckling strength included the influence of stress ratio of shell and ring stiffener, and to propose the simplified formula for predicting the elastic ring frame tripping strength combined the proposed formula of the elastic shell buckling strength. The elastic shell buckling strength and the elastic ring frame tripping strength predicted by these simplified formulas are compared with the result of finite element analysis by changing various parameters, such as the scantling of ring stiffener and shell thickness. From the comparison of these results, accuracy of these simplified formulas is examined.
