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[Objectives] This paper focusses on deriving formulations to predict the ultimate strength of both intact and damaged steel ring-stiffened cylinders subjected to hydrostatic pressure. The damage scenario considered in this study can represent the collision accidents of marine ring-stiffened cylinder structures,such as submarines,semi-submersibles,spars and more recently as buoyancy columns of floating offshore wind turbine foundations with supply vessels.[Methods] To validate the numerical modelling and the analytical techniques, six small-scale ring-stiffened cylinder models were tested. Drop tests with a knife-edge indenter were conducted to generate the damage on four cylinder models. Then, all models were tested under hydrostatic pressure for assessing the ultimate strength in intact and damaged conditions. Next,the rigorous parametric studies on actual design full-scale ring-stiffened cylinder examples were performed by changing the design variables. Based on the parametric study results, the simple design equations are derived for predicting the residual strength of damaged ring-stiffened cylinders.[Results] The accuracy and reliability of the derived formulation has been compared with those of test and finite element analysis results,which has a good agreement for all cases of loadings.[Conclusions] Therefore, it can be useful for design purpose and serviceability limit state assessment of ring-stiffened cylinders in the risk conditions of marine structures.