Absrtact The pressure vessels are as most useful structures in submarine industrials. Occurrence of buckling is the main factor in failure of pressure hulls and design of them is based on prevention of Occurrence of buckling.In this research other types of stiffeners are studied. In the seventies, Harri wrote to Ross and made the suggestion that the strength to weight ratio of a pressure hull could be dramatically improved if tubes placed in a longitudinal direction replaced the stiffening rings. Ross replied to Harris, in the same year suggesting that the placement of such tubes would better serve pressure vessels if they were in a circumferential configuration, as such pressure vessels generally fail due to the stresses they experience in the circumference.In an extension to this work, Ross suggested that if the internal tubes were themselves subjected to internal pressure, then the subsequent reaction in the circumferential direction would place the pressure hull in an initial state of tension,and thus in a state best able to resist the compressive forces, associated with external hydrostatic pressure. For this reason, he tested a Semi Elliptic shell under external pressure and calculated the buckling pressure in cases of without stiffener, tube stiffeners with and without internal pressure. This research associated with strength of tube stiffeners, tube stiffeners under internal pressure and spiral stiffeners. For study of tube stiffeners, tubes are modeled in nine categories that each category represents a certain radius of the tube cross section to radius of the shell ratio. Each category is divided into six cases of different thickness, that each case represents a certain radius of the tube cross section thickness.The results were depicted in the form of buckling pressure plots on the M/L ratio using a diagram of LPF(load factor in terms of arc length). By reviewing this plots for nine categories, difference in behavior of tubes for both local and overall buckling can be found.Also, by comparing the buckling pressure values ??in all cases of linearity, geometric non linearity and geometric - material nonlinearity, the effect of geometric and material nonlinearity on buckling pressure have been studied. In the next step for five categories from those nine categories, the equivalent rings for tubes have been modeled and analyzed like those tubes. Using the plots of buckling pressure on the M/L ratio, efficiency of tubes and equivalent rings are compared. In the next step, the internal pressure applied to the tubes, and compared with the case of without internal pressure. Finally, for three cases, the equivalent spiral tubes have been modeled and analyzed with tubes. Key Words cylindrical shell, stiffener, buckling, nonlinear analysis.
