Buckling Design of Stringer and Orthogonally Stiffened Cylinders
Authors: Ellinas C.P. , Supple W.J. , Walker A.C. 
Published: December 2007

This paper reports a study carried out on behalf of the U.K. Dept. of Energy to update its Guidance Notes relating to the buckling design of offshore steel structures. The results presented here refer to stringer and orthogonally stiffened cylinders and are compared with the predictions from various design codes. Conclusions are predictions from various design codes. Conclusions are drawn as to the validity and conservativeness of the code recommendations.

Introduction

Stringer together with ring stiffeners form the main stiffening elements of fabricated cylinders used as compression members in steel offshore construction. Stringer stiffeners normally are used to provide additional stiffness in axially compressed members. However, it is usual to restrain stringer-stiffened cylinders circumferentially at regular intervals by the use of ring stiffeners: effectively, the shell then becomes orthogonally stiffened. The shell segments between adjacent stringer and ring stiffeners are effectively unstiffened cylindrical panels that merit special attention. It is normal practice in offshore design to suppress buckling modes that involve buckling of the ring stiffeners; therefore, in such cases collapse is determined by the resistance of the stringer-stiffened cylinder buckling. Orthogonally stiffened cylinders formed one of the five main groups of structural elements investigated in a study carried out on behalf of the U.K. Dept of Energy to update the Guidance Notes 2 relating to the buckling design of offshore structures. An extensive literature survey was performed, and available experimental data on the buckling of stringer and orthogonally stiffened cylinders were collected. processed, and analyzed. The data were obtained mainly from past research programs, but an important contribution has come from recent offshore-related research carried out in Europe and in the U.S. The processed experimental evidence has formed the basis of extensive comparisons with the design code recommendations. This has allowed critical comments and concise recommendations to be made on the validity and apparent safety in predicting strength by the codes. Experimental investigations on stringer-stiffened shells were restricted to mainly axial compression studies in the past. It is only recently that offshore design requirements past. It is only recently that offshore design requirements have led to research into other loading cases involving, external pressure. This is reflected in available experimental information, which is extensive for axial compression but rather limited for other loading cases.

Experimental Information

The collection and critical evaluation of available experimental data is one of the important aspects of this work because it forms the basis of correlation and comparisons with proposed and established design approaches. The methods used in collecting, processing, and interpreting available and reliable information have been described in detail. The processed information corresponds mainly to stringer-stiffened shells. A small number of test results from orthogonally stiffened cylinders is available and also has been considered. In total, about 250 test results were processed for this study. It would be impractical to list processed for this study. It would be impractical to list here the wide range of shell geometries and large number of references from which the results were obtained. However, extensive information about this data base is given in Ref. 1.

Buckling Behavior of Orthogonally Stiffened Cylinders

Orthogonally stiffened cylinders, as used in offshore construction. are subject to a variety of loadings applied either uniaxially or in combination. including axial compression, external pressure, bending, and torsional loads. Of these only the first two and their combination are considered in this paper. Stringer and orthogonally stiffened cylinders may collapse in any number of different ways according to the type of loading to which they are subjected and their geometries, boundary conditions, material characteristics, shape imperfections, and welding-induced stresses. The main collapse modes are:local panel buckling of the plating between stiffeners, which on its own does not plating between stiffeners, which on its own does not usually cause collapse (Fig. 1);stringer-stiffened cylinder buckling, in which one or more of the stringers buckle together with the shell plating between ring stiffeners, which can precipitate catastrophic collapse (Fig. 2);general instability, in which both stringer and ring stiffeners buckle together with the shell plating, causing catastrophic collapse (Fig. 3);, andlocal stiffener tripping (Fig. 4).

It is well known that the nature of buckling in orthogonally stiffened cylinders can be very complicated, with complex forties of interaction between the different buckling modes taking place.

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