Strength of Ring-Stiffened Cylinders Under Combined Loads
Authors: Frieze P.A. , Cho S.A. , Faulkner D.A. 
Published: October 2008

In the first part of the paper, the background, model parameters', model fabrication, test rig and instrumentation relating to combine axial compression and radial pressure tests on eighteen stress-relieved three bay ring stiffened cylinders is described. Four different geometries were involved and the results in terms of axial and hoop stresses at the collapse limit state are presented non-dimensionalised with respect to the corresponding static tensile yield stresses; Interaction curves for the four geometries are presented.

The second part of the paper is concerned with deriving a strength formulation for ring stiffened cylinders subjected to axial loading and radial pressure.

The basic formulation approaches a linear interaction for slender cylinders when the axial load is compressive and the von Mises ellipse' for-stocky cylinders. A quadratic interaction between yield and imperfect elastic buckling is used. By comparison with test data, the knock-down factors on perfect elastic buckling are derived in a format simpler-than that in existing codes. The accuracy of the resulting formulation is compared with that obtainable using the DnV Rules for axial compression and BS 5500 for hydrostatic loading.

The geometries-of the ring stiffened cylinders examined in both aspects of the work correspond to those used in present and planned semisubmersibles and TLPs.

INTRODUCTION

The first TLP to be commissioned is now reaching the final stages of fabrication. The design of this North Sea platform occupied many man hours mainly because of its uniqueness, but also because of the dearth of knowledge concerning- its loading, response, strength and fatigue behaviour. Only strength is of concern here so the other aspects will not he considered further.

The main structural components of this first TLP and, indeed, most semi-submersibles, are the deck, columns and pontoons. The deck is a reasonably conventional arrangement and could have been designed using one of many structural steel or bridge codes. The pontoons are basically rectangular in cross-section but their corners are-rounded. Bridge technology combined with ship and other platform design concepts were available for the design of these c6mpcnents, although ship specifications cannot necessarily be relied upon to give real guidance on strength.

The columns are circular in cross-section and are stiffened. The DnV Rules were available for their design if ring frames were to be the only form of stiffening. These particular Rules, however, were derived from-data pertaining to elastic structures (air craft) and have received little substantiation for their use in plasticity-dominated response regimes because of the lack of experimental data.

As an alternative to design rules, experimental data could have been adopted as the basis for the strength considerations. However, very little data exist on stiffened cylinders subjected to combined loads and failing in the plastic regime. Considerable data are available on ring framed cylinders subjected to hydrostatic pressure which were brought together to form the basis for the design procedure in BS 55002.