ABSTRACT

This paper proposes an estimated formula for the tripping buckling strength of the ring-stiffener attached in stiffened cylindrical shells under external pressure. The proposed formula is derived assuming the tripping buckling as the buckling of only the flange supported by springs and considering effects of cylindrical shape and torsional stiffness which are not included in a conventional formula. The accuracy of the proposed formula is examined by comparing with calculation results by finite element analyses.

INTRODUCTION

Ring-stiffened cylindrical shells are often used in offshore and submergible structures which are exposed to external pressure. To estimate the buckling strength of ring-stiffened cylindrical shells is important to ensure their structural safety. Although the buckling strength can be accurately calculated using the finite element analysis (FEA), it takes a lot of effort and much time for modeling and calculation. Thus, practical estimation formulae for the buckling strength have been often used at an initial stage of structural design.

Buckling modes of ring-stiffened cylindrical shells under external pressure can be categorized as follows:

  1. the shell buckling (the local buckling of the cylindrical shell between ring-stiffeners);

  2. the overall buckling (the buckling of the entire region of the cylindrical shell including ring-stiffeners);

  3. the tripping buckling of the ring-stiffener; and

  4. the local buckling of the web or the flange of ring-stiffeners.

To lengthen the bulkhead space is recently required from viewpoints of economy and functionality. In this case, it may be thought that ring-stiffeners having larger dimensions are adopted to secure sufficient strength. If the dimensions of ring-stiffeners increase, the overall buckling strength increases, whereas the tripping buckling strength decreases. The estimation formula is required to estimate the tripping buckling strength accurately. Additionally, two types of arrangement of ring-stiffeners attached on inside or outside of the cylindrical shell are adopted. Thus, it is necessary to consider the effect of both inside and outside ring-stiffeners in the formula.

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