Hydrostatic and Axial Collapse Tests of Stiffened Cylinders

Abstract

This paper describes the results of collapse tests on two models of longitudinally and circumferentially stiffened leg sections of an offshore platform. Methods of predicting buckling loads are discussed and predicted platform. Methods of predicting buckling loads are discussed and predicted buckling loads are compared with the model test results. Introduction Structural elements can collapse because of material failure or geometric buckling. Cylindrical shells of low diameter-to-thickness ratios (d/h less than 60), such as tabular members of conventional offshore platforms, generally do not buckle locally and can be designed on the basis of Euler column buckling or material failure. Cylindrical shells of relatively high ratios (d/h greater than 60), on the other hand, may be subject to local shell buckling. Unstiffened thin-walled cylinders are prone to sudden and disastrous failures at loads well below buckling loads predicted by classical small-deflection theory. The predicted by classical small-deflection theory. The behavior of unstiffened cylinders is especially unpredictable when they are subjected to axial compression and bending loads. In fact, the scatter of test data may range up to 500 percent or more. Donnell and Wan showed theoretically, by means of an imperfection analysis, that a possible reason for the discrepancy was initial imperfections caused by manufacturing tolerances. This hypothesis now is generally accepted. The effect of imperfections is most severe for axially compressed, unstiffened cylinders. Similar discrepancies, though generally not as severe, have been observed for other loading conditions and shell shapes. In addition to geometric imperfections, experimental and theoretical evidence also has shown that the buckling load may be affected by the boundary conditions and by residual stresses introduced during the manufacturing process. Residual stresses change the effective process. Residual stresses change the effective stress-strain curve of the material and cause inelastic action to begin before the yield point is reached. As a result, the buckling process is hastened. The elastic postbuckling behavior of perfect and imperfect, axially compressed, unstiffened thin cylinders is demonstrated qualitatively in Fig. 1. There is a sudden decrease in load carrying capacity upon buckling. Because there is no postbuckling reserve strength, the buckling of an axially compressed, unstiffened cylinder is coincident with failure. Moreover, the failure is sudden and drastic. For this reason, the relative confidence level in the buckling load should be higher for unstiffened cylinders than for most other structural elements. This is made difficult by the lack of agreement between theoretical and experimental results for unstiffened cylinders. Therefore, these cylinders should be designed conservatively. To improve their resistance to buckling, large-diameter cylinders frequently are stiffened using longitudinal stiffeners (called stringers) and circumferential stiffeners (called rings). Stiffened cylinders behave better than their unstiffened counterparts, as demonstrated qualitatively in Fig. 2. Two aspects of the behavior of stiffened cylinders are significant. First, the detrimental effect of geometric imperfections is less pronounced than that for unstiffened cylinders. Second, the decrease in load capacity on buckling is much smaller than that for unstiffened cylinders. Thus, stiffened cylinders are not characterized by sudden drastic failures compared with unstiffened cylinders. JPT P. 668