The compressive strain at which local buckling occurs, depends on many factors. These are the geometry (diameter to wall-thickness ratio, imperfections), the loading conditions (bending moment, normal force, internal or external pressure, etc.), residual stresses, the shape of the stress strain diagram and the strength of the steel in longitudinal and circumferential direction. Test results show considerable scatter. In a research program for PRCL four bending tests on 20-inch pipes were carried out. Also a survey of relevant test results from other sources was carded out. It appears that the manufacturing method (seamless, UOE) has a considerable influence on the critical buckling strain. These were compared with available prediction formulae for the bending moment capacity and the critical strain. Statistical and probabilistic evaluations were carried out to determine the best design formulae and appropriate safety factors. An overview is given of the bending tests on three 20-inch UOE manufactured pipes and one seamless pipe with D/t ratios 45, 27, 22 and 29. The main results of the statistical and probabilistic evaluations are presented. The factors that govern the performance in bending moment capacity and the critical buckling strain are briefly discussed.
In the past 20 years, much research has been conducted on buckling and collapse of pipelines under external pressure, bending moment, tension or compression and combinations of these loads. The available test results show considerable scatter, which is considered to be caused by variations in the stress-strain relationship, the anisotropy of the steel, the Bauschinger effect, geometrical deviations, residual stresses, test conditions, etc. The manufacturing method (seamless, UO, UOE) has a considerable influence on these properties and on the local buckling and collapse resistance. For UOE manufactured pipe, a significant reduction in collapse strength has been observed compared to not expanded pipe or seamless pipe (Gresnigt, 2000).