Chapter 4 - Buckling and Collapse Under External Pressure

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Pipe collapse under external pressure is one of the main limit states that govern the design of offshore pipelines. This chapter uses results from careful experiments and several levels of analysis to study the dependence of the collapse pressure on the main problem parameters. Thinner pipes used in shallower waters buckle elastically, but collapse due to postbuckling inelastic action. The classical elastic buckling pressure is derived, followed by the derivation of Timoshenko's design formula for the onset of collapse of an initially ovalized pipe due to material yielding. Thicker pipes used in deeper waters buckle and collapse in the plastic range. Plastic bifurcation buckling formulation is used to derive buckling equations useful in design. However, plastic buckling and collapse of long tubes under external pressure involves several nonlinearities and is best handled numerically. To this end, the formulation and solution procedure of the nonlinear code BEPTICO is presented. The fidelity of this type analysis is demonstrated by a series of collapse pressure calculations covering a range of D/ts, diameters and yield stresses, with excellent agreement to measured values. The code is also used to establish the effect on the collapse pressure of pipe D/t, initial ovality, wall thickness eccentricity, material yield stress, the shape of the stress-strain response, and residual stresses. The chapter also outlines how actual pipe imperfections can be measured, important to both the manufacture and design of pipelines, and finishes with design recommendations.