Chapter 7 - Buckling and Collapse Under Combined External Pressure and Tension

Abstract

This chapter examines the extent to which tension lowers the collapse pressure of long tubes and pipes. Building on the results of collapse under pure external pressure of Chapter 4, the effect of tension on first the elastic and then elastic-plastic bifurcation buckling pressure is presented. Timoshenko's formula for the collapse of tubes with initial ovality that buckle elastically due to material yielding is modified to include the effect of tension and evaluated. The effect of tension on plastic collapse, more relevant to offshore pipelines, is modeled through an extension of the nonlinear formulation presented for the corresponding external pressure problem. The extension is correspondingly reflected in the custom numerical code BEPTICO. The high fidelity of this formulation is demonstrated by the excellent prediction of tension-pressure collapse envelopes from experiments of several tube D/ts. Additional parametric studies show that the tension-pressure interaction collapse envelope is affected by initial ovality, by the yield stress and shape of the stress-strain response, and to a lesser extent by circumferential bending-type residual stresses and other factors. It is concluded that these effects are most effectively addressed through a uniform tube-type nonlinear formulation.