On the dynamics of propagating buckles in pipelines

A combined experimental and analytical study of the dynamic propagation of buckles initiated in long pipes under external pressure is presented. The experiments involve measurement of the steady-state velocity of buckles initiated in stainless steel tubes with D/t=27.9. Results from tubes pressurized by air or water are presented for pressure levels ranging from the propagation pressure to the collapse pressure of the tube. The buckle velocity in air was found to be significantly higher than in water at the same pressure. The flip–flop mode of buckle propagation was found to take place for pressure levels 13% below the collapse pressure and higher. A finite element model, capable of simulating the dynamic initiation and propagation of such buckles has been developed. The model accounts for the inertia of the pipe, the nonlinearity introduced by contact between the collapsing walls of the pipe while the material is modeled as a finitely deforming elastic–viscoplastic solid. The buckling and collapse are assumed to take place in vacuum. The model is shown to reproduce well the dynamic initiation and propagation of buckles in air and the predicted velocities are in good agreement with those measured in this medium. The buckle was found to sharpen significantly with pressure. This sharpening, coupled with higher pressure, causes an increase in the amplitude of reverse ovality in a zone just ahead of the propagating buckle front. At higher pressures, the reverse ovality is shown to be high enough to initiate collapse at a new site. The new collapse is at 90° to the original one. This sequence of events is repeated, resulting in the flip–flop mode of buckle propagation.