Buckling and Collapse Under Combined Bending and External Pressure

This chapter deals with the mechanics of inelastic bending in the presence of external pressures and associated limit states. The development of this interaction starts with the behavior under pure bending and pure external pressure. When two loads are applied together, they interact strongly through ovalization. Consequently, bending a tube under pressure results in a lower collapse curvature. Correspondingly, pressurizing externally a bent tube results in a lower collapse pressure. Although wrinkling bifurcation remains possible in relatively high D/t tubes bent under relatively low external pressure, in commonly used pipeline geometries, limit load instabilities dominate the behavior. Bending in the presence of external pressure is experienced by pipelines during their installation and subsequently during their operation. In installation methods such as S-lay, J-lay, and reeling, the pipe is bent under relatively high external pressure in the sagbend. The pipeline is also bent under external pressure as it conforms to surface undulations on the seafloor. Bending at the sea floor can also be experienced because of snaking resulting from pipe expansion caused by the passage of hot hydrocarbons (in some cases, purposely induced, in others accidentally). It is also a condition that can develop in case of upheaval buckling of a section of a buried pipe.