Arctic pipelines buried across permafrost will be subjected to fluid (oil or gas) pressure as well as bending due to differential thaw settlement or frost heave in the soil. The circumferential pipe stress due to the pressure is allowed, according to ASME B31–8, to be as high as 0.80 SMYS. When combined with bending, the relatively high circumferential stress may affect pipe buckling in the plastic range. This paper describes an axisymmetric bifurcation analysis in the plastic range for predicting the critical maximum axial stress (or strain) occurring under the combined loading. The rationale for this simplified analysis is based on reported investigation results, both theoretical and experimental, for buckling of pipes subjected to pure bending. This analysis indicates that the critical strain under the combined loading can be considerably greater than under pure bending only.
Large pipe bending is considered to be the primary concern for the structural integrity of arctic pipelines buried across permafrost. Hot oil or chilled gas transported through the pipelines can cause differential thaw settlement or frost heave in the soil. This soil movement may induce large pipe bending leading to local buckling of the pipe. Local buckling of pipes subjected only to bending has been investigated by many researchers. The investigations show that pipe buckling is possible in two different modes: limit point type and bifurcation type of instability. Cross-sections of idealized pipes will deform by ovalizing (flattening) under moment. As the deformation increases with increasing moment, it acts to reduce the moment-carrying capacity of the cross-section until a limit point is reached where the moment curvature Curve has zero slope. The loss of pipe stability beyond this point is called limit point buckling. The limit point buckling may occur in the elastic or the plastic range.
