Upheaval Buckling

Upheaval buckling mostly is referred to in case of instability of an offshore pipeline. Onland it is often named overbending. The phenomenon occurs if the pipeline has an internal compressive axial force, is buried and the pipeline or trench has some imperfection leading tot out of straightness of the pipeline axis. The backfill in an offshore trench often is loose material, for instance sand. The backfill material provides an overburden load on the pipeline, but the (pipe-upward) stiffness of this material is quite low, due to the loose skeleton of the grains. The upward soil resistance at the top of the pipe is low compared to horizontal and downward soil resistances and if the pipeline due to the imperfection tends to buckle it will buckle upward. The magnitude of the buckle depends on the axial force in the pipeline and the soil friction that determines the feeding length. In fact there is a shift from an axial equilibrium to a lateral equilibrium state.

OffshoreBuckle

The buckle may rise out of the sea bottom and as such becomes an obstacle for trawl fishing and anchors may hook the pipeline causing serious damage. An example of a subsea pipeline buckle is shown.

OffshoreSnaking

 In case of an unburied or only partly buried pipeline the compressive force in the pipeline may release itself in sideward buckling (snaking). Depending on the bending stiffness of the pipe and the soil friction including some horizontal soil resistance, the buckle may be smooth or become more sharp.