Monotonic and cyclic soil responses to upheaval buckling in offshore buried pipelines

Alex J Baumgard, Cambridge University
Geotechnical Engineering Group


In the last few decades, the operating temperatures and pressures for subsea oil and gas pipelines have dramatically increased. This change, combined with a trend towards using pipes of smaller diameter, has increased the propensity for such pipelines to buckle in accordance with the greater axial loads. For buried offshore pipelines, upheaval buckling is the dominant buckling mode, in which the pipeline deflects vertically, potentially extricating itself from its protective cover of soil. By understanding the ability of the soil to restrain this pipe motion, such buckling can be prevented.

A series of full-scale monotonic uplift tests has been performed under submerged conditions on a variety of materials with different particle sizes. In addition to the particle size, the effects of relative density, pipe uplift speed and burial depth were investigated with respect to their influence on the development of peak uplift resistance and its rate of mobilisation. A series of monotonic uplift tests was also performed on a variety of recovered offshore soils at reduced-scale in a geotechnical centrifuge. These two series of tests, in addition to a finite element analysis, were used to verify the accuracy of several existing uplift resistance models, where good agreement between one such model and the experimental results was observed.

The phenomenon of cyclic creep was introduced, where under repeated loading cycles, a pipeline is able to extricate itself gradually through a cover of soil. Full-scale cyclic tests under both load-control and displacement-control were performed on several soil types and densities, and the significance of the basal cavity created beneath the uplifted pipe was investigated. A novel numerical model has been proposed, in which a combination of soil particle migration around the periphery of the pipe and the existence of a basal cavity act to govern whether or not cyclic creep occurs. This numerical model was compared with the experimental results, and was found to predict successfully the occurrence of cyclic creep in the experiments.

Finally, the related problem of pipeline flotation during backfilling was investigated. Full-scale experimental tests were carried out in which the backfilling velocity and bedding gap separation were varied. A simple analytical model has been proposed, formulated using the principles of fluid dynamics. This model provides a close correlation with the experimental results, thus enabling the flotation forces acting on the pipeline during backfilling to be predicted.

Keywords: backfill, full-scale, cyclic creep, mobilisation distance, pipe flotation, pipe pull-out, soil resistance, submarine buried pipeline, upheaval buckling, uplift resistance.