Thesis: Jens Schupp

Doctor of Philosophy, Mansfield College, University of Oxford, Michaelmas Term 2009

Upheaval Buckling and Flotation of Buried Offshore Pipelines

The declining production rate of major oilfields in the North Sea required the increasing exploitation of smaller oil fields, which necessitated new production techniques to secure competitive hydrocarbon production. Autonomous seafloor based production facilities were installed and their product stream tied back with small diameter pipelines (tiebacks) to existing super structures. These tiebacks have to be embedded into the seafloor for several reasons and to reduce installation costs, modern installation methods, such as jet trenching, are used. The jet trenching method fluidises the soil in which the pipeline can sink to depth. The fluidised soil then resettles on top of the pipe in a very loose state, making the backfill material very susceptible to flow liquefaction. The current research investigates two possible failure mechanisms - unzipping and flotation - that can arise from these backfill conditions. Both are thought to have the potential to resurface long continuous pipeline sections.

The first scenario describes an unzipping mechanism, by which an initial buckling event triggers soil liquefaction that propagates along the pipe as the pipeline resurfaces, driven by buoyant forces. Laboratory investigations could prove that high pipeline pullout rates can cause liquefaction in very loose saturated sand. Upheaval buckling tests and pullout tests on long slender pipes also indicated the occurrence of soil liquefaction. However, the onset of a resurfacing mechanism that propagates along the pipe could not be triggered, and so the testing could not verify the feasibility of the proposed unzipping scenario.

The second model describes a flotation mechanism, by which dynamic pipeline excitations, as caused by slug flow (oil occlusions separated by gas bubbles), trigger flow liquefaction in the proximity of a pipeline. If the triggered liquefaction extends to the surface, pipeline flotation, where long lengths of pipeline resurface simultaneously, appears to be possible. In laboratory tests, flotation of short pipe segments could be triggered by applying a mechanical impulse. A model was developed to describe the testing results, which then allowed modelling of a case study that reports a significant pipeline failure, where several kilometres of pipeline resurfaced. The close match between simulation and case study suggests the feasibility of the proposed flotation scenario.