In deep water, the pre-service embedment of on-bottom pipelines is mainly controlled by the lay process and its effects on the soil strength. Predicted ranges in local pipeline embedment are often very wide due to uncertainties associated with the surface soil properties, the influence of the sea state and vessel motions on soil remolding, and the complexity of the large-deformation process of pipe-seabed penetration. Pipeline embedment has a significant impact on pipe-soil interaction resistance — commonly described by the ‘friction factors’ used for pipeline design. Most design checks, such as for controlled lateral buckling, need to be satisfied under both low and high extremes of pipe-soil resistance and therefore pipe embedment. A wide range in pipe embedment can create significant design challenges for pipeline engineers, for example where engineered buckle mitigation structures are required to alleviate potential high strains in the pipeline.
Data obtained from as-laid field surveys of subsea pipeline embedment have been analyzed to illustrate quantitatively how embedment varies along a pipeline route. This variation is linked to both anthropogenic influences, for example as a result of the stop-start pipeline construction process, as well as natural variability in (i) soil conditions along the route and (ii) sea state conditions during laying. Accounting for this variability through statistical analysis of as-laid embedment, and incorporation of appropriate length scales relevant to each design check, can have significant beneficial impacts on pipeline design through reductions in the ranges of pipe-soil friction factors. In some cases these may reduce or negate the need for expensive mitigation. The approaches illustrated in this paper have already found beneficial use on real projects.
