Pipeline Embedment in Deep Water: Processes and Quantitative Assessment

Abstract

Abstract The paper outlines the processes that determine pipeline embedment in softsediments typical of deep water, and describes quantitative methods forassessing the contribution to pipeline embedment from each process. The mainprocesses considered are: self-weight, force concentration during installation, and cyclic lateral and vertical motion during pipe-laying. For each process, non-dimensionalized solutions are provided to evaluate embedment for givenpipeline and soil properties, and different conditions operative duringpipe-laying. Methods to establish a penetration resistance profile using modernpenetration testing techniques are described, and the up-scaling of thesemeasurements to pipeline behavior is discussed. The important effects of heaveand soil self-weight are also quantified, and the effects of remolding andsubsequent reconsolidation following a cyclic event are also demonstrated. Finally, the influence of cyclic motion within the touchdown zone duringpipe-laying is explored. Examples of numerical and physical modeling of the layprocess show that these dynamic effects can have a dominant influence on theas-laid embedment. Approaches for quantifying this aspect of behavior in designare discussed. Introduction Engineering design of a pipeline in respect of external interactions, such asstability, lateral buckling, axial friction, heat transfer and exposure tosubmarine slides, requires an assessment of the as-laid pipeline embedment. Indeep water, pipelines are commonly laid on the seabed without specific actionsdirected at embedding the pipeline, and without additional overlyingprotection. As such, embedment or partial penetration of the pipeline into theseabed is a function of the self-weight of the pipeline relative to thestrength of the seabed, but is complicated by the laying process, which leadsto enhanced contact stresses in the region where the pipeline meets the seabed(the so-called touchdown zone), and during which dynamic motion of the layvessel and suspended part of the pipeline can lead to significant additionalpenetration as the seabed soils are remolded locally. It is necessary to bound the pipeline embedment, and hence the anticipatedaxial and lateral pipe-soil resistance, in order to ensure a safe design. It isnot possible to adopt ‘conservative' extreme design values, since low and highresistance can work for or against a particular design consideration, dependingon the limit state under consideration. For example, the different influencesof high and low pipe-soil interaction forces on the phenomena of buckling andwalking are summarized by Bruton et al. [1]. Another example where embedmenthas a conflicting influence on design requirements is stability versus thermallosses. High embedment and therefore high lateral resistance improves on-bottomstability. However, it also reduces convective heat losses, leading to higheroperating temperatures distant from the wellhead, and therefore greater thermalexpansion. The feasibility of a particular design solution is strongly affected by thedesign values for pipe-soil interaction forces. If a controlled lateralbuckling solution cannot be adopted to accommodate the thermal pipelineloading, then alternative solutions of seabed anchors or expansion spools arerequired. These devices increase the installation cost of the pipelinesignificantly, since additional construction processes and seabed interventionsare needed. In a recent development offshore West Africa, it was found thatsignificant cost savings, in excess of US$50 million, could arise for thepipeline system from relatively minor refinements of the design parameters forpipe-soil interaction. Improved methods are necessary to raise the reliabilityof these designs in marginal cases, enhancing the viability of offshorehydrocarbon developments in deep water.