Consolidation Around Seabed Pipelines

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Abstract The build-up of pipe-soil resistance that can be mobilized on a seabed pipeline, particularly in the axial direction, is affected by consolidation of the seabed beneath the pipeline after it is laid. On fine-grained soils the consolidation period may extend well beyond the laying period, and involve a significant gain in pipe-soil resistance: a process analogous to the set-up of piles driven in clay. The available level of pipe-soil resistance affects many aspects of pipeline design, such as stability on route curves and down slopes, geohazard vulnerability, lateral buckling and axial walking behaviour. This paper considers consolidation around on-bottom pipelines through a numerical study, supported by field data. The soil is modeled as an elastic material with coupled Biot consolidation. Pipe embedments of up to 0.5 diameters have been considered and the effect of soil berms adjacent to the pipe, formed of the displaced soil, has been explored. The solutions and guidance provided in this paper allow pipeline designers to estimate the ‘set-up’ periods over which the effective stress at the pipe-soil interface will rise in the period after laying, indicating the build-up of available axial pipe-soil resistance. The results account for the enhancement of this resistance through a ‘wedging’ action. The dissipation process is retarded by a factor of up to 2 if the displaced soil forms a berm that is collapsed against the pipe wall. Similar solutions for the analogous situation of consolidation (‘set-up’) around driven piles have existed for many years. This paper provides the equivalent design tool for on-bottom pipelines. Background Subsea pipelines are commonly laid directly on the seabed. The axial resistance that can be mobilized between the pipe and the seabed is an important consideration for both the installation and the in-service behaviour. It is an input into the following design aspects, and others:Resistance during shore pulls and bottom-tow installation.Stability of route curves during laying.Stability on a sloping seabed.End expansions and buckle feed-in in response to internal temperature and pressure.Restraint to support the pipe adjacent to externally-loaded sections - such as when impacted by submarine slides. When a pipe is laid on the seabed it penetrates into the surficial soils, which are typically very soft in deep water. In finegrained clayey soils the usual rate of laying is such that the embedment response is undrained. Excess pore pressure is generated in response to the imposed weight of the pipeline, and due to shearing of the soil. The effective stress in the soil remains low, as it was in situ, until drainage occurs. After full dissipation of the lay-induced pore pressure, the sum of the vertical effective stress around the pipe-soil contact equals the pipe weight.