Pipeline Floatation Assessment in Liquefiable Soil

Abstract

Abstract This paper presents an original analysis of the flotation potential for a pipeline buried in soil which may not be fully liquefied, but nevertheless exhibits a significant excess pore pressure induced by wave-induced soil shearing. For pipelines that are light enough to be subjected to flotation, the accumulation of pore pressure enhances the flotation potential in two ways:The pore pressure accumulation in the seabed increases the upwards buoyancy force exerted on the pipeline.The increase of excess pore pressure in the soil reduces the soil resisting force which restrains the pipeline to prevent flotation. Ultimately, the excess pore pressure may become equal to the initial effective stress level, which implies full liquefaction. It is commonly accepted that this implies a high risk of pipeline flotation if the submerged specific gravity of the pipe is less than the total unit weight of the soil. However, the combination of an increased buoyancy force and a reduced soil resistance as excess pore pressures accumulate mean that the buried pipeline may reach an unstable state before full liquefaction is reached. Hence pipelines may also float in soils that are not fully liquefied. This paper will present the theoretical background for assessing both the uplifting force exerted on the pipeline and the corresponding soil resisting force, as a function of both the excess pore pressure level in the seabed and the cover depth over the crown of the pipeline. The model used for assessing pore pressure accumulation in the seabed is also described and then an example application is presented, comprising a typical pipeline in a backfilled trench subject to a design storm. Introduction Offshore pipelines in shallow waters are often buried in order to protect them against damage by fishing equipment or drag anchors and to improve their on-bottom stability under storm conditions. A vital aspect of buried pipeline design is the assessment of pipeline flotation potential under design storms or seismic events since these may govern the pipe specific gravity, the burial depth and the selection of engineered backfilling materials. Two fundamental components need to be addressed in a pipeline flotation assessment: what is the storm generated excess pore pressure distribution in the surrounding soil and what resistance can be mobilised in the soil in order to resist pipeline flotation. While the mechanisms of excess pore pressure generation under seismic or storm conditions have been studied extensively (e.g. Seed & Idriss, 1971; Ishihara and Yamazaki, 1983), the existing methods for assessing the potential for soil liquefaction have mainly been developed for silica sands with relative density or cone resistance as the fundamental input soil parameter. In addition, the standard procedures are only applicable where fully undrained conditions pertain in the soil, whereas partially drained conditions would generally be expected under storm conditions for most liquefiable soil types. For the carbonate soils encountered in tropical waters (e.g. on the North West Shelf of Australia), these standard approaches are not applicable since the liquefaction properties of carbonate soils are quite different to those for a silica soil of equivalent cone resistance. In addition, it is often found that carbonate materials with a high fines content are much more liquefiable than equivalent non-carbonate clayey soils. Hence, the potential for pipeline flotation during a storm is generally much higher in uncemented carbonate soils compared to most silica sands.