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Abstract Severe environmental loads on exposed pipelines can lead to excessive horizontal displacements or free spanning with subsequent flow induced vibrations. The interaction between a submarine pipeline and an erodible sea bed has important implications for design choices relevant to concrete coating thickness (and consequently for the required laying equipment) and/or pipeline burial depth. For pipelines resting on or buried in erodible seabeds affected by severe environmental conditions, design should be founded on a careful analysis of the following aspects:The soil erodibility, the bedform activity and relevant time scales.The maximum curvature which the pipeline crossing active sand waves and sand dunes can achieve.The initial pipeline-sea bed configuration and that expected during the pipeline life time.The maximum length of potential exposure and/or suspension expected to develop with respect to permissible lengths from a structural integrity view point.The soil liquefaction potential and its effects on the pipeline or adjacent slopes. The present paper summarizes the possible scenarios for pipelines laid on erodible sea beds, the techniques used to evaluate the onset and extent of erosion and other forms of pipeline-sea bed interaction and the design criteria applicable for the different configurations which can occur. A probabilistic approach to assess the structural integrity of scour induced free spans is introduced. Introduction In submarine pipeline design, sea bed stability, short and long term structural integrity and protection against human intervention (such as fishing, maritime traffic and marine construction) that Can damage Or be damaged by the pipeline Dresence (Camaneto and Tominez, 1991), have to be taken into account. Environmental conditions determine the mobility of the sediments and the characteristic forms of the sea bed, (Anselmi and Bruschi, 1993). On highly erodible sea beds the pipeline causes a strong perturbation of the near bottom hydrodynamics thus leading to soil erosion accumulation close to the Depending on the consequences in terms of unexpected exposure and/or suspension, these phenomena can be detrimental to the pipeline integrity, (Bruschi et al., 1994). For instance, the development of long free spans implies that the pipeline can subsequently be exposed to dynamic amplification of the vibrations induced by the hydrodynamic field (Bruschi et al., 1986). In other cases, the interaction between the pipeline and the sea bed may result in the partial burying of the pipeline in a relatively short time compared to the operating life of the pipeline. The latter case offers the possibility of considerable savings in stabilisation works such as trenching, overweighing or other types of intervention (Bruschi et al., 1993). In addition, the mobility of the characteristic morphological features of an erodible sea bed, for instance sand waves and sand banks, are relevant during the pipeline planning stage. |
