Interaction Between Offshore Pipelines and Migrating Sand Waves

Abstract

Large areas of shallow, sandy seas are covered by migrating tidal sand waves. Sand wave migration rates are on the order of 10s of meters per year, with heights between 10 and 30% of the water depth. If such regions are traversed by pipelines, the dynamic interaction between the rock-berm protection of the pipelines and the migratory sand waves must be accounted for to assure the long term stability of both the rock-berms and pipelines. This study employs a 2DV model to demonstrate the hydrodynamic and morphodynamic interaction between migrating sand waves and a rock-berm constructed perpendicular to the migration direction. The timescale of sand waves and the design life of rock-berm are similar, consequently, rock-berms in sand wave regions experience a change in bed level approximately equal to that sand wave height. Due to the large difference in temporal scales between local erosive processes and sand wave migration, the passing of a sand wave is manifest as a general rising or falling of the ambient seabed, while a rock-berm is fixed at its construction elevation. Consequently, the critical design case is for a rock-berm constructed at a sand wave crest since the surrounding bed level decreases throughout the operational life of the pipeline. A conservative design approach is to construct rock berm protection in a sand wave trough, resulting in rising ambient seabed levels throughout the operational lifetime of the underlying pipeline or electrical cable.