Predicting the As-Laid Embedment of Surface-Laid Pipelines and Cables in Sand Wave Regions

Predicting the as-laid embedment of surface-laid pipelines and cables is important for estimating their in-service response. To date, design approaches have only considered flat seabed conditions. The as-laid embedment is estimated using vertical ‘lay down’ seabed springs that are calculated allowing for the pipeline geometry and weight as well as its interaction with the seabed during the dynamic lay process. This paper addresses the important and different case where pipelines and cables are laid on a seabed with bedform features (e.g. sand waves, mega ripples or ripples). The measured as-laid embedment for a pipeline installed within a region of sand waves on the North West Shelf of Australia is examined and compared with numerical simulations of the lay process. Good agreement with the field embedment is obtained for both flat and sand wave zones. However, this is achieved using soil springs that are around 10 times softer than prescribed by current recommended practice. This is attributed to the neglect of dynamic lay effects in design guidance for sands and the high compressibility of carbonate soils. The field data and the simulations both show that the pipe embedment varies throughout each sand wave. The maximum embedment near the sand wave peak is greater by a factor of up to 5 compared to the minimum embedment found near the sand wave trough. The embedment profile is phase-shifted from the sand wave profile by 10–20% of the sand wave length.