Pipeline scour and self-burial on a thin veneer of sand overlying rock due to steady currents

Draper, S., Cheng, L., Sun, W., An, H., White, D., Bransby, F., Borges-Rodriguez, A. and Griffiths, T. (2015) Pipeline scour and self-burial on a thin veneer of sand overlying rock due to steady currents. Cheng, Liang, Draper, Scott and An, Hongwei (eds.) In Scour and Erosion: Proceedings of the 7th International Conference on Scour and Erosion, Perth, Australia, 2-4 December 2014. CRC Press. pp. 601-610 .

Abstract

At many locations on the NorthWest Shelf of Australia the seabed can be characterised as a thin veneer of sand overlying rock or cemented sediment.Very little is known about the potential for pipelines to selfbury in this configuration, despite the implications on pipeline stability design and pipeline buckle reliability. In this paper we describe a series of model scale laboratory experiments that have been performed to explore the mechanisms of scour due to steady currents for a pipeline placed on sand overlying rock. First we show (using a simple quasi steady experiment) that the onset of scour due to piping requires much larger current velocities when there is only a thin veneer of sand overlying rock compared with an infinitely deep layer of sand. This result can be explained in terms of the seepage flow net beneath the pipe. Secondly we show, using experiments in a recirculating (O-Tube) flume, that the scour mechanisms for a pipeline placed on sand overlying rock can be summarised as (i) tunnel scour truncated by the rock as the pipeline lowers; (ii) lee wake scour following lowering of the pipe; and (iii) sand transport over the pipeline (providing far field sediment is mobile, i.e. live bed scour). The final ‘burial’ state of the pipeline resulting from these mechanisms may or may not include sediment either side of the pipe, with the outcome depending on the initial thickness of the sand veneer relative to the pipe diameter and the contact achieved between the pipe and the underlying rock layer as the pipe lowers. We conclude by discussing the implications of these findings for pipeline design.

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Contributors

Author: D. White

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