UWA Oceans Institute

Observations on sedimentation-induced burial of subsea pipelines


OCEANS ONLINE     ISSUE 9. AUGUST 2016

A team of UWA researchers have explored subsea pipelines along the North West Shelf of Australia and discovered that fish can play an important role in increasing holes beneath pipelines.

This remarkable interplay between the marine sciences and pipeline engineering was unravelled via the OI’s cross-disciplinary links that brought together, PhD student Simon Leckie, Dr Henning More, Dr Scott Draper, Dr Dianne McLean, Professor David White and Professor Liang Cheng to analyse not only the effect of marine life on the subsea pipelines, but to also take a detailed look at other aspects affecting their embedment.

After pipelines are laid on the seabed, the level of the seabed around the pipeline (the pipeline ‘embedment’) can change due to transport of sediment towards (sedimentation) and away (scour) from the pipeline. These changes are poorly understood, which has prevented them being properly considered in pipeline design.

Significant numerical and physical modelling of these processes has been carried out. However, until now, there has been very few comparisons to field data. In this study, the changes to pipeline embedment induced by sediment mobility were analysed for three pipelines that lie on Australia’s North West Shelf and upper continental slope. The pipelines traverse a range of soil, metocean and marine habitat conditions, providing a unique opportunity to study the influence of these variables on pipeline embedment change.

In previous work, scour – that is, erosion of sediment from around the pipeline – has been thought to dominate the changes, but in this case direct sedimentation against the pipeline was shown to be the most important driver of embedment change. The degree of embedment change was seen to be highly dependent on the variation of currents along the pipeline, with the greatest effects occurring in an area of the outer continental shelf where high velocity, short duration currents associated with internal waves occur.

A series of laboratory tests using UWA’s O-tube facilities confirmed the sedimentation process, and allowed the time dependency of the embedment change to be quantified. By carefully accounting for laboratory scaling, both the amount of the embedment change and the rate of change were shown to be consistent between the field and laboratory.

Finally in a collaboration between engineers and a marine ecologist, fish were shown to play an important role in increasing holes beneath the pipeline – an influence that was overlooked in previously published physical and numerical modelling work, disseminated in the engineering literature. These holes interact with the sediment transport processes driven by the tide and internal waves, and correlated with areas of increased marine life adjacent to the shelf break and adjacent to some shallow reefs.

The full report can be found here.

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