Predicting the rate of scour beneath subsea pipelines in marine sediments under steady flow conditions

Mohr, H., Draper, S., Cheng, L. and White, D.J. (2016) Predicting the rate of scour beneath subsea pipelines in marine sediments under steady flow conditions. Coastal Engineering, 110, 111-126. (doi:10.1016/j.coastaleng.2015.12.010).

Abstract

Model scale experiments of scour beneath a submarine pipeline, coupled with erosion testing, have been undertaken using two marine sediments and five artificial sediments having a wide range of grain size. The experiments reveal that for both the marine and artificial sediments the time scale of scour beneath the pipeline depends on the erosion properties of the sediment. For coarser sediments, mobilised mainly in transport along the bed, the rate of scour is found to agree well with the existing empirical formula of Fredsøe et al. (1992). In contrast, for finer sediments that are mobilised mainly through entrainment into suspension and can have relatively high erosion resistance, the rate of scour is different to that predicted using the same empirical formula. To explain this result, theoretical arguments are used to relate the rate of scour beneath a pipeline to the fundamental erosion properties of the sediment; namely the transport rate along the bed and the true erosion rate of the sediment. These arguments lead to two new empirical formulas that may be used to predict the time scale of the scour process beneath subsea pipelines. The first formula is appropriate when the sediment scours predominantly via transport of sediment along the bed, and is consistent with the empirical formula due to Fredsøe et al. (1992). The second formula is appropriate when sediment erodes mainly via entrainment into suspension, as is often the case for fine or 'cohesive' sediments. Collectively, the two formulas may be used in practice to make predictions of the rate of scour for pipelines in marine sediments and artificial sediments, provided erosion testing results are available.

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Contributors

Author: D.J. White

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