(1) Predicting scour beneath subsea pipelines from existing small free span depths under steady currents

Jun Y. Lee ^{a , ∗},Jasmin McInerney ^b , Remo Cossu ^{c , a} , Zhi Q. Leong ^a , Alexander L. Forrest ^{b , a}
a National Centre for Maritime Engineering and Hydrodynamics, Australian Maritime College, University of Tasmania, 1 Maritime Way, Newnham Tasmania 7248, Australia

b Department of Civil and Environmental Engineering, University of California –Davis, 1 Shields Avenue, Davis, CA 95616, United States of America

c School of Civil Engineering, University of Queensland, St Lucia Queensland 4072, Australia

Received 5 October 2016; received in revised form 9 February 2017; accepted 3 March 2017
Available online 18 March 2017

Abstract

An equation was developed to predict current-induced scour beneath subsea pipelines in areas with small span depths, S . Current equations for scour prediction are only applicable to partially buried pipelines. The existence of small span depths (i.e. S / D < 0.3) are of concern because the capacity for scour is higher at smaller span depths. Furthermore, it is impractical to perform rectification works, such as installing grout bags, under a pipeline with a small S / D . Full-scale two-dimensional computational fluid dynamics (CFD) simulations were performed using the Reynolds-averaged Navier–Stokes approach and the Shear stress transport k –ω turbulence model. To predict the occurrence of scour, the computed maximum bed shear stress beneath the pipe was converted to the dimensionless Shields parameter, and compared with the critical Shields parameter based on the mean sediment grain size. The numerical setup was verified, and a good agreement was found between model-scale CFD data and experimental data. Field data were obtained to determine the mean grain size, far field current velocity and to measure the span depths along the surveyed pipe length. A trend line equation was fitted to the full-scale CFD data, whereby the maximum Shields parameter beneath the pipe can be calculated based on the undisturbed Shields parameter and S / D .
© 2017 Shanghai Jiaotong University. Published by Elsevier B.V.
This is an open access article under the CC BY-NC-ND license. ( http://creativecommons.org/licenses/by-nc-nd/4.0/ )
Keywords: Sediment scour; Subsea pipelines; Small span depths; Steady currents.

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