Proceedings of the Thirty-second (2022) International Ocean and Polar Engineering Conference
Shanghai, China, June 5–10, 2022
Copyright © 2022 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-81-4; ISSN 1098-6189

www.isope.org

Numerical study of local scour of subsea pipelines in combined steady and oscillatory flow
Vatsal Dhamelia, Ming Zhao, Pan Hu
School of Engineering, Design and Built Environment, Western Sydney University
Penrith, NSW, Australia

scour around a vibrating pipeline in steady current. It was found thatthe maximum scour depth occurred slightly behind the pipeline rather
than directly below it. Zang et al. (2021) studied vortex induced
vibration of a pipeline on a silty seabed and its effects on local scour
under steady current in a clear water condition. Their results indicate
different velocity has different scour profiles; bedload is dominant forsmall velocity whereas suspended load is dominant and scour profiles
are irregular in high velocity cases.

ABSTRACT
Submarine pipelines have played a significant role in coastal
engineering, marine engineering, and offshore engineering to transport
offshore oil and gas to shore. Over the past decades, identifying thecause of pipeline failure has been a top interest of many researchers.
Local scour is reported to be one of the main causes that threaten thesafety of subsea pipelines. A great number of experimental andnumerical studies have been performed to investigate local scour
around subsea pipelines. Oscillatory and steady flows are commonlyused to model flow near the sea floor induced by the motion of waves
and currents, respectively. However, the studies of local scour aroundcombined steady and oscillatory flow are limited. In this study, local
scour around subsea pipelines under combined steady and oscillatory
flow is investigated using numerical method. The numerical modelcomprises RANS flow model, k-ω turbulence model, sedimenttransport model and seabed evolution model. The effects of thecombined flow ratio of the steady to oscillatory flow velocities on thescour are quantified and discussed.

INTRODUCTION

Wave-induced flow is generally modelled as oscillatory flow whenpipeline scour is studied under waves. Yang et al. (2019) investigatedscour around a piggyback pipeline under oscillatory flow conditions.
They mainly focused on the influence of the gap ratio and KeuleganCarpenter (KC) number on the scour depth. The KC number is a
nondimensional number for a pipeline in oscillatory flow defined as:(1)
where Uw and T are the amplitude and the period of oscillatory flow,respectively, and D is the diameter of the pipeline. An increase in KC
increases the scour depth of a pipeline in oscillatory flow (Yang et al.,2019). Liu et al. (2016) proposed a numerical model to investigate
nonlinear wave induced local scour around pipeline on flat and sloping
bed conditions. Their numerical model suggested that the maximum
scour depth of a pipeline on a flatbed is proportional to the square of
the KC number. It is found the KC number, Shields parameter and theinitial gap between pipe and bed play important roles in the scour
process.

Over the last few decades, local scour below submarine pipelines havebeen studied extensively because it has strong potential of causing
pipeline failure in ocean engineering. If a pipeline is shallowly buried,
the scour occurs due to the pressure difference between upstream anddownstream of the pipeline exceeds a limit (Zang et al., 2021). Afterthe onset of scour, the increased flow velocity through the small gap
between the pipeline and erodible bed leads to strong amplification ofshear stress on the bed, which is the acting force for sediment transportand scour. These problems have attracted many researchers to perform
extensive experimental and numerical studies over the past years.
Many scholars have conducted investigations of the development oflocal scour near subsea pipelines under steady current. Liu et al. (2021)
developed a numerical model by solving incompressible Navier-stokes
equation with the shear-stress transport k-ω model to investigate local

In many engineering applications, pipelines are subjected to the
combined action of waves and currents. The complexity of theinteraction between steady and oscillatory flows makes the scour more
complicated (Sumer and Fredsøe, 1996, Cevik and Yuksel, 1999,Ahmad et al., 2019, Zhao et al., 2021). In combined steady andoscillatory flow, a flow ratio is defined as:r(2)where r is the combined flow ratio, Uc is the steady current velocity.
The flow ratio r represents the contribution of steady flow velocity.
Ahmad et al. (2019) performed a computation fluid dynamics (CFD)
study of local scour under combined steady and oscillatory flows using
open source REEF3D. It was further confirmed that an increase in KC
number increases the scour depth below the pipeline. In addition, thedifference between current alone and oscillation alone reduces with theKEY WORDS: Subsea pipeline; scour; steady flow; oscillatory flow;
combined flow.

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