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

Combined wave-current induced oscillatory seabed responses around two pipelines in tandem
Xu Chen and Dong-Sheng Jeng
School of Engineering & Built Environment, Griffith University Gold Coast Campus
Queensland, Australia

ABSTRACTTwo categories associated with the mechanisms of wave-induced seabedliquefaction have been identified (Zen and Yamazaki, 1990; Nago et al.,1993): oscillatory and residual liquefaction, which are in accordancewith the generated procedure of pore water pressure. The oscillatoryliquefaction normally appears instantaneously under wave troughs invery dense sand deposits with linear reversible soil characteristics,
which is related to phase lag and amplitude decay in the oscillatory
pore pressures (Madsen, 1978). The residual liquefaction is generally
caused by the build-up of the excess pore pressure under the volumetricwave loading, which is usually observed within the fully saturated
seabed (Seed and Rahman, 1978; Sumer, 2014). In this study, only
the oscillatory liquefaction mechanism with transient pore pressure isexamined.

The seabed stability in the vicinity of submarine pipelines under waveplus-current conditions is one of the major issues in offshore projects.
Previous works mainly focus on the evaluation of the seabed response
around a single pipeline. In this study, the previous model (PORO-FSSIFOAM) will be adopted to investigate the effect of the gap ratios (G/D)
of twin pipes on the wave & current-induced transient seabed response.
Based on numerical examples, the following conclusions were found:(i) the effect of different G/D between twin pipes on the distributionof excess pore-water pressure and resulting seabed liquefaction cannot
always be ignored; and (ii) as the gap ratio (G/D) is 1.25, the soil response beneath both pipelines is significant than in the condition of asingle pipeline.

Based on Biot’s poro-elastic theory (Biot, 1941), numerous numericalstudies for the Wave-Seabed-Structure Interactions (WSSI) have been
available in the literature. Among these, Jeng and Cheng (2000)
proposed an FDM model to discuss the possibility of wave-driven shear
failure around a pipeline. They found that as the burial depth and pipe
radius increased, the potential shear failure around a pipeline rises in theseabed with a lower degree of saturation. A few FEM studies have been
implemented to explore the wave-induced seabed responses around aburied or trenched pipeline (Gao et al., 2003; Gao and Wu, 2006; Zhang
et al., 2011). However, the aforementioned investigations have notconsidered the combined loadings of waves and currents. Liang et al.
(2020) proposed an OpenFOAM model to study the mechanism of soil
response and liquefaction induced by the combined waves and currents.
In their research, they pointed out the liquefaction phenomenon waseasier to observe around the inlet of the currents.

KEY WORDS: pipelines in tandem; excess pore-water pressure; horizontal distance between twin pipes; OpenFOAM; seabed liquefaction;
2D modelINTRODUCTION
Pipelines are one of the essential installations for the oil and gas
industry, which have been used for the transportation of oil and gas
from offshore to onshore. Since the first offshore pipeline was built byBrown Root to carry oil in 1954, the development of the submarine
pipeline networks has across the globe, even were regarded as “lifelines”
of the oil industry (Sumer and Fredsøe, 2002). The existence of thesubmarine pipeline does not only alter the nearby flow morphology
but also lead to the surrounding seafloor instability (including soilliquefaction, scour and shear failure) and ultimately cause damage orfailure of the pipeline (Sumer, 2014). In general, when the seabed isexposed to the wave, the generated dynamic pressures along the seafloor
can induce the pore pressures, effective stresses and soil displacements
within the seabed (Jeng, 2012). Then, the seabed in the vicinity of thepipeline could become unstable or even liquefied due to the increasing
excess pore pressures and the reducing vertical effective stresses. Oncethe liquefaction occurs, the soil will behave like a kind of heavy fluid
without any shear resistance, which greatly enhances the risk for thepipeline to sag.

Due to technical and economic factors, two pipelines are occasionallylaid in parallel. In the existing design method, the two tandem pipelines
were processed individually because of the large gap between them inpractice. However, Li et al. (2020) revealed the twin pipes become
more dependent with decreasing horizontal distance, which means that
it is not reasonable to treat them separately. The previous research
related to the two pipelines that are laid in parallel mainly focus on thelocal scour under different flow conditions. For example, Zhao et al.
(2015) investigated the scour around the twin pipelines under steadycurrent and various pipeline gap conditions. The numerical results

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