Proceedings of the Twenty-first (2011) International Offshore and Polar Engineering Conference
Maui, Hawaii, USA, June 19-24, 2011
Copyright © 2011 by the International Society of Offshore and Polar Engineers (ISOPE)
ISBN 978-1-880653-96-8 (Set); ISSN 1098-6189 (Set); www.isope.org
Cyclic Movement Behavior of Shallowly-Embedded Offshore Pipeline on Carbonate Seabed
Tomiya Takatani
Department of Civil Engineering, Maizuru National College of Technology
Maizuru, Kyoto, Japan
bility of a shallowly-embedded offshore pipeline resting on the carbonate sandy seabed under severe storm condition by an advanced finite
element analysis using a non-linear spring element to represent a nonlinear relationship between pipeline and seabed surface.
In general, the soil resistance is strongly influenced by the increasing embedment of pipeline with cyclic horizontal movement behavior. Pipeline movement behavior due to cyclic loading strongly depends on a pipe-soil interaction and also needs some appropriate friction factors, which are different from the conventional values used in
the assessment of the on-bottom stability of an offshore pipeline under
hydrodynamic loading. It is, therefore, very important to investigate the
large-amplitude and cyclic pipe-soil interaction behavior using experimental tests and numerical analyses in order to accurately evaluate the
pipeline movement behavior during cyclic loading.
The purpose of this paper is to investigate the pipeline movement
behavior during cyclic loadings, focusing on the cyclic drag and lift
forces with the submerged self-weight of pipeline and the pipe-soil
interaction effect. In this paper, a two-dimensional non-linear finite
element analysis based on an effective stress theory is employed for a
pipe-soil interaction problem. To investigate the influences of bonding
at pipe-seabed soil interface, non-linear joint elements are used at pipeseabed soil interface. Pipe is assumed to be 1.0m diameter and 0.25m
for its initial depth, and seabed is assumed to be a carbonate soil with
two relative densities Dr=60% and 80%. Cyclic loadings due to both
drag and lift forces for a 100-year return period storm condition are
evaluated using the Fourier decomposition method (Sorensen et al.,
1986). In particular, the rotation behavior of pipeline may be generated
by a cyclic twisting moment force induced by the initial embedment of
pipe and cyclic loadings. Shallowly-embedded offshore pipeline stability during cyclic horizontal and vertical loadings is numerically investigated by taking into consideration the carbonate soil conditions, the
amplitude of constant vertical loading, and the cyclic rotation behavior
of pipeline.
ABSTRACT
A non-linear finite element analysis for a pipe-seabed interaction problem based on an effective stress theory was carried out in order to investigate the stability of a shallowly-embedded offshore pipeline on the
carbonate sandy soil under the hydrodynamic environment. Pipeline
movement behavior during cyclic loadings due to both drag and lift
forces caused by waves and currents was numerically simulated
through some carbonate soil conditions, the cyclic horizontal and vertical loadings under constant vertical loading. Pipeline movement behavior greatly depends on the carbonate soil conditions, the amplitude of
vertical loading, and the cyclic rotation behavior of pipeline.
KEY WORDS: Offshore pipeline; carbonate seabed; relative density;
cyclic loading; Fourier force model; non-linear finite element method;
joint element.
INTRODUCTION
In general, a shallowly-embedded offshore pipeline is directly exposed
to the drag and lift forces induced by the hydrodynamic environment.
The pore pressure accumulation in the soil around the pipeline induced
by these forces will reduce the effective strength of the soil and degrade
the bearing response of the pipeline. In particular, the reduction of bearing capacity in the soil and the cyclic movement of pipeline due to both
drag and lift forces caused by waves and currents will lead to a large
deformation of pipeline and a sudden break-out, which has a serious
influence on the safe operation of the pipeline. Recently, Dingle et al.
(2008) observed the deformation mechanism during cyclic lateral
movement of pipeline through some centrifuge tests. White and Cheuk
(2008) proposed a simplified modeling of cyclic lateral pipe-soil interaction, based on the accumulation and deposition of berm materials.
Zhou et al.(2008) and Wang et al.(2009) conducted numerical simulations of static and dynamic pipe embedment in softening soil, and also
obtained the seabed deformation and the localized remoulding behavior
during pipe embedment to compare with centrifuge modeling study.
Chatterjee et al.(2010) studied pipe–soil interaction during vertical penetration and large amplitude lateral pipe motion by a large deformation
finite element analysis, and investigated differences in behavior between light and heavy pipes. Takatani et al.(2008) investigated the sta-
ANALYTICAL METHOD
A non-linear finite element analysis based on an effective stress theory
for a pipe-seabed interaction problem is conducted in order to investigate the stability of a shallowly-embedded offshore pipeline on the
carbonate sandy soil under the hydrodynamic environment. In this
analysis, both drag and lift forces evaluated from current velocity
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