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Numerical investigation of dynamic embedment of offshore pipelines
Étude numérique de l’ancrage dynamique de conduites enterrées maritimes
Dutta S., Hawlader B.
Memorial University,
St. John’s, Canada.
Phillips R.
C-CORE,
St. John’s, Canada.
ABSTRACT: Pipelines are one of the key components of offshore oil and gas development programs. Deep water pipelines are often
laid on the seabed and penetrate into soil a fraction of their diameter. High operating temperature and pressure generate axial stress
that could buckle the pipeline laterally. The embedment and formation of soil berm have a significant effect on lateral resistance. The
embedment of a pipeline depends on stress concentration at the touchdown point (TDP) and dynamic laying effects. In this study,
large deformation finite element modelling of dynamic penetration of offshore pipeline is presented. The Coupled Eulerain
Lagrangian (CEL) technique is used to develop finite element model. The pipe is first penetrated into the seabed followed by a small
amplitude cyclic lateral motion. Results from the finite element models are compared with centrifuge test results. High plastic shear
strain is obtained around the pipeline during cyclic loading which causes significant pipe embedment. The shape of soil berm is
different from that of monotonic pipe penetration.
RÉSUMÉ : Les conduites enterrées sont un des éléments clés des programmes de développement de pétrole et de gaz. Des conduites
enterrées en eau profonde sont souvent mises sur le plancher océanique et pénètrent dans le sol sur une fraction de leur diamètre. La
température et la pression de fonctionnement élevées génèrent une contrainte axiale qui peuvent déformer la conduite latéralement.
L'ancrage et le sol encaissant ont un effet significatif sur la résistance latérale. L'enfouissement d'une conduite dépend des
concentrations de contraintes et des effets dynamiques de la pose. Dans cette étude, une modélisation par éléments finis en grande
déformation de la pénétration dynamique de la conduite est présentée. Une technique de type Eulérien Lagrangien (CEL) est utilisée
pour développer le modèle éléments finis. Le tuyau est d'abord mis en place dans le fond marin puis subit un mouvement cyclique de
faible amplitude latérale. Les résultats des modèles éléments finis sont comparés avec les résultats d’essais en centrifugeuses.
D’importantes valeurs de la déformation plastique sont obtenues autour de la canalisation lors du chargement cyclique ce qui
nécessite un ancrage suffisant de la conduite. La forme du sol encaissant est différente de celle du tuyau mis en place statiquement.
KEYWORDS: pipelines, dynamic embedment, clay, large deformation analysis.
1 INTRODUCTION
As-laid pipelines are commonly used in deepwater. During
installation the as-laid pipeline could be penetrated a fraction of
its diameter into the seabed (Bruton et al. 2006), and a soil berm
could be formed. The soil around the pipelines provides not
only the thermal insulation and hydrodynamic stability to the
pipe but also resistance to pipeline walking and lateral buckling
during high operating temperature and pressure. Accurate
assessment of as-laid pipe embedment is extremely difficult.
Depending upon sea state, vessel conditions, pipe stiffness and
soil conditions, the pipeline might experience both in-plane and
out-of-plane cyclic motion during installation (Westgate et al.
2010, 2012), which causes dynamic embedment of the pipeline.
The penetration of a pipeline under static load can be
obtained using bearing capacity theory, analytical solution or
finite element techniques. In the current engineering practice,
two additional factors are used to estimate the embedment of
pipelines: (a) additional vertical force near the TDP (the point
where the pipe first touches the soil) due to catenary effects and
(b) dynamic lay effects. A number of methods have been
proposed in the past to estimate these factors (Carneiro et al.
2010, Oliphant and Yun 2011). For example, Randolph and
White (2008) proposed an empirical equation to calculate the
touchdown lay factor (
f
lay
) using pipe submerged weight,
bending rigidity, horizontal component of effective tension, lay
angle, water depth and seabed stiffness. The embedment factor
for dynamic lay effects (
f
dyn
) varies between 2 and 10 (Lund
2000, Bruton et al. 2006). This wide range of variation in this
factor makes the assessment of pipe embedment very difficult.
During installation, both vertical and lateral pipe motions
can soften the seabed soil near the pipe. Soil
softening/remolding together with water entrainment can reduce
the undrained shear strength of soil. Field observation
(Westgate et al. 2010) and physical modeling using
geotechnical centrifuge (Cheuk and White 2011) show that the
horizontal cyclic motion, although small amplitude, has a
significant effect on pipe embedment.
The main purpose of this study is to conduct large
deformation finite element (FE) analysis for dynamic events
during the installation of pipeline. Coupled Eulerian Lagrangian
(CEL) technique is adopted in the analysis using ABAQUS FE
software. Four FE models are developed for two different soils:
kaolin and high plasticity clays (plasticity index for kaolin is 34
and for high plastic clay is 100-130, Cheuk and White 2011).
The results are compared with the centrifuge test results
vailable in the literature.
a
2 PROBLEM DEFINITION.
The problem considered in the present finite element (FE)
modelling is shown in Fig.1. During laying, offshore pipelines
usually penetrate vertically into the seabed due to its self-weight
and catenary effect near the touchdown zone (TDZ). The vessel
movement from wave loading could cause small amplitude
cyclic motions in the
x
-direction. As the pipeline is under a
vertical load (
p
0
), the lateral movement in the
x
-direction could
cause additional vertical penetration as shown by Stage-II and
III in Fig. 1.
