2311
Influence of jack-up footprints on mudmat stability – How beneficial are 3D effects?
Influence des dépressions laissées par les jack-ups sur la capacité portante des mudmats – quels
sont les effets bénéfiques d’une analyse en 3D?
Ballard J.-C., Charue N.
Fugro GeoConsulting Belgium
ABSTRACT: Jacket platforms are piled into the seabed but need to be supported temporarily by mudmats during installation. They
sometimes need to be located next to seabed features such as pug marks formed by previous deployments of jack-up rigs. These
features may influence the bearing capacity of the mudmats. This is a 3D problem for which simplified approaches are unsatisfactory,
simplified 2D plane strain simulations can lead to over-conservative results. This paper presents a project example in very soft clay
for which the software package Plaxis 3D has been successfully used. The presence of a pug mark was found to degrade significantly
the yield surface in the VHM load space. A comparison between 2D and 3D analyses shows that the beneficial 3D effects are
substantial, especially when the pug mark is located at the corner of the mudmat. The zone of influence of the pug mark is also much
more limited when the problem is modelled in 3D.
RÉSUMÉ : Les platesformes de type « Jacket » sont fondées sur pieux mais nécessitent d’être supportées temporairement pendant
l’installation par des mudmats (fondations de type superficiel). Ces jackets sont parfois situées à proximité de dépressions laissées par
l’installation antérieure de jack-ups. Ces dépressions peuvent influencer la capacité portante des mudmats. Il s’agit d’un problème 3D
typique pour lequel aucune solution simplifiée n’existe. Une approche 2D (en état plan de déformation) peut même mener à des
résultats trop conservatifs. Cet article présente un exemple dans de l’argile molle pour lequel la suite de logiciels Plaxis a été utilisée
avec succès. Les conclusions sont les suivantes : la présence des dépressions modifie singulièrement la surface de rupture dans
l’espace VHM. Une comparaison entre les approches 2D et 3D montre que les avantages à faire appel au 3D sont substantiels,
spécialement quand la dépression est située à proximité du coin du mudmat. La zone d’influence de la dépression est aussi bien plus
limitée lorsque le problème est modélisé en 3D.
KEYWORDS: Pug mark, mudmat, stability, VHM, 2D, 3D, Finite Element Analysis, soft clay, remoulded, jack-up, mesh
1 INTRODUCTION
Jacket platforms are the most common type of offshore
structure in the offshore hydrocarbons industry (Dean, 2010).
They consist of open-framed steel structures made of tubular leg
chords, horizontal bracing, and diagonal bracing. These
structures are piled into the seabed but need to be supported
temporarily by mudmats during installation. Mudmats are
essentially flat stiffened metal plates attached to legs or the
lower braces. In soft soils, mudmats can cover the entire surface
between the legs to maximise the bearing area. They are
generally subjected to combined Vertical, Horizontal and
Moment (VHM) loads induced by the jacket weight, wind,
waves and currents.
Jacket platforms are not always installed on a virgin seabed
and are sometimes located next to features such as pug marks
formed by previous deployments of jack-up rigs. A jack-up is a
mobile, self-elevating offshore platform consisting of a hull and
three or more retractable legs passing through the hull
(McClelland et al, 1982). A unit moves onto location, sets its
legs onto the seabed, and raises its hull out of the water. The
legs are supported on independent foundations called spudcans.
Penetration and extraction of spudcans in soft grounds create
zones of remoulded soil and seabed depressions (Hossain et al,
2012). These seabed features potentially influence the bearing
capacity of the mudmats and need to be accounted for in the
stability verification.
Mudmats subjected to combined VHM loads and located
next to a jack-up footprint is a 3D problem for which simplified
approaches for analysis do not exist. Simplified 2D plane strain
simulations are generally performed but they can lead to over-
conservative results. This type of problem is better analysed by
means of 3D Finite Element (FE) analyses.
This paper presents a project example in very soft clay for
which the software package Plaxis 3D (Plaxis, 2011) has been
used successfully. The analysis allowed confidence to be
established for the selected location of the mudmat with respect
to a pug mark. In contrast, a simplified 2D analysis suggested
that the proximity of the mudmat to the pug mark was
unacceptable.
It is shown for this particular example how the presence of a
pug mark degrades the yield surface in the VHM load space. 3D
analyses are compared with 2D analyses to quantify the
beneficial 3D effects for different pug mark locations
.
2 PROBLEM GEOMETRY AND SOIL CONDITIONS
A 30 m by 30 m square mudmat is considered. The mudmat is
located next to a circular pug mark of 30 m in diameter.
Analyses were performed for 2 positions of the mudmat. The
first position considers a pug mark located along the width of
the mudmat while the second position considers a pug mark
located at the corner of the mudmat, as illustrated on Figure 1.
The distance d between the edges of the mudmat and the pug
mark is varied in the analysis.
Soil conditions in this example consist of very soft clay. The
soft deposit is considered to be 15 m thick and underlain by
stiffer soils, which are not modelled. The intact undrained shear
strength increases linearly with depth according to s
u
= 4 + 0.8z
(in kPa), where z is the depth below ground level in meter. This
gives a strength heterogeneity
= 6 where
= kB/s
u0
, k is the
