Analysis of spudcan–footprint interaction in a sin

Authors: DongFeng Mao MingHui Zhang Yang Yu MengLan Duan Jun Zhao

Publish Date: 2015/01/24

Volume: 12, Issue: 1, Pages: 148-156

PDF Link

Abstract

The footprints that remain on the seabed after offshore jackup platforms completed operations and moved out provide a significant risk for any future jackup installation at that site Detrimental horizontal and/or rotational loads will be induced on the base cone of the jackup platform leg spudcan in the preloading process where only vertical loads are normally expected However there are no specific guidelines on design of spudcan reinstallation very close to or partially overlapping existing footprints This paper presents a rational design approach for assessing spudcan–footprint interaction and the failure process of foundation in a single layer based on nonlinear finite element method The relationship between the distance between the spudcan and the footprint and the horizontal sliding force has been obtained Comparisons of simulation and experimental results show that the model in this paper can deal well with the combined problems of sliding friction contact fluid–solid coupling and convergence difficulty The analytical results may be useful to jackup installation workovers close to existing footprintsReinstalling a spudcan very close to or partially overlapping existing footprints is generally not recommended in the guidelines SNAME OC7 panel 2007 Hossain and Randolph 2008 In a situation where this is inevitable the guidelines recommend the use of an identical jackup same footing geometries and leg spacing and locating it in exactly the same position as the previous unit where possible However it is unlikely that two jackup units have an identical design because the structures of most units are often custommade and the deployments of units are subject to availability It is evident that existing guidelines are not adequate for rig operators to install jackup units in close proximity to existing footprints safelyFootprint issues involve soil elastoplasticity material and geometric nonlinearities fluid–solid coupling friction contact during spudcan preloading and difficult convergence of numerical solutions Hanna and Meyerhof 1980 Kellezi and Stromann 2003 DeJong et al 2004 Deng and Kong 2005 Leung et al 2008 Previous research mainly focuses on the spudcan–footprint interaction through the centrifuge model test Murff et al 1991 Hossain et al 2005 Cassidy et al 2004 2009 Teh et al 2010 Gan 2009 Gan et al 2012 Kong et al 2010 2013 Xie et al 2012 conducted a series of drum centrifuge model tests to investigate spudcan–footprint interaction and the effect of leg stiffness on spudcan–footprint interaction With the centrifuge model tests Stewart and his coworkers Stewart 2005 Stewart and Finnie 2001 studied the effect of bending rigidity of legs on spudcan–footprint interaction and the influence of the distance between the spudcan and the footprint on sliding Dean and Serra 2004 discussed the effect of equivalent stiffness of legs on spudcan–footprint interaction Teh et al 2006 reported a set of test results investigating the effects of sloping seabed 30° inclined to the horizontal and footprint on loads developed in jackup legs They found that the effect of the footprint is much greater than that of the seabed slope This indicates that the footprint problem is more serious than a sloping seabed Other researchers have tried to investigate the footprint problem with numerical simulation Zhang et al 2011 2014 Jardine et al 2002 simplified a threedimensional model to a plane strain one to deal with footprint issues The current understanding of this topic is still insufficient and only a small number of studies of the footprint problem are available in the public domain Although it is a great challenge to obtain a converged numerical solution a good numerical model and solution is very important because it is able to achieve more accurate estimation of carrying capacity of spudcans and better explanations for tests This paper takes various factors including failure process of foundation nonlinearity sliding friction contact and fluid–solid coupling into account It discusses the finite element model of spudcan–footprint interaction in spudcan reinstallation near an existing footprint as well as handling relative parameters With the model of the spudcan–footprint interaction the changes of horizontal sliding force on the spudcan at different offset distances between the spudcan and the footprint were analyzed with ABAQUS software The finite element model was validated by comparing the simulation result with experimental resultsSince the ultimate bearing capacity would be underestimated if the initial geostress equilibrium were not considered in numerical simulation this paper deals with the initial geostress equilibrium first and imports a stress file with an ‘initial conditions’ method This is instead of the ‘Geostatic’ way a commonly used geostress equilibrium analysis method in general simulation involving in soil that is difficult to deal with for such a complex problem as spudcan–soil interaction with an existing footprint In addition because of serious soil deformation under a large spudcan penetration depth in order to avoid huge warping and ensure accuracy of calculation ALE selfadaptive meshes are employed

Keywords: