Effect of time on spudcan–footprint interaction in clay

Abstract

Mobile drilling platforms often return to sites where previous installation, operation and extraction have formed footprints on the seabed. Owing to soil consolidation during the jack-up operational period and the intervening period before reinstallation, the interaction between a new spudcan installation and an existing footprint is complex and time dependent. This paper presents a series of drum centrifuge model tests to investigate the changes in the shear strength of soils beneath and adjacent to a spudcan footprint in normally and overconsolidated clays. The changes with time after two different jack-up operational periods are presented. The results reveal that the soil beneath a footprint generally loses some strength initially, owing to soil remoulding, but it subsequently regains its strength with time as it reconsolidates. The soil remoulding and subsequent strength gain are found to be more significant in normally consolidated clay than in overconsolidated clay. A longer jack-up operational period has an effect of strengthening the underlying soil below the spudcan in both clays. The vertical load, induced horizontal load and moment on a spudcan during its reinstallation into an existing footprint at different times after footprint formation are studied. Compared with the load for installing a spudcan for the first time, the load required to reinstall the same spudcan to the same depth is smaller in normally consolidated clay if the elapsed time between footprint formation and spudcan reinstallation is relatively short. The required load for spudcan reinstallation subsequently increases with footprint elapsed time. For overconsolidated clay, the load required for spudcan reinstallation is always smaller than that for initial spudcan installation, irrespective of the time between installations. The effects of original in situ soil strength, changes in soil strength with time and footprint elapsed time on the interaction between spudcan and footprint are examined.Les plates-formes de forage mobiles reviennent souvent sur des sites où des installations, l'exploitation, et des opérations d'extraction précédentes ont formé des empreintes sur le fond marin. Du fait de la consolidation du sol survenue au cours des périodes opérationnelles sur installations de forage autoélévatrices, et de la période qui s'écoule avant la réinstallation, l'interaction entre l'installation d'un nouveau caisson d'enfichage et une empreinte existante est à la fois complexe et tributaire du temps. La présente communication illustre une série d'essais sur maquette centrifuge à tambour effectués pour examiner les variations de résistance au cisaillement des sols sousjacents et adjacents à l'empreinte du caisson d'enfichage, dans des argiles à consolidation normale ou surconsolidées. Les changements survenus en fonction en temps, à la suite de deux périodes opérationnelles avec installations de forage autoélévatrices, sont présentés. Les résultats révèlent que le sol situé sous une empreinte perd génér-alement, dans un premier temps, ...

Centrifuge and Numerical Modelling of Reaming as a Mitigation Measure for Spudcan-Footprint Interaction

“…The sign conventions shown in the right hand side of Figure 7 are used for plotting the VHM forces during spudcan repenetration. In general, these observations are consistent with those from existing studies (e.g., Stewart and Finnie, 2001;Cassidy et al, 2009 andGan, 2009). Positive vertical pressure (V) indicates that the spudcan resists upward acting ground reaction.…”

“…Depending on availability and work requirements, the same jackup, or most often, a different jack-up with different design, capability and size of spudcan footing may be deployed. Other researchers have also identified important variables that influence the spudcan reinstallation behavior, for instances, leg-hull fixity (Gaudin et al, 2007 andKong, 2011), preload level and leg bending stiffness (Cassidy et al, 2009), soil shear strength mapping across the footprint (Gan et al, 2012) and crater geometry of footprint (Kong et al, 2013). Osborne (2005) reported a survey on incidents related to geotechnical aspects of jack-up units.…”

Failure mechanisms of a spudcan penetrating next to an existing footprint

“…Reinstallation next to these footprints is a problematic operation because the spudcan is subjected to eccentric and inclined loading conditions. Previous experimental studies have attempted to measure the combined loading on a spudcan reinstalling at different offset distances [3][4][5][6][7][8].…”

Analysis of spudcan–footprint interaction in a single soil with nonlinear FEM

“…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 (2010Kong et al ( , 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.…”