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.