Centrifuge Experiments to Study Extraction of a Deeply Embedded Spudcan Using Top Jetting

Extracting the spudcan footings of mobile jack-up rigs from the seabed at the end of their operations is challenging when the capacity of the rig to pull is low compared to the extraction resistance of the spudcans. This is particularly the case when the spudcans are deeply embedded in soft clay and subjected to long periods of operation that place load on the foundations and allow for consolidation to occur in the soil. A technical solution used by the offshore industry to overcome spudcan extraction resistance is to use a water jetting system that ejects pressurised water through nozzles on the spudcan face. The aim of using water jetting with nozzles located at the top of the spudcan is to reduce extraction resistance through fracturing and softening of the upper soil. However, the efficiencies of such systems are not known to offshore jack-up operators. Top jetting efficiency is therefore addressed in this paper, which reports a series of physical experiments on jetted spudcan extraction conducted in a geotechnical beam centrifuge. The efficiency of water jetting is reported for extraction from depths of up to three diameters in normally consolidated clay, for different jetting flow rates. The excess pore pressure and maximum breakout force measured reveal insights into the extraction process with top jetting. The maximum extraction resistance is shown to be unaffected by top jetting but relates to the suction developed at the spudcan base, which can be reduced by jetting at the spudcan base (Bienen et al. 2009; Gaudin et al. 2011). Top jetting can, however, reduce the extraction resistance post breakout as indicated by the experimental results of this study.