Jack-ups usually operate at a number of sites during their design life, which necessitates their retrieval at the end of deployment on any of those locations. In clay soils, the process may however be difficult due to the potentially high extraction resistance. One option believed to ease the spudcan extraction is to use a jetting system at the invert of the spudcan to break the suction generated during the extraction and hence to reduce the extraction resistance. The feasibility and the performance of such a jetting system have been investigated in a geotechnical centrifuge. At 200g, the reduced scale spudcan models a 17.1 m diameter spudcan, which is penetrated up to 1.5 diameters into normally consolidated clay and held at operation load for the equivalent of 4.5 years. The main parameters investigated include the flow rate during jetting, the extraction rate and the direct load applied during extraction. The results demonstrate the feasibility of reducing the extraction resistance through jetting provided that the jetting flow rate is sufficiently high with respect to the extraction rate and that the extraction rate is high enough to ensure undrained behaviour.
The majority of the world's offshore drilling in shallow water (up to 120 m) is performed from self-elevating mobile units, which are generally referred to as ‘jack-up’. The units consist of a buoyant triangular platform resting on three independent truss-work legs, each with a rack and pinion system used to jack the legs up and down through the deck. The foundations of independent-leg jack-up platforms approximate large inverted cones, commonly know as ‘spudcan’. In modern jack-ups the spudcan diameter can be in excess of 20 m. When a jack-up rig is relocated, its legs have to be extracted from the seabed.
