Effect of spudcan geometry on penetration and extraction resistance in clay

Spudcan foundations of various geometries are commonly used in the field for supporting offshore oil and gas drilling (jack-up) rigs. This paper investigates the effect of the spudcan and its base geometry on the penetration and extraction resistance in soft normally and lightly overconsolidated clay. A series of centrifuge tests was undertaken on model spudcans of three different geometries typically used in the field. The spudcans were extracted from penetrations of 2·64∼3 (area equivalent) diameters and after an operation period of ∼19 months. Additional large-deformation finite-element analyses allowed for exploring a range of spudcan base geometries. Corresponding field data were accumulated. The results from this study have confirmed the accuracy of the assumption of a circular conical-based footing and the use of area equivalent diameter for assessing spudcan penetration resistance, as suggested by ISO guidelines 19905-1. Simulation of a flat-based circular plate resulted in 19% higher resistance compared to that of conical-based footings. For spudcans with cutouts and with a peripheral skirt, maximum penetration resistances were, respectively, 14 and 12% lower, and maximum extraction resistances were 20 and 14% lower compared to those for a general spudcan. The corresponding required jack-up leg length was 9 and 7% higher. The effect of peripheral skirt length was shown to be minimal (at least for length relative to spudcan diameter ≤ 0·42).