Breakout force required for jack-up spudcan extraction from sand-over-clay seabeds

Spudcan retrieval from clay soils remains a major concern offshore as the extraction force required to overcome suction and soil resistance often exceeds the pulling capacity available on the mobile jack-up, causing extensive delays. Although methods to calculate extraction resistance have been recently suggested for seabeds of pure clay, to date there is no guidance available for the commonly encountered sand-over-clays. Based on failure mechanisms observed in half-spudcan visualisation tests, and calibrated against an extensive geotechnical centrifuge database of precisely measured extractions, this paper presents a method for calculating the force required to extract the spudcan foundations of mobile jack-up platforms after they have penetrated through a sand layer into underlying clay. Complexities, such as the strength degradation and strength recovery of the underlying clay soil, that occurs during spudcan installation and jack-up operations, are accounted for. Validation of the proposed method is demonstrated by retrospective prediction of the centrifuge testing database. The method outlined will allow operators of jack-up platforms to assess the extraction force prior to jack-up installation and to plan operational scenarios based on seabed conditions.