Experimental Investigation of the Effect of Cyclic Loading on Spudcan Extraction

Abstract

Selfelevating mobile jackup units have been employed in offshore exploration and development in shallow waters at depths of up to approximately 150 m. Jackups are designed to move to a new site after operations are completed. The spudcan footings, which can be embedded up to three diameters deep in soft soil, must therefore be extracted by jacking down the hull into the water and then floating it beyond the neutral draft. This provides the maximum pullout force to overcome the soil resistance to the jackups, but this force may not be sufficient. Problematic cases of this offshore are reported to take up to 10 weeks to extract, a costly exercise for the industry. A method sometimes used offshore is to cycle the spudcans vertically in an attempt to free them. This can be achieved by pushing and pulling the leg by leaving the hull afloat in the water and allowing the impact of small amplitude waves on the hull to generate cyclic loads on the spudcan. This paper reports a series of centrifuge tests investigating the ability to extract a spudcan under regular and irregular cyclic loading. Spudcan extraction tests were performed from a depth of three spudcan diameters in normally consolidated clay in a geotechnical beam centrifuge. The results demonstrate that successful extraction is dependent on the combination of mean pullout load and the amplitude of the cycling. It is also shown that insufficient tensile static loads and prolonged small cyclic loads result in the dissipation of the negative excess pore pressure at the spudcan invert caused by the buoyancy of the hull in excess of neutral draft. It results in consolidation of soil and changes in the shear strength of the soil and consequently either extraction of the spudcan after a long period of time or unsuccessful leg extraction.