Spudcan extraction from deep embedment in soft clay: Fid-Bau Portal

Spudcan extraction from deep embedment in soft clay

Kohan, Omid / Gaudin, Christophe / Cassidy, Mark J. / Bienen, Britta

Highlights • The mechanism at the maximum extraction resistance involves a reverse end bearing mechanism associated with plug uplift. • The mechanism is valid for embedment ratio up to 3 times the spudcan diameter and is independent of the operation period. • The peak suction at the spudcan invert is a function of the operation period. • The difference in pore pressure between the end of the operation period and the peak suction is approximately constant.

Abstract After drilling is completed, spudcan footings of mobile jack-up rigs are extracted from the seabed before the jack-up is manoeuvred to a new location. In some instances, the extraction may prove to be difficult and time consuming, especially when the spudcans are deeply embedded, because the pull-out capacity of the rig is less than the extraction resistance of the spudcans. In soft soil, the extraction resistance may be significantly augmented by the development of suction at the spudcan invert. To investigate this phenomenon, a deeply embedded 30mm diameter model spudcan was extracted in a series of physical model experiments conducted at an acceleration of 200× g in a geotechnical beam centrifuge. The spudcan, instrumented with two pore pressure transducers, one at the top and one at the bottom face, was extracted from normally consolidated clay and under undrained conditions. Eight tests are reported exhibiting embedments ranging from 1.5 to 3 spudcan diameters and varying operation periods. The excess pore pressure and maximum breakout force measured reveal insights into the magnitude of the suction forces at the spudcan invert, which were observed to increase with the embedment depth. No change in failure mechanism was observed between 1.5 and 3 spudcan diameters depth.