Influence of base suction on extraction of jack-up spudcans

Operators of mobile jack-up rigs often face difficulties when extracting spudcan foundations of jack-up rigs in the field. Centrifuge model tests were carried out to evaluate the uplift resistance of a jack-up spudcan in normally consolidated clay during extraction. The top and bottom faces of the model spudcan were instrumented with total pressure and pore pressure transducers. These pressure readings and pore pressure changes in the soil beneath the spudcan were monitored throughout the installation, operation and extraction of the spudcan. In the present study, the operation stage was simply simulated by maintaining a constant vertical load on the spudcan. Tests were conducted to examine the effects of duration of operation period and ratio of operational working load to installation load on the uplift resistance of the spudcan. The test results revealed that suction developed at the interface between the spudcan base and the underlying soil, and the magnitude of base suction increased with the operation period of the spudcan. It is noted that, during extraction, the resistance to overcome the soil resistance above the spudcan increases only slightly with spudcan operation period. Hence the increase in base suction is established to be the key contributor for the larger breakout force required to extract spudcans with longer operation periods.