Investigation of the potential of bottom water jetting to ease spudcan extraction in soft clay

Abstract

Retrieval of jack-up rigs at the end of their operations is common practice in the offshore industry, notably to move the rigs to another drilling location. In some cases the process is difficult, time consuming and costly, because the high extraction resistance on the jack-up's spudcan footings can exceed the capacity of the rig to pull. This is particularly the case in soft clay, where significant suction is developed at the spudcan invert. The main option available to operators to ease the spudcan extraction resistance is to use a jetting system at the spudcan invert to attempt to break the suction generated. However, there is a general consensus within the offshore industry that jetting systems, under their current configurations, have a limited efficiency. Centrifuge experiments have been performed at the University of Western Australia in order to understand the mechanisms taking place during jetting extraction, and to provide recommendations to optimise the jetting performance. A reduced-scale spudcan model simulating a 17·11 m diameter prototype spudcan with jets has been tested at 200 times the acceleration of Earth's gravity (known as 200g). It was extracted from penetrations of up to 1·5 diameters in normally consolidated clay at variable extraction rates and variable jetting flow rates. Measurements of the generated suction and the total extraction resistance after a preloading period have provided insight into the extraction mechanisms with jetting. The study has demonstrated that jetting can lead to significant reduction in extraction resistance, provided that the extraction rate is fast enough to ensure undrained extraction, and that a sufficiently high flow rate is applied with respect to the extraction rate.