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.

Jack-up push-over analyses featuring a new force resultant model for spudcans in soft clay

“…In this model, χ is assumed to be 0.6 based on centrifuge tests in kaolin clay (Zhang et al, 2013) when w p Z H c þ0.5D. This ratio is consistent with other experimental researches on extraction behavior of spudcan footing in soft clay (Purwana et al, 2005;Bienen et al, 2009a;Gaudin et al, 2011). For penetration depths in the transitional zone, i.e., H c ow p o H c þ0.5D, a simple linear interpolation is proposed to calculate the net and total vertical compressive capacities.…”

“…This is indeed indicated in the work by Templeton et al (2005), Templeton (2009) and Zhang et al (2011Zhang et al ( , 2012a, though the soil disturbance caused by spudcan installation was neglected in these numerical studies. The backflow also potentially acts as a "seal" on top of the spudcan, allowing suction to be developed, as observed in centrifuge experiments (Cassidy et al, 2004a;Purwana et al, 2005;Gaudin et al, 2011). Therefore, tensile loading of the jack-up leg is possible, though it is not currently considered in conventional site specific assessment.…”

The effect of water jetting on spudcan extraction from deep embedment in soft clay

“…Scale factors for other parameters such as load, pressure, and time, can be found in Garnier et al (2007) who made an inventory of the scaling laws and similitude questions related to centrifuge modelling. Gaudin et al (2011a) established the scaling factor on the flow rate to be N 2 /α 3 , where α is the reduction factor between the correctly scaled and the actual model nozzle diameters. For instance, prototype jetting nozzles of 38 mm diameter would require 0.19 mm diameter nozzles in a 1/200th reduced scale model.…”

“…This is in contrast to the effectiveness of bottom jetting which has been demonstrated through centrifuge tests to reduce the extraction resistance by diminishing the negative excess pore pressure, or suction, generated at the spudcan invert. The tests conducted at an acceleration of 200 g modelled a 17.11 m (prototype) diameter spudcan equipped with three sets of twelve jetting nozzles at the spudcan base (Bienen et al, 2009;Gaudin et al, 2011a). The embedment depth was up to 1.46 spudcan diameters corresponding to 25 m in prototype scale.…”

“…The reduction in extraction resistance was demonstrated to depend on the filling ratio rather than the jetting pressure, and at a filling ratio of 0.7 the jetting flow rate was found to negate the generation of negative excess pore pressure at the spudcan invert. Bienen et al (2009) and Gaudin et al (2011a) proposed a conceptual framework to estimate the required bottom jetting flow rate based on the centrifuge experimental data. Note that the conclusions are valid only if the behaviour of the surrounding soil is considered undrained during extraction process.…”