Centrifuge model study on spudcan footprint interaction 6

Chapter – Effect of Time on Spudcan-footprint Interaction Chapter Effect of Time on Spudcan-footprint Interaction 5.1 Introduction Footprints formed after the extraction of a jack-up unit have two main characteristics: ground depression and non-uniform soil strength beneath the depression. It has been established in Chapter that the physical profile of the footprint depression is the dominant factor in the spudcan-footprint interaction in firm/stiff clays. On the other hand, the soil strength variation in the footprint is dominant for spudcan-footprint interaction in soft clays. More importantly, the degree of soil strength variation changes with time as excess pore pressures generated by the previous spudcan installation dissipate (Gan et al., 2007 and Leung et al., 2007). The soil undergoes re-consolidation during the operational period and after the extraction of the spudcan. This has significant implications on spudcan-footprint interaction. This chapter presents experimental data on the soil characteristics beneath and around a footprint at various times and its influence on an offcentred spudcan re-installation. The centrifuge tests presented in this chapter were conducted on the University of Western Australia (UWA) drum centrifuge. It is worth highlighting that the tests presented in Chapter were conducted in NUS whereas in this chapter, the tests were conducted in UWA. 148 Chapter – Effect of Time on Spudcan-footprint Interaction It is noted that the spudcan, soil and preload pressure were different for both sets of experiment. The effect of these parameters will be studied and generalized by dimensionless analysis in Chapter 6. The schematic model setup of test conducted in UWA is shown in Fig. 5.1(a). The soil shear strength profiles at various positions from the footprint centre were evaluated by ball penetrometer tests. The effect of the footprint on the new spudcan installation was evaluated in terms of vertical and horizontal forces and rotational moment acting at the spudcan reference point with the sign convention as shown in Fig. 5.1(b). The primary objectives of the experiments are to investigate: i) the soil shear strength changes with elapsed time after the formation of a footprint due to spudcan installation and extraction, ii) effects of jack-up operational period on soil strength characteristics, and iii) effects of time on spudcan-footprint interaction To accomplish these objectives, a total of 10 experiments were conducted in both normally consolidated and over-consolidated clays. The raw data was reported in Gan et al. (2008a). 5.2 Test programme 5.2.1 Selection of jack-up operational period and elapsed time after a footprint is formed In general, the operational period depends on the type of work the rig is engaged in. Depending on the task involved, the operational duration of a mobile jack-up rig in the field can vary from weeks to as long as years. Typically, a jack-up rig operates at a site from a week to year for exploring oil and gas and up to years for drilling production wells at a new platform. 149 Chapter – Effect of Time on Spudcan-footprint Interaction On the other hand, the elapsed time between visits of jack-up rig depends on the production targets. In general, a future rig will visit an established site for workover after to years. Occasionally, a rig is required to return quickly to a site for production enhancement. On other occasions, the well may be left undisturbed without jack-up intervention for a period of over years. As such, two operational periods (see Fig. 5.2) that were considered to bound the problem were selected in the present study: a) Case OP - a very short ( . Effect of Time on Spudcan-footprint Interaction 148 C C h h a a p p t t e e r r 5 5 Effect of Time on Spudcan-footprint Interaction 5.1 Introduction Footprints formed after the extraction of a jack-up. Time on Spudcan-footprint Interaction 161 where c is the adjusted coefficient of consolidation, t is the consolidation time and R is the radius of the circular footing. For simplicity, in this study, . during spudcan penetration and extraction. However, when the time is long enough Chapter 5 – Effect of Time on Spudcan-footprint Interaction 166 for a significant degree of u e dissipation to take place,