ABSTRACT

The reinstallation of jack-up rigs near an existing footprint has been identified as one of the key challenges in the offshore oil and gas industry. This study introduces the effect of spudcan shapes to minimise spudcan-footprint interaction. A series of 3D large-deformation finite-element (LDFE) analyses and centrifuge tests have been carried out varying combination of spudcan shape factors:

  • underside profile;

  • skirt length; and

  • number of holes.

The results from this study indicate that subtle changes in spudcan shape can ease the spudcan-footprint interaction issues.

INTRODUCTION

Mobile jack-up platforms often return to sites where previous jack-up installation, operation and extraction have left craters, commonly referred to as footprints, on the seabed. The reinstallation of jack-up rigs near an existing footprint has been identified as one of the key challenges in the offshore oil and gas industry (Hunt and Marsh 2004). This is because, during this reinstallation, the spudcan tends to slide towards the centre of the footprint and it induces excessive lateral forces and bending moments on the leg (see Fig. 1). The frequency of offshore incidents during installation near footprints has increased by a factor of four between the period 1979∼88 and 1996∼06 (Osborne, 2005) and at an even higher rate over 2005∼2012 (Jack et al. 2013).

This issue has received significant attention from researchers, with most measuring the induced loads on a leg with a generic shaped spudcan (or flat plate) and analysing its consequence on the jack-up structure (Gaudin et al. 2007; Leung et al. 2007; Cassidy et al. 2009; Gan et al. 2012; Kong et al. 2013; Zhang et al. 2015). Systematic investigations on how to mitigate spudcan-footprint interaction issues under field conditions are notably limited. Industries have been conducting trials such as stomping and successive leg repositioning (Brennan et al. 2006) and water jetting with the generic spudcans (Handidjaja et al. 2009). However, those concepts require additional operations and hence are expensive.

This study focuses on modifying spudcan shapes to ease the spudcan-footprint interactions without any additional operations. First numerical investigation was carried out through 3D large deformation finite element (LDFE) analyses varying spudcan shapes, leading to select the most effective shape at mitigating spudcan-footprint interaction issues. A series of centrifuge tests at 200g was then carried out to verify the performance of the selected novel spudcan shape.

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