The requirement to re-install spudcan foundations close to existing footprints has a significant and often detrimental effect both structurally on the jack-up leg's load and stability and, geotechnically, on the bearing capacity of the spudcan. The influence of an existing footprint on the bearing capacity and potential horizontal displacement of the spudcan have therefore been investigated experimentally using a geotechnical centrifuge. The experimental arrangements feature a fully instrumented jack-up leg, measuring axial forces and bending moments, coupled to a sliding device that allows free lateral displacement of the spudcan (advancing previous experimental and finite element studies that have prevented movement of the jack-up leg). This paper presents experimental results obtained by varying the offset distance between the spudcan and the footprint for normally consolidated clay. Implications for the reinstallation of jack-ups under these conditions are discussed.

Jack-up and spudcan background

Jack-up units are commonly used for oil or gas exploitation in water depths up to 120m (Figure 1). They consist of a buoyant triangular hull connected to three independent truss-work legs, with a conical shape foundation (known as a spudcan) at the base. During installation, the legs are lowered into the seabed independently and usually one after each other. The loading process includes stages of preloading where additional load (in comparison to the in-service load) is applied on the spudcan. Once the drilling or work-over is complete, the jack-up unit is removed, leaving on the seabed footprints which may be up to 10m deep and 20m wide in soft clay1.

Reinstallation of jack-up units nearby pre-existing footprints is one of the challenges currently faced by the jack-up industry (Figure 2). During the installation process, vertical load is applied directly though the centre of the spudcan. In the case of installation near a pre-existing footprint, where the soil surface is uneven, an eccentric and/or inclined reaction from the soil will be applied to the spudcan. This will tend to cause tilting of the spudcan that is resisted by the development of bending moment in the leg, leading potentially to overloading of the legs. Although this problem has been clearly identified2, 3, there are still no guidelines to assist operators in a safe reinstallation, aside from the recommendation to monitor leg loads via rack phase difference (RPD) during installation.

Previous investigations carried out by Clunie-Ross4 and Stewart and Finnie5 showed that the critical offset ratio, ß, when the maximum leg bending moment is developed is 0.75 (measured as the centre-to-centre offset distance between the two installations, a, divided by the spudcan diameter, D ? see Figure 2). On the other hand, the highest horizontal load was recorded at a normalised offset distance of about 1.25. A more recent study6 aimed to quantify the bending moment developed in the leg during reinstallation using centrifuge testing. The parameters investigated were the stiffness of the leg, the spudcan geometry and offset ratio, and the pre-load level. The results showed similar trends with a critical offset ratio around 0.5, independent of the flexural stiffness of the leg.

This content is only available via PDF.
You can access this article if you purchase or spend a download.