This paper presents a numerical soil-structure investigation of two potential solutions to the problem of installing a jack-up platform at the North Everest, North Sea, site, where prior operations had left an unmatched set of spudcan craters in the seabed clay layers. The first potential solution considered was infilling the craters with granular material, whereas the second solution involved capping the infilled craters with gravel loading platforms. This numerical investigation shows that, for the eccentric cases examined, both options are likely to lead to unacceptable platform leg forces and moments developing during installation to the desired pre-load level. The findings of this study led to further examination of stomping, which was combined with seabed excavation to achieve a successful solution on site.
Installing jack-up platforms at sites where previous installations have left seabed craters can be problematic. The difficulties are magnified if the new spudcan locations do not coincide with the existing craters (see Figure 1). Possible solutions to the problem are stomping and crater infilling. In many cases, the option of stomping may not be desirable due to rig-time considerations, weather dependency and possible adverse rig leg loads. The option of prior crater infilling would be attractive if it could result in a flat seabed and make normal installation procedures possible.
Crater infilling was investigated by Jardine et al.1 It was shown that, for the firm to stiff clay stratigraphy of one Central North Sea site and the structure of the rig proposed for the site, the option of infilling a set of eccentrically located craters with a granular material would result in lateral and rotational movements of the spudcans, as well as lateral forces and bending moments in the leg which might be unacceptably large. The findings of that study led to conventional stomping finally being adopted as a solution to the particular case history1. However, these findings could be specific to the site and project circumstances and may not be generally applicable.
The North Everest study described herein considered again the option of infilling old craters, before considering a second option of capping the infilled craters with gravel loading platforms. It was thought possible that the existence of a dense sand layer at shallower depths at North Everest might lead to more favourable results than were found in the earlier project study.
Figure 1: Eccentric installation of spudcan into existing crater (available in full paper)
It was planned to bring the GlobalSantaFe (GSF) Galaxy-I jack-up, a mobile offshore drilling unit (MODU), to the North Everest site to conduct well work-overs in late 2005?early 2006. However, earlier operations between February 1993 and April 1994 involving the lighter GSF Magellan MODU had left a set of craters close to the intended GSF Galaxy-I spudcan locations. The craters were formed in a shallow firm to stiff clay layer (around 5.3m thick). The maximum pre-load pressures applied in 1993 were around 323kPa, with a lighter maintained load (185kPa) being applied