Jack-up reinstallation near existing footprints

[Truncated abstract] Jack-ups are self-elevating mobile units operating in offshore oil and gas fields situated in water depths up to 120m or so. A typical jack-up unit consists of a floatable hull platform and three independent retractable truss-work legs each resting on a spudcan footing. The spudcans are typically between 10 to more than 20m in diameter. During the removal of a jack-up unit from a site, the extraction of spudcans leave significant seabed depressions. In the offshore industry, these are referred to as “footprints”. Jackup units often have to return to the same site and be installed near or into existing footprints. This is a problematic operation because the spudcan located near the footprints is subjected to eccentric and/or inclined loading conditions. This can lead to structural failures within the jack-up legs. Furthermore, movements can occur in the spudcan and jack-up legs, with potential risks that the jack-up hull may hit the fixed platform. The jack-up reinstallation problem is perceived by the industry as a significant problem with impact on time, costs and safety of structures and personnel. Jack-up reinstallation response is relatively complex and in this study three controlling parameters were identified: the footprint’s geometry, the footprint’s soil properties and the structural properties of the jack-up unit. This study reports a comprehensive series of experimental investigation on each of these three parameters, first in isolation and finally in combination. The motivation is that with an improved knowledge of the role of these parameters, prediction method can be developed and the effectiveness of mitigation measures can be assessed, ultimately reducing the failure rate of jack-up reinstallation.