The large footprints that remain on the seabed after offshore mobile jack-up platforms have completed operations present hazardous conditions for any future jack-up installation at that site. The slope of the footprint causes detrimental horizontal and moment loads to be induced on the spudcan during the preloading process where only vertical loads are expected. This paper reports model tests exploring the efficiency of innovative spudcan foundations and perforation drilling to mitigate this issue. The soil conditions tested simulate soft seabed strength profiles close to the mudline, varying the undrained shear strength. The most critical reinstallation location and existing footprint depth were investigated. In all of the experiments, an initial footprint was created. Skirted, innovative and generic spudcan models were then offset and reinstalled on nonperforated and perforated sites, with the vertical and moment loads on the spudcan recorded using a highly instrumented shaft. The innovative spudcan and the removal of soil inside the spudcan perimeter, with an area of 9%perforated, reduced the induced moment significantly.
Most offshore drilling in water depths up to 150m is performed from self-elevating jack-up rigs because of their proven flexibility, mobility and costeffectiveness. These jack-ups typically consist of three independent truss legs, each attached to a large 10–20m diameter inverted conical footing, colloquially known as ‘spudcans’. Unlike a fixed platform or gravity structure, a jack-up unit is mobile in nature. The changing of drilling locations necessitates retrieval of the unit at each site.
On removal of the jack-up unit, the legs are retracted from the seabed leaving depressions (referred to as a crater, or ‘footprint’) at the site. The depth and configuration of a footprint are a function of several factors.
