MJ Reardon, BSc, CEng, MICE. This paper was first published in GE’s October 1986 edition.
The use of mobile jack-up drilling units for offshore oil and gas exploration has increased considerably in recent years. At present, approximately 400 units are available world-wide, with the potential to capture 60% of the exploration drilling market. The remaining offshore exploration is performed from drill-ships and semisubmersible vessels.
A typical independent leg jack-up is shown in Fig. 1. In 1985 units were being constructed with leg lengths of 157m and with a total weight during installation of 350MN. The cost of such a unit may range from 70 to 90 million US dollars depending on the customer preference of on-board equipment.
The foundations of jack-up units (sometimes called ‘spudcans’) can take many forms. Most modern footings are polygonal in plan, and in elevation have gently sloping soil contact surfaces with a central point or tip to provide horizontal restraint. The spudcans of the larger units may have plan areas of 260m2 and during installation develop foundation contact pressures of 440kPa.
It is the engineering analysis of these footings which is often of interest to offshore geotechnical engineers, particularly as the safety and success of a project can be crucially dependent upon the unit’s foundation performance.
Before addressing the geotechnical aspects of jack-up operations it may be useful to briefly discuss some of the practical points relating to the installation of these units.