Spudcan-footprint interaction during jack-up workovers

It is becoming increasingly common for jack-up rigs to drill additional wells, or service existing wells at established platform locations. Such 'workovers' require the jack-up to be located immediately adjacent to the platform, with positional tolerances at times less than 1 metre. A hidden danger exists where the seabed consists of soft to firm clay, in the form of old 'footprints' remaining from previous jack-up activities. Preloading of a jackup spudcans near to these footprints can result in uncontrolled penetration, slewing of the rig and collision with the platform. However, there is little published information that can be used to assess the likely field behaviour. In this paper, some centrifuge model tests aimed at examining the effect of penetration of spudcan footings adjacent to footprints are described. Data from the experiments are presented to illustrate the likely magnitude of lateral forces and moments that may be induced in a jack-up leg. A simple method of analysis for this problem is outlined, and it is shown that the trends from this approach agree broadly with the experimental data.