The installation of jack-up platforms on sites containing old footprints is known to be problematic. One approach is to place granular infill in the old footprints to produce an approximately level seabed. Jardine et al (2001) recognised that this procedure could lead to uncertain results when the new locations are offset from the crater centre, and when the infill and original seabed soils had different properties. They performed numerical analyses to explore the forces and moments developed by a typical structure installed over infilled craters formed at a North Sea location where clay soils dominated the soil profile, considering an offset between the old and new foundation positions of approximately one-quarter of a footing diameter. Jardine et al (2001) showed that the problem could be treated analytically. This paper presents an updated version of the analysis in which the finite element mesh employed was further refined. It is shown that the composite foundation capacity is reduced by prior cratering and infilling. The reduced capacity is associated with significant lateral and rotational footing displacements, with large lateral forces and bending moments developing in the jack-up's leg.


It is known that problems may occur when installing a jack-up at a location where footprints from previous jack-up operations exist, for example North Sea, or Southeast Asia. As a result there has been a tendency for operators to select either jack-ups with the same footing pattern as previously utilised, or to site subsequent units such that overlap of the foot prints is avoided or minimised. When working over a platform, the geometric constraints are often such that when different jack-up designs are deployed there is no alternative other than to accept that the footprints will overlap, as shown in Figure 1.

In such instances the footings have a tendency to move into the old footprints. Whilst the degree of movement is, in some way, related to the footing shape, it is generally not practical to modify the footings when a unit is already contracted for the deployment. There are then only two real options for mitigating the problem, ‘stomping’ and infilling. (Fig. 1 and 2 are available in full paper) Infilling has the advantage that the incoming unit is presented with a relatively flat seabed and can then be positioned using normal procedures infilling of old footprints formed in a coarse grained sea-bed with granular materials has been undertaken previously, and it can be intuitively understood that this should not pose particular problems if the material characteristics are similar. However, many North Sea platforms are founded on layered cohesive materials and it is not obvious how these will behave when infilled with granular materials. It is this situation that was investigated in the study described here.

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