Prediction of Punch-through for Spudcan Penetration in Sand Overlying Clay

Abstract

Abstract Installation of spudcans in sand overlying clay sites is often fraught with potential punch-through problems which can cause serious damage to the jack-up rigs. A reliable evaluation of the punch-through potential of the spudcan requires an accurate prediction of spudcan bearing capacity-depth profile at the site. This paper proposes a new design method, which improves on the methods recommended by SNAME (2002), to estimate the spudcan bearing capacity as it penetrates a soil profile with sand overlying clay by considering the actual failure mechanisms of the soil caused by a continuously penetrating spudcan The proposed method generalizes the spudcan bearing capacity depth profile into three different phases namely, prepeak, peak and post-peak phases as the spudcan penetrates the sand layer into the underlying clay. The development of the bearing capacities in the first two phases is associated with the strength variance between the upper sand and lower clay soil layers as well as the ratio of sand layer thickness to spudcan diameter; whereas for the post-peak phase, the bearing capacity is significantly affected by the presence of a sand plug trapped underneath the penetrating spudcan. The solution for estimating the bearing capacity of each phase is provided. The performance of the proposed method is illustrated by back-analyzing experimental and field data. Comparisons with the SNAME (2002)'s recommended methods are also included. In general, the proposed method shows a significant improvement in predicting the spudcan bearing capacity-depth profile, which could not be achieved by the existing methods based on a wished-in-place shallow foundation assuming a single failure mechanism applied repeatedly at different penetration depths. By considering the presence of a trapped sand plug, the spudcan final penetration in the underlying clay is better predicted by the proposed method. The proposed method captures the essential characteristics of spudcan penetration behavior in sand overlying clay which, in turn, enables the potential jack-up rig punch-through failure and the corresponding spudcan rapid settlement to be better assessed. Introduction Spudcan punch-through is one of the most critical foundation failures experienced by offshore mobile jack-up rig (HSE, 2004). Osborne & Paisley (2002) defines the failure as " an unexpected jack-up footing rapid settlement resulting in consequential loss in drilling time??. Figure 1 shows a record of punch-through failure reported up to year 2008. The actual causes for these failures are not completely known as they were not disclosed. However, a seabed profile comprises strong overlying weaker soils is an inherent factor may lead to punch-through failure (HSE, 2004). This is because there is a tendency that the applied load would exceed the maximum bearing capacity offered by the stronger layer and hence causing the spudcan to penetrate rapidly into the lower soil layer which has a much lower bearing capacity. Depending on the magnitude of the unexpected rapid settlement and the leg jacking rate to sustain the hull trim, different degrees of rig damage will be encountered. Sand overlying clay is one of those problematic soil profiles indentified by HSE (2004) which may cause punchthrough. Figure 2 shows the distribution of sand overlying clay at areas where offshore oil and gas activities are. To install a spudcan in sand overlying clay, an accurate prediction of bearing capacity (Qv) - penetration depth (d) profile is of utmost importance so that the spudcan penetration behavior can be monitored confidently. [Take note that in this paper, d is defined as the distance measured from the lowest level of spudcan maximum sectional area to the seabed surface, see Figure 3].