On the VHM capacity of a buried spudcan in soft clay as it relates to the new ISO 19905-1 guidelines

In soft clays, the spudcan foundations of mobile offshore jack-up platforms penetrate deeply. During the installation process in these soft soils, backflow around the spudcan seals the cavity. Although it is widely perceived that the backflow increases the horizontal and moment bearing capacities, definition of the magnitude is not well understood. Current ISO guidelines allow the consideration of the influence of backflow based on limited finite element analyses. This paper presents recommendations on the bearing capacity surface of the spudcan under combined vertical (V), horizontal (H) and moment (M) loading based on results of a comprehensive numerical and experimental study. These recommendations are for a typical spudcan at embedment depths up to three footing diameters and soil sensitivity from one to five in soft clay. While additional capacity (compared to the case without backflow) is confirmed, detailed comparison with the capacity surface set out in the ISO document, indicates that the current guidelines are overly optimistic. BACKGROUND Jack-up platforms (Figure 1a) are widely used in the offshore industry. Before its installation in each new site, a site specific assessment is performed to evaluate the suitability of the platform for the proposed site. In terms of geotechnical considerations, there are mainly three tasks: i) prediction of footing penetration during preloading; ii) jack-up foundation stability during operation (particularly due to storm loading); iii) other aspects of foundation performance, such as extraction, scour, seabed instability, during and after preloading (SNAME 2008; ISO 2012). This paper is concerned with the second task, i.e. the stability of the spudcan foundations during operation. In the operational mode, a jack-up platform is subjected to significant environmental loading, such as wind, wave and current, beside its self-weight. These loads are ultimate transferred to the spudcan foundations, imposing combined vertical (V), horizontal (H) and moment (M) loading conditions (Figure 1b). The evaluation of the foundation stability is thus closely influenced by the adopted bearing capacity of the spudcan foundation under combined VHM loading.