Tolerably mobile subsea foundations – observations of performance

Increasing demands for oil and gas exploration in deep water with soft seabed conditions are resulting in the size and weight of subsea shallow foundations stretching the capabilities of installation technologies. One innovation to reduce foundation footprints involves designing foundations to move in a tolerable manner to absorb applied loads rather than being engineered to resist these loads and remain stationary. Critical design considerations are the evolution of foundation capacity and the mode of foundation displacement. The foundation should be designed to slide with acceptable settlement and rotation to prevent overstressing the joints with connected pipelines. This paper presents observations from centrifuge model tests of a mat foundation designed to slide under applied loading. The foundation is subjected to a simulated lifetime of operation, with many cycles of sliding and intervening periods of consolidation. The results provide insights to assist design, including a remarkable rise in the lateral foundation resistance over the sliding events, through repeated episodes of shearing and reconsolidation, and quantification of the accumulated settlements and rotations. The foundation is shown to translate with minimal rotation. The settlement between sliding events is more significant. This is due to the tendency for soft clay to contract on shearing, as excess pore pressures generated during sliding subsequently dissipate. The sliding-induced consolidation settlements control the tolerability of the performance of the mobile foundation.