The uplift of offshore shallow foundations

Shallow foundations have been widely used in the offshore oil and gas industry as the support of gravity-based structures, fixed platforms or subsea infrastructure such as pipeline end manifolds and terminations. Depending on their application, they may be subjected to uplift loadings due to buoyancy, overturning moments or retrieval operations. Under uplift, negative excess pore pressures (also refer to suctions) may be generated in the soil, which contribute to increasing the uplift resistance. This is beneficial during the operation of offshore structures, e.g., to resist cyclic wave loading, but is impeditive for retrieval of subsea structures. Traditional methods only consider the uplift of shallow foundations under either purely drained or purely undrained soil conditions, in which the pore pressure mechanism is ignored and the soil is simplified as a single-phase material in numerical modelling. This thesis focuses on the uplift resistance of offshore shallow foundations in clay and the underlying fundamental generation and dissipation of pore pressures.

Centrifuge and numerical modelling were performed to investigate the mechanism of uplift. The uplift resistance was found to be a function of the rate of uplift, which governs the suction generation and dissipation in the soil and the occurrence of breakout between the foundation invert and the soil. The suction and the associated uplift resistance are highly rate dependant and closely related to the preloading history prior to uplift. With increasing uplift velocity, the resistance increases due to the generation of a higher magnitude of suction from the reduced rate of excess pore pressure dissipation and viscous enhancement of the soil strength. The application of preloading may decrease or increase the subsequent uplift capacity depending on the magnitude and duration of preloading, from which the soil may be remoulded and strengthened. The breakout, which occurs prior to or after a peak loading, limits the sustainability of suction and consequently the magnitude of the uplift resistance. Breakout is generated by either the vertical effective stress at the foundation invert reducing to zero, or a change in boundary conditions resulting in the separation of the foundation invert from the soil. A combination of eccentric uplift and perforation can facilitate the development of a breakout and significantly reduce the uplift capacity.

The research presented in the thesis provides a comprehensive understanding of the suction generation and dissipation mechanisms in clay, and enables an integrated approach to predict the uplift resistance of shallow foundations. Mitigation measures were developed to establish an optimal strategy to facilitate the retrieval of shallow foundations used to support subsea infrastructure.