Assessment of uni-directional foundation skirts to enhance performance of tolerably mobile subsea foundations

The advantages in terms of cost and safety of tolerably mobile foundations supporting subsea structures are agreed consistently in the offshore literature. Nonetheless, this technology is yet to be employed by industries due to concerns on the effects of environmental snag loads that may cause misalignments, loss of the functional configuration and eventually collapse of the structure. The technology that can be adopted to develop performances of tolerable mobility is the implementation of uni-directional foundation skirts. This technology is assessed by modelling skirted foundations under undrained conditions in the Abaqus software environment, a suite of simulation programs based on the finite element method. The main outputs being assessed are the upper bound of the dynamic resistance in the non-mobile direction, H_(x,ult), available to withstand environmental snag loads, and the static mobility performance ratio, H_(x,ult)/H_(y,ult), taken as an indicator of the displacement complaint behaviour. An extensive campaign including 185 three-dimensional finite element models has been designed in order to perform a sensitivity analysis to the following parameters: (i) number of uni-directional internal skirts, (ii) depth of foundation skirts, (iii) undrained soil strength heterogeneity of the soil, (iv) vertical eccentricity of horizontal loads. A comprehensive guidance on modelling geotechnical problems using the software Abaqus is also provided.