Jack-up platforms rely on the bearing capacity offered by spudcan foundations to withstand metocean loading of cyclic nature. The damage caused by significant cyclic events (e.g. storms) is routinely taken into account adopting accumulation procedures. A cyclically-degraded soil strength, accounting for the strength loss due to cyclic excess pore pressure generation, is then typically adopted to calculate the spudcan capacity. However, this approach ignores the potentially beneficial effects of soil consolidation that may occur during small-amplitude cyclic loading (i.e. in calm waters). Since consolidation increases the soil strength, ignoring this component leads to a certain degree of conservatism. This paper investigates the potential to reduce conservatism in the design of spudcan foundations by considering post cyclic consolidation, using finite element modelling. An advanced, yet easy-to-calibrate, hypoplastic constitutive model is utilised in combination with the large deformation finite element method RITSS to model the response from very small to large-strain of a jack-up foundations undergoing various episodes of cyclic loading. The results quantify the increase in spudcan penetration resistance due to post-cyclic consolidation, which can unlock more eventual bearing capacity to offset the detrimental effect of soil degradation generally considered in standard approaches.
Raffaele Ragni, Joe Tom, Britta Bienen and David Mašín
Keywords: spudcan, cyclic loading, consolidation, hypoplasticity, finite element
Publisher: DFI