The Finite Element Analyses Of The Failure Mechanism Of Spudcans Penetrating Near Existing Footprints During Jack-ups' Reinstallation

Jack-ups are self-elevating mobile units explorating and producting in offshore oil and gas fields. Its design depth is 10m to 160m generally. A typical jack-up consists of a floatable full platform and three independent retractable truss-work legs, the bottom of each leg connect to a single spudcan footing. The significant depressions that remain on the seabed after offshore mobile jack-up platforms have completed operations from a site. This seabed depression is reffered to as a "footprint". As more and more demand for oil and gas resources, the operating frequency in offshore sea is increasing, jack-ups often have to return to the same site and to be installed near to or into existing footprints. The spudcan located near the footprints is subjected to eccentric and/or inclined loading conditions that slipping into footprint. This can lead to structural failures within the jack-up legs and platform overturning, even the jack-up hull may hit the fixed platform. This dangerous condition called " step in footprint". Most of the existing researches on the interaction between spudcans and footprint are obtained by model tests. The failure mechanisms during reinstallation are still not very clear.The paper uses the Coupled Eulerian-Lagrange (CEL) large deformation finite element method to analyze the failure mechanism of spudcans penetrating near existing footprints during jack-ups'reinstallation. Firstly, Simulating the process of spudcans installed in homogeneous soil, quantitatively analyzing the influence of mesh technology, mesh density, penetration velocity and geometry of base model on the ultimate bearing capacity of spudcans. And on the basis of it, Simulating spudcans penetrating close to existing footprints to analyze the instability mechanism of reinstallation in three aspects.(1) The offsets of reinstallation, that is, the distance between the center line of spudcans and footprints. The different offsets lead to different peak horizontal force and peak moment, then investigating the critical offset where generated the maximum peak loads on spudcans.(2) The depths of footprints. Investigating the footprints of same depths, different slopes and diameters, the ratios of offset to diameter are equal to 1/2. Combining the failure mechanism to analyze the influence of different depths of footprints on the bearing capacity of spudcans.(3) The size of footprints. Invesgating the same offsets, the footprints of same slopes, different depths and diameters. Combining the failure mechanism to analyze the influence of different sizes of footprints on the bearing capacity of spudcans.