1. Modelling of Spudcan and Suction Caisson Performance M.J. Cassidy, M.F. Randolph (University of Western Australia) and B.W. Byrne (Oxford University) Project: An intensive study of shallow foundation behaviour Purpose: Make a comparative assessment of the response of similarly sized spudcans and caisson. Motivation: Investigation of shallow foundation alternatives for the long-term deployment of mobile offshore drilling units For more details visit us at www.cofs.uwa.edu.au Investigating Spudcan and Caisson Foundation Response in the UWA Drum Centrifuge Drum Centrifuge installed 1998 1.2m diameter; soil sample channel 300mm high (vertically) and 200mm radial depth max acceleration 485g SOIL The drum centrifuge at UWA has been used to comparatively assess the performance of spudcan and caisson foundations in both normally consolidated kaolin clay and normally consolidated calcareous silt (from the Gorgon area). The tests were designed to investigate footing behaviour under combined loading, and to provide data that can be used to validate existing as well as develop new plasticity models. The novelty of the testing was to compare the response of skirted and non-skirted foundations and therefore assess whether improvements in performance could be obtained by the addition of a skirt. In order to apply appropriate loading combinations to the model scale footings in a centrifuge, a hinged loading arm was developed. This allowed higher ratios of moment to horizontal load to be applied to the footings than the more conventional fixed arm approach. Loading legs used in the experimentsModel spudcan and caisson footings Example vertical installation data Example combined loading data of a caissonAll normalised results against theoretical yield surface Analysis of jack-up at the Sunrise site Based on the experimental results (and existing theory) a force-resultant model for describing the behaviour of caissons in clays has been developed. Though plasticity models depicting spudcan behaviour exist they have been limited to the traditional inverted conical spudcan footings shape and the extension to the caisson configuration here represents a significant advance. As the model is developed within strain-hardening plasticity theory it allows it to be implemented with structural analysis codes. (1) The model has been verified by retrospective simulation of the centrifuge experiments. (2) Its practical application shown by a series of example analyses of the proposed production jack-up at the Sunrise site. (3) The benefits of the caisson configuration compared with typical spudcan shapes have been shown. Simplified model of proposed Sunrise jack-up and footing options Deck displacement response for 100-year conditions for different footing options Project Conclusions and Benefits 1.The caisson in the centrifuge tests displayed increased moment stiffness, increased horizontal capacity and larger combined load capacity under tension, but surprisingly no significant improvement in moment capacity. 2.A simple strain-hardening plasticity model for the analysis of caisson was developed and implemented in UWA’s jack-up analysis package. 3.For the Woodside’s proposed Sunrise site, (a) optimum caisson geometries (b) penetration and relative movement of the foundations, including permanent deformation in extreme events (c) comparative behaviour to equivalent sized spudcan footings, were studied and the advantages of caisson foundations quantified. (after Stewart, Boyle & Randolph, Centrifuge 98)