Centrifuge study of lattice legs effect on spudcan fixity under cyclic combined loading conditions

Abstract

In the design of offshore jack-up rigs, it is commonly assumed that the lattice legs have no effect on the fixity and other foundation behavior of the spudcan, presumably because the opening ratio of lattice legs is typically quite large. This paper describes a centrifuge experimental study to explore the effect of lattice legs on spudcan fixity under cyclic loading. The modeling equipment is first described. A novel feature of this equipment is its ability to maintain a constant holding vertical loading on the spudcan leg while subjecting the latter to rocking over many cycles. The centrifuge model results and discussion presented show that the presence and configuration of the lattice leg has a significant influence on spudcan foundation behavior in several different aspects. The effect is most significantly manifested in the fixity and bending moment. The fixity of the entire spudcan foundation is increased substantially by the presence of the lattice. The bending moment at the spudcan-lattice connection is also reduced by the lattice leg, with the maximum moment occurring slightly above the spudcan. Both of these effects can potentially lead to substantial cost savings in jack-up structures.