M–H Yield Envelope for Deeply Embedded Spudcan–Lattice Leg Assembly in Normally Consolidated Clay

Abstract

This paper examines the effect of lattice leg–soil interaction on the horizontal force and bending moment on deeply embedded lattice legs of jack-up rigs in soft clay. Centrifuge model tests results showed that there is considerable soil resistance on the leg, even in the case of open lattices. The main effect of this is to lead to a decrease in bending moment at the leg–spudcan connection, so that the maximum moment is actually reached above the spudcan. An approach for assessing the horizontal force and bending moment in the lattice leg is proposed, based on consideration of soil resistance on the lattice leg, which is treated as an assemblage of chords and braces, and the sleeved leg treated as a tubular body. It then leads to the development of a procedure to calculate the moment–lateral load (M–H) capacity of a leg–spudcan assembly, which incorporates the yield envelope of the spudcan. The calculation process can be more conveniently implemented through an iterative process, although it can be solved analytically. Comparison with centrifuge data showed reasonably good agreement. The M–H envelope of an embedded rectangular lattice leg–spudcan assembly is also computed for combinations of lateral load and moment applied at mudline level. This combined envelope is shown to be significantly larger than that of the spudcan alone, indicating substantial contribution of the lattice legs to M–H capacity. This potentially allows the embedded portion of the leg and spudcan to be represented by an equivalent foundation at the mudline level with an enlarged M–H envelope, for structural design.