Undrained capacity of a hybrid subsea skirted mat with caissons under combined loading

Skirted mudmats are commonly used to support offshore infrastructure. However, these may not be able to provide sufficient capacity to notably withstand the lateral loads applied by jumpers and connectors. The concept of hybrid foundation system was therefore proposed to enhance the capacity in a targeted fashion, through addition of internal suction caisson compartments. This paper presents a numerical investigation into the undrained bearing capacity of a rectangular-shaped hybrid foundation with two caisson units. Uniaxial capacities under vertical, horizontal, and moment loading are reported, but the focus lies on the horizontal-moment planes due to the anticipated low vertical load on the foundation. Detailed discussion highlights the contribution of the internal caisson compartments to the significant increases in horizontal capacity compared to a simple rectangular skirted mat through changes in the failure mechanism. This results not only in changes in size, but also in shape, of the capacity envelope. The study was extended to more general horizontal-moment loading conditions, where the shape of the capacity envelope was found to remain approximately constant. This applies to the hybrid foundation and the rectangular skirted mat in the homogeneous soil considered here. An expression is proposed that provides a slightly conservative approximation of the capacity envelope under combined horizontal and moment loading from any direction.