Study on combined loading capacity of Spudcan on soft over stiff clay

Abstract

Jack-up type offshore platforms like rig, wind turbine installation vessel (WTIV) are being used in a wider field than fixed offshore platforms because of their ability to move. The installation process of the jack-up consists of "retrieval" moving to the target field, "preloading" penetrating the Jack-up leg, "unloading" removing the preload and lifting the hull, and finally the "operation" starting into the function of the platform. Especially, in case of the penetration of the leg in the operation phase, Jack-up is subjected to an environmental load, which is transmitted to the soil through Spudcan under the legs. These forces on the soil can be simplified to two dimensions of vertical, horizontal and rotational directions. Generally, a soil model in the structural strength assessment of Jack-up has been modeled with a pinned condition or linear spring. However, under the vertical, horizontal, and rotational combined loads, the soil shows nonlinear behavior where the strength becomes weak. Design regulations reflect this disturbance and recommend the soil model using the concepts of yield surface and secant stiffness. This proposed soil model can be just applied to a single-layered soil, and there is no clear analytical equation for the multi-layered soil. Therefore, in this paper, the yield surface and the ultimate capacity of the soft over stiff clay soil, which is mainly distributed in the southwestern sea of S