The spudcan foundation has been widely used in offshore engineering for jack-up rigs. However, "punch through" failure often occurs during marine operation in areas where a stronger soil layer overlays a softer soil layer. Punch-through analysis of jack-up rigs is required before conducting marine operation. The traditional analytical method as indicated in some regulations such as "SY/T6707-2008 Specification for marine well site survey" is the projected area method, which is based on theories by Hanna and Meyerhof. In this paper, a non-linear numerical calculation by the finite element software ABAQUS is performed to simulate the penetration of spudcan. The displacements caused by loading on the stronger soil layer are calculated, and a more accurate result is obtained by applying The projected area method.
In consideration of mobility, jack-up rigs have been a popular choice in areas of shallow to medium water depth in offshore oil/gas exploration at present. Most jack-up units have three or four individual legs with a conical footing as its spudcan (LIU Jun, HU Yuxia, KONG Xianjing, 2005). However, "punch through" failure often occurs in jack-up rigs with the spudcan foundation where a stronger soil layer overlays a softer soil layer. After initial set down of the legs on the sea bed and loading on the legs, the spudcans will penetrate the soils until equilibrium is reached between the capacity of the soil and the forcing load. If a soft layer exists below a stronger layer such as granular soils, the profile of capacity with depth may reduce over a range of depth. The failure usually occurs in this case, with a sudden penetration within a short time —"punch through" failure (Health & Safety Executive, 2004).
