The suction caisson is a relatively short, large diameter, open-ended cylindrical pile that can provide ideal anchors for laterally moored production structures in the deepwater such as the Gulf of Mexico. However, there is a paucity of performance data on suction caisson behavior and our current level of understanding is limited.
This study applies the upper bound method of plasticity theory using a three-dimensional collapse mechanism described by Murff and Hamilton (1993) to estimate the ultimate capacity of laterally loaded caissons under undrained conditions. The mechanism combines a deforming conical soil wedge in the near surface with plane strain deformation at depth. The study is concerned with predicting the holding capacity of suction caissons applied through catenary mooring lines. For the analysis, the caisson is assumed to rotate about a horizontal axis through its center line due to a horizontal mooring load. The effects of load-point application depth, caisson length to diameter ratio, soil strength characteristics, tip rotational resistance and soil unit weight effects are examined.
Plane strain and three dimensional finite element analyses are used to verify collapse loads estimated from plastic limit analyses. The two methods of analyses compare favorably in both translating and rotating conditions. Sensitivity studies were carried out to achieve a better understanding of the collapse mechanism and to develop simplified solutions for preliminary or conceptual design.
The plastic limit solutions presented herein are capable of modeling for complexities of real soils such as non-uniform soil strength, caisson-soil adhesion, and suction on the back of the caisson.