Suction caissons have been used as mooring systems for offshore structures since the mid-1980s due to their advantages over driven piles and drag embedment anchors Over the last five years, suction caissons have become the foundation of choice for anchoring deepwater floating structures (e.g., SPARS, DDCVs, semis) or supporting seabed founded systems (e. g, well heads, SDUs, UTAs). As for driven piles, the installation of suction caissons induces excess pore pressures in normally consolidated. (NC) clays. The distribution of the excess pore pressures and their dissipation with time have a significant effect on the caisson's pullout capacity, and very few studies have been conducted on these subjects. In order to investigate the behavior of suction caissons installed in NC clay, a series of centrifuge tests were carried out at C-CORE'S centrifuge facility. The model caissons were installed in carefully prepared clay testbeds through a combination of self-weight and active suction. This paper first presents briefly C-CORE'S centrifuge facility and describes the centrifuge tests, including the clay testbed and model caissons, the multi-function test set-up, the testing procedure and the monitoring of excess pore pressures. The results of the centrifuge tests regarding the distribution of excess pore pressures induced during installation of the caisson and their dissipation with time are then discussed. The experimental results are analyzed using the cavity expansion theory, which was first used for driven piles. Finally, the experimental dissipation results are compared with a modified version of the Bogard and Matlock's empirical hyperbolic equation for consolidation time, corrected for the diameter and wall thickness of the caisson. Key words: Suction caisson; centrifuge test; excess pore pressure, cavity expansion theory, consolidation, clay
Suction caissons are large cylindrical foundations with an open bottom and a closed top. They have several advantages compared to other mooring systems.
The installation of these foundations, through a combination of self-weight and active suction, can be adapted to a number of offshore support spreads, resulting in lower costs and shorter time for installation;
There is a large area on top of the caisson where ballast can be stored.
When a vertical or an inclined uplift force is applied by the floating structure, an underpressure (passive suction) m the soil inside and at the tip of the caisson is generated The resulting combination of passive suction, additional dead-weight or ballast and the soil hction along the caisson wall, collectively defined as the caisson pullout capacity, reacts to keep the structure in place.
Suction caissons can achieve substantial horizontal holding capacity or mobilize significant pullout load capacity.
Suction caissons have a high positioning accuracy, and require no drag-in operations or proof-loading tests. As a result, this type of mooring system is generally less expensive than other traditional foundations and anchors. During the last decade, suction caissons have been used in more than 36 fields with water depths as deep as 1650 m (Andersen and Jostad, 2002).