Laboratory experiments are being conducted to study the behavior of suction caissons used for deep offshore moorings. The tests are conducted in large tanks of normally consolidated kaolinite. The 100- mm diameter caisson prototype is inserted using either dead weight or dead weight followed by suction, with measurements of displacements, axial forces, and pore water pressures. During suction installation, the test results show reduced pore pressures both inside and outside the caisson and increased penetration resistance. The development of a large hydraulic gradient inside the caisson was also observed during suction installation. These results illuminate how the applied suction affects the soil inside and surrounding a caisson during installation.
A suction caisson is a closed-top steel tube (typically more than 2 m in diameter) that is lowered to the seafloor, allowed to penetrate the bottom sediments under its own weight, and then pushed to full depth with differential pressure produced by pumping water out of the interior. Suction caisson anchors (SCAs) are an attractive alternative for providing fixed anchorage for floating structures in deep water offshore. They can be installed in water depths greatly exceeding the feasible limits for driving piles, can be inserted reliably at pre-selected locations and penetration depths with minimal seafloor disturbance, have a much larger holding capacity than drag embedment anchors, and can be recovered for re-use by pressurizing the interior. The large axial and lateral capacity of an SCA permits the design of taut mooring systems with smaller seafloor footprints, which are less likely than catenary systems to encroach upon adjacent tracts or facilities. Sparrevik (2001) estimates that there are over 300 suction caissons in operation around the world. Most worldwide experience with this technology has involved relatively short, large-diameter caissons with vertical loads applied at the center of the top plate.
