ABSTRACT

The relative inefficiency of piles in resisting lateral forces has led the offshore industry to consider alternative anchorage systems such as suction caissons. Suction caissons can be installed very quickly and precisely at the desired location with less heavy installation equipment and at lower cost. Therefore they are considered as a viable anchorage system in a wide variety of soils ranging from soft clay to dense sands and overconsolidated clays and for a wide variety of structures ranging from floating exploration platforms to permanent production facilities. The development of suction caissons in recent years has seen them used around the world in more than 36 fields in the last decade alone (Andersen et al., 2002). Suction caissons are large cylindrical shells, with an open bottom and a closed top fitted with valves. The aspect ratio of these piles, defined as the length to diameter ratio, is relatively small when compared with the aspect ratio of conventional piles, typically six or less (Andersen et al., 2005). Internal stiffeners are usually added, to resist buckling during the installation process, since the caisson walls are relatively thin. They are installed partly by self weight and partly by differential pressure between the surrounding environment and the inside of the skirted foundation. In some cases, dead weights can be applied on the top of the cap to ensure that compressive loads are acting on the suction anchors (Zdravkovic et al., 2001). Once full penetration has been achieved, the valve is closed. Any vertical movement during service will result in the generation of suction pressure inside the anchor which will mobilize the reverse end-bearing mechanism, as it is described by Byrne and Finn (1972). Foundations for offshore structures, however, experience significant environmental loads from waves, currents and wind giving

rise to lateral loads. The direct consequence of that is that resultant loads can be inclined to the vertical. It is well documented that suction anchors are capable of resisting both lateral and axial loads as well as inclined loads.