Suction caissons are being used increasingly for the anchorage of large compliant offshore structures. Uplift capacity of the suction caissons under inclined loading is a critical issue in these applications, and reliable methods of predicting the capacity under this form of loading are required in order to produce reliable designs. An extensive theoretical investigation has been carried out of suction caissons in uniform soils subjected to inclined uplift loading for cases where the behaviour of the seabed soil is undrained. A brief review of previous research on the behaviour of suction caisson under uplift loading is included, and results of finite element modelling are presented. A simplified method for the estimation of the uplift capacity for suction caissons is described, based on the results of the finite element study. The expressions developed in this paper take into account the influence of the aspect ratio of the caisson, the point of application and angle of inclination of the loading, and the undrained shear strength of the soil.
Suction caissons often provide a cost effective alternative for catenary, taut leg, tension leg moorings and jacket structures, compared to conventional piles and drag anchors, particularly in areas such as the North Sea, the Gulf of Mexico and North-West Shelf in Western Australia, and for water depths in the 300m to 3000m range. Suction caissons are normally cylindrical units made of concrete or steel. A suction caisson can penetrate partly into the soil under its own weight. Further penetration can be achieved by pumping the water out of the caisson, creating an inside under-pressure relative to the outside water pressure (active suction). The installation phase is where the suction caissons gain many of the advantages over piled foundations, as the suction installation can take place in a relatively short time.
