An extensive theoretical investigation has been carried out of suction caissons subjected to vertical uplift loading for cases where the behaviour of the seabed soil is undrained, partially drained or drained. Theoretical solutions for the uplift capacity are derived, based on the results of a finite element study, and then validated by previous field experiments and model test results. The expressions developed in this paper for the uplift capacity take into account the influence of the aspect ratio of the caisson, the shear strength parameters of the soil, the soil permeability and the loading rate.
Compliant offshore structures, like tension leg platforms (TLPs), are usually subjected to considerable uplift forces. These structures require foundations that can anchor them to competent strata. It has been common in the past to use piles to provide such a foundation. However, there are difficulties associated with the installation of the long piles that are usually necessary, particularly in deep waters. Largely because of these difficulties a new type of foundation, the suction caisson, has been developed and used to provide uplift resistance for a variety of in situ soil conditions. A suction caisson, open at the bottom and closed at the top, is designed to penetrate to the sea floor by its own weight and by also creating an inside under-pressure relative to the outside water pressure. The latter is known as the active suction installation method. As soon as there is any indication of pullout movement, the suction caisson mobilises significant pullout capacity through the development of negative changes of pore water pressure inside the soil plug and at the bottom of the caisson depending on soil conditions. This is known as the passive suction condition.
