Comparison of Analytical and Centrifuge Model Tests for Suction Caissons Subjected to Combined Loads Available to Purchase

The holding capacity of suction caissons increases significantly when the mooring line is attached below the mudline. In addition to the position of the attachment point, the soil profile, caisson geometry and the loading angle at the attachment point (padeye) will also influence the holding capacity. Various combinations of these parameters will control the failure mechanism that determines the holding capacity. The failure capacity can be dominated by the vertical or horizontal failure mechanisms — or a combination of the two. When the combination controls, the caisson response is referred to as being in the ‘interaction’ region’. If the suction caisson response is in the interaction region, methods of analyses required to predict the ultimate holding capacity are more complex than for pure vertical or horizontal failure mechanisms. This paper considers the response of suction caissons for all three failure mechanisms. The various failure mechanisms were primarily investigated through comparisons of an upper bound plasticity model and centrifuge model tests. Finite element results were also obtained and compared to the plasticity model solutions. The general foundation conditions investigated are considered fairly typical of suction caissons currently being used to anchor deepwater facilities in normally consolidated soils. The caisson scaled diameters investigated were slightly less than 5.36 m. with embedment (L) to diameter (D) ratios (L/D) slightly less than 5 for comparisons between the plasticity and centrifuge models. Comparisons were made between the finite element and plasticity models for an L/D ratio of 6. The padeye attachment point was about 2L/3 below the mudline for the plasticity and centrifuge comparisons and 0 (mudline) and L/2 for the finite element and plasticity model comparisons. The finite element estimates were slightly less than the plasticity model estimates (3 to 9%) with the largest differences in the interaction region. Both the plasticity and centrifuge models indicated that the transition from a vertical mechanism to an interaction failure mechanism occurs at a load angle of 40 to 45° from the horizontal. In the interaction region the centrifuge results were generally less than the plasticity model predictions. Centrifuge tests with a bevel at the caisson tip were inconclusive in terms of establishing the effectiveness of the bevel for displacing more soil to the outside of the caisson and thereby increasing the external skin friction.