Analysis of Suction Caisson Capacity in Clay

Abstract The paper addresses analysis techniques and design parameters for suction caissons in clay for loading conditions ranging from horizontal to vertical. A three-dimensional upper bound approach is described for caissons undergoing significant horizontal motion or rotation, and the relative accuracy of the analysis is assessed through comparison with independent semi-analytical 3-dimensional finite element analyses. For vertical loading, design parameters for shaft friction and end-bearing are deduced from results of physical model tests, for both sealed and unsealed caissons and for short-term and long-term loading. Interaction between horizontal and vertical loading is discussed, and aspects such as strength anisotropy of the soil, mis-alignment of the caisson (vertically and in plane), and orientation of the applied load are considered. Introduction Suction caissons are now used widely in the offshore industry for anchoring systems, with applications ranging from quasihorizontal loading for catenary mooring lines (Colliat et al, 1996) to quasi-vertical loading for tension leg platforms (Andersen et al, 1993; Clukey et al, 1995). Between these two extremes, the use of suction caissons for taut wire moorings with loading angles of 30 to 50° is becoming increasingly common (Mello et al, 1998). As for any form of foundation system, evaluation of the capacity of a suction caisson requires (a) a conceptual and analytical model of the collapse mode, and (b) decisions on appropriate model parameters (e.g. bearing capacity factors) and soil parameters (e.g. shear strength, interface friction). A review of these issues and other aspects of suction caisson design including the installation phase was presented by Andersen and Jostad (1999). For quasi-vertical loading, the failure mode is similar to that for a pile, with the main difference being the lower aspect ratio, which in turn leads to reliance on development of internal suction below the caisson lid in order to mobilise the end-bearing capacity over the full area of the caisson. Under quasi-horizontal loading, the failure mode is less straightforward, involving potential rotation of the caisson and a more complex three-dimensional flow mechanism. It transpires that the collapse mechanism is amenable to analytical treatment, making use in particular of flow fields proposed for laterally loaded piles in clay (Randolph and Houlsby, 1984; Murff and Hamilton, 1993). The final consideration for general loading conditions is the degree of interaction to be expected between vertical and lateral collapse modes. This is critical for taut wire moorings, where an interaction diagram in vertical and horizontal loading space is needed for design. These different aspects of suction caisson capacity are considered in detail in the following three sections. Quasi-Horizontal Loading The most common use of suction caissons is as anchors for catenary mooring lines, where the chain angle at the mudline is close to zero. Allowing for the curvature of the chain within the soil, the loading angle at the caisson padeye will fall within the range 10 to 20° from the horizontal. For such loading angles, the failure mode of the caisson will involve horizontal translation and rotation with little vertical movement.