Measured Horizontal Capacity of Suction Caissons
Authors: Coffman Richard A. , El-Sherbiny Rami M. , Rauch Alan F. , Olson Roy E. 
Published: October 2008

Laboratory tests were conducted to study the behavior of suction caissons when loaded horizontally below the mudline. The experiments were performed using a 4-in. diameter caisson installed to a depth of 32 in. in normally consolidated kaolinite clay. The test prototype is representative of caisson geometries commonly used in mooring systems for deep offshore locations with soft seafloor sediments. Nine horizontal load tests were conducted with the loading cable attached at different points along the lower half of the prototype caisson. In each test, the caisson was installed to half its length using dead weight, followed by the application of suction to achieve full penetration. The loading cable was then pulled to a horizontal position and excess pore water pressures on the caisson exterior were allowed to dissipate. The caisson was loaded rapidly to failure under undrained soil conditions. Data are presented showing the horizontal load capacity, top cap displacement, caisson tilt, and excess pore water pressure on the caisson wall during loading. The results indicate that the maximum horizontal capacity occurs when the load is applied to the caisson between two-thirds (21 in.) and three-quarters (24 in.) of the embedded length. The failure mechanism and the developed pore pressures depend on the position of the load application.

Introduction

A suction caisson is a large diameter, closed-top, steel pipe that is lowered to the seafloor, allowed to penetrate the bottom sediments under self weight, and then pushed to full penetration using differential pressure produced by pumping water out of the interior. Suction caissons are widely used as foundations and mooring anchors for offshore structures in deep water. They can provide larger holding capacities than drag anchors, can be installed in water depths greatly exceeding the feasible limits for driving piles, can be installed reliably at pre-selected locations with minimal seafloor disturbance, and can be recovered for re-use. In the Gulf of Mexico and elsewhere, suction caissons are widely used as foundation anchors in normally consolidated and lightly overconsolidated clays below deep water. Typical diameters of current caissons are 12 to 25 ft and length to diameter ratios are about six. Suction caissons in these offshore applications are subjected to a wide range of loading conditions. Loads are vertical for tension leg platforms, inclined in taut mooring systems, and mostly horizontal in catenary systems. Numerous investigations have focused on different aspects of suction caisson performance. Axial capacity has been studied using 1-g laboratory experiments [1-6], model tests in geotechnical centrifuges [7-9], field tests [10], and numerical simulations [11-12]. Inclined loading capacity has been investigated using centrifuge tests [13-14] and finite element analyses [15]. Aubeny et al. [16] developed a plasticity solution for predicting the capacity of suction caissons subjected to both inclined and horizontal loading. Comparisons between centrifuge tests, finite element analyses, and plasticity solutions for different load inclination angles [17, 18] showed that the use of plasticity solutions resulted in higher capacities than when finite elements were used.