The Pullout Capacity of Suction Caisson Foundations For Tension Leg Platforms

The main focus of the "lnnovative Foundations for the Deep Ocean" project has been to investigate the behavior of suction caisson foundations for tension leg platforms (TLP) in water depths of 2000 to 3000 m (6000 to 10,000 ft) in the Gulf of Mexico. A set of model caisson foundations was designed, fabricated, and instrumented for testing in the laboratory under simulated TLP loading. The caissons were made with length-to-diameter ratios of 2, 4, 6, and 12 to study the effect of increasing caisson length on penetration resistance and pullout capacity, as well as the feasibility of using suction as the method of installation. The simulated TLP loading was in the form of static and cyclic tension. Measurements of total foundation capacity, displacements, and pore water pressures inside and in the vicinity of the model caissons were made. The pullout capacity of the model caissons was clearly defined under both drained and undrained loading conditions. Results of the experimental program have shown that the rate of pullout loading had a significant influence on the capacity. The long-term pullout capacity represented the upper limit on peak cyclic loading. Introduction Traditionally, suction caissons have been built with length-to-diameter (LID) ratios, or aspect ratios, of approximately 0.5 to 2, e.g., Gullfaks C (Tjelta et al., 1990), Snorre TLP (Fines et at., 1991; Stove and Christophersen, 1992), Europipe (Tjelta, 1994), etc. In the North Sea, the foundation material in the deeper waters is predominantly dense sand and stiff clay. In normally consolidated clay, however, foundation capacity is derived mainly from increased soil shear strength with depth. It was, therefore, deemed necessary to examine the possibility of extending the range of aspect ratios of suction caisson foundations beyond that used in the North Sea.