The Response of Suction Caissons in Normally Consolidated TLP Loading Conditions

Abstract

ABSTRACT This paper describes a series of seven cyclic loading tests performed in a centrifuge to evaluate the behavior of steel suction caissons under uplift loadings with soil conditions typical of deepwater Gulf of Mexico (GOM). Suctioncaissons are large cylindrical foundation units installed with active suction after padial penetration by deadweight. They derive most of their uplift resistance from passive suction. The tests were performed to simulate a prototype caisson with a 50-ft diameter and about a 100-ft penetration be/ow the mud/ine. A primary objective of the testing programs involved the application of cyclic loads representative of the actual /oads predicted for a deepwater Tension Leg Platform (TLP) during a design storm. Tests were conducted for a range of inclinations from vertical including: 1) purely vertical, 2) at a 6" inclination, and 3) at an inclination angle that varied during the test. The results obtained from these tests established the combination of the cyclic load ratio (cyclic load divided by the static uplift resistance) and the number of applied cycles that cause failure of the foundation. For cyclic load ratios of 0.48, 0.34 and 0.27 with no fixed static load the number of cycles required to cause failure were approximately 100, 500, and 1,000 respective/y. The tests where the inclination angle varied experienced the most reduction in capacity. Finally a 55-fl diameter by 11O-ft long caisson in normally consolidated soils was shown to satisfy design criteria for a 100-year design storm in the Gulf of Mexico. INTRODUCTION Background One alternative for TLP anchorages is a relatively new concept referred to as a suction caisson. Suction caissons, like piles, are generally cylindrical in shape but have larger diameters and subsequently shallower penetration depths. The foundations are usually constructed by attaching several cylindrical units side by side. The term "suction" refers to two different factors affecting the foundation system, i.e., 1) the controlled reduction of pressure at the caisson head during the installation process, and 2) the passive reduction of pressure occurring during uplift. This passive reduction in pressure mobilizes a reverse-end bearing at the bottom of the caisson. Suction caissons offer several advantages. The first relates to the installation process. Installation involves penetration by deadweight and the reduction of pressure under the caisson head creating a downward differential pressure. The installation therefore relies on pumps rather than pile hammers required for deepwater pile installation. Once installed, the pull-out resistance benefits from the larger diameter, which:provides a substantial area at the top of the caisson for ballast and,mobilizes a significant amount of reverse-end bearing or passive suction during uplift. In the late 1980s Exxon became involved with these foundations when they were first proposed for the Snorre TLP in the North Sea. The Snorre foundation installation along with the Gulfaks C field tests (Tjelta et al., 1986) demonstrated installation feasibility of large-scale suction caissons. Additional information on the Snorre foundation design and installation is described by Cottrill (1991) and Fines et al.(1991).