Application of Suction Caisson Foundations in the Gulf of Mexico
Authors: El-Gharbawy Sherif L. , Iskander Magued G. , Olson Roy E. 
Published: October 2008

In the last decade, use of suction caissons (suction piles) has increased in offshore arena. Suction caissons have the appearance of inverted buckets with sealed tops and are installed by pumping water out of them. Pumping creates a differential pressure across the top pushing the caisson into place, and thus eliminating the need for driving. Suction can be used also to resist axial tensile loads. Suction caissons realize economical advantages over traditional driven piles due to the speed of installation, elimination of the pile driving process, and reduction in material costs. There are a number of uncertainties in the design of suction caissons. The state of stress and soil conditions adjacent to a suction caisson may be different from those surrounding driven or bored piles; thus experience-based design methods may not work well with suction caissons. The tensile load capacity of suction caissons depends primarily on the hydraulic conductivity and the shearing strength properties of the foundation material, drainage length, and rate of loading. The relationship between the various parameters affecting the tensile capacity is not clearly understood. Furthermore, during pullout, volume change characteristics of the surrounding soils may change the theoretical suction pressures. A review of the existing knowledge relating to the design and construction of suction caissons is presented in this paper. Experimental results from a number of laboratory studies in sand and clay are also presented along with case histories.

Introduction

In the last the decade, the use of suction pressure to install the foundations of offshore structures has been transformed from a novelty concept to being a viable alternative to pile driving. This effort has been motivated by the depletion of oil reserves in shallow waters and the need to install offshore platforms at greater depths. The cost of traditional fixed jacket platforms (FJP) increases exponentially with depth due to the increase in material and labor cost. Bullwinkle, the largest FJP, has been installed in 400 m (1350 ft.) of water. Floating structures, such as the tension leg platforms (TLP) are becoming, in some cases, the only economically viable design alternative. For example, the estimated cost of using an FJP in 872 m (2860 ft) of water at the site of the Auger TLP would have exceeded the estimated cost of the oil reserves. Auger, which was ASCE 1995 Civil Engineering Project of the Year was installed at a cost of $1.2 billion (1). Foundations of TLPs are typically installed at great depths and are subjected to tensile and cyclic loading. In this environment, suction caissons realize economical advantages over traditional driven piles due to the speed of installation, elimination of the pile driving process, and reduction in material costs (2). Suction pressures may also be used for developing the axial tensile load capacity of suction caissons.

Suction Caissons

Suction caissons have been also known as suction piles, suction anchors, bucket foundations, and skirt foundations. Suction caissons can be distinguished from traditional piling in a number of ways. Geometrically, suction caissons have been larger in diameter and shorter in length than traditional piling.