Suction Anchor Installations for Deep Gulf of Mexico Applications
Authors: El-Gharbawy Sherif L. , Olson Roy E. , Scott Shaesby A. 
Published: October 2008

A cost-effective solution to mooring problems in the deep and ultra-deep waters of the Gulf of Mexico (GoM) is the suction anchor system. Suction caisson foundations, also known as skirt piles and bucket foundations, have been used with success in the North Sea for major structures such as the Gullfaks C gravity platform (1989), the Europipe fixed steel jacket (1994), and the Snorre tension leg platform (1991). In the normally consolidated clays of the deep GoM, the new foundation concept is implemented with mooring applications where the holding capacity derives from increased resistance with depth to pullout or lateral bearing capacity failure mechanisms. Knowledge of the geotechnical engineering properties of the deep GoM clays is key to accurate and efficient foundation design. An effective way to determining these properties is by way of correlation with actual caisson behavior during installation. The results of a laboratory modeling program of suction caisson installations are used in conjunction with observations from about half a dozen actual deployments to back-figure the geotechnical properties of the foundation materials. The model caissons were fabricated with length-to-diameter ratios ranging between 2 and 12 to investigate the effect of increasing caisson aspect ratio on the feasibility of using suction as the method of installation. In the field, the installed caissons had a diameter of 12 ft (3.7 m) and an aspect ratio of 5. The caissons were installed in water depths ranging between 4000 to 10,000 ft (1200 to 3000 m). In general, self-weight penetration accounted for approximately one half the installation depth and the suction penetration data were utilized to sketch profiles of undrained shear strengths with depth at the installation locations.

Introduction

One of the objectives of this project has been to investigate the installability, using suction, of caissons with increased length-to-diameter (aspect) ratios over those used in the past. Suction caissons with aspect ratios ranging between 0.35 and 2.0 had been used successfully in the North Sea, where the foundation material was mainly stiff clays and dense sands. In the Gulf of Mexico, however, in water depths of 4000 to 10,000 ft (1200 to 3000 m), the soil is composed primarily of soft, normally consolidated clays. To make use of the greater shear strength characteristics of the soil at increased depths, caissons with greater aspect ratios were introduced.

To examine the potential of the research program, a 4-in. (100-mm)-diameter model caisson with an aspect ratio of 8 was fabricated and successfully installed using suction in a laboratory-prepared soil sample. From the results of this test, the components of the resistance to penetration were identified and incorporated into a limit equilibrium model. This model was utilized in selecting the dimensions of model caissons to be used in the testing program (El-Gharbawy, 1998a). The successful installation and testing of the model caissons with greater aspect ratios than those typically used for North Sea applications has paved the way for the inception of the Suction Anchor System for the mooring of mobile drilling units in the GoM (El-Gharbawy, 1998b).

In this paper, the driving and resistance forces acting on the caisson during installation are identified. Plots of caisson penetration versus time for the different caissons are presented to substantiate the limit equilibrium model. Soil shearing resistance profiles are estimated on the basis of installation observations and thence utilized in design.