Introduction of a New Offshore Cone Penetrometer (CPT) Device for the Gulf of Mexico
Authors: Meyer Barry J. , Harman Don E. , King Philip G. 
Published: October 2008

A downhole cone electrometer system called Swordfish has been developed by McClelland Engineers, Inc., for use in the Gulf of Mexico. This tool advances a standard cone electrometer 10 ft into the soil below the base of a borehole. The tool is also capable of obtaining 3-in.-diameter push samples.

The development and testing of this equipment included extensive onshore testing at McClelland Engineers' Houston office and offshore testing at sites in the Gulf of Mexico. The cone electrometer test (CPT) data from an offshore site are presented and compared to the results obtained from an adjacent soil boring.

Introduction

Offshore construction is continuing to expand in the Gulf of Mexico. Numerous structures are being installed in both shallow and deep waters. The fixed template type platform with a pile foundation is still the predominant structure in the Gulf of Mexico. A site investigation that includes at least one deep soil boring is required for the placement of a fixed structure in the Gulf of Mexico. These investigations are normally conducted from an oilfield supply boat. The wire-line percussion sampler which was developed in the early 1960's is still the most commonly used technique for acquiring soil samples. With this technique, sample quality is sacrificed to allow the boring to be conducted in the most expeditious manner.

Cone electrometer data can aid foundation design by providing information on in situ properties of both cohesion less and cohesive soils. This testing technique which is widely used around the world can yield a more cost effective reliable design. A downhole cone electrometer system called Swordfish has been developed by McClelland Engineers. To provide continuous cone electrometer data in 10-ft increments and to take 3-in.-diameter push samples.

This paper describes the new system and its operation. Also, the cone electrometer data are compared to results from an adjacent conventional soil boring.

Review Of Site Investigation Techniques In The Gulf Of Mexico

In the period of 1947 to 1962, a conventional land method of taking soil samples was used offshore (McClelland, 1972). As water depths increased beyond 100 ft, this relatively slow method became impractical for an offshore environment. To overcome this problem, a wire line percussion sampling technique was developed (McClelland, 1972). Although the percussion sampling technique is practical, numerous authors (Terzaghi and Peck, 1967; Erich, 1971; and Young, Quiroz, and Ehlers 1981) have shown the deleterious effect that this procedure can have on sample quality.

In situ testing techniques can be used to overcome the effect that the percussion sampling technique has on sample quality. The most widely used technique in the Gulf of Mexico is the vane shear test. The Remote Vane, developed by McClelland Engineers in collaboration with Shell Development Company (Doyle, et al 1971), measures the shear strength of cohesive soils in situ. Another technique that has been used in the Gulf of Mexico more recently is the cone electrometer test. This test provides continuous quantitative information on the in situ properties of both cohesive and cohesion less soils. The newly developed Swordfish system can provide this information.