Seafloor Cone Penetrometer For Deep Penetration Measurements Of Ocean Sediment Strength
Authors: Ferguson George H. , Bell Wiley D. 
Published: October 2008

Foundation design and construction planning for offshore structures are rapidly becoming more complex, involving a wide variety of foundation types, foundation elements, installation procedures and subsurface conditions. Maximum water depths and foundation loads are increasing, and severe environmental conditions producing multiple-hazards are more common. Sophisticated analyses developed for these conditions require accurate and thorough soil property evaluation. In situ testing of soil properties, such as Remote Vane and cone penetrometer tests, enhance the information obtained from laboratory tests on soil samples.

A remotely-controlled, hydraulically-operated, seafloor-based system called Stingray has been developed by McClelland Engineers which operates in conjunction with offshore drilling equipment to provide continuous cone penetrometer information, high-quality soil samples and other in situ measurements to any drillable penetration from a floating base. This powerful capability of testing below formations which normally cause cone penetrometer refusal assures information for deep as well as shallow foundation designs.

Results are presented from the onshore development and testing of the Stingray system, and from the offshore use of the cone penetrometer, mechanical and readout subsystems.

INTRODUCTION

Offshore construction for the oil industry had its significant beginning in 1947 off the coast of Louisiana in the Gulf of Mexico. From that time to the present, many changes have taken place. Offshore construction has expanded geographically to areas around the world, and continues to expand with the search for oil. The wide variety of geographic locations involves many different subsurface geologic conditions. The maximum water depth of offshore construction has increased steadily since 1947 -soon to exceed 300 meters. Greater water depths, together with severe environmental conditions found in many areas, lead to increased foundation loads. A variety of foundation types, foundation elements and installation procedures have been developed. All of these changes, together with another design consideration in some geographic areas -- seismic loads, have required the development of increasingly complex and sophisticated design procedures. These procedures, in turn, often require greater accuracy, sophistication and thoroughness in the measurement of the engineering properties of foundation soils.

To accommodate these needs, site investigation techniques have evolved through several stages of development (McClelland, 1975). An important technique for measuring soil properties which is receiving considerable attention is one of making the measurements in place, or in situ. This technique can significantly reduce sources of inaccuracy associated with testing soil samples, such as sample disturbance and stress relief caused by removing samples from their natural environment.

A device which offers an advantage of continuous in situ information with depth is the cone penetrometer. By pushing the penetrometer into the soil at a controlled and constant rate, simultaneously measuring both point resistance and sleeve friction, information is obtained which can be related to soil strength and other soil properties. These relationships permit a numerical evaluation of the in situ properties of cohesionless, as well as cohesive soil formations -- an important advantage of cone penetrometer testing.