Geotechnical and Engineering Geological Investigations of Deep-Water Sites
Authors: Campbell Kerry J. , Dobson B. Mark , Ehlers Clarence J. 
Published: October 2008

Comprehend I?ve, multitasking primary invest Imation?s are needed to assess conditions at deep-water sites on the continental slope. This is because geologic conditions on the continental slope can be complex, and evidence for seafloor instability is far more common than on the continental shelf. Deep-water site investigations are operationally difficult, require special techniques and equipment, and are technically complex. As a result, these Investigations take longer and cost more than similar investigations done on the continental shelf. The phases of a deep-water site investigation and the sequence of execution are:review of existing data and planning;geophysical survey Inga to give an overview of site conditions;geophysical data interpretation and selection of boring locations;soil sampling and In-situ testing;laboratory testing and data analysis; andIntegration of geological, geophysical, and geotechnical data and engineering assessment of site conditions. Comprehensive deep-water site Investigations require careful planning and coordinate Ion and can take 6 to 12 months or more to complete.

Introduction

The Cognac, Cerveza, and Hondo platforms are among the few major offshore production structures which have been installed at deep-water sits. Other deep-water structures are being planned and the number of Installations Is expected to Increase rapidly during the next few years. Deep-water sites are generally def Inked as sites where the water depth is greater than 600 feet. The 600-foot water depth contour often marks the seaward edge of the gently sloping continental shelf and the top of the steeper continental slope. As nearly all production structures have been installed at sites where the water depth is less than 600 feet, techniques of site investigation and methods of geotechnical analyses for sites on the continental shelf have been refined and are more-or-less routine. In contrast, techniques and methods of analyses for investigating deep-water sites on the continental slope are still in their early stages of development.

Generally, geotechnical site investigations for deep-water sites are more comprehensive and techsIca11y more camp1ex than for sites on the continental shelf. There are two primary reasons for this. First, to protect the extraordinarily high Investments in deep-water structures, a thorough understanding of the site?s engineering geologic and geotechnical conditions Is necessary.

Thus, comprehensive site investigations are warranted. Second, engineering geologic conditions in deep-water areas are generally more diverse and less well understood than in shallow-water areas. For example, the evidence for seafloor Instability is widespread in deep-water areas compared to most shall own-water cease. Large mass I?ve failures have been documented Including one 500-foot thick slide which covers an area of about 18 standard lease blocks in the northern Gulf of Mexico2. Many of these slope failures occurred thousands of years ago and are now stable. However, our knowledge of deep-water environments is still too limited for us to readily predict which areas may be unstable. As a result, multidisciplinary geophysical, geological, geotechnical, and oceanographic investigations are required in most deep-water areas.