Abstract

A deepwater integrated geotechnical and geophysical site investigation took place m the central region of the Gulf of Mexico The purpose of the site investigation was to evaluate soil properties and shallow geohazards for future foundation design of a floating production facility. Acquiring the necessary data for foundation design in deepwater involves challenges not normally encountered in more shallow water depths To meet these challenges a new non-tethered geophysical tool was developed and a geotechnical tool not normally used in the Gulf of Mexico was adapted to obtain cost effective data These tools were used to identify and characterize subsurface conditions at the site. They proved to be effective in delivering key data m a cost effective manner Integrating results from these tools proved successful in providing crucial information for evaluating seafloor and sub-bottom features and soil conditions that influence foundation design. Integration of the geotechnical and geophysical data as it was collected provided the flexibility to modify the Initial program to ensure that conditions were characterized sufficiently for design Some geophysical data were specifically collected at the foundation locations to enhance the geotechnical program. Results from the targeted geophysical data identified three geohazards that could affect the design of the sucbon caisson foundations intended to moor the host facility. Additional geotechnical data were gathered and integrated with the geophysical data to fully assess these geohazards. The integration of the data collected during this site investigation provided the information required for installation and design of the suction caisson foundations We note, however, that the relatively benign surface and subsurface conditions at this site were conducive to the tools and methodologies chosen to characterize this site. At other more complex deepwater sites the tools and methodologies used at this site may have to be modified.

Introduction

To site a floating facility m deepwater a number of issues best resolved using geophysics and geotechnics must be addressed. These include (1) understanding soil properties to facilitate the design of the foundations, (2) understanding shallow geohazards that may effect installation and service of the foundations for subsea equipment and the host facility By integrating geophysics and geotechnics an enhanced understanding of both soil properties and shallow geohazards is obtained (e.g Jeanjean et al, 1998) Geotechnical and geophysical information is obtained through a specifically designed site investigation program Careful consideration of the capabilities of available geotechnical and geophysical tools should be made to maximize value while providing acceptable information for design. An example of a successful integrated site investigation that meets these criteria is presented in this paper.

Deepwater offers several unique challenges beyond those encountered in shallower waters. To meet these challenges cost effectively new tools must be developed and existing tools adapted to perform in these harsh environments. A recent integrated site investigation in southern Green Canyon, Gulf of Mexico was undertaken using new and recently adapted cost effective tools.

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