Analysis of Spatial Variability in Deepwater Geotechnical Data for Foundation Design

Abstract eng:
The design of foundations for oil and gas deepwater developments typically requires soil characterization over large areas. Often, in uniform geological settings, it is neither practical nor economical to acquire geotechnical data at every foundation location. Additionally, the zone of interest for the foundation may extend deeper than the available geotechnical data. The question then arises as to how to design a foundation when information is not available for its exact location. This paper describes an analysis of spatial variability in geotechnical properties for foundation design in a deepwater region. The geology consists of normally to slightly overconsolidated marine clays that have not been affected by major faulting, mass transport or erosion. Data are summarized for about 100 locations with soil borings, jumbo piston cores and cone penetration tests. Geostatistical models that describe spatial variations in the design undrained shear strength versus the distance away from the available information are formulated and calibrated with the database. Models to estimate end bearing, side shear and lateral resistance are developed. These geostatistical models are incorporated into a reliability-based framework to account for uncertainty due to spatial variability in foundation design. Design examples are provided to illustrate the framework. In this setting, depth-averaged values of undrained shear strength are correlated over longer distances than point values due to stratigraphic features. There is greater variation and less spatial correlation in the more recent hemipelagic deposits (upper 20 m) than deeper turbidite deposits and along the continental shelf versus off of the shelf. The increased conservatism required in foundation design due to spatial variability when site specific strength data are not available is generally small in this setting. The proposed framework has potential applications in other deepwater settings, provided that a basin-specific soil database is available.

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