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The AAPG/Datapages Combined Publications Database
Houston Geological Society Bulletin
Abstract
Abstract: Deepwater Geohazards and Engineering Geology: Meeting
Tough Challenges
By
Fugro-McClelland Marine Geosciences
Petroleum exploration and development are now being carried out in water depths of 5,000 feet and more. Investments for these offshore activities are huge: rates for some deepwater drilling rigs alone are currently $100,000 per day or more, and total investment in some deepwater developments has been as much as $1.2 billion.
To complicate matters, shallow geologic and soil conditions at deepwater sites are often complex and difficult compared with conditions typically found on the continental shelves. These complex conditions present serious engineering and safety challenges that require careful application of geoscience techniques as a basis for avoiding or reducing hazards and to help minimize the cost of deepwater development. Reliable engineering-geologic characterization of site conditions, being increasingly based on 3-D seismic data, is essential to optimize siting, design, and operation of facilities and to thus maximize value to investors.
Complete characterization of offshore
sites includes defining water depths and
seafloor topography, determining soil
geotechnical properties and relationships
among soil strata, and making an engineering
assessment of geologic conditions.
Geophysical
data define geologic
features and general stratigraphy, whereas
borehole data define detailed stratigraphy
and soil geotechnical properties at specific
points; alone, neither completely characterizes
a site. Among the activities
requiring offshore site characterization,
including geohazards assessment, are well
planning and exploratory drilling, pipeline
routing and design, and facilities siting
and design.
Complex deepwater (water depths >600 feet) conditions in the Gulf of Mexico that can cause engineering difficulties include 1) steep and potentially unstable slopes of 10 degrees or more; 2) irregular, commonly rocky, topography with sharp relief ranging from a few feet to several tens of feet; 3) faults, many of which appear to be active, with seafloor scarps ranging up to more than 200 feet high; 4) both modem and ancient landslides covering large areas; 5) gas hydrates (solid, ice-like mixtures of gas and water found in water depths >1,500 feet) that may be subject to reduced shear strength and thaw settlement when heated; 6) overpressured sands at relatively shallow depths; 7) erosion of tens of feet of seafloor sediments; and 8) soil conditions ranging from weak, underconsolidated soils to rock. Similar difficult conditions are found in many other deepwater areas outside the Gulf of Mexico as well.
Several geophysical
tools are typically
used to help characterize offshore
sites: a narrow-beam water-depth
recorder with velocimeter calibration
and/or a swath-mapping bathymetric
system; a side-scan sonar to show a
plan view of the seafloor and features
on it; a shallow-penetration subbottom
profiler (3.5 kHz) to show geologic
conditions to penetrations of up to
about 200 feet; an intermediate-penetration
profiler (minisparker or small
air guns, as examples) to show conditions
within the foundation zone (to
penetrations of about 500 feet); and a
deep-penetration profiler (air-gun
array with multi-channel digital
recording, for example) to show deep-seated
faults, buried landslides, and
gassy sediments (to penetrations up to
4000 feet).
Results of marine engineering geophysical
site surveys include a variety
of color graphics such as water-depth
map and 3-D perspective views of the
seafloor, seafloor gradient map,
seafloor soil and soil province maps,
soil cross sections, geologic structure
and features maps, and hazards maps,
including drilling risk and development
favorability maps. Results 1) are
presented using simple, straight-forward
terminology and not in technical
jargon the end-user (engineers) may
not be familiar with; 2) focus on the
important engineering issues and not
on survey documentation and methodologies;
3) are presented as quantitatively
as possible; and 4) are integrated
with geotechnical data for a reliable
definition and engineering assessment
of both soil and geologic conditions.
Developing trends include increasing use of 3-D seismic data and workstation analysis; site-survey data increasingly being recorded in digital format; integration of site-survey results in GIS data bases; and application of exploration and development techniques (including attribute analysis and geostatistics for direct characterization of materials using seismic data) to shallow engineering concerns.
Characterizing deepwater sites around the world, and developing new techniques and technologies for doing so quantitatively, will continue to provide marine engineering geoscientists with tough challenges into the next century.
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