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OCR for page 98
THE CHALLENGES OF EEZ USE
The EEZ seabed represents a diverse, poorly understood, difficult, and sensitive environment
that differs fundamentally from coastal and terrestrial areas, where engineering practice and
experience are more advanced (see Chapter 2~. Present and potential uses of the EEZ seabed will
involve a wide range of activities that may include trenching, excavation, drilling, pile driving, and
anchoring. These activities will support the placement of cables, pipelines, instruments, and
structures (see Chapter 3~.
The major challenge is to achieve the efficient use of the EEZ seabed, which will require a
thorough understanding of seabed characteristics and processes at prospective sites for specific
activities. Achieving this goal entails two interrelated tasks:
1. The impact of seafloor characteristics and processes on the proposed engineering activity
must be rigorously defined for cost-effective and safe planning, design, construction, and
maintenance.
2. The impact of the proposed activity on seafloor characteristics must be carefully determined
and monitored to minimize use-related changes and environmental degradation.
Accomplishing these tasks depends on the ability to gather various kinds of data and integrate
them into a framework for site evaluation. The common elements of such a framework and the
overarching technology systems needed to acquire such data across various uses are analyzed in this
chapter.
A FRAMEWORK FOR SITE EVALUATION
A realistic assessment of the constraints to engineering development and the impacts of EEZ
use at specific sites will require a systematic integrated approach. An organized framework of
investigation involving oceanographic, geologic, geotechnical, and biological data is needed to
recognize the interrelationship of each on overall site performance. This approach involves the
development of a site performance model in which seafloor characteristics are quantified and
interrelated to give predictive capability (Figure 5-1~. Each model relates to a specific use or
combination of uses and is intended to focus on the possible constraints to and impacts of use and
the data needed for assessment. Such models are generally independent of resource evaluation.
For any EEZ use, all the components of a particular location on the seafloor need to be
evaluated, especially linkages and feedbacks. Clearly, the effects of geology, geotechnical properties,
oceanography, and biology will vary from one site to another.
98
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99
Geology
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Geotechnology |
~ Recognition ~
| Oceanography\
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, - -I
Site Performance Model |
Use ~
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FIGURE 5-1 Site performance model.
For example, a prospective location for deepwater oil and gas development may be geologically
active or benign, possess strong or weak sediments, experience imperceptible or energetic currents,
and have simple, resilient, or complex and vulnerable biological communities.
A first step in an integrative process (Figure 5-1) is recognizing the balance of conditions at a
site, which can be achieved by mapping and sampling. Measurement of seabed conditions and
processes involves a range of in situ sampling, monitoring, and laboratory techniques. Data for
geologic characteristics (origins, ages, and process activity), geotechnical properties (strengths,
variability), near-bottom oceanography (physical and chemical), and biological populations (structure
and interdependence) are combined and analyzed into a comprehensive assessment of the site. The
more detailed and integrated the assessments in terms of history and present behavioral status, the
more powerful its predictive capability.
The site-performance model leads to optimum design and planning of engineering tasks. As
development proceeds, site monitoring ascertains site behavior and departures from that predicted by
the model, enabling remedial action. Experience has shown that this type of integrative, focused
framework is essential for efficient engineering of the seabed. This approach has been used to
successfully design, install, and maintain oil and gas facilities in hazardous offshore regions and is
the basis of site assessments in such frontier areas as the continental slope of the Gulf of Mexico.
The technical constraints associated with specific development activities indicate that there is a
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100
shared need for site performance models for each use, based on very similar or identical suites of
information. Following is a general review of the major categories of information needed.
Geology
Analysis of the geologic characteristics of engineering sites aims at comprehensive evaluation of
present three-dimensional sediment and structural geometry, site history, and future geologic
behavior. Geologic data allow identification of constraints to engineering provided by existing site
conditions and forecasts of possible magnitudes and distributions of geologic processes during the
engineering design lifetime.
Each use of the seabed will be affected by different properties of the site in different ways. For
any development area, however, there is a shared need for information on bottom roughness,
seafloor slope, sediment types and geometries, seafloor and near-surface Processes. and the regional
and subsurface context.
Bottom roughness is the local variability in seafloor morphology antis determined from sonar or
camera imagery. Roughness varies in relief scale and origin. Low-relief roughness can be due to
sediment texture contrasts (e.g., clay to boulders) or to current-induced bedforms (ripples and
dunes). Large-relief roughness can be caused by concretionary areas, carbonate mounds, or clefts,
fissures, and steps on rock outcrops. Roughness affects the design and emplacement of structures
that rest on the seafloor, such as templates, pipelines, and cables.
Seafloor slope is determined from bathymetric profiles or high-resolution swath bathymetry and
is usually represented as three-dimensional displays. Slope variability can be extreme. Shelves,
basins, or rise areas can be flat or very gently sloping. Faults, diapir slopes, or eroded canyon walls
can be very steep (up to 40 degrees). Present seafloor engineering practice for templates, piles, and
seafloor-supported structures is to avoid gradients greater than 10 to 15 degrees. Pipelines and
cables are less constrained by absolute gradient than by abrupt changes.
Near-surface sediment distributions and geometries are usually determined by acoustic profiling
and sampling. Geologic properties of sediments reflect their origins, modes of deposition, and post-
depositional modifications, with corresponding variability in geotechnical properties. Geologic site
mapping usually distinguishes conformable layered sequences, unconformities indicating erosion or
nondeposition, chaotic or deformed sedimentary units, and acoustically transparent, homogeneous
layers or zones. There is often considerable ambiguity in interpretation of acoustic data, and the
engineering constraints to site development can be determined only by combinations of geological
and geotechnical information.
Geologic processes that can constrain development include faulting, subsidence, tectonic uplift,
erosion, turbidity currents, and various types of mass movement. Where engineering design does not
match the magnitude and frequency of process action. the natural geologic processes can be
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hazardous to seafloor development activities. Prediction of process magnitude and frequency by
both geologic and geotechnical data and incorporation into design leads to reduced hazard potential.
The development of oil and gas resources using specially designed and constructed platforms in the
Mississippi delta mudslide area is an example of practical, economic engineering in a geologically
active region.
Geologic information for development sites aims at recognizing processes and assists in
analyzing their potential effects. Seafloor morphology and near-surface sediments contain the
accumulated partial signatures of long-term environmental and geologic process events. Hindcasting
and reconstruction, supported by geologic dating, reveals the types, magnitudes, sequences, and
distribution of past processes. When such information is combined with geotechnical,
oceanographic, and biological data (Figure 5-1), future trends for site behavior may be anticipated.
For example, evidence from former landslide deposits, erosional unconformities, or fault
displacement rates can be used to forecast landsliding, erosion, or faulting.
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Prudent site evaluation studies also consider the regional geologic contm of neighboring areas
and the deeper subsurface geologic conditions. Regional geologic data provide perspectives on
geologic processes that may extend into the engineering site. For example, pipelines, cables, waste
disposal, and mining activities may be impacted by landslide processes initiated upslope from the
actual seafloor use. Similarly, deep subsurface conditions, such as faulting, may impact seafloor
engineering because of upward extension of displacements or gas leakage.
Geotechnical Properties
Most activities planned for the EEZ will require some type of installation or facility that will
depend on seabed foundation support or subsurface installation. There are a large number of
different foundation types and installation procedures required to accommodate the wide range of
sediment conditions encountered throughout the EEZ
Present geotechnical engineering practice offers highly advanced field investigation methods and
analytical procedures available for design that will help achieve risk-free installation and
performance. Good quality geotechnical data required at the development site most often will
consist of the sediment profile (stratigraphy), sediment type (classification), and sediment
engineering properties (density, strength, Reformational characteristics, and permeability). These
data are traditionally acquired by performing laboratory tests on recovered samples, although in situ
testing in recent years has provided an improved method for obtaining meaningful measurement of
sediment characteristics.
Geotechnical data requirements will vary depending on the type of activity planned at the site.
For example, uses by the military most often will require data to only a few meters depth for the
small foundations and cables placed on or near the seafloor. On the other hand, deep foundations,
such as the piles supporting the massive production platforms for the oil and gas industry, often
require geotechnical data to depths of at least 200 m. Disposal of wastes within the subsurface
sediments may require different types of data (such as permeability) to tens of meters below the
seafloor. Thus, each proposed activity will require that geotechnical data be acquired specifically to
the depth of interest for the proposed activity at the site. A brief description of how the
geotechnical properties influence various types of seabed uses is presented in the following sections.
Sediment profiles are needed for almost any activity associated with the seabed that requires a
thorough understanding of the variation of sediment conditions with depth below the seafloor
(stratigraphy). Sediment stratigraphy is important because the physical, chemical, and biological
characteristics of marine sediments are closely linked to the overall engineering behavior associated
with in situ stresses applied to the sediment strata by loads imposed from various activities. Since
sediment types may range from extremely weak sensitive clays to highly cemented rocks, it is
essential to understand the variation of the sediment throughout the profile. Depending on the type
of sediment, the type and size of foundation or anchor, and the applied loads, a geotechnical
investigation will be required using sampling techniques described in Chapter 4.
Sediment classification tests are performed to categorize the sediments according to color, grain-
size distribution, and plasticity characteristics once sediment samples have been obtained at a
prospective site. These tests provide the basic framework to understand composition and consistency
necessary for assigning other types of laboratory tests. Classification test results are also important
to the design of the foundation support/restraint system, because they provide the first indication of
whether the sediment behaves as a cohesive or noncohesive material.
Sediment engineering properties are required to allow clear accurate analysis of the behavior of
marine sediments, which depends on appropriate modeling of the strength and stiffness variation
with depth in the deposit. For example, the design of foundation piles to support various types of
structures will require shear strength parameters and stress-strain characteristics to significant depths
below the seafloor to predict the pile performance under various axial and lateral loads.
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Consolidation tests also may be required if load-settlement characteristics are to be investigated for
a gravity-type platform, or if stress history of the sediment is important in interpreting strength data.
For some uses, such as burial of wastes in sediment or mining, it is necessary to determine
other properties and behavior. For example, permeability (or hydraulic conductivity) is a key
parameter in the assessment of the possible migration of contaminants through the sediment. For
some types of mining activities it is necessary to determine the engineering properties of the
surficial sediments (upper 0 to 1 m) in order to assess trafficability characteristics for bottom-
crawling equipment and to assess rippability of surface ore bodies.
The laboratory tests should model the initial and final status of stress, -drainage, and loading
conditions within the sediment mass as accurately as possible with respect to field conditions. Thus,
a thorough understanding of environmental conditions is essential to ensure that the impact of the
proposed activity on the seabed leads to risk-free performance in terms of both engineering and
biological impact.
Oceanography
For a prospective EEZ seabed utilization site, the principal components of oceanographic
information critical to recognition of the balance of conditions and to the resulting site-performance
model are water depth, seawater motion, sediment chemistry, and variability at or near the site.
Some of the information on the geology and biology of the site also has oceanographic relevance.
Water depth is important for several reasons. First and probably foremost, seawater is an
"overburden" that has to be dealt with during any use of the EEZ seabed because, a peon, a greater
water depth implies greater problems in the installation and operation of facilities and equipment
for seabed resource recovery. In addition, there are depth donations to the physical, chemical,
biological, and geological processes that may affect the site use and provide insight into the
processes to be investigated.
Seawater motion at a particular site has several components that should be considered in use-
related planning. The most obvious is the current pattern, which is the distribution of discrete, near-
bottom seawater flows moving in identifiable directions. Sometimes these flows are large and
reasonably well defined-e.g., where the Gulf Stream impinges on the southeastern continental
margin, or where the western boundary undercurrent contacts the Atlantic continental slope. Other
flows may be much smaller and have a shorter lifetime, such as the storm-wind-generated
downwelling on the northeastern U.S. continental shelf. Another important process is wave-bottom
interaction, derived either from wind-driven surface waves on shallow continental margins or from
various forms of internal waves. A seawater motion that occurs everywhere in the EEZ is near-
bottom turbulence, the random motion of water parcels of widely varying size in the vicinity of the
seabed. All of these forms of seawater motion can contribute to or influence the sediment transport
rate, which is one of the most critical geological parameters relating to the use of the EEZ seabed.
Many aspects of the sediment chen~isay impact seabed use. Mineralogy and chemical composition
affect geotechnical properties, sediment erodibility, and the production and community structure of
benthic organisms. A particularly important chemical parameter is the sedimentary oxidaiion-
reduction profile. The thickness of the upper oxidizing zone and the contrast between the abundance
and nature of its oxidizing compounds and the abundance and nature of the underlying reducing
compounds influence the corrosion rates of emplaced structures, the vertical transport of
sedimentary pollutants like metals, and the functioning of benthic communities. Bioturbation rate,
the Stirring speed" of sediments by organisms, both alters and is in turn affected by sediment
chemistry.
The variability of the diagnostic parameters just discussed is also a critical part of the
oceanographic information necessary for the responsible recovery of EEZ seabed resources. Without
adequate knowledge of the range and frequency of the variations in those parameters, it will not be
possible to forecast the duration and consequences of a proposed use, construct an effective site
OCR for page 103
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performance model that also minimizes environmental damage, or determine if a particular use has
caused any subsequently observed environmental changes.
Biology
Typically, biological characteristics of a particular site are measured and incorporated into an
assessment of possible environmental impacts from a proposed use. Unfortunately, these exercises
usually provide a picture of the biological characteristics and site properties at only one time. Even
less appreciated are the effects that organisms have on the geological, geotechnical and
oceanographic properties of the EEZ seafloor. A fuller understanding of biological effects on
seafloor uses and, conversely, the possible impacts of a use on biological communities, requires
knowledge of the temporal variability in communities, production rates of benthic organisms, and
the structure of benthic food webs and bioturbation rates.
The notion of community structure embodies a variety of concepts, most notably the number of
species present and the relative abundance of individual species. Changes in community structure
over time are expressed as the stability and resilience of the community. In a stable community, the
structure changes little, either because the organisms in the community are long-lived or because
loss rates and recruitment are approximately balanced. Resilience refers to the ability of a
community disrupted by either natural or anthropogenic disturbances to return to its original
structure. The tolerance of individual species to disturbances and the rates at which they can
recolonize the seafloor determine community resilience. Most research on the stability and
resilience of benthic communities has focused on nearshore rocly habitats; understanding of the
dynamics of benthic communities throughout the EEZ is limited. For example, the long-term effects
on community structure of sediment disruption or resuspension arising from a use such as seafloor
mining cannot be predicted with any confidence.
Food web structure of a biological community consists of the connections between benthic prey
and predators. Many commercially valuable species prey on organisms living in sediments. In turn,
these prey often obtain their nutrition from organic matter in sediments. This network takes on
crucial importance if it becomes a pathway to mobilize materials placed in or on the seafloor.
Furthermore, the production rate of benthic organisms must be considered, since it will affect the
rate of transfer of materials through a food web. For example, highly productive areas of the
seafloor might be more likely to lead to food-web magnification of waste materials deposited on the
seafloor. As a rule of thumb, the production rate of benthic communities decreases with increasing
water depth or with increasing distance from land, but localized exceptions to this general trend can
occur.
Bioturbation, the mixing and stirring of sediments by organisms, alters geotechnical properties of
sediments, such as water content, shear strength, and near-surface stratigraphy. Bioturbation also
modifies sediment credibility and the direct physical exchange of dissolved and particulate materials
between the seafloor and the water column. Thus any use of the seafloor requiring evaluation of
these characteristics (for example, pipelines, anchors, waste disposal, etc.) must consider the role of
bioturbation. Bioturbation rates and vertical extent in the sediment column depend on the species
present and their abundance (community structure), and the physical and chemical properties of
sediments. Understanding these dynamic processes, both in general and for specific sites, is a
necessary component for determining long-term effects of developing and using seabed resources.
TECHNOLOGY SYSTEMS FOR THE SEABED
Evaluating the suitability and vulnerability of particular EEZ sites for use and development is a
complex task. Predictive site performance models require integrated geologic, geotechnical,
oceanographic, and biologic data at scales, coverages, and resolutions appropriate to the site and the
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proposed use. The acquisition of such suites of data for the EEZ seafloor involves three
interrelated and complementary technologies: surveying and mapping, sampling and in situ
measurement of seafloor properties, and monitoring of changes in processes. The specific
technology requirements for particular uses is described in Chapter 3. A comprehensive examination
of technologies for mapping and subverting, acquiring geotechnical data, and monitoring is presented
in Chapter 4. Following is an overview of the present status of and development needs for major
technology systems required for seafloor investigation.
Mapping and Surveying
The acquisition of task and site-specific seafloor data for mapping is a fundamental necessity for
EEZ development. Locations and time frames for needed data will be determined by the specific
development or engineering activity. For example, mapping in support of oil and gas development
is dictated by exploration targets and production schedules.
The specific types of mapped data needed for each EEZ development activity require further
definition. The survey tools and grids will vary with the local characteristics of the area and the
proposed activity. But for oil and gas production systems, mining and waste disposal sites, military
installations (in fact, for most of the anticipated EEZ uses), there is overlap and commonality in the
maps, scales, and resolutions required. Each use shares the need for mapping for site selection, site
appraisal, evaluation of direct engineering effects, and long-term site behavior. Combinations of
mapped data, including bathymetry, seafloor imagery, near-surface sediment profiles and properties,
are appropriate.
The capability of marine survey techniques has developed rapidly over the last two decades. A
powerful suite of acoustic tools exists, able to address survey needs at various scales and resolutions.
Both reconnaissance-scale and task-specific survey systems are presently at work, mapping the EEZ
seabed over the full range of water depths. Multisensor systems and combined-mission cruises offer
economy in data acquisition, providing complementary data sets from a single survey traverse.
Intermediate-scale regional mapping of hiph-interest areas has already proven extremely valuable
_ ., , ~ ,
in support of EEZ activities. Regional acoustic surveys over medium-spaced line grids (e.g., < 10
km x 10 km) represent a balance between high-resolution data, density of coverage, and time and
costs of surveying. Such selected-area surveys give the regional and technical context for design and
interpretation of site-specific surveys. Reconnaissance mapping provides a general overview of large
areas of the EEZ or its entirety. Operational and cost factors dictate either generalized coverage of
large seafloor features (e.g., GLORIA) or very lengthy surveys (e.g., Sea Beam). Reconnaissance
mapping provides a general background for more detailed site- and task-specific surveying.
The principal technical constraints to surveying and mapping the EEZ for development are now
mainly operational, involving questions of rates of coverage, resolution, system efficiency, and cost-
effectiveness. Some of the required technical developments include
.
.
rapidly traversing bottom vehicles capable of acoustic/in situ property cross correlation;
deep-tow systems with multifrequency, multisensor packages in autonomous vehicles or
improved towing and cable technology;
· multipurpose systems with vessels dedicated to routine multipurpose surveying with a wide
range of tools aimed at extensive suites of seafloor data acquired in single traverses;
· multipurpose sensors from which simultaneous bathymetry, seafloor imagery, and subbottom
profiles can be obtained;
· improvements in acoustic sources, arrays, variable and multifrequency profiling systems,
together with real-time processing, to address difficult geologic terrain and the requirements for
engineering data at greater depths below the seafloor;
· common, mutually compatible formats for digital recording and processing to facilitate
exchange of data; and
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· new geologic interpretation modes (involving synthesis of traditional acoustic data, with
statistical analysis procedures and integration with geotechnical data) for effective use of survey data.
The principal nontechnical constraints to surveying and mapping of the EEZ are primarily
organizational, involving questions of planning; prioritization of surveying technology resources;
cooperation among different agencies, groups, and organizations; and data availability. Expense,
different time frames for development, and the challenges of a largely unknown, difficult
environment all dictate planning and organization of effort for successful and cost-effective EEZ
mapping. Further improvements in the technical and operational effectiveness of surveying methods
will be costly. Because of overlap in interests, missions, and mapping needs, the benefits of
improved technology will be shared among government agencies, the military, and industry.
Geotechnical Data
Certain types of geotechnical data are necessary to characterize the sediment properties at the
site of a proposed activity and within the general area. This detailed knowledge of seabed sediments
will require meaningful measurement by sampling, in situ testing, and experimental testing. The
ability to acquire geotechnical data by these three methods varies according to the area being
investigated. The various data acquisition systems are highly developed for water depths less than
200 m, whereas only moderate or little development has occurred with acquisition systems that can
be used in arctic or deepwater regions. Present systems can be summarized as follows:
.
deployment systems, which can be divided into three broad categories: (11 self-contained
units, (2) drilling rigs, and (3) submersibles (ROVs and AUVs);
· sampling systems, which may be divided into downhole or seafloor deployment modes;
in situ testing, which is used to determine sediment properties and has been emphasized
increasingly in the past decade because stress relief and disturbance associated with recovering
samples from boreholes in deep water often alters sediment physical and chemical properties; and
· expenmental model testing systems which began to be used during the last decade in the
marine environment to develop methods of predicting the field performance of various foundation
design elements. Another long-term experimental testing system utilizes instrumentation to measure
thermal, geochemical, and geotechnical responses of sediments when an experimental heat source is
embedded into the seafloor sediments. Results of all these types of small-scale experimental tests
look promising and have provided insight into the behavior of full-scale foundations and the effects
of various activities on sediment properties.
.
.
c7 ~,
Development of the EEZ in the Arctic and off the continental shelf in water depths exceeding
200 m will depend on further development of the following systems:
.
instruments capable of providing data for analyzing long-term sediment stability;
· improved sampling and in situ testing devices for determining sediment properties in more
difficult offshore environments; improved experimental testing methods that will allow the properties
and behavior of the seabed to be reliably observed and predicted in a more controlled environment;
and
· compact deployment systems that can operate in a rapid transit mode and that will
automatically measure seafloor properties or sample the sediments.
A further nontechnical issue for acquiring geoscience data is the need to develop a coordinated
research and development plan involving different agencies and industry user groups and academia
in a cooperative effort to develop improved systems for sampling and performing in situ and
laboratory tests on seafloor sediments.
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Monitoring
The environmental consequences of expansion of U.S. activities into the EEZ are difficult to
predict in advance. Monitoring projects on the scale of years to decades will be necessary to acquire
real data on seafloor process activity. Such data on seafloor behavior, processes, and causative
factors is a key element in predictive site evaluations and will expand the-usefulness of mapped data.
Technology will, in a sense, Striver environmental studies. As new measurement technologies
are perfected so that they can be routinely used, new monitoring will be possible. There are several
technology needs associated with conducting monitoring programs in the EEZ
· A satisfactory method of measuring pollutants in EEZ sediment needs to be developed.
· The ability to quickly assess populations or organisms in the seabed during monitoring
studies needs improvement.
· Time and space capabilities of the sampling device or platform need to be matched with
the time and space domains of the phenomenon of interest.
SUMMARY
Collecting and disseminating data from EEZ mapping, surveying, research, and monitoring
activities as a base for planning various uses of the EEZ seabed represent a scientific and technical
challenge similar to the challenge of exploring space. The magnitude of data to be analyzed and the
needs for technology entail long-term national commitments of funds and expertise. Such
commitments require planning and coordination among the various communities who are involved in
this undertaking. These issues are addressed in the next chapter.
Representative terms from entire chapter:
eez seabed
