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8
Assessment of Technical
Considerations and Needs
to be Met in Dredging U.S. Ports
This chapter focuses on engineering design elements of navigational
facilities, maintenance dredging, the capability of the dredging
industry of the United States, and needs for research and
development. Several technical considerations important to
engineering design and dredging activities are treated in Chapter
7--estuary hydraulics, for example, and the site and nature of the
disposal site for dredged materials. Another most important
consideration--the institutional framework--is discussed in Chapter 7.
ENGINEERING DESIGN OBJECTIVES FOR DREDGED
NAVIGATIONAL FACILITIES
There are two important design objectives for navigational
£acilities-accommodating the maneuvering requirements of vessels, and
reducing as much as possible the future maintenance dredging
required. Some general considerations of the vessel in the waterway
and sedimentation are briefly described in succeeding subsections. It
should be kept in mind that any engineered structure represents many
compromises among these and other objectives, and for navigational
facilities in particular, many unique local features have to be
understood and taken into account.
Maneuvering Requirements of Vessels
Dramatic changes occur in a vessel's response characteristics in
shallow water, and unique disturbing forces act on the vessel that
have no counterpart in the open ocean. Vessels are primarily designed
for the open ocean, however, so their accommodation in confined waters
depends on adequate design of navigational channels and operational
practices.
95
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Entrance
For most ports there is a critical entrance (and exit) area seaward of
the protecting headlands, rock, breakwaters, or jetties where both
shallow-water effects and those of waves, swell, wind, and currents
may act on the vessel. Where entrance channels are dredged in these
transitional areas, greater depths and widths must be provided than
those of channels within the port's sheltered areas, owing to the
vessels' tendency to heave, pitch, roll, and drift (Marine Board,
1981; 1983).
Sinkage
Inside the entrance, shallow-water effects are accentuated by
decreasing depths and widths of navigational channels. The velocity
of water flowing around the sides and under the hull of the vessel
must accelerate, with corresponding lowering of pressure (by
Bernoulli's Law). The vessel sinks lower in the water with (usually)
trim by the bow. For the same reasons, sinkage increases with the
narrowness of the channel and with the vessel's forward speed. If
underkeel clearance is small, vessel speed must be reduced to
counteract linkage, but it should be noted that minimum speeds must be
maintained to counteract the forces acting on the vessel and to
maintain headway. Some ships' engines (particularly the diesel
engines favored in new ships) have minimum operating speeds. Sinkage
is also affected by water density, and will increase in freshwater as
compared to seawater (this is important to ports on river or estuarine
systems, in which a change in water density will be experienced in a
vessel transit).
Bank Effects
While water flow past the sides of the hull is symmetrical if the
vessel is on the channel's centerline and aligned with it, moving off
the centerline will decrease the flow area between the vessel and the
near bank, causing the flow rate on that side to accelerate, with
corresponding loss of pressure. This unequal pressure regime causes -
bank-suction force aft, and a yaw moment turning the vessel back
toward the centerline, as well as a sideslip velocity toward the near
bank, that together with the vessel's forward velocity, induces a
small drift angle toward the near bank; this also induces a small
moment toward the opposite bank. Uncorrected, a vessel once off
centerline would sheer from the near to the far bank, and back, or
ground. Bank effects are forcing functions acting on a vessel that
must constantly be corrected by steering changes.
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Vessel Interactions
In passing or overtaking in navigational channels, vessels experience
unique disturbing forces never experienced in the open ocean (the
effects of vessel-vessel and vessel-bank interactions, since vessels
must move off the centerline to pass or overtake). These effects
develop fully after the vessels have passed, and in any area of
passing or overtaking, sufficient width and length must be provided
for some distance to allow controlled recovery.
Decreased Turning Performance
Vessels at sea have a turning radius comparable to their length, owing
to the continuous sideslipping of water under the keel. This ability
is lost in shallow water, particularly if underkeel clearance is very
small, because water flow under the keel is constricted. Vessels in
ballast also have decreased turning performance.
Winds and Currents
In winds or currents acting at an angle to the vessel, a compensating
yaw (or "crab") angle must be achieved and maintained. This means
that the vessel will "sweep out" a path broader than its beam. The
very high superstructures of some vessels that have most of their
profiles above water, such as containerships and car carriers, present
considerable windage area, and may require more channel width than
their narrow beams would suggest. Even vessels that have little
profile above water fully loaded, such as tankers, may present
considerably more windage area in ballast. The critical relationship
appears to be the ratio of wind speed to ship speed: at ratios of
wind speed/ship speed of about 6 to 7, great difficulty can be
expected in controlling lightly loaded vessels or those with high
windage areas, and at ratios of about 10, control of most fully loaded
vessels will likely be impossible.
Prevailing winds blowing over long periods can also raise or lower
water levels (wind setup or letdown).
Irregularities
A feature of navigation in channels and maneuvering areas that is
often mentioned by pilots but that has not received much systematic
study is the effect on vessels of bottom and bank irregularities.
Modelling of navigational channels usually assumes uniform side slopes
and unvarying bottoms, but general and local conditions usually favor
rapid shoaling on one or another side of a bend or turn, or formation
of a spit that encroaches on the channel at breakwaters or jetties,
with the result of narrowing the width or reducing the depth of
channels in locations where width and depth are most critical to
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maneuverability. Dand (1976) gives an example of ship collision
caused by shoaling in a turn.
Piloting
Maneuvering a vessel in the shallow waters of a navigational channel
and other port facilities is entirely different from maneuvering a
vessel at sea in deep water with infrequent and distant traffic.
Unlike a vessel at sea, a vessel in the confined waters of a
navigational channel requires constant steering to counteract the
number and magnitude of hydrodynamic forces acting on it. Harbor
pilots familiar with the port and experienced in maneuvering vessels
in confined waters board vessels and guide their transits in and out
of the port. All the ports of the United States serving oceangoing
traffic require pilotage.
Successful shiphandling by a pilot in navigational channels demands
smooth, skillful integration of several very important elements:
directing vessel movements; assessing other traffic movements in
meeting and overtaking, as well as crossing traffic; evaluating waves
and surges created by the ship; assuring that the helmsman clearly
understands and executes rudder commands and steering directions
without error; analyzing radar information; knowing the magnitude and
effects of currents, wind, the hydrodynamic interaction of ship and
channel; and anticipating possible changes in high-shoaling areas.
Harbor piloting in ports of the United States is typically of
foreign vessels, of unknown maneuvering characteristics, designed and
equipped primarily for the deep ocean. The pilot will therefore spend
some time on boarding a vessel testing its responsiveness (and that of
the helmsman), and checking the radar and other equipment. The ship's
radar, in particular, might be in any state of repair or calibration.
In poor visibility, the pilot must rely on the radar heading line, and
a problem that frequently occurs with poor radar calibration is
bearing resolution error. An undetected error in bearing resolution
of 2°, for example, will place a vessel 200 ft out of position in just
one mile. In some wind and sea conditions, and in heavy rain or snow,
a "clutter" zone will appear on the radar screen representing the area
around the ship. Activating the clutter-supression controls often
eliminates small targets from the screen, such as buoys and fishing
vessels. Losing buoys from the screen, the pilot may attempt to use
the radar to determine the ship's position by estimating distances
from prominent features of the landscape. An error in the ranging
mechanism of just .05 mile will cause a position error of 300 ft.
Lack of Minimum Standards for Vessel Maneuverability
Considerably complicating the job of both pilot and channel designer
is the lack of minimum standards for vessel maneuverability (Landsberg
et al., 1983; Webster, 1983; Card et al., 1979~. Even very modern
vessels, and vessels in the same class, show wide variation in
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response characteristics, from relatively controllable to unwieldy.
Recent efforts by the Society of Naval Architects and Marine Engineers
(SNARL) and the International Maritime Organization (IMO) show promise
of achieving such standards, but these efforts will take time.
Criteria for Dimensions of Dredged
Navigational Facilities
General guidelines for the dimensions of dredged navigational
facilities have been developed taking into account the over-all vessel
maneuvering requirements described in preceding subsections. In the
United States, the guidelines are developed by the U.S. Army Corps of
Engineers (1983~; consensus standards are also developed and updated
by international organizations, such as the Permanent International
Association of Navigation Congresses (PIANC), and the International
Association of Ports and Harbors, and by other maritime nations.
These standards (see Appendix B) are similar in most respects: those
of the Corps tend to offer more guidance for smaller vessels, and
those of international organizations to concentrate on large,
full-form vessels, such as tankers.
The guidelines are based on selection of a design vessel or
vessels, and calculating needed widths and depths for linkage, passing
or one-way traffic, wind and current effects, etc. The general
guidelines offer a useful first approximation that must be refined
with site-specific information and design validation.
The general criteria also provide standards for an initial
assessment of existing facilities. Using the guidelines of PIANC and
the Corps, the technical panel of the committee made a summary
assessment of U.S. ports and a more detailed assessment of the
navigational facilities of six ports, two on each coast, taking as the
design vessels those that use the ports frequently. The results are
briefly summarized in the succeeding subsection.
SUMMARY OF ASSESSMENT OF NAVIGATIONAL
FACILITIES IN U.S. PORTS
Most navigational channels in the United States are made up of
relatively short, straight sections between 1.5 and 1.7 nmi (nautical
miles) in length, connected by turns and bends. A survey of all those
with straight sections at least 30 ft deep (Atkins and Bertsche, 1981)
indicates that the majority are less than 600 ft wide; the greater
number of these being either between 350 ft and 400 ft or between 550
ft and 600 ft wide. More than 75 percent of the turns are 40° or
less, 34 percent are between 20° and 40°, and 43 percent are 20° and
less.
In comparison to the general criteria for navigational channels
established by international organizations and the U.S. Army Corps of
Engineers (1965, 1983), these dimensions are at or below the
geometrical limits for the average-size vessels using the channels.
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The technical panel of the committee found that ports in the United
States generally lack adequate emergency anchorage areas, and that
turning basins are few and minimal in dimensions for the vessels (not
the largest) using the port. The question thus arose in the panel's
investigation: what was the design basis--particularly the design
vessel--for which existing navigational facilities were designed?
Table 16 (Appendix G) shows the year of authorization for major
navigational channels and turning basins at their present dimensions.
Of 154 authorizations, only 34 have occurred in the past 20 years, 12
since 1970, and none since 1976. Some date from 19th century sailing
ships.
Despite the paralysis In authorizations since 1976 (and some that
were authorized in that and previous years have never been built),
studies continue to be conducted of needed improvements (Table 17,
Appendix G). All these proposed improvements were designed by the
guidelines of the 1965 Engineer Manual, which predates the Corps'
current 1983 Engineer Manual. While awaiting authorization (and as
funds permit), updating occurs in the district offices by the new
Engineer Manual. The Norfolk district, for example, indicates that in
the interval awaiting authorization, studies have been undertaken to
refine the design basis using the 1983 Engineer Manual, and
alternative configurations for this project, as well as for the
improvement of Mobile Harbor, have been tested using the full-scale
vessel simulator (CAORF) of the U.S. Maritime Administration.
Many of the projects, however, represent minimal improvements for
existing vessel traffic: the design basis assumes, for example, that
design vessels will not be fully loaded, or width calculations are
minimal, assuming tug escort. In general, many proposed improvements
are for relatively modest sizes of vessels (which may or may not be
appropriate), and not all proposals allow these vessels to be fully
loaded.
It must be borne in mind that there are constraints on widths,
depths, and diameters in many areas: existing berths, piers, and
other structures; harbor and bay tunnels, bridges; submarine pipelines
and cables; salinity locks, and water-supply intakes.
Nevertheless, the dredging projects have yet to be initiated to
match shoreside improvements or the needs of vessels now calling
regularly on ports of the United States. The principal engineering
problem in the design of dredged facilities is time. As the proposed
improvement progresses through successive stages of the process for
gaining authorization and funding, the engineering refinement or
redesign that might be undertaken is limited by the project dimensions
established in the initial stages. Two proposed improvement projects
now in progress through the decision making system have been succeeded
by proposals for additional dimensions. In a previous study (Marine
Board, 1983), the time and nature of the decision making process were
found to discourage research and innovation, and to impose limits on
engineering, owing to the long times that elapse between the initial
assessments of need and the initiation of dredging. More importantly,
the time scale of the process was found to exceed the time scale of
major changes in the world fleet.
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Thus, while the general criteria for the design of navigational
facilities have been brought up to date, institutional issues have
impeded their effective application.
Operational Adequacy
The time and nature of the institutional process for achieving
improvements in navigational facilities, and the funding stalemate of
the past 10 years imply increasing obsolescence of the ports'
waterways. Significant obstacles prevent the systematic application
of engineering and construction dredging to ensure navigational
adequacy. The burden to achieve navigational adequacy then falls on
operations--on the conditions and practices used in individual ports.
To gain an understanding of the operators' views of navigational
facilities in U.S. ports, the technical panel sent a questionnaire to
the pilots organizations (Appendix C) requesting information about
channel size and design, maneuvering problems, aids to navigation,
maintenance dredging, and operational strategies used, if any, to
compensate for perceived physical inadequacies. Of the organizations
responding, only 2 judged the channels adequate for present vessel
traffic; 3 suggested that channels would be adequate if maintenance
dredging were performed on a regular basis; and 7 indicated that the
channels were inadequate.
Vessels named by the pilots as being most difficult to handle
divide into two groups (some organizations mentioned both): the
largest, deepest-draft vessels they handle, owing to small underkeel
clearance, and lightly loaded vessels with high, flat sides, such as
containerships and car carriers (as well as a passenger vessel, in one
case, having a high abovewater profile). Among the areas in their
ports pilots most frequently cited as critical were jettied entrances,
followed by narrow sections and tight turns. Other critical areas
mentioned were those where crosscurrents or crosswinds are
encountered. One pilot group in the Pacific said their entrance
channels were adequate in normal conditions, but inadequate in
swells. The pilots were unanimous in the judgment that improved aids
to navigation cannot substitute for channel improvements.
All respondents indicated that special operating arrangements have
been established by the pilots organizations to compensate for
inadequate channel dimensions: one-way traffic, restricted passing
and overtaking in bends and turns, transit with high tide for
underkeel clearance, and use of tugs. In certain channels, pilots use
hydrodynamic interactions with banks and other vessels to execute
meeting and passing situations, or to round a turn of inadequate
radius of curvature (using sheering effect to augment the decreased
turning performance of a large vessel with small underkeel clearance).
It is important to understand these operational practices in the
design or improvement of navigational channels; for example,
observation of critical maneuvers often shows less variation in swept
paths among pilots than in less-critical maneuvers, but this may
indicate an area that needs widening, rather than one that could be
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narrower. Control of a vessel in critical maneuvers is often achieved
by a great many rudder commands and a higher average value of rudder
angle (Hoofs et al., 1978~.
In a review of channel design, Hooft (1981) recommends a
sensitivity analysis of the vessel's controllability as a function of
external factors (such as wind or current) and channel width. Where
two-way traffic is frequent and widening is indicated but not
possible, it is helpful to have emergency anchorages alongside the
channel.
It should be noted that all calculations or estimates having to do
with the navigational requirements of vessels will be accompanied by
some uncertainty:
The behavior of vessels in channels (although better understood
today than in the recent past) is still very much in need of
further study. Little exact guidance is available, and actual
behavior may differ from predicted behavior owing to a number
of complex and interactive factors.
Computer-aided vessel simulation has improved in recent years,
offering the potential for engineering design verification of
alternative dimensions and layouts. Caution must be exercised
against excessive fineness in the determination of channel
dimensions through vessel-transit simulation, as even the most
sophisticated simulator is accurate only within about a 20
percent range.
Local conditions of the physical environment are important but
highly variable. The ship's response, in turn, is affected by
its velocity, hull configuration, propulsive mechanism,
loading, and underkeel clearance.
There are no minimum standards for vessel maneuverability.
· Even more importantly, there are no consensus standards for
navigational safety. This was identified as a top priority for
the design of entrances to ports and harbors by an
interdisciplinary meeting (Marine Board, 1981~. Some
shipowners have developed probabilistic methods to enable their
ship's masters to calculate underkeel clearance and thus
determine the advisability of entering ports around the world
(Kimon, 1982~. This method is data dependent, and can be
improved with more and better data.
DESIGN OF NEW CONSTRUCTION DREDGING
PROJECTS FOR MINIMAL MAINTENANCE DREDGING
As pointed out in Chapter 9, thorough understanding of local tidal
hydraulics and circulation is necessary to design dredged navigational
facilities for minimal shoaling (see also Marine Board, 1983~.
Site-specific hydrographic surveys, measurements of currents, and an
understanding of existing patterns of sedimentation in the port are
all necessary; in addition, a physical model can be a helpful tool in
assessing interactions of the facility with currents and (possibly)
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effects of the facility on salinity distributions. If an evaluation
of the effectiveness of the design is needed, models of the currents,
the salinities, and the sediment transport will be required. The
Corps has conducted research, development, and field studies for many
years to improve its ability to model the hydrodynamics, salinities,
and sediment transport in waterways, and the private sector also has
the capability to make measurements and provide physical and
mathematical modeling services. Many of the processes of aggregation,
deposition, and erosion important to an understanding of sediment
transport, and thus, the management of sediment deposition have been
incorporated in mathematical descriptions for quantitative evaluation
of design and management alternatives (Ariathurai and Krone, 1976;
McAnally, 1984~. The perpetual nature of maintenance dredging argues
for investment in site studies and models to guide design and
subsequent management.
New construction dredging projects offer the opportunity to reduce
subsequent maintenance dredging by design and management strategies.
In this connection, it might be noted that in many ports, federal
projects and local projects (particularly side channels), together
with the location and orientation of piers, wharves, and other
pile-supported structures, are incompatible. That is, one causes
accelerated shoaling for the other. A coordinated plan would be
helpful in reducing these incompatibilities and reducing maintenance
dredging.
A more difficult conflict is that between the need for emergency
anchorages and the disproportionate amount of maintenance dredging
these facilities typically require. The same is true of turning
basins, but their economic yield in terms of accommodating vessels is
perhaps more evident. Little engineering attention has been given to
emergency anchorages and turning basins. One possible solution for
some ports would be to dredge the facilities with flatter, stepped
side-slopes. The design would have a higher initial cost, but far
lower maintenance cost. Another interesting possibility is being
investigated by the Norfolk District of the Corps: using anchor buoys
similar to those developed for offshore oil loading/unloading
(described in Chapter 5) for offshore anchoring.
MAINTENANCE DREDGING
Table 18 (Appendix G) shows the annual average maintenance dredging
costs for each port. As the total approaches a half-billion dollars a
year, reducing the sedimentation associated with navigational
facilities, and achieving the lowest-cost maintenance dredging program
are important dredging needs.
Determining the most cost-effective program of maintenance dredging
depends on detailed site-specific knowledge (Herbich et al., 1981;
Marine Board, 1983~. As indicated in Chapter 9, navigational
facilities change the preexisting sediment regime; therefore, an
important consideration in reducing maintenance dredging requirements
is the siting and design of these facilities. Other considerations,
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such as improvements in dredging plant and its use, are discussed in
succeeding sections.
Rates of deposition and types of sediments vary greatly from port
to port, and man's activities near and far from the port, as well as
natural causes, make significant contributions that cannot always be
predicted or controlled. In some areas, most of the annual sediment
movement will occur during a few storms. Waves and surges generated
by the vessels can also move sediments; over time, bank erosion from
these forces can modify the channel's side slopes (Herbich and
Schiller, 1984~. As a result of these and other in-channel forces,
the channel ages and changes shape, with corresponding shifts in areas
and rates of sedimentation.
Thus, determining an effective maintenance dredging program in a
particular port depends on a great deal of historical and current
local knowledge, and frequent hydrographic surveys. The usual case
for an existing navigational facility is that some areas have higher
shoaling rates than others, and deciding when and how much additional
dredging they should have also depends on frequent hydrographic
surveys. Trawle and Boyd (1978) found hydrographic surveys to be
infrequent in the Corps districts, and substantial variation among the
districts in the methods used to calculate the amount of additional
dredging needed in these areas. Since the 1978 report, the Corps has
made considerable investments in vessels and survey equipment for the
districts. The information collected by the committee and technical
panel indicates that survey practices and advance maintenance dredging
(deeper dredging in selected areas) still vary from district to
district.
One impediment to more efficient maintenance dredging (discussed in
a succeeding section) is the year-to-year budget of the Corps. As
funding for operations and maintenance has declined in constant
dollars, the Corps has distributed the gap among the districts. In
reading the yearly reports of Corps activities, it can be seen that
the major projects not being maintained at project depth change from
year to year, as some will be dredged and others allowed longer times
between maintenance dredging.
An important set of considerations that is sometimes not addressed
by maintenance dredging programs is that the most efficient operation
of the port depends on assured access by vessels at the drafts
specified in port guidelines. Port calls by liner operators, in
particular, are scheduled months in advance. Many ports allow transit
of deeper-draft vessels at high water, or in one-way traffic, or some
other combination of operational practices to ensure passage at small
underkeel clearance. These smaller underkeel clearances--about 2.5
percent of vessel draft--mean that the vessels are transiting at
closer tolerances than those for which the channel was designed, and
maintenance dredging is more critical. Even if vessels avoid
grounding in areas of higher deposition, the presence of shoaled areas
can affect their response characteristics, and this can be equally
critical in narrow channels at small underkeel clearance.
Assessment of the adequacy of maintenance dredging in the ports of
the United States would entail detailed port-by-port analysis and site
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studies, including a review of historical data on dredged volumes and
frequencies, and of the changes over time in the facilities and their
use. The committee and technical panel gained the general impression
that maintenance dredging was a high priority in some districts,
characterized by frequent surveys, long-term planning, and advance
maintenance dredging, and a lower priority in others, characterized by
a more reactive program of responding to the needs expressed by repre-
sentatives of the port, pilots, or local U.S. Coast Guard.
CAPABILITY OF THE DREDGING INDUSTRY
A survey for the International Association of Dredging Contractors
(Prognos, 1984) indicates that every developed maritime nation funds
the new construction and maintenance dredging of its major
navigational facilities, and that every nation is concerned to keep
down the costs. The report recommends that (where appropriate)
dredging be contracted to the private sector, a solution that has
already been instituted for the most part in the United States.
Several questions have been asked about the dredging industry in the
United States: If a significant number of new construction dredging
projects were initiated, would the industry have the capability to
perform the work? What can be done to lower the cost of dredging?
What technical improvements can be made for greater efficiency and
productivity? These questions are taken up in the following sections
Equipment and Procedures
The dredging of sedimentary deposits within ports and navigational
waterways is accomplished by one of two primary techniques, hydraulic
or mechanical. Within each class, a number of functionally different
systems are available (see Figure). The ultimate selection of the
operating system is based primarily on the sediment type, water depth,
sea conditions, location and proximity of the disposal area, and to
Hydraul ic
Hopper ~ Sidecast, ng Agitation
Plain Cutterhead Dustpan
Suction
Dredging Systems
Dipper Bucket Ladder
Dragline Clam Orange
Shell Peel
some extent, the availability of equipment. In addition, the
contamination levels of the sediment and the need to minimize
near-field resuspension and far-field dispersion may be considered (as
indicated in Chapter 9~.
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The majority of dredging projects in the United States employ
hydraulic dredging techniques (Table 19, Appendix G). These
techniques are particularly well suited for use in areas characterized
by a high degree of sediment mobility where virtually continuous
dredging is required and the dredged material is either moved from
the channels to disposal areas in deeper water or placed in reasonably
proximate shoreside containment areas. Mechanical techniques are more
frequently employed in areas of slower sedimentation. These
techniques also appear to be favored if coarse-grained material is to
be dredged, or if high contaminant levels require minimal agitation or
fluidization of the sediments and a general retention of the cohesive
character of in-place, fine-grained materials. These latter
characteristics, in combination with the limited number of alongshore
disposal areas, have historically favored the use of mechanical
dredging techniques in New England.
Structure of the U.S. Dredging Industry
The U.S. dredging industry consists of approximately 190 firms*
competing primarily for federal contracts. The ten larger companies
account for 56 percent of dredging under federal contracts.
A recent study by the Small Business Administration concludes that
federal procurements account for about 75 percent of all dredging in
the United States. Given average annual federal contracting of $331
million for the period 1980-83, the industry performs about $440
million of work annually. Additionally, the Corps of Engineers
operates a fleet of 13 dredges which performed an average of $86
million per year for the same period. The following table summarizes
dredging revenues in the U.S.
Annual Average Value of Dredging Work in U.S. (1980-1983)
(millions of dollars)
Contractor Corps of Engineers Total
Federal contracts $331 $86 $417
Private contracts 110 0 110
$441 $86 $527
SOURCES: Federal contract dollars from U.S. Army Corps of
Engineers. Private contract dollars from Small Business
Administration, 1984.
*Small Business Administration (1984) estimates 250, but without
evidence. A total of 163 bid successfully on federal contracts from
(1980-1984~; 31 more bid unsuccessfully on at least one project.
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The distribution of cubic yards dredged in federal projects from
1980 through 1983 is shown below.
Total Amount of Dredging (106 yds3) (Federal Projects),
1980-1983
Dredged 1980 1981 1982 1983
By contractors 225 281 220 250
By USCE 84 88 60 50
Total 309 369 280 300
Four-Year
Total Average
976 244
282 70
.
1,258 314
SOURCE: U.S. Army Corps of Engineers
Data on cubic yards dredged under private contracts are not readily
available. If they were roughly proportional to the average price per
cubic yard of federal contracts, they would not exceed 75 million
cubic yards annually. In all likelihood, the true figure is much
lower because most private contracts are for relatively small
quantities with correspondingly higher unit costs than federal
contracts.
Having applied average price per cubic yard to work performed under
federal contracts to estimate work in the private market, a note of
caution needs to be added about making dollars-per-yard comparisons
between the contractor fleet and the Corps fleet.
Comparisons using annual averages or totals are virtually
meaningless owing to differences in types of projects, measurement of
yards dredged, and equipment utilization rates. Contractor dredges
perform virtually all cutter and bucket work, half the hopper work,
and about one-fourth of the dustpan work, and Corps dredges perform
the balance. Cutter jobs often involve sizable preparation of
disposal areas that account for 10 to 20 percent of contract price
while material dredged by bucket, hopper, and dustpan dredges is
usually transported to a deep-water disposal site. Contractors
normally work on unit-price contracts and are most often paid on the
basis of quantities determined by before-dredging and after-dredging
surveys. Corps dredges, on the other hand, work until surveys or the
depths of operations show that desired depths and widths have been
achieved. The Corps is less concerned about overdredging, which is
uneconomic for a contractor. Contractor dredge production is usually
measured by net pay yardage while Corps dredge production is measured
by gross yards removed. Finally, the Corps fleet has about a 70
percent utilization rate while contractor dredges average less than 50
percent utilization and thus must spread their fixed costs such as
depreciation, insurance, and interest over proportionately fewer yards.
In a presentation to the Dredging Committee of the American
Association of Port Authorities in 1981, two industry representatives
described the U.S. dredging fleet and its annual production capacity
in millions of cubic yards as follows:
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Large Cutter Dredges (18" to 42" Diameter Discharge)
Small Cutter Dredges (Discharge ~ 18")
Large Bucket Dredges (12 to 22 c.y.)
Small Bucket Dredges ~ 5 to 10 c.y.)
Hopper Dredges (1,200 to 12,00 c.y.)
Dustpan (38" Discharge)
Number
of Annual
Dredges Capacity
101 453
150
18
60
11
341
164
83
11
711
The most sigificant change to the contractor fleet since then has
been the addition of two more hopper dredges (4,000 and 2,800
cubic-yard capacity, respectively) and the keel-laying for a third
hopper dredge of about 4,000 cubic-yard capacity. Since few dredges
have retired or left the country to work overseas, industry capacity
has remained in the neighborhood of 700 million cubic yards per year.
Based on the peak workload of 281 million yards dredged by contractor
plant for the federal government in 1981, utilization stands at about
40 percent of physical capacity. Thus, the industry has substantial
extra capacity available for private work and for new work dredging.
The Corps of Engineers' fleet consists of 13 dredges:
Large Cutter Dredges (~18" Discharge) 2
Hopper Dredges
Dustpan Dredges
Sidecaster Dredges
Special Purpose Dredge
Total
4
3
l
13
One cutter dredge is scheduled for retirement during fiscal year
1985. The total does not include a number of small two-man cutter
dredges which have very low utilization. As recently as 1980, the
Corps fleet consisted of 27 active dredges. The Corps has retired
dredges as contractors have built new plant under the terms of the
Industry Capability Program discussed in more detail in a succeeding
subsection. The current fleet of 13 dredges includes 3 hopper dredges
launched in 1982 and 1983. The average annual workload of $86 million
gives the Corps about 16 percent of the U.S. dredging market. Only
one contractor performs a larger share of total U.S. dredging work
than the Corps of Engineers' fleet.
Improving the Economy and Efficiency of Dredging
Greater economy and efficiency in dredging can be achieved by
replacement of plant with modern dredges, application of available
technology (instrumentation, automation), and integration of project
planning. These are briefly discussed in succeeding sections.
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Replacement of Dredging Plant
Most of the dredging in the United States is performed by
cutter-suction dredges, with hopper dredges claiming the next-highest
percentage, and dustpan, clamshell, and dipper dredges the remainder.
The dredging industry in the United States has invested substantial
sums in recent years to replace the entire hopper dredge fleet with
modern, technologically efficient dredges. Therefore, the most
effective improvement in over-all dredging efficiencies can be
realized in the modernizing the cutter-suction fleet.
Cutter-Suction Dredges: Problems and Opportunities
The cutter-suction dredges of the United States are relatively old.
Only 5 of the 20 largest were built in the last 10 years. This fleet,
therefore, lacks most of the technology developed in the last decade.
Another characteristic of cutter-suction dredges that contributes to
inefficiency is their general-purpose nature. They were usually
designed to handle the "typical" project rather than to have the
optimum capabilities for a specific project. They are normally too
powerful for the simpler projects or too weak for the more difficult
jobs.
The benefits of replacing this equipment are many. Available
technology increases productivity at reduced operating cost, and
design features can be added that expand capabilities and enhance
safety.
The high capital cost of this plant is the major impediment to
replacement. Attracting the necessary capital to build the new
dredges will require changes in the market for which they compete (as
described in a succeeding section).
Among the new equipment for cutter-suction dredges are the dredging
wheel and suction tube position indicator system.
Dredging Wheel The dredging wheel replaces the cutter on a cutter
dredge. In the dredging wheel, the buckets are bottomless. By
placing the buckets close together and overlapping, a tunnel is
created, the inner limitation of which is the suction mouth itself.
The dredging sequence is mechanical excavation followed by hydraulic
suction.
Position Indicator System This type of indicator provides the
operator with immediate visual indication of the position of the
suction pipe, depth of the suction head and the angle of the lower
part of the suction pipe in both the horizontal and vertical planes.
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Recent Improvements in Hopper Dredges
Although no single type of dredge will ever be universally superior to
all others, the hopper dredge is the only general-purpose plant that
can work effectively in open water subject to the action of waves.
The three most important parts of a trailing suction hopper dredge
are the hopper, the suction draghead, and the dredge pumps. Recent
improvements have been made to these parts (Herbich and Brahme, 19807.
Hopper Turbulence in the hopper maintains the dredged material in
suspension: to allow the material to settle quickly, it is important
to keep the turbulence to a minimum. Recent developments (Brahme and
Herbich, 1977) include installation of the discharge pipes farther
down into the water at mid-depth, or even below, and discharging
sideways at the aft end of the hopper. Gratings have also been
provided on two sides to reduce turbulence.
Draghead-Mounted Dredge Pump One of the significant improvements
in recent years is installation of a dredge pump on the draghead. As
a result, the suction pipe has become a delivery pipe. It was
possible to achieve a specific gravity of 1.4 in the solids-water
mixture, even when the dredging depth was increased.
Active Draghead This new type of draghead was developed to achieve
economically acceptable output from a hopper dredge operating in
clay. The draghead is called the "active rotary draghead." It
incorporates a rotating cylinder with a number of knives that slice
the clay layers.
Venturi Draghead The Venturi draghead consists of three parts the
pivoting part, called a visor, the fixed part, which contains the
water jets, and an elbow transition between the fixed part and the
actual suction tube.
The operating principle of the Venturi draghead is based on
creating negative pressure immediately above the seabed by converting
part of the pressure energy into kinetic energy. It appears from the
field tests that the production in fine sand can be increased by 30 to
40 percent. However, no increase in production was observed in
dredging of coarse sand.
Automatic Draghead with Winch Control System The automatic
draghead winch controller was developed to regulate the movements of
the suction pipe and draghead throughout the dredging cycle. It is
programmed to swing the pipe outboard, to lower the pipe, and, in
conjunction with the swell compensator, maintain the correct pressure
of the draghead on the bottom.
The installation of an automatic suction pipe controller has
simplified the operational procedures, thus enabling the operator to
concentrate on obtaining the maximum output of solids. It has also
enabled the vessel to continue operations in bad weather, while
minimizing the risk of damage.
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Split-Hull Hopper Dredge A hopper dredge divided longitudinally
into two parts, which are joined by hinges at the main deck level, is
emptied by allowing the two halves to swing apart. The main advantage
of this type of dredge is the fast disposal of material and easy
disposal of sticky clays, clay loam, and silt.
Underwater Pump in the Suction Pipe A pump in the suction pipe
supported by a ladder not only increases the dredging depth but also
increases the efficiency of the dredging process (Herbich, 1975~.
Automation
The chip-based microcomputer technology opened the way for automation
in the dredging industry. Automation assists the operator, but does
not replace him. By taking over data acquisition, providing real-time
data analysis and displaying operations of the various elements, the
operator will be able to follow the process more carefully, and will
be able to take steps to improve the efficiency of the dredging
project.
Individual automatic systems that have already been developed are
vacuum-relief valve, bypass valve, automatic draghead winch
controller, automatic light mixture overboard, and draghead visor
controller (Van Zutphen, 1983~. For example, the automatic suction
pipe controller moves the suction pipe. The controller actuates the
winches to swing the suction pipe outboard or inboard and alters its
position during dredging. It also controls the swell compensator and
incorporates a number of safety systems.
Production Instrumentation
Production Meter
A production meter system provides continuous indication of density,
total flow, and solids mass-flow rate of the material pumped by a
hydraulic suction dredge (Figure 4, Appendix G). A production meter
system can also give total solids production (Erb, 1981~.
Two types of density gauges are commercially available: a
differential-pressure gauge and a nucleonic density gauge (Figure
15~. The magnetic flow meter measures the total flow rate. Other
types of meters, such as the sonic flowmeter and Doppler flowmeter,
are under development, or have recently become available (Roskam et
al., 1983~.
As shown in Figure 5 (Appendix G), the leverman may operate as part
of a feedback loop. The leverman monitors the operation of the dredge
by watching the indicator and adjusting the controls as necessary.
Usually three parameters are displayed to the leverman:
slurry density,
flow velocity, and
solids flow rate.
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The information is best displayed by use of a crossed pointer display
of the type shown in Figure 6 (Appendix G).
Nuclear Silt Density Meter A nuclear silt density meter
(Belgraver, 1983) has been used to measure the material in situ in
connection with the "nautical depth" concept (Marine Board, 1983~.
Economies in Efficiency
Dredging operations in Europoort-Rotterdam were significantly improved
by the introduction of modern partly automated dredges and the
installation of modern instrumentation. The costs of annual
maintenance dredging were reduced 40 percent in spite of inflation and
fuel-price increases. This is a good indication that significant
economies can be achieved by modernization of the equipment used.
Market Incentives and Effects
Market expansion, such as the port deepening projects being considered
in the United States, would likely stimulate investment. Caution must
be expressed, however, that this type of sudden expansion will also
result in higher prices for dredging in the short term. The
longer-term result would probably be an improved fleet capable of
producing more efficiently. Capacity will increase with demand and
force prices to moderate. This pattern is suggested by examining the
international dredging market in the mid-1970s. There was an
unprecedented rise in demand owing to the port development programs in
the Middle East. This forced prices up and encouraged investment in
new plant and equipment.
Three years later, a new fleet of dredges was available. This
increase in supply and a moderating market combined to bring the price
level well below that which had prevailed. The users of dredging
services paid a premium for several years but now are enjoying savings
generated by a more efficient fleet.
Changing market conditions for hopper dredging in the United States
produced a marked difference in the 1970s. Until the late 1970s,
hopper dredge work was performed by a fleet owned entirely by the
government. In 1979, legislation was enacted to allow the private
sector to compete for a majority of this work. This was a deliberate
decision to promote private-sector capability by sacrificing
short-term savings from lower-cost (in depreciation and interest),
but obsolete dredges. In 5 years, the dredging industry invested more
than $250 million in efficient, productive hopper dredges.
Integrated Projects
With few exceptions, dredging work in the United States is carried out
under short-term contracts. The majority of dredging work is funded
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and administered by the U.S. Army Corps of Engineers, and for a number
of economic and regulatory reasons, the projects are segmented. A
typical contract is for about 4 months of work, for $2 to $3 million.
The proliferation of smaller projects prompts use of older,
less-sophisticated equipment. Although dredging costs are kept low
over the near term, this practice discourages investments in new
dredging plant and equipment, and in research and development.
There are several specific costs associated with small segmented
projects:
.
.
· Mobilization--This category includes all costs associated with
transporting equipment, people and materials to and from the
site. They also include setting up or rigging the dredges for
operation, establishing supply lines and complying with the
lengthy administrative procedures associated with each
contract. These costs often account for 15 to 20 percent of
the costs on small contracts.
Le rning Curve--Each project is different. Often crews are
unfamiliar with the idiosyncrasies of a project and must gain
experience with the project before the dredge output reaches
its maximum. This cost can be quantified. In comparing
average production rates, it is often found that production is
as much as 50 percent higher during the second half of a
project than it is during the first. Additionally, costs per
unit of time are lower during the later stages of most projects.
Advance Maintenance--Additional material can be excavated by
dredging somewhat deeper without decreasing the forward
progress of the dredge. This deepening can be added at little
extra cost, and will increase the interval between dredging.
CONCLUSIONS
Although general criteria have been developed for the design of
dredged navigational facilities, their application in the United
States is impeded by the length and character of the decision making
process.
One of the consequences of the long lead times for decisions about
port dredging is to discourage systematic engineering for port
development. The concept of the design vessel in studies of dredging
projects is hardly applicable to a world fleet that has a half-life of
10 years when the approval process takes more than 20 years. None of
the existing authorizations for dredged navigational channels is as
recent as the advent of large dry-bulk carriers or the
latest-generation containership. The lack of timely improvements
places the burden on local pilots, the ports, and U.S. Coast Guard
(but primarily the pilots) to develop operational practices that
enable vessels to transit obsolete navigational facilities safely.
Besides the reduction of safety margins that would otherwise be
achieved through engineering design and maintenance dredging, these
practices are uneconomic in many ports.
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The institutional process is also project-specific rather than
programmatic. A programmatic approach is needed to achieve optimal
port des gn to minimize maintenance dredging as well as port
efficiency and navigational safety.
Each port is unique; thus, site studies are most important in
defining dredging problems. Much remains to be learned about the
maneuvering requirements of vessels, and collaborative
interdisciplinary efforts are needed to achieve better understanding
and to refine the tools of design verification and analysis (such as
vessel simulation). Mathematical and physical models, field
measurements, and engineering observation are well developed and need
to be employed to understand local sedimentation and to minimize the
maintenance dredging required by new construction projects.
The dimensions of dredged navigational facilities in the U.S.
appear minimal for the vessels now using them, and emergency
anchorages are small or lacking. Depending on vessel traffic and
other local conditions, these minimal dimensions may be adequate, but
the institutional process for improvements will impede needed
improvements if they are inadequate. The process is insufficiently
flexible to allow timely spot improvements, such as widening a turn.
The committee did not attempt a thorough evaluation of the nation's
maintenance dredging program, as this would have entailed very
detailed port-by-port analysis. The committee notes, however, that
the year-to-year budget of the Corps and its declining level in
constant dollars is an impediment to the efficiency of the maintenance
dredging program and to the operations of the ports. That is, the
Corps attempts to achieve equity among the ports by lengthening the
time between dredging intervals at successive ports, and those
suffering the lack of authorized depths must restrict ship drafts
during those periods. Greater use might be made of advance
maintenance dredging in high-shoaling areas, and to widen turns.
Existing institutional arrangements also restrict the Corps from
making the most effective use of dredging resources.
Most of the dredging in U.S. ports is carried out by the private
sector under short-term, unit-price contracts to the Corps (and a
smaller amount under local port contracts or by dredges owned by the
port). Greater economy and efficiency in dredging can be achieved by
the replacement of existing plant with modern dredges, application of
available technology in instrumentation and automation, and
integration of project planning. The U.S. fleet of hopper dredges is
modern and technologically efficient: the principal opportunities for
improvement are in the cutter-suction fleet. The proliferation of
small dredging projects (owing to economic and regulatory constraints)
prompts the use of older, less-sophisticated equipment, and additional
costs for repeated mobilization, inability to maximize the
productivity associated with the later stages of a larger project, and
loss of the opportunity to perform advance maintenance dredging at
small additional cost. Positive changes will be needed to provide the
market incentives for investment in new dredging plant, and changes in
institutional arrangements will be needed for other improvements to be
made.
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REFERENCES
Ariathurai, C. R. and R. B. Krone (1976), "Finite Element Model of
Cohesive Sediment Transport, "J. Hydr. Div., ASCE, HY3: 323-338.
Atkins, D. A. and W. B. Bertsche (1981), "Evaluation of the Safety of
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Be=
National Academy Press).
Belgraver, N. J. (1983), "A Nuclear Silt Density Meter," Dredging
Engineering Short Course Notes, Texas A&M University, College
Station, Texas.
Brahme, S. B. and J. B. Herbich (1977), "Dredging in India,
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Card, J. C. et al. (1979), "Report to the President on an Evaluation
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Dand, I. (1976), "Hydrodynamic Aspects of Shallow Water Collisions,"
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Englehardt, E. H. L. (1983), "Draghead Position Indicating System,"
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Erb, T. L. (1981), "Production Meter Systems for Suction Dredges,"
Dredging Engineering Short Course, Texas A&M University, College
Station, Texas.
Heiberg, E. III (1983), "Recent Port Planning Developments in the
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United States Harbors," 8th International Harbour Congress, 2.109.
Herbich, J. B. (1975), Coastal and Deep Ocean Dredging (Houston,
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Herbich, J. B. and R. E. Schiller (1984), "Surges and Waves Generated
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Herbich, J. B. and S. Brahme (1980), "Modern Developments in Dredging
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Herbich, J. B., W. R. Murden, and C. C. Cable (1981) "Factors in the
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Hooft, J. P. (1981), "Ship Controllability," Problems and Opportunities
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Channels," Presentation to STAR Symposium, Society of Naval
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Marine Board (1983), Criteria for the Depths of Dredged Navigational
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(1983), "Flow Metering in
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Dredging Systems," Dredging and Port Construction, February.
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for the Purpose of Setting a Size Standard," Washington, D.C.
Trawle, M. J. and J. A. Boyd, Jr. (n.d.), Effects of Depth on Dredging
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U.S. Army Corps of
Representative terms from entire chapter:
navigational facilities
