Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.
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
96 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.
97 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
98 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
99 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.
100 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.
101 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
102 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)
103 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,
104 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
105 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~.
106 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.
107 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:
108 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.
109 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.
110 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.
111 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.
112 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
113 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.
114 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.
115 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 Ship Navigation in Harbors," Problems and Opportunities in the Design of Entrances to Ports and Harbors (Washington, D.C.: 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, Suggested Improvements in Techniques and Equipment,' Report No. CDS 204, Texas Engineering Experiment Station, College Station, Texas. Card, J. C. et al. (1979), "Report to the President on an Evaluation of Services and Techniques to Improve Maneuvering and Stopping Abilities of Large Tank Vessels," Report No. CG-M-4-79, Washington, D.C., U.S. Coast Guard. Dand, I. (1976), "Hydrodynamic Aspects of Shallow Water Collisions," Naval Architect, 6: 323-346. Englehardt, E. H. L. (1983), "Draghead Position Indicating System," Dredging_and Port Construction, February. 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 United States: Status of Channel Deepening Proposals for Major United States Harbors," 8th International Harbour Congress, 2.109. Herbich, J. B. (1975), Coastal and Deep Ocean Dredging (Houston, Texas : Gulf Publishing Cop. Herbich, J. B. and R. E. Schiller (1984), "Surges and Waves Generated by Ships in a Constricted Channel," Presentation to 19th International Conference on Coastal Engineering, Houston, Texas, September 1984 (proceedings in press). Herbich, J. B. and S. Brahme (1980), "Modern Developments in Dredging Equipment," Dredging Engineering Short Course, Texas A&M University, College Station, Texas. Herbich, J. B., W. R. Murden, and C. C. Cable (1981) "Factors in the Determination of Cost-Effective Dredging Cycle," Proc. XXV Int. Nau. Cong., Inland and Maritime Waterways and Ports, PIANC, Section II, Vol. 2, Edinburgh, Scotland, May 10-16. Hooft, J. P. (1981), "Ship Controllability," Problems and Opportunities in the Design of Entrances to Ports and Harbors (Washington, D.C.: . National Academy Press). Hooft J. P. et al. (1978), "Horizontal Dimensioning of Shipping Channels," Presentation to STAR Symposium, Society of Naval Architects and Marine Engineers, New London, Conn., April 1978. Landsburg, A. C. et al. (1983), "Design and Verification for Adequate Ship Maneuverability," Presentation to Annual Meeting, Society of Naval Architects and Marine Engineers, New York, November 9-12, 1983.
116 Marine Board (1983), Criteria for the Depths of Dredged Navigational Channels (Washington, D.C.: National Academy Press). McAnally, W. H., Jr. (1984), Hydraulic, Salinity, and Sediment Models, Report No. TABS2, Waterways Experiment Station, U.S. Army Corps of Engineers, Vicksburg, Miss.. Prognos, AG, (1984), A New Policy to Cut Harbour Expenses (Basle: Prognos AG). Roskam, A. K., C. J. Hoogenkiji, L. IJmker (1983), "Flow Metering in Dredging Systems," Dredging and Port Construction, February. Roskam' A. K., C. J. Hoogendijk, L. IJmker (1983), "Flow Metering in Dredging Systems," Dredging and Port Construction, February. Small Business Administration (1984), "Profile of the Dredging Industry 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 Frequency, Report 1, Survey of District Offices (Vicksburg, Miss: U.S. Army Engineer Waterways Experiment Station). U.S. Army Corps of Engineers (1983), Engineer Manual: Hydraulic Design of Deep-Draft Navigation Projects, EM 1110-2-1613 (Washington, D.C.: U.S. Army Corps of Engineers). U.S. Army Corps of Engineers (1965), Engineer Manual: Tidal Hydraulics, EM 1110-2-1607 "Washington, D.C.: Engineers). Van Zutphen (1983), "Automation in the Dredging Industry," Dredging and Port Construction, February 1983. Webster, W. C. (1983), "Steering and Maneuvering: State of the Art Report," Presentation to American Towing Tank Conference, Washington, D.C., August 1983. U.S. Army Corps of