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Shiphandling Simulation: Application to Waterway Design 2 Waterway Design Process The design of a waterway is a highly complex and demanding exercise. The process involves an amalgam of economics, engineering, environmental and social aspects, political considerations, and historical precedence. Shiphandling simulation plays a role in a small but very important part of this process. This chapter describes the state of practice of waterway design to establish the context in which computer-based shiphandling simulations are applied. Also examined are typical design issues that appear to lend themselves to assessment through shiphandling simulation as well as design elements and data that are critical to successful simulations. The discussion provides a basis for understanding the advantages and limitations of shiphandling simulations and the potential for advances in the underlying technology. The central role of the U.S. Army Corps of Engineers (USACE) in the waterway design process in the United States has produced a somewhat different institutional process (described in Appendix B) in comparison to the rest of the world. However, the engineering concepts used in waterway design are essentially the same. A waterway design defines the form and dimensional boundaries required to meet functional objectives consistent with fundamental civil engineering practices and construction options. Construction includes excavation (dredging), manipulation of earth and rock, and the erection of heavy structures. As with other areas of civil engineering, the actual construction
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Shiphandling Simulation: Application to Waterway Design design is developed by the application of well-known principles of the physical sciences, such as hydraulics, geotechnics, and properties of materials. Most of the effort in waterway design has historically concentrated on these civil engineering aspects. Sedimentation has received particular attention, including the means to reduce or control it and its effects which might result from changing hydraulics within the waterway system relative to the tidal prism (National Research Council [NRC], 1981, 1987; USACE, 1977). However, the true challenge of waterway design is to balance the civil engineering requirements with those of form and function, including environmental considerations. THE DESIGN CHALLENGE The distinctive and unique thrust of waterway design is to quantify the factors that are used to determine the form and its dimensions for navigation. The process is difficult and involves complicated hydrodynamic reactions between the waterway and vessels. The difficulty is compounded by the fact that the vessels are independently controlled by human operators and sensitive to the pilots' reactions to varying operational demands. This fact, under any operating conditions, results in a certain lack of precision or certainty in determining vessel paths (Atkins and Bertsche, 1980; McAleer et al., 1965; Norrbin, 1989). Therefore, margins for safety need to be provided in the principal waterway dimensions. Estimation of appropriate margins is a key design function. Principal design elements of form that are required to be determined and dimensions that must be developed for a given waterway are the following (Atkins and Bertsche, 1980; Dand, 1981; Marine Board, 1985; McAleer et al., 1965; McCartney, 1985; Norrbin, 1986; USACE, 1983): location orientation or alignment depth width radius of curvature of bends tangent distance between bends aids to navigation These elements and their dimensions are primarily a function of the dimensions of the design vessel, its track, and its expected vertical and horizontal movement as it transits the waterway. Additional clearance dimensions to allow for uncertainty of position, operational safety, and hydrodynamic requirements are also required. Depth may also include a preinvestment factor to allow for sedimentation during intervals between intermittent maintenance dredging.
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Shiphandling Simulation: Application to Waterway Design Vessel motions and path and clearance requirements can be estimated by calculation, physical tests, or semiempirical methods. The operating environment is so variable, and the calculations so complicated, that considerable judgment is usually required for design by traditional guidelines. In practice, it appears that most estimates have been made by applying semiempirical methods and judgment (Atkins and Bertsche, 1980; Dand, 1981; NRC, 1981; Norrbin, 1986). Closely related to design is the operational analysis of a given waterway to appraise its capacity in terms of vessel size, traffic pattern, or density (Atkins and Bertsche, 1980). Operational analysis is applied to alternate design options, and the results are considered in optimizing design for safety and cost-effectiveness and when designing a navigational aids system. Operational analysis is also used by vessel operators (for example, shipping companies) to appraise the suitability of a waterway for a particular vessel and its loading limitations. A special case is forensic analysis where the conditions for an accident are deduced and re-created. Much of the same technology is used for operational analysis as for design, but application techniques and methodologies may vary to reflect the somewhat different objectives. Acceptable tolerances for calculated results may also differ (Gress and French, 1980). For this study, operational analysis is considered a special case of design and is not explicitly discussed. DESIGN ISSUES The issues to be addressed in waterway design are both technical and institutional (Herbich, 1986; NRC, 1983, 1985; Olson et al., 1986). Technical Issues Key technical issues include the following (McCartney, 1985): A design vessel or vessels must be selected with dimensions and characteristics around which the design is to be developed. The design vessel may be an existing vessel, a new vessel in planning or under construction, a conceptual ship of the future, or a composite of critical dimensions and properties of several vessels. Selection of the design vessel is a defining decision in the design process, regardless of design aids used (USACE, 1983). Dynamic behavioral characteristics must be determined for the design vessel (or vessels) as the vessel transits the waterway subject to various external forces and its own hydrodynamic and inertial properties. Related is the question of whether the vessel is to be maneuvered with or
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Shiphandling Simulation: Application to Waterway Design without the assistance of tugs (Armstrong, 1980; Brady, 1967; Crenshaw, 1975; Reid, 1975, 1986). The actions of a vessel's pilot must be determined relevant to dynamic behavioral characteristics (Armstrong, 1980; Crenshaw, 1975; Hooyer, 1983; Norrbin, 1989). Piloting skills resident in a local pool of pilots are not necessarily a critical factor in channel design studies. If a ship can be proved to be adequately handled by an experienced pilot, and thus the physics of the transit problem are not critical, then it follows that other pilots may be trained to operate the vessel safely, although better navigational aids might be required. Operating requirements must be noted, including the required speed of vessel in transit, density of vessel traffic in the waterway, traffic mix, special safety requirements, and degree of tolerance for risk of operating interruption (such as a grounding or collision). Assumptions for environmental conditions and limits must be assessed including oceanographic, hydrological (for example, tidal prism, currents, water levels), atmospheric, meteorological, and ecological factors, as well as time of day as it affects visibility. Costs must be determined for construction, maintenance, environmental and social impacts, and for vessel operations, together with their allocation and the assignment of benefits. Levels of risk that are acceptable must be determined. Acceptable Levels of Risk A special technical issue is risk. Tradeoffs made in design result in channel and waterway configurations that can be characterized as achieving an acceptable level of risk. There are no guidelines about what the acceptable risk level should be; thus the determination is highly subjective. During the assessment and based on its collective experience, the committee observed that port and public officials are reluctant to concede that some level of risk is an element in any port and waterway design. Reasons for this include concern over liability, project permitting, and interport competition. This general attitude has impeded the use of risk analysis with or without shiphandling simulation. Insight on risk can potentially be addressed by using simulation to identify maneuvering problems that may be associated with design alternatives or may be induced by certain physical conditions in the waterway environment. Moreover, the use of simulation has the potential to reduce the extra margins traditionally used to overcome uncertainty, thereby reducing construction and maintenance costs. Although this benefit may appeal to the project sponsor, it is the significant design refinement opportunities afforded by simulation that lead directly to the question of whether the fidelity of the technique justifies reliance on it over
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Shiphandling Simulation: Application to Waterway Design or in addition to traditional design practices to achieve adequate margins of safety. Institutional Issues Institutional issues are more difficult to define. Although they influence and are influenced by the technical issues, institutional issues are often the overriding and decisive factor in many waterway designs. They are usually associated with methods of finance, special environmental or social concerns, litigation, or legislation (Kagan, 1990; McCartney, 1985; NRC, 1987; Olson et al., 1986; Rosselli et al., 1990). Within the past several decades, competition for use of coastal areas has greatly increased. Competition exists between residential, industrial, recreational, and conservation uses. As a result, waterway development processes have come under much greater scrutiny by local interest groups, resulting in a lengthening of the already long approval process (Kagan, 1990; NRC, 1987). In the United States, for example, the time interval between design study and construction for a federal waterway project is frequently more than 20 years (NRC, 1985), which means that the assumed technical issues will likely have changed greatly by the time construction is completed. Typically, the original design vessel becomes obsolete (and may no longer be in service), shipping practices change, new supportive technology is developed, and cost relationships are altered. There is no reliable methodology for projecting future vessel design or operational trends when planning waterways or for adequately accommodating changes that occur. Thus, original technical issues can be quickly overtaken by events. Although an extreme, this situation in the United States is merely an exaggeration of global historical trends. Technological development in ships and in shipping operations have repeatedly stretched the technical limits and dimensional margins of waterways and harbors (McAleer et al., 1965; McCallum, 1987; Permanent International Association of Navigation Congresses [PIANC], 1980). The trend has been magnified during the past century and a half in response to the industrial revolution and expansive trends in world economic activity. The dilemma for waterway designers is to balance the costs and benefits. Shipowners are the direct beneficiaries of increased efficiencies gained from larger vessels. Others may receive direct or indirect benefits but may also bear the costs of providing facilities (NRC, 1985), which affects the resources available for a project. Institutional pressures and counter pressures (NRC, 1985) affect the designers' ability to implement a design of maximum utility and overall economic benefit. Ports and harbors are one element and represent a small share of the overall investment in the worldwide seaborne transportation
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Shiphandling Simulation: Application to Waterway Design system. However, construction, operation and maintenance cost are usually a major issue for local and national authorities responsible for funding. This fact, coupled with environmental and social concerns that have potentially significant cost implications, means that waterway development will inevitably remain under pressure for provision of minimal facilities or deferment (Kagan, 1990; NRC, 1985, 1987). As a consequence, economic pressure to use ships larger than design ships into existing waterways will continue (Jensen and Kieslich, 1986). Because the United States has few natural deep-water harbors, waterway designers have had to continually reappraise vessel size and operating limits for existing waterways and have developed minimal incremental improvements for extending those limits, usually for economic purposes (Atkins and Bertsche, 1980). However, there has been no impetus for use of simulation for project-specific design reevaluation or safety appraisal. Assessing the effectiveness of waterway design once a project is constructed, the adequacy of the design for use by vessels exceeding design vessel characteristics, and the accuracy of simulation predictions are not elements of current practice. Furthermore, no one, including the Army Corps of Engineers, Coast Guard, or project sponsors, appears to be overseeing operations to assure that vessels using a waterway remain within the vessel operating characteristics for which the waterway was designed. In fact, there is an economic incentive for shippers and port authorities to exceed the design parameters of the waterway in order to accommodate the latest generation ships, thereby maintaining a competitive advantage relative to other ports and maximizing the amount of cargo that can be accommodated. DESIGN PROCEDURES The classic full-effort design procedure consists of the following steps (Dand, 1981; McAleer et al., 1965; Norrbin, 1986; Sjoberg, 1984): establishing various trial design alternatives to meet both civil engineering and navigation requirements; comparing their estimated capital and maintenance costs, benefits, and other factors; and selecting a best alternative. Further incremental improvements to the selected alternative are usually considered and made by an iterative process until the design team is satisfied. By weighing tradeoffs in the cost-benefit analysis, the process approaches optimization (Burgers and Loman, 1985; Olson et al., 1986). True optimization of waterway design is seldom achieved or even attempted because of the lack of data, particularly data that are reliable and accurate and relating to accidents.
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Shiphandling Simulation: Application to Waterway Design DESIGN PARTICIPANTS CONGRESS Legislates project authorizations and appropriations for federal share of project funding. U.S. ARMY CORPS OF ENGINEERS (USACE) Plans, constructs, and maintains federal projects in navigable waterways. Conducts technical research in waterway design and construction techniques. U.S. COAST GUARD (USCG) Plans, constructs, maintains, and operates federal aids to navigation in navigable waterways; administers federal regulations pertaining to marine safety, security, and marine environmental protection. U.S. MARITIME ADMINISTRATION (MARAD) Shiphandling simulation research and development; steering and maneuvering properties of ships. Provides advisory support in the design process. PROJECT SPONSORS The local or regional organizations or authorities who contribute nonfederal funding to a specific waterway project. May include state, port, and local authorities. LOCAL INTERESTS Segments of the local community with interests in waterway construction, operation, and maintenance who may act as petitioners or advisers for waterway projects. May include state and port authorities, terminal operators, shipping companies, and pilot associations. PUBLIC INTEREST GROUPS Organized representatives from the public sector who have a direct or indirect interest in waterway projects. Interests include social, political, and environmental issues. DESIGN ENGINEERS Technical design consultants to the USACE, sponsors, and other interested parties on a contractual basis. Principally involved in providing full technical support for waterway projects outside of the United States because most countries do not have the equivalent of the USACE and many foreign ports—especially in developing countries—are owned by private companies. For public waterway projects in the United States, there is some attempt to follow this classic procedure, but with significant variations. The design process prescribed by the USACE has six phases: reconnaissance feasibility
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Shiphandling Simulation: Application to Waterway Design SOME TECHNICAL TERMS USED IN THIS REPORT DRIFT The sideways motion of a vessel from its track as it makes its transit. DRIFT ANGLE The angular between a vessel's heading and its track. SWEPT PATH A trace of the paths of the extremities of the vessel plan form as it makes its track while it transits the waterway. Account is taken of drift, drift angle, and yaw. SWEPT PATH ENVELOPE The outer boundaries of the swept paths with the most extreme deviations from target track that encompass all of the swept paths of the vessels that transited the waterway. TRACK A trace of the path of a vessel as it makes its transit of a waterway. TRANSIT A passage of vessel from point to point in a waterway. YAW The angular rotation of a vessel's longitudinal axis from the desired line of track. preconstruction engineering and design real estate acquisition construction operation and maintenance The first two phases listed are theoretically where the waterway form and dimensions are determined (Olson et al., 1986). In practice, form and dimensions are fixed in the construction phase because of actual or perceived inadequate consideration of the interests of participants in earlier phases. In some cases, considerable delays in project approvals and implementation have occurred, resulting in a range of both constructive and detrimental effects (Kagan, 1990; NRC, 1985). A brief description of USACE design process mechanics is provided in Appendix B. Developing a reasoned and sound technical design (which accommodates engineering, operational, safety, and environmental factors) as early in the process as possible establishes a solid basis for subsequent refinements. An issue is whether existing design tools are adequate to the challenge.
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Shiphandling Simulation: Application to Waterway Design DESIGN TOOLS AND TECHNIQUES The tools available to the waterway designer for technical solutions have improved markedly in recent years (Gress and French, 1980; McCartney, 1985; Norrbin, 1986; Olson et al., 1986). Before the ready availability of computers, designers were limited to carrying forward previous experience by judgment alone—with the aid of experiments with physical scale models—or by laborious mathematical calculations. Graphical methods with paper plots of position were often used to help visualize the pilot's task, the interaction of the forces on the vessels, and the vessel's resulting path. For practical reasons—usually monetary and time constraints—the number of model tests that were run and the variations that could be tested were usually limited. Similarly, the complexity of the mathematical solutions and applied formulae limited their number and required major simplifications to be usable. The calculations typically were used to check and verify previous assumptions rather than as a primary determinant. The capacity and speed of the modern computer has changed the designer's task dramatically. Mathematical solutions are now practical from the initial stages of design. The relative ease of changing input conditions has broadened the feasible alternatives to be considered (Burgers and Loman, 1985; Gress and French, 1980). Even with the modern tools available, the waterway designer must carefully input parameters and interpret results. To assist the designer, various groups, including USACE, PIANC, and International Association of Ports and Harbors (IAPH), have developed guidelines for design dimensions (PIANC, 1980, 1985; USACE, 1983). Although these guidelines are often helpful for visualizing a new waterway for initial studies, they are too general to assure an optimum design for a given condition. There are many examples of workable waterways that do not meet the guidelines by wide margins (Jensen and Kieslich, 1986; NRC, 1985). No substitute has been developed to replace intelligent and skillful analysis by a qualified, experienced waterway design engineer (Dand, 1981; Norrbin, 1986; Sjoberg, 1984). Not all of the elements of a waterway are equally amenable to analysis by modern tools and technology. Basic data gaps and incomplete theories still exist. Although the technical press reports some study of the subject in recent years, considerable approximation and applied judgment are required for some elements and conditions. It is beyond the scope of this study to examine design factors in detail. However, several elements are important in considering the appropriateness of design tools and techniques, of which computer-based shiphandling simulation is one option. The elements are depth, width, aids to navigation, environmental data and civil engineering, and design vessel.
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Shiphandling Simulation: Application to Waterway Design Depth Depth is the key waterway dimension. It usually results in the greatest cost impact, establishes the character of the port and its traffic, establishes the initial and maintenance dredging requirements, and affects the horizontal controllability and resultant swept paths of the vessels that use the waterway. In the United States, project depth is determined by some technical analysis, but primarily by political and administrative means. Technical requirements for depth include allowances for (NRC, 1983; USACE, 1983) the vessel's expected draft; the vessel's vertical motion from squat or sinkage as it moves through the water and from pitch, roll, and heave caused by waves and other external forces; an under-keel clearance for hydrodynamic reasons; and an extra clearance to account for errors in measuring channel depth and vessel draft and for dredging tolerances. For new projects, an extra depth allowance may be included to allow for sedimentation to occur between intermittent maintenance dredging. The primary technical tool for estimating depth requirements is designer judgment. Calculation of depth requirements involves the determination of critical sea and meteorological conditions, vessel operations, and other factors that affect the vertical motions and chance dimension errors. Because it is unlikely that maximum conditions for all factors will occur simultaneously, some designers have attempted to determine depth requirements by probabilistic forecasts. For example, studies for the Panama Canal Company in 1975 involved a special probabilistic approach related to pilot variance in compressed-time simulations (Norrbin et al., 1978). However, in practice, probabilistic forecasting has had mixed acceptance by designers. Even where practiced, considerable human judgment is still required for both input and evaluation. Guidelines help, but in actual practice in many waterways, ship drafts consistently exceed those indicated as allowable by guidelines published by USACE, PIANC, and IAPH. Ships are routinely brought into ports with drafts that exceed project depths by taking advantage of daily tides and river stages (MacElrevey, 1988; NRC, 1983, 1985; Plummer, 1966). In practice, the only consistent, albeit informal, control over maximum draft on port entry or departure seems to be exercised by local pilots who make expert judgment calls on under-keel clearances that will permit safe movement of each vessel. Although the published guidelines offer a reasonable if imprecise gauge for safe under-keel clearances, economic criteria are applied by shipping interests.
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Shiphandling Simulation: Application to Waterway Design Width The required design width includes one or more vessel maneuvering lanes plus allowances for side clearances from the design vessel to the edge of the channel, other vessels, banks, structures, or natural features of the waterway. Width of the maneuvering lane is determined by the horizontal dimensions of the design vessel, its varying orientation in the waterway, and its deviations or drift from the desired track (Marine Board, 1985; USACE, 1983). A trace of the design vessel's extremities outlines and defines its swept path. The maneuvering lane is intended to provide an envelope of all the expected swept paths of the vessels that will transit the waterway under the various assumed design conditions. It is desirable that the lane's alignment is as close to a straight line as possible. Deviations in path alignment to avoid obstructions, take advantage of natural features, reduce dredging and sedimentation, or improve vessel operations are made with allowance for the design vessel's turning ability. The side clearance dimension from an obstruction or bank provides a minimum path for the return flow of water displaced by a vessel as it moves along the edge of the maneuvering lane. It also provides a safety allowance for potential errors in the vessel's position. Deviations from desired orientation and vessel track are caused by a vessel's inherent stability or instability, the effects of external forces from wind, wave, current, and hydrodynamic reactions, and the applied control efforts by the pilot. The degree of vessel control applied by the pilot is a major variable assumed by the designer. Unlike vertical motions, a vessel's horizontal motions and deviations can be anticipated and compensated for by pilot action. The effectiveness of this action is dependent on the pilot's level of skill, perception, and reactions, and following execution, on the inherent controllability and responsiveness of the vessel. Determining the degree of vessel control is a difficult challenge for the designer. As with depth, actual practice has indicated that widths of much narrower dimensions than those recommended by traditional guidelines are both feasible and practicable. Some waterways such as the Houston Ship Channel fall into this category and are operated successfully (Jensen and Kieslich, 1986), although not without risk (Gates, 1989). Although technological gaps in the science still exist, there has been considerably more work done regarding width in recent years than there has been on the vertical phenomena. Calculation is feasible with a reasonable level of confidence. Special cases, such as basins where low speed maneuvers are planned, bends and turns in channels, and passages through bridges, require special study. However, the design tools are generally the same and are available. Weaknesses in the technological base include a lack of definitive data
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Shiphandling Simulation: Application to Waterway Design on maneuvering of vessels with very small under-keel clearances, especially in confined waterways. Also, quantitative guidance on the effects of different bottom material and contour forms and the effects of pitch, roll, and heave on track keeping in shallow water are scanty. Designer judgment and the transfer of prior experience are the principal tools to account for these conditions at present. Navigational Aids Aids to navigation systems are an important but frequently overlooked element of waterway design (Atkins and Bertsche, 1980). By integrating aids to navigation into the waterway design, the effectiveness and possible precision of vessel position fixing is improved and the designer can allow for tighter margins in waterway dimensions if they are validated by some means. Available navigational aids range from traditional aids to navigation, such as buoys and ranges, to electronic position fixing devices, such as loran and differential GPS (global positioning system). All aids require human perception and reactions for maneuvering the vessel. Quantifying and evaluating behavioral modifications associated with use of aids to navigation is a particularly difficult challenge to the waterway designer. Normal design procedure is to solicit the opinions and judgment of experienced mariners as a guide. Although this method frequently is satisfactory, it does not fully evaluate navigational systems in the context of a new or modified design. Shiphandling simulation has been applied and demonstrated to be of value for assessing aids to navigation (Atkins and Bertsche, 1980). The waterway designer must carefully allow accuracy tolerances for behavioral modifications relevant to maneuvering strategies that may result from the type and placement of aids to navigation. Because unbroken delineation of channel boundaries and traffic lanes is typically not feasible in a waterway or fairway, the relationship of the vessel to its intended track is determined either by electronic or visual fixes (with some lag behind actual positions due to human and electronic processing time) or by expert estimations based on all information available. The pilot's strategy is therefore based on the perception of position and the onward track. Any lack of precision widens the track requirements. Environmental Data and Civil Engineering Navigational and civil engineering (including construction) aspects of waterway design require considerable data relating to the environment, both above and below the surface. Ideally, the data would be drawn from analy
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Shiphandling Simulation: Application to Waterway Design sis of meteorological and hydrographic measurement records coupled with up-to-date physical surveys. Such records are not always available with the detail required for a specific project site. Physical scale and mathematical hydraulic models have sometimes been used to interpolate general data regarding site-specific estimates of currents, sedimentation, and wave patterns. Similarly, mathematical models have been used with general synoptic charts for estimating meteorological conditions (Seymour and Vadus, 1986; USACE, 1977). Random or selective field measurements are usually advisable for verifying such estimates. As with all other aspects of waterway design, the engineering skill of the designer, together with clear and complete analytical reasoning, are prerequisites for success. Design Vessel Selection of the design vessel, or vessels, is one of the most critical decisions in waterway design (Dand, 1981; McAleer et al., 1965; USACE, 1983). Vessel dimensions and maneuvering characteristics are key to the required waterway geometry and dimensions, no matter what design method is used. The design vessel might be an actual vessel based on proposed operations or a hypothetical vessel. In accepted practice, the design vessel is selected to represent a combination of the largest ship with the least controllability that will require the greatest depth and largest width of the waterway, considering both swept path and clearances. It may not necessarily represent either the largest specific ship or the least controllable ship, although both are normally considered before a selection is made. Ideally, vessel size and characteristics are based on forecasts of operations, considering world trends in shipping, and on forecasts of trade and traffic for the port. In actual practice, vessels used in most waterways differ substantially from what the designer had forecast 20 or more years earlier. In the committee's view, major reasons for this discrepancy include: dramatic changes in the form and composition of the national and worldwide merchant fleets made available through modern technologies; the time scale of the waterway development process, which is longer than the working life of a typical ship; and the absence of a waterways management regimen that restricts vessel access only to vessels that do not exceed design vessel characteristics (which could have the potential side effect of impeding development of maritime technologies). Because of the inexact forecasts of future actual ships or vessels and the wide degree of variation in handling characteristics even of similar
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Shiphandling Simulation: Application to Waterway Design ships due to such factors as loading, the exact modeling of a particular design vessel is excessive. A reasonable approximation is sufficient. The objective is to model a representative vessel with typical behavior under the control of typical pilots under the conditions being studied. Dimensions and other particulars for existing and new designs can be gleaned from naval architecture journals and publications from the classification societies. Estimates of the far future can be based on interviews with shipping interests and on deductive reasoning. In all cases, verification of handling characteristics by experienced mariners is of great assistance. SUMMARY Waterway design, whether for new construction, improvement of an existing waterway, or appraisal of the capacity of a waterway, involves estimating the navigation requirements of an assumed vessel or vessels, coupled with estimates of the civil engineering factors. Present technology allows calculation and mathematical modeling of the factors that affect waterway width and form in the horizontal plane, but considerable judgment still needs to be applied. Depth and other elements, including the need for aids to navigation, are still estimated and based primarily on human judgment. Human reactions by vessel pilots are an important ingredient, and their assessment and accommodation present a particularly difficult challenge to the designer. Optimization of design, wherein all elements are appraised in terms of the others and alternate solutions are compared for maximum cost effectiveness, is not usually practical because of insufficient data and imperfect technology. Optimized designs in the United States are difficult to achieve because of institutional factors, such as increased emphasis on social and environmental objectives in design and the long lead times before implementation of a project after planning. Design tools or techniques are needed that can give reasonably correct technical solutions quickly and early in the process to provide a more scientific and technical basis for accommodating competing objectives that affect the waterway development process.
Representative terms from entire chapter: