Shiphandling Simulation: Application to Waterway Design (1992)

Shiphandling simulators have gained considerable acceptance worldwide as a useful aid in harbor and waterway design, albeit slowly and not yet universally. Detailed examination of representative simulation applications is useful to

• evaluate overall simulation usefulness,

• determine how simulation results were applied to the design process,

• develop an understanding of some of the problems encountered and how they were addressed,

• understand how simulation validity was verified, and

• assess the impact of different levels of simulator sophistication on simulation results.

Six simulation projects were selected for examination. These simulations were performed for projects at

• Oakland Harbor, California;

• Upper San Francisco Bay (Richmond Long Wharf), California;

• Grays Harbor, Washington;

• Norfolk-Hampton Roads, Virginia;

• Coatzacoalcos, Mexico; and

• Gaillard Cut of the Panama Canal.

The projects were selected for several reasons, including

• the variety of navigational-harbor design problems that the simulations addressed,

• the different levels of sophistication applied in several of the applications, and

• the firsthand knowledge of many of the simulations by the committee.

A summary of each of the six case studies is included in Appendix C. Each was reviewed with particular attention given to specific lessons learned that might be generalized for application to other simulations. These lessons, both technical and administrative, are included in the descriptions in Appendix C and are consolidated into the five findings that follow.

In each of the six applications of simulators that were examined, the project sponsors were able to modify the waterway design and/or operation to achieve: significant cost savings, improvements in ship safety, and/or reduction in environmental risk. Cost savings were generally the result of reduced dredging or shifting of dredging activity to less costly sites. In one case, Coatzacoalcos, the cost savings resulted from being able to use larger ships safely without additional dredging.

Increased ship safety was achieved by identifying critical navigational areas during the simulation process. For example, in Oakland, significant safety benefits were derived by widening the bar channel and the entrance channel beyond the width initially proposed. Although this widening required an extra cost for additional dredging, it was offset by reduced dredging in other areas where the simulation had indicated it was not necessary. (Although the simulation was successful, port-sponsored project construction on an accelerated schedule has been discontinued because of legal constraints and the inability of the sponsor to develop a plan for disposal of dredged material that was acceptable to all parties [Appendix C]).

Environmental risks may be reduced by simulations in two ways. Improved ship safety contributes to a reduction in environmental risk because it reduces the probability of spillage of oil or other toxic substance that might result from ship groundings or collisions. Simulation can also reveal channel configurations that have smaller dredging requirements and which therefore minimize the environmental impact.

The committee cannot say with certainty that the cost, safety, and environmental benefits observed in these case studies would not have been achieved without the use of simulations, that is, if more intensive design reviews and audits had been conducted in the base case. However, it was observed that these benefits were not forthcoming before undertaking the simulations. Therefore, the committee finds it appropriate to credit the simulations for these benefits.

Successful harbor and waterway design involves the effective interaction of many different parties, including officials from federal and local governments, community and public interest (including environmental) groups, port operators, shipowners, hydraulic and civil engineers, naval architects and hydrodynamicists, environmental engineers, and pilots.

In the projects reviewed, simulation effectively focused the attention of these various parties on the harbor and waterway project. It provided a common framework for describing the project and the related problems as perceived by the various groups involved. In essence, it permitted these groups to communicate with each other with a common language and understanding that might not have been otherwise possible.

The use of simulation to focus communications demonstrated the potential of simulation to contribute measurably to successful, cost-effective harbor and waterway design and development. This benefit is separate from the research or engineering contributions that are usually expected of simulation.

The issue of simulation validity received considerable attention throughout the study. During initial assessments, it was determined that an accurate mathematical modeling of the ship-channel interactive process was not possible, given current knowledge. The broad mathematical principles underlying the physical situation are generally well defined. However, a valid simulation in the strict engineering or scientific sense requires the measurement of environmental forces, their interaction, and the response of the ship to a degree that has never been attempted because of the great technical difficulty and costs involved.

In each of the six projects, local pilots were extensively used, not only as participants in the simulation process, but also, in essence, as the final arbiters of the validity of the simulation. Although the use of local pilots in

this manner is not ideal because of the subjectivity involved, this limitation was recognized in the six projects and was addressed by incorporating greater safety margins than might have been necessary if the simulation validity could have been objectively measured. The risk exposure when applying simulation results, even with current knowledge, is considered less (at times significantly less) than that without simulation. Application of appropriate safety margins appears to be a reasonable way to deal with the current inability to measure objectively the validity of simulation.

Different levels of simulation were used in many of the case studies or, often, even within a single case study. Significant differences in the sophistication of the displays used for real-time simulation were noted between the various facilities. For example, some facilities had greater than 270° fields of vision, while others had only a straight-ahead display. Some facilities complemented the simulated radar screen with a bird's-eye presentation of the ship's position in the harbor, while others did not. Meaningful results were obtained from less-sophisticated simulations for certain problems. For example, fast-time simulations (no person in the loop) were used extensively for the Thimble Shoal Channel and Atlantic Ocean Channel simulations during the Norfolk-Hampton Roads project conducted at the Computer Aided Operations Research Facility (CAORF) for the State of Virginia and the U.S. Army Corps of Engineers. Navigation in these entrance channels does not require the use of any visual references from landmarks. Therefore, a real-time simulation with sophisticated displays was deemed unnecessary and costly. In other studies, high-quality visual displays were felt to be extremely important. This was the case for Grays Harbor, Washington, where it is necessary to navigate through two bridges that are offset from one another immediately after the vessel completes a sweeping turn in the river.

Simulation was demonstrated in application to be a source of guidance for the designer or user of a harbor and waterway. Simulation results must be interpreted and applied with care because their accuracy cannot be objectively verified. Although this limitation seemed to have been recognized by participants in each of the six projects, no comprehensive methodology was found to be in place for guiding designers on the establishment of safety

margins or dealing with the uncertainties inherent in simulator results. This is a significant gap in the state of practice.

Simulations typically were used more often for design verification and refinement rather than for developing basic design parameters and limitations. This situation may change as harbor and waterway designers become more familiar with the usefulness of simulations and more skilled in their application. Some movement in this direction by designers seems to be taking place.