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DGPS-Based Survey Systems for Small Dredging Projects

Source: International Dredging Review

Moderately-priced, leased GPS-based positioning systems are now available for small dredging projects. A typical system includes a standard PC compatible computer, with Trimble 4000 series differential GPS and Windows-driven software such as HYPACK.

A properly-qualified technician can install a basic system on a small to medium-sized dredge in one or two days. Once the system is set up, a person with basic knowledge of electronic navigation equipment (such as Loran) can be trained to operate the system in less than two days.

As with all electronic equipment, GPS systems are not without a few problems, but a properly installed system should be at least 90 to 95 percent available, and this average is constantly improving. There are strengths and weaknesses, and some project applications are better suited than others.

Advantages
Properly set up and installed, in most marine environments GPS systems will report positions in real time with one second updates within one meter or less of true position. This is far more reliable than either microwave or physical ranges.

The dredge or cutterhead position is shown in real time on monitors that can be installed in almost any weatherproof location on the dredge. Positions can be selectively logged by the computer, and recalled later to review work progress. This single feature has proven invaluable in keeping track of dredge activities. It is very easy to plot dredged areas, reposition the dredge and log obstructions. Besides the dredge position, the monitor can graphically show the outline of the dredged area, channel limits, stationing, shoals and obstructions.

GPS is an all-weather system. While accuracy may be reduced somewhat in heavy cloud cover, snow or fog, it is designed to operate with sufficient accuracy under most conditions.

There are no physical markers, ranges or shore stations to be maintained, surveyed or serviced (where differential corrections are available on public airways). System maintenance is minimal. After completing the initial startup and training, a typical project can be handled with less than one service call per month and an occasional call to a technician.

Precautions
GPS loses accuracy or can operate erratically when "multipath" conditions exist. (Multipath is caused when satellite signals reflect off large metal surfaces.) Sources of multipath in the marine environment include large container cranes, ships and fuel tanks. In these cases, the operator should stop operation until the multipath source is removed from the area, or resort to limited use physical ranges.

The dredge must have a reasonable clean, dry area to house the computer and equipment. Standard computers do not work well in extreme heat or cold, around excessive vibration or when wet. Conversely, more rugged and weatherproof computers are available; they just cost more. (Standard systems have been installed in and work well in the cab of an Ellicott® Series 370 10" dredge.)

Dredge operators need to be trained in the use of computer and software. While the newer programs are "user friendly," training time can vary considerably, depending on the aptitude of the individual. Generally speaking, there should be one full time person on the project with fair to good computer skills.

Although greatly simplified from equipment of the past, it is best if these systems are installed, set up and maintained by professionals. The company that leases the system should provide this service at a reasonable cost.

Even though the equipment is reliable, failure can occur. Be sure to know where to get a backup system (usually from the leasing company).

In the past five years, CLE has installed a number of electronic positioning systems on dredges and survey boats. We have converted our lease pool inventory exclusively to Trimble GPS systems, and Coastal Oceanographics HYPACK software.

A recent installation was for AGM Marine for a dredging project in Provincetown, Massachusetts. AGM used an Ellicott® Series 370 dredge, which has limited but sufficient cabin space. The computer was installed on a shelf hung from the cabin ceiling, with the monitor suspended directly below it. The GPS and power supply were installed in a waterproof box behind the operator's seat. A small generator was used to supply power.

The contract was for improvement dredging of the 250-foot-wide entrance channel to Provincetown Harbor. The area to be dredged was irregular shaped. CLE created an image on the computer monitor that showed the channel limits, 50-foot offsets, 50-foot stations and an outline of the exact dredging and overdepth areas. This display made it easy for the dredge superintendent to visualize the position of the dredge in relation to the plans, and to bypass "non-digging" areas. Once the dredge was on line, normal operation of the computer was confined to eight keys: <+> and <-> to zoom in and out, four arrow keys to reposition the screen image as the dredge moved, and when logging a sweep of the cutter, the "s" and "e" keys for the start and end of the sweep.

Besides the obvious benefits, the contractor was able to gain additional benefits that he had not anticipated. Due to extreme tides, boat traffic and severe weather, the dredge had to be moved off station regularly. In each case, the dredge superintendent was able to annotate the exact position of the dredge before moving it, and bring it back to the exact same location when work was resumed. In the course of the project, the dredge encountered numerous obstructions in the form of abandoned moorings and debris. Again the superintendent was able to precisely report the locations of these obstructions to the Corps resident engineer by reading the coordinates directly off the monitor. After completing the initial post dredge survey, CLE was able to provide a new display for the monitor that precisely outlined each area that required re-dredging. This allowed the superintendent to quickly and expeditiously move to each location with no guesswork.

AGM had used microwave positioning systems in the past, and found that the GPS package was considerably less expensive and more reliable and user friendly. Also, the cost of installation, setup, training and equipment lease was less expensive than the cost of surveying positions for the placement of physical ranges.

There is no doubt that the potential applications of GPS to dredging have only begun to emerge. As equipment and software continue to improve, new competitive advantages will be gained by those who use it to take the lead. The reality is that as environmental constraints continue to tighten, dredging costs per cubic yard will continue to rise. The ability of the dredge plant to control the amount of overdredging and reduce the time spent re-dredging is becoming the deciding factor as to who wins or loses a project. Precise real time positioning of the dredge is fundamental to controlling both these factors.

Excerpted from International Dredging Review

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