Dredges
General Dredging
Each year, the U.S. Army Corps of Engineers' hopper dredges (ESSAYONS and YAQUINA) work on West Coast ports from Grays Harbor, Washington to San Diego, California. Other contract dredging work is done by hydraulic pipeline dredges (OREGON), hopper dredges, and mechanical clamshell dredges.
Hopper dredge - propelled floating plant which is capable of dredging material, storing it onboard, transporting it to the disposal area, and dumping it.
Clamshell dredge - mechanical cable excavator dredge that uses a single bucket attached to the dredge crane with cables. The dredge operates by lifting the bucket (the clamshell), dropping it into the bottom sediments, lifting the bucket and dredged material to the surface, and emptying the dredged material into a nearby disposal facility or scows or barges for transportation to a disposal facility.
Pipeline dredge - dredge whose prime function is to excavate and move material hydraulically to another location without rehandling. The dredge is generally comprised of a ladder, cutter, suction pipe, cutter motor, hull, lever room, main pump and engine, spuds and discharge line. A pipeline dredge having a discharge diameter of 24 inches or more is called a large dredge. A class of smaller pipeline dredges that are designed to be easily disassembled for overland transporting to the next dredge site are called portable dredges. Portable dredge hulls are built in section that can be bolted together. Usually sections consist of a center hull for machinery, pump, an other equipment, and two more pontoon side sections.
Safety Tips Around Dredges
Dredges monitor marine VHF radio channels. You may contact a dredge on channels 13 or 16. It's important to know and understand the following whistle blasts from the dredge:
- One blast: Pass on the left (port) side
- Two blasts: Pass on the right (starboard) side
- Three blasts: Dredge is backing up
- Five blasts: Danger! Watch out!
Keep a reasonable distance away from the hopper dredge when it is operating. There are six hazards you may confront if you travel too closely or in the working area. They are:
- Propellers at the back of the dredge generate strong water currents that can capsize a smaller vessel.
- Propellers at each side of the boat at the front also can generate strong water currents that can capsize smaller vessels.
- Large volumes of water may overflow from the hoppers underwater at the center of the dredge.
- Powerful streams of water are automatically released from the holes at either side of the front of the dredge. These streams are powerful enough to capsize a boat or push a person under water.
- Dredged material relocation sites may be shallower than ocean or river charts may indicate.
- Coastal or off-shore relocation sites, which are marked on coastal charts, may have rough sea conditions due to shallower water.
Each year, the U.S. Army corps of Engineers' hopper dredges work on West Coast ports from Grays Harbor, Washington, to San Diego, California. Other contract dredging work is done by hydraulic pipeline dredges, hopper dredges and mechanical clamshell dredges.
Pipeline dredges have long pipelines that extend from the dredge to the shore, with underwater cutter heads sweeping back and forth across the channel. Both the dredge and pipelines are held in place with spuds and anchor lines. The anchor lines are marked for safety with yellow anchor buoys.
When passing the pipeline dredges, remain on the clear side and watch for the anchor buoys. When going downstream or with the current, do not approach the pipeline. If your motor stops, the current will pull your vessel under the pipeline.
Hopper dredges move like a ship. When dredging, they move very slowly. Normally, you can tell when they are dredging by the signals on the main mast. During the day, a black ball over a black diamond over another black ball will be shown. At night, a red light over a white light over another red light means the dredge is working. These signals also indicate that the dredge is restricted in its ability to maneuver and you must stay clear of the vessel.
When the dredge's hopper is loaded, the dredge maneuvers both in and out of the channel to reach the relocation site. During this time, the dredge may move much faster and may turn frequently.
Hopper Dredge Essayons
The Dredge Essayons is the latest dredge to be built for the U.S. Army Corps of Engineers. Delivered to the Portland District in 1983, the Essayons helps to maintain the entrance bars, rivers and harbors on the coasts of California, Oregon, Hawaii and Alaska, and in emergencies, the Mississippi River. Because of its size and dredging depth, the Essayons is particularly well-suited for dredging the larger coastal entrances. Designed to operate anywhere in the world, the Essayons is automated for operation with an automatic dragarm handling system. Sophisticated instrumentation allows constant production monitoring and enables the dredge crew to maintain maximum dredging efficiency 24 hours a day. The Essayons is also equipped for direct pumpout.
Hopper Dredge Yaquina
The Yaquina was designed by the U.S. Army Corps of Engineers and constructed by the Norfolk Shipbuilding and Dry-dock Corporation of Norfolk, Virginia, in 1981. Delivered to Portland District in 1981, the Yaquina helps to maintain the entrance bars, rivers and harbors on the California, Oregon and Washington coasts. Because of its size, the Yaquina is particularly well suited for dredging the smaller, shallow coastal entrances.
The Yaquina is automated for operations with an unattended engine room and semiautomatic dragarm handling system. Sophisticated instrumentation allows constant production monitoring and enables the dredge crew to maintain maximum dredging efficiency 24 hours a day. The dredge normally works continuously, tying up eight hours or less per week for fuel, water, supplies and maintenance.
Dredge Oregon (Port of Portland)
The Dredge OREGON is owned and operated by the Port of Portland to maintain the Port’s wharf and berth areas and to provide sand fill to develop waterfront property.
Several months of the year it is leased to the U.S. Army Corps of Engineers on a non-profit, cost-reimbursable basis for maintaining portions of the Lower Columbia and Willamette Rivers’ 40-foot-deep channel under a requirement of “Local Cooperation.”
The Dredge OREGON’s main working features are a cutterhead, suction pipe, centrifugal pump, pumping engine and discharge pipe. The suction pipe, cutterhead motor and drive shaft, and the cutterhead are mounted on a steel frame called the “ladder.” The ladder is hinged on the bow with the cutterhead end supported by wire ropes from the “A” frame overhead.
The cutterhead is a basket frame of spiral blades mounted around the end of the suction pipe. When turned by a shaft from the cutterhead motor at the top of the ladder, the cutterhead dislodges riverbed material.
A suction pipe leading from the cutterhead to a large centrifugal pump picks up the loosened material and it is pumped through the pipeline at a rate of 2,000 to 3,000 cubic yards an hour.
Because the dredge is non-propelled, it is held in place with stern anchors, swing lines and anchors, and spuds. Swing Lines are cables that lead from the ladder to anchors placed several hundred feet to each side of the dredge. The swing lines are controlled by winches on each side of the bow and pull the dredge from side to side. Spuds are long, heavy steel tubes that can be lowered to penetrate the river bottom. A spud stands at each side of the stern and can be raised or lowered independently to serve as a pivot for the dredge while it is working.
By pivoting on the right spud and pulling on the right swing line, the cutterhead can be "swung" in an arc across the river bottom. At the appropriate point of the swing, the left spud is lowered and the procedure is reversed causing the dredge to “walk” forward. The distance that the dredge is swung to the right before the left spud is lowered determines the distance the dredge advances. Small tugs occasionally reposition the swing anchors to keep up with the dredge.