A backhoe is a type of grab crane dredging pontoon. It consists of a hydraulic grab crane mounted on a dredging pontoon. The dredging pontoon is usually held in place by three spud poles. Backhoes are used to dredge heavy clay, soft stone, blast rock and soil thought to contain boulders, for example in foreshore protection operations.
Cutter Suction Dredger
A cutter suction dredger is a stationary or self-propelled vessel that uses a rotating cutter head to loosen the material in the bed (‘cutting’).
A suction inlet located beneath the cutter head (known as the suction mouth) is connected by a suction tube directly to one or more centrifugal pumps. The vacuum force at the suction inlet sucks up the loosened material. The suction tube and cutter head are attached to a ladder. The ladder with cutter head is positioned at the fore of the vessel.
On the aft side, the cutter generally has two spud poles. One spud pole (the auxiliary spud) passes straight through the vessel, while the other is mounted on a movable spud carriage, which can be moved lengthwise along the vessel or pontoon. Steel cables are used to move the ladder or cutter head back and forth, with the spud in the spud carriage as the centre of each concentric circle that it describes. Moving the spud carriage causes the cutter suction dredger to move as well (‘stepping’). The cutter suction dredger discharges the dredged material directly to shore via a floating pipeline or into a barge with a special loading system.
Van Oord: Dredger “Artemis” Commissioned (The Netherlands)
Cutter Suction Dredger Castor from Van Oord on Vimeo
Trailing suction hopper dredger
A trailing suction hopper dredger has large, powerful pumps and engines that enable it to suck up sand, clay, sludge and even gravel from ocean or river beds. One or two suction pipes run from the vessel to the bed. A draghead is attached to the end of the pipe and lowered to just above the bed, making it possible to regulate the mixture of sand and water that it takes in. A trailing suction hopper dredger generally stores the dredged material in its own hopper and discharges the left-over water overboard.
A trailing suction hopper dredger can empty its hopper in a variety of different ways:
- Depositing вЂ“ Opening doors or valves on the bottom of the vessel so that the hopper contents drop out.
- Pumping вЂ“ Using jet pumps or water jets to pump water into the hopper at high pressure so that the sand becomes ‘fluid’ again. The dredge pumps can then pump the resulting mixture through a pipeline which is connected to the vessel.
- Rainbowing вЂ“ This method is the same as pumping, except that the hopper contents are not pumped through a pipeline, but are sprayed over the vessel’s bow directly at the desired location.
Van Oord: TSHD “Vox Maxima”
Water injection dredger
Water injection dredgers are often used in small, shallow ports and marinas because they have good manoeuvrability and can dredge very close to embankments and quay walls. An injection beam located underneath the vessel injects large volumes of water under low pressure into silt or fine sand in order to resuspend it. The sediment mixture turns into a density current, which is then removed with the help of gravity.
Some water injection vessels are built as demountable pontoons. The Wodan is a demountable T-shaped pontoon with a U-shaped WID pipe. It is 12 m wide, 32 m long and has two HK700 pumps. The pontoon consists of nine pontoon containers, one winch container and another six containers with dredging pipes, pumps and other equipment. The powerful winch makes it possible to quickly adjust the dredging depth from 1 to 20 m. The pontoon is towed or pulled.