For a dredging project, a range of auxiliary equipment is deployed in addition to dredgers. Based on the project requirements, the type of dredgers used, the distance to the disposal site, the accessibility of the dredging area or encountered materials, any of the following equipment can be required.
Pipelines can be used for the transport of dredged materials, both in combination with cutter suction dredgers as well as hopper dredgers. A floating pipeline is connected to the onboard pipeline of a CSD to directly transport the slurry to the disposal site. Trailing suction hopper dredgers can also discharge by pipeline by connecting a floating pipeline to the bow coupling at the bow of the vessel. With TSHD’s, discharge by pipeline is used to deposit the soil to shore. Pipelines come in all shapes and sizes as different circumstances require different configurations. There are floating and sinking types, different materials are chosen for wear-resistance, and different connectors and discharge hoses are used.
Booster stations are units that are added to pipelines to extend the distance over which the dredged material can be transported. They basically consist of an engine room and dredge pump that is either installed on a pontoon or ashore. The booster station adds extra pumping power to the pipeline. They are mostly used in combination with cutter suction dredgers. It can be very useful to match the booster station with the dredger, so that the dredger and booster have largely the same parts. That way, the number of spare parts needed on a project is reduced. There are also containerised booster stations available on the market to ease transport.
Instead of booster stations it is also possible to install additional dredge pumps inside the dredger to pump the material directly over the required distance and to control all the engines on board the dredger.
Dredgers without any means of propulsion rely on work boats, push boats or tugs to position them in the dredging area. Work boats are also used for support operations, such as hoisting the cutterhead and pump parts, anchor handling, handling of floating pipelines and the transportation of people, fuel and goods. They are usually equipped with a hydraulic crane and anchor handling winches.
Dredged material can be transported by the dredger itself if it has a hopper, otherwise hopper barges can be used. Hopper barges come in all shapes and sizes and can be self-propelled or non-self-propelled, seagoing or suitable for use in inland waterways only. They can have their own means of disposal, such as bottom doors or a split hull, or separate unloading equipment can be required.
When hopper barges are deployed for the transport of dredged materials to a reclamation project, a special barge-unloading dredger is often used for the re-handling of the materials. This type of dredger empties the barges and pumps the materials ashore via a pipeline. The barges can come alongside the dredger and a suction arm with water jets is lowered into the hopper of the barge. The barge is unloaded by pumping the mixture of sand and water into the pipeline. Usually the barge-unloading dredger is positioned on spud poles.
A bed leveller is a simple yet effective item of dredging equipment. It is basically a tugboat with an A-frame, winch and submerged drag. It can be used to level high spots in areas where the average dredging depth is sufficient, to dig soil from quay walls and jetties to the centre of the channel in ports, or to lower the top of slopes in shallow areas where hopper dredgers have problems with the available depth. The natural current in the channel will then flush away the sediment or a trailing suction hopper dredger will pick it up. Bed levellers are particularly useful in nautical channels and busy ports, where stationary equipment like grab dredgers are considered to be obstacles and therefore not preferable.
Water injection system
Water injection dredging is used to transport the sediment in a small port or channel to a nearby and deeper area. This dredging principle can also be used to lower slopes. The injection system consists of a self-propelled pontoon with a submersible pipe and a simple water pump. When the arm is lowered to the bottom, the water jet brings the material in suspension just above the river or seabed. The liquefied material will then flow to the deepest point in the vicinity. A water injection system can operate extremely economically, because the energy consumption is rather low.
Hard, solid rock cannot be dredged, not by even the strongest of cutter suction dredgers. When these boulders or rock faces are also too large to lift using a mechanical dredger, such as a backhoe or grab dredger, the removal of rock has to start with pre-fracturing. This can be done either with explosives or by deploying a rock breaker.
A pneumatic rock breaker consists of a pontoon with a central well. Mounted lengthways on the pontoon is a gantry carrying two travelling cranes, from each of which a large pneumatic hammer with a special rock chisel is suspended. The hammers are lowered to break up areas of rock, after which the fragments can be removed with grabs, clamshells or a backhoe dredger.
Dredging contaminated soil requires high levels of accuracy with low generation of turbidity and low spillage. To achieve this, special environmental dredgers are used. Various types of environmental dredgers exist, but they have one thing in common. They are equipped with a high degree of instrumentation and automation in order to fulfil the requirements of precise dredging.