Role and Types of Dredgers: A dredger is a maritime vessel equipped with one or more devices for scraping or sucking the silt that has accumulated on the sea bed (the tool used for excavating and scraping the sea bottom is called the Dredge).
More broadly, a ship fitted with an excavation tool that can remove deposits from the bottom, such as sand, gravel, sediments, etc., is called a dredger ship, or simply a dredger.
The role of dredgers is crucial because they guarantee the safe bottom clearance required for safer travels.
Dredging is the process of excavating in freshwater or shallow water to collect sediments from the bottom for later disposal.
The sediments could be collected for the following uses:
- making water navigation or fishing easier in shallow waters
- for replenishing the sand on public beaches which might have undergone severe coastal erosion
- Gold and coal mining
- Removal of contaminants from the sea bed
- Reclamation of areas damaged by oil spills or natural calamities
- Creation of new harbours
Dredging may be the only choice in some circumstances, even though it can have extremely negative consequences on the marine and aquatic ecosystem.
Dredger types can be broadly categorized into three groups based on how dredged material is transported from the sea floor to the water’s surface:
The mechanical dredgers are convenient for clearing rubbish or hand-packed material since they can operate in small spaces.
Other dredgers do not fall into the first two types; instead, hydraulic dredgers operate on the premise of introducing substantial volumes of process water to alter the initial structure of the sediments.
The various kinds of dredgers that aid in the removal of seabed sediments, whether they be mechanical or hydraulic, are:
1. Mechanical Dredgers
These come in a range of designs, but they all operate on the same “hand-packing” basis. These have a grab or bucket attached to them that is pushed over the loose bed sediments, fills with material, and is then elevated to transfer it to the appropriate disposal location. An overview of a few popular mechanical dredgers is provided below:
Dredgers in buckets
These mechanical dredgers are the most ancient kind. These are regarded as stationary dredgers because they are anchored to anchors, but winches allow them to be moved around semi-arcs when dredging.
In bucket dredgers, an unending chain of buckets is provided. These buckets are used to scrape off and fill the loose material that has been dredged; once full, the buckets can be deposited into barges by simply tipping the bucket upside down over the barges.
Dredgers with bucket ladders
These dredgers are essentially modified versions of traditional bucket dredgers; the buckets in these dredgers are arranged in a row and are attached to a wheel that automatically gathers sediments. They are more effective and capable of tearing out strong, tough corals. These dredgers’ poor output, increased need for anchor lines, and loud operation are their only drawbacks, which have rendered them outdated.
They are strong enough to tear out the corals and can be used on a broad range of materials, including soft rock. However, because to their high noise level, poor productivity, and requirement for anchor lines, their utilization has significantly decreased in recent years.
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Dredgers to Grab
Clamshells, also known as grab dredgers, are stationary dredgers that are secured to spud poles or anchors. These dredgers use a grab as their cutting tool, which consists of two half-shells that are wire-operated.
Once the dredged material is filled into these half-shells, the material is loaded onto barges. Grab is installed on a hydraulic excavator or a dragline. These can come in a variety of forms, including waterproof grabs, (top) closed grabs, and (top) open grabs.
While the capacities of grab hopper draggers range from 100 to roughly 2,500 m³, grab dredgers can have capacities ranging from 1.0 to 20 m³. The power of the crane has a major impact on their capacity. Grabs are useful for clearing debris from the corners of basins and docks, and they also facilitate excavating near quay walls.
A grab dredger is a rotating crane with a grab that is positioned on a pontoon or hopper vessel. With a clam-like motion, it gathers sediments from the seabed and releases them, as the name implies. It helps pick up loose sand and clays, and is frequently used for excavating bay mud.
Backhoe Dredgers
These are also known as Dipper dredgers and resemble onshore excavators in certain ways. These are employed in shallow dredging and harbor maintenance. These excavators have hydraulic power and a digging bucket or half-open shell that can dig through a variety of materials.
When the shell or bucket is full, it is pushed in the direction of the machine and unloaded onto barges. The capacity of a bucket can vary from 0.5 to 13 m³. It is important to use caution when dropping this large, stiff bucket because it may harm the quay walls or the canal lining.
Similar to certain onshore excavators, backhoe dredgers are equipped with a digging bucket that may be used to delve through various materials. Once the bucket is fully excavated, it is removed and stored on board barges. Even if they have few restrictions when it comes to deep dredging, deeper excavation is now very simple thanks to some newer, more advanced dredgers.
2. Hydraulic Dredgers
These dredgers’ primary characteristic is that the material they sift is lifted through a pumping system and sent to outlet pipes while in suspension.
Since fine materials are easier to hold in suspension than heavy gravels, these are best suited for displacing fine materials.
Using stronger pumps, gravel and other heavy materials can also be extracted with a hydraulic dredger. Here are some examples of typical hydraulic dredgers:
Suction Dredgers
Usually, these are used to clear the seafloor of sand or silt deposits. Dredged material is sucked with or without a water jet using a vertical suction pipe that is pushed vertically inside the sand deposit. Dredged material is transported straight to the reclamation area in barges or containers.
There are two varieties of these suction dredgers: cutting suction dredgers and profile or plain suction dredgers. The sole distinction between a cutter suction dredger and a plain suction dredger is that the former has a cutting tool, which could be a swinging arc. Other than that, both operate and look similar.
The so-called CSDs have a cutter head at the suction intake that aids in removing dirt and moving it to the suction mouth. When applied to hard surfaces like rock, CSDs use a wear-resistant pump to draw up the dredged soil, which is subsequently released through a pipeline or a barge.
Dredger with a trailing suction hopper
A Suction That Trails Hopper A dredger is a boat that operates on its own. It is made up of trailers or hoppers equipped with bottom gates or valves. After the hopper or trailer is filled, the bottom valves or gates are closed, and the hopper is elevated using cranes or winches. At this point, the material is put into the hopper hydraulically. The most usual places for this dredger to be employed are open waters, such as rivers, canals, estuaries, and open ocean dredging.
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A hopper dredger is a self-propelling vessel that stores its load in a sizable onboard hold known as the hopper. It is mostly useful for harbor maintenance and pipe trenching. They are able to pump the load offshore or open the bottom ports to unload the load after carrying it over long distances. Due to their high output rates, hopper dredges are mostly used for dredging soft non-rock soils, making land reclamation projects simple to complete.
Dredger with Water Injection
This self-driven dredger extracts sediments by using powerful water jets. A strong water jet transforms sediments into suspension; since this suspension is heavier than water, gravity and water currents carry it away and dispose of it at a designated location. This kind of dredger is more frequently employed for harbor upkeep and is typically utilized to dredge mud or fine sand bottoms.
Water injection dredgers, which are frequently utilized for projects with environmental concerns, function by forcing water into the bed material to fluidize it. After it becomes fluid, it is either transported away by the natural current or displaced by another burst of water.
3. Other types of dredgers
These are dredgers that are made for a particular use and do not fit into the hydraulic or mechanical dredger categories. These additional dredger types can all be used in reservoirs, industrial lagoons, and small canals that are shallow waterways. Among them are:
Dredgers that are air-lift and jet-lift
The Venturi effect is the basis of operation for jet-lift dredgers. Injecting a high-speed stream causes the surrounding water and bed material to be forced into a delivery pipe.
With the exception of using high-pressure air as the channel for inducing water or material and disposing of it through a suction pipe, air-lift dredgers are similar to jet-lift dredgers. Both air-lift and jet-lift dredgers are unsuitable for handling strong or hard materials.
Augur suction dredgers
These dredgers function similarly to mechanical cutter suction dredgers, except they have an Archimedean screw that rotates and is positioned perpendicular to the suction pipe. The material is removed by rotating the augur-like screw, which then feeds the removed material into the centrally located suction pipe. When precise vertical and horizontal measurements are required for the material to be dredged, this dredging method is utilized.
Dredger for Reclamation
Typically, this kind of dredger is employed in reclamation operations. Barge unloading dredgers is another name for these.
Dredges powered by air
These operate according to the same theory as a vacuum. They are made up of a high vacuum pressure chamber that is suspended with the use of a crane. Bed material is pumped via vacuum pressure into the inlet chamber, which is then raised by a crane and dumped at the designated location. These dredgers are exclusively used for materials that flow readily.
Dredges that are afloat
These represent a significant technological leap because they can function in both elevated and submerged environments. To do the dredging, these have grabs, buckets, or a shovel installation installed.
A bed leveler or plow
This particular kind of dredger is equipped with bars or a long cutting blade that is fixed to a boat. The dredger scrapes the bed level to the necessary depth as it moves across the bed. It just uses a blade to scrape the dredged material a specific distance, making it ideal for short-length direct dredging.
