Last Updated on June 15, 2022
Have you ever wondered how people carve out and remove material from the seafloor? The big answer comes in the form of a powerful machine. Dredgers are watercraft that remove and relocate the contents (sand, sediment, mud, etc.) from the bottom of a body of water.
An area is dredged for various reasons, including:
- lowering the bed level (for safer navigation by boats or to allow for larger vessels)
- creating new harbors
- removing contaminants
- replenishing beach sand
Dredgers come in a variety of sizes and types. Names for types of dredgers reflect either the design, the application of the machine, or the material being manipulated.
There are three classifications of dredgers: mechanical, hydraulic, and other. Almost all dredgers fall into either the category of mechanical, which is sometimes called bucket. The other category is hydraulic, which is also called suction.
Dredgers are either stationary, portable, or self-propelled. Stationary dredgers stand on columns (called spuds) and need to be constructed in place. Portable dredgers can move, but not by their own power. Self-propelled dredgers can be driven (using their own power) across waterways.
Hydraulic dredgers usually use suction to remove material such as sand, silt, even gravel, from the seabed. A pipe extends down from the dredger and sucks the seafloor at a vertical angle. Pumps in the pipe create suction, which brings the material to the surface. Then the material is oftentimes laid onto barges and transported elsewhere or sent into pipelines. They don’t work well with large rocks and compacted earth.
Their vertical suction pipe pushes inside the sand deposit and the dredged material is sucked with or without a water jet. Many hydraulic dredgers are equipped with one or more jet water pumps to assist either the beaching process or to improve the mixture forming process near the suction mouth.
There are several types of hydraulic dredges, including plain suction dredges, cutterhead suction dredges, trailer suction hopper dredgers, and water injection dredgers.
1. Plain suction dredgers
A plain suction dredger is always stationary and is capable of achieving good production when the material is loose. This machine reaches depths of more than 300 feet. They are frequently used for reclamation and to gather sand for the concrete industry.
There are three types of plain suction dredgers: standard, deep, and dustpan. The standard plain suction dredger has been described above. The deep suction dredger is similar to the plain suction, but is better equipped for greater depths, especially in excess of 100 feet. And the dustpan suction dredger comes equipped with a dustpan-shaped nozzle. This provides a low and wide cut compared to the traditional, circular-shaped nozzle on other plain suction dredgers.
2. Cutterhead suction dredgers
A cutterhead suction dredger is similar to the plain suction dredger, but is differentiated by being equipped with a cutterhead located at the suction opening. The cutterhead loosens the material and the cutting knives help bring the material to the suction opening. Due to its cutting power, these dredgers are used for hard surfaces and rocks. They can either be stationary or portable. Portable cutterhead suction dredgers are the industry standard for large-scale production projects. Examples include dredging sand and gravel, deepening channels or canals, and reclaiming land lost to erosion.
3. Trailing suction hopper dredgers
The trailing suction hopper dredger (THSD) is self-driven. This dredger can be used at sea or on inland waterways, and is sometimes confused for a large boat or barge. The dredger comes equipped with a suction pipe, a hopper (or trailer) for holding material, and a means of emptying the hopper. Dredging is achieved at a very low speed-between two and three knots per second. When the hopper reaches capacity, the dredger stops dredging, lifts the pipe, and moves to the location where the material will be deposited. Gates (or valves) open to empty the material and then the machine returns to dredging.
The size of the hopper volume of a TSHD is its most important size metric. Sizes range from a few hundred cubic yards up to more than 30,000 cubic yards. The machine works best with soft clays, silt, sand and gravel. Since the THSD is a free-moving vessel, it doesn’t hinder other shipping while dredging. That’s why this dredger gets used in higher traffic areas, such as harbors and shipping channels.
4. Water injection dredgers
The water injection dredger is a hydraulic dredger, since it uses water as a dredging tool. This machine doesn’t use suction. Instead these mechanisms “liquify” the sea floor by injecting water below its surface. This causes the top portion of the sea bed to mix with the water—called fluidization—and the current removes a lot of the material downstream. This means you don’t have to worry about relocating it yourself. This process is often used for environmentally sensitive projects or at smaller ports for maintenance dredging.
Mechanical dredgers use a bucket to cut into the surface floor and raise the material above sea level. They are often called bucket dredgers, because that is the most popular dredging tool on mechanical dredgers. However, a variety in the types of buckets including grapples, dippers, and ladders, allows for greater diversity and better performance from these machines.
Although mechanical dredgers can be stationary, portable, or self propelled, they are always stationary when dredging. They are moored either by poles or by anchors. The reason is because the activity of cutting into the sea floor and raising the mechanical arm to the surface can reposition the dredger. The dredger operator needs to reposition the buckets right next to the spot where they just cut, and the best way to do that is to remain in the same spot. Because the arm on a bucket dredger rotates and the dredger remains positioned for several cuts, a group of cuts will form an arc shape. Depth is only determined by the length of the arm, however, accuracy decreases with depth.
1. Dipper dredgers
Dipper dredgers are also known as scoop dredgers and backhoe dredgers. They derive their name from a mechanical arm attached to a bucket, similar to a backhoe, with which they scoop the sea floor. These machines bring the material to the surface, and dump it into barges. Bucket capacities range from less than one cubic yard to 17 cubic yards. These dredgers are well-suited for harbor maintenance and shallow dredging.
2. Clamshell dredger
Also known as grab dredger or a grapple dredger, this type of dredger is the most commonly used dredger in North America.
This dredger features two clamshell-looking buckets fastened to the end of a mechanical arm extended below the surface of the water. The buckets can move to and from one another; like the jaws of a mouth. The buckets open up to take a “bite” into the sea floor by closing the buckets, and then the arm brings the load above surface.
Bite size varies from a few cubic feet to a few hundred cubic feet. Larger clamshell dredgers are used for bulk dredging. Smaller dredgers are used for jobs such as dredging hard to access places in harbors, small quantities with strongly varying depth levels, along quay walls, and for sand and gravel. This mechanism works best when dredging materials such as soft clay, sand, or gravel. For soft soils lighter dredgers are favorable, whereas smaller, heavier dredgers are preferred for heavier soils.
A variation of the clamshell dredger is the orange-peel dredger. The only distinction is the shape of the buckets, which resemble a portion of an orange peel versus a clam’s shell.
3. Ladder dredgers
Also known as elevator dredgers, ladder dredgers use numerous buckets positioned one in front of another, attached to an oval-shaped line called a ladder. The ladder reaches as low as the seafloor to as high as several feet above deck. The buckets move on the line into the water, down to the sea floor, where it scoops material, and up to the surface, where it dumps it, before following the line back down to the bottom. Having multiple buckets operating in succession makes for near continuous dredging operation.
Ladder dredgers are classified as either stationary, self-propelled, barge-loading, or sea-going hoppers. Self-propelled dredgers often operate in rivers and calm waters. The second and third types are sea-going dredgers, however, the second one is limited to calm waters, such as ports or estuary channels. The third is a full sea-going vessel, comprising both barge and dredge in one.
4. Sand dredgers
A sand dredger is either being used to dredge sand or has features or components that make it optimized for dredging sand. They can be suction/hydraulic, since hydraulic dredgers perform well when moving loose, fine material. Or they can be mechanical and come equipped with various attachments like buckets, blades, or nets for altering sand. Sand dredgers are used extensively for the construction and landscaping industries, since sand can be used in the creation of concrete or as a decorative material.