What is Dredger and Types of Dredgers Used in the Maritime Industry? Let’s find out

    Dredger is a marine vessel fitted with a device(s) to scrap or suck the sediment deposition over sea bed is known as a dredger (The device used for excavation and scraping of the sea bed is called the Dredge).

    Dredger is Dredging
    Dredger in progress

    In a more general sense, a ship equipped with excavation tool which is capable of weeding off depositions such as sand, gravel, sediments, etc. from the seabed is referred as dredger ship or more commonly a dredger.

    Dredgers are of great importance, as they serve the purpose of ensuring the necessary safe bottom clearance for the safer voyages.

    The excavation carried out in either shallow or fresh waters with the aim to gather up the sediments located in the bottom to dispose off at another place is called Dredging.

    The sediments might be gathered for purposes like:

    • making the 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

    Although dredging can have very harmful effects on the marine and aquatic environment, in some situations it may be the only option available.

    Possible reasons for dredging include improving existing water features; reshaping land and water features to alter drainage, navigability, and commercial use; constructing dams, dikes, and other controls for streams and shorelines; and recovering valuable mineral deposits or marine life having commercial value. In all but a few situations the excavation is undertaken by a specialist floating plant, known as a dredger.

    Dredging is carried out in many different locations and for many different purposes, but the main objectives are usually to recover material of value or use, or to create a greater depth of water. Dredges have been classified as suction or mechanical.

    Dredging has significant environmental impacts: it can disturb marine sediments, leading to both short- and long-term water pollution, destroy important seabed ecosystems, and can release human-sourced toxins captured in the sediment.

    Type of Dredgers

    Dredgers

    Broadly the types of dredgers are classified into three categories (On the basis of the method employed for transportation, of dredged material from the bottom of the sea to the surface of the water):

    • Mechanical dredgers which are suited for working in confined areas and are useful for removing the hand-packed material or debris,
    • Hydraulic dredgers which work on the principle of adding large amounts of process water to change the original structure of the sediments, and
    • Other dredgers which do not fit into the above two categories.

    Whether mechanical or hydraulic, the different types of dredgers that help in the removal of the seabed sediments are:

    1. Mechanical Dredgers

    These are available in a variety of forms, but each has the same working principle of “hand-packing”. These are equipped with a grab or a bucket, which is driven on the loose bed sediments, then material get filled in the bucket and the bucket is then raised to transport it to the requisite disposal site. Some common mechanical dredgers are briefed as follows:

    (i) Bucket Dredgers

    These are the oldest type of mechanical dredgers. These are fixed on anchors so considered as stationary dredgers but can be moved along the semi-arcs while dredging, with the help of winches.

    An endless chain of buckets is provided in bucket dredgers, these buckets scrap off and fill the loose dredged material in them and after being completely filled, these buckets can be emptied into barges by turning the bucket upside down over the barges.

    (ii) Bucket Ladder Dredgers

    These kind of dredgers are just modification of conventional bucket dredgers, the series of buckets used in these dredgers are mounted on a wheel which mechanically picks up the sediments. They are more efficient and can be used to rip out even powerful and hard corals. The only limitations of these dredgers are their low production, the requirement of more anchor lines and high level of noise, which made them obsolete these days.

    They can be used for a wide variety of materials including soft rock material and are powerful enough to rip out the corals as well. But because of their low production, high level of noise and the need for anchor lines, their use has hugely diminished in recent times.

    (iii) Grab Dredgers

    Grab Dredgers (or Clamshells) are also stationary dredgers fastened either on anchors or on spud-poles. The cutting tool, of these dredgers, is a grab which consists of wire operated two half-shells.

    These half-shells after filling dredged material in them, load the material in barges. Grab is mounted either on a dragline or hydraulic excavator. These can be of different types such as (top) open grab, (top) closed grabs and watertight grabs.

    Grab dredgers can be of different capacity ranging between 1.0 and 20 m³ whereas the capacity of grab hopper draggers ranges from 100 to about 2,500 m³. Their capacity largely depends upon the crane power. Grabs are efficient in removing material in corners of dock and basins and also these make excavation closer to quay walls easier.

    A revolving crane, fitted with a grab, placed on a hopper vessel or pontoon is known as a grab dredger. As the name suggests, it picks up the sediments at the seabed with a clam grabbing motion and discharges the contents. Often used for excavating bay mud it also is useful to pick up clays and loose sand.

    Dredgers working

    (iv) Backhoe Dredgers

    These are also referred to as Dipper dredger and are somewhat similar to onshore excavators. These are used for harbour maintenance and shallow dredging. These are hydraulically driven excavators and consist of a half-open shell or a digging bucket which is capable of digging across a wide range of materials.

    The shell or bucket is moved toward the machine and when filled, emptied in barges. Bucket capacity ranges between 0.5 and 13 m³. Care should be taken while dropping this heavy and rigid bucket as it can cause damage to canal lining or quay walls.

    Like some onshore excavators, Backhoe dredgers have a digging bucket attached to it which digs through a wide range of materials and when it is excavated it’s brought out and placed on the onboard barges. Although they have few limitations where deep dredging is concerned but with some recent modern dredgers, deeper excavation is made quite easy.

    2. Hydraulic Dredgers:

    The key feature of hydraulic dredgers is that the material dredged by these type of dredgers is in suspension form and raised through the pumping system and fed to outlet pipes.

    These are most suitable for dislodging fine materials because it is easier to hold fine materials in the suspension than heavy gravels.

    Gravels and other powerful material can also be removed through hydraulic dredger by using greater power pumps. Some common hydraulic dredgers are as under:

    (i) Suction Dredgers:

    These are generally employed to remove sand or silt deposits from the seabed. They have a vertical suction pipe, which is pushed vertically inside the sand deposit and dredged material is sucked with or without water jet. The dredged material is laden into barges or can directly to the reclamation area.

    These suction dredgers can be of two types viz. profile or plain suction Dredgers and Cutting Suction Dredgers. Rest of the working and design of cutter suction dredger is similar to the plain suction dredger but has the only difference that cutter suction dredger is equipped with a cutting tool which may be swinging arc.

    CSD, as they are normally called, have a cutter head at the suction inlet which helps to loosen the earth and take it to the suction mouth. Used for hard surfaces like rock, CSDs suck up the dredged soil with the help of a wear-resistant pump and then discharge it through a pipeline or a barge.

    (ii) Trailing Suction Hopper Dredger:

    A Trailing Suction Hopper Dredger is a self-driven dredging vessel. It consists of hoppers or trailers with bottom gates or valves. The material is loaded in hopper hydraulically when this hopper or trailer is filled, the bottom valves or gates are closed and the hopper is raised up with cranes or winches. This dredger is most commonly used in open water: rivers, canals, estuaries and the open sea dredging.

    Suitable mostly for harbour maintenance and pipe trenching, a hopper dredger is a self-propelling vessel that holds its load in a large onboard hold knows as the hopper. They can carry the load over large distances and can empty it by opening the bottom doors or by pumping the load offshore. Hopper dredges mostly dredge the soft non-rock soils and because of their high production rates can carry out land reclamation projects easily.

    (iii) Water Injection Dredger:

    It is a self-driven dredger which excavates sediments with strong water jets. Strong water jet converts the sediments into suspension, as this suspension is heavier than water, it is carried away by water currents and gravity and disposed-off at a specific site. This type of dredger is generally used to dredge mud or fine sand bottoms and are more commonly used for harbour maintenance.

    Often used for environmentally sensitive projects, water injection dredgers work by fluidizing the material by pumping water into the bed material. Once it is fluidized it is either moved by a second burst of water or is carried away by the natural current.

    Purposes of Dredger & Dredging

    • Capital dredging: dredging carried out to create a new harbour, berth or waterway, or to deepen existing facilities in order to allow larger ships access. Because capital works usually involve hard material or high-volume works, the work is usually done using a cutter suction dredge or large trailing suction hopper dredge; but for rock works, drilling and blasting along with mechanical excavation may be used.
    • Land reclamation: dredging to mine sand, clay or rock from the seabed and using it to construct new land elsewhere. This is typically performed by a cutter-suction dredge or trailing suction hopper dredge. The material may also be used for flood or erosion control.
    • Maintenance: dredging to deepen or maintain navigable waterways or channels which are threatened to become silted with the passage of time, due to sedimented sand and mud, possibly making them too shallow for navigation. This is often carried out with a trailing suction hopper dredge. Most dredging is for this purpose, and it may also be done to maintain the holding capacity of reservoirs or lakes.
    • Harvesting materials: dredging sediment for elements like gold, diamonds or other valuable trace substances. Hobbyists examine their dredged matter to pick out items of potential value, similar to the hobby of metal detecting.
    • Fishing dredging is a technique for catching certain species of edible clams and crabs. In Louisiana and other American states, with salt water estuaries that can sustain bottom oyster beds, oysters are raised and harvested. A heavy rectangular metal scoop is towed astern of a moving boat with a chain bridle attached to a cable. This drags along the bottom scooping up oysters. It is periodically winched aboard and the catch is sorted and bagged for shipment.
    • Preparatory: dredging work and excavation for future bridges, piers or docks or wharves, This is often to build the foundations.
    • Winning construction materials: dredging sand and gravels from offshore licensed areas for use in construction industry, principally for use in concrete. This very specialist industry is focused in NW Europe, it uses specialized trailing suction hopper dredgers self discharging the dry cargo ashore. Land based old river beddings can be processed in this manner too.
    • Contaminant remediation: to reclaim areas affected by chemical spills, storm water surges (with urban runoff), and other soil contaminations, including silt from sewage sludge and from decayed matter, like wilted plants. Disposal becomes a proportionally large factor in these operations.
    • Flood prevention: dredging increases the channel depth and therefore increase a channel’s capacity for carrying water.

    Environmental impacts of Dredgers

    Dredging can create disturbance to aquatic ecosystems, often with adverse impacts. In addition, dredge spoils may contain toxic chemicals that may have an adverse effect on the disposal area; furthermore, the process of dredging often dislodges chemicals residing in benthic substrates and injects them into the water column.

    The activity of dredging can create the following principal impacts to the environment:

    • Release of toxic chemicals (including heavy metals) and from bottom sediments into the water column.
    • Collection of heavy metals lead left by fishing, bullets, 98% mercury reclaimed [natural occurring and left over from gold rush era].
    • Short term increases in turbidity, which can affect aquatic species metabolism and interfere with spawning. Suction dredging activity is allowed only during non-spawning time frames set by fish and game (in-water work periods).
    • Secondary impacts to marsh productivity from sedimentation.
    • Tertiary impacts to avifauna which may prey upon contaminated aquatic organisms.
    • Secondary impacts to aquatic and benthic organisms’ metabolism and mortality.
    • Possible contamination of dredge spoils sites.
    • Changes to the topography by the creation of “spoil islands” from the accumulated spoil.
    • Releases toxic compound Tributyltin, a popular biocide used in anti-fouling paint banned in 2008, back into the water.

    The nature of dredging operations and possible environmental impacts cause the industry to be closely regulated and a requirement for comprehensive regional environmental impact assessments with continuous monitoring. The U.S. Clean Water Act requires that any discharge of dredged or fill materials into “waters of the United States,” including wetlands, is forbidden unless authorized by a permit issued by the Army Corps of Engineers. As a result of the potential impacts to the environment, dredging is restricted to licensed areas only with vessel activity monitored closely using automatic GPS systems.

    Major dredging companies

    According to a Rabobank outlook report in 2013, the largest dredging companies in the world are in order of size, based on dredging sales in 2012.

    • China Harbour Engineering (China)
    • Jan De Nul (Belgium)
    • DEME (Belgium)
    • Royal Boskalis Westminster (Netherlands)
    • Van Oord Dredging and Marine Contractors (Netherlands)
    • National Marine Dredging Company (United Arab Emirates)
    • Great Lakes Dredge & Dock Company (United States)
    • Manson Construction Co. (United States)
    Share.

    Leave A Reply