Have you ever wondered how the famous Dubai Palm Islands were created?
Palm Jumeirah, Deira Island, and Palm Jebel Ali, as well as the Second Suez Canal and many other artificial islands, channels and ports, are a result of big-scale capital dredging projects. Dredging, however, isn’t only found in remarkable land-shaping operations – it is used to build dikes that keep us safe and deepen the water channels which are used to deliver everyday products to our supermarkets.
Dredging operations take place all around us, wherever there is sufficient water depth to allow a dredger to operate. They serve different purposes and are performed using different dredging styles and different types of dredging.
What is dredging? What are the benefits of dredging?
Dredging is an operation of excavating material from water environments, usually in order to improve existing water features. Dredging processes are used during different projects: deepening water channels, drainage, constructing dams, artificial land, ports, dikes, and other controls for streams and shorelines. These dredging processes remove unwanted material from a specific site to consequently place it in another desired location. On the other hand, dredging can also be used to extract valuable mineral deposits with commercial value, for example, gold.
Dredging activities can, therefore, fall into these categories:
Maintenance Dredging - which is the activity of maintaining existing watercourses: rivers, channels, harbors at the required depth allowing vessels to pass. Tidal movements deposit sand in port basins and waterways, causing their depth to gradually decrease. Maintenance dredging ensures navigable depth, allowing vessels to continue operating.
Capital Dredging is used to describe processes including new civil engineering works, building new islands, harbors, channels, and deepening the existing ones. If the dredging activity changes the map of that region, it is capital dredging. Land reclamation is a perfect example of a capital dredging project. Land reclamation is constructing new land for industrial purposes, in order to build ports, airports or simply residential areas where land is scarce. Frequently, these projects use the soil (sand, clay, rock) dredged at another deepening project nearby.
Mineral Dredging, as the name suggests doesn’t only remove the material to dispose of it, but does it for the purpose of acquiring minerals with an economic value. It can be dredging to extract gold, diamonds, tin, mineral sands but also construction sand, clay, and gravel.
And last but not least, the Environmental Remedial Dredging, which consists of removing polluted soil, which might cause a hazard to public health.
Dredging is also used for coastal protection and offshore trenching. Offshore trenching is part of bigger pipeline or offshore wind park construction projects. For example, whenever the offshore power cables from the Offshore High Voltage Station (OHVS) need to be securely buried in the seabed in order to connect the wind park with the land.
The dredging process consists in general of the following four steps: loosening, extraction, transportation and disposal of the material. A special vessel can loosen and extract the material (sand, mud or rock) and the transportation of the material can be done by a barge or through long pipelines floating on the water surface. It is possible that you have seen such lines during your visit by the seaside.
It is also possible, that you have noticed an interesting action of a vessel spraying sand material in a high arc. This part of the process is called rainbowing. During rainbowing, the unwanted dredged material mixed with water is being emptied from the tanks. The collected material, usually sand, is used to infill sites, build new lands and islands or to replenish eroded sand that has been lost to coastal erosion.
How dredging works
Loosening, extraction, transportation, and disposal of the dredged material would not be possible without proper equipment. There are some main principles of dredging, but how dredging exactly works depends a lot on the equipment used in the process.
What is dredge equipment?
Dredge equipment is all equipment used to perform dredging works, ranging from small, mechanical river barges to large and impressive trailing suction hopper dredger vessels (TSHD) or cutter suction dredgers (CSD). They differ in size and dredging method they were designed to perform:
Types of dredging methods
How do dredgers execute their work? Some dredgers use a hydraulic system and suction (cutter suction dredging, trailing suction dredging), while others remove the material mechanically with the use of backhoes, buckets, grabs, and other equipment.
Having so many options, what influences the choice of the right dredging method?
The choice for the right dredging method is made based on the estimated volumes (cubic metres) and nature / physical characteristics of the material to be excavated. Dredging operations are usually preceded by a geotechnical survey and soil sampling. Different dredging method will be advantageous in case of loose deposits, such as sand and a different in case of rock. Access to and location of the dredging site, as well as depth, also plays a role.
Some of the most common dredging methods fall into these categories:
- trailing suction dredging;
- cutter-suction dredging;
- pontoon-mounted grabbing;
- bucket dredging;
- backhoe dredging;
- airlift dredging
A range of specialized dredging equipment is used to perform each of the mentioned dredging methods.
Types of dredging equipment
Dredgers are usually classified into two groups: mechanical or hydraulic, based on the method used to move the dredged material from the sea-bed to the surface.
Hydraulic dredgers / Suction dredgers
Suction dredgers are efficient dredgers which use powerful centrifugal pumps to raise dredged material through a pipe system. The two most popular types of suction dredger vessels are so-called TSHD’s (trailing suction hopper dredgers) and CSD’s (cutter suction dredgers).
What is a TSHD?
A TSHD, or trailing suction hopper dredger is a vessel equipped with a long pipe. At the end of the pipe, a so-called dredge drag head is installed.
A trailing suction hopper dredger simply trails this long pipe during operation, sucking material from the bottom of the waterbed just like a vacuum cleaner sucks in the dirt. It mixes the sand with water, creates a slurry, which then is moved up the suction pipes.
Just like a vacuum cleaner has bags or bins for the temporary storage of dirt, a trailing suction hopper dredger has hoppers- big storage tanks. When these get full, the TSHD vessel sails towards a disposal area. The collected material is usually dumped in once through special doors in the hull, or pumped out, although some vessels are also equipped with using drag buckets and conveyors.
Bottom discharge is possible through the bottom doors or conical bottom valves. Bow discharge equipment and the bow-coupling system enables a THSD to discharge the slurry via a floating dredge line or by means of a rainbow nozzle. Rainbowing, as we already mentioned, is a directly controlled discharge of the slurry to the shore without floating lines, resulting in arched-shape disposal.
If you are looking for purchasing IHC’s discharge equipment spares, whether bottom or bow discharge, check out our Discharge Equipment catalogue.
Trailing suction dredgers are very efficient when the dredged soil is loose. When faced with firmer soils and rock, a cutter suction dredger (CSD) will be employed instead.
What is a CSD?
A CSD (cutter suction dredger) is a vessel with a characteristic, cutting mechanism visible at its front, on what is called a cutter ladder. Cutter suction dredgers combine the two basic principles of dredging: excavation and suction.
Just like a previously described TSHD was compared to a powerful vacuum cleaner, a CSD can be compared to a big drill, however, as the name suggests, a cutter-suction dredger vessel not only drills but also sucks in the drilled sand through the long pipe.
The cutting mechanism, usually a big cutter head or a dredge wheel, is needed in order to dredge in project sites containing hard surfaces such as rock. The cutting mechanism first loosens the hard bed material, which is then mixed with water and sucked through the suction mouth and a pipe. The sucking process is possible thanks to a sturdy, wear-resistant centrifugal pump.
Different cutter heads, equipped with different types of cutter teeth are needed for different soil types. For example, the teeth used for dredging sand have a different shape than the teeth that need to cut through hard rock. Some cutter heads are equipped with exchangeable cutter teeth, which makes it possible for the vessel to adjust the cutter head to the soil.
For more information on the types of cutter teeth, please go to our post: Buyer’s Guide To Cutter Teeth.
Some cutter heads are designed to be more effective for a particular soil type. For example the newly developed Lancelot cutter head which, when used for loose soil, makes better use of the available power, resulting in a higher output thanks to its cylindrical shape and additional arms.
It is also worth mentioning that the cutter head is not fixed in position. It is moved from left to right in a swinging movement and the movement is controlled by anchors and winches. Thanks to this system, it can dredge a big area of the waterbed.
As you might have noticed, there is no letter “H” in CSD, as cutter suction dredgers do not have a hopper. Where is the material stored? Instead of a hopper, the collected material is emptied through a dredge line.
Dredge Lines and Dredge Line Components
Dredge lines are the wear-resistant pipelines and rubber hoses floating behind the cutter suction dredger, used to transport the slurry to the shore.
The slurry is a mixture of solids and water, which was created to make the movement of solids (sand, clay) through a hose possible.
The use of dredge lines calls for reliable and effective flow control, and that is why the system is equipped with wear-resistant valves. Turning glands are also parts of the system, allowing pipes and components to safely rotate around their axes, ensuring maneuverability and greater safety. Different dredge line components, including hoses, pipes, gate valves, and floaters as well as discharge equipment can be purchased easily on ShipSupport’s store.
SEE ALSO: How to replace rubber rings in a gate valve?
Other types of hydraulic dredgers
Other, less common types of suction dredgers are auger dredgers, jet lift dredgers, and air-lift dredgers.
An auger suction dredger functions similarly to the cutter suction dredger, but it has an environmentally sound cutting tool that lies horizontally on the seabed, working precisely to loosen thin layers of sediment. The jet-lifts working method is different – a jet-lift uses a high-speed stream of water to pull the nearby water mixed with bed material, into a pipe. An Air-lift works by blowing air into the pipe, which rises inside the pipe, dragging water with it.
The efficient operation of hydraulic (suction) dredger depends to a great extent on its pump. Their role is vital, as dredge pumps create a vacuum that sucks the material from the seabed up into a suction pipe.
How a dredge pump works
Dredge pumps are centrifugal pumps with a built-in impeller, which is connected to the drive motor through a shaft. By moving, the impeller creates a centrifugal force, which causes the vacuum to suck up the slurry. The capacity of a dredge pump is expressed in maximum cubic metres per hour.
Every suction dredger has at least one dredge pump installed, and some of them might have and additional submersible dredge pumps. Jet pumps can also be encountered on dredging vessels.
Water jet pumpHow does a water jet pump work? It is an axial-flow pump, a centrifugal pump, or a mixed flow pump which consists of a water intake and a nozzle to direct the flow of water out. Water jet pumps are very common in the maritime world. These are centrifugal pumps usually used for propulsion. In the case of dredging, they also have another application. Used mostly on trailing suction hoppers they use water pressure to liquefy the sand. They can be installed on a dredger drag head, where they create jet water which helps lift up the sand, or, inside the hopper, where they are used to liquefy the collected sand which became too solid and make it suitable for discharge.
Mechanical dredgers usually fall into three categories: backhoe dredgers, bucket dredgers and grab dredgers.
What is a Backhoe Dredger (Backhoe/Dipper Dredger)?
A backhoe dredger is a hydraulically operated excavator that is performing the dredging works using a half-open shell. Similarly to land digging machines, it excavates the ground, lifts it up and loads it, usually on barges or onshore. This machine is mainly used in harbors and other shallow waters. The size of backhoe dredgers is expressed by their bucket capacities.
What is a Grab Dredger?
Similar to land excavator grabs seen on construction sites, a grab dredger’s dredging tool; a grab, usually consisting of two half-shells operated by wires or (electro)-hydraulically. The grabs can be installed on backhoe type hydraulic excavators or draglines.
Grab dredgers can be found on pontoons or on self-propelled vessels which have a hopper to store the dredged material. In the case of pontoon type grab dredgers, their size is determined by the capacity of their grab bucket. In case of self-propelled grab hopper dredgers size is expressed in terms of the hopper, as they might have more than one grab installed onboard.
What is a Bucket Dredger?
Bucket dredgers are one of the oldest types of dredgers. A bucket dredger has an endless chain with buckets that scrape over the bottom of the waterbed, scoop the material, move it up, tip over and empty once they reach the barge. Once emptied, each bucket returns empty on the underside of the chain to the bottom of the ladder where it is ready to scrape another portion of the soil. The size of bucket dredgers is usually expressed by the capacity of their buckets.
Other, less common, mechanical dredger types are bed leveler dredgers, water injection dredgers, and pneumatic dredgers.
Every day, dredging project members and dredger vessel crews work hard in remote locations around the world to perform maintenance dredging works or bring new capital dredging processes to life. What makes dredging interesting is that it has a direct effect on the world around us. It keeps countries safe from rising water levels, it maintains water channels operational but it also reshapes existing lands, altering the maps of the world.
We hope that this article gave you a better insight into what dredging is and how it is performed. Would you like to know more about upgrading your dredging equipment or purchasing dredger spare parts? Do not hesitate to contact us.
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