Anchor-boom or anchor handling boom
A boom used to place and reposition the anchors.
A backhoe dredger consists of a hydraulic crane with a heavy bucket that is mounted on a pontoon or a self-elevating platform. A backhoe dredger is mainly used to dredge hard materials like rock, boulders, underwater dams, removal of old revetments etc.
Barges are ancillary equipment, used for the transport of soil. There are all kinds of possibilities to use barges. One of the most common is, that a dredging installation dredges the soil and disposes it in the barge. As soon as the barge is loaded, it sails with its own propulsion installation or with the help of a tug boat, to the dumping place. Barges discharge by means of bottom doors, they can also be split barges and there are also barges with no means of discharging at all. These will be unloaded with the help of for instance a barge unloading dredger.
The dredged material can be transported over long distances via pipelines. However, the pressure in pipelines gradually, due to friction and bends in the pipeline. This limits the distance over which the soil can be transported. A booster station can be added to the installation to extend the transport distance. A booster station is basically a combined engine and pump room, which can be installed in a pontoon or ashore to boost the pressure.
Part of a TSHD to which a floating discharge hose can be connected. As the name says the bow coupling installation is located at the bow of the dredger and is used to discharge the soil via shore pumping/pipelines.
Bow thrusters are propulsion devices that are installed in the bow of a vessel to increase maneuverability. They are ideal for navigation in confined spaces, such as port basins or narrow channels, and they are also used to keep the dredger in place during rain bowing or discharge by pipeline.
These are natural (quarry stone) or artificial (concrete blocks) elements that make up part of the layers for mound dikes and for other marine (foundation haunches for example) or hydraulic (dam layers) works, orderly or randomly positioned as a protection against the erosion caused by the swell. Concrete blocks come in a large variety of shapes that are selected according to their different properties for each job: dolos, cubes, tetrapods, quadripods, tribars, etc.
A hollow structure that forms part of the marine works’ section, (dikes, quays, etc.) after being transported and delivered to the site on the ground or in the sea, subsequently being filled with selected material.
Dredging activities focused on creating new harbors, berths or waterways, or to deepen or widen existing facilities.
Centrifugal dredge pumps
Centrifugal dredge pumps lie at the heart of every dredging challenge. The pumps provide the suction power to transport the soil mixture from the bottom of the dredge site to a hold or the discharge site through pipeline. Their power, capacity, resistance to wear and malfunction, and general robust and reliable character are essential features. Dredge pumps come in a variety of sizes, designs and materials and can be installed onboard, inboard, submerged and in booster stations.
A charter contract means that the customer hires the equipment and personnel of the contractor and informs the contractor where to dredge. The contractor is hired at a basic rate per day or per hour.
Christmas tree installation
In conditions with waves or swell, positioning a dredger by means of spuds is not advisable. In that case, a central anchor system, also known as a Christmas tree installation can be used.
Coastal protection is an essential process to provide a defence against flooding and the erosion of land caused by waves, tides, currents and wind. Beach re-nourishment and dike construction are some examples of coastal protection works. Both require the recovery and transportation of large volumes of sand.
The critical velocity is the velocity below which the grains in the mixture start falling out of suspension and start forming a layer at the bottom of the pipeline.
Cutter suction dredger
The cutter suction dredger (CSD) is a type of dredger that uses a rotating cutting tool to dislodge the soil in order for it to be drawn into a suction pipe. A CSD typically consists of a non-self-propelled pontoon with in a pivoting ladder that houses the cutter and suction pipe. The types of soil it can handle are sand, gravel, clay, sandstone and soft rocks. Cutter suction dredgers are widely used for land reclamation projects, the construction of dams, dikes and breakwaters, the excavation of new harbour basins and canals, the improvements of the course of a river, etc.
A cutterhead is a piece of dredging equipment used on cutter suction dredgers. The cutterhead dislodges the soil through a rotating motion, so that the slurry can be drawn into the suction pipe. Since the dredging process starts with excavation and slurry creation, a CSD’s performance is mainly defined by its cutterhead. Different soil types demand different forms of penetration. Therefore, there is a wide variety in cutterheads available.
With a design/construct contract the customer will supply a concept with the main requirements of the project. Contractors have to include a detailed project design in the tender offers or pre-qualification documents.
A trailing suction hopper dredger can empty its hopper in various ways. Depending on the project specifications, the discharge takes place by opening the hopper doors, pumping the material to shore or by ejecting it using a rainbow installation.
For a cutter suction dredger transport of dredged materials goes through pipelines. With several pumps placed in series the dredged materials can be transported by pipelines over long distances.
At the entrance of each suction arm of a trailing suction hopper dredger (TSHD), a draghead is connected. This draghead collects the soil just like the mouth of the vacuum cleaner at home. As the first point of contact with the soil a draghead is of vital importance to the entire dredging process and it comes in many forms. Dragheads can be fitted with tools such as water jets, blades and rippers.
This is a barge used for the transport and dumping of dredging or filling debris. It is made up of some central open-air containers on a floatable perimeter which are limited by sluices incorporated in the hull and operated by chains and winches, so that when these are opened they unload the material letting it fall. Depending on the size of the boat it will have one or various individually operated compartments. The dragline excavators are towed or autonomous and they can also be used for transporting rock material. They are loaded at the sides of the dredges or from quays when dealing with fillings.
Dredging can be defined as the underwater removal of soil and its transport from one place to another for purposes of deepening or making profitable use of the removed soil.
Dredging control systems
Dredging control systems enable efficient monitoring and control of vital dredge process equipment either manually, automatically or through the use of artificial intelligence.
A dredging wheel is a special type of equipment used on cutter suction dredgers or specialized wheel dredgers to excavate soil. The wheel consists of a crown of bucket shaped blades, rotating in the direction of the ladder. Dredging wheels can be used for different soil types, from peat and clay to sand and so rock. Due to its operational features like high concentration dredging and high selectivity, a dredging wheel is often used in mining applications.
Environmental dredging targets polluted materials in sections of ports and rivers, often within industrial areas. These remediation projects require accurate dredging, low generation of turbidity and low spillage in order to remove contamination without harming the surrounding environment.
A frame contract is often used in large dredging projects. The contract describes the overall scope of the work and a global timing. The customer and the contractor (or a consortium of contractors) will then negotiate a detailed share contract for the different parts of the project.
The structure that holds the suspension gear of the ladder.
Small dredging operations and parts of large projects are executed with grabs or clamshells. The grabs can be mounted on stationary pontoons, either with anchors or spuds, or on self-propelled vessels. Especially near constructions in ports and rivers the grab can be operated accurately and safely.
Grain size distribution
Each soil consists of grains of different sizes. The size of the smallest grain and of the largest grain may vary widely from the average size. That is why the distribution of the grain sizes of the soil must be known. To determine this, the soil is sieved. The result of this operation is represented as a grain size distribution diagram.
This is a structure constructed for coastal protection whose purpose is to collect the coastal deposition or delay erosion. When various groynes are necessary to act together this is known as a groyne field.
Head of a dredge pump
The head is a characteristic that is used to define a dredge pump’s capacity. This is the height of the liquid column that a pump could create, and is measured in metres.
The hold of a dredging vessel in which the dredged soil is stored for transport. The spoil can either be discharged at a dumping site by opening doors or valves in the hopper bottom, or delivered to a shore pipeline with the aid of the vessel’s pumps.
Hydraulic dredging is dredging by sucking up the soil, much like a vacuum cleaner at home.
This is a fixed or floating structure, normally an open construction or on posts, that penetrates the water from the water’s edge serving as a berthing quay or as a landing stage.
The ladder of a cutter suction dredger is the arm that houses the suction pipe. The cutterhead is also connected to the ladder, near the entrance of the suction pipe. The ladder can be accurately lowered or hoisted with a gantry and winches. One or two centrifugal dredge pumps are installed in the pump room and some dredgers also have a submerged pump on the ladder for additional pumping power.
Lakes and reservoirs dredging
With an increasing demand for clean energy and drinking water, the construction and maintenance of water reservoirs and lakes used for hydropower is on the rise . Dredging also tackles the growing problems of aquatic weed infestation and excessive sedimentation in existing water reservoirs, improving the availability of clean drinking water and the reliability of the power supply.
The process of creating new land in the ocean, riverbeds or lakes. The dredging process uses sand, clay or rock from the seabed to create new land elsewhere.
Lump sum contract
A lump sum contract is also known as a fixed price contract. Customers and contractors agree that the project will be carried out at a fixed total price.
Dredging work aimed at maintaining a navigable depth in waterways and access channels that are threatened by siltation. Maintenance dredging is also necessary in water reservoirs and hydropower lakes where sedimentation occurs.
Some customers, such as public companies and other governmental organisations, own dredging equipment, but lack the knowledge or do not have the right staff to carry out a dredging project efficiently. In that case, the customer can hire a contractor to operate and maintain the dredging equipment.
Marine aggregates dredging
Marine aggregates, such as sand and gravel, play a crucial role in modern day life. These natural resources are widely used in the construction industry and, with the global population rising, the demand for construction aggregates is also increasing. Dredgers are deployed to collect and transport the sand and gravel from offshore extraction areas.
In most countries, the contractors have to operate under the conditions of the international ‘MARPOL convention’. This means that all the waste from the dredger has to be collected and transported to a designated deposit. Bilge water must be treated with separators before pumping overboard.
General basis of the classification of soils as composed by the PIANC (Permanent International Association of Navigation Congresses) based on particle size. The main soil types are: boulders/cobbles, gravels, sands, silts, clays and peats and organic soils.
Mechanical dredges dredge by means of excavation. Typical mechanical dredgers are: the bucket dredger, the backhoe dredger and the grab dredger. The cutter suction dredger uses a combination of both hydraulic and mechanical dredging techniques.
Dredgers are used for wet excavation in the mining industry. Dredgers excavate the natural resources from a river, near shore or from man-made ponds and pump them directly into mining separation plants for processing.
There are two basic types of dikes, which are mainly shielding structures (although they can also act as coastline barriers and/or coastal protection): ones with a vertical face and ones with a sloping face (mixed types exist, that work in one way or another depending on the sea level). The first ones, mound dikes or breakwaters, normally consist in a one-piece stone element protected by one or more blocks, or special pieces of concrete breakwater layers of a larger size on which the waves break, and can have a screen wall on their crown.
Offshore power cables, pipelines and umbilicals are buried in the seabed to keep them in place and protect them against heavy objects. Dredgers are used to create a trench in which the cable or pipeline is then buried.
A procurement strategy in which all contractors can bid on a dredging project.
A mechanism found in hopper dredgers that releases the surplus of water in the hold or hopper. While loading, sand mixed with water is pumped into the hopper. At a certain moment the maximum load of the hopper is reached which consist of sand and water. To load more sand the excessive water must be flushed overboard. This is possible by using the overflow.
Dredging activities for port construction and expansion include the creation of access channels, berthing places and turning basins, reclaiming land for yard and terminal activities, and the deepening and widening of existing waterways.
The process of selecting a contractor for a certain dredging project.
This is a structure constructed at the side of a port, river or canal where boats (crafts) can berth and operate from. It can also be a floating structure.
Rainbowing is a discharge method used by trailing suction hopper dredgers in shallow locations or ashore, in for instance land reclamation and beach nourishment projects. The sand from the hopper well is ejected out of the rainbow installation at the bow of the vessel in the shape of an arc.
On some dredging projects, hard rock needs to be removed before dredging operations can begin. Even the strongest cutter suction dredgers will fail in solid, hard rock. The removal of rock then has to start with pre-fracturing. This can be done with the aid of explosives or mechanical rock breakers. A rock breaker generally 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.
A type of dredger that is equipped with propulsion devices to allow independent movement.
A procurement strategy in which only pre-selected contractors are invited to bid on a project.
To transport the dredged materials such as sand or clay through a pressurised pipeline, it has to be mixed with water. This mixture of solids and water is known as slurry. Transport of the dredged material via a pipeline is therefore referred to as slurry transport.
Spill or spillage is the quantity of material that is cut loose by a dredger, but doesn’t end up at the discharge site. In all earth moving equipment a certain loss or spill occurs.
Split hopper or split hull vessel
Split hopper dredgers of barges are divided over their entire length and spoil discharge is achieved by swinging the two halves apart.
When a dredger is equipped with a spud carriage system the working spud is placed in a carriage which, with the aid of a hydraulic cylinder, can travel over several metres in longitudinal direction of the pontoon. That way, the dredger can move forward, without having to reposition the spuds. A number of cuts can be executed without using the walking spud, making the dredging process more efficient.
A spud pole is a pole that is driven into the seabed or riverbed in order for a dredger to remain stationary during dredging operations.
A spud system is used to position a stationary dredger. A spud system can be found on a cutter suction dredger or any other pontoon-like dredger. On CSD’s a spud system consists of two heavy spud poles, the so-called working spud and the auxiliary or walking spud. One of the spud poles is always lowered into the soil in order to keep the dredger in position. With two side anchors, connected on either side of the ladder, the dredger can swing an arc around the working spud pole, excavating almost any type of soil in a very accurate way. Lowering the second spud -the walking spud- during a short period enables the operator to reposition the dredger, making a walk-like motion.
In the course of years, dredging components have been developed and dredgers designed capable of producing the best possible results in the widely differing circumstances in which they were required to operate. Many dredging components have been standardized, and today a custom-built dredger can often be built up from or incorporate standard parts, with a consequent reduction of building costs and delivery period.
Stationary or non-self-propelled dredger
A dredger without propelling devices.
Submerged dredge pump
In addition to the dredge pumps in the pump room, some cutter suction dredgers have an additional submerged pump installed on the ladder. This submerged pump provides the dredger with additional pumping power.
Swell compensators compensate the movement of a vessel caused by swell and therefore enables trailing suction hopper dredgers to dredge in heavy weather and swell. The compensation mechanism is integrated in the draghead gantry and ensures a flexible connection between ship and suction tube, in such a way that the draghead remains uninfluenced by the vertical movements of the dredger. The pressure of the draghead on the bottom of the river or the sea bed remains practically the same while the ship is rolling in a swell, and the stresses in the wire ropes by which the suction tube is suspended vary only within narrow limits.
Tender with pre-qualifications
When this procurement strategy is used, the tender process starts with a pre-qualification phase, in which contractors have to prove that they are qualified to execute the dredging project. This ensures that unqualified contractors are filtered from the tender process.
Trailer suction hopper dredger
A trailer suction hopper dredger (TSHD) unites dredging, transport and discharge in one single piece of equipment. A TSHD is a self-propelled vessel with a conventional hull shape and a so-called ‘hopper’ or hold. It is equipped with one or two suction pipes with drag heads that are lowered onto the seabed from the sides of the vessel. The dragheads trail over the seabed and suck up the soil for it to be stored in the hopper or hold of the vessel. Once the hopper is full, the vessel sails to the discharge site where the dredged material is released either by opening the hopper doors or by using a pumping system. Trailing suction hopper dredgers can easily handle sand and silt and are widely used for dredging works in ports and nautical channels, offshore works and other dredging projects where transport distances are long.
Infrastructures or treatment activities can demand the discharge of the un-treated or partially purified water at considerable distances from the coastline (or at the coastline in cases of large freshwater bodies) by means of pipes called underwater pipelines. These installations on the sea bed can either be freely supported or embedded in an excavated or dredged trench which is then filled, protecting it against possible scouring, and in both cases with anchoring elements. The outlet can be equipped with volute chambers to help with the discharge dilution within the environment.
Unit rate contract
In a unit rate contract the contractor is paid for the dredged volumes at a fixed price per unit.
Velocity is the speed of the slurry in a certain direction.
There are two basic types of dikes, which are mainly shielding structures (although they can also act as coastline barriers and/or coastal protection): ones with a vertical face and ones with a sloping face (mixed types exist, that work in one way or another depending on the sea level). The first ones or reflective dikes are normally made up of a wall built with concrete blocks or caissons with its foundations on a breakwater haunch and they are designed to a certain depth so that the waves cannot break over them but are reflected by them. In some cases they can be directly used as quays.
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.