What Are Cutting Suction Dredgers?

Dredging is the removal of natural deposits from the bed of a sea, river, or lake. These natural deposits include clay, mud, silt, sediments, sludge, rocks, and debris, which are stripped away after being accumulated in large amounts over prolonged periods on river or sea beds.

Dredging is beneficial for a host of purposes including:

• To maintain navigable water channels or waterways such that the depth of these environments remains high, and such depth values essentially cater to the minimal design parameters of the vessels frequenting them, like draft, for ensuring efficient propulsion and manoeuvring, as well as mitigate risks of incidents such as grounding.
• Land reclamation and reforms: exploitation of these natural deposits as a reserve for raw materials for new building civil constructions for habitation.
• Paving the way for new water bodies to be artificially created for the development of ports, harbours, berths, docks, piers, and channels or waterways. This is defined as capital dredging.

  

• Protection of aquatic natural habitats and ecosystems as removal of these deposits, which are often polluted or contaminated, and sources of toxins, leads to the creation of more unsullied environments for underwater flora and fauna.
• Tapping into these deposits to extract rich mineral resources that are entrapped within them, and other biological consumables like oysters, crabs, clams, shells, and certain breeds of fish that subsist on such natural reserves.
• Other construction purposes like dams, lock gates, and laying underwater cables or piping.

Cutting Suction Dredgers

In a cutting suction dredger, the entire dredging equipment and setup is mounted atop a buoyant vessel. Depending on the size and complexity of the dredging system, the hull form is decided, ranging from a simple barge-like pontoon (usually unpropelled) to a ship-like vessel (mostly propelled).

CSDs are very versatile and can handle all forms of earth materials irrespective of size, distribution, complexity, hardness, cohesion, density, and so on. They are the most advanced forms of dredgers and are often supplants of other categories like hopper or trailing suction owing to their operational efficiency and versatility.

Let us now briefly touch upon the working principle and parts of a Cutter Suction Dredger.

Other than the buoyant hull on which the entire dredging system is mounted, the CSD is characteristic of the following important components:

1. The most distinct feature of Cutting Suction Dredgers is the presence of the large cutting head apparatus in the front known as the rotating cutter head.

This is mostly like a large rotating drill bit that grinds and pulverizes any form of geological or topographical deposits in the river or seabed, aiding any form of material deposits to be loosened or broken down into smaller fragments such that they are stripped out of their resting place with the help of the associated suction mechanism.

The cutter head rotates about its axis at very high velocities by electrical power and drills into the deposits. The rotating cutter head is characteristic of multiple teeth or sharp blades that grind through the submerged deposits and help in releasing adhesion and cohesion forces of the materials such that they are easily sucked up into the suction lines.

By the combined effort of the drilling moments of this arm and the impact forces of the teeth that break through the deposits, any form and amount of material can be handled with ease. At a more localized level, the working forces at any instant can be simplified into axial forces or normal forces of the cutter head, and the tangential forces of the rotating blades (producing moments).

The size and the design complexity of these cutter heads depend on the working conditions, the nature of the deposits (hard, cohesive, non-cohesive), and the size of the dredger vessel.

2. The cutter head is mounted on the cutter ladder, which is like an elongated arm projecting out of the dredger hull in the bow region at an angle, as shown.

3. The cutter head is driven by what is known as the cutter head drive.

4. Suction Pipe: They comprise the network of piping connected directly to the cutter head. They entrain the broken or loosened seabed or riverbed material in the wake of the cutter head action and transport it away for discharge. The opening of the pipe through which the material is sucked in is called the suction inlet. These piping are present inside the cutter ladder itself and are often composed of either high-grade steel or superior rubber material.

5. Pumps: The Cutting Suction Dredger has multiple pumps. The first pump performs the role of suction, that is, slurping in the broken or loosened material above the sea bed. They are mostly centrifugal pumps having high suction or NPSH (Net Positive suction head) characteristics.

In small CSDs working in very shallow conditions, and with short suction piping, sometimes the intake pump is absent and the material is pushed through the suction pipe using the physical nature of the pressure differentials itself.

The other pump(s) like dredge pumps/discharge pumps/inboard pumps are present on board and help in transporting the mixture of deposits and water towards the outlet of the dredging system.

6. Onboard and discharge piping: Once the mixture of the bottom material and water is entrained into the dredging system by the centrifugal pumps at the inlet of the suction ladder, they are transported into another network of pipelines present onboard. The runoff material then passes through these pipes and is released at the outlet region of the dredging system (at stern) or transferred to any other system that further handles these materials.

7. Spuds and anchorage: Spuds are slender legs that help in fixing the dredger vessel to the river or sea bottom by directly penetrating down the layers of the ground material. These spuds serve a two-fold purpose: 1) Positioning the vessel at a fixed location for operational purposes. 2) Acting as support for working the cutter suction head.

While the first purpose can also be achieved with the help of other means ranging from traditional anchors to modern-day DPS like in most conventional vessels of today, the second purpose mandates the usage of only spud and related systems.

Spuds in CSDs

For a buoyant vessel floating in the water, the hull is a simply supported beam in a state of total hydrostatic equilibrium.

Thus, when the cutter head ladder is lowered from the bow region to drill into the rocky or silty material below, it imparts various forces and moments, and from first principles, the equivalent set of reactions produced as a result must be balanced from a fixed support somewhere to satisfy a state of mechanical equilibrium.

Imagine drilling a hole in your wall. The handle of the drilling machine essentially receives the necessary fixity from the grasp of your hand.

In the absence of any fixity, and the water being a fluid, one end of the vessel here acting as a beam would remain free. Every time the cutter head or ladder imparts the forces and moments on the working ground, due to a mechanical unbalance, not only will the cutting operation be hampered but also owing to the reaction forces at the point of contact, the vessel would tend to move or drift away instead of being fixed to a particular location. Such necessary fixity is provided by these spuds acting as hinges.

CSDs have either of two spud configurations: a) Fixed spud, and b) Spud Carriage.

In fixed spuds, there are two pillar-like spuds at the aft of the dredger. During operations, one of the spuds is lowered into the ground and the other remains free. This spud, known as the working spud, acts as a hinge and provides the necessary fixity at one end.

Now the dredging operation must be carried out over an extent of area. Thus, after the cutter head has dredged one finite patch of seabed or riverbed within its mechanical scope, it needs to shift to the rest of the swathe.

Uncoupling the spud, momentarily powering up or towing the vessel, and precisely positioning the dredger and the cutter head over the adjacent working patch is a cumbersome process often prone to inaccuracies and errors.

In a fixed spud arrangement, after one round of work, the working spud is disengaged and lifted, and at the same time, the auxiliary spud is lowered. Now, the latter becomes the working spud and the former, auxiliary. The cycle repeats. In this process, a reasonable extent of the seafloor or riverbed is dredged in a single go.

This process is identical to how we walk! The cyclic alternating lifting and lowering of our foot in tandem make us move, and in this same manner, a fixed spud arrangement moves over a definite stretch gradually combing through every part of the topological distribution underwater.

The spud carriage is an improvement over the traditional fixed spud arrangement. Here a sliding arrangement mounted on rollers, rails or electrically driven conveyors consisting of all components of dredging operation is present.

During operations, this arrangement known as the carriage juts out of the hull gradually based on requirement. A spud attached at the end of this carriage pins itself to the ground, acting as a fixed support.

When the carriage has covered all underlying areas within its maximum mechanical scope, that is within its greatest outreach, the arrangement retracts, and the vessel repositions itself to the next new location. This process is more time-efficient than the fixed spud.

You might also like to read-

• Different Types of Dredgers Used in the Maritime Industry
• Top 12 Dredging Companies in the World
• What is Dredging – History, Importance And Effects
• Effects of Dredging on the Marine Environment
• How Dredging Anchor is Used for Maneuvering Ships?

About Author

Subhodeep is a Naval Architecture and Ocean Engineering graduate. Interested in the intricacies of marine structures and goal-based design aspects, he is dedicated to sharing and propagation of common technical knowledge within this sector, which, at this very moment, requires a turnabout to flourish back to its old glory.

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