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.