Dredging is the process of removing sediment and debris from the bottom of bodies of water, such as seas, rivers, and lakes. This material can include clay, mud, silt, sediments, sludge, rocks, and other accumulated debris over time. Dredging is often necessary to maintain navigation channels, improve water flow, or reclaim land from the seabed, according to Marine Insight.
Benefits Of Dredging
- 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 maneuvering, 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, harbors, berths, docks, piers, and channels or waterways. This is defined as capital dredging.
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.
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.
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.
The cutter head is driven by what is known as the cutter head drive.
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.
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.
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.
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Source: Marine Insight
