Different kind of dredgers

Dredging is a technique in which you clean the water bottom by moving the dirt. It is done for various purposes. One of those goals is to maintain a minimum depth of canals and harbors where boats can sail.

Maintenance dredging is now one of the basic tasks, while the demand for other tasks is rapidly increasing, including:

• creating new land for port and industrial development

• Trenching, backfilling and protection operations for offshore pipelines, coastal runoff pipelines and for cables laid on the seabed

• environmental dredging and clearing of contaminated sediments

• replenishment of beaches and coastlines, not only for coastal protection, but also for recreational use.

There are two different types of dredging: mechanical digging and hydraulic digging.

Mechanical excavation is mainly applied to cohesive soils. The dredge is excavated and removed using mechanical parts such as grabs, buckets, cutter heads or scoops.

Hydraulic digging, on the other hand, is done with special water jets in soils that are not cohesive, such as silt, sand and gravel. The dredged material removed from the seabed is sucked up and transported further as a mixture with centrifugal pumps.

Mechanical dredgers

There are several mechanical dredgers, including:

1. Backhoe dredger

The name “backhoe dredger” is based on the large land-based excavator mounted on one end of a spud-rigged pontoon. The main advantage of this dredger is that it can dredge a wide variety of materials, including debris and soft, weathered or broken rocks. Partly that makes this dredger very suitable for different types of soils.

2. Bucket chain dreger

The bucket chain dredger is a dredger equipped with an endless chain of buckets carried by a ladder. The buckets are attached to a chain and sorted by size (from 200 to 1000 liters). These buckets are used to scoop material from the bottom and lift it above the water. The buckets are then discharged into special bins. These bins can eventually be transported.

Bucket chain dredgers are held in place by giant anchors. Today this classic ship is mainly used for environmental dredging projects.

3. Cutter suction dredger

The cutter suction dredger is a dredger equipped with a cutting head, which first excavates the soil before it is sucked up by the strong flow of the dredge pump. The cutter suction dredger moves during dredging. These dredgers are regularly used to dredge trenches for pipelines and channels in hard, solid ground. Dredging on hard soils is very demanding from a dredger, so it is important that the dredging equipment is replaced regularly. Sea-going cutter suction dredgers have different propulsion than non-sea-going cutter suction dredgers.

4. Grab dredger

A grab dredger uses a grab mounted on cranes or outriggers. This grab loads the dredged material into independently operating inland vessels.

Grabs can handle both silt and hard objects (blocks of rock, wrecks). This makes grab dredgers ideal for cleaning up difficult-to-access waters, such as canals in cities. Grab dredgers are also suitable for gravel extraction and maintenance dredging on uneven soils.

Suction dredgers

There are also different suction dredgers:

1. Plain suction dredger

A Plain suction dredger is a stationary suction dredger with at least one dredge pump connected to the suction line. The dredged material can be discharged via pipelines or by barge.

2. Trailing suction hopper dredger

The Trailing suction hopper dredger is a non-stationary dredger, that means that it is not anchored with wires or spud but is dynamically positioned. The Trailing suction hopper dredger has the shape of an actual ship, with hopper-type cargo holds to store the dredged material. There is a suction arm on both sides of the ship, which consists of a lower and higher part. Trailing suction hopper dredgers are mainly used for maintenance work and the dredging of trenches in softer soils.

How is the dredged material disposed of?

When the vessel needs to be unloaded, jet pumps in the hopper are used to dilute the dredge so that it can be pumped ashore or discharged from the sea bed. What also occasionally occurs is that the dredge is not discharged, but is accurately moved on the water bottom via the suction pipes.

Dredging transport equipment requires a ship-to-shore pipeline, a powerful pump and a special connection between pipeline and ship – called the bow coupling.

The mixture can also be sprayed forward over the ship’s bow via a mixture nozzle (rainbowing).