Dredging is the process of extracting or removing sediments and other materials under bodies of water and transporting them to another location.
Extracted deposits from seabed includes silt, sand, soil, trash, mud, gravel, debris and marine organisms.
Dredgers are not hard to find once spotted. You can even check their latest projects using free vessel tracking websites.
Dredging can be also done on land. But for the purpose of this article, let’s focus on dredgers operating in rivers, lakes, canals, channels, estuary, harbors and sea floor
What are Dredgers?
Dredgers are watercraft that carry out dredging.
They have various designs depending on their capability, purpose and method of extraction.
Some dredgers are purposely built ships for the sole task of dredging. They have built in power and can move on their own, even sailing from one port to another
Other dredgers are heavy construction equipment found on land. Examples of these are excavators or cranes with a clam shell bucket.
They are placed on top of self propelled barge or barge towed by tugboat to be able to conduct dredging in bodies of water.
There are many reasons why dredging is carried out. Most of the time, the main and most common objective is to deepen certain parts of rivers, harbors, canals or sea.
Before a port or canal is created, the area has to be dredged so huge ships can enter and leave safely.
Natural sedimentation crops up waterways thereby endangering vessels against grounding or touching bottom. Maintenance dredging solves this problem.
Dredgers are used to create trenches for submarine pipelines underwater.
Additionally, companies use them to “dig holes” for foundational support of structures built on top of water such as bridges and windmills.
Mining and Recovery
Another reason for dredging is to gather minerals from underwater much like mining does. Corporations usually carry this venture.
Dredging also serves to recover priceless materials which have been blanketed in sediments underwater.
Think about the valuable things sunken ships carry with them a long time ago. Sediments would have creep on top of these precious items due to currents and siltation.
Some construction materials like gravel and sand can be found underwater. Recovering them through dredging is a known method.
Dredging also maintains ecosystem by removing trash and debris as well as restores shorelines to their former glory.
Choosing the Right Dredger
Dredging is a very complicated operation involving lots of calculated steps.
Choosing the type of dredger depends on many factors most of which revolves around the construction environment and project details.
Deciding on the wrong one is costly, gives long delays and damage to the dredger as well.
Nature of Soil
Soil or sediment is the material that we have to take out underwater.
Understanding the type of sediment and its density is a huge factor for efficient dredging.
Taking that into account, dredgers produce maximum output when working with soils within their handling design.
Trailing Suction Hopper Dredger for debris and rock dredging is inefficient. It is best to use mechanical dredgers.
Area and Volume of Dredged Material
Different types of dredgers have different capacity. The area to be dredged plays an important part in choosing the dredger.
Long barges in river dredging may be difficult to turn around.
Using large seagoing dredger for huge areas is economical.
The same seagoing dredger may not be able to enter inside rivers, channels and canals.
Depth of Water
How deep is the water? Moreover, how shallow is it?
Its not enough that the dredger can reach the seabed. Its also important that its suction pipes or grabs can achieve the target depth.
A Backhoe Dredger may not be able to operate in a depth of more than 20 meters. Even if it can reach that level, it may not fulfill the task of dredging to 25 meters.
Besides, one must consider the freeboard where it’s mounted.
Part of dredging is handling the dredged soil and transporting it somewhere.
How far is the disposal area?
Do we need a barge or just lay out a pipeline?
If we use barge, how many do we need in proportion to the production rates?
How long will the pipe be if disposal method is via pipeline?
Which is more economical for the duration of the project? How about the traffic in the area?
Performance of dredgers, their operators and the company behind them plays an important role on this subject.
A well experienced company may give you insights and solutions not found in the books.
Furthermore, their crew should have the required training and experience just like the crew organization of any vessel.
Dredging affects marine life.
It increases the turbidity (clearness) of water in the area. This have direct consequences to visual predators like birds and other marine predators.
Increased suspension of sediments reduces light penetration in the water. This exposes the vulnerability of certain planktons and other benthic species.
Additionally, the disturbance created during dredging affects critical life stages of certain marine life.
Lastly, dredging could carry out for months or even years. Their engine noise has potential impact to marine mammals like whales and dolphins which uses low sonar levels to navigate their way.
Types of Dredgers
Dredgers are categorized according to the methods used in extracting dirt from underwater and delivering it up to the surface.
There are two main types of dredgers, though a third one is recognized for those who doesn’t fall on either of the two.
1. Mechanical Dredgers
In the most basic explanation, mechanical dredging involves the use of grabs or buckets.
It “scoops” up the sediments and delivers them up for disposal.
I many cases, mechanical dredging prides in mobility and precision. It is robust as it is able to remove rock, gravel and other “cohesive” materials.
This is why they are mostly used in harbors, rivers, channels and canals.
Mechanical dredgers are further classified according to their built.
a. Grab/ Clamshell Dredger
When it comes to precise dredging, Grab or clamshell dredger does the job.
They are basically clamshell crane redefined for dredging and then installed on a barge.
The grab is suspended with a wire, is lowered into the seabed and excavates the sediments once it touches them.
Due to that mechanism, it has the advantage of dredging on deep spots with pinpoint accuracy. They can work with clay, gravel, and even broken rocks but not particularly effective in fine silts.
The excavated material is then hauled up vertically and discharged in a separate barge or hopper.
We will talk about hoppers later on.
b. Dipper Dredger
This type of dredger is very similar to a Backhoe dredger. The only difference is the position of the bucket.
Remember how a shovel works? You thrust the blade into the ground in a forward motion in order to take out the soil.
Or think about a bulldozer which drives forward to collect materials.
A Dipper Dredger works in a similar principle. It is a backhoe mounted on a barge and gathers dirt thru forward thrust.
To stabilize the barge, it may be fitted with spuds on its forward and aft part.
These spuds moor the barge by standing on the seabed.
Companies use Dipper Dredger when the water ahead is too shallow for the barge to float.
c. Backhoe Dredger
Backhoe Dredgers (BHD) are hydraulic excavators used on land but slightly modified to do dredging works.
They are mounted on a barge and works similar to Dipper Dredger except for the method it scoops up sediments.
While Dipper Dredger gathers seabed thru forward thrusts, Backhoe Dredger collects them thru backward movement.
A transport barge is also moored alongside the dredger where the sediments are accumulated before disposing elsewhere.
d. Bucket/ Bucket Ladder Dredger
Just like merchant ships, naming dredgers can be straightforward too.
Bucket ladder dredgers are composed of two primary things which set them apart from other dredgers.
They have, well, buckets and a ladder.
Series of buckets are installed around a chain in an endless loop.
These buckets rotate in one direction and goes up on a ladder. They scoop up sediments upon reaching the bottom of the seabed and bring it on top.
Before rotating back downward, the sediments fall off a chute which goes to a barge for disposal.
2. Hydraulic Dredgers
Aside from grabs, buckets or scoops, pumps and pipelines are also used to to extract sediments from underwater to the surface.
These types of diggers are called Hydraulic Dredgers. They come in many forms but shares a common principle.
Hydraulic dredgers use suction to bring the dredged sediments up. Think of it like a vacuum cleaner operating underwater.
Loosened sediments mixed with water enables sucking them out possible. Hence, they work best in cohesion-less soils like sand, silt or even gravel.
Hydraulic Dredgers are sometimes called Suction Dredgers.
a. Plain Suction Dredger (PSD)
It is called PSD for very simple reasons- it only consists of suction pipes, pumps and discharge pipes. These components are installed in a barge or pontoon.
The vessel is kept stationary through the use of spuds. In some scenario, complex mooring is involved.
During operation, the suction pipe lowered to the bottom using wires. It sucks the sediments while releasing water jet.
This disturbs the soil thereby creating suspension for easy pumping.
The discharged material is transferred to a barge. Or it is pumped directly to the disposal area using extended pipelines fitted with booster pumps along the way.
b. Cutter Suction Dredger
In some instances, sediments hardens overtime making PSD less effective. This is where Cutter Suction Dredger (CSD) comes in.
CSD is similar to PSD but differs what’s on the tip of its suction pipe.
It uses a a rotating cutter to excavate the material thereby loosening them for easy pumping.
The disturbed sediments is discharged to a barge or directly to the disposal site using pipelines much similar with PSD.
Cutter Suction Dredgers use spuds to keep them in position. Aside from that, it lowers its anchors on port and starboard side.
CSDs use these anchors for swinging the ship with the pivot point being the stud. This type of dredging covers huge areas while keeping the depth at uniform level.
c. Trailing Suction Hopper Dredger
We mentioned “hopper” in the beginning of this article. Here, it has its own meaning.
Hoppers are container for bulk materials like trash, rocks, silt, sediments, sand or grain. This is similar to the cargo storage of tankers and bulk carriers except for one difference.
Its main feature is the opening bottom doors as its discharge mechanism.
Trailing Suction Hopper Dredger (TSHD) is a capable, self-powered seagoing vessel equipped with hopper type cargo holds.
TSHD doesn’t use spuds or requires barge for discharging. It slowly navigates or “trails” in a pre-determined route.
After lowering its trailer arm on its side, it slowly moves forward while sucking down the material along the way.
The dredged soil can be dealt with in three ways.
- Stored in the hopper and disposed in a designated area
- Connected to a pipe leading to the discharged site
Rainbowing is spraying out the dredged material to the discharge location.
Other types of dredger can not be classified between mechanical and hydraulic due to their special characteristics and working mechanisms.
Generally, these types are small compared to the two.
And these dredgers also have small production output. They are suitable for ponds, lakes and rivers.
Due to their size, these ships can be transported by trucks and hauled in and out of the water using a crane and sometimes by itself.
a. Auger Suction Dredger (ASD)
Auger Suction Dredger operates on the same principle as a Cutter Suction Dredger (CSD). The two differs on what’s in their suction head.
While CSD uses circular rotating blades to dredge the bottom, ASD operates with a horizontal screw blades called Archimedean Screw.
Archimedean Screw works in a way that when it rotates, the sediments are guided to the middle where the suction head is attached.
The ship dredges as it moves forward. Its dredging pattern is like plowing a field compared to CSD’s swinging pattern.
b. Jet-Lift Dredgers
This type of dredger doesn’t use pump to directly suck the water-sediment mixture. It uses the Venturi Effect much like how eductors work.
The suction head has no moving parts. Streams of water going at high-speed pulls out the surrounding water together with sediments around it.
This is only used on small scale dredging activity.
c. Air Lift Dredgers
This type of dredger works in a similar way with Jet Lift Dredger. It has two main components; a Pulse tube and a Lift Pipe.
Pulse tube is where high pressure air is introduced at pulsating intervals between 1 second to three seconds.
The compressed air travels to the Lift Pipe while displacing the water.
As soon as the air inside the lift pipe bubbles up to the water surface, it draws in water-sediment mixture.
The cycle repeats during the dredging process.
d. Amphibious Dredgers
By the name itself, these types of dredgers are capable of operating on land and water.
They are basically heavy machinery excavator fitted with studs, floaters or legs that enable them to “walk, stand or float”.
These types works predominantly on rivers, lakes, swamps or ponds.
e. Water Injection Dredger
When we talk about dredgers, what usually comes to mind is a mechanism that picks up something from the seafloor and brings it up on the surface.
Water Injection Dredger (WID) is an outlier as it functions very differently.
Instead of grabs or suction pumps, this dredger does not pick up sediments for disposal.
This is how it operates.
It lowers a horizontal jet bar with series of nozzles to the sediment layer and beams volumes of low pressure water.
This “disturbs” the soil and creates fluidized sediments which creeps up horizontally away from the dredged area into the lower part of the water column.
The slope of sea bed, hydrostatic difference and tidal currents drive this phenomena.
Hence, Water Injection Dredging is also called “Natural Dredging”.
Would you like to work on a dredger? Which shipping companies hire crew for dredger vessels?
May the winds be in your favor.