Types of Dredgers and their uses in Marine Operations
Dredgers are a vital component of the worldwide shipping fleet, with various dredger types performing excavations and scraping of the seabed for a multitude of reasons. Without them, many of the coastal shipping channels and harbours around the world would be unusable. This would have a huge knock-on effect on the entire global supply chain.
If you’re interested in getting the inside scoop on dredgers and their uses, keep on reading, as we’ll be giving you the lowdown of these hugely important vessels on the page below.
What is a Dredger and What Are its Uses?
Let’s start by looking at what a dredger is exactly. Defining them is actually very simple, as they’re vessels that can remove and relocate materials, such as sediment and mud, from the bottom of a stretch of water. But as with many aspects of the maritime world, things do get more complex when looking at types of dredgers – something we’ll run you through further down this page.
But why are dredgers in such demand? The answer lies in the multitude of jobs they can complete, most of which are vital to shipping and industry. Some of the more common jobs carried out by dredgers include:
- The deepening of stretches of water to ensure sufficient depth for vessels.
- The creation of new harbours, which require specific water depths to function effectively.
- Mining of various materials from under the water.
- Removing any contaminants that might be found at the bottom of a stretch of water
- Collecting sand to be used to rebuild beaches, which are regularly eroded and therefore need additional sand.
- Collecting sand to be turned into concrete.
As you can see, dredgers really are a vital tool for the shipping industry. Many dredgers also need support from tugboats, as well as boats for the transferring of workers from vessel to shore, and vice-versa. This is something we facilitate throughout the Middle East, so please get in touch to find out more about tugboats for hire and ship chartering.
Types of Dredgers
Let’s now look at the various types of dredgers you’ll find available, all of which have pros and cons and fulfil different requirements.
Firstly, it’s important to point out that dredgers fit into one of three categories. These are:
- Stationary. These dredgers are constructed in place and are supported by spuds – columns projecting from the bed.
- Portable. These are dredgers that can move from location to location, but aren’t powered, and therefore need to be moved by other vessels, such as tugboats.
- Self-propelled. These dredgers can also move and can do so using their own power.
All dredgers can also be separated into three other categories: mechanical, hydraulic/suction, and other. Find out more about mechanical and hydraulic/suction dredgers below.
Most types of dredgers are mechanical, but what does this mean? Well, they all use a bucket to complete whatever the job in question might be. This is why they’re also regularly referred to as bucket dredgers.
You won’t just find a standard bucket attached to a mechanical dredger – there are also a variety of other attachments that can be used, making them exceptionally versatile vessels. These include dippers, grapples, and ladders.
You will find stationary, portable, and self-propelling mechanical dredgers, but when completing a job, they must remain in one place, moored using an anchor or poles. Mechanical dredgers need anchoring as they easily change position, due to the pressure of the bucket on the bed. By anchoring, it remains in the same position and therefore functions more effectively.
Mechanical dredgers can be broken down into various types. You can see some of the more commonly used mechanical dredger types below.
Dipper dredgers are some of the most seen dredger types in the world. They’re attached to barges and work in a simple way, using a bucket to scoop material forwards, away from the dredger – much like you would do when using a shovel in your back yard. They use powered spuds, which increase the power they possess. While not self-propelled, they can move forward using the spuds.
Dredgers of this type are particularly effective in confined spaces, plus their power makes them the dredger of choice when removing particularly hard material, such as clay, stone, and blasted rock. They can also be used to remove obstructions from waterways, such as disused structures. On the downside, they cause lots of sediment disturbance, plus they only work to a limited depth.
Backhoe dredgers are very similar to dipper dredgers, as they too dig down in an arc using a bucket. The difference with backhoe dredgers is that they scoop towards the dredger, much like a standard digger would do on a construction site. Dredged materials are placed onto a barge and then transported to a designated location.
This type of dredger is exceptionally popular and can be used in a variety of situations, as well as for dredging a wide range of materials. Their most common use is in foreshore protection projects. More modern backhoe dredgers can be used to dredge at depths of 30 metres or more. Most backhoe dredgers are not self-propelled, so they must be towed into place.
These are also often called elevator dredgers and use several buckets to remove materials. These buckets are all attached to a ladder in an oval shape. The ladder then moves, much like a caterpillar track, and each bucket hits the bed and removes material in the process. The material is then dumped automatically on the way round. This process continues until the ladder is turned off.
The clear advantage to ladder dredgers is that they facilitate fast dredging. They are most used by mining companies for mining minerals, often while attached to a sea-going vessel.
As the name suggests, dredgers of this type are mainly used for moving sand. Sand dredgers can either be mechanical or hydraulic, with hydraulic dredgers preferable when moving fine material. Mechanical dredgers can come with many attachments, including buckets, nets, and blades.
These dredgers are mainly used by the construction industry, as sand is a vital component of many building materials. Interestingly, sand dredgers are currently being used widely by China, as they dredge sand from Taiwan’s waters, degrading the island’s waters and providing the Chinese with valuable sand in the process.
Next, we move onto hydraulic dredgers, which are not as common as mechanical dredgers, but still prevalent on seas, rivers, and lakes.
But what are hydraulic dredgers? Well, they use suction to remove materials from the bed, sometimes with the assistance of a jet of water. A pipe hangs down from the dredger to the bottom of the water. Pumps then cause the material to be sucked up through the pipe. The material can then be dumped into barges or sent elsewhere via pipes.
Hydraulic dredgers can be stationary, portable, and self-propelled. As with mechanical dredgers, there are many types of hydraulic dredgers, and we’ve explained the more commonly found ones below.
Plain Suction Dredgers
A plain suction dredger uses a pipe, which is pushed into the material and then sucks it out, sometimes with the help of a water jet. All reclaimed material is then placed onto a barge or sent straight to a reclamation area.
Plain suction dredgers are mostly used to clear sand and silt and can reach materials that are over 300 feet below the water level. Land reclamation projects use dredgers of this type heavily, plus they are also used to gather sand for the creation of concrete.
There are three different varieties of plain suction dredgers. Standard dredgers of this type do exactly as described previously, while deep suction dredgers are used in deep water, typically deeper than 100 feet. Dustpan suction dredgers have a nozzle shaped like a dustpan, allowing the dredger to make a different type of cut into the material.
Cutterhead Section Dredgers
Cutting suction dredgers – often simply called CSDs – work in roughly the same way as plain suction dredgers, but with one significant difference: they have a cutting tool, most commonly a swinging arc, attached to the head of the pipe, which loosen material so it can be easily sucked up the pipe.
As you can probably imagine, this type of dredger is used for clearing harder materials, such as rocks. Dredgers of this type are widely used for larger projects, including large-scale dredging of sand and gravel, and the deepening of sea channels. They are also commonly used for reclamation.
Trailing Suction Hopper Dredgers
Trailing suction hopper dredgers – more commonly called TSHDs – are self-driven dredgers, which can look like large boats and barges. They move at an exceptionally low speed – somewhere around 2 to 3 knots is typical – and have three main components: a pipe for removing material, a hopper, and a mechanism to dump collected material. This dredger type is most used in harbours and other areas with high traffic.
The dredger moves automatically throughout its assigned area, dragging its pipe. The pipe collects material, which is most commonly silt, sand, soft clay, and gravel, and stores material it collects in the hopper, which can be around 30,000 cubic yards in the largest TSHDs. When the hopper is full, the pipe is lifted, and the dredger takes the material to a designated spot, where it’s dumped.
Water Injection Dredgers
Water injection dredgers are a specialist dredger type, used for projects that are environmentally challenging. They can also be used in smaller ports for regular dredging duties. There’s no suction involved in the operation of this dredger – in fact, the dredged materials never even make it above the water line.
So, how do they work? Well, a strong jet of water is injected under the material to be dredged, which causes the material to become agitated. Much of the material then mixes with the water in a process called fluidisation and is taken away by the tide. This means that there’s no requirement for the dredged material to be relocated.
Choosing the Right Type of Dredger
Now you know all about the most commonly found dredgers, this question has to be asked: which dredger vessel is best for your requirements? When deciding this, it is vital to consider several factors: material transport, vessel transport, location, material to be dredged, the environment, and the financial side of things. Let’s look at all four in greater depth.
Where will the dredged material go once it has been removed from the bed? Will you be working in open water and therefore need the material to be transported elsewhere using barges? Or are you closer to land, meaning that material can be moved using pipes?
Before you start dredging, you’ll need to get your dredger into place. If you decide on a portable dredger, you’ll need to hire other craft for ocean towage, such as tugboats. If you instead decide on a self-propelled dredger, there will be no requirement for extra vessels.
Where is the material you need to dredge located? If you’re working in a harbour or other area with high levels of traffic, a stationary dredger might get in the way, meaning a TSHD might be the best choice. If you’re working in a confined a space, a dipper dredger might be the prudent choice.
You also need to think about water depth. Are you dredging in an area of deep water? If so, you’ll need a dredger that has long reach, such as a deep suction dredger.
Material to be Dredged
Different dredgers are used to move different material types, and it’s vital you pick the correct option. Will you be dredging an area of hard rock? If so, a cutterhead suction dredger might be appropriate. Are you attempting to remove fine silt? A plain suction dredger might be the best choice in this situation.
It might even be that you’re not looking to perform a typical dredging activity. Instead, you might be engaged in marine salvage or mining. These will also require specific characteristics from a dredger.
The protection of the environment is becoming more and more important and damaging the environment unnecessarily can reflect very badly on a company. Therefore, you must consider your environmental impact. For example, you might choose to use a water injection dredger, in order to minimise the environmental affect.
Your finances will also play a large role in the type of dredger you choose. Quite simply, some dredgers are worth more than others, therefore they will cost more to hire. You’ll also need to think about crewing requirements and other expenses, such as the costs to transport and dump cleared material. Also, is the dredger you need located close to you, or will you have to pay for it to be transported to your location?
If you’re going to be using a dredger for a prolonged period, the best choice might be to purchase, rather than hire. If you go down this route, don’t just think about the purchase price – also factor in considerations such as maintenance, fuel, insurance, crewing, and costs for ship lay up management, if required.
As you can see, dredgers come in a huge variety of shapes and sizes, each with their own individual uses. The type of dredger you choose will really be down to your own requirements – there is no one size fits all, so considerable thought should go into the dredger types available.
At Dexter Offshore, we’re here to support your dredging efforts, offering several types of vessels often required to support dredging activities, such as tugboats and barges. Please don’t hesitate to get in contact if you need to hire any of our vessels, or if you simply need some advice on dredging.
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