Types of dredger
The principal feature of all dredgers in this category is that the loosened material is raised from its in-situ state in suspension through a pipe system connected to a centrifugal pump. Various means can be employed to achieve the initial loosening of the material. If it is naturally very loose, suction alone may be sufficient, but firmer material may require mechanical loosening or the use of water jets. Hydraulic dredging is most efficient when working with fine materials, because they can easily be held in suspension. Coarser materials - and even gravel - can be worked but with a greater demand on pump power and with greater wear on pumps and pipes.
A Suction Dredger is a stationary dredger used to mine for sand. The suction pipe is pushed vertically into a sand deposit. If necessary water jets help to bring the sand up. It is loaded into barges or pumped via pipeline directly to the reclamation area.
Profile or Plain Suction Dredger
In its most simple form the Profile or Plain Suction Dredger consists of a pontoon able to support a pump and suction pipe and to make the connection to the offloading pipe. More sophisticated vessels have separate suction and delivery pumps, water jets at the suction inlet and articulated suction pipes. While working, a dredger may be held in position by one or more spuds or, in deeper water, by a complex system of moorings. Plain suction dredgers are mainly used to win fill material for reclamation, with the material being placed ashore through a floating pipeline. Very long distances can be pumped by the addition of booster pumps in the line. Material may alternatively be loaded directly into barges moored alongside. The normal measures of size are the diameter of the offloading pipe, which can vary between 100 and 1,000 mm, or the installed horsepower.
Another use of plain suction dredgers - common in the USA - is to dredge from the navigation channel of a river and side cast the material to nearer the bank through a short pipeline or simply by jetting. In this role they are more commonly known as dust-pan dredgers.
Modern suction dredgers can recover material from great depths and can also extract sand from below a clay overburden. Known as a deep suction dredger, this type offers the potential to recover fill material from depths up to 100 m. Production is very dependent upon the permeability of the material dredged and is best in clean sands.
Cutter Suction Dredger
A Cutter Suction Dredger is a stationary dredger which makes use of a cutter head to loosen the material to be dredged. It pumps the dredged material via a pipeline ashore or into barges. While dredging the cutter head describes arcs and is swung around the spud-pole powered by winches. The cutter head can be replaced by several kinds of suction heads for special purposes, such as environmental dredging.
When the in-situ material is too compact to be removed by suction action alone, some form of mechanical loosening must be incorporated near the suction mouth. The most common method is a rotating cutter: the main feature of the cutter suction dredger. This is mounted at the lower end of the ladder used to support the cutter drive and the suction pipe. The loosened material then enters the suction mouth, passes through the suction pipe and pump (or pumps) and into the delivery line.
Cutter suction dredgers operate by swinging about a central working spud using moorings leading from the lower end of the ladder to anchors. By pulling on alternate sides the dredger clears an arc of cut, and then moves forward by pushing against the working spud using a spud carriage. A generally smooth bottom can be achieved, and modern instrumentation allows profiles and side slopes to be dredged accurately. Some of the larger cutter suction dredgers are self-propelled to allow easy movement from site to site.
The size of a cutter suction dredger is measured by the diameter of the suction pipe and by the installed machinery power. Pipe diameters are in the range 100 to 1,500 mm. A modern highly automated cutter suction dredger is capable of achieving high outputs over sustained periods and production rates of around 500,000 m³/week are possible under good conditions.
Cutter suction dredgers can be used to deliver through a pipe- line or to load barges. They may also be used simply as loosening devices for material to be re-handled by another type of dredger, in which mode offloading is directly over the stern to the sea. Pipeline offloading is most common but is vulnerable to waves and currents and causes an obstruction to other vessels. To avoid these problems part of the pipeline may be submerged and laid on the channel-or sea-bed.
Cutter suction dredgers are mainly used for capital dredging, especially when reclamation is associated with the dredging. Smaller vessels can be dismantled into sections and moved by road or rail for work in inland waterways, sludge lagoons, reservoirs and similar isolated areas. Large heavy-duty cutter dredgers are capable of dredging some types of rock which have not been pre-treated.
An alternative form of loosening is the use of a rotating bucket wheel at the suction mouth. Bucket wheel dredgers are most commonly used in mineral extraction operations and to date have not found general favour among the major international dredging contractors.
Trailing Suction Hopper Dredger
A Trailing Suction Hopper Dredger is a self-propelled ship which fills its hold or hopper during dredging, while following a pre-set track. The hopper can be emptied by opening bottom doors or valves (offloading) or by pumping its load off ashore. This kind of dredger is mainly used in open water: rivers, canals, estuaries and the open sea.
Trailing suction hopper dredgers, commonly known simply as ‘hoppers’ or ‘trailers’, have a hull in the shape of a conventional ship, and are both highly seaworthy and able to operate without any form of mooring or spud. They are equipped with either single or twin (one on each side) trailing suction pipes. Material is lifted through the trailing pipes by one or more pumps and loaded into a hopper contained within the hull of the dredger. The measure of size of a hopper or trailer dredger is the hopper capacity. This may range from a few hundred cubic metres to over 40,000 m³ - increasingly larger vessels have been constructed in recent years to allow economic transport of the dredged material, especially for reclamation projects.
The suction pipe terminates in a drag-head, which may be of the plain type or may incorporate a water jet system, blades or teeth, or other means of dislodging compacted material. The function of the drag-head is to allow the material to flow to the suction inlet as efficiently as possible.
A trailing suction hopper dredger operates very much like a floating vacuum cleaner. It sails slowly over the area to be dredged filling its hopper as it proceeds. On completion of loading the dredger sails to the relocation site where the cargo can be offloaded, either by opening the doors or valves in the hopper bottom, by using the dredging pump to deliver to a shore pipeline, or directly to shore by using a special bow jet. This last technique is known as “rainbowing” and is commonly used for reclamation and beach nourishment.
Some trailer dredgers split over their entire length to achieve a rapid offloading of material which may be difficult to unload through doors. Such special vessels are understandably more expensive to build than those with a rigid hull.
Trailing suction hopper dredgers operate best by skimming off layers of material in long runs, such as might be found in channel dredging. They are unable to get into corners and may be difficult to manoeuvre in confined spaces close to quays and jetties. They are not very effective on hard materials such as the stiffer clays, but can dredge rock which has been blasted, or loosened by a cutter dredger. These dredgers are very efficient for the materials they can handle effectively. Most harbour maintenance dredging today is carried out by trailers, and they are also employed for capital projects, pipe trenching and reclamation.
A Reclamation Dredger is a stationary dredger used to empty hopper barges. A suction pipe is lowered into the barge. Extra water can be added by water by water jets to facilitate the suction process. The dredged material is pumped by pipeline ashore, to a reclamation area, or to a storage depot.
Barge Unloading Dredger
Barge unloading dredgers are used to transfer material from hopper barges to shore, usually for reclamation. A barge unloader is basically a pontoon supporting a suction pump for the unloading, and a high pressure water pump used to fluidise the barge contents by jetting. The mixture is then pumped through a pipeline to the point of reclamation or relocation.