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b. Hydraulic 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 for greater
demand on pump power and with greater wear on pumps and pipes.
SUCTION DREDGER
A stationary dredger used to mine for sand. The suction pipe is
pushed vertically into a sand deposited. 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 this type consists of a pontoon able to
support a pump and suction pipe and to make the connection to the
discharge 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 discharge pipe, which can vary between 100
and 1000 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 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 spudpole 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 1500 mm. A modern highly automated cutter
suction dredger is capable of achieving high outputs over sustained
periods and production rates of around 500000 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 discharge is directly over the stern to the sea. Pipeline discharge
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 dredging operations and to date have not found general
favour among the major international dredging contractors.
TRAILING SUCTION HOPPER DREDGER
A self propelled ship which fills its hold or hopper during dredging,
while following a pre-set track. The hopper can be emptied by o
bottom doors or valves (dumping) or by pumping its load 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 discharged 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 20000 m’
– 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 draghead, 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
draghead 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 disposal site where the cargo can be discharged, 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 discharge of material which may be difficult to discharge
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 manœuvre
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.
RECLAMATION DREDGER
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 discharge, and a high
pressure water pump used to fluidize the barge contents by jetting.
The mixture is then pumped through a pipeline to the point of reclamation
or disposal.
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