248
R.H. Charlier and Chr. P. De Meyer
Discharge is normally effected by means of bottom dumping, either through
valves or sliding doors or by splitting (splithopper) or where reclamation is
desirable by pumping out the hopper into a land pipeline at a specially designed
berth.
The shortest operational cycle of the trailing suction hopper dredger is a single
run across the dredging area with following sequence : lower draghead ; sail
across the site (dredging): ; raise draghead ; turn. The main cycle, for bottom
dumping, then becomes : repeat subcycle as necessary ; sail to dump ; dump ; sail
to site. The number of subcycles in a main cycle will depend not only on how
long it takes to fill the hopper, but also to what extent the hopper is filled. When
dredging fine materials, it is often economically desirable to sail to the dump
with a load which is less than the possible maximum. Some dredgers are
equipped with an adjustable overflow which allows to load a higher volume for
lower densities in the hopper.
When the trailing suction hopper dredger is being used for reclamation purposes
and is self-discharging ashore, the main cycle, for pumping ashore, is modified to
the sequence : repeat the subcycle as necessary- ; sail to berth ; moor and connect
to pipeline ; pump ashore ; disconnect pipeline ; sail to site.
Trailer dredgers are usually rated by their hopper capacities (in tons) and the
power of the sandpumps and propellers. Hopper capacities may vary from some
300 to 11.000 m 3 (10,600 to 38,.900 ft3).
3.1.7.1.2 Cutter suction dredger
The cutter suction dredger is the most common of all dredger types. A cutter
suction dredger has two main components : the cutterhead and the dredging
pump. The cutterhead, located at the entrance of the suction pipe, is used to
agitate soft materials or to cut harder materials to bring them to a suitable state
for removal by hydraulic means. Cutters are usually rotated at between 10 and 30
rpm, and the rotary motor is located either directly behind the cutter in a
submersible drive unit, or at the upper end of the ladder (fig. 29b).
The dredging pump, in the body of the dredger, creates a vacuum in the suction
pipe and draws the soil up the pipe and through the pump. The soil is then
discharged by being pumped through a pipeline. The raising of the dredged
material is due to the pressure of the atmosphere acting on the water column in
the suction pipe, thus there is a limit to which dredging depths can be extended
before cavitation occurs. This explains why many modern cutter suction dredgers
have a dredging pump situated well below water level on the ladder that supports
the suction pipe, thereby, effectively reducing the heights of the suction column.
R.H. Charlier and Chr. P. De Meyer
Discharge is normally effected by means of bottom dumping, either through
valves or sliding doors or by splitting (splithopper) or where reclamation is
desirable by pumping out the hopper into a land pipeline at a specially designed
berth.
The shortest operational cycle of the trailing suction hopper dredger is a single
run across the dredging area with following sequence : lower draghead ; sail
across the site (dredging): ; raise draghead ; turn. The main cycle, for bottom
dumping, then becomes : repeat subcycle as necessary ; sail to dump ; dump ; sail
to site. The number of subcycles in a main cycle will depend not only on how
long it takes to fill the hopper, but also to what extent the hopper is filled. When
dredging fine materials, it is often economically desirable to sail to the dump
with a load which is less than the possible maximum. Some dredgers are
equipped with an adjustable overflow which allows to load a higher volume for
lower densities in the hopper.
When the trailing suction hopper dredger is being used for reclamation purposes
and is self-discharging ashore, the main cycle, for pumping ashore, is modified to
the sequence : repeat the subcycle as necessary- ; sail to berth ; moor and connect
to pipeline ; pump ashore ; disconnect pipeline ; sail to site.
Trailer dredgers are usually rated by their hopper capacities (in tons) and the
power of the sandpumps and propellers. Hopper capacities may vary from some
300 to 11.000 m 3 (10,600 to 38,.900 ft3).
3.1.7.1.2 Cutter suction dredger
The cutter suction dredger is the most common of all dredger types. A cutter
suction dredger has two main components : the cutterhead and the dredging
pump. The cutterhead, located at the entrance of the suction pipe, is used to
agitate soft materials or to cut harder materials to bring them to a suitable state
for removal by hydraulic means. Cutters are usually rotated at between 10 and 30
rpm, and the rotary motor is located either directly behind the cutter in a
submersible drive unit, or at the upper end of the ladder (fig. 29b).
The dredging pump, in the body of the dredger, creates a vacuum in the suction
pipe and draws the soil up the pipe and through the pump. The soil is then
discharged by being pumped through a pipeline. The raising of the dredged
material is due to the pressure of the atmosphere acting on the water column in
the suction pipe, thus there is a limit to which dredging depths can be extended
before cavitation occurs. This explains why many modern cutter suction dredgers
have a dredging pump situated well below water level on the ladder that supports
the suction pipe, thereby, effectively reducing the heights of the suction column.