Although systems for
describing dredgers vary, generally three broad classifications are recognized
based on the means of excavation and operation. These are known as mechanical
dredgers, hydraulic dredgers, and hydrodynamic dredgers. Mechanical dredgers come in a variety of forms, loosening the in-situ material and raise and transport it to the surface. Most common mechanical dredgers are (1) the bucket ladder dredgers, (2) the grab dredger, (3) the self-propelled
grab hopper dredger, and (4) dipper dredger.
1.1 General
The bucket ladder
dredger (or called bucket chain dredger) is one of the mechanical dredgers. This is a
stationary dredger that is equipped with a continuous chain of buckets, which
are carried through a structure, the ladder. The ladder is mounted in a
U-shaped pontoon. The drive of the bucket chain is on the upper side. The
bucket dredger is anchored on six anchors. During the dredging, the dredger
swing round the bow anchor by taking in or paying out the winches on board. The
bucket, which are filled on the underside, are emptied on the upper side by
tipping their contents into a chute along which the dredged material can slide
into the barges moored alongside. The chain is driven by the so-called upper
tumbler at top of ladder frame, which is connected either via belt to the
diesel or directly to an electro motor or hydro-motor.
Picture 1: The Bucket Ladder Dredger
Picture 2: A Continuous Chain of Buckets at A Bucket Ladder Dredger
Picture 3: Simplified Diagram of A Barge Loading Bucket Dredger
Picture 4: Positioning of The Dredger in Cut
Since 1960, bucket
ladder dredgers have been almost entirely replaced by either backhoe dredgers
or trailing suction hopper dredgers or cutter suction dredgers. The reason is
that the bucket dredger, with its six anchors, is big obstacle to shipping.
Moreover, maintenance costs are high and the bucket dredger requires many highly
skilled operatives. Bucket chain dredgers’ production has not kept
pace afterwards, compared trailing suction dredgers.
1.2 Area of Application
Bucket dredgers are only
used in new or maintenance dredging projects when the initial depth of the area
to be dredged is too shallow for TSHDs (trailing suction hopper dredgers) and
the distance involved are too long for hydraulic transport. For environmental
project, which requires the dredging of “in situ densities”, the bucket dredger
is suitable.
Bucket dredgers also
come in a variety of types. For example:
1) Dredgers with or without
the means of propulsion.
2) Dredgers with a conveyor
belt.
Picture 5: A Bucket Ladder Dredger without The Means of Propulsion
Picture 6: Floating Conveyor Belts (Catamaran Type)
The maximum dredging
depth is highly dependent on the size of the dredger. There are dredgers with a
maximum dredging depth of more than 30 meters. For such large dredgers, the
minimum depth is often 8 meters.
Bucket dredgers can be
used in almost every type of soil, from mud to soft rock. When rock has been
fragmented by blasting, bucket dredgers are often used because of their
relative lack of sensitivity to variations in the size of the stones. Bucket dredgers can’t be
used in areas with waves and swell. Furthermore, because of the amount of noise
they produce, in urban areas they are often subject to restrictions in relation
to the working time or the permitted number of decibels measured at a specific
distance from them. The capacity of a bucket dredger is expressed in terms of
the content of the buckets. The capacity of bucket can vary between 30 and 1200
liters.
1.3 Working Method
When a bucket dredger is
working, the anchoring plays an important role in both positioning the dredger
in the cut and in the excavation by the buckets. As mentioned previously, the
dredger swings round the bow anchors. The bow wire has a length of 1 to 2 times
the bucket capacity in liters. This means that for the large dredgers it may be
1 to 2 km long. It will be clear that with such great lengths measures must be
taken to prevent the radius of the swing circle from being reduced by the bow
wire being dragged over the bottom. Over water, therefore, one or more
pontoons/float/bow barges are positioned under the bow wire. If the wire runs
mainly over land, it is placed on drummer roller.
The swinging of the
dredger and the provision of the excavation forces is mainly carried out by the
side winches. The side winch velocity used depends on the type of soil and also
on the "step length" and the height of the cut. For the most effective possible
transition of forces the side wires must make an angle with the bow wire that
is a little smaller than 90 degree.
When swinging round the
bow anchor, the swing angle (ß) that the dredger makes with swing circle must be kept as
constant as possible. The choice of the swing angle is related to the clearance
between the buckets on the lower part of the chain over the bottom or the
slope. If this is not done, it is possible that the bucket chain will run off
the bottom tumbler as a result of the lateral forces that act on it. At the
beginning of a new cut the swing angle is brought to the desired value as
quickly as possible. If there is a current in the dredging area, the swing
angle must be kept as large as possible, at 90 degree. The stem winch controls
the swing angle. The stem anchor is used to obtain the required tension in the
bow wire.
The step length, the cut
thickness and the swing velocity along the cut determine the amount of soil
that is cut per unit of time. This amount must be at least in balance with the
number of buckets per unit of time multiplied by the capacity of the buckets.
In other words the bucket capacity and the bucket speed are related to the
factors mentioned above. Some dredgers have more than one type of bucket, so
that, depending on the soil type, the capacity can be adapted to the expected
production. Because with high excavation forces the dredger will not be able to
completely fill the buckets, so that they are partly filled with water. This is
of course not economical. The position of the
ladder, particular the ladder angle, also effects the maximum filling degree of
the buckets. If the bucket rim is not horizontal, fluid soil will partly flow
out of the bucket.
1.4 The Design
When designing bucket
dredgers the following design parameters are important:
1) Productivity capacity
2) Dredging depth (minimum
and maximum)
3) Soil type
4) The discharge of the
dredged material (barges or via pipelines)
As previously mentioned,
the bucket dredger can be used in all types of soil from clay to soft rock
which hasn’t been blasted and hard rock which has been fragmented by blasting.
The type of soil to be dredged has a big influence on the design and the
construction of the dredger. Considerable forces arise during the dredging of
rock. For all types of soil it is necessary to know the required cutting
capacity and the energy that is needed to transport the dredged material via
the bucket chain to the upper tumbler.
1.5 The Soil
The influence of the
soil to be dredged is seen in the power of the upper tumbler, the strength of
the ladder, links and buckets and also in the bucket capacity and shape. If a
bucket dredger is equipped with buckets for both soft soil and rock, the
capacity of the rock buckets is roughly 60 to 70% of that of the soil buckets.
1.6 The Winches
The winches on a bucket
dredger have various functions and therefore various requirements with regard
to the power, the forces and band velocity, which differ from winch to winch.
1.6.1 The Ladder Winch
The ladder winch, which
is used to adjust the required dredging depth is usually mounted on the ladder
gantry of the larger bucket dredgers, while the smaller demountable dredgers
usually have the ladder winch mounted on deck.
Owing to the greater
weight of the ladder and the buckets this is the strongest winch on the bucket
dredger. The ladder winch velocity is roughly between 6 and 10 /min. The drive is
usually a slow running electric or hydraulic engine. Because of the need
setting the dredging depth, it is necessary to have an adjustable winch.
1.6.2 The Bow Side
Winches
The installed bow side winch power is between 10% and 20% of the main drive. The
side winch velocity of the bucket dredger is generally lower than that of the
cutter suction dredger. Nominal side winch velocities lie between 10 and 15
m/min. It will be clear that the excavation process requires a winch that can
be well controlled and adjusted.
1.6.3 The Stern Side Winches
The stern side winches
have a secondary function and do not determine the production. The stern
winches control the dredger with regard to the cut (swing angle ß). The requirements
relating to the control and the forces are thus considerably less than for the bow
side winches. The power is roughly half that of the bow side winches. The
nominal side winch velocities are of course equal. The stern side winches are
usually mounted on the afterdeck. To avoid hindering the arrival and departure
of barges, as well as the warping of the barges alongside the dredger, the side
wires are led down to a sufficient depth directly beside the dredger in
vertical guides, also called wire spuds.
Picture 7: The Wire Spud Construction
1.6.4 The Bow Winch
The bow winch is used to
pull the dredger forwards when a new cut is started. The required force for
this lies in the same order of magnitude as for the side winch. The required
velocity, however, is considerably lower (nominally 2 – 3 m/min). Higher speed
are, of course, necessary when positioning the bow anchor.
Picture 8: Main Features of Bucket Dredger
1.6.5 The Stern Winch
The function of the
stern winch is to ensure the required tension in the bow wire. This
consideration demands the required force which is roughly equal to that of the
bow wire. However, the need to move the bucket dredger backward quickly to the
adjacent cut places higher demands on the velocity (5 – 10 m/min).
