MECHANICAL DREDGERS: THE BUCKET LADDER DREDGERS

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).

SOURCE: Delf Open Course