The present invention relates to dredging apparatus.

Known suction dredgers remove material from the sea bed by stirring up the sea bed and by pumping water that is adjacent to the sea bed to the surface through a pipe. The moving water entrains silt or sand from the sea bed and pumps it to the surface with the water as a slurry or suspension. The silt or sand is then separated from the water which is returned to the sea. The dredged solid is then retained and later dumped at another location. Alternatively, the pumped suspension can be conveyed by pipeline to another location and dumped.

The term suspension is to be taken to mean solid particles that are at least temporarily suspended in a body of liquid. The term emulsion is used to mean a finer suspension.

A problem with both of these methods is the transportation of the dredged material to a dumping location. To do this the dredging vessel must travel to the dumping ground, or barges must be provided, or a slurry pipeline must be provided. All of these transport methods add considerably to the dredging costs.

The present invention seeks to provide a dredging apparatus and method which does not require apparatus to carry the dredged material to a dumping location.

According to a first aspect of the invention there is provided an apparatus for dredging the bed of a body of

liquid comprising means for dredging solid material from the bed, means for agitating the dredged material with liquid to form an emulsion of solid particles and means for releasing it into the body of liquid.

In a preferred arrangement, the means for dredging pumps the solid to a dredging platform entrained in part of the liquid.

Preferably, the means for agitating the material comprises a duct or tank containing one or more rotor means.

Preferably, the dredging platform comprises one or more pontoons. The means for dredging may comprise one or more pipes extending to the bed. The pipe(s) may be mounted on a movable member. The member may comprise a crane jib, preferably a back-hoe crane jib.

Preferably, the the means for releasing further agitates the emulsion prior to discharge. The pumping rate may equal the release rate.

Preferably, the means for releasing comprises diffuser means, through which the pumped solid and liquid passes.

According to second aspect of the invention there is proved a method of dredging the bed of a body of liquid which flows at least part of the time, comprising dredging solid material from the bed, agitating the solid material with liquid to form an emulsion of solid particles at a desired concentration, and then

releasing the emulsion at a desired velocity into the

body of liquid at a time at which it is flowing.

The liquid is preferably water and the body of liquid may be a harbour, river or any littoral region where there is a tidal flow.

According to a third aspect of the invention there is provided a device to agitate the particles in an emulsion comprising a duct along which emulsion can flow and exit means in the walls of the duct through which the emulsion can leave the duct.

In a preferred arrangement, the exit means comprise a plurality of outlet pipes. The diameter of the outlet pipes is small, relative to the width of the duct.

Preferably, a surface is positioned adjacent to the end of each outlet pipe, whereby the particles in the emulsion collide with the surface as they leave the device.

According to a fourth aspect of the invention there is provided a device to reduce the particle size of solids in a solid/fluid mixture comprising a flow path, along which the mixture passes, passing through size reduction means, wherein the size reduction means comprise one or more rotating shafts each having one or more blades mounted thereon.

In order that the invention and its various other features may be understood more easily, an embodiment thereof will now be described by way of example only.

Fig.l is a side elevation of a dredging barge according to the invention in operation; the sea and sea bed are

shown in section;

Fig.2 is a view from direction 2 in Fig.l;

Fig.3 is a schematic plan of the dredging barge dredging a harbour;

Fig.4 is a plan view of a diffuser unit comprising part of the dredging barge;

Fig. 5 is a schematic side view of the device shown in Fig. 4;

Fig. 6 is a plan view of a liquidiser unit comprising part of the dredging barge, with one part of the unit shown in cross section; and

Fig. 7 is a section on 7-7 in Fig. 6.

The dredging barge shown in Fig.l comprises a dredging unit 10 and control unit 30 mounted on two general work pontoons 40.

The dredging unit 10 comprises a rotatable fifteen to twenty ton crane body 11 and a back-hoe excavator jib

12. On the end of the jib is mounted a submersible

DP150B pump 13. A pipe 14 leads from the pump 13 along the jib. The pump conveys a suspension of water and the bed material through the pipe. A radioactive density meter 31 measures the solid concentration in the suspension. A 25mm grill 34 passes across the pipe.

The suspension is mixed with compressed air and passed through a liquidiser unit 90. The suspension then passes to a diffuser unit 71 mounted on the stern of the pontoons. A bypass valve 32 can be opened to pass

some of the suspension through a return pipe 33.

The control unit 30 contains all of the equipment needed to navigate the dredging barge. It also provides a vantage point for overseeing dredging operations.

The dredging barge can be propelled by two motors 41 which can position the barge. The pontoons 40 contain ballast tanks and crew accommodation quarters. In use, the barge is moored to two spud posts 43.

During dredging the excavator is moved through the sea bed 60 by moving the jib 12, which stirs up sand and silt on the sea bed. The silt and sand are then pumped through the pipe 14 by the pump 13, along with a quantity of sea water, up to the dredging barge. The pumped mixture forms a suspension which is passed through the liquidiser unit 90 to reduce the size of particles in the mixture to produce an emulsion. The mixture is then delivered to the diffuser unit 71 which mixes the emulsion by further agitating it before it is discharged back into the sea.

The grill 34 catches stones and other large particles that are brought up from the sea bed. To clear the grill bypass valve 32 is opened and the stones pass along the return pipe 33 and into the sea.

The crane body 11 can be rotated to dredge different sections of the sea bed. The ballast tanks allow the trim of the barge to be altered to compensate for different crane body and jib 12 positions.

Fig.3 illustrates the principle of operation of the

dredging barge 80. The barge is operated at the edge of the shallows 81 (indicated by dot-dash lines) in a harbour. The barge only operates when the tide is ebbing and water is carried out of the harbour mouth 82.

The dredger pumps material from the sea bed and releases it back into the flowing tide stream so that, rather than settling back onto the bed of the harbour, the material is carried out to sea by the ebb tide.

In order to achieve this, it is necessary to carefully control the concentration of solid material returned to the sea. If the particles concentration is too low the dredging operation will be uneconomic. However, if the particle concentration is too high the settling rate of the particles will be too high, and settling may occur before the material has left the harbour.

In order to reduce the rate of settling it is necessary to thoroughly mix the emulsion prior to its discharge. This is done by agitating the emulsion by passing it through the diffusor unit 71.

The present pump has an output of 720 m /hr and is operated with a solids concentration in the range 19-35% (weight %) . A range of 19-27% is preferred and 19% is considered optimal.

The structure of one type of diffuser unit 71 is shown in Figs.4 and 5. The unit comprises a manifold pipe 72 from which outlet pipes 73 extend at right angles. An end plate 74 is positioned at the end of each outlet pipe. At the end of the manifold is a removable end cover 75, secured by a clamp assembly 76.

The principle of operation of this type of diffuser unit is illustrated in Fig 5. Pumped emulsion from pipe 14 enters the manifold pipe 72 and passes into the outlet pipes 73. There is a slight reduction in area passing from pipe 14 to the manifold which accelerates the emulsion. The emulsion leaves the outlet pipes through apertures 77. The narrow outlet pipes further increase the velocity of the emulsion. As the emulsion leaves the apertures it impinges upon the end plates 74. The particles in the emulsion collide with the end plates, thereby further agitating the emulsion.

The unit is positioned just below the sea level 100 so that the emulsion is discharged upwards to form a cloud of particles 101 within the body of liquid.

The removable end plate 74 allows easy access to the interior of the diffuser unit to clear any pumped debris that may block the unit.

The diffuser unit shown in Figs. 1 and 2 is of a different type, comprising a single duct branching into two exit pipes 78 to form a Y shape. There is an end plate at the end of each exit pipe. The two exit pipes eject plumes of emulsion over the surface of the water.

Figs. 6 and 7 illustrate the principle of operation of the liquidiser unit 90. The unit comprises four drums 91 having flanged ends which are linked by ducts 92, 93, 94. The unit is supported by brace members 95. Each drum contains a shaft 96 driven by an air motor 97. On each shaft are mounted eight elongate rotor blades 98 (internal details only shown in one drum in Fig. 6, rotor blades not shown in Fig. 7). Compressed air is passed into the unit along air inlets 99 in the

inlet duct 92. The exit duct 94 has a smaller flow area than the other ducts 92,93.

The function of the liquidiser unit is to break up large lumps of sea bed material, often clay, which would otherwise pass straight into the sea and sink immediately. A further function of the unit is to increase the degree of agitation of the suspension.

Suspension enters the unit through the inlet duct 92. The suspension passes from drum 91 to drum 92, through ducts 93. In each drum the suspension is acted on by the rotors 98 to produce a fine emulsion. Compressed air is injected into the emulsion to increase the break up of the lumps of bed material. The compressed air also oxygenates the emulsion.

An alternative configuration for the liquidiser unit is a single pipe having a number of shafts extending radially across it, each shaft carrying a number of rotor blades.

The present dredging apparatus and method has the advantage that the dredged material is carried to the dumping site by the action of the tide. This greatly reduces the cost of dredging.

As the dredging barge remains stationary it does not interfere with traffic movement in the harbour. A number of modifications are possible to the dredging barge and method described. The configuration of the dredging barge can be altered. The barge can have only a single hull. For some applications the dredging unit 10 can be mounted on a jetty or on a dockside.

The back-hoe excavator jib 12 can be replaced by any device which stirs up the sea bed, such as cutter head or a water jet.

The diffuser unit can be replaced by any suitable device which further agitates the pumped mixture.

In some applications it is envisaged that the diffuser unit can be dispensed with. For instance, if the distance to the dumping site is short.

The dredging barge can be fitted with a holding tank to hold pumped mixture the contents of which can be agitated, e.g by stirring, before release into the water. In some applications it may be necessary to agitate the pumped mixture continuously to prevent settling. This allows dredging to be carried out at any time, in particular in daylight and good weather, and release to take place on the ebb.

The use of a holding tank makes the basic dredging method applicable to other dredging techniques, in particular grab dredging and endless bucket dredging. With these, the dredged material is mixed with water in a holding tank, agitated, and is released on the ebb tide. No diffuser unit is needed.

The dredging method can be used on inland rivers. Here, continuous operation is possible. To enhance the rate at which material is removed from the dredging site the engines can be run during operation.

The pipe 14 can be replaced by a plurality of pipes connected in parallel.

For some applications it may only be necessary to raise the pumped mixture part way to the surface.

The outlet pipes 73 in the diffuser unit 71 can be replaced with slots in the wall of the duct 72.

The number and shape of the rotor blades 98 in the liquidiser unit 96 can be varied. The blades can be bent at their ends, The blades can incorporate chain sections to form flails. The blades can be replaced with lengths of chain.

The flexibility of the apparatus allows it also to be used for conventional dredging. The return pipe 33 can be used to pass the suspension into a barge for settling and transport to a remote dumping site. The submersible pump can be replaced with a grab for work on rocky areas of the seabed.

As the liquidiser unit oxygenates the silt this biologically regenerates the solid material in the emulsion. This enhances the growth of marine life when the silt is redeposited.