GB2300014A - Dredging - Google Patents

Abstract

In a method of dredging using a plough 3 to scoop up material from the bed of a body of water, jets 8 of water are squirted from nozzles 9 located on the back of the plough so as to loosen the layer of sediment 12 directly below the layer 13 which is being removed. A vibrator or rake may be used instead of the jets of water to loosen the sediment layer 12. The softer, prepared sediment 12 is easier to scoop up during the subsequent passes of the plough 3 and therefore requires a less powerful tugboat 1 to perform the dredging operation.

Description

Dredging System The present invention relates to a dredging system.

In bodies of water, such as harbours, basins and lakes, solid particles are carried by means of a water flow, and then settle on the bottom of the relatively still portions of the water body. In due course, the solid particles form a thick layer which is not disturbed by further flow movements. Such a water body might also be subjected to tidal flows, wherein during the entering flood-tide flow, particles are fed in, but during the returning ebb-tide flow, not all the particles are fed back, so that deposition takes place.

It is known to use a plough to level off the seabed, riverbeds, channels and harbours and/or reduce the level thereof by scooping up the material and transporting it elsewhere. The plough, which has the shape of a wide dragline bracket, is lowered to the bottom or bed of the water body and is towed through the material. When the plough is full, it is raised from the bed and transported to a dumping area where its contents are dumped. When a thick layer has to be removed the plough has to make several passes over the same area.

The dumping area is usually lower than the area being dredged. Ideally, the dumping area is not only a lower area in the vicinity of the dredging location e.g.

a harbour basin, but is also adjacent to a river. This river may then absorb the sediment particles and subsequently discharge them in its own natural way.

Other methods employed by many harbour authorities in their constant battle to maintain adequate depth at their ports, is to use bucket-, grab-, backhoe- or suction hopper dredgers. Vessels of this kind, and in particular hopper dredgers are very expensive to run and therefore make the dredging work costly.

It is also known to completely disintegrate and disperse the layers of sediment by using powerful vibrators, or by injecting water or a combination of water and air into the mud, silt or clay sediment layers. Such action puts the sediment back into suspension with the water, and usually relies upon the flow of water to carry the sediment away downstream to a location where it will not pose a problem.

Alternatively water could be injected into the sediment to produce a density current which can be used to drive the sediment to the required location.

The main disadvantage with this type of dredging is that it can be harmful to life in the body of water because it produces a large underwater cloud of suspended sediment, which can suffocate fish and other aquatic life. Also there is the problem that eventual deposition of the sediment is unpredictable and it may simply lead to the sediment being carried downstream and deposited in a basin or harbour causing further problems.

Thus, viewed from a first aspect, the present invention provides a method of dredging material disposed on the bed of a body of water, comprising urging a material engaging member across the bed to scoop up material, wherein the material is loosened by loosening means prior to being scooped up.

Viewed from a second aspect, the present invention provides apparatus for dredging material disposed on the bed of a body of water, comprising a material engaging member which can be urged across the bed to scoop up material, and loosening means to loosen the material prior to its being scooped up.

An advantage of the present invention is that it enables the material engaging member, which is preferably in the form of a plough, to be more easily urged through the material because the material has been previously loosened. Hence, a less powerful propulsion unit is required, thereby reducing the cost of dredging.

Alternatively, existing towing vessels may be used to tow larger ploughs or other engaging members.

This invention is based on the principle that the mud, silt, clay layers remain intact and are only made softer for easy removal by a plough or other engaging member. The material can be transformed into a spoil with a lower internal friction. The sediment substances are loosened or softened into a more liquified condition, making it easier to scrape them up because less force is required. This may be particularly effective on deep layers of sediment where the weight of the water and sediment above has compressed and hardened the sediment.

The loosening means may be a vibrator or rake like device, but preferably comprises a device which directs jets of fluid at the layer of sediment. Such a device may comprise a plurality of laterally spaced nozzles.

Preferably the fluid is water, but it may also be a water/air combination. Disintegration of the layers of sediment may be avoided by careful regulation of the fluid supply and pressure at e.g. jetting nozzles. The aim is to simply reduce the density of the material which makes its removal by mechanical means easier, thereby increasing production.

Preferably the loosening means is arranged to loosen material behind the engaging member. Thus, in use, the material is loosened by the loosening means during a first pass over the bed by the engaging member and loosening means, and is scooped up by the engaging member during a second such pass over the bed. Hence, the loosening means will loosen the layer of material immediately below that which has just been scooped up by the engaging means during a particular pass. It is also preferable that the loosening means does not extend beyond the sides of the engaging member, as this not only keeps the drag to a minimum, but also reduces the likelihood of damage to the equipment from striking underwater debris.However, other embodiments are envisaged in which the loosening means is positioned to one side of the engaging member to prepare a layer or strip of bed material adjacent to the one being removed.

Alternatively, it may even be positioned directly in front of the engaging member. However, both of these arrangements will make the loosening means more susceptible to damage from underwater debris and are likely to increase the drag, requiring a more powerful and expensive towing vehicle, making them less desirable.

The present invention may be used in bodies of water in which there is a flow or where a flow is absent.

In a preferred embodiment, a row of waterjets are fitted to the back of a plough. These jets preferably squirt pressurised water in a jetting action to loosen the material behind the plough as it passes over an area of bed material making the second and subsequent passes much easier, thereby enhancing the production and efficiency. The jetting action of this preferred apparatus may be suspended if the sediment is too soft and the production is not enhanced by its action. The jetting action is directed downwardly towards the seabed or riverbed or the like preferably through a series of nozzles fitted to the bottom of a tube which is mounted to the back of the plough. Preferably, these are level with the bottom of the plough to have the most impact.

The length of the tube is preferably the same as the width of the plough. The size, number and the distance between the nozzles can be varied depending for example on hardness of the material.

Preferably the fluid is pressurised by means of a pump located on board the vessel towing the engaging member. Alternatively, the pump may be mounted on the plough and draw water from close to the bed of material to be dredged. Other methods of producing jets of fluid are envisaged, for example by fitting a number of downwardly directed impellers which direct water flow through a baffle arrangement into a series of water jets.

When dredging in accordance with a preferred method of the present invention, it is best not to use the jetting action during the final pass. Preferably dredging is achieved by removing the spoil strip-wise and layer by layer, with the layer thickness being related to the size of the plough or other extracting means, and the hardness of the material.

Preferably, the engaging member is urged across the bed in a direction towards a dumping area. By this method, each subsequent pass can follow the same route, and avoid spillage of sediment over the already dredged area(s). For example, in a harbour basin situated adjacent to a deeper flowing body of water, dredging may start from a position away from the mouth of the harbour basin and then progress along a path which is directed towards the mouth of the basin.

The sediment (spoil) is taken from the basin bed, transported and may be disposed of in a river, for example, where it is taken away by the flowing river water. With some harbour basins it may be prudent and more desirable not to deposit the material into the river but to discharge the sediments/spoil in a different way e.g. in a recess or well in the bottom of the area to be treated. Then the mouth of a suction pipe may be placed into this recess or well to suck up the excavated material which is subsequently transported away for disposal elsewhere. Alternatively, the sediment can be strongly diluted by adding additional water and pumped into a river to be carried away by the current. In this way the sediment particles will be taken up in the upper water layers and be discharged at sea. The chance of renewed settlement within the river is then small.

Some preferred embodiments of the present invention will now be described by way of example only and with reference to the accompanying drawings, in which: Figure 1 shows a plan view of an area to be dredged with its adjacent disposal area; Figure 2 is a cross-sectional view of Figure 1 along the line A-A showing the utilisation of the preferred embodiment; Figure 3 is an enlargement of the preferred apparatus shown in Figure 2; Figure 4 is a plan view of the preferred apparatus of Figure 3; Figure 5 is a perspective view of the preferred embodiment showing a plough with a preferred attached water injection system; and Figure 6 is a side view of another embodiment showing a plough having the waterjet nozzles of the water injection system raised above the bottom of the plough.

Figure 1 shows a spoil tip 16 as well as an area to be treated 17 (the high area). The high area is, in plan, divided into parallel strips i.e. a, b, c, d, e, f and g. The width of the strips is related to the width of a plough 3. The plough 3 has the shape of a wide dragline bucket (see Figure 5) and is operated in much the same way, albeit under water. Figure 2 shows the layers to be removed i.e. 10, 11, 12 and 13. The thickness of the layers depend on the size of the plough and the hardness of the sediment.

When the top layer 13 is being removed by the plough 3, waterjets 8 are activated to loosen the material in the underlying layer 12. This is repeated for the underlying layers 11 and 10 on subsequent passes. The water for the waterjets 8 is pushed through nozzles 9 which are fitted at the bottom of a transfer tube 7 which is fitted to the back of the plough 3, generally level with the bottom or inserted into the bed material for maximum effect. However, in some sediment types it may be beneficial to have the transfer tube 7 fitted at a higher position so that the jets 8 are off the bed as this will produce less disturbance. This is shown in Figure 6. The nozzles 9 preferably point vertically downward. The water may be pressurised by a suitable pump 15 located on the boat 1, the pump 15 pumping water to the nozzles 9 via a flexible tubing 6.

The plough 3 is raised and lowered by a hoisting wire 4 which is led over a hoisting frame 2. Wire 5 is the pulling wire. Both the pulling wire 5 and the hoisting wire 4 may be made rigid by utilising steel girders, holding the plough more firmly on the bed.

The equipment is either mounted on a tugboat 1 or a pontoon. The latter could be self-propelled or it could be pulled or pushed by a tugboat. Both the tugboat and the plough move ahead in the direction of the arrow labelled 14.

The plough 3 with activated jets 8 is pulled ahead in direction 14 at the required depth until the plough is full of spoil, at which point it is lifted clear of the bed by the hoisting wire 4 and transported to the spoil tip 16 where the spoil is tipped. These motions are repeated until the required depth has been obtained over the whole of the area.

The volume of water and the amount of pressure applied at the bottom rear end of the plough 3 depends on many factors such as the type of material to be treated, its density, and the depth of the water.

The nozzles 9 are interchangeable i.e. different sizes can be used and nozzles may be closed off depending on the condition of the sediment.

When the plough 3 with attached tube 7 is lowered on the bed and water is injected into the mud, silt or clay layer, the layer(s) will expand due to the supply of water and the internal friction amongst the particles will be reduced for easy subsequent removal by the plough 3.

It will thus be seen that in the preferred embodiment described above the waterjets enable the material of the riverbed, seabed or the like to be loosened very easily and controllably without the production of a large sediment cloud. It is not the intention to loosen the sediment to an extent where it becomes waterborne once more and disperses, but only to lessen the internal friction of the material so that it may be easily scooped up by the engaging member. Hence, the loosening means needs only to loosen the material and does not have to be as powerful as those devices which completely disintegrate and disperse the layers of sediment.

Thus, at least in the illustrated embodiments of the present invention there is provided a simple and comparatively cheap method to remove deposits of mud, silt, clay and sand.

Claims (12)

Claims

1. A method of dredging material disposed on the bed of a body of water, comprising urging a material engaging member across the bed to scoop up material, wherein the material is loosened by loosening means prior to being scooped up.

2. A method as claimed in claim 1, wherein the material is loosened by the loosening means during a first pass over the bed by the engaging member and loosening means, and is scooped up by the engaging member during a second such pass over the bed.

3. A method as claimed in claim 1 or 2, wherein the material is loosened by directing a fluid into it.

4. A method as claimed in claim 1, 2 or 3, wherein the loosened material is scooped up by a plough.

5. A method as claimed in any preceding claim, wherein the engaging member is urged across the bed in a direction towards a dumping area.

6. Apparatus for dredging material disposed on the bed of a body of water, comprising a material engaging member which can be urged across the bed to scoop up material, and loosening means to loosen the material prior to its being scooped up.

7. Apparatus as claimed in claim 6, wherein the loosening means is arranged to loosen material behind the engaging member.

8. Apparatus as claimed in claim 6 or 7, wherein the loosening means comprises a device for directing a fluid into the material.

9. Apparatus as claimed in claim 8, wherein the device comprises a plurality of laterally spaced nozzles.

10. Apparatus as claimed in any of claims 6 to 9, wherein the engaging member comprises a plough.

11. A method substantially as hereinbefore described with reference to Figures 2 to 6.

12. An apparatus substantially as hereinbefore described with reference to Figures 2 to 6.