Device and method for dredging bed material under water

Abstract

 The invention relates to a device and method for dredging bed material under water. The device (1) comprises a vessel (4) which can be anchored to an underwater bed (2) and has a ladder (8) which can be positioned downwards at an angle (6) into the body of water (7) from the vessel (4) and which is provided with a cutting tool (10) at its free end, the cutting tool (10) comprising a number of excavating elements (103, 105) arranged around a common rotation axis (101). The cutting tool (10) comprises a passage (106) for dug-up bed material which adjoins an inlet end of a discharge means (13) for the dug-up bed material, which discharge means (13) comprises a carrier (13) for the bed material which is driven by means of a mechanical drive, which carrier comprises two conveyor belts which rotate in opposite directions of rotation across each other at the same time. The invention makes it possible to dig up underwater beds while preventing sediment spill.

Description

[0001] The invention relates to a device for dredging bed material under water. The invention also relates to a method for dredging bed material under water using the inventive device.

[0002] Dredging operations commonly form part of hydraulic engineering activities, such as for example in port development, tunnel construction, urban development, beach replenishment, coastal protection, installation of power plants, such as wind turbines, mining, environmental improvement and the like.

[0003] With dredging, it is obviously important to work efficiently and to remove as much bed material as possible per unit time using as little power as possible. However, in the last few years, increasing attention has been paid to the effects of dredging on the surrounding environment, in particular the flora and fauna. This applies in particular to dredging operations which are being carried out in the vicinity of nature reserves.

[0004] A significant problem in this context is sediment spill. During digging, dredging, the transportation and/or dumping of dug up bed material, part of the latter will remain behind in the water as relatively fine sediment. Such sediment clouds can significantly cloud the water and this increased turbidity disturbs the natural environment.

[0005] A known device for dredging an underwater bed is described in NL-1031253. The known cutter head is a rotating body which is rotatable about an axis and is formed by a base ring and a boss which is arranged concentrically at a distance therefrom, between which a number of arms extend which are provided with cutting tools, such as teeth. Through-openings are provided between the arms via which the dredged bed material can be discharged. The cutter head is used in combination with a cutter suction dredger (also referred to as a cutter dredger). A cutter dredger comprises a vessel which is anchored to the floor by means of spud piles. The anchorage absorbs the reactive forces occurring during dredging and transmits these to the floor. A suction pipe is attached to the ladder of the cutter dredger which is connected to the cutter head and via which the dredged bed material, mixed with water, is removed. During dredging, the cutter head is made to rotate and lowered into the water together with the ladder and the suction pipe, usually at an oblique angle, until it touches the floor. The cutter head is then dragged through the floor while rotating by alternately shifting the ladder from port to starboard using winches, in which case the cutter head performs a lateral slewing movement. By in each case shifting the cutter dredger by a certain distance and repeating the above-mentioned slewing movement, it is possible to dredge a complete floor surface. The suction pipe is connected to a pump which then pumps the dredge spoil to a barge which is moored alongside via a pressure pipe or directly to a dumping zone.

[0006] The hydraulic transportation of the known cutter dredger has the drawback that, depending on type of floor, a relatively large amount of fine clay and silt particles are brought into an aqueous suspension and the removal of these particles therefrom is very time-consuming and laborious. This may result in a relatively large amount of sediment spill during loading of the barges or when forcing up in the dumping zone. This technique can therefore not always be used in the vicinity of nature reserves.

[0007] It is an object of the present invention to provide a device for dredging bed material under water which, in particular, makes it possible to dredge underwater beds in an efficient manner without causing a great degree of sediment spill and, in particular at relatively large depths and with relatively hard beds, with a significantly greater efficiency than can be achieved with other mechanical excavators, such as dredging cranes or bucket dredgers.

[0008] To this end, the invention provides a device, comprising a vessel which can be anchored to an underwater bed and having a ladder which can be positioned downwards at an angle into the body of water from the vessel and which is provided with at least one rotating cutting tool at its free end, the cutting tool comprising a number of excavating elements arranged around a common rotation axis, and having a passage for dug-up bed material which adjoins an inlet end of a discharge means for the dug-up bed material, which discharge means comprises a carrier for the bed material which is driven by means of a mechanical drive. An outlet end of the discharge means can discharge at any desired position, but will usually be situated at the location of the vessel, for example on the deck of the vessel.

[0009] The carrier preferably comprises at least two conveyor belts which rotate in opposite directions of rotation across each other at the same time. In such an assembly of two or more conveyor belts which are mutually stacked, the conveyor belts run substantially parallel with one another in the same direction of movement. By having a second conveyor belt rotate (counterclockwise) at the same time at least above the conveyor belt, the dredged bed material, which is caught between the two conveyor belts, is transported away in an upward direction more efficiently. Further, it is found that the amount of sediment spill is substantially reduced. To further improve on preventing sediment spill, the conveyor belts are preferably curved in the transverse direction (perpendicular to the direction of movement), and more preferably such that the mutual distance between the conveyor belts is largest at their mid-section and decreases towards the longitudinal edges of the conveyor belts.

[0010] The rotating cutting tool may in principle comprise any cutting tool which is able to dig up bed material. In an embodiment, the at least one rotating cutting tool may comprise a rotating body which is rotatable about an axis and is formed by a base ring and a boss which is arranged concentrically at a distance therefrom, between which a number of supporting arms extend which are provided with cutting tools, for example in the form of cutter bits.

[0011] In another, more advantageous embodiment, the rotating cutting tool comprises a cutter-wheel dredger, in which the cutter-wheel dredger comprises a wheel which is rotatably driven about a horizontal axis and which is provided on the outer circumference with a series of excavating buckets which are distributed along the circumference and whose inlet opening faces forwards viewed in the direction of rotation, and in which the outer circumference of the wheel comprises a passage for dug-up bed material, which passage adjoins the inlet end of the discharge means for the dug-up bed material.

[0012] Surprisingly, combining the rotating cutting tool with the described discharge means for the dug-up bed material results in a lower sediment spill than is the case with the known cutter dredger. In addition, using the latter, it is possible, in particular at relatively great depths and with relatively hard beds, to achieve a significantly greater efficiency than is the case with other mechanical excavators, such as dredging cranes or bucket dredgers. The inventive device is therefore advantageously used with a method for dredging bed material under water which comprises the steps of lowering the ladder and the discharge means of the device into the water; loosening the bed material by rotating the cutting tool; carrying away dredged bed material onto the carrier of the discharge means via the passage; and transporting the dredged bed material by driving the carrier.

[0013] The increased efficiency of the inventive device compared to the known dredging cranes, in particular at relatively large depths, is achieved by the fact that the present invention makes a virtually continuous digging process possible. When using dredging cranes, the digging process is only a part of the dredging cycle. This is due to the fact that a relatively large proportion of the cycle is spent displacing (lifting, slewing, easing) and dumping the dredged bed material and excavating bucket. As excavating progresses to greater depths, the proportion of the digging process in the dredging cycle gradually decreases and the dredging cycle thus becomes less efficient. Using the present invention, it is in addition possible to achieve a significantly better dredging tolerance compared to dredging cranes, which is very important, for example when dredging tunnel trenches. In addition, due to the improved dredging tolerance, the efficiency is increased further since overdredging of the floor is prevented.

[0014] The improved efficiency compared to the bucket dredger known from, for example, US 1,540,276, in particular with relatively hard floors, is due to the fact that the drive for the digging process and the drive for the (mainly vertical) transportation of the dredged floor have been disconnected. With a bucket dredger, the available digging capacity is relatively limited, since the forces for the digging process have to be transmitted by the drive wheel of the bucket chain, by the bucket chain itself and by each of the buckets in turn. Using larger digging capacities would require each of these parts having to be made stronger, which would result in an excessively heavy and expensive construction. A further advantage of the inventive device is that the speeds of the rotating cutting tool (the rotational speed) and those of the discharge means (the discharge speed) can be selected independently from one another. This further increases efficiency.

[0015] In an embodiment of the invention, a device is provided in which the carrier of the discharge means is provided with a covering. This has a positive effect in further reducing the sediment spill.

[0016] In another embodiment of the invention, the device comprises a suction pipe which is connected to pump means and which extends from the vessel up to the vicinity of the inlet end of the discharge means. Such a suction pipe makes it possible to suck up any bed material originating from the discharge means which has collected at the location of the inlet end and discharge it to the vessel. In addition, the suction pipe can also create a slight underpressure at the location of the inlet end and the rotating cutting tool, creating an inflow by the cutting tool which can further reduce the turbidity in the surrounding area. In order to achieve said effects, the cross section of the suction pipe and the capacity and flow rate of the pump means can be significantly lower than the cross section and capacity of a the suction pipe or pump means which are common for a cutter dredger, for example less than 25%.

[0017] In yet another embodiment of the invention, a device is provided in which at least one outlet end of the discharge means is provided with a scraping device. This makes it possible to catch more sticky bed material, such as clay, as a result of which the efficiency of the dredging process for this type of bed material is increased further.

[0018] To this end, the mechanical drive of the carrier of the discharge means may comprise any drive which is suitable therefor. A suitable embodiment of the device according to the invention comprises a mechanical drive in the form of endless chains running between two wheels, in which at least one wheel is drivable, and the carrier is connected to the chains.

[0019] The carrier of the discharge means can also be chosen within wide limits, as long as the discharge of dug-up bed material does not take place hydraulically, causing substantial mixing with water. In this context, a suitable embodiment of the device comprises a carrier in the form of a conveyor belt. Such a carrier makes it possible to discharge dug-up bed material substantially continuously. In this case, it may be advantageous, in particular, to make transportation at a relatively steep angle possible if the conveyor belt is provided with retaining means for the bed material on a load-bearing surface, such as for example V-shaped ribs.

[0020] It is advantageous to use an embodiment of the device according to the invention in which the carrier comprises a series of buckets connected to the mechanical drive, in particular for digging up bed material from a relatively great depth, in particular from depths greater than 25 m from the water surface. This is due to the fact that the discharge means can be placed at a relatively steep angle into the body of water by means of such a carrier, as a result of which it is possible to reach greater depths with the same length of the discharge means. Preferably, said buckets are of a relatively lightweight design, as a result of which they cannot be used to dig up bed material, but only to transport away the bed material which has been dug up by the cutter-wheel dredger.

[0021] In a suitable embodiment, the device according to the invention comprises one cutting tool which is rotatable about a horizontal axis, and in particular a cutter-wheel dredger which comprises one wheel which is rotatably driven about a horizontal axis and the outer circumference of which is provided with a series of excavating buckets which are distributed over the circumference and whose inlet opening faces forward in the direction of rotation, and in which the outer circumference of the wheel comprises a passage for dug-up bed material. The axis of rotation of the wheel is substantially horizontal, meaning that said axis runs parallel to the water surface. In this case, the axis of rotation may be situated in the plane of the ladder to which the cutter-wheel dredger is attached, but preferably extends at right angles to this plane. The cutting tool can also cut in the direction of the axis of the ladder, but in such an embodiment, it is necessary to arrange (inside the digging device) a relatively short mechanical escalator means, such as for example an Archimedean screw part, in order to take the dredged material up to the discharge means.

[0022] In an embodiment according to the invention, a device is provided which comprises two cutting tools arranged on either side of the ladder, preferably cutter-wheel dredgers. By means of such a device, it is possible, in certain circumstances, to increase the production width, in particular if the cutting tools are moved across the underwater bed in a direction which differs from the direction of their (common) axis of rotation. In addition, more cutting tools also provide better screening and protection of the discharge means.

[0023] A cutter-wheel dredger of an embodiment of the inventive device comprises a wheel which is rotatable about a horizontal axis and is provided with a series of fitted excavating buckets on the outer circumference. The number of excavating buckets can be selected according to the circumstances and the latter are preferably evenly distributed along the circumference. The excavating buckets may be in the form of buckets, with excavating buckets which are substantially U-shaped in the radial cross section of the wheel being preferred. In this case, the legs of the U shape adjoin said outer circumference. Such excavating buckets lack a bottom, thus facilitating the discharge of the dug-up bed material. For discharge purposes, a passage is provided in the outer circumference, preferably along the entire length between the legs of the buckets, which adjoins a collecting unit which extends into the wheel and which in turn adjoins an inlet end of a discharge means, for example in the form of a number of driven buckets.

[0024] In operation, the cutting tool, preferably the cutter-wheel dredger, may rotate in such a manner that the excavating buckets move from the bottom to the top, but it is also possible that they operate in the opposite direction. During operation of the device, the position of the vessel with the ladder to which the cutting tool is attached has to be fixed in order to be able to excavate in an accurate manner. At relatively shallow depths, the vessel can be anchored by lowering spud piles and/or cables. At greater depths, for example more than 35 metres below the water surface, the use of cables is preferred. The occurring reactive forces act (depending on the direction of rotation of the rotating cutting tool) both in a downwards direction, leading to a tendency to pull the vessel deeper into the water, and in a horizontal direction, leading to a tendency to bring the cutting tool out of engagement with the underwater bed to be treated. The horizontal reactive forces can usually be absorbed in a satisfactory manner using spud piles, but at relatively great depths, the bending moments acting on the spud pile become excessive, as a result of which it is no longer economical to use spud piles. It has been found, that the device according to the invention can also be stabilized in a satisfactory manner using cables.

[0025] A simple embodiment of the device according to the invention is characterized by the fact that the ladder which can be positioned downwards at an angle from the vessel into the body of water comprises the discharge means. The ladder and the discharge means then form part of the same structure which, for example, comprises a truss structure, in which the carrier of the discharge means is accommodated, preferably provided with a covering.

[0026] In yet another embodiment of the invention, a device is provided in which the outlet end of the discharge means adjoins a second discharge means which is situated on deck of the vessel. This makes semi-continuous discharge of dug-up bed material possible, which discharge is facilitated further by an embodiment in which the second discharge means comprises a carrier which is driven by means of a mechanical drive.

[0027] A further embodiment of the device according to the invention comprises a device in which the second discharge means extends in the transverse direction of the vessel, and can be moved in translation in the transverse direction. It is then advantageous if the device furthermore comprises a discharge vessel for the dug-up bed material which is moored on at least one side of the vessel, in which an end of the second discharge means adjoins a well of the discharge vessel.

[0028] Using the inventive device, it is possible to dig up an underwater bed in an efficient and environmentally friendly way, in which an embodiment of the method according to the invention is characterized by a method in which the vessel is anchored to the underwater bed by lowering a spud pile from the vessel, and in which the cutter-wheel dredger is moved across and into the bed by alternately shifting the vessel from port to starboard using winches, in which case the vessel rotates about the spud pile and the cutter-wheel dredger performs a lateral slewing movement.

[0029] In an alternative embodiment of the method according to the invention, the vessel is anchored to the underwater bed by using anchors which engage with the bed and which are connected to the vessel by means of anchor ropes, and the cutter-wheel dredger is moved across and into the bed according to a path which is determined by the shifting of the vessel by means of the anchor ropes.

[0030] Both methods make it possible to excavate an underwater bed without causing a large amount of sediment spill. A further advantage of the device and method according to the invention is the fact that the excavation can be performed to a very precise depth, as a result of which the dug-up underwater bed will exhibit a relatively well-defined and flat surface. This important advantage prevents the unnecessary supply of expensive hydraulic engineering material. Thus, for example, the gravel bed required for the foundation of a tunnel construction will require a smaller supply of gravel than is the case with the known method.

[0031] The invention will now be explained in more detail with reference to the attached figures without, incidentally, being limited thereto. In the figures:

Fig. 1 shows a diagrammatic side view of an embodiment of the device according to the invention;

Fig. 2 shows a diagrammatic top view of another embodiment of the device according to the invention; and finally

Fig. 3 shows a diagrammatic perspective view of an embodiment of a cutter-wheel dredger according to the invention.

[0032] Referring to Figs.1 and 2, a device 1 is shown for dredging bed material 2 below the water level 3. The illustrated embodiment comprises a vessel 4 which is anchored to the underwater bed 2 by means of spud piles 5 (the rear spud piles 5 act as an aid for anchoring the vessel temporarily). The vessel is furthermore provided with a ladder 8 which is positioned downwards into the body of water 7 at an oblique angle 6. The ladder 8 comprises a truss structure which is constructed from mutually connected tie members and compression members 80, and which is provided with one cutter-wheel dredger 10 at its free end. However, it is also possible to provide the ladder 8 with cutter-wheel dredgers 10 on either side. At its other end, the ladder 8 is connected to the vessel 4 so as to be rotatable about a shaft 9. The ladder 8 can be rotated about the horizontal axis by means of cables 11, which can be hauled in, for example, using the winches present on deck (in which case the ladder 8 is pulled up and the angle 6 reduced), or paid out (in which case the ladder 8 is lowered and the angle 6 increased).

[0033] The ladder 8 furthermore comprises a discharge means for the dug-up bed material in the form of a carrier 13 for the bed material which is driven by means of a mechanical drive 12. In the illustrated embodiment, the carrier 13 comprises a conveyor belt which conveys dug-up bed material from the bottom to the top in the direction 14 indicated. The conveyor belt is provided with a(n invisible) covering to further prevent sediment spill. On the load-bearing (upper) surface, the conveyor belt 13 is provided with V-shaped ribs 15 which prevent bed material from sliding downwards.

[0034] In the illustrated embodiment, the outlet end 130 of the conveyor belt 13 adjoins a second discharge means in the form of conveyor belt 16. The latter is situated on deck of the vessel 4 and extends in the transverse direction 17 of the vessel 4. The conveyor belt 16 can preferably be moved in translation in the transverse direction 17, both to port and to starboard, as is diagrammatically illustrated by means of the double arrow 18. The translation of the conveyor belt 16 makes it possible for an end 180 of the conveyor belt 16 to adjoin the well 19 of a discharge vessel 20, which is moored on one side of the vessel 4 and which can receive the dug-up bed material. By means of the device according to the invention, dug-up, relatively dry bed material can be transported to a treatment area by the discharge vessel 20 in a simple manner. Due to the fact that the bed material is relatively dry, dumping it in the treatment area will result in relatively little sediment spill. Thus, it is for example possible to moor the discharge vessels 20 on a quayside and empty them by means of cranes.

[0035] Referring to Fig. 3, a cutter-wheel dredger 10 according to the invention will now be described in more detail. The illustrated cutter-wheel dredger 10 is connected to the bottom end of the ladder 8 by means of a supporting structure 105 and comprises a wheel 102 which is rotatably driven about a horizontal axis 101 and which is provided on its outer circumference with a series of excavating buckets 103 which are distributed along the circumference and whose inlet opening 105 faces forwards (upwards) viewed in the direction of rotation 104. In radial cross section (along plane 107), the excavating buckets 103 of the wheel 102 are U-shaped, with the legs of the U shape adjoining said outer circumference. The excavating buckets 103 are thus not provided with a bottom, but taper slightly in order to make it more difficult for dug-up bed material to fall out. In order to discharge the bed material present in the excavating buckets 103, a passage 106 is provided in the outer circumference of the wheel 102, preferably along the entire length between the legs of the buckets 103, which passage 106 adjoins a collecting unit 108 which extends into the wheel 102. The bed material is pushed out of the excavating buckets 103 by a pushing lip 109 which extends substantially along the peripheral direction of the wheel 102. The bed material pushed out of the buckets 103 lands in the collecting unit 108 which in turn adjoins an inlet end 131 of the discharge means 13 which, in the embodiment illustrated in Fig. 3, has a carrier in the form of a series of holders 132 connected to a mechanical drive, the mechanical drive comprising endless chains 133 extending between two gear wheels (not shown), in which case at least one gear wheel is driven.

[0036] The wheel 102 of the cutter-wheel dredger 10 is rotated about the axis 101 by means of a rotary motor 110 in the illustrated direction 104, in which case the bottom edges of the excavating buckets 103 come into contact with the underwater bed 2 and detach material therefrom. This material is conveyed onto the deck of the vessel 4 in the above-described manner via the driven holders 132 and, if desired, to the well 19 of the discharge vessel 20.

[0037] According to the invention, a method is provided in which the ladder 8 is lowered under water by paying out the cables 11 until the cutter-wheel dredger 10 reaches the bottom, in which case the ladder 8 will make a certain angle 6 with the horizontal direction. The cutter-wheel dredger is then made to rotate by means of the rotary motor 110, in which case bed material is detached and collected in the excavating buckets 103. This bed material is discharged to the holders 132 via the passage 106, which holders 132 then convey the dredged bed material onto the deck of the vessel 4 by means of the mechanical drive (12, 133).

[0038] In this case, the vessel 4 can be anchored to the underwater bed 2 by lowering spud piles 5, in which case the cutter-wheel dredger 10 is moved across and into the bed by alternately shifting the vessel 4 from port to starboard by means of anchor ropes 30 operated by winches (which are in turn anchored to the underwater bed 2 by anchors 31), in which case the vessel 4 rotates around the spud pile 5 and the cutter-wheel dredger 10 performs a lateral slewing movement.

Claims

1. Device for dredging bed material under water, comprising a vessel which can be anchored to an underwater bed and having a ladder which can be positioned downwards at an angle into the body of water from the vessel and which is provided with at least one rotating cutting tool at its free end, the cutting tool comprising a number of excavating elements arranged around a common rotation axis, and having a passage for dug-up bed material which adjoins an inlet end of a discharge means for the dug-up bed material, which discharge means comprises a carrier for the bed material which is driven by means of a mechanical drive, which carrier comprises two conveyor belts which rotate in opposite directions of rotation across each other at the same time.

2. Device according to claim 1, in which the rotating cutting tool comprises a cutter-wheel dredger, in which the cutter-wheel dredger comprises a wheel which is rotatably driven about a horizontal axis and which is provided on the outer circumference with a series of excavating buckets which are distributed along the circumference and whose inlet opening faces forwards viewed in the direction of rotation, and in which the outer circumference of the wheel comprises a passage for dug-up bed material, which passage adjoins the inlet end of the discharge means.

3. Device according to claim 1 or 2, in which the carrier of the discharge means is provided with a covering.

4. Device according to any one of the preceding claims, in which the device comprises a suction pipe which is connected to pump means and which extends from the vessel up to the vicinity of the inlet end of the discharge means.

5. Device according to any one of the preceding claims, in which at least one outlet end of the discharge means is provided with a scraping device.

6. Device according to any one of the preceding claims, in which the mechanical drive comprises endless chains running between two wheels, in which at least one wheel is drivable, and the carrier is connected to the chains.

7. Device according to any one of the preceding claims 7, in which a conveyor belt is provided with retaining means for the bed material on a load-bearing surface, such as for example V-shaped ribs.

8. Device according to any one of the preceding claims, comprising two cutting tools arranged on either side of the ladder.

9. Device according to any one of the preceding claims, in which the carrier comprises a series of buckets which are connected to the mechanical drive.

10. Device according to any one of the preceding claims, in which the ladder which can be positioned downwards at an angle from the vessel into the body of water comprises the discharge means.

11. Device according to any one of the preceding claims, in which the outlet end of the discharge means adjoins a second discharge means which is situated on deck of the vessel.

12. Device according to claim 12, in which the second discharge means comprises a carrier which is driven by means of a mechanical drive.

13. Method for dredging bed material under water by means of a device according to any one of claims 1-12, comprising the following steps:

- Providing a device according to any one of claims 1-12;

- Lowering the ladder and the discharge means of the device under water;

- Detaching the bed material by rotating the cutting tool;

- Discharging dredged bed material up to the carrier of the discharge means via the passage;

- Conveying the dredged bed material by driving the carrier.

14. Method according to claim 13, in which the vessel is anchored to the underwater bed by lowering spud piles from the vessel, and the cutting tool is moved across and into the bed by alternately shifting the vessel from port to starboard using winches, in which case the vessel rotates about the spud pile and the cutting tool performs a lateral slewing movement.

15. Method according to claim 13, in which the vessel is anchored to the underwater bed by using anchors which engage with the bed and which are connected to the vessel by means of anchor ropes, and the cutting tool is moved across and into the bed according to a path which is determined by the shifting of the vessel by means of the anchor ropes.

16. Method according to claim 15, in which the path is substantially straight.

Drawing