EP3259409B1

EP3259409B1 - Suction head of a dredging vessel and method for dredging using said suction head

Info

Publication number: EP3259409B1
Application number: EP16715344.4A
Authority: EP
Inventors: Bruno Tack
Original assignee: Baggerwerken Decloedt and Zoon NV
Current assignee: Baggerwerken Decloedt and Zoon NV
Priority date: 2015-02-18
Filing date: 2016-02-17
Publication date: 2019-04-10
Anticipated expiration: 2036-02-17
Also published as: BE1022377B1; WO2016132299A1; LT3259409T; EP3259409A1; DK3259409T3

Description

TECHNICAL FIELD OF THE INVENTION

The invention relates to a suction head of a dredging vessel configured for movement in a dragging direction, wherein the suction head comprises a connecting construction connectable to a suction conduit and a visor rotatable relative to the connecting construction and having a suction opening directed toward the bottom for suctioning up bottom material.. The invention likewise relates to a dredging device comprising the suction head and a method for dredging bottom material under water while making use of the suction head.

BACKGROUND OF THE INVENTION

A known suction head is for instance described in EP-A-0892116 . The suction head described herein is connected to a suction conduit of a dredging vessel, for instance a trailing suction hopper dredger. The suction head is provided with a visor which can be rotated between a lowered and a raised position around a shaft running transversely of a dragging direction. The visor has a suction opening directed toward the bottom for suctioning up bottom material dislodged by a lower edge of the visor. During dredging the suction head connected to the suction conduit is lowered underwater and in the lowered position dragged in a dragging direction of the trailing suction hopper dredger over the bottom for dredging, wherein bottom material is dislodged and suctioned away with a quantity of water to a bin of the dredging vessel through the suction conduit connected to a suction pump.
It is important during dredging to achieve a relatively high dredging efficiency, this being understood in the context of this application to mean the volume of bottom material dredged per unit of time and unit of power. The dredging efficiency depends on, among other factors, the ground condition, wherein a hard and/or highly compacted bottom generally results in a low dredging efficiency. Such a bottom is indeed difficult to dislodge, whereby relatively little bottom material is suctioned up (and a large amount of water). The dredging efficiency can optionally be increased by allowing the bottom material collected in the bin to settle and discharging the water layer resulting on the settled bottom material via an overflow received in the bin. Such a method cannot always be applied and can be deemed harmful to the environment. Smaller bottom material particles, which can cause a high degree of turbidity in the surrounding water, may indeed be entrained by the water.
AU 2005200784 A1 in figure 5 discloses a carrier beam 27 provided with a number of cutting teeth 28. The position of the carrier beam 27 can be adjusted by a piston cylinder assembly 30. The carrier beam 27 and teeth 28 dislodge bottom material.
US 4,150,502 discloses provided with a dislodging roll 22 to disturb the structure of stony ground prior to sucking it up.
EP 0 892 116 A1 shows a drag head having a foot section 15' adapted to provide a fixed support on the ground while dredging.
US 2014/0250742 A1 D4 finally discloses a drag head wherein the teeth are actuated by the same actuator that also controls movement of the visor of the drag head.

SUMMARY OF THE INVENTION

The present invention has for its object, among others, to provide a suction head of a dredging vessel which, compared to the known suction head, has an increased dredging efficiency, in particular for hard and/or highly compacted bottoms. Another object relates to obtaining an increased dredging efficiency in combination with a relatively low, or even non-existent bin overflow (also referred to by the skilled person as spill).
The suction head according to the invention comprises for this purpose a suction head according to claim 1. The suction head comprises a connecting construction connectable to a suction conduit and a visor rotatable relative to the connecting construction between a lowered and a raised position around a shaft running transversely of a dragging direction P and having a suction opening directed toward the bottom for suctioning up bottom material dislodged by a lower edge of the visor, wherein the suction head further comprises a precutting device and positioning means configured to carry a lower edge of the precutting device to a greater depth than the lower edge of the raised visor so that bottom material can be dislodged with the lower edge of the precutting device. A suction head according to the invention allows the bottom for dredging to first be dislodged with the pre-cutting device suitable for the purpose, wherein the dislodged bottom material is preferably not or only very partially suctioned up through the suction conduit. The bottom material dislodged by the precutting device can subsequently be suctioned up with the same suction head. Because the bottom material is first dislodged, the dredging efficiency will be higher here.
The precutting device can in principle take any form as long as a lower edge of the precutting device can be carried to a greater depth than the lower edge of the raised visor, and the precutting device comprises a holder body which is rotatable with the positioning means relative to the connecting construction between a lowered and a raised position around a shaft running transversely of the dragging direction P. The holder body is configured to be able to receive and at least temporarily hold dislodged bottom material (for instance via an open wall part). A suitable holder body comprises for instance a digger bucket. The dredging efficiency for bottoms which comprise (undesired) objects not suctionable through the suction conduit, such as rocks, stones and the like, is further improved in this way. As it moves through the bottom, the precutting device can indeed pick up and discharge the objects not suctionable through the suction conduit. This can for instance prevent or delay blockage of the suction opening of the visor.
An embodiment of the invented suction head has for this purpose the feature that the positioning means are configured to carry the lower edge of the precutting device to a lesser depth than the lower edge of the visor in the lowered position of the visor, so that bottom material can be dislodged with the lower edge of the visor and can be suctioned up through the suction conduit.
The invented suction head is based on the insight that the dredging efficiency can be increased by separating cutting of the bottom from the suctioning and carrying of the cut bottom material to the bin. A shorter suctioning time can also be achieved, which is favourable for the energy consumption of the dredging vessel and can drastically reduce the spill.
During dislodging of the bottom using the precutting device a lower edge of the precutting device is carried to a greater depth than a lower edge of the visor. Carrying to a greater depth means that the lower edge of the precutting device penetrates deeper into the bottom than the lower edge of the visor. This latter can even (and will generally) be situated above the ground level.
During further dislodging of the bottom and suctioning up of the bottom material using the visor a lower edge of the precutting device is carried to a lesser depth than a lower edge of the visor. Carrying to a lesser depth means that the lower edge of the precutting device penetrates less deeply into the bottom than the lower edge of the visor. The lower edge of the precutting device can, and will even preferably be situated above the ground level.
Another embodiment according to the invention provides a suction head wherein a lower edge of the precutting device comprises a series of cutting tools which extends in a line transversely of the dragging direction for the purpose of penetrating into the bottom. According to this embodiment the cutting tools can be adapted to the set purpose, i.e. precutting of the bottom. The geometry of the cutting tools can for instance be adapted here to the ground condition.
An embodiment of the suction head according to the invention comprises a precutting device, the cutting tools of which comprise a concave cutting surface in the dragging direction P. It has been found that such cutting tools are particularly suitable for dislodging hard and/or highly compacted bottom material. A known suction head can be provided with a visor which is provided on a lower edge thereof with a series of cutting tools which extend in a line transversely of the dragging direction for the purpose of penetrating into the bottom. Usual cutting tools or teeth for a visor generally have in the dragging direction P a substantially flat or slightly convex cutting surface.
The dimensions of the cutting tools of the precutting device (in particular their length and width transversely of the dragging direction) can be chosen within broad limits. According to an embodiment, the dimensions of the cutting tools of the precutting device are at least twice as great, more preferably at least three times and most preferably at least four times as great as the dimensions of the cutting tools of the visor. The same applies to the volume of the cutting teeth, which can for instance be determined by immersing the associated cutting tools in water and measuring the water displacement.
The greater depth to which a lower edge of the precutting device can be carried relative to the lower edge of the raised visor can also be selected within broad limits. Provided in an embodiment is a suction head wherein the depth at which the lower edge can be situated is at least twice as great, more preferably at least five times as great, and still more preferably at least eight times as great as the depth at which the lower edge of the raised visor is situated.
A further improved embodiment for discharging undesired objects is obtained with a suction head wherein a rear wall of the precutting device comprises passage openings. Less resistance is hereby generated during the forward movement, while the rear wall simultaneously functions as capture wall for the undesired objects. The terms front and rear wall must be understood in relation to the dragging direction, wherein the dragging direction points from the rear wall to the front wall.
A useful embodiment comprises a suction head wherein the rear wall comprises a latticework of a number of intermediate walls which are placed at a distance from each other and between which the passage openings are located. A number of intermediate walls are optionally connected removably to the holder body. This has the additional advantage that the size of the captured objects can be adjusted. The intermediate walls can be designed in any manner. It is thus possible for instance to apply rigid elements such as rods and plates, though flexible elements such as chains and slings are also possible.
The positioning means with which the holder body can be rotated preferably comprise hydraulic cylinders, piston rods of which engage on an upper side of the holder body. Some or all intermediate walls can if desired be connected by means of rods to a fixed part of the suction head, for instance to the connecting construction of the suction head. The holder body can be carried to the retracted position by retracting piston rods or be carried in the direction of the lowered position by pushing piston rods out. The stationary intermediate walls will be able to move relatively unobstructed here relative to the holder body. This enhances the escape of undesired objects from the holder body when the holder body is moved to the raised (or open) position.
In order to enable a lower edge of the holder body to be carried to a greater depth the holder body is rotated around a shaft running transversely of the dragging direction into a lowered position of the holder body. It is possible for the holder body to be rotated around the same shaft with which the visor is rotatable. A more suitable embodiment comprises a suction head wherein the shaft around which the holder body is rotatable differs from the shaft around which the visor is rotatable. Greater depths can hereby be reached. There are further practical advantages when the shaft around which the holder body is rotatable is received in the visor, preferably on a downstream side (rear side) of the visor. A downstream side forms a side of the visor facing away from the dragging direction.
As already stated above for an embodiment in which the precutting device comprises a holder body, the positioning means of the precutting device can comprise any means with which a lower edge of the precutting device can be carried to a greater depth than the lower edge of the raised visor and can be held with a sufficient force at this depth during movement through the bottom. It is thus possible to move the precutting device by means of cables, screws, gear rack connections and the like. A preferred embodiment comprises a suction head wherein the positioning means of the precutting device comprise hydraulic cylinders. The hydraulic cylinders can for instance be connected with a movable rod side to a wall part of the precutting device. The wall part can be moved by pushing out or conversely retracting the cylinder rod.
A particularly suitable embodiment of the invention provides a suction head provided with means for rotating the visor, wherein these means are utilized as positioning means to carry a lower edge of the precutting device to a greater depth. The drive of a visor generally comprises protruding mounts which are arranged on the connecting construction and which are connected using remotely controllable hydraulic cylinders to protruding mounts arranged on (an upper wall of) the visor. By extending the hydraulic cylinder rods the distance between the protruding mounts of the connecting construction and those of the visor is increased, whereby the visor rotates around a shaft which runs transversely of the dragging direction and with which the visor is connected to the connecting construction. In the present embodiment the hydraulic cylinders used for rotation of the visor are also utilized to move the lower edge of the precutting device in the direction of the bottom, for instance by rotating the precutting device around a shaft running transversely of the dragging direction. A wall part of the precutting device can be provided for this purpose with protruding mounts to which the hydraulic cylinder rods used for rotation of the visor can be connected. A part not for rotation (visor or precutting device) can be blocked, and a part for rotation (precutting device or visor) can be unblocked by changing the position of pins. This embodiment has the additional advantage that the (remote) control of (the hydraulic cylinders of) the visor can remain unchanged relative to the known suction head, and that a separate or separated drive is not necessary for the precutting device.
In order to be able to produce the forces required for the precutting the hydraulic cylinders, protruding mounts and hydraulic drive can take a heavier form than is the case in the known suction head.
The suction head according to the invention preferably comprises a visor provided with a series of cutting tools or teeth which are disposed in a line transversely of the dragging direction and which, during suctioning up of dislodged ground material, can penetrate partially into the ground dislodged by the precutting device.
The invention likewise relates to a dredging device comprising a dredging vessel which is configured for movement in a dragging direction P and to which a suction head according to the invention is connected.
Provided in yet another aspect of the invention is a method for underwater dredging of bottom material with such a dredging device. The suction head is dragged here in the dragging direction P over a bottom for dredging, wherein the lower edge of the precutting device is carried to a greater depth than the lower edge of the visor in the raised position of the visor, and is moved through the bottom in order to dislodge bottom material with the lower edge of the precutting device. In an embodiment of the method a suction pump connected to the suction conduit is set here to the OFF-position whereby substantially no bottom material is suctioned through the suction conduit. A rear wall of the precutting device preferably comprises passage openings, and during movement through the bottom the precutting device carries with it and discharges objects not suctionable through the suction conduit, preferably to a position outside the area for dredging.
Once the bottom has been sufficiently precut in this way, and the non-suctionable objects have been removed from the precut ground, in yet another embodiment of the method the visor is rotated into the lowered position of the visor, the lower edge of the precutting device is carried to a lesser depth than the lower edge of the lowered visor, a suction pump connected to the suction conduit is set to the ON-position and the lower edge of the visor is moved through the bottom in order to suction up the bottom material dislodged by the precutting device.
The method according to the invention is particularly suitable for dredging bottom material comprising rocks and/or stones, and/or for dredging hard and/or highly compacted bottoms.
Stated expressly is that the embodiments of the invention described in this patent application can be combined in any possible combination of these embodiments, and that each embodiment can individually form the subject-matter of a divisional patent application.

BRIEF DESCRIPTION OF THE FIGURES

The invention will now be further elucidated on the basis of the following figures and description of a preferred embodiment, without the invention otherwise being limited thereto. In the figures:

Figure 1 is a schematic side view of a dredging device according to the invention;
Figure 2 is a schematic perspective side view of an embodiment of a suction head according to the invention;
Figure 3 is a schematic side view of the suction head according to the invention shown in figure 2;
Figure 4 is a schematic perspective rear view of the suction head according to the invention shown in figure 2; and
Figures 5A to 5G show in schematic manner different steps of a possible embodiment of the method according to the invention.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

Figure 1 shows a dredging vessel 1 which is provided with a motor, not shown in the drawing, for driving a propeller 2 via a propeller shaft for the purpose of propelling dredging vessel 1. Also present are devices, not shown in the drawings, for steering dredging vessel 1, such as a rudder and transversely placed propellers for facilitating manoeuvring.
A dredge pump, not shown in the drawings, is arranged in dredging vessel 1. Arranged against a side wall of the dredging vessel is a suction conduit 3, one end of which is connected to the suction connection of the dredge pump. In the present embodiment the suction conduit 3 comprises two members 3a and 3b which are connected to each other by means of a coupling allowing some relative angular displacement. The connection between upper member 3a of suction conduit 3 and the vessel also allows angular displacement in the vertical plane and about the axis. For support of the movable end of upper member 3a of suction conduit 3 this member is connected to a cable 4a, the other end of which is connected to a winch 5a. For support of the movable end of lower member 3b of suction conduit 3 this member is likewise connected to a cable 4b, the other end of which is connected to a winch 5b. It is thus possible using winches 5a, 5b to vary the height of suction conduit 3. It will be apparent that, also subject to the depth of the bottom for dredging, the number of members of suction conduit 3 can be increased or decreased, with a corresponding adjustment of the number of cables 4 and winches 5. A suction head 6 according to the invention is arranged on the free end of second member 3b of suction conduit 3.
Referring to figure 2, an embodiment of suction head 6 is shown. Suction head 6 comprises a connecting construction 7 connectable to suction conduit 3 and a visor 9 rotatable relative to connecting construction 7 around a shaft 8 running transversely of dragging direction P and having a suction opening directed toward the bottom for suctioning up bottom material. Visor 9 of suction head 6 is provided with a series of cutting tools in the form of teeth 10 which are attached to a toothed beam 11 running transversely of the dragging direction P and which can penetrate partially into the ground during dredging.
Suction head 6 further comprises a precutting device which in the shown embodiment comprises, among other parts, a holder body 12 which can be rotated relative to connecting construction 7 around a shaft 14 running transversely of dragging direction P between a lowered position as shown in figures 1, 2, 3, 4, 5A to 5C, 5E and 5F and a raised position as shown in figures 5D and 5G. Shaft 14 is received in (a flange of) visor 9 and differs from shaft 8 around which visor 9 can be rotated. As shown in figures 3 and 5G, visor 9 can be rotated around shaft 8 from a raised position shown in figure 3 to a lowered position shown in figure 5G. In the raised position visor 9 is fixed by a pin-hole connection (15a, 15b) although another manner of fixing is also possible. When pin-hole connection (15a, 15b) is released visor 9 can be rotated around shaft 8 by means of positioning means which are formed in the shown embodiment by a pair of hydraulic cylinders 17 which are generally remotely controllable and which are connected with a movable piston rod outer end to protruding mounts 18 attached to an upper wall of holder body 12 and on a cylinder housing side to tubular mounts 19 attached to connecting construction 7. After visor 9 has been blocked by blocking pins (15a, 15b) and hydraulic cylinders 17 have been extended, holder body 12 rotates around the shaft 14 running transversely of dragging direction P with which holder body 12 and fixed visor 9 are mutually connected.
Holder body 12 is for instance embodied as a digger bucket and comprises two side walls (12a, 12b), a front side (facing in the dragging direction P) allowing entry of bottom material and a rear wall facing away from the dragging direction P. The rear wall is provided with passage openings 20 formed by a number of intermediate walls (21a, 21b) which are placed at a distance from each other and between which passage openings 20 are located. Suction head 6 is further provided with positioning means in the form of a number of hydraulic cylinders 17 with which holder body 12 can be rotated around shaft 14. The piston rods of hydraulic cylinders 17 are connected to holder body 12 by means of connections 18. Holder body 12 can be carried to the retracted position shown in figures 5D and 5G by retracting the piston rods of cylinders 17. Holder body 12 is lowered in the direction of the bottom by pushing the piston rods out. A number or all of the intermediate walls 21b are further connected pivotally at the bottom to holder body 12 and likewise connected pivotally on a top side to connecting construction 7 of suction head 6 by means of rods 22 and hinges 23. Intermediate walls 21 can hereby move relative to holder body 12. During raising and lowering the holder body rotates around shaft 14 and intermediate walls 21b slide in slots 24 arranged in an upper wall of holder body 12 (see figure 5C). Due to the relative movement of holder body 12 and intermediate walls 21 non-suctionable objects 50 collected in holder body 12 during the precutting can be released relatively easily when holder body 12 is raised, whereby these objects 50 can exit holder body 12 in simple manner and can be offloaded outside the dredging zone (figure 5D).
In the shown embodiment a lower edge 25 of holder body 12 comprises a series of cutting tools 38 which extends in a line transversely of dragging direction P and which in use can penetrate into the bottom. Cutting tools 38 can comprise any cutting tool suitable for precutting the bottom. In a preferred embodiment cutting tools 38 are provided with a cutting face 39 with a concave shape in the direction of the dragging direction P as shown in figure 3. This facilitates dislodging of hard and/or highly compacted ground.
When holder body 12 is lowered a lower edge 25 of holder body 12 is carried to a greater depth 30 than the depth 31 at which a lower edge 26 of the raised visor 9 is situated (see for instance figure 3). In the shown embodiment the depth 31 of the visor lower edge 26 is positive relative to ground level 32 and edge 26 is situated above ground level 32. In this position lower edge 25 lies lower than lower edge 26 of visor 9 and also lower than ground level 32. Lower edge 25 of holder body 12 can hereby be brought into contact with the bottom and dislodge bottom material during movement of the suction head in dragging direction P. The depth 30 at which lower edge 25 can be situated amounts to at least five times the depth at which lower edge 26 of visor 9 is situated in the raised position of visor 9.
As shown in figure 5G, lower edge 25 of holder body 12 can be carried to a lesser depth 30 than lower edge 26 of visor 9 in the lowered position of the visor so that bottom material can be dislodged with lower edge 26 of visor 9, wherein teeth 10 enhance dislodging by penetrating into the bottom. In the shown embodiment lower edge 26 of visor 9 comprises a series of cutting tools 10 which extends in a line transversely of dragging direction P and which in use can penetrate into the bottom. Cutting tools 10 can comprise any cutting tool suitable for shovelling up the precut ground in relatively simple manner.
Using the suction head of the invention bottom material can be dredged under water. A suction head 6 connected to a dredging vessel 1 and lowered under water can here be dragged over the bottom in dragging direction P, wherein lower edge 25 of holder body 12 is brought to a greater depth 30 than lower edge 26 of visor 9 in the raised position thereof. Referring to figures 5A to 5F, lower edge 25 of holder body 12 is moved through the bottom in dragging direction P in order to dislodge bottom material. Holder body 12 is held in a lowered position here by hydraulic cylinders 17. A suction pump (not shown) connected to suction conduit 3 is in the OFF-position here, whereby substantially no bottom material is suctioned by suction conduit 3 during dislodging of the bottom.
A known suction head is often not capable of penetrating deeply into hard and/or highly compacted bottoms. A cutting depth of for instance only 1-15 cm can thus be reached in so-called glacial till. In a typical method according to the invention lower edge 25 of suction head 6 is moved several times over part of a bottom, wherein ground layer thicknesses in a range of for instance 1-50 cm are dislodged during each passage. Although the invention is not limited to dislodging specific ground layer thicknesses, ground layer thicknesses of 0.5 m to at least 2 m can be dislodged in this way.
As shown in figures 5A to 5C, holder body 12 is provided with a rear wall with passage openings 20 present between intermediate walls (21a, 21b) for water and for objects of dimensions smaller than the distance between intermediate walls (21a, 21b). Larger objects 50 which cannot be suctioned up by suction conduit 3 are carried along as holder body 12 moves through the bottom (figures 5B and 5C). This can already take place during a first passage as shown in figures 5A to 5C wherein the depth at which lower edge 25 of holder body 12 is situated amounts to the order of magnitude of 10-15 cm. Objects 50 can for instance comprise stones or rocks present in the bottom with dimensions of 30 cm and more. When holder body 12 is filled with objects 50, these objects 50 are discharged to a suitable location outside the area for dredging, as shown in figure 5D. The degree of filling of holder body 12 can be measured with a measuring device suitable for the purpose, the measurement of which is visible to the operator of suction head 6.
After a first passage of suction head 6 a layer thickness of dislodged ground material is obtained of 10-15 cm. The dislodged ground material remains lying on the bottom as a relatively soft layer of fragments of bottom material, for instance glacial till fragments. During a subsequent passage holder body 12 will penetrate easily through the dislodged ground layer. After several passages lower edge 25 of holder body 12 will have penetrated to a depth of for instance 1.5 m into the dislodged ground, as shown in figures 5E and 5F.
Once the desired layer thickness has been reached, suction head 6 is changed from the precutting position, in which holder body 12 is in the lowered position, to a suction position wherein visor 9 is brought into the lowered position of visor 9. This can take place in simple manner by raising holder body 12 with hydraulic cylinders 17, and connecting it rigidly in the raised position to visor 9 by means of a pin-hole connection. Pins 15a are subsequently removed from holes 15b, whereby pin-hole connection (15a, 15b) is released and visor 9 can be moved freely relative to connecting construction 7 using hydraulic cylinders 17. Lower edge 25 of holder body 12 is brought to a lesser depth 30 here than lower edge 26 of the lowered visor 9 (the depth 30 is thus negative). Lower edge 26 of visor 19 is held at a depth 31 with hydraulic cylinders 17 and moved through the bottom in dragging direction P in order to suction up the bottom material dislodged by holder body 12. A suction pump (not shown) connected to suction conduit 3 is placed for this purpose in the ON-position. Because the dislodged ground material forms a soft bottom layer, it is suctioned up with increased efficiency.
The invention is not limited to the above described embodiment and also comprises modifications thereto to the extent these fall within the scope of the claims appended below.

Claims

Suction head (6) of a dredging vessel (1) configured for movement in a dragging direction P, wherein the suction head (6) comprises a connecting construction (7) connectable to a suction conduit (3) and a visor (9) rotatable relative to the connecting construction (7) between a lowered and a raised position around a shaft (8) running transversely of the dragging direction P and having a suction opening directed toward the bottom for suctioning up bottom material dislodged by a lower edge of the visor (9), wherein the suction head (6) further comprises a precutting device and positioning means (17) configured to carry a lower edge of the precutting device to a greater depth than the lower edge of the visor (9) in the raised position of the visor (9) so that bottom material can be dislodged with the lower edge of the precutting device, characterized in that the precutting device comprises a holder body (12) which is rotatable with the positioning means (17) relative to the connecting construction (7) between a lowered and a raised position around a shaft (14) running transversely of the dragging direction P, wherein a rear wall of the precutting device comprises passage openings (20) and the holder body (12) is configured to receive and hold dislodged bottom material.
Suction head (6) as claimed in claim 1, wherein the positioning means (17) are configured to carry the lower edge of the precutting device to a lesser depth than the lower edge of the visor (9) in the lowered position of the visor (9), so that bottom material can be dislodged with the lower edge of the visor (9).
Suction head (6) as claimed in claim 1 or 2, wherein a lower edge of the precutting device comprises a series of cutting tools which extends in a line transversely of the dragging direction for the purpose of penetrating into the bottom.
Suction head (6) as claimed in claim 3, wherein a lower edge of the visor (9) comprises a series of cutting tools which extends in a line transversely of the dragging direction for the purpose of penetrating into the bottom, and the cutting tools of the precutting device differ from the cutting tools of the visor (9).
Suction head (6) as claimed in any of the foregoing claims, wherein the depth at which the lower edge of the precutting device can be situated is at least five times as great as the distance from the lower edge of the visor (9) in raised and lowered position.
Suction head (6) as claimed in claim 1, wherein the shaft (14) around which the holder body (12) is rotatable differs from the shaft (8) around which the visor (9) is rotatable.
Suction head (6) as claimed in claim 6, wherein the shaft (14) around which the holder body (12) is rotatable is received in the visor (9).
Suction head (6) as claimed in any of the foregoing claims, wherein the positioning means (17) of the precutting device comprise hydraulic cylinders.
Suction head (6) as claimed in any of the foregoing claims, wherein the suction head (6) is provided with means for rotating the visor (9), and these means are utilized as positioning means (17) to carry a lower edge of the precutting device to a greater depth.
Suction head (6) as claimed in any one of the preceding claims, wherein the rear wall comprises a latticework of a number of intermediate walls which are placed at a distance from each other and between which the passage openings (20) are located.
Suction head (6) as claimed in claim 10, wherein the positioning means (17) comprise hydraulic cylinders which engage on an upper side of the precutting device and the intermediate walls are connected in stationary manner to the connecting construction (7) of the suction head (6).
Dredging device, comprising a dredging vessel (1) which is configured for movement in a dragging direction P and a suction head (6) as claimed in any of the claims 1-11 connected to the dredging vessel (1).
Method for underwater dredging of bottom material with a dredging device as claimed in claim 12, wherein the suction head (6) is dragged in the dragging direction P over a bottom for dredging, wherein the lower edge of the precutting device is carried to a greater depth than the lower edge of the visor (9) in the raised position of the visor (9), and is moved through the bottom in order to dislodge bottom material with the lower edge of the precutting device.
Method as claimed in claim 13, wherein a suction pump connected to the suction conduit (3) is set to the OFF-position whereby substantially no bottom material is suctioned through the suction conduit (3).
Method as claimed in claim 13 or 14, wherein a rear wall of the precutting device comprises passage openings (20), and during movement through the bottom the precutting device carries with it and discharges objects not suctionable through the suction conduit (3).
Method as claimed in claim 13, wherein the visor (9) is brought into the lowered position of the visor (9), the lower edge of the precutting device is carried to a lesser depth than the lower edge of the visor (9) in the lowered position of the visor (9), a suction pump connected to the suction conduit (3) is set to the ON-position and the lower edge of the visor (9) is moved through the bottom in order to suction up the bottom material dislodged by the precutting device.