Method for performing an underwater dredging operation, and dredging installation

Abstract

 In a dredging operation, a cutting member (8) exerts a cutting force on a parcel (34) of pack bottom material. Thereby, in increase of the mutual distance of the bottom material particles (33) is generated, leading to a pore pressure decrease in the water field pores between said particles. By directing a water jet (30) towards said parcel of bottom material, a water flow is introduced into said pores thereby at least partly eliminating the pressure decrease and enabling a cutting operation with a decreased cutting force.

Description

[0001] The invention is related to a method for performing a dredging operation by means of a drag head which is connected to a dredging vessel by means of a suction tube and which is dragged over the bottom by the vessel. The drag head has a number of teeth for cutting the bottom material. A water supply system is connected to the drag head, in such a way that by means of a pump, water is pumped towards the drag head for assisting the cutting action of the teeth and for transporting said cut bottom material to the suction tube. The vessel has onboard storage means, such as a hopper, for collecting the cut material.

[0002] Through the water supply pumps, dredging pumps and the propulsion installation of the vessel, energy is supplied to the dredging system, whereby a specific energy consumption (i.e. the ratio between the supplied energy and the amount of cut bottom material) is obtained. Generally, it is attempted to operate dredging systems in such a way that relatively large amounts mixtures of water and cut bottom material with a high specific density and a relatively low speed are transported towards the hopper. Thereby, it is possible to deposit a relatively large amount of cut bottom material in the hopper at moderate cost together with reduced teeth wear.

[0003] In this connection, the energy which is consumed in the cutting process plays an important role. The cutting forces, which are only available to a limited extent due to geometric properties of the suction tube arrangements, are obtained by the propulsion unit of the vessel, which requires a certain specific energy consumption to that end. Also the pumps for generating water jets and for transportation of the mixture of cut material and water consume energy. The amount of energy required is greatly increased in the case of relatively hard packed, very fine and highly cohesive water bottoms. The reason for this high energy requirement is the following.

[0004] As a result of mechanically cutting and removing material from the water bottom, the particle structure thereof is disturbed in such a way that the bottom material may deform according to shear faces. However, these shear faces can only come into being when simultaneously the volume of the pores in the bottom material is increased. In this connection, the pore water which is present in the pores between the particles of the bottom material plays an important role.

[0005] In saturated water soils, the pore volume is usually filled with said pore water. In case a pore volume increase is imposed, a demand for water is generated in the pores. This demand results in a pressure decrease. The phenomenon of pore volume increase and pore pressure decrease is referred to as dilatation or dilatantion.

[0006] The degree to which this water demand can be satisfied depends on the permeability of the bottom, layer thickness and the propagation speed of the cutting process. The permeability of the bottom material is influenced in a negative way by the fine and packed character of the particle bottom material. In case the water demand is satisfied only partly as a result of poor permeability, the underpressure quickly increases up to the vapour pressure and almost vacuum. Such large underpressure gives rise to accordingly large normal forces, which result in large friction forces both between the particles and between the bottom material and the cutting member. The sum of these forces is equal to the cutting force.

[0007] The aim of the invention is to provide a dredging operation by means of which the cutting forces can be reduced and the efficiency of the dredging system as a whole can be increased. Said aim is obtained by a method for performing a dredging operation in a water body by means of a drag head, the bottom of said water body comprising a packed bottom material of particles which enclose pores which are filled with pore water, said drag head comprising at least one cutting member for cutting the bottom material, jetting means for treating the bottom material by means of the at least one water jet, and suction means for removing the cut bottom material, said method comprising the steps of:

• dragging the drag head along the bottom,
• exerting a cutting force on a parcel of packed bottom material by means of the cutting member, said cutting force causing an increase of the mutual distance of at least part of said particles of the parcel of bottom material and thereby causing a pore pressure decrease in the water filled pores between said particles,
• directing at least one water jet towards said parcel of bottom material so as to introduce a water flow into said pores between said particles and thereby at least partly eliminating the pressure decrease in said pores caused by the said displacement of bottom material, said elimination resulting in a decreased specific cutting force,
• subsequently cutting said parcel of bottom material by means of the cutting member.

[0008] In the method according to the invention, negative pore pressures are prevented or mitigated by directly supplying a water flow to the pores concerned. The demand for pore water is thus satisfied by the water flow which introduces water into the pores. It is sufficient for said flow to have a limited magnitude, e.g. 15% of the cut material flux. An effective flow can be obtained by means of a water jet, the output pressure of which is between 2 and 5 bars above static water pressure. It is to be noted that such water lacks a cutting effect; in contrast, water jets for cutting bottom material indeed have a significantly higher output pressure of e.g. 400 bar.

[0009] In particular, the cutting member can generate a cutting face in the parcel of bottom material, at least the pores at said cutting face and/or bordering said cutting face being subjected to a pore pressure decrease caused by the cutting member and a pressure increase caused by the jet water.

[0010] Preferably, the water jet is essentially about horizontally directed towards the bottom material in front of the cutting member, whereby the angle between the water jet and the vertical is between 45° and 135°. In order to obtain a water flow in close proximity of the bottom material which is in need of additional pore water as a result of a cutting action, the water flow can be generated at or near the cutting member.

[0011] Preferably, the flow of the water jet is between 3 % and 40% of the cut material flux. More preferably, the flow of the water jet is between 4% and 35% of the cut material flux. Most preferably, the flow of the water jet is between 5% and 25% of the cut material flux.

[0012] In a specific embodiment of the invention, the method according to the invention comprises the steps of:

• applying at least two cutting members at different height levels,
• directing at least two water jets at different height levels towards the bottom material in front of a respective cutting member,
• subsequently cutting the less densely packed bottom material which is in front of the respective cutting member.

[0013] According to a further development of the method according to the invention, at least one secondary water jet is generated for diluting the cut bottom material enabling the flow of said material towards the suction means.

[0014] The invention is furthermore related to a dredging installation for carrying out the method as described before, comprising a dredging vessel provided with a tube which at one end is connected to the vessel and which at the other end carries a drag head, pump means for generating a flow through the tube from the drag head towards the vessel, as well as a conduit for feeding a water flow towards the drag head, said drag head comprising cutting means for cutting the bottom of a water body and jet means connected to the conduit for subjecting said water bottom to at least one water jet. According to the invention, the jet means are carried out for introducing a water flow into the pores between the particles of the bottom material and thereby at least partly eliminating the pressure decrease in said pores which is caused by the cutting action of the cutting means.

[0015] Furthermore, the invention is related to a drag head for a dredging installation as described before, and for carrying out the dredging operation according to the invention.

[0016] The invention will now be described further with reference to an embodiment of the drag head according to the invention.

Figure 1 shows a view in perspective of the drag head.

Figure 2 shows a front view of the drag head.

Figure 3 shows a side view of the drag head.

Figure 4 shows a cross section according to IV - IV of figure 2.

Figure 5 shows an enlarged cross section according to V - V of figure 2 of a tooth incorporated in the drag head.

Figure 6 shows a cross section through the drag head similar to the cross section shown in figure 4, in operation.

Figure 7 a, b show details of a particle bottom material.

Figure 8 shows a detail of the cutting action.

[0017] The drag head shown in figures 1 up to 3 comprises a main body 1 at one end of which the mounting flange 2 is mounted. By means of the mounting flange 2, the drag head can be connected to a suction tube and a dredging vessel (not shown). At the other end, the body 1 carries a so called visor 3 through the hinge connection 4. The angle orientation of the visor 3 can be adjusted with respect to the body 1 by means of the hydraulic piston/cylinder devices 5.

[0018] The visor carries two rows 6, 7 of cutting members 8, each provided with a jet nozzle 36 (other numbers of rows are possible as well). These cutting members are shown on an enlarged scale in figure 5. Each cutting member 8 comprises a cutting tooth 9 as well as a tooth holder 10. The tooth holders 10 have a mounting part 11 by means of which they are connected to a cross bar 12 of each row 7, 8. Furthermore, each tooth 9 has a cavity 14 in which the pointed end 13 of the mounting part 11 is tightly fitted. By means of a pin (not shown) extending through the holder and the tooth, axial displacements are prevented.

[0019] The tooth holders 10 each have a channel 15 which at one end opens out in a chamber 16 and the other end of which receives a feed pipe 17. The feed pipe 17 is tightly fitted within the tooth holder channel 15 by means of a sealing system 18. Furthermore, the tooth 9 has a tooth channel 19, which has a somewhat curved shape and into which a correspondingly curved section of the feed pipe 17 extends as well. At its free end, the feed pipe 17 is guided with respect to the tooth channel 19 by means of a guiding sleeve 20.

[0020] As shown in figure 4, the tooth holder chamber 16 of each tooth holder 10 is in communication with a feed opening 21 in the cross bars 12. These cross bars each delimit a plenum chamber 22, which plenum chambers 22 are fed with pressurized water by means of the respective feed lines 23, 24. These feed lines 23, 24 are each connected to a manifold 25 which in turn, by means of supply tube 26, is supplied with pressurized water.

[0021] Furthermore, jet nozzles 28 are connected to the body 1, which jet nozzles 28 are in communication with the supply pipe 26 as well through plenum chamber 27.

[0022] The process of cutting the bottom of a water body is now explained further with reference to the figures 6 up to 8. Figure 6 shows the general orientation of the drag head according to the invention with respect to the bottom 29 of a water body during a cutting operation. The visor 3 is oriented with respect to the body 1 through the piston/cylinder devices 5.

[0023] For clarification, figure 7a, 7b and 8 show details of the bottom material and the cutting process. Figure 7a shows the usual packing of particles 33 of the bottom material. In a normally packed bottom, said particles 33 are firmly urged onto each other in a random way. Upon exerting a cutting action on the bottom material as shown in figure 8, parcels 34 of bottom material are sheared with respect to each other by the cutting tooth 8. This shearing becomes possible as soon as the particles 33 are oriented along shear or glide surfaces 35. In such condition, parcels 34 can be moved with respect to each other.

[0024] It will be noted in figure 7b that the mutual distance between the particles 33 has been increased. As a result, the pore size in figure 7b is generally larger than in figure 7a. By introducing a water flow from the teeth 8 into the pores through the jets 30, the water pore volume is supplied with additional water which makes the formation of shear faces 35 easier. As a result, the cutting force of the teeth 8 is decreased.

Claims

1. Method for performing a dredging operation in a water body by means of a drag head, the bottom (29) of said water body comprising a packed bottom material of particles (33) which enclose pores which are filled with pore water, said drag head comprising at least one cutting member (8) for cutting the bottom material, jetting means (36) for treating the bottom material by means of the at least one water jet, and suction means for removing the cut bottom material, said method comprising the steps of:

- dragging the drag head along the bottom (29),

- exerting a cutting force on a parcel (34) of packed bottom material by means of the cutting member (8), said cutting force causing an increase of the mutual distance of at least part of said particles (33) of the parcel of bottom material and thereby causing a pore pressure decrease in the water filled pores between said particles (33),

- directing at least one water jet (30) towards said parcel (34) of bottom material so as to introduce a water flow into said pores between said particles (33) and thereby at least partly eliminating the pressure decrease in said pores caused by the said displacement of bottom material, said elimination resulting in a decreased specific cutting force,

- subsequently cutting said parcel (34) of bottom material by means of the cutting member (8).

2. Method according to claim 1, wherein the water jet (30) is directed towards the bottom material in front of the cutting member (8) at an angle between 45°and 135° to the vertical.

3. Method according to claim 1, wherein the water jet (30) is essentially horizontally directed towards the bottom material in front of the cutting member (8).

4. Method according to any of the preceding claims, wherein the water flow is generated at or near the cutting member (8).

5. Method according to any of the preceding claims, wherein the flow of the water jet is between 3 % and 40% of the cut material flux.

6. Method according to any of the preceding claims, wherein the flow of the water jet is between 4% and 35% of the cut material flux.

7. Method according to any of the preceding claims, wherein the flow of the water jet is about 15% of the cut material flow.

8. Method according to any of the preceding claims, wherein the water flow is between 0,5 and 1,5 cubic meter per second.

9. Method according to any of the preceding claims, comprising the steps of:

- applying at least two cutting members (8) at different height levels,

- directing at least two water jets (30) at different height levels towards the bottom material in front of a respective cutting member (8),

- subsequently cutting the less densely packed bottom material which is in front of the respective cutting member (8).

10. Method according to any of the preceding claims, wherein the cutting member (8) generates a cutting face in the parcel (34) of bottom material, at least the pores at said cutting face and/or bordering said cutting face being subjected to a pore pressure decrease caused by the cutting member and a pressure increase caused by the jet water (30).

11. Method according to any of the preceding claims, wherein the output pressure of the water jet (30) is between 2 and 5 bar above static pressure.

12. Method according to any of the preceding claims, wherein at least one secondary water jet (28, 31, 32) is generated for diluting the cut bottom material enabling the flow of said material towards the suction means.

13. Dredging installation for carrying out the method according to any of the preceding claims, comprising a dredging vessel provided with a tube which at one end is connected to the vessel and which at the other end carries a drag head (1, 2), pump means for generating a flow through the tube from the drag head (1, 2) towards the vessel, as well as a conduit (28) for feeding a water flow towards the drag head (1, 2), said drag head (1, 2) comprising cutting means (8) for cutting the bottom (29) of a water body and jet means (36) connected to the conduit for subjecting said water bottom to at least one water jet (30), characterised in that jet means are carried out for introducing a water flow into the pores between the particles (33) of the bottom material and thereby at least partly eliminating the pressure decrease in said pores which is caused by the cutting action of the cutting means (8).

14. Dredging installation according to claim 13, wherein the jet means are carried out for delivering water jet (30) with an output pressure between 2 and 5 bar above static water pressure.

15. Drag head for a dredging installation according to claim 13 or 14.

Amended claims

Amended claims in accordance with Rule 86(2) EPC.

1. Method for performing a dredging operation in a water body by means of a drag head, the bottom (29) of said water body comprising a packed bottom material of particles (33) which enclose pores which are filled with pore water, said drag head comprising at least one cutting member (8) for cutting the bottom material, jetting means (36) for treating the bottom material by means of the at least one water jet, and suction means for removing the cut bottom material, said method comprising the steps of:

- dragging the drag head along the bottom (29),

- exerting a cutting force on a parcel (34) of packed bottom material by means of the cutting member (8), said cutting force causing an increase of the mutual distance of at least part of said particles (33) of the parcel of bottom material and thereby causing a pore pressure decrease in the water filled pores between said particles (33),

- directing at least one water jet (30) towards said parcel (34) of bottom material so as to introduce a water flow into said pores between said particles (33) and thereby at least partly eliminating the pressure decrease in said pores caused by the said displacement of bottom material, said elimination resulting in a decreased specific cutting force,

- subsequently cutting said parcel (34) of bottom material by means of the cutting member (8),

- wherein said water flow is generated at the cutting member (8).

2. Method according to claim 1, wherein the water jet (30) is directed towards the bottom material in front of the cutting member (8) at an angle between 45°and 135° to the vertical.

3. Method according to claim 1, wherein the water jet (30) is essentially horizontally directed towards the bottom material in front of the cutting member (8).

4. Method according to any of the preceding claims, wherein the flow of the water jet is between 3 % and 40% of the cut material flux.

5. Method according to any of the preceding claims, wherein the flow of the water jet is between 4% and 35% of the cut material flux.

6. Method according to any of the preceding claims, wherein the flow of the water jet is about 15% of the cut material flow.

7. Method according to any of the preceding claims, wherein the water flow is between 0,5 and 1,5 cubic meter per second.

8. Method according to any of the preceding claims, comprising the steps of:

- applying at least two cutting members (8) at different height levels,

- directing at least two water jets (30) at different height levels towards the bottom material in front of a respective cutting member (8),

- subsequently cutting the less densely packed bottom material which is in front of the respective cutting member (8).

9. Method according to any of the preceding claims, wherein the cutting member (8) generates a cutting face in the parcel (34) of bottom material, at least the pores at said cutting face and/or bordering said cutting face being subjected to a pore pressure decrease caused by the cutting member and a pressure increase caused by the jet water (30).

10. Method according to any of the preceding claims, wherein the output pressure of the water jet (30) is between 2 and 5 bar above static pressure.

11. Method according to any of the preceding claims, wherein at least one secondary water jet (28, 31, 32) is generated for diluting the cut bottom material enabling the flow of said material towards the suction means.

12. Dredging installation for carrying out the method according to any of the preceding claims, comprising a dredging vessel provided with a tube which at one end is connected to the vessel and which at the other end carries a drag head (1, 2), pump means for generating a water flow through the tube from the drag head (1, 2) towards the vessel, as well as a conduit (28) for feeding a water flow towards the drag head (1, 2), said drag head (1, 2) comprising cutting means (8) for cutting the bottom (29) of a water body and jet means (36) connected to the conduit for subjecting said water bottom to at least one water jet (30), which jet means are carried out for introducing a water flow into the pores between the particles (33) of the bottom material and thereby at least partly eliminating the pressure decrease in said pores which is caused by the cutting action of the cutting means (8), characterised in that said jet means are located at the cutting member and are carried out for delivering a water jet (30) with an output pressure between 2 and 5 bar above static water pressure.

13. Drag head for a dredging installation according to claim 12, comprising cutting means (8) for cutting the bottom (29) of a water body and jet means (36) connectable to a pressurized water source for subjecting said water bottom to at least one water jet (30), which jet means are carried out for introducing a water flow into the pores between the particles (33) of the bottom material and thereby at least partly eliminating the pressure decrease in said pores which is caused by the cutting action of the cutting means (8), characterised in that said jet means are located at the cutting member and are carried out for delivering a water jet (30) with an output pressure between 2 and 5 bar above static water pressure.

Drawing