WO2011122942A1

WO2011122942A1 - Submersible dredging device, assembly of a riser system and submersible dredging device, vessel and method of driving a slurry pump

Info

Publication number: WO2011122942A1
Application number: PCT/NL2011/050167
Authority: WO
Inventors: Stanislav Verichev; Hendrik Anton Albers; Robert Gerard Van De Ketterij; Cornelis Hendrik Van Den Berg
Original assignee: Ihc Holland Ie B.V.
Priority date: 2010-03-31
Filing date: 2011-03-11
Publication date: 2011-10-06
Also published as: NL2004484C2

Abstract

The present invention relates to a submersible dredging device (12) for dredging slurry from a waterbed of a water volume to a water surface, comprising: - a slurry pump inlet (15) and a slurry pump outlet (16); - a slurry pump impeller (25) for transporting the slurry from the slurry pump inlet to the slurry pump outlet; and - a drive assembly (26) for driving the slurry pump impeller; wherein the drive assembly comprises a reinjection channel (2) connecting a drive assembly inlet and a drive assembly outlet, the drive assembly further comprising a drive impeller arranged in the reinjection channel and coupled to the slurry pump impeller so as to drive the slurry pump impeller.

Description

SUBMERSIBLE DREDGING DEVICE, ASSEMBLY OF A RISER SYSTEM AND SUBMERSIBLE DREDGING DEVICE, VESSEL AND METHOD OF DRIVING A SLURRY PUMP The invention relates to a submersible dredging device, an assembly of a submersible dredging device and a riser system, a vessel having a riser system with a submersible dredging device and a method of driving a slurry pump.

Submersible dredging devices may be used for delivering slurry from a waterbed of a water volume (for instance the sea) to a water surface (for instance the sea surface). The submersible dredging device may be arranged to deliver subsea slurry from a seabed via a riser system to the sea surface.

Subsea slurries can be delivered to the sea surface using submersible centrifugal dredge pumps installed over a vertical deep sea riser system. These pumps are normally driven by electric or by hydraulic motors. Energy for the motors is supplied using electric and hydraulic umbilicals, respectively. Power is generated at the vessel by combustion engines, converted into electric (or hydraulic) power, which is then transferred subsea to the pump drive converting electric (or hydraulic) into the motion.

Submersible dredging devices, such as submersible dredge pumps, are supposed to provide high slurry production rates and therefore can have a power of up to 2 MW or even more. Combined with the necessity to install such pumps at large depths (for instance in the order of several kilometers), this assumes that high power motors must be employed to drive these pumps. Electric or hydraulic motors can be used.

In case of electric motors, power is usually supplied using one or more electric umbilicals. A disadvantage of this approach is the need to seal the electric motors and the associated electric control system and/or to provide for an ambient pressure compensation system. Furthermore, the umbilicals may become complex and/or expensive as both the needed power and the depth increase. Possible failure of the electric system in this case may also be an issue.

In the case of hydraulic motors, there could be two approaches: oil hydraulics and water hydraulics. According to the first approach, there is provided a submersible hydraulic system powered from the vessel using electric umbilicals. A disadvantage of this approach is the increased number of systems so that the reliability of the system may be compromised as the number of system components that could fail is increased. Also, any system has efficiency less that 100%, which means that a larger number of systems generally results in a smaller overall efficiency.

Alternatively, according to a second approach, water hydraulics systems are used. According to this approach, positive-displacement type hydraulic motors are to be used since most of the other types of hydraulic motors do not deliver sufficiently high powers and torques. Normally, such motors deliver either sufficiently high power but small rpm (in this case, a gear system is to be installed in between of pump and motor), or sufficiently high rpm but small power (in this case, a number of motors driving the same pump is to be installed in a series). Furthermore, hydraulic fluid is normally not allowed to contain solids (particles) with sizes (diameter) larger than 100 microns. Also, solids content in a power fluid should be kept sufficiently small.

GB 1505449 describes an apparatus for the hydraulic raising of solids. The document discloses a delivery line running from a sea bottom to a ship. At an intermediate location along the delivery line a pump and a turbine are provided. The pump is driven by the turbine. The turbine is driven by a working fluid that is provided to the turbine via a working fluid pressure line coming from the ship. The working fluid is brought to the required pressure by means of a pump provided at the ship.

The working fluid is drawn in by the ship via a separate line and is disposed in the sea at the intermediate location along the delivery line. Alternatively, the working fluid is in a closed loop and is fed back from the intermediate location to the pump provided at the ship.

In subsea slurry production roughly 50% of water is produced. This water can be separated at the vessel from the useful solid material and the separated waste water can be reinjected back into the water volume.

Consequently, if one supposes to use this waste water as a drive fluid, then a considerably complex filtering and separation system needs to be installed.

According to GB 1505449 the separated waste water is discharged from the ship via discharge openings near the sea surface.

It is an object of the present invention to provide an improved drive assembly for driving a pump.

It is a further object of the invention to provide a submersible dredging device wherein the disadvantages of the prior approaches have been reduced or even completely removed. It is a further object of the present invention to provide an improved method of driving a pump, for instance a slurry pump in a submersible dredging device.

It is a further object of the invention to provide a submersible dredging device having an improved subsea power generation and transfer.

According to a first aspect of the invention at least one of the objects is achieved in a submersible dredging device for pumping slurry through a riser system, the dredging device comprising:

- a slurry pump for pumping slurry through the riser system, the slurry pump comprising a pump inlet and a pump outlet, the pump inlet and pump outlet being configured to be connected to a dredging pipe of the riser system; and

- a drive assembly for driving the pump, wherein the drive assembly comprises a reinjection channel connecting a drive assembly inlet and a drive assembly outlet, the drive assembly outlet and drive assembly inlet being configured to be connected to a water reinjection pipe of the riser system for discharging water into the water volume, the drive assembly further comprising a drive impeller arranged in the reinjection channel and coupled to drive the pump.

The pump could be any type of pump, for instance a piston type pump or a centrifugal pump. In embodiments of the present invention the pump is a slurry pump comprising a slurry pump impeller for transporting slurry from the slurry pump inlet to the slurry pump outlet and wherein the drive impeller of the drive assembly is coupled to the slurry pump impeller so as to drive the slurry pump impeller.

In an embodiment the reinjected water is discharged near the water bed. In a further embodiment the reinjected water is discharged into the water bed, for instance into the sea bed. In another embodiment the reinjection water is used to release slurry from the sea bottom. In an embodiment a drive impeller is present in the reinjection channel, the drive impeller forming a drive, for instance a generator, for the slurry pump. The flow of water through the reinjection channel is used as a power source. This reduces, and in specific embodiment avoids, the use of separate umbilicals for electrical or hydraulic supply of energy. According to this embodiment, it is assumed that reinjection water is used as power fluid to drive the submersible dredging device.

In an embodiment reinjection water or waste water containing relatively large particles and relatively high solids content can be used to power the system. In an embodiment the slurry channel is part of the dredging pipe of the riser system for delivering slurry upwards from the waterbed to the water surface, while the reinjection channel is part of reinjection pipe for transporting a flow (of water) from the surface downwards towards (near) the waterbed.

In an embodiment the slurry pump comprises a slurry pump impeller. The submersible dredging device comprises at least two impellers, at least one in an upward channel and at least one in downward channel. The at least two impellers can be present in separated casings in a single house, or in an embodiment in separated (generally connected) housings.

In an embodiment the drive impeller and slurry pump impeller have several blades. In embodiments the drive impeller is arranged to be driven by the stream of water inside the reinjection pipe of the riser system and the slurry pump impeller is arranged to generate a stream of slurry in the dredging pipe of the riser system.

Depending on the application, impellers can have different embodiments: open and closed type, radial, axial, helical, or radial-axial type (e.g. like Francis turbine) etc., different sizes (for instance diameters) and/or different shapes.

In an embodiment the drive impeller is mechanically coupled with the slurry pump impeller. This allows the use of a submersible dredging device that is purely mechanical and does not need electrical power and/or additional hydraulical power to function. The power can be generated at the vessel using the submersible dredging device to which it is coupled via the riser system.

In an embodiment the drive impeller and slurry pump impeller are positioned coaxially. Both impellers may be arranged in one or more housings by using suitable bearings. The bearings can be aligned. The coaxial position allows a simple coupling between the driving impeller (drive impeller) and the driven impeller (slurry pump impeller).

In an embodiment rotating parts are made of wear-resistant material. In an embodiment rotating parts can have wear resistant coatings or can be composed of several parts with different materials.

In an embodiment a drive shaft couples the drive impeller and slurry pump impeller. The drive impeller is driven by the waste water, which results in rotational motion of the second impeller transmitted via the common shaft. The number of systems and energy conversions is reduced here as much as possible. In an embodiment the drive shaft comprises two rigidly coupled shafts having a rotary seal in between. An advantage of this embodiment is that one of the devices (i.e. slurry pump or drive assembly) may be replaced without needing to lift the other one.

In an embodiment a transmission, for instance a gear transmission or a similar transmission, couples the drive impeller and slurry pump impeller. This allows different rotational speeds of the impellers as well as possibility to install both devices not coaxially but, for instance, at the same angle.

In an embodiment the water reinjection pipe of the riser system has a water reinjection outlet directed at the waterbed. In an embodiment the outlet may comprise a nozzle allowing water from the reinjection pipe to be discharged at high speed into the volume of water. In this way slurry from the waterbed can be released before it is sucked into the slurry channel.

According to an aspect of the invention an assembly of a riser system and a submersible dredging device is provided. The submersible dredging device can have any of the features of the submersible dredging device described in this application. In an embodiment the assembly further comprises a water pump connected to the reinjection channel via the riser system. The water pump is arranged to provide a flow of (reinjection) water through the riser system towards the submersible dredging device. The impeller in the reinjection channel will rotate as a result of the water flow. This allows to provide a system wherein the pump is positioned at a distance from the submersible device, said pump forming the original generator for pumping the slurry. Further this allows providing the submersible dredging device without electrical or additional hydraulic inputs.

In embodiments of the present invention the assembly comprises a first housing for accommodating the at least the drive impeller of the drive assembly and a second housing for accommodating at least the slurry pump impeller, wherein at least one of said housing is connected to the riser system. The connection between the housing and the riser system may be a substantially rigid connection (for instance, screwed, via rigid pipes). In another embodiment the connection is a flexible connection (for instance, flexible hoses). In the latter case, the housing may be dynamically decoupled from the riser system. It could also be top-tensioned to the vessel or a buoyancy module or just attached to the buoyancy module. The assembly may further comprise a reinjection pump configured and arranged to receive a liquid fraction of the slurry pumped up by the slurry pump and to direct the liquid fraction through the reinjection channel. The liquid fraction may be liquid (for instance water) originating from the mixture of liquid and solid (i.e. the slurry) and being arrived at the vessel via the riser system. In many practical situations the reinjection pump is arranged at or in the vessel, the pump may be positioned elsewhere as long as it is possible the transport the liquid fraction downward through the reinjection pipe with a view to discharge the liquid anywhere in the water volume.

According to another aspect of the invention a drive assembly is provided for driving a pump, for pumping a mixture through a first pipeline, the pump comprising a pump impeller, a pump inlet and a pump outlet which inlet and outlet are connectable or connected to the pipe line, wherein the drive assembly comprises:

- a channel connecting a drive assembly inlet and a drive assembly outlet, the drive assembly outlet and drive assembly inlet being configured to be connected to a second pipeline through which a liquid may be caused to flow;

- a drive impeller arranged in the channel so as to be driven by the liquid flow; wherein the drive impeller is coupled to drive the pump impeller of the pump.

The pump may or may not be of a submersible type. For instance, the pipelines may be (at least partly) land-based. The pump may be a slurry pump to transport slurry to a discharge location, for instance on board a vessel or land-based location. The pipelines may be arranged to transport any type of medium, for instance oil, water, slurry, etc. In a specific embodiment the first pipeline transports a mixture of liquid and solids (for instance slurry from the bottom of a volume of water) in a first direction, while the second pipeline transports another liquid or the liquid part of this earlier- mentioned mixture in a second direction, opposite the first direction, to return the liquid to a specific location. The liquid flow in the second pipeline may then be used to energize the pump in the first pipeline. The first and second pipelines may be part of an upright riser system, but may also be part of different systems, for instance systems for transporting slurry in a more horizontal direction.

In embodiments of the invention connection means are provided to

mechanically couple the drive impeller with the slurry pump impeller. The the connection means may be configured to cause rotation of the pump impeller by the rotation of the drive impeller. The connections means may comprise a transmission gear to transmit the rotation of the drive impeller to rotation of the pump impeller according to a specific transmission ratio.

According to an aspect of embodiments of the invention a vessel (for instance a ship, pontoon or the like) is provided having a (vertical sub sea) riser system with a submersible dredging device described herein. In an embodiment the vessel comprises a water pump connected to the reinjection channel via the riser system. This embodiment allows a reduced number and/or complexity of umbilicals.

In an embodiment the vessel has a separator for separating water from the slurry received via the riser system. The separator can separate the water from the delivered slurry and in an embodiment a water outlet of the separator is connected with the reinjection channel. The water taken from the waterbed may be discharged again in the volume of water, for instance near the water bed.

In an embodiment preprocessed sea water only or a combination of waste water and preprocessed sea water is used in the riser system to drive the impeller and to form a drive for the slurry pump as well.

According to a further aspect a riser system for dredging is provided having a dredging pipe and a discharge channel (for instance - but not limited to- a reinjection pipe), wherein the discharge channel comprises a discharge impeller forming a drive for a pump arranged in the dredging pipe of the riser system.

In an embodiment the riser system and/or the submersible dredging device comprises multiple pumps wherein a subset of the pumps is driven by waste water, while other pumps are driven by sea water, for instance pre-processed sea water following ecological restrictions.

According to a further aspect a method of driving a slurry pump in a submersed dredging device coupled via a riser system to a vessel is provided. The method comprises providing a water flow from the vessel to the submersed dredging device and driving the slurry pump by means of the water flow. The method involves a "water mill" like principle wherein the dredging pump is operated by the mechanical forces generated by a stream of water driving flowing past an impeller of a driving assembly.

According to an embodiment the method may comprise separating water from the slurry received through the riser system and pumping the separated water back into the riser system so as to generate the water flow driving the slurry pump. Although the invention is described in connection with several embodiments thereof, it will be clear that the invention is not limited to the embodiments shown. The skilled person will recognize this disclosure encompasses all explicit and implicit combinations of features indicated in this application.

Further features, advantages and details of the present invention will also become apparent from the following description of embodiments thereof, wherein reference is made to the accompanying figures.

Fig. 1 shows a ship having a vertical sub sea riser system and a submersible dredging device according to an embodiment of the present invention;

Fig. 1A shows a detail of the embodiment of the submersible dredging device according to figure 1 ; and

Fig. 2 shows a schematic cross sectional view of an embodiment of the submersible dredging device.

FIG 1 provides a schematic view of a dredging vessel having a vertical riser system having a power fluid channel and a dredging device at the lower end. The dredging vessel (v) is provided with a riser system 20 comprising a dredging pipe 1 , a discharge pipe 2 and means to connect both pipes to the vessel. The dredging pipe 1 has a suction end 3 at which slurry (s) can be sucked into the pipe. In the embodiment shown in figure 1 the slurry is dredged using a submersible remote-controlled vehicle 6 that is provided with dredging equipment for removing slurry from the sea bed. In other embodiments of the invention other types of dredging methods and/or dredging equipment may be used.

The dredging pipe 1 has a discharge end 4 at which the pumped slurry may be discharged in the vessel's hold 9. The slurry may be pumped upward by one or more dredge pumps 5, for instance -but not limited to- one or more centrifugal pumps. The dredge pump 5 may be arranged at any position along the dredging pipe 1 , but usually is arranged close to or at the suction end 3 of the pipe. The discharge pipe 2 is connected to a water pump 7. The water pump 7 can generate a downward water flow along the discharge pipe 2 towards a nozzle 8. The water discharged from the nozzle 8 may be discharged at a depth within the boundaries provided for by international maritime regulations.

The water pumped by the water pump into the discharge pipe may be water that has been separated from the slurry that has been pumped up by the dredge pump 9. The water is herein also referred to as "waste water" or "reinjection water". The latter term may be used since this water is sucked up by the dredging pipe and once it has arrived at the sea surface and has been separated by a separator in the vessel, is pumped into in the discharge pipe, also called the reinjection pipe, and reinjected into the sea close to the suction end of the dredging pipe.

The vessel (v) further comprises an active control system 11 (schematically shown) for controlling the flow rate of the waste fluid. If there is not enough waste water produced at the vessel, the missing volumes are to be sucked from the sea. On the contrary, if the flow rate of waste water is higher than required, the waste water is to be discharged using a relief valve (not shown in the figures) installed in the discharge pipe.

Figures 1 A and 2 schematically show a view and a cross section of a submersible dredging device 12 in accordance with an embodiment of the invention. In the shown embodiment the device 12 comprises a water driven submerged centrifugal slurry pump. It can be used to deliver slurry from a sea bed to the sea surface if connected to a vertical sub sea riser system.

Referring to figure 2, the dredging device 12 is shown in more detail. The dredging device 12 comprises a housing 13 divided in a first compartment 14 for accommodating the dredge pump 5 and a second compartment 15 accommodating a drive assembly for driving the dredge pump, as will be explained hereafter. The compartments or casings are separated by a partition wall 36. The housing 13 comprises a dredge pump inlet 15 and dredge pump outlet 16, both openings being in fluid communication with the first compartment 14. Similarly, the housing comprises an inlet 17 and an outlet 18, both openings being in fluid communication with the second compartment 15.

A lower part 19 and an upper part 20 of the dredging pipe 1 are connected to the inlet 15 and outlet 16 of the first compartment, respectively. Similarly, an upper part 21 and a lower part 22 of the pipe 2 are connected to the inlet 17 and outlet 18 of the second compartment 15, respectively.

Rotatably attached to the housing and arranged inside the first compartment 14 thereof is a slurry pump impeller 25 of the slurry pump 5, while in the second compartment 15 a drive impeller 26 is arranged. The drive impeller 26, more specifically, in the present embodiment, the vanes or blades 38 thereof, is arranged to be rotatable in the direction denoted by arrow 28 around axis 29. The slurry pump impeller is connected with the drive impeller by a common shaft 28 (or two coupled shafts with a gear box in between (not shown in the drawings), and rotatable in the same direction as the drive impeller.

Depending on the application, drive impeller 26 and slurry pump impeller 25 are placed either in separate casings (pump housings) or at one casing with separated chambers or compartments 14,15. More specifically, slurry pump impeller 25 is present in a slurry channel (not shown) formed between slurry inlet 15 and outlet 16, while the drive impeller is present in a similar channel extending between inlet 17 and outlet 18 of compartment 15. In the partition wall 36 a shaft sealing 37 is provided to allow shaft 28 to be rotated without slurry from compartment 14 entering compartment 15 or vice versa. Also, depending on the application, drive impellers can be different: open and closed type radial, axial, and helical, or of the turbine type etc.

As mentioned above, the water pump pumps water into the upper part 21 of the reinjection pipe 2. The pressurized water flow then enters the second compartment 15 and causes the reinjection or drive impeller 26 to rotate. After the water flow has provided motion for the reinjection or drive impeller 26 of the dredging device 12, it leaves the compartment 15 and enters the lower part 22 of the reinjection pipe 2, Finally the water is discharged by nozzle 8 and directed to the seabed (s).

In the embodiment shown in figure 2 the impellers 25 and 26 are positioned coaxially and are coupled directly via mechanical means. More specifically, rotational motion of the drive impeller 26 is transmitted into rotational motion of the slurry pump impeller 25 via the rigid shaft 28. Alternatively, the shaft may be embodied as two (or more) rigidly coupled shafts having a rotary seal in between. In another embodiment a gear transmission is provided between both impellers so that the rotational movement of the drive impeller may be transmitted in a suitable transmission ratio to the slurry pump impeller. The rotation of the impeller 25, more specifically vanes or blades 27 thereof, causes the slurry present in the dredging pipe 1 to be pumped towards the vessel.

The driving force exerted by the drive impeller 26 on the dredging pump impeller 25 of the dredging pump 5 is sufficient to cause the dredging pump to transport the slurry upward towards the dredging vessel (v). There is no need anymore for a separate drive mechanism, such as an electric or one or more separate hydraulic motors. The entire system may be purely mechanical with rigid couplings and all the power is generated at the vessel. At the same time, waste water containing relatively large particles and relatively high solids content can be used to power the system. There is no requirement to loop power fluid so the waste water after passing the drive impeller may be continuously discharged into the sea bottom at high flow rates.

In some embodiments only one pump, for instance one slurry pump, is used. In other embodiments more than one pump is used. They can be distributed along the riser system at certain mutual distances and/or large booster stations may be located somewhere along the riser system. The waste water (which is to be used as (a part of) the motive fluid) is discharged at any location, for instance as deep as possible. The channels in which the individual reinjection impellers driving the respective pumps are positioned, could each be connected to a separate supply line or could be fed from one main supply line (with certain pressure), for instance via a system of valves.

Operating parameters of the three devices, i.e. a first pump (e.g. slurry pump) for pumping a medium through the first pipeline, a second pump (e.g. reinjection pump) for pumping a medium through the second pipeline and the driving assembly for driving the second pump are to be chosen in such way that best efficiency points of their efficiency curves either match each other or belong to more or less the same range of flowrates, which guarantees optimal energy consumption and dynamic stability if the flowrates (both slurry production rate and, consequently, flowrate of the drive fluid) are varied within the range required by the operational conditions.

While preferred embodiments of this invention have been shown and described, modifications thereof can be made by one skilled in the art without departing from the spirit or teaching of this invention. The embodiments described herein are exemplary only and are not limiting. Many variations and modifications of the system and apparatus are possible and are within the scope of the invention. Accordingly, the scope of protection is not limited to the embodiments described herein, but is essentially limited by the claims, the scope of which shall include all equivalents of the subject matter of the claims.

Claims

1. Submersible dredging device for dredging slurry from a waterbed of a water volume, such as subsea slurry from a seabed, via a riser system to a water surface, the dredging device comprising:

2. Dredging device as claimed in claim 1, wherein the pump is a slurry pump comprising a slurry pump impeller for transporting slurry from the slurry pump inlet to the slurry pump outlet and wherein the drive impeller of the drive assembly is coupled to the slurry pump impeller so as to drive the slurry pump impeller.

3. Submersible dredging device according to claim 2, wherein the drive impeller is mechanically coupled with the slurry pump impeller.

4. Submersible dredging device according to claim 2 or 3, wherein the drive impeller and slurry pump impeller are arranged coaxially.

5. Submersible dredging device according to any of the claims 2-4, wherein a common drive shaft couples the drive impeller and slurry pump impeller to transmit rotational motion of the drive impeller to the slurry pump impeller.

6. Submersible dredging device according to claim 5, wherein the drive shaft comprises two rigidly coupled shafts having a rotary seal in between.

7. Submersible dredging device according to any of the previous claims, comprising a gear transmission between the drive impeller and slurry pump impeller.

8. Submersible dredging device according to any of the preceding claims, wherein the drive impeller and slurry pump impeller comprise several blades.

9. Submersible dredging device according to any of the preceding claims, comprising a first housing for accommodating the at least the drive impeller of the drive assembly and a second housing for accommodating at least the slurry pump impeller.

10. Submersible dredging device according to any of the claims 1-8, comprising a combined housing for at least the slurry pump impeller and the drive impeller.

11. Submersible dredging device as claimed in any of the claims 2-10, wherein the drive impeller and pump impeller are of different shapes and/or sizes.

12. Assembly of a riser system and a submersible dredging device according to any of the preceding claims, wherein the assembly further comprises a water pump connected to the reinjection pipe of the riser system.

13. Assembly as claimed in claim 12, comprising a first housing for accommodating the at least the drive impeller of the drive assembly and a second housing for accommodating at least the slurry pump impeller, wherein at least one of said housing is connected to the riser system, wherein the connection is a rigid connection or a flexible connection.

14. Assembly according to claim 13, wherein the water reinjection pipe has a water reinjection outlet arranged to be directed at the waterbed.

15. Assembly according to any of the preceding claims, comprising a reinjection pump configured and arranged to receive a liquid fraction of the slurry pumped up by the slurry pump and to direct the liquid fraction through the reinjection channel.

16. Drive assembly for driving a pump, for instance a slurry pump, for pumping a mixture through a first pipeline, the pump comprising a pump impeller, a pump inlet and a pump outlet which inlet and outlet are connectable or connected to the pipe line, wherein the drive assembly comprises:

- a reinjection channel, connecting a drive assembly inlet and a drive assembly outlet, the drive assembly outlet and drive assembly inlet being configured to be connected to a second pipeline, being a water reinjection pipe of a riser system, through which a reinjection liquid may be caused to flow;

- a drive impeller arranged in the channel so as to be driven by the liquid flow;

wherein the drive impeller is coupled to drive the pump impeller of the pump.

17. Drive assembly as claimed in claim 16, comprising connection means to mechanically couple the drive impeller with the slurry pump impeller, wherein the connection means are configured to rotate the pump impeller by the rotation of the drive impeller.

18. Drive assembly as claimed in claim 16 or 17, further being characterized by the features of the drive assembly as claimed in any of claims 1-15.

19. Drive assembly as claimed in any of claims 16-18, in combination with the first and second pipeline.

20. Vessel having a riser system with a submersible dredging device according to any of the claims 1-14, wherein a reinjection water pump on the vessel is connected to the reinjection channel via the reinjection pipe of the riser system.

21. Vessel according to claim 20, comprising a separator for separating water from the slurry received via the riser system.

22. Vessel according to claim 21, wherein a water outlet of the separator is connected with the reinjection channel.

23. Riser system for dredging slurry from a water bed to a water surface, the system comprising a dredging pipe for transporting slurry upward towards the water surface and a discharge pipe, being a reinjection pipe, for transporting water downwards to the water bed, the system further comprising a discharge impeller arranged in the discharge pipe for driving a slurry pump in the dredging pipe.

24. Method of driving a slurry pump in a submersed dredging device coupled via a riser system to a vessel by a providing water flow from the vessel to the submersed dredging device and driving the slurry pump by means of the water flow.

25. Method as claimed in claim 24, comprising separating water from the slurry received through the riser system and pumping the separated water back into the riser system so as to generate the water flow driving the slurry pump.

26. Method as claimed in claim 24 or 25, wherein a dredging device as claimed in any of the claims 1-16 is employed.

27. Use of a dredging device, vessel and/or a drive assembly as claimed in any of the preceding claims.