EP4408733A1

EP4408733A1 - A hull cleaning device and a method of cleaning a hull

Info

Publication number: EP4408733A1
Application number: EP22790540.3A
Authority: EP
Inventors: Henrik NEUSCHÄFER LARSEN; Jens SEVERIN STEPHENSEN
Original assignee: Hydro Hull Cleaning AS
Current assignee: Hydro Hull Cleaning AS
Priority date: 2021-09-30
Filing date: 2022-09-29
Publication date: 2024-08-07
Also published as: WO2023052496A1

Abstract

A submergible cleaning device (1) comprising a frame (2) and at least one cleaning unit (5) attached to the frame (2). The cleaning units comprises: a housing (6) defining a lower edge (7) extending about an opening (8) into an inner cavity (9), an outlet (10) through a surface (11) of the housing, a power-driven pumping arrangement to provide a fluid flow from the cavity through the outlet (10), a water injection system with at least one nozzle (12), a rotary disc (13) defining an outer surface (14) and on opposite inner surface (15) and arranged in the opening (8) with the inner surface (15) facing the cavity (9), the rotary disc (13) being arranged for rotation about a rotor axis (16) and carrying the at least one nozzle (12) to thereby generate a circulating flow of water during rotation, and power-driven means (17) arranged to rotate the rotary disc about the rotor axis (16). To provide better flow conditions from the cavity into the outlet and thereby to enable a reduced power-driven pumping and thereby reduces suction, better manoeuvrability, and reduced power consumption, the radial distance from the outlet (10) to the rotor axis (16) is larger than a radial distance from a periphery (17) of the rotary disc (13) to the rotor axis (16).

Description

A HULL CLEANING DEVICE AND A METHOD OF CLEANING A HULL

INTRODUCTION

The present disclosure relates to a submergible cleaning device for cleaning under water structures such as ship hulls with high pressure water. Further, the disclosure relates to a device comprising a frame with one or more cleaning units each having rotating discs with cleaning nozzles. The disclosure further relates to the cleaning unit, and to a method of cleaning an underwater surface.

BACKGROUND

Clean and smooth underwater surfaces on marine vessels are essential for ensuring good performance and low fuel consumption. Underwater drones and Marine ROVs are already used for hull cleaning between dockings. Such cleaning is typically carried out while the ship is attached to a mooring.

While reducing fouling and friction and thereby contributing to environmental, green, behaviour, the underwater cleaning process also releases contaminants and potentially pollutes the environment where the cleaning takes place. For environmental reasons, it is desirable to efficiently collect material which is released from the hull.

It may be a solution to create a very strong suction of water away from the cleaning device. The water which is sucked away from the device may be filtered or cleaned in any desirable manner. However, this solution may create undesired strong adhesion by suction of the device onto the surface which is cleaned and consequently reduce manoeuvrability or damage the surface to be cleaned.

SUMMARY

It is an object of embodiments of the disclosure to enable a more efficient collection of material during underwater cleaning. It is a further object to increase efficiency and manoeuvrability of the device along the surface to be cleaned, and to protect the surface better against damage.

According to these and other objects, the present disclosure, in a first aspect, provides a submergible cleaning device wherein each cleaning unit defines a housing comprising a lower edge extending about an opening into an inner cavity. An outlet provides a passage for water through a surface of the housing, and a power-driven suction arrangement provides a fluid flow from the cavity through the outlet. A water injection system provides high pressure water through at least one nozzle, and a rotary disc is arranged in the opening and carries the nozzles.

The rotary disc is arranged such that water which is ejected by the nozzles, creates a circulating flow of water when the rotary disc rotates.

The device comprises power driven means, particularly an electric or hydraulic motor etc, arranged to rotate the rotary disc about a rotor axis, e.g., at 350-700 rpm.

A radial distance from the outlet to the rotor axis is in the size of, or larger than a radial distance from a periphery of the rotary disc to the rotor axis. Accordingly, the outlet is displaced radially away from the rotor axis at least to the location of the periphery or further away from the centre.

Bernoulli's principle states that an increase in the speed of an incompressible fluid occurs simultaneously with a decrease in static pressure.

Consequently, within a horizontal fluid flow, the highest speed occurs where the pressure is lowest, and the lowest speed occurs where the pressure is highest. In a rotating, circular, flow of water, the highest flow speed occurs at the periphery of circulating body of water. At the centre of the body, the speed is zero, and consequently, the Bernoulli's principle dictates a pressure profile with low pressure at the periphery and raising pressures towards the centre of the rotary disc.

By Bernoulli's principle, the circulating flow created by the nozzle(s) on the rotating rotary disc defines a flow speed profile and creates a pressure profile in the housing which primarily seeks to maintain the water in that radial distance from the centre of the rotating disc.

In traditional cleaning devices, the outlet is often located at or near the centre, i.e. , at the rotor axis. However, due to the Bernoulli's principle, this is not the location the water would seek automatically. On the contrary, the pressure profile provided by the Bernoulli's principle requires an excessive low pressure in the outlet to overcome the low pressure at the location where the speed of the water is highest, namely at the periphery of the rotating disc.

Accordingly, a strong suction force provided by powered pumps at the outlet is typically necessary to conquer the low pressure at the periphery, and such strong suction forces may negatively impact the ability to manoeuvre the cleaning device and it may potentially damage the surface which is cleaned by the device.

By the location of the outlet at the periphery or at a radial distance exceeding the radial distance to the periphery, the pressure occurring due to the rotation of the body of water facilitates an easier evacuation of the water through the outlet, and the power pumping structure may be operated at a lower suction pressure leading to increased manoeuvrability, reduced risk of damaging the surface, and a reduced power consumption of the pump structure.

The system makes use of the fact that the liquid in the chamber rotates strongly due to the rotating nozzle(s). An associated horizontal gravitational field is therefore created in the chamber. All material which is released from the cleaned surface is carried by the centrifugal force into a strong tangential water flow along the edge of the cleaning chamber, where it is directed to the outlet.

The power-driven pumping arrangement can benefit from the water velocity at the outlet, with the associated locally generated negative pressure in relation to the surrounding water masses. This reduces the need for a large static pump generated negative pressure at the outlet.

The rotating water flow and the location of the outlet provides a device which can operate with a very moderate negative pressure in the cleaning disc, and thus a very moderate pressure against the surface to be cleaned.

By radial distance to the outlet is herein meant a radial distance from the rotor axis to the centre of the outlet.

The rotary disc may have a size and location such that a radially extending gap is formed between an outer periphery of the rotary disc and an inner surface of the housing. The gap extends in a radial direction relative to the rotor axis and therefore defines a distance between the periphery of the rotary disc and the rim of the housing. The outlet is arranged in direct communication with at least a section of the gap meaning that water can flow directly and unobstructed from that section of the gap to the outlet. Herein, we refer to that section of the gap as "a collection zone".

Since the collection zone of the gap is in direct communication with the outlet, water can be repelled directly from the gap into the outlet by centrifugal force generated by rotation of the rotary disc. Considering a specific suction pressure in the outlet, this provides an increased flow of fluid through the outlet.

If water is sucked through the outlet at a relatively strong suction pressure, the resulting suction force onto the surface to be cleaned may hinder or at least reduce the ability to move the cleaning unit along the surface to be cleaned.

The outlet may extend from the collection zone in the direction of the rotor axis. This provides a change in the flow direction. Firstly, the water and removed material flows tangentially, or circumferentially along the outer periphery of the rotary disc until it reaches the collection zone, and subsequently, it changes flow direction and flows via the outlet in the axial direction of the rotor axis. This potentially improves the control of the pumping and water removing from the housing.

Particularly, the specific location of the gap relative to the outlet further enables a relatively moderate suction pressure to be applied by the pumping structure. Particularly, a balance between the size and location of the collection zone, the rotational speed of the rotary disc, the water pressure and flow rate of water through the nozzles, and the suction pressure applied by the pumping structure may provide a good balance between efficient collection of contaminants and suction of the device onto the hull. With the claimed structure, it is possible to select a rotor speed, a nozzle pressure, and a pumping pressure which provides adequate suction onto the surface to be cleaned and provides good collection of the material which is removed from the surface to be cleaned.

When reference herein is made to "radial direction", it is a direction which is perpendicular to the rotor axis. The rotary disc may be circular but could also have other shapes, e.g., an oval shape.

A gap extending from the periphery of the rotary disc and radially towards an inner surface of the housing has a size which increases in a direction towards the collection zone. This may increase the ability to collect contaminants at the collection zone. The collection zone could be defined as that part of the gap where the gap is largest. And the collection zone may be terminated with a radially extending wall forcing the water into the outlet.

The housing may form a bottom portion and a rim portion extending between the bottom portion and the lower edge. The outlet may be formed in the bottom portion, and the cavity may be a unitary cavity facilitating free flow between the housing and the inner surface of the rotary disc. If, the outer periphery is followed in the direction of rotation of the rotary disc, and if the position where the smallest gap between the rim and the peripheral edge of the rotary disc is called the start point at zero degrees, then the collection zone could be e.g., between 270- 360 degrees, and the aforementioned radially extending wall terminating the collection zone could be in the interface between the start point and the collection zone, i.e. at 360 degrees (or zero). In the collection zone, the gap is larger than elsewhere, and it may particularly be more than twice the size of the gap at start point, or even more than three times the gap at the start point.

Following the direction of rotation from the start point to the collection zone, the gap may increase gradually or stepwise towards the collection zone. In one embodiment, the circumference could be split into at least three sectors, one extending from the start point to a first intermediate position. In this sector, the gap could be constant. A second sector extending from the first intermediate position to the start of the collection zone. In this, second, sector, the gap may increase, and a third sector may define the collection zone. In this zone, the gap may either increase further, or it may be constant.

The collection zone allows the collection of the material released from the surface and provides an improved way of removing the material through the outlet.

The rotary disc may form an offset surface portion displaced in the direction of the rotor axis into the cavity relative to the outer periphery. The offset surface portion may be a planar portion, and it may define a proximal space between the rotary disc and the surface to be cleaned. That proximal space may reduce suction of the device onto the surface to be cleaned. The offset surface portion may be a completely closed portion, or at least a portion preventing flow across the rotary disc from the outer surface to the inner surface.

The offset surface portion may e.g., be perpendicular to the rotor axis.

The nozzles may be arranged in the outer periphery and thereby arranged to eject water from an ejection point displaced relative to the offset surface portion. This allows the nozzles to be relatively close to the surface to be cleaned, and thereby provides a good cleaning effect of the nozzles. Further, it places the nozzles at the periphery where the speed caused by the rotation is highest. That increases the effect of the Bernoulli's principle.

Each nozzle may be arranged for ejecting water from the outer surface of the rotary disc in the direction of the rotor axis, i.e., typically perpendicularly relative to the surface to be cleaned. This provides a good cleaning effect compared e.g., to an inclined angle of the nozzle relative to the surface to be cleaned. The device may comprise a pressure release structure with a non-return valve allowing a flow into the cavity for reducing a pressure difference between a suction pressure at a central portion of the outer surface of the rotary disc and a surrounding pressure outside the housing.

The non-return valve may be located at a radial distance from the rotor axis which is smaller than the radial distance to the outlet, i.e., such that the non-return valve is closer to the centre.

The non-return valve may be controllable, e.g., electrically, to provide controllable reduction of the pressure difference between the suction pressure at the central portion of the outer surface of the rotary disc and the surrounding pressure outside the housing.

The rotary disc may form a pressure equalizing passage from the inner surface to the outer surface, e.g., in the form of one or more holes through the rotary disc. Such one or more holes may be located at or near the rotor axis, or radially displaced away from the rotor axis, and it may provide a flow of water from the inner surface of the rotary disc to the outer surface of the rotary disc. This flow may particularly be caused by the Bernoulli's principle, and by the suction from the pumping structure at the outlet. This may increase a radially outwards flow of water over the outer surface of the rotary disc.

The cleaning device may comprise a down-flow structure configured to provide a flow through the pressure equalizing passage out of the cavity, i.e., configured to provide a downwards flow from the inner surface to the outer surface of the rotary disc. Such a down-flow structure may e.g., comprise a propeller structure formed on the rotary disc and configured to provide the flow by rotation of the rotary disc about the rotor axis. The downwards flow may e.g., be provided in a central opening of the rotary disc.

Each cleaning unit may be attached movably to the frame whereby they can move relative to the surface to be cleaned independent on the location of the frame relative to the surface to be cleaned.

The device may e.g., comprise an elevation structure configured to move the cleaning unit between a lowered position and a raised position.

The frame may define a working surface to be arranged against the surface to be cleaned. The lowered position is a position where the outer periphery is closer to a working surface of the frame and thereby closer to the surface to be cleaned, and the raised position is a position where the cleaning unit is moved away from the working surface and thus the surface to be cleaned.

The elevation structure may be configured to move the cleaning units by a pressure of water applied to the nozzles of that cleaning unit. The elevation structure may e.g., comprise a hydraulic actuator working against the force of a spring, where the spring urges the cleaning unit towards the raised position, and the hydraulic actuator urges the cleaning unit towards the lowered position, and where the hydraulic actuator is actuated by the water which is applied to the at least one nozzle.

A rigid beam structure may be arranged to connect a plurality of cleaning units to the same elevation structure such that they are all raised and lowered simultaneously. The rigid beam structure may constitute said frame.

The device may comprise a manifold connecting the outlets of a plurality of cleaning units to define a mutual outlet. The manifold may be constituted at least partly by said rigid beam structure.

The power-driven pumping arrangement which provides suction through the outlet may comprise at least one suction pump, e.g., an electrically driven pump. The manifold may be arranged between the outlets of the plurality of cleaning units and the at least one suction pump such that all cleaning units share the same power-driven pumping arrangement.

A waste transport section, e.g., in the form of a flex hose, e.g., a hose of a considerable length may extend from the outlet to a base station. The base station may e.g., be a floating device, a small barge or ship which controls operation of the cleaning device, and the waste transport section may extend from the cleaning device to that base station. In case of large ships, the need for long distance transport e.g., up to 200-400 meters may be considered.

An additional suction pump, e.g., an electrically driven pump, may be arranged at the base station to provide a flow in the transport section.

The ratio between pressure and flow rate of water through the nozzles may be changed by replacing the nozzles with nozzles of different cross-sectional size. This may e.g., include replacement of nozzles with nozzles of different sizes.

The cleaning device may comprise an exchangeable rim element defining the lower edge, and the lower edge of the exchangeable rim element may be elastically deformable by the suction of the device onto the surface to be cleaned during operation. This may minimize or prevent a gap between the lower edge and the surface to be cleaned and thereby further facilitate efficient collection of contaminants. The rim may e.g., be made of a polymer or rubber material.

In a second aspect, the disclosure provides a cleaning unit for a cleaning device according to the first aspect of the disclosure and comprising potentially any of the elements and features mentioned relative to the cleaning unit of the cleaning device of the first aspect of the disclosure.

In a third aspect, the disclosure provides a method of cleaning an underwater surface by use of a cleaning device according to the first aspect. The method comprises applying water at high pressure through the nozzles while rotating the rotary disc about the rotor axis and collecting water through the outlet by use of at least one pump.

BRIEF DESCRIPTION OF DRAWINGS

The invention will now be described by way of example with reference to the following figures in which:

Figs. 1 and 2 illustrate a cleaning device;

Figs. 3 and 4 illustrate a cleaning unit for the cleaning device;

Figs. 5-8 illustrate details of the interior of the housing of the cleaning unit;

Fig. 9 illustrates details of the rotary disc; and

Fig. 10 illustrates suspension of the cleaning unit on a frame of the cleaning device.

DESCRIPTION OF AN EMBODIMENT

Further scope of applicability of the present disclosure will become apparent from the following detailed description and specific examples. However, the detailed description and specific examples, while indicating preferred embodiments, are given by way of illustration only, since various changes and modifications within the scope of the disclosure will become apparent to those skilled in the art from this detailed description. Fig. 1 illustrates a submergible cleaning device 1 which can be used for cleaning underwater surfaces such as hulls of ships or foundations of marine constructions etc. The device comprises a chassis 2 forming a remote operated vehicle (ROV). The ROV includes a frame e.g., in the form of the rigid beam 101 shown in Fig. 10. The frame holds several cleaning units, and various additional components for moving the cleaning units relative to the frame and thus relative to the chassis. The chassis holds various equipment for supporting the cleaning and manoeuvring of the ROV.

The ROV comprises four vertical thrusters 3, and two horizontal thrusters 4. The thrusters are used to allow the ROV to be manoeuvred independent from tugs or cable suspensions and give the ROV abilities to be positioned at specific locations, e.g., under the bottom of a ship. The thrusters are tunnel thrusters and comprise propellers installed in a tunnel.

Each thruster is designed for continuous or intermittent operation and the device may include a computer configured for coordinated control of the Thrusters to provide easy and precise manoeuvring of the ROV.

The ROV comprises four cleaning units 5, other numbers of cleaning units could be considered, e.g., any number from 1 to 10 cleaning units.

The cleaning units are exposed on a working surface 6 of the ROV, c.f. Fig. 2.

The working surface includes a distance structure in the form of wheels 7 arranged to roll on the surface to be cleaned and thereby create a distance between the working surface of the ROV and the surface to be cleaned.

Peripheral equipment such as lamps 8 and cameras 9 are included to facilitate remote monitoring and operation from a base station, e.g., in the form of a barge floating aside of that ship which is being cleaned. The base station could, alternatively, be constituted by that ship itself.

A mutual outlet 10 collecting water and contaminants from all cleaning units is provided at an upper surface 11 facing away from the working surface 6.

Figs. 3 and 4 illustrate perspective views of a cleaning unit. In Fig. 3, the cleaning unit is seen from above, and in Fig. 4, the cleaning unit is seen from the working surface. The cleaning unit comprises a housing 30 defining a lower edge 31 extending about an opening 32 into an inner cavity 33.

An outlet 34 provides an opening through a surface 35 of the housing, which surface 35 is the bottom portion 48 of the housing.

Each cleaning unit comprises a set of wheels 37 allowing precise adjustment of the distance between the surface to be cleaned and the lower edge 31.

A rotary disc 36 defines an outer surface 41 and on opposite inner surface. The inner surface faces towards the surface 35 of the housing.

The rotary disc is arranged for rotation about the axis illustrated by the dotted line 42 and it is located in the opening 32. The motor 38 rotates the rotary disc, and the valve system 39 feeds water into the cleaning unit.

Four nozzles 43 are arranged along a peripheral edge of the rotating disc. The nozzles are connected to a water pressure system, e.g., a conventional pressure washer and receives pressurized water via the valve system 39. The pressure may e.g., be 300-500 bar water pressure. When the rotary disc is rotated about the rotation axis, the nozzles generate a circulating flow of water.

The dotted line 44 indicates a radial distance from the outlet 34 to the rotor axis 42, and the dotted line 45 indicates a radial distance from a periphery of the rotary disc 36 to the rotor axis 42. The radial distance 44 is larger than the radial distance 45.

The outlet is arranged in direct communication with a collection zone 46 formed by a gap extending from the periphery of the rotary disc and radially towards an inner surface of the housing, particularly a rim portion 47 of the housing. The rim portion forms a skirt extending towards the surface to be cleaned.

The gap between the periphery of the rotary disc and the radial inner surface of the housing has a size which increases in a direction towards the collection zone, i.e., at the location of the outlet 34, the radial gap 61 is largest. The increasing size of the gap 61 is illustrated also in Figs 6 and 7. The collection zone terminates in a radial wall 62 forcing the water out through the outlet 34. The outlet 34 extends from the collection zone 46 in the axial direction., i.e. in the direction of the rotor axis. Figs. 6 and 7 illustrate the rotary disc seen from above, i.e. seen from the inner surface of the rotary disc, and the housing is illustrated in a cross section perpendicular to the rotor axis, i.e. without the bottom portion 48.

Particularly, the radial gap 61 increases from the intermediate location 63 towards the start 64 of the collection zone 46. From the start point 65 to the intermediate location 63, the gap has a constant size.

The device may further comprise a pressure release structure with a non-return valve (not shown). The pressure release structure may allow a flow into the cavity for reducing a pressure difference between a suction pressure at a central portion of the outer surface of the rotary disc and a surrounding pressure outside the housing. The outlet may be placed radially further away from the rotor axis than the pressure release structure, and that allows a radially outwards flow of water from the pressure release structure to the outlet.

Fig. 5 illustrates the connection between the motor and the rotary disc including a coq wheel 51, 52. Fig. 5 also illustrates the water flow channel 53 extending through the centre of the rotor shaft 54 for feeding the nozzles.

Figs. 5 and 7 illustrate that the outlet is formed in a bottom portion of the housing, and that the cavity is a unitary cavity facilitating free flow between the housing and the inner surface of the rotary disc, i.e., there are no internal wall structures except what is formed by the rotary disc.

Fig. 8 illustrates that the rim 47 terminates towards the surface to be cleaned with a rubber edge 81. Fig. 8 illustrates that the rotary disc 13 is a closed shell without openings allowing water to pass from the outer surface of the rotary disc to the inner surface of the rotary disc.

Fig. 9 illustrates the rotary disc, and particularly illustrates that the rotary disc forms an offset surface portion 91 displaced in the direction of the rotor axis 42 into the cavity relative to the outer periphery 92. The offset surface portion is a planar portion which increases the distance between the rotary disc and the surface to be cleaned. That may reduce the suction of the device towards the surface to be cleaned. The offset surface portion is perpendicular to the rotor axis.

Fig. 9 illustrates four holes 93 in the rotary disc. These holes form a pressure equalizing passage from the inner surface to the outer surface and further reduces the suction pressure of the device against the surface to be cleaned. The holes are located at a distance from the rotor axis, i.e., radially displaced relative to the rotor axis. In an alternative embodiment, or as an addition to the radially displaced holes, the rotary disc may include a central hole. During use, water will flow downwards through the holes towards the surface to be cleaned and that will reduce the suction. To enhance such a downwards flow, the rotary disc may include a propeller structure such that the rotation of the rotary disc increases the downwards flow.

Fig. 10 illustrates a frame 101 in the form of a rigid beam structure. The frame connects all four cleaning units to an elevation structure with water hydraulic actuators 102 arranged to move the frame 101 with all cleaning units between a lowered position and a raised position. When the water pressure for the nozzles is activated, the pressure of the water acts on the water hydraulic actuators 102, and in response they move the frame and the cleaning units closer to the working surface and thus the surface to be cleaned. When the water pressure is stopped, springs will move the frame 101 and the cleaning units away from the working surface and thereby increase manoeuvrability of the cleaning device.

A wobble hinge may allow each cleaning unit to wobble relative to the rigid beam and thereby facilitates movement across surface irregularities of the surface to be cleaned, e.g., across anodes, valves and other obstacles sometimes found on the bottom of ships.

The frame 101 is made as a rigid beam structure, and it forms a manifold connecting the outlets of all cleaning units and connects them to the mutual outlet 10 illustrated in Fig. 1. In alternative embodiments, the frame could be a lattice structure and the manifold could be a pipe structure attached to the lattice structure.

Claims

1. A submergible cleaning device (1) comprising a frame (2) and at least one cleaning unit

(5) attached to the frame (2), each cleaning unit comprising :

- a housing (6) defining a lower edge (7) extending about an opening (8) into an inner cavity (9),

- an outlet (10) through a surface (11) of the housing,

- a power-driven pumping arrangement to provide a fluid flow from the cavity through the outlet (10),

- a water injection system with at least one nozzle (12),

- a rotary disc (13) defining an outer surface (14) and on opposite inner surface (15) and arranged in the opening (8) with the inner surface (15) facing the cavity (9), the rotary disc (13) being arranged for rotation about a rotor axis (16) and carrying the at least one nozzle (12) to thereby generate a circulating flow of water during rotation, and

- power-driven means (17) arranged to rotate the rotary disc about the rotor axis (16), wherein a radial distance from the outlet (10) to the rotor axis (16) is equal to, or larger than a radial distance from a periphery (17) of the rotary disc (13) to the rotor axis (16).

2. The cleaning device according to claim 1, forming a radially extending gap between an outer periphery of the rotary disc and an inner surface of the housing.

3. The cleaning device according to claim 1, wherein the outlet is arranged in direct communication with a collection zone formed by a section of the gap.

4. The cleaning device according to claim 2 or 3, wherein the gap has a size in the radial direction between the periphery of the rotary disc and the inner surface of the housing which increases in a direction towards the collection zone.

5. The cleaning device according to any of the preceding claims, wherein the housing forms a bottom portion and a rim portion extending between the bottom portion and the lower edge.

6. The cleaning device according to claim 5, wherein the outlet is formed in the bottom portion.

7. The cleaning device according to claim 6, wherein the outlet extends from the bottom portion in the direction of the rotor axis (16).

8. The cleaning device according to any of the preceding claims, wherein the cavity is a unitary cavity facilitating free flow between the housing and the inner surface of the rotary disc.

9. The cleaning device according to any of the preceding claims, wherein the rotary disc forms an offset surface portion displaced in the direction of the rotor axis into the cavity relative to the outer periphery.

10. The cleaning device according to claim 9, wherein the offset surface portion is a planar portion.

11. The cleaning device according to claim 9 or 10, wherein the offset surface portion is perpendicular to the rotor axis.

12. The cleaning device according to any of claims 9-11, wherein the nozzles are arranged in the outer periphery and thereby arranged to eject water from an ejection point displaced relative to the offset surface portion.

13. The cleaning device according to any of the preceding claims, wherein each nozzle is arranged for ejecting water from the outer surface of the rotary disc in the direction of the rotor axis.

14. The cleaning device according to any of the preceding claims, comprising a pressure release structure with a non-return valve allowing a flow into the cavity for reducing a pressure difference between a suction pressure at a central portion of the outer surface of the rotary disc and a surrounding pressure outside the housing.

15. The cleaning device according to claim 14, wherein a radial distance from the outlet to the rotor axis is larger than a radial distance from the non-return valve to the rotor axis.

16. The cleaning device according to claim 14 or 15, wherein the non-return valve is controllable to provide controllable reduction of the pressure difference between the suction pressure at the central portion of the outer surface of the rotary disc and the surrounding pressure outside the housing. 15

17. The cleaning device according to any of the preceding claims, wherein the rotary disc (13) forms a completely closed disc preventing flow of water from the outer surface to the inner surface.

18. The cleaning device according to any of the preceding claims, wherein the rotary disc forms a pressure equalizing passage from the inner surface to the outer surface.

19. The cleaning device according to claim 18, wherein the pressure equalizing passage is radially offset from rotor axis.

20. The cleaning device according to claim 19, comprising a down-flow structure configured to provide a flow through the pressure equalizing passage out of the cavity.

21. The cleaning device according to claim 20, wherein the down-flow structure comprises a propeller structure formed on the rotary disc and configured to provide the flow by rotation of the rotary disc about the rotor axis.

22. The cleaning device according to any of the preceding claims, wherein each cleaning unit is attached movably to the frame.

23. The cleaning device according to claim 22, comprising an elevation structure configured to move the cleaning unit between a lowered position and a raised position, wherein the outer periphery is closer to a working surface of the frame in the lowered position than in the raised position, the elevation structure being arranged to move each cleaning unit by a pressure of water applied to the nozzles of that cleaning unit.

24. The cleaning device according to claim 23, wherein a plurality of cleaning units are connected to the same elevation structure.

25. The cleaning device according to any of the preceding claims, comprising a wobble hinge allowing each cleaning unit to wobble relative to the frame.

26. The cleaning device according to any of the preceding claims, comprising a manifold connecting the outlets of a plurality of cleaning units to define a mutual outlet.

27. The cleaning device according to claim 16, wherein the power-driven pumping arrangement comprises at least one suction pump, and wherein the manifold is arranged between the outlets of the plurality of cleaning units and the at least one suction pump. 16

28. The cleaning device according to any of the preceding claims, comprising a waste transport section extending from the outlet to a base station, and an additional suction pump arranged at the base station to provide a flow in the transport section.

29. The cleaning device according to any of the preceding claims, wherein the water injection system is configured for change between different pressures and different flow rates of water through the at least one nozzle.

30. The cleaning device according to any of the preceding claims, comprising an exchangeable rim element defining the lower edge.

31. The cleaning device according to claim 30, wherein the lower edge of the exchangeable rim element is elastically deformable by the suction of the device onto the surface to be cleaned during operation.

32. A cleaning unit for a cleaning device according to any of the preceding claims.

33. A method of cleaning an underwater surface by use of a cleaning device according to any of claims 1-31, the method comprising applying water at high pressure through the nozzles while rotating the rotary disc about the rotor axis and collecting water through the outlet by use of at least one pump.

34. The method according to claim 33, wherein the water is caused to rotate within the housing to an extend where the rotating water provides a lower pressure along an outer rim of the rotary disc than at the centre of the rotary disc.

35. The method according to claim 33 or 34, wherein the water is prevented from flowing from the outer surface to the inner surface.

36. The method according to claim 33-34, wherein the device is according to claim 18, and the water is allowed to flow from the inner surface to the outer surface via the pressure equalizing passage.

37. The method according to claim 33-36, wherein the water is allowed to flow radially outwards over the outer surface of the rotary disc.