WO1999025609A1

WO1999025609A1 - Method and apparatus for use of gases in liquids

Info

Publication number: WO1999025609A1
Application number: PCT/GB1998/003425
Authority: WO
Inventors: Roger Arthur Robert Hall
Original assignee: Individual
Current assignee: Individual
Priority date: 1997-11-18
Filing date: 1998-11-18
Publication date: 1999-05-27
Anticipated expiration: 2000-05-18
Also published as: GB9724240D0; AU1440799A

Abstract

Methods and apparatus are disclosed based upon dispersing bubbles in a body of liquid so as to modify the mechanical properties of the liquid. In one embodiment, a bubble concentrator (10) is interposed between a gas supply conduit (11) and the body of liquid to produce bubbles (12) of a selected size and at a suitable location to modify natural waves (20) before the waves impact a marine structure (18).

Description

1 Method and Apparatus for use of Gases in Liquids 2

3 This invention relates to uses generating or

4 introducing gases into a liquid. More particularly,

5 this invention relates to a device for generating or

6 introducing (directly or by concentration and release)

7 bubbles of gas or gases into any liquid or mix of

8 liquids for reducing the density of the liquid or mix

9 of liquids, which has the consequence of reducing one

10 or more of the following: buoyancy forces; forces due

11 to movement of liquid; and forces due to waves or wave

12 trains. 13

14 In this field it is already known that the buoyancy

15 force or the dynamic force on an object on, in, or

16 within a liquid can be altered by increasing or

17 decreasing the density of the liquid on a permanent

18 basis. It is also known that the force of, or the

19 effect of, surface waves upon such an object can be

20 reduced by placing a secondary object or objects

21 upstream in order to absorb or dissipate the forces 22_. acting upon the object.

23

24 The disadvantages in such known methods are that

25 altering the density of a liquid on a permanent basis 1 is cumbersome, expensive and wasteful and has the

2 potential for causing pollution and destruction of life

3 forms, substrate layers and food chains by the addition

4 of higher or lower density liquids or soluble solids.

5 There is also the potential for long term, long

6 distance effects upon the environment as a result of

7 such additives, often unconnected with the area where

8 the liquid density has been altered. 9

10 In the case of placing secondary objects, such objects

11 are invasive and potentially damaging to the

12 environment. Furthermore, their fabrication,

13 transportation and construction relies heavily upon

14 natural or man made materials, thereby involving the

15 destruction of terrestrial sites and the excessive use

16 of non-renewable energy. Also, the installation of

17 such secondary objects is not an option in remote areas

18 and locations which experience hostile sea and ocean

19 conditions . This may result in the need to suspend or

20 abandon operations such as mineral and oil exploration

21 and production for long, costly periods. 22

23 A suitable solution to coastal erosion has not been

24 discovered. This has particular consequences in areas

25 having low population densities, where it is not cost-

26 efficient to prevent or ameliorate erosion, resulting

27 in much valuable land and property being lost to the

28 sea. 29

30 Wave action also causes discomfort to crew and

31 passengers aboard marine vessels as fuel consumption 32_. and power is. increased to control pitch and roll

33 through high, steep-sided waves. 34

35 Scouring around bridge piers, supports and other

36 structural members in marine or brackish waters or in 1 rivers due to turbulent flow around and downstream of

2 such objects causes problems of instability or

3 over-engineering to counteract the resultant upstream

4 force. Dissipaters and/or reducers placed to combat

5 such effects also suffer scouring, saltation and

6 redistribution of mixed particle sizes of bed load.

7 This in itself can lead to upstream pressures upon the

8 structures which such dissipaters and/or reducers are

9 supposed to protect. 10

11 Oil spillage dispersal often requires spraying an oil

12 slick surface with a suitable detergent.

13 Emulsification occurs during contact between the

14 dispersing detergent and the oil. This is limited to

15 the area of surface in contact with the atmosphere and

16 consequently, emulsification is delayed. Costly

17 mechanical devices exist which have the purpose of

18 increasing the oil surface area, but such devices can

19 only be applied to spills that have been mechanically

20 contained by boom-type floating structures. 21

22 Icebergs have a high mass, in conjunction with a low

23 velocity. The resultant momentum of even relatively

24 low volume icebergs is huge. Consequently little or no

25 attempt is made to divert icebergs from a collision

26 course with other floating marine or seabed fixed

27 structures. Instead, preference is given to moving the

28 structures out of collision course. The downtime for

29 moving such structures has expensive consequences due

30 to the time which is wasted during such operations. It

31 is also well known that floating ice can extend

32_. downward to drag along the seabed, and it is possible

33 that such an iceberg base may collide with and cause

34 damage to seabed production fixtures or pipelines with

35 the inevitable loss of valuable oil or gas and

36 resultant pollution. 1 Mineral separation processes in which minerals are

2 separated from their gangue using a liquid of constant

3 density require combinations of costly mechanical

4 separation and liquid separation techniques which

5 require that the minerals be transported from one

6 apparatus location to another with the associated cost

7 and time losses . 8

9 According to a first aspect, the present invention

10 provides a method of altering the mechanical properties

11 of a body of liquid by passing bubbles of one or more

12 gaseous substances therethrough. The method may

13 comprise the steps of: 14

15 i) locating one or more gaseous medium conduit

16 means in a selected location within or

17 adjacent to the body of liquid; and 18

19 ii) supplying compressed gas via the conduit

20 means so as to disperse bubbles from one or

21 more locations on the conduit means. 22

23 The compressed gas emitted from the one or more

24 locations may be compressed air. Alternatively the

25 compressed gas may be a mixture of one or more gases

26 which may include air. 27

28 The step of locating the conduit means may include

29 locating the conduit means on the seabed. The conduit

30 means may be temporarily or permanently retained on the

31 seabed and may include a number of portions shaped for 32. providing bubbles from one or more locations, thereby

33 reducing the forces upon an object positioned above the

34 conduit means due to varying weather and tide

35 conditions. 1 The bubbles may be provided by means of a bubble

2 concentrator interposed between the conduit and the

3 main body of liquid. 4

5 The invention may be used to provide a method of

6 altering the buoyancy forces which act upon an object

7 or series of objects in the body of liquid. 8

9 Alternatively, it may provide a method of decreasing

10 the forces which act upon the object or series of

11 objects due to the motion of the body of liquid. 12

13 As a further alternative, it may provide a method for

14 decreasing the forces upon the object or series of

15 objects in the body of liquid due to the impact of one

16 or more surface waves upon the object or series of

17 objects. 18

19 In another alternative, it may provide a method of

20 reducing coastal erosion. 21

22 In yet another alternative, it may provide a method of

23 decreasing the forces which act upon a mobile marine

24 object due to the impact of one or more surface waves. 25

26 In still another alternative, it may provide a method

27 of reducing structural damage to permanent or temporary

28 structures located within a body of liquid. 29

30 In still another alternative, it may provide a method

31 of enhancing oil slick dispersion. 32.

33 In still another alternative, it may provide a method

34 of altering the path of an iceberg. 35

36 In still another alternative, it may provide a method 1 of separating minerals from their gangue .

2

3

4 According to a second aspect, the present invention

5 provides a gaseous medium conduit means consisting of .

6 one or more hollow conduits having holes therein for

7 the dispersal of bubbles of one or more gaseous

8 substances therethrough. 9

10 The conduits may be arranged in a structure for the

11 purpose of supplying bubbles into a body of liquid to

12 alter the mechanical properties of the body of liquid. 13

14 The shape and dimensions of the structure may depend

15 upon the shape of the one or more objects located on or

16 in the body of fluid. It may also depend upon the

17 direction of the prevailing current flow or tidal

18 action of the body of liquid. Alternatively, the shape

19 and dimensions may depend upon the requirement to

20 account for current flow or tidal action upon the

21 object from any direction. 22

23 The structure may be permanently or temporarily

24 retained upon the bed of the body of liquid. 25

26 The structure may be a web consisting of an array of

27 conduits arranged in substantially concentric closed

28 paths and radial supply conduits. The supply conduits

29 may enable a gaseous medium to be transported to one or

30 more sectors of the web structure, the sectors being

31 defined by the concentric conduit and supply conduits. 32.

33 One or more objects on or in the body of the liquid may

34 be located above the structure such that the buoyancy

35 forces of the object and/or the forces acting upon it

36 due to the motion of the body of liquid and/or due to 1 the impact of one or more surface waves upon the object

2 may be decreased on passing bubbles of one or more

3 gaseous substances through the conduit means . 4

5 The mechanical properties which are altered by passing

6 bubbles into the body of liquid may include the density

7 of the body of liquid and/or the buoyancy force and/or

8 the wave forces acting upon one or more objects on or

9 in the body of liquid. 10

11 The conduits may be arranged to reduce the coastal

12 erosion due to the motion of a body of liquid and/or

13 due to the impact of one or more waves. 14

15 Alternatively, the conduits may be arranged in a

16 structure capable of co-operating with a mobile marine

17 vessel and may incorporate a framework of conduit pipes

18 spaced from the hull, submerged or semi-submerged

19 structure of the mobile marine vessel. The structure

20 may project forward from the bow of the vessel to

21 decrease the forces due to motion of the body of fluid

22 in which the vessel is travelling and/or the forces due

23 to impact of waves upon the vessel. The structure may

24 also extend athwart and astern of the vessel; this

25 decreases the effect of waves from any direction. 26

27 The invention further provides a bubble concentrator

28 for use in a body of liquid, the concentrator

29 comprising a chamber formed by side walls and a lid,

30 the lid comprising one or more sections hinged to the

31 side walls, and means for producing bubbles within the 32. chamber, the arrangement being such that said bubbles

33 are concentrated under the lid until a sufficient

34 buoyancy is established to overcome a closing force of

35 the lid. 36 The lid may suitably be in the form of two lid sections which are equally weighted to provide a desired closing force.

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings of which:

Fig 1 is a schematic of an embodiment of the invention;

Fig 2a is a plan and Fig. 2b a side view of another embodiment of the invention;

Fig 3 is a plan view of a further embodiment of the invention;

Figs 4a and 4b are a side view and a plan view, respectively, of another embodiment of the invention;

Fig 5a is a schematic side view of a further embodiment; and Fig 5b is a cross-section on B-B of Fig 5a.

In all embodiments of the invention, a supply of compressed gas is conducted to an array of bubble generators. The gas is typically air, but any suitable gas or mixture of gases can be utilised. The gas is delivered into a liquid which may be salt water, fresh water or brackish water, with the purpose of temporarily and locally decreasing the density of the liquid. This has the effect of reducing the forces acting upon an object located within and/or on the liquid. The forces are static forces, such as the buoyancy force acting upon an object in the liquid, or transient forces due to the motion of the body of liquid, or surface waves. The force due to the 1 expanded, bursting bubble at the surface of the liquid

2 within any wave or waves can be utilised to reduce the

3 momentum of any such wave or waves. 4

5 Referring to Fig 1, a device 10 generates or

6 concentrates bubbles 12 at a suitable depth between the

7 atmosphere/water surface 14 and the water/bottom 16

8 interfaces, at a rate of production and size so that

9 the bubbles rise, expand, and on bursting through any

10 wave or wave/atmosphere interface, cause the waves to

11 collapse or partially collapse, thereby altering the

12 momentum of the wave or waves. This has a resultant

13 effect upon the force on any structure 18 on or in the

14 body of liquid, and can be used to reduce the force of

15 waves upon permanent or semi-submersible structures

16 such as rigs used in oilfield production and/or

17 exploration for sub-seabed oil reservoirs. Typically,

18 air is compressed on site upon such structures, before

19 being conducted to the bubble generators that are

20 situated at a suitable distance away from the

21 structure, and either fully surround or by a movable

22 segment afford protection across the widest dimension 23 of such structures. The original waves and the

24 collapsed waves are shown schematically at 20 and 22,

25 respectively. 26

27 By generating a suitable size of bubble, an effect is

28 achieved whereby the bubble expands as it rises and, on

29 approaching the surface, forces water out of the

30 surface into the air space of the bubble to form a

31 relatively flat water surface. 32.

33 The device 10 permits bubbles 12 of a desired size to

34 be generated. The device 10 comprises a perforated air

35 conduit 11 which forms streams of small bubbles 13

36 within a housing 15 the top of which is closed by 1 hinged doors 17 provided with counterweights 19. Thus,

2 the small bubbles 13 are concentrated beneath the doors

3 17 until they form a sufficient quantity of air to give

4 buoyancy to open the doors 17, releasing the air as one

5 or two large bubbles 12 (as indicated in broken lines

6 in Fig. 1), following which the doors 17 are closed by

7 the counterweights 19. 8

9 Referring to Fig 2, a device 24 is placed upstream in a

10 single direction flow stream 26 or uptide and/or

11 downtide of any structure such as a bridge pier 28 or

12 harbour pile to entrain bubbles 30 which reduce the

13 overall density and thus the scouring force and other

14 dynamic forces (for example, pressure on a pile) due to

15 the turbulent and/or laminar flow of liquid upstream,

16 beside and/or downstream of the interface of the flow,

17 and the solid or semi-solid bed and the structure. 18

19 The device consists of a series of hollow conduits 32

20 arranged in a structure which is designed to co-operate

21 with the bridge pier 28, harbour pile or the like and

22 provides a curtain of bubbles to reduce the scouring

23 forces on the structure due to the current and tidal

24 flow of the body of liquid. The conduits 32 are

25 interconnected, having holes 34 therein for the

26 dispersion of the bubbles 30 of a gas, which in this

27 embodiment is compressed air. The compressed air is

28 supplied from the surface via hollow supply conduits

29 36, from which it is dispersed around the conduits 32

30 before subsequently passing into the body of fluid.

31 The device is located on the river bed 38 and surrounds 32. the bridge pier 28 or the like. Some of the conduits

33 32 are inclined to provide drainage of liquid from

34 within the conduits 32 during use, the conduits 32

35 having drainage holes 40 for this purposes. 36 1 The advantages of the invention and/or the ways in

2 which the disadvantages of previously known

3 arrangements are overcome, include the following. Most

4 if not all existing wave-calming or destroying devices

5 operate only in relatively shallow water. Most involve

6 the interaction with solid or semi-solid, frequently

7 bulky structures often of a permanent nature, such as

8 breakwaters or rock and concrete or alternatively

9 submerged or semi-submerged, captive buoyant or

10 semi-buoyant devices placed within the amplitude of

11 average to above average amplitude waves. 12

13 As an alternative to the arrangement shown, the bubble

14 generating device could be positioned separately from

15 the structure, and could be secured on the bed by

16 weights or anchors, or be a floating device tethered to

17 the bed. 18

19 No such devices exist in deep water for the protection

20 of oil drilling or production rigs and the like, or

21 converted boats or ships used for floating oil

22 production. 23

24 The present invention affords protection to permit

25 offshore oil operations that until now have had to

26 cease due to excessive forces and wave pressure. The

27 present invention also enables a more comfortable and

28 stable work environment to be created. 29

30 Referring to Fig 3 the device is located on the seabed,

31 and is used to reduce the forces acting upon a marine 32. installation 42 such as a moored semi-submersible

33 oilfield production rig or the like in a body of

34 liquid. The device consists of a series of conduits

35 which are constructed in rings 44, having connecting

36 radial supply conduits 46. In this embodiment, the 1 rings 44 are hexagonal, but other polygonal or

2 triangular forms may be used. The conduits forming the

3 hexagonal rings 44 are supplied with a gas, which in

4 this embodiment is compressed air, via the supply

5 conduits 46. The device forms a series of sectors, and

6 it is possible to isolate one or more of such sectors

7 such that compressed air is supplied to one or more of

8 the sectors, individually or in combination. In this

9 way, the forces on the marine installation 42 due to

10 current and tidal motion of the body of liquid, or due

11 to wave forces, can be controlled by supplying

12 compressed air to one or more of the sectors, depending

13 upon the flow direction of the liquid, or the weather

14 conditions causing the surface waves. 15

16 The compressed air required for operation of the device

17 is produced on site at the marine installation 42 or on

18 a secondary installation. The device can incorporate

19 bubble concentrators such as that shown in Fig 1 if

20 desired. 21

22 The device can also be employed in shallow water where

23 its effects may be used to calm or ameliorate the force

24 of wave erosion to cliffs and coastlines, in a single

25 or multiple linear array generally parallel to the

26 coastline allowing preservation or a possible

27 reclamation of valuable agricultural land adjacent to

28 the sea. 29

30 In a modification of the foregoing embodiment, a web-

31 like structure similar to that of Fig 2 but smaller in 32. extent is not secured to the sea bed but is provided

33 with propulsion and buoyancy means (in a manner similar

34 to a remotely operated vehicle) to enable it to be

35 maneouvered to a desired location, which will typically

36 be upstream of a structure to be protected. Shapes and forms other than web-like structures may be used in a similar way.

Thus far no device has been applied ahead of waterborne vessels, ships, boats etc. Even moderate winds with long fetches (distances over which waves can build unhindered) can and do build the most forceful of wave types known as steep waves with high amplitude that can cause all types of vessels to experience high stresses and strains to their structures causing reduced travel velocities, and sometimes even causing vessels caught in such conditions to make slow, cautionary way or to heave-to, or suffer stresses great enough to induce welds and/or joins to open, finally resulting in failure and sinking. Under such conditions there is serious threat to the safety of the vessel or its deck cargo, or of serious damage to these.

An arrangement of the device may project forward and/or variably athwart of the bows and sides of a mobile marine vessel, being employed up weather or up current.

Injecting bubbles afterward, in the directional sense of from the bows towards the stern, contributes to the forward motion by the jet (action/reaction) effect, and when set to an appropriate bubble size, delivery rate and depth, destroys partially or wholly the steepness and force of any impinging waves.

Referring to Fig 4, the device consists of an articulated framework which is moveable via joints 50, and which is fixed to the hull of a mobile marine vessel such as the ship 52 of Fig 4. The framework incorporates downwardly and forwardly angled conduits 56 with horizontal conduits 58 attached thereto, some of these being provided with holes therein (as 1 indicated by dashed lines in Fig. 4) for the dispersion

2 of bubbles of a gas, which is intended to be compressed

3 air. The compressed air is supplied to the conduits

4 via one or more suitable supply conduits (not shown) .

5 In this way, bubbles are directed down out of selected

6 angled conduits 56 and horizontal conduits 58,

7 subsequently rising to the surface of the body of

8 liquid to reduce the forces due to current and/or tidal

9 flow of the body of liquid, and the forces due to 10 impact of surface waves upon the marine vessel. 11

12 The device also has a second support means in the form

13 of one or more hydrofoils 60, to provide an upthrust

14 force on the device when the liquid flows over the

15 hydrofoil 60. The hydrofoils may be fixed, or may be

16 variable geometry (hinged) . 17

18 A further advantage of the device is its application to

19 the problem of scouring around solid or semi-solid

20 structures wholly or partly immersed in, on or within

21 flowing liquids downstream of permanent or temporary

22 structures such as river bridge or harbour supports.

23 In such circumstances, smooth streamline flow often

24 becomes turbulent at certain depths at and near the

25 bed/liquid interface, causing scouring of the bed load,

26 with resultant effects upon such structures. 27

28 Thus far nothing has the ability to significantly

29 ameliorate the drag force of the turbulent flow from

30 scouring - where solid or semi-solid upstream

31 structures have been tried they have themselves

32. suffered scouring and often removal from their supposed

33 position of efficacy. Indeed even streamlining of say

34 bridge piers/supports in fast flowing rivers fails to

35 prevent scouring. 36 1 By using the device of Fig 2 to partially and

2 temporarily reduce the liquid density by introducing

3 bubbles at a suitable position upstream of the

4 turbulent flow regimes reduces the overall liquid mass

5 and thus reduces the scouring forces . 6

7 Current attempts to disperse accidental oil spills upon

8 water take various forms, with a favoured one being

9 detergent spraying and/or containment within a

10 temporary system of oil booms. Dispersal by spraying

11 such superficially homogeneous masses is delayed

12 somewhat since the area of detergent in contact with

13 the oil is limited to the semi-planar upper surface of

14 the oil. 15

16 Introducing bubbles into the liquid upon which the oil

17 floats causes break-up of the slick into variably sized

18 masses that present a larger surface area to the

19 detergent, thus increasing the rate of emulsification

20 and eventual dispersal. 21

22 Subsea oil exploration and production must enter ever

23 more hostile areas of the earth as the more accessible

24 source regions become depleted. One such area may be

25 the northern North Sea and the adjacent Atlantic

26 regions west of the Shetland Isles, generally in a

27 north to north-west direction as a continuous band

28 across and through the arctic and polar regions to the

29 north west of Canada and Russia. 30

31 The device not only has application for wave-calming in

32. these regions of severe and hostile weather, but also

33 has uses in its ability to temporarily reduce liquid

34 density to deal with the very real danger of iceberg

35 collision. 36 The only way of avoiding inevitable, collision between exploration or production rigs has been to design rigs that can within a matter of twelve hours temporarily abandon their position in a collision path with icebergs having a huge momentum.

Steering all but the relatively tiny icebergs out of collision vectors is, where viable, done by powerful vessels, pushing or pulling the icebergs aside through resistant seawater.

The device when used to reduce the seawater density in a suitable position relative to the iceberg will reduce resistance to vector it out of collision course.

In this way the costs due to lost time and the damage risk is significantly reduced as operations can continue uninterrupted.

The present invention may also be used in the field of the separation of mineral ores. It is known, in general, to separate mineral from gangue by flotation methods using liquids or using froth. The present invention contemplates the use of controlled bubble formation to provide a controllably varied density in a liquid and thus to facilitate separation by density.

Such a process is illustrated in outline in Fig 5. A ground mineral ore in a stream of water is introduced to a series of V-shaped troughs 70 each of which has an outlet slot 72 along its bottom and compressed air conduits 74 for forming streams of bubbles in the mixture flowing through the troughs 70. The air pressure to each individual trough 70 and the size of the air outlets from the conduits 74 of each individual trough 70 can be chosen to give a desired alteration in buoyancy, such that materials of selected density tend to be discharged from the slot 72 of that trough 70.

A similar effect could be achieved in a vessel which combines vortex-inducing vanes, bubble formation , and multiple outlet ports.

In addition to, or instead of, the use of bubbles, the ore separation may make use of water currents of variable velocities to wash forward lower density materials, allowing higher density material (ore) to drop into a collecting vessel.

The foregoing description refers to conduits placed on a surface such as the seabed. Conduits could equally be buried in the seabed or the like, so long as means of communication are provided to conduct gas from the conduit into the body of liquid to form bubbles.

Other modifications and improvements may be made to the foregoing embodiments within the scope of the present invention.

Claims

1. A method of altering the mechanical properties of a body of liquid by passing bubbles of one or more gaseous substances therethrough.

2. The method of claim 1, comprising the steps of:

i) locating one or more gaseous medium conduit means in a selected location within or adjacent to the body of liquid; and

ii) supplying compressed gas via the conduit means so as to disperse bubbles from one or more locations on the conduit means.

3. The method of claim 2, in which the compressed gas emitted from the one or more locations is compressed air.

4. The method of claim 2, in which the compressed gas emitted from the one or more locations is a mixture of one or more gases which may include air.

5. The method of any of claims 2 to 4 , in which the step of locating the conduit means includes locating the conduit means on the seabed.

6. The method of claim 5, in which the conduit means includes a number of portions shaped for providing bubbles from one or more locations, thereby reducing the forces upon an object positioned above the conduit means due to varying weather and tide conditions .

7. The method of any of claims 2 to 6 , in which the bubbles are provided by means of a .bubble concentrator interposed between the conduit and the main body of liquid.

8. The method of any preceding claim, used for one of the following: reducing coastal erosion; decreasing the forces which act upon a mobile marine object due to the impact of one or more surface waves; reducing structural damage to permanent or temporary structures located within a body of liquid; enhancing oil slick dispersion; altering the path of an iceberg; or separating minerals from their gangue.

9. A gaseous medium conduit means consisting of one or more hollow conduits having holes therein for the dispersal of bubbles of one or more gaseous substances therethrough, the conduits being arranged in a structure for supplying bubbles into a body of liquid to alter the mechanical properties of the body of liquid.

10. The structure of claim 9 adapted to be permanently or temporarily retained upon the bed of the body of liquid.

11. The structure of claim 9 or claim 10, in the form of a web consisting of an array of conduits arranged in substantially concentric closed paths and radial supply conduits.

12. The structure of any of claims 9 to 11, in which the conduits are arranged to reduce the coastal erosion due to the motion of a body of liquid and/or due to the impact of one or more waves.

13. The structure of any of claims 9 to 11, in which the conduits are arranged for co-operation with a mobile marine vessel.

14. The structure of claim 13, in which the conduits are arranged in a framework spaced from the hull, submerged or semi-submerged structure of the mobile marine vessel.

15. The structure of claim 14, in which said framework projects forward from the bow of the vessel to decrease the forces due to motion of the body of fluid in which the vessel is travelling and/or the forces due to impact of waves upon the vessel.

16. The structure of claim 15, in which said framework also extends athwart and astern of the vessel.

17. A bubble concentrator for use in a body of liquid, the concentrator comprising a chamber formed by side walls and a lid, the lid comprising one or more sections hinged to the side walls, and means for producing bubbles within the chamber, the arrangement being such that said bubbles are concentrated under the lid until a sufficient buoyancy is established to overcome a closing force of the lid.

18. A bubble concentrator according to claim 17, in which the lid is in the form of two lid sections which are equally weighted to provide a desired closing force.