WO1987002962A1

WO1987002962A1 - Diving apparatus and methods of operating same

Info

Publication number: WO1987002962A1
Application number: PCT/GB1986/000685
Authority: WO
Inventors: Malcolm Johnson; David William Kirkley
Original assignee: Ocean Technical Services Limited
Priority date: 1985-11-07
Filing date: 1986-11-06
Publication date: 1987-05-21
Also published as: GB8626558D0; GB2217367A; EP0245343A1; GB2182967B; GB2217367B; GB8910558D0; GB8527415D0; GB2182967A

Abstract

A method and apparatus for carrying out air breathing diving operations using a wet diving bell (1) capable of housing a diver and having a closable access aperture (2) in which method the diver is deployed to a depth with the diving bell (1) with the access aperture (2) open and the bell flooding to level (M), the diver thereafter being recovered in a normal mode, by raising the bell and diver with the access door (2) open, and subject to decompression stops, to a recovery depth, thereafter clearing the chamber of water (W), closing the access door (2) and raising the bell and diver to the surface, connecting the bell aperture (4) to a decompression chamber and transferring the diver to said chamber all under recovery depth pressure.

Description

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TITLE

DIVING APPARATUS AND METHODS OF OPERATING SAME

This invention relates to diving apparatus and to a method of carrying out underwater diving operations usin such apparatus.

Commercial air diving in the 0 to 50m depth range is mostly conducted using surface-orientated techniques, in which a diver returning from a work station at depth will be raised with one or two brief in-water stops, the last of which will usually be at approximately 10m^' depth. He is then li ted rapidly to the surface .before entering a recompression chamber which is pressurised to approximately 14m of water. In the following decompression modified US Navy Diving Tables are used with the diver breathing air or pure oxygen. The technique is based on the observations that, provided the diver is recompressed quickly, for example within 3 minutes or so, he does not start to suffer from decompression sickness or "the bends". The safety of these techniques as far as the long-term health of the diver is concerned is questionable. It is desirable to provide an alternative to the current surface-orientated techniques both as regards increased safety and from the point of view of more efficient working.

Wet bells have been in use for a number of years with the inherent advantages of being able to transport and protect divers through the surface interface and providing a refuge for divers at the worksite.

There is increasing concern over the incidence of bends at depths up to 50 metres when using the OS Navy or similar surface air or surface oxygen decompression schedules.

In most diving operations the US Navy surface decompression schedules are modified based on trial and error. The factor added can vary greatly depending on experience and medical advice available to the company safety officer.and operations personnel. The procedure forms a very imprecise art in which the requirement of bringing the diver to atmospheric pressure and then recompressing to continue decompression ves concern.

With wet bell diving the usual method is. to make in-water decompression stops as prescribed by diving tables. The bell is then lifted out of the water to the diving station and the divers quickly enter the decompression chamber. They are recompressed: to a pressure greater than that existing during the last in- water stop. Decompression then continues according to - 3 -

the schedule, prescribed in an appropriate diving table. In this method the diver is brought to atmospheric pressure and then recompressed and no improvement in safety is achieved except that physical protection is 5. given to the diver by the wet-be11 structure when going to the worksite below water and returning.

Deep diving bells are used at greater depths,, these being in the nature of capsules which can be sealed under pressure at the working depth. They are 0 "then brought to the surface without in-water decompression stops and decompression takes place slowly either in the dry diving bell or in a decompression chamber into which a transfer is made at the pressure existing in the bell. No decompression to surface 5 pressure followed by recompression in a chamber is necessary.

It is an object of this invention to provide an improved wet diving bell and a method of diving up to 50m water depth using air. 20 According to one aspect of this invention there is provided a wet diving bell comprising a pressure ,. resisting chamber to accommodate one or more divers, a closable aperture to allow a diver to enter the chamber, means to allow the chamber to flood, at least in part, 25 on immersion in water and means to substantially clear the chamber,of water whilst submerged. Preferably the wet diving bell comprises a water pressure resisting chamber to accommodate one or more divers, a lower access aperture preferably in the base of the chamber to ^~ allow a diver to enter the chamber, a door sealingly engaging the aperture to close same and a further upper .closable aperture preferably in the side of the. chamber to allow flooding of the chamber.

The chamber may be spherical, cylindrical, hemispherical, ellipsoidal or other closed shape or combination of shapes with the access aperture at a normally-lower surface which aperture is sufficient size to permit entry and exit of a diver but which may be closed so as to seal the internal chamber of the diving ^" bell. Means may be provided within the diving bell to accommodate the diver. Preferably the construction of the bell will include a platform suspended beneath the diving bell on which a diver may stand and which can also be used as a work station. - This invention also provides a method of performing air breathing diving operations using a wet diving bell capable of housing a diver and having a closable access aperture, in which method the diver is deployed to depth with the diving bell preferably with the access aperture open and the bell flooding, the diver therereafter being recovered, in a normal mode, by raising the bell and diver with the access door open, and subject to decompression stops, to a recovery depth, thereafter clearing the chamber of water, closing the access door, and raising the bell and diver to the ^•surface, and preferably connecting the bell to a decompression chamber and transferring the diver to said chamber all under recovery depth pressure or alternatively decompressing the diver within the bell. In this method a wet diving bell can accommodate a diver in the chamber and the chamber flooded with water or the chamber emptied of water by means of air pressure thereby creating a breathable atmosphere within the chamber. The bell may be raised with a closable aperture open and decompression stops made at depths as prescribed by diving tables. Before the diving bell reaches the surface the divers enter and at the last prescribed in-water decompression stop close the aperture. The diving bell is then raised to the surface and decompression continues in the diving bell at the surface or alternatively the divers transfer under pressure to a decompression chamber at the same pressure as the interior of the diving bell and decompression is carried out therein. Thus the divers are not subject to decompression to surface pressure followed by recompression and further decompression.

Because divers are recovered at pressure and transferred, under pressure to a decompression chamber, it has been found, and according to another aspect of 5" this invention, that a modified decompression can advantageously be used wherein decompression is effected gradually or with smaller and more frequent steps than ^'hitherto and. customary diving tables need not be followed. The method may also embody decompression

10 using oxygen in whole or in part.

This invention therefore provides what may be termed a "third" method of carrying out a diving operation which lies between the traditional wet bell methods and the deep water pressurised capsule methods

15 using special breathing mixtures.

The diving bell will, according to yet a further aspect of this invention, be associated with lifting and winching gear whereby the bell may be brought to the ^"surface diving station and positioned so that an

2.0 aperture is then directly connectable with a decompression chamber. Preferably the structure of the bell will incorporate an exit aperture at a side thereof for the purpose of transferring to the decompression chamber. 5 Apart from increased safety considerations the use of the bell according to this invention for air diving has several benefits. Two divers may work outside the wet bell, each acting as standby diver for the other with each being in visual contact. Thus 100% diver utilisation is achievable. The umbilical to the surface is connected to the wet bell, not the diver, therefore he is not subjected to the same current drag forces as a surface orientated diver. The diver's effective working bottom-time can be increased, particularly in situations where tidal currents exist. The wet bell provides a useful store for working tools and inspection equipment, thereby largely eliminating the time spent in running tools. Possibilities exist to develop the wet bell into a versatile work station. Clear communications to the surface are available if required to discuss detailed operations, ensuring that there is complete understanding of the situation. A safe sanctuary is nearby in the event of a mishap or the diver feeling unwell. In this invention the e^ bell has a much larger dry cavity than is normal. Whereas in a normal wet bell the dry space is only sufficiently large for a diver's head, the modified wet bell will have a dry space large enough to accommodate the bodies of two divers completely. -^c The bell has preferably two doors, one on the side and one at the bottom of the dry space. When these ^' doors are shut this space becomes pressure tight with an excess pressure internally but the doors will open if excess pressure develops externally.

Ordinarily the bell will be used in the same way as a known wet bell. Divers will use it as a deployment aid and will enter the water with- both doors open so that it will flood as it passes through the sea surface leaving only a small air space at the top.

When the divers are due to return to the surface, .they will enter the bell dry space and shut the side door. The bell will then be raised at normal ascent rates with in-water stops as necessary according to US Navy Standard Air Decompression Tables, or similar, until approximately 13m is reached, at which depth any remaining water is blown out and the lower door shut. For emergency use the bell can be blown out at full operating depth. From 13m the pressure in the cavity is kept constant and the bell lifted -as quickly as possible to a diving station where either decompression is carried out on deck in the bell, although this will probably be an emergency measure only, or the bell is mated to a decompression chamber and the divers transfer under pressure through the side door. They are then decompressed in the chamber according to the tables. This invention also provides an apparatus which is a self-contained unit comprising the pressure facility, wet bell handling system, and dive control station which is suitable for installation in a variety of locations.

The advantages of this invention are that the modifying of wet bell diving techniques to permit '- sealing the dry space with the divers inside does not require new technology. The modified wet bell is to be distinguished from the usual type of open-sructured wet bell and also from the dry bell which is used in heliox bounce and saturation diving at depths beyond 50m. In this invention the dry space should be as small as possible.

In the present invention a preferred system provides the following features: a) The wet bell is designed to acommodate two or possibly three divers and their equipment. b) It is to be used to a depth of 50m only. c) diving is carried out using compressed natural air as breathing medium. d) The main use will be in areas where tidal currents are experienced. e) Operations are possible in all normal diving situations, such as platforms, drilling rigs, vessels, f) A complete modular system may be provided comprising wet bell, handling system, decompression chamber, control system, air compressor set, umbilicals, guidewire arrangements and, secondary recovery equipment. The shape of the wet bell is preferably such that the volume is a minimum to reduce the buoyancy force in the dry condition and therefore minimise the dry bell weight with the drag coefficient being low so that use in tidal conditions is improved. The size and shape is also arranged to accommodate two men and their equipment while permitting operation of doors and internal controls.

Further, the shape should withstand internal pressure corresponding to the maximum working depth of 50m which is 5.5 Bar without needing more than the minimum shell thickness. The bell may be constructed from lightweight high- strength polymer materials such as polycarbonate which has the advantage of transparency or other materials such as glass or Kevlar-reinforced polyester and epoxy. Steel has the advantage of being easily and cheaply formed using well-known codes and, more importantly, its high ductility makes it very suitable in situations where rough handling is the rule. For these reasons, steel is the preferred material.

The bell should be slightly negatively-buoyant, when dry and strongly negatively buoyant during deployment. This is achieved by leaving both side and lower doors open resulting in very rapid flooding when passing through the surface of the water.

The doors in the bell may be of the self-sealing type kept closed by the increasing internal pressure as the bell is brought up. Although the bell will be supplied with air through an umbilical, compressed gas supplies need to be carried on board. Several cylinders of air or breathable gas mixtures can be carried, possibly strapped to external supports or inside said supports. Arrangements to control the gas flow from inside the bell cavity will be provided.

The access apertures or doors in the bell chamber will be held closed by the excess internal pressure and hence pressure resisting fastenings of strength are not essential. In a preferred arrangement simple clamps or clips of limited strength are used to retain the doors closed initially, major retention being effected by internal pressure. The clips or clamps may be arranged to yield on application of limited external force thus, in an emergency, access to the divers in the chamber can be obtained quickly by breaking the door fittings.

Divers ' umbilicals have take-off points external

5. to the bell. Once in the bell, the divers may remove helmets or masks and store these together with the coiled umbilicals outside and adjacent to the lower door. The inside of the bell is preferably equipped '- with its own communications link and built-in breathing

10 system for each diver.

The handling system, may be a relatively simple arrangement and for platform or drilling rig work a fixed gantry may be provided. The problem arises from the need to raise the bell well clear of the mating

15 flange on the associated decompression chamber. The bell then needs to be brought into precise juxtaposition so that the door flanges can be mated with a simple clamp.

A preferred method of mating the door flanges

20 makes use of guide rods including a dog leg which causes the bell to be brought closer to the decompression chamber during the closing stages. Typical lateral movement would be 5 cm. to prevent damage caused by impact between the mating flanges.

Z5 The bell may be raised and lowered using a lift - 13 -

wire having'its own winch which can be electrical, hydraulic, or pneumatic. Pneumatic drives only require an adequate storage capacity of compressed air which is easily achieved using a low-pressure storage tank and are preferred.

For shipboard use it is necessary to ensure that the bell is deployed well clear of the ship's stern or side and the handling system uses either a swinging A- frame, or telescopic gantry. The decompression chamber has a suitable flange and clamp on the entry door to permit mating with the bell. The flange itself may be a standard NATO specification to permit attachment of a one-man rescue chamber as an alternative to the bell. The clamp itself should be of a type already in use with dry-bell systems, such as a split type with a manually operated screw action.

If continuous or "back- o-back" diving is required, however, a standard two-compartment decompression chamber will not suffice. For baσk-to- back work a three-compartment chamber is used in which entry or exit is possible at each end. To avoid having a single very heavy three-compartment chamber the unit can be split into a two-compartment unit with a separate single compartment unit bolted to it on site, or three /02962

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single chambers.

Whichever type of chamber is used it will be fitted with equipment such as BIBS, emergency heating, medical locker, bunks, seats and the like. The control system may be standard although there are more control functions than usual with a normal wet bell operation. The complete system of control cabin, decompression chamber, and handling system may be mounted on a single skid or frame, which may be broken down into individual modules for transportation and handling.

The use of one or more guidewires is preferable to ensure that the bell is lowered under control horizontallly as well as vertically. This is particularly important when diving in marginal weather conditions and when there is a significant tidal current. The usual technique is to lower a heavy gravity anchor (clump weight) to the seabed and maintain the lowering cable under tension. The wire passes over ~- sheave or roller attached to the bell thereby ensuring that close contact is maintained. Whereas basket deployment is frequently carried out with only one guidewire, a two-wire system is to be preferred for a bell to give more precise control and prevent rotation. A typical clump weight system for use with a bell consists of the weight in the form of a heavy bar or concrete-filled tube slightly longer than the width of the bell. A wire runs from a fixed point adjacent to the side of the bell in its pre-launch position, through a sheave at one end of the clump weight, along the weight to the sheave at the other end and then back to the surface sheave giving a lead to the guidewire winch. The clump weight needed is determined by the fluid drag on the bell and its umbilical. If the bell hoist wire should fail a secondary method of bell recovery is needed. With a guidewire and clump weight system such as that described either the bell can be lowered onto the clump weight by means of the umbilical (which must contain a strength member) or the clump weight can be slowly raised to engage with the lower bell buffer ring. The guidewire winch is then used to raise the bell to the surface. It must of course be rated to lift both bell and clump weight together. If a clump weight is not ^;in use a second idle wire can be attached to the bell which can be used to lift it in an emergency. It will require its own winch. Alternatively, the secondary recovery wire can be kept above water and attached by the standby diver if needed. The procedure of this invention whereby divers returning to the surface use the bell as a conventional wet bell until approximately 13m depth has been described. At 13ra the divers shut the lower door thereby converting the bell into a dry bell (pressure 1,33 Bar). The bell is raised in this form to the surface and mated to the decompression chamber. The entry lock is then pressurised with its door open to 1.33 Bar when the side door of the bell will open enabling the divers to transfer into the decompression chamber entry lock. They will shut the entry-lock door behind them thereby isolating the chamber from the bell.

The bell can then be depressurised and preparations for the next dive begin.

Once the divers have isolated themselves in the decompression chamber, the procedure is the same as for any surface-orientated dive. The pressures in the entry lock and main lock are equalised and the divers transfer into a large compartment, again shutting the door behind them. The entry lock can then be depressurised and is available for locking-in an assistant or medical doctor if necessary.

In the case of back to back diving with a three- compartment chamber, the divers will undergo part of their decompression in the centre or second compartment but will transfer into the third compartment at an intermediate chamber stop. This leaves the first and second compartments free to receive divers returning from the second dive. On termination of the first, decompression schedule, divers exit from the chamber at the remote end.

This invention is further described and illustrated by way of example and with reference to the accompanying drawings. In the drawings, Figure 1 shows a side elevation of a diving- bell according to this invention, Figure 2 shows a cross section with a diver in a seated position, Figure 3 shows a side elevation of a shipboard ■mounted winch and decompression chamber, Figure 4 shows an end elevation of the apparatus shown in Figure 3, Figure 5 shows schematically a guide wire and weight arrangement, Figure 6 shows schematically a modification of the diving bell, Figure 7 shows a side elevation of a second embodiment of apparatus, and Figure 8 shows an end elevation of the embodiment of Figure 7. Referring to the drawings and more particularly to Figures 1 and 2, the wet bell comprises a spherical pressure chamber 1 which incorporates a lower access aperture 2 provided with an internal closable door 3 which may be shut to seal the chamber. A further side aperture 4 is also provided again with an internally closing and sealable door 5.

Viewing ports 6 can be provided as required with- the top of the bell having a winch cable anchor point 7. The aperture-4 is designed to mate with a suitable aperture on a decompression chamber (see Figures 3 and 4). Beneath the bell 1 is provided a circular platform 8 on which divers 9 may stand to be partly housed within the sphere. The platform 8 is supported by leg structures 10 also supporting an annular buffing ring 11 on which various tools such as a vice 12 can be mounted, the platform 8 also carries a gas cylinder 13 for compression of the chamber.

In normal use the internal water pressure will be approximately at the point W some third of the way up from the bottom of the chamber. The maximum water level will be at M which is level with the top of the aperture 4.

The chamber 1 will also incorporate a seat 14 and headrests 15. Thus, for emergency purposes divers may enter the chamber, close the door 3 and rest whilst the structure is brought to the surface maintaining the pressure.

Referring to Figures 3 and 4, a shipboard mounted winch system may be provided comprising davits 30 which may be swung out over the side of the vessel using hydraulic rams 31. The wet bell chamber 1 may then be lowered on the cable 32 suitably guided to the underwater work point by means of guide cables 33. The bell 1 when winched on board the vessel can be secured and the aperture 4 is arranged to be brought adjacent an aperture 34 associated with a two or three compartment decompression chamber 35.

The wet bell will be deployed from a gantry system complete with-twin guide wires. A pneumatic man-riding winch will lower the bell into the water with both doors 3 and 5 open and with the divers standing on the platform. An air pocket will remain at the top of the bell so that buoyancy will be minimal. Both divers will be fully dressed in (and breathing from) bandmasks.

On completion of the work underwater the divers will return to.the bell, coil up their umbilicals and close the side door if not already closed. They will blow out the water using air from cylinder 13 or using air supplied through an umbilical tube from the surface dive statioη. and proceed to carry out in-water decompression at the stipulated stops. At a convenient depth such as the depth corresponding to the last in-, water decompression stop they will enter the chamber. The bottom door 3 will be closed and the chamber will proceed to the surface for complete decompression.

On the surface a diving supervisor will make the decision whether to lock the divers into the surface air decompression chamber 35 or continue decompression in the submersible chamber 1.

In this invention the proven practice of wet bell operations with the technique of transforming a wet bell into a submerged decompression chamber prior to it being lifted clear of the water are combined. The wet bell will be normally limited to operations in the air range and as a decompression chamber only will not^" usually be subjected to external pressure. The shell construction can thus be lightweight reducing the size and weight of the support gantry carrying the winch which need not be welded to the platform and can be removed to various platform locations relatively easily.

The construction of the chamber 1 will, however, be sufficient such that it may be brought to the surface at higher pressures if required in an emergency, thus the divers may be very rapidly taken to safety without the usual decompression recompression problems.

Figure 5 shows a schematic arrangement of the wet diving bell using a clump weight which serves to anchor two parallel guide lines to the sea bed. As shown in the drawing, the bell 1 has two sideways extending arms 50 each of which has a guiding means such as a roller 51 in contact with a respective guide wire 52. One of these guide wires 52 is anchored at 53 to the shipboard structure and the wire extends through pulleys 54 which carry the clump weight 55. The return wire 52 passes over a further pulley arrangement 56 to a winch 57. The bell 1 has a cable 58 passing over pulley 59 and running to a second winch 60. With this arrangement the clump weight 55 may be lowered to the sea bed with the two parallel cables 52 forming a positioning and guiding device.

As hereinbefore mentioned the cables 52 with winch 57 and clump weight 55 may form an emergency retrievel system for the bell in which case the clump weight can be raised to contact the lower^, part of the bell structure and thereafter used to bring same to the surface.

Figure 6 shows a modification of the diving bell 1 having aperture 4 for coupling with a decompression chamber and the aperture 2 for entry and exit of the divers. In the construction illustrated here the platform structure 8 is augmented by means of a basket 60 having an access door 61 and which may be. used to store equipment whilst maintaining a clea access to the entrance aperture 2 'of the bell 1. This construction utilising the basket may be developed further so as to provide a number of specially constructed add on modular units which can be coupled with the platform 8 with each unit being adapted for a particular purpose. Figures 7 and 8 show a further embodiment which is particularly adapted for an offshore platform. The general construction and operating method is similar to that previously described.

Referring to Figures 7 and 8, the wet diving bell 70, in this case of cylindrical construction with a spherical top dome includes an entrance aperture 71 again closeable by a door with a side aperture 72 with a similar door. The bell is raised and lowered by means of cable 73 utilising winch 74. A clump weight 75 is provided with side guide cable^:s 76, one of which is secured to the platform structure at 77 and the other of which is coupled to winch 78. The bell 70 includes guides 79 which run along the cables 76. To guide the bell during the final stages posts 76a are provided which are engaged by arras 79. The posts preferably have a dog leg 76b positioned so that after the flanges between the side door 72 and a transfer chamber 87 have overlapped initially, the bell is moved laterally by the dog leg to bring the flanges together. this avoids the risk of the edges of the flanges initially contacting with resulting damage.

The bell with its associated winch structures is located at an end 80 of a lattice work modular support unit 81 which is secured by cross-bearing members 82 to an appropriate part 83- of an offshore rig. Located within the lattice structure 81 is a decompression chamber 84 and a second larger decompression chamber 85 interconnected by means of a transfer air lock 86. The third chamber 87 is provided for transferring divers under pressure from the bell 70 through the access aperture 72 and into the decompression chamber 84 via a further air lock 88.

A particular feature of the construction shown in Figures 7 and 8 is the modular arrangement whereby the decompression chambers together with a control room 89 are located within a sub-frame structure 81 which can be readily transferred in sections to the offshore platform and thereafter assembled to form a complete operational unit. The dive control room 89 will provide facilities for monitoring divers in the decompression chambers together with usual regulation and control equipment foj operating the chambers and the wet bell 70.

Claims

CLA IMS

^'• 1. A wet diving bell comprising a pressure resisting chamber to acommodate one or more divers, a closable

₅ aperture to allow a diver to enter the chamber, means to allow the chamber to flood, at least in part, on immersion in water and means to substantially clear the chamber of water whilst submerged.

₁0 2. A wet diving bell in accordance with Claim 1 comprising a pressure resisting chamber to accommodate one or more divers, a lower access aperture in the chamber to allow a diver to enter or exit the chamber, a door sealingly engaging the aperture to close same and a t₅ further closable upper aperture in the chamber to allow flooding of the chamber.

3. A wet diving bell in accordance with Claim 2, wherein the lower access aperture is in the base of the

:Q chamber.

4. A wet diving bell in accordance with Claim 2 or 3, wherein the upper aperture is in the side of the chamber.

5. A wet diving bell in accordance with Claim 2,3 or 4, wherein the upper aperture allows rapid flooding of a major part of the chamber, with a dry upper cavity being maintained.

6. A wet diving bell in accordance with any preceding Claim 2 to 6, wherein the upper aperture and/or possibly the lower aperture is adapted for connection with a decompression chamber.

7. A wet diving bell as claimed in any preceding claim, constructed for use up to the air diving limit of approximately 50m water depth.

8. A wet diving bell as claimed in any preceding claim, wherein an external support platform structure is provided beneath the access aperture.

9. A wet diving bell as claimed in any preceding claim, wherein the chamber is of spherical, cylindrical or ellipsoidal shape or a combination thereof.

" 10. A wet diving bell in accordance with Claim 9, wherein the chamber has a cylindrical body with a part spherical top dome.

11. A wet diving bell as claimed in any preceding claim, wherein the bell, when dry, is negatively

₅ buoyant.

12. A wet diving bell as claimed in any preceding claim, wherein the upper aperture includes a door.

₀ 13. A wet diving bell in accordance with Claim 12, wherein the upper aperture is of a size to allow- a diver to pass therethrough.

14. A wet diving bell in accordance with any preceding ι_ claim, wherein an aperture closable by an access door is adapted to be coupled with a decompression chamber, the aperture enabling a diver to transfer from the bell to the decompression chamber.

₀

15. Apparatus for performing; diving operations, comprising a wet diving bell according to any preceding claim, a winching means for raising or lowering the bell and a decompression chamber connectable with an aperture

_* in the bell whereby a diver may transfer from the bell to the decompression chamber.

16. Apparatus in accordance with Claim 15, wherein the bell is connected with a pressure transfer chamber which in turn is connected with a decompression chamber.

17. A wet diving bell or apparatus in accordance with any preceding claim, wherein a compressed gas source is provided to clear the chamber of water.

18. A method of performing air breathing diving operations using a wet diving bell capable of housing a diver and having a closable access aperture, in which method the diver is deployed to depth with the diving bell, the diver thereafter being recovered, in a normal mode, by raising the bell and diver with the access door open, and subject to decompression stops, to a recovery depth, thereafter clearing the chamber of water, closing the access door and raising the bell and diver to the surface, connecting the bell to a decompression chamber and transferring the diver to said chamber all under recovery depth pressure. :

19. A method in accordance with Claim 18, wherein, for emergency recovery, the diver and bell are raised from depth to the surface with the access door closed and at operating depth pressure, the bell being connected to the decompression chamber and the diver transferred to •said chamber all under operating depth pressure.

20. A method in accordance with Claim 18 or 19, wherein the diver is deployed to depth partly or wholly within the bell chamber, with the access aperture open and the bell chamber flooding, a dry space being maintained in an upper part of the chamber.

21. A method in accordance with Claim 18, 19 or 20, wherein after closing the access door the chamber is cleared of water.

22. A method in accordance with any preceding claim 18 to 21, wherein the diver is subject to decompression in the bell without transfer to a separate decompression chamber.

23. A method according to any one of Claims 18 to 22, carried out using the diving bell or the apparatus of any preceding claim.

24. A wet diving bell constructed and arranged to function as described herein and exemplified with reference to the drawings.

25. An apparatus for carrying out diving operations as described herein and exemplified with reference to the drawings.

26. A method of carrying out diving operations as described herein and exemplified with particular reference to the drawings.