GB2588690A - Releasing buoyancy during subsea installation - Google Patents

Abstract

A buoyancy release system comprises a transmission system that conveys a common rotary drive from a drive interface to operate one or more buoy-release mechanisms. Aspects of the invention include the buoy-release mechanisms comprise respective latch members that are each movable between a latch position to retain a respective buoy and a release position to release that buoy. The latch members are movable in a release direction toward their release positions in unison from different starting latch positions relative to the buoys or at different speeds relative to the speed of the rotary drive. Preferably, the buoy-release mechanism are configured such that their buoy-release latches will reach the release position at different times. Thus, the buoys may be released from the support simultaneously or at different times or in any desired sequence. The drive interface conveniently comprises a spigot that is configured for engagement by torque tool, for example of a ROV (Remote Operated Vehicle). The system may be used to release multiple floats, buoyancy modules or buoys from subsea structures when they are to be sunk to the seabed or otherwise lowered from the surface.

Description

I

Releasing buoyancy during subsea installation This invention relates to the challenges of releasing buoyancy when installing large structures underwater. Examples of such structures are pipeline bundles and other heavy items of equipment as used in the subsea oil and gas industry.

The invention is particularly concerned with the need to release multiple floats, buoyancy modules or buoys from subsea structures when they are to be sunk to the seabed or otherwise lowered from the surface. In US 4786207, for example, buoys are released in order to sink a pipeline after towing the pipeline to an installation site.

The prior art proposes various ways to release buoys from a subsea structure. For example, US 3727417 describes simply pulling a cable or a chain that connects multiple buoys. Similarly, in US 4107802 and GB 2221247, pulling a cable opens clamps that retain buoys.

Buoy release is also achieved in the prior art by cutting a cable or a similar retaining or fastening element that joins a buoy to a structure. For example, US 4436450 discloses the use of a wedge to break straps that hold buoys.

A disadvantage of any system that comprises flexible cables is the risk of entanglement or jamming. Cables may also be damaged by accidental clashes with sharp edges, for example when towing out a pipeline bundle from an onshore fabrication yard into the sea.

In US 4563108, a pressure-sensitive link between a structure and a buoy breaks upon reaching a predetermined depth so as to free the buoy. In US 4974995, the buoys themselves are designed to collapse upon reaching a predetermined depth. However, both of these approaches require the buoy to be submerged already to a sufficient depth and so are of no use to initiate sinking of a subsea structure.

Hydraulic or pneumatic systems are also known in the art, for example as disclosed in EP 1022501 or US 4127007, either to adjust the content and hence the buoyancy of a buoy or to release a buoy. However, such fluid-actuated systems are complex, sensitive to water depth and susceptible to puncture or leakage.

In some circumstances, it may be desirable to release multiple buoys gradually or intermittently or in a particular sequence so as to reduce buoyancy progressively rather than suddenly and to ensure that loads remain balanced and controlled. In those circumstances, simple buoy release systems that release all of the buoys together therefore suffer a significant disadvantage. More sophisticated and flexible buoy release systems that address this disadvantage suffer the further disadvantages of complexity of design and operation and consequent unreliability.

There remains a need for a simple and reliable buoy release system that can be set up, operated and controlled easily and predictably, especially without having to provide multiple release inputs at multiple locations.

The invention has been devised against this background. From one aspect, the invention provides a buoyancy release system, comprising: at least one buoy-release mechanism; and a transmission that is arranged to convey drive from a rotary drive interface to operate the or each buoy-release mechanism. The drive interface conveniently comprises a spigot that is configured for engagement by torque tool, for example of an ROV.

There may be a plurality of the buoy-release mechanisms, which may conveniently be disposed in a linear array. Elegantly, a common transmission may be arranged to convey the drive to all of the buoy-release mechanisms. Such a transmission may comprise at least one shaft that is coupled to the drive interface and that extends between the buoy-release mechanisms. More generally, the transmission may comprise at least one shaft that is coupled to the drive interface and to the or each buoy-release mechanism.

The or each shaft may comprise a screw that is in threaded engagement with the or each buoy-release mechanism to convert rotary drive of the shaft into linear drive of the or each buoy-release mechanism. The thread of such a screw may have a variable pitch along its length.

The or each shaft may be coupled to the or each buoy-release mechanism by a belt or chain that is arranged to convey rotary drive of the shaft to rotary drive of the or each buoy-release mechanism.

The or each buoy-release mechanism suitably comprises a latch member that is movable between a latch position to retain a buoy and a release position to release that buoy.

Where there is more than one buoy-release mechanism, their latch members may be movable in unison in a release direction toward their release positions. In that case, before movement in the release direction, the latch member of a first buoy-release mechanism may be closer to its release position than a latch member of a second buoy-release mechanism. Conveniently, at least one spacer may be inserted to offset the latch member of the second buoy-release mechanism away from its release position.

In another approach, the latch members of the buoy-release mechanisms may be movable at different speeds in a release direction toward their release positions. For example, the transmission may apply respectively different gear ratios between the drive interface and the buoy-release mechanisms.

The transmission may comprise one or more belts or chains extending between the drive interface and the or each buoy-release mechanism.

The system of the invention may further comprise at least one buoy that is engaged with the or each buoy-release mechanism. In that case, a frame of the system may support the or each buoy, in addition to the or each buoy-release mechanism, the transmission and the drive interface.

The inventive concept embraces a subsea structure, such as a pipeline bundle, fitted with the system of the invention.

The inventive concept also extends to a corresponding method of releasing buoyancy from a subsea structure, the method comprising operating at least one buoy-release mechanism to release at least one buoy by conveying rotary drive from an interface to the or each buoy-release mechanism. For example, a common drive may be conveyed to a plurality of the buoy-release mechanisms. Rotary drive may be applied by engaging a torque tool with the interface and then activating the torque tool, for example using an ROV.

At least one buoy may be released with initial rotary drive and then at least one further buoy may be released with continued or additional rotary drive.

At least one latch member may be moved between a latch position that retains a buoy and a release position that releases the buoy. For example, the latch members of multiple buoy-release mechanisms may be moved in a release direction toward their release positions in unison.

Movement of a first latch member of a first buoy-release mechanism may start from a first latch position and movement of a second latch member of a second buoy-release mechanism may start from a second latch position. In that case, the distance between the first latch position and the release position of the first latch member may be less than the distance between the second latch position and the release position of the second latch member. For this purpose, the latch member of the second buoy-release mechanism may be offset away from its release position by adding at least one spacer to the mechanism.

The invention may therefore be exemplified by a rotary ROV-drivable interface and a transmission system for transmitting rotary drive from the interface to a plurality of buoy-release mechanisms. Each of the buoy-release mechanisms is driven by that common drive to move a respective buoy-release latch between a latch position and a release position. Preferably, the buoy-release mechanisms are configured, or configurable, such that their buoy-release latches will reach the release position at different times.

Thus, the invention provides a largely mechanical buoy release system that solves the problems of releasing more than one buoyancy unit remotely, for example using an ROV, from a single activation point at different times.

In the examples to be described, the invention is embodied as a mechanical device in which an ROV can simply rotate a torque bucket to operate multiple buoy-release mechanisms. The device can be extended and adjusted easily to either simultaneously or sequentially release a number of buoyancy sections or buoys. In a first embodiment, the release sequence is set by simple insertion of fixed-length spacers into each mechanism. Depending upon how and where the spacers are inserted, simultaneous, multiple or sequenced individual buoys can be released without the need for the ROV to move from a single activating position or to change its application of rotary drive.

Embodiments of the invention provide a remotely-actuated buoyancy release mechanism that comprises: a frame mounted on an elongate member or structure such as a pipeline bundle; at least one buoy release device interfacing with at least one buoy and mounted on the frame; at least one shaft extending longitudinally inside the frame able to rotationally engage the buoy release device; and an operating interface to couple with a remotely-operated tool, such as a torque tool of an ROV, and thereby to rotate the at least one shaft.

The buoy release device may, for example, comprise gears and a threaded central stem that is retracted away from a buoy holder when the gears are rotated. The buoy holder is suitably integral with the buoy and may be a U-shaped plate or bar that is engaged by the central stem when the central stem is in an extended position.

The mechanism suitably comprises a plurality of buoy release devices to hold and release several buoys. The thread pitch and/or length of the threaded central stem may be different for each buoy in order to organise simultaneous or sequential release.

Embodiments of the invention also implement a method to release remotely at least one buoy that is temporarily or provisionally attached to an elongate member, the method comprising: coupling a remotely-operated tool to an end shaft of a transmission system; and rotating the remotely-operated tool until at least one buoy is released.

In summary, a buoyancy release system of the invention comprises a transmission system that conveys a common rotary drive from a drive interface to operate one or more buoy-release mechanisms. The buoy-release mechanisms comprise respective latch members that are each movable between a latch position to retain a respective buoy and a release position to release that buoy. The latch members are movable in a release direction toward their release positions in unison from different starting latch positions relative to the buoys or at different speeds relative to the speed of the rotary drive.

In order that the invention may be more readily understood, reference will now be made, by way of example, to the accompanying drawings in which: Figure 1 is a top plan view of a buoyancy release system in a first embodiment of the invention; Figure 2 is a sectional side view of the system of Figure 1; Figure 3 is an enlarged plan view corresponding to Detail Ill of Figure 1; Figure 4 is an enlarged a partial end view of the system of Figure 1; Figure 5 is an enlarged sectional side view corresponding to Detail V of Figure 2, taken on line V-V of Figure 4; Figures 6a to 6f are enlarged sectional side views taken on line VI-VI of Figure 4; Figures 7a and 7b are further enlarged side views corresponding to Figure 6b; Figures 8a and 8b are schematic side views of a buoyancy release system in a second embodiment of the invention; and Figures 9a and 9b are schematic side views of a buoyancy release system in a third embodiment of the invention.

Referring firstly to the first embodiment of the invention shown in Figures 1 to 7b of the drawings, a buoyancy release system 10 comprises a support 12 that releasably retains a plurality of buoyancy modules or buoys 14 mounted atop the support 12. The support 12 may be fixed to a subsea structure such as pipeline bundle or an item of subsea equipment that requires additional buoyancy temporarily, for example during towing or lowering to a subsea installation site. When required, the buoys 14 may be released from the support 12 to remove or reduce the buoyant upthrust that they apply temporarily to the subsea structure via the support 12.

Advantageously, the invention enables the buoy-release operation to be activated and controlled remotely in a single movement applied at a single location. Yet, the buoys 14 may be released from the support 12 simultaneously or at different times or in any desired sequence to achieve a predetermined gradual or progressive reduction in the aggregate upthrust applied to the structure. To do so, an unmanned underwater vehicle such as an ROV engages a torque tool with a drive interface 16 of the support 12 to effect single-point activation and control of the buoy-release operation. Of course, in principle, a diver could instead apply a torque tool to the drive interface 16, if the diver's safety can be assured in an environment in which buoys 14 are being released nearby.

In this example, the support 12 is elongate, the buoys 14 are disposed in a series or straight-line array and the drive interface 16 is disposed at a proximal end of the support 12. The support 12 extends distally away from the drive interface 16 at the proximal end toward a distal end of the support 12. The length of the support 12 between its distal and proximal ends is proportional to the number of buoys 14 and so is indeterminate.

The elongate configuration of the support 12 shown in this example is not essential but is advantageous because it facilitates a modular arrangement in which the support 12 can be lengthened or shortened at its distal end to suit a greater or lesser number of buoys 14. Indeed, the principle of the invention could be applied to the release of as few as one or two buoys 14.

In this instance, there is a total of six buoys 14 on the support 12, each buoy 14 having a typical size of 1.8m x 1.8m in plan view and being centred on a respective tubular core post 18. The longitudinal centre-to-centre pitch from buoy to buoy is slightly greater than the diameter of each buoy 14, for example 1.9m, hence defining small gaps between the buoys 14. The gaps ensure reliable release of the buoys 14 from the support 12 by preventing interference between the buoys 14.

A securing eye 20 depends centrally from the underside of each buoy 14 in alignment with the core post 18. The securing eye 20 is penetrated longitudinally by an aperture 22 that can receive and be engaged by a respective longitudinally-extending release pin 24 of the support 12. Each release pin 24 serves as a movable latch member that, in a lock or latch position, retains the associated buoy 14 and, in a release position, releases the associated buoy 14. Specifically, in the latch position, the release pin 24 is engaged with the securing eye 20 to hold the buoy 14 relative to the support 12 against buoyant upthrust. When activated via the drive interface 16, the release pin 24 is movable longitudinally into the release position, disengaged from the securing eye 20, to release the buoy 14. The buoy 14 is then free to float away from the support 12 under the influence of buoyant upthrust.

In this example, the support 12 comprises an elongate ladder frame 26 including parallel longitudinal C-section members 28 that are held spaced apart by pairs of cross-members 30 at longitudinal intervals. The cross-members 30 are parallel to each other and are orthogonal to the longitudinal members 28. The underside of each buoy 14 rests on top of the longitudinal members 28. The securing eyes 20 of the buoys 14 hang between the longitudinal members 28 and between the paired cross-members 30.

A parallel pair of lead screws 32 extends longitudinally along the frame 26, extending through and supported by bushings in the cross-members 30. Optionally, as shown, the lead screws 32 are formed of sections joined end-to-end by keyed or splined shaft couplings 34. This supports modular lengthening or shortening of the support 12 to suit a different number of buoys 14.

The drive interface 16 at the proximal end of the support 12 drives the lead screws 32 to turn about their respective longitudinal axes. For this purpose, as best seen in Figure 4, each lead screw has a pinion gear 36 at its proximal end that forms part of a gear set of the drive interface 16. The gear set further comprises a central pinion gear 38 that is disposed between, and meshed with, the pinion gears 36 of the lead screws 32. The central pinion gear 38 turns about an axis that is parallel to and disposed between the longitudinal axes of the lead screws 32.

A drive shaft or spigot 40 fixed to the central pinion gear 38 is shaped for engagement by a torque tool of an ROV. On being engaged with the spigot 40 and activated, the torque tool turns the central pinion gear 38 and hence, via their meshed driven gears 36, causes the lead screws 32 to contra-rotate relative to the frame 26 and each other.

The support 12 further comprises a series of buoy-release mechanisms 42, one for each buoy 14. In this example, the buoy-release mechanisms 42 comprise respective longitudinally-spaced carriages. Each carriage comprises a respective one of the release pins 24 and a transverse guide plate 44 that carries that release pin 24.

The guide plate 44 of each carriage bridges the lateral spacing between the lead screws 32 and is movable longitudinally by and along the lead screws 32. The release pin 24 of each carriage penetrates the guide plate 44, extending both distally and proximally from the guide plate 44 on a longitudinal axis parallel to, equidistant between and coplanar with the longitudinal axes of the lead screws 32.

It will be apparent from Figures 1, 3 and 6a to 6f that the longitudinal offset of the release pins 24 relative to the guide plates 44 differs from carriage to carriage. In particular, the release pin 24 of the first carriage at the proximal end of the series extends distally from the guide plate 44 to a lesser extent than the release pin 24 of the second carriage of the series, and so on moving distally from carriage to carriage along the series. Nevertheless, conveniently, all of the release pins 24 may be substantially identical.

As best appreciated in Figures 7a and 7b, each release pin 24 comprise a male-threaded proximal portion 24P and a wider plain cylindrical distal portion 24D. When in the latch position as shown in Figure 7a, the distal portion 240 extends through and engages with the aperture 22 of the securing eye 20. The distal portion 240 also extends, as a sliding fit, through aligned holes in the paired cross-members 30 that embrace the securing eye 20 between them.

A shoulder 46 is defined by the step difference in diameter between the proximal portion 24P and the distal portion 24D of the release pin 24. The guide plate 44 is clamped between the shoulder 46 and a lock nut 48 that is engaged with the thread of the proximal portion 24P.

The guide plate 44 may be sandwiched directly between the shoulder 46 and the lock nut 48, as can be seen in relation to the first carriage in Figure 6a. In that example, the lock nut 48 bears against the proximal side of the guide plate 44 and the shoulder 46 bears against the distal side of the guide plate 44. However, Figure 6b and the corresponding enlarged views of Figures 7a and 7b show that the second carriage has a short spacer sleeve 50 around the proximal portion 24P of the release pin 24, interposed between the shoulder 46 and the distal side of the guide plate 44.

The spacer sleeve 50 determines the longitudinal offset of the release pin 24 relative to the guide plate 44, in effect shifting the release pin 24 distally by the length of the spacer sleeve 50. Thus, the third carriage shown in Figure 6c has a longer spacer sleeve 50 that shifts its release pin 24 distally to a correspondingly greater extent. Figure 1 shows that the sequence of ever-longer spacer sleeves 50 defining ever-greater distal offsets of the release pins 24 continues through the fourth to sixth buoy-release mechanisms 42.

The spacer sleeves 50 may, for example, be in threaded engagement with the proximal portions 24P of the release pins 24, in which case each spacer sleeve 50 may simply be advanced along the thread until the spacer sleeve 50 bears against the shoulder 46. Alternatively, the spacer sleeve 50 may comprise split half-shells that are clamped around the proximal portion 24P of the release pin 24.

The lead screws 32 have mutually-opposed male threads in engagement with respective complementary female threads of the guide plates 44 and/or of nuts 52 fixed relative to the guide plates 44. Contra-rotation of the lead screws 32 therefore causes the guide plates 44 to move longitudinally in unison relative to the frame 26. Thus, the carriages are movable relative to the frame 26 by the lead screws 32 acting as a common drive for the buoy-release mechanisms 42. The lead screws 32 therefore exemplify a drive transmission system from the drive interface 16 to the buoy-release mechanisms 42.

Distal movement of the guide plates 44 is prevented by stop plates 54, shown in the drawings partially cut away, that extend inwardly from the longitudinal members 28 of the frame 26. The guide plates 44 abut the stop plates 54 to define start positions for the carriages.

The arrangement is such that the guide plates 44 all move proximally away from the stop plates 54, toward the drive interface 16, when the central pinion gear 38 and hence the lead screws 32 are turned by a torque tool of an ROV. As the threads of the lead screws 32 have a constant pitch along their length, the carriages all move proximally at the same speed.

It will therefore be apparent that the differing distal extents of the release pins 24 from carriage to carriage defines the order in which the release pins 24 will disengage from the securing eyes 20 of the respective buoys 14, hence freeing the buoys 14 in the corresponding sequence. In this respect, as a comparison with Figures 6a, 6b and Sc makes clear, Figures 6d, 6e and 6f show the first to third carriages moved proximally in unison to the extent that the release pin 24 of the first carriage has disengaged from the securing eye 20 of the first buoy 14. The securing eye 20 of the first buoy 14 has therefore been lifted clear of the support 12 in Figure 6d under the buoyant upthrust of the first buoy 14, now released.

In the position shown in Figures 6d, 6e and 6f, the release pins 24 of the second and third carriages have slid through, but remain engaged with, the securing eyes 20 of the second and third buoys 14. It will be apparent that continued proximal movement of the second and third carriages will firstly disengage the release pin 24 of the second carriage from the securing eye 20 of the second buoy 14, as shown in Figure 7b, and then will disengage the release pin 24 of the third carriage from the securing eye 20 of the third buoy 14.

In general, with reference to Figures 7a and 7b, if a proximal movement P is sufficient for the first buoy-release mechanism 42 to release the first buoy 14, the spacer sleeve of the second buoy-release mechanism 42 should have a length of P. Similarly, the spacer sleeve 50 of the third buoy-release mechanism 42 should have a length of 2P, the spacer sleeve 50 of the fourth buoy-release mechanism 42 should have a length of 3P, and so on through the series of buoy-release mechanisms 42.

The order in which the buoys 14 are released may be determined simply by fitting appropriately-sized spacer sleeves 50 to the release pins 24. Release pins 24 fitted with no spacer sleeves 50 or shorter spacer sleeves 50 will disengage from the securing eyes 20 of their associated buoys 14 sooner than the release pins 24 fitted with longer spacer sleeves 50. In this respect, sooner means that fewer turns of the spigot 40 of the drive interface 16 will be necessary to effect release of some buoys 14 than others.

Spacer sleeves 50 can be positioned in any arrangement required to achieve a desired sequence for releasing the buoys 14. For example, with suitably-positioned spacer sleeves 50, it would be possible to release buoys 14 simultaneously at the proximal and distal ends of the series and then to release buoys 14 in pairs moving inwardly from the ends of the series. It will therefore be evident that the buoys 14 need not only be released in succession moving from the proximal end of the series to the distal end of the series. It will also be apparent that the buoyancy release operation can be paused before all of the buoys 14 have been released, simply by stopping or disengaging the torque tool that drives the buoy-release mechanisms 42. For example, the torque tool could be stopped or disengaged after completing a certain number of turns required to release some but not all of the buoys 14.

The use of spacer sleeves 50 provides a convenient, repeatable and accurate way of determining the order in which buoys 14 are released, while being easy to assemble, to adjust and to check. However, other arrangements are possible. For example, the release pins 24 could be attached to the guide plates 44 by threaded engagement. This would allow the release pins 24 to be turned and advanced longitudinally to determine their distal extent relative to the guide plates 44, and hence to determine the order in which the buoys 14 will be released.

The skilled reader will appreciate that the arrangement shown in Figures 1 to 7 could be adapted such that buoys 14 are released by distal rather than proximal movement of the release pins 24. Also, with additional gear arrangements to change the direction of the rotary drive, it would be possible for the release pins 24 to move transversely to release the buoys 14.

Many other variations are possible within the inventive concept. For example, it would be possible for the thread pitch to vary from carriage to carriage along the length of the lead screws 32. This would cause the guide plates 44 and hence the release pins 24 to move longitudinally at different speeds, and so would obviate the need for the release pins 24 to have differing distal extents. The aforementioned option of assembling the lead screws 32 from sections joined end-to-end would be apt to achieve variable thread pitch for this purpose. Lead screws 32 assembled from sections would also facilitate changing the thread pitches of different sections to change the order in which buoys 14 are released.

More generally, the lead screws 32 of the first embodiment are only one option for a transmission system that uses a common rotary drive to operate multiple buoy-release mechanisms 42. Further examples are shown in the second embodiment of Figures 8a and 8b and in the third embodiment of Figures 9a and 9b. For simplicity, these schematic views show just two buoys 14 and omit the structure of the support 12. Figures 8a and 9a show a torque tool 56 of an ROV approaching the drive interface 16, whereas Figures 8b and 9b show the torque tool 56 engaged with and turning the spigot 40 of the drive interface 16.

Figures 8a and 8b show a transmission system 58 that transmits rotary drive from the drive interface 16 along a series of buoy-release mechanisms 42 using a series of chains or belts 60. Here, the spigot 40 turning on a longitudinal axis turns a meshed pair of bevel gears 62 that redirect the drive through 900 to turn a pulley 64 about a transverse axis. Each buoy-release mechanism 42 comprises a further pulley 64 that turns a transverse drive shaft 66. Each drive shaft 66 carries a pinion gear 68 that meshes with a rack 70. The rack 70 is integral with a latch member 72 that moves relative to a respective buoy 14 in response to rotation of the drive shaft 66 and hence the pinion gear 68. A series of belts 60 carries the drive from one pulley 64 to the next.

It will be noted that the pinion gear 68 on the drive shaft 66 of the second buoy-release mechanism 42 has a smaller diameter than the pinion gear 68 on the drive shaft 66 of the first buoy-release mechanism 42. Consequently, the latch member 72 of the second buoy-release mechanism 42 will travel more slowly than the latch member 72 of the first buoy-release mechanism 42 for each rotation of their drive shafts 66. A similar effect could be achieved by changing the relative diameters of the pulleys 64 of the first and second buoy-release mechanisms 42.

To illustrate this, Figure 8a shows the latch members 72 fully engaged with the buoys 14 in latch positions. Conversely, Figure 8b shows the latch member 72 of the first buoy-release mechanism 42 in a release position disengaged from the first buoy 14, whereas the latch member 72 of the second buoy-release mechanism 42 has also moved, but more slowly, and so remains in a latch position engaged with the second buoy 14. The second buoy 14 is therefore retained while the first buoy 14 floats free as shown.

Finally, Figures 9a and 9b show a transmission system 74 that combines a longitudinal shaft 76 with pulleys 64 and chains or belts 60. Here, the shaft 76 transmits rotary drive from a drive interface 16 along a series of buoy-release mechanisms 42. Each buoy-release mechanism 42 comprises a respective drive pulley 64 and transverse belt 60 that takes drive off the shaft 76. Each buoy-release mechanism 42 further comprises a driven pulley 78 that is driven by the belt 60. Each of the driven pulleys 78 is in threaded engagement with a latch member 72 that is thereby driven to move relative to a respective buoy 14 in response to rotation of the shaft 76 and hence of the driven pulley 78.

It will be noted that the driven pulley 78 of the second buoy-release mechanism 42 has a greater diameter than the driven pulley 78 of the first buoy-release mechanism 42. Consequently, the latch member 72 of the second buoy-release mechanism 42 will travel more slowly than the latch member 72 of the first buoy-release mechanism 42 for each rotation of the pulleys 64 on the shaft 76. A similar effect could be achieved by changing the relative diameters of the pulleys 64 on the shaft 76.

Like Figure 8a, Figure 9a shows the latch members 72 fully engaged with the buoys 14 in latch positions. Conversely, Figure 9b shows the latch member 72 of the first buoy-release mechanism 42 in a release position disengaged from the first buoy 14, whereas the latch member 72 of the second buoy-release mechanism 42 has also moved, but more slowly and to a lesser extent, and so remains in a latch position engaged with the second buoy 14. Again, therefore, the second buoy 14 is retained while the first buoy 14 floats free as shown.

In the arrangements shown in Figures 8a, 8b, 9a and 9b, the order in which the buoys 14 are released can be modified by changing the gearing effected by the relative sizes of the pulleys 64, 78 and the pinion gears 68 and/or the pitches of the pinion gears 68, the racks 70 or the threads.

In a further variant, each buoy-release mechanism 42 could comprise a respective gear train that takes a common drive and applies a selectively different gear ratio to drive differential movement of latch members 72 acting on the buoys 14. The gear ratio of each gear train could be adjusted by meshing different combinations of gears. Alternatively, a continuously-variable transmission could be used instead of a gear train.

Claims (30)

1. Claims 1. A buoyancy release system, comprising: at least one buoy-release mechanism; and a transmission that is arranged to convey drive from a rotary drive interface to operate the or each buoy-release mechanism.
2. 2. The system of Claim 1, comprising a plurality of the buoy-release mechanisms.
3. 3. The system of Claim 2, wherein the buoy-release mechanisms are disposed in a linear array.
4. 4. The system of Claim 2 or Claim 3, wherein a common transmission is arranged to convey the drive to the buoy-release mechanisms.
5. 5. The system of Claim 4, wherein the common transmission comprises at least one shaft that is coupled to the drive interface and that extends between the buoy-release 20 mechanisms.
6. 6. The system of any of Claims 2 to 5, wherein the buoy-release mechanisms comprise respective latch members that are each movable between a latch position to retain a respective buoy and a release position to release that buoy.
7. 7. The system of Claim 6, wherein the latch members of the buoy-release mechanisms are movable in unison in a release direction toward their release positions.
8. 8. The system of Claim 7, wherein before movement in the release direction, the latch member of a first buoy-release mechanism is closer to its release position than a latch member of a second buoy-release mechanism.
9. 9. The system of Claim 8, further comprising at least one spacer that offsets the latch member of the second buoy-release mechanism away from its release position.
10. 10. The system of any of Claims 2 to 5, wherein the latch members of the buoy-release mechanisms are movable at different speeds in a release direction toward their release positions.
11. 11. The system of Claim 10, wherein the transmission applies respectively different gear ratios between the drive interface and the buoy-release mechanisms.
12. 12. The system of any preceding claim, wherein the transmission comprises at least one shaft that is coupled to the drive interface and to the or each buoy-release 10 mechanism.
13. 13. The system of Claim 12, wherein the or each shaft is a screw that is in threaded engagement with the or each buoy-release mechanism to convert rotary drive of the shaft into linear drive of the or each buoy-release mechanism.
14. 14. The system of Claim 13, wherein the screw has a thread with variable pitch along its length.
15. 15. The system of any of Claims 12 to 14, wherein the or each shaft is coupled to the or each buoy-release mechanism by a belt or chain that is arranged to convey rotary drive of the shaft to rotary drive of the or each buoy-release mechanism.
16. 16. The system of any preceding claim, wherein the transmission comprises one or more belts or chains extending between the drive interface and the or each buoy-release mechanism.
17. 17. The system of any preceding claim, wherein the drive interface comprises a spigot that is configured for engagement by an ROV torque tool.
18. 18. The system of any preceding claim, further comprising at least one buoy engaged with the or each buoy-release mechanism.
19. 19. The system of Claim 18, further comprising a frame that supports the or each buoy-release mechanism, the transmission, the drive interface and the or each buoy.
20. 20. A subsea structure fitted with the system of any preceding claim.
21. 21. A method of releasing buoyancy from a subsea structure, the method comprising operating at least one buoy-release mechanism to release at least one buoy by conveying rotary drive from an interface to the or each buoy-release mechanism.
22. 22. The method of Claim 21, comprising conveying a common drive to a plurality of the buoy-release mechanisms.
23. 23. The method of Claim 21 or Claim 22, comprising moving at least one latch member between a latch position that retains a buoy and a release position that releases the buoy.
24. 24. The method of Claim 23, comprising moving the latch members of multiple buoy-release mechanisms toward their release positions in unison in a release direction.
25. 25. The method of Claim 24, comprising starting the movement of a first latch member of a first buoy-release mechanism from a first latch position and the movement of a second latch member of a second buoy-release mechanism from a second latch position, wherein a distance between the first latch position and the release position of the first latch member is less than a distance between the second latch position and the release position of the second latch member.
26. 26. The method of Claim 25, preceded by adding at least one spacer to offset the latch member of the second buoy-release mechanism away from its release position.
27. 27. The method of any of Claims 21 to 23, comprising moving the latch members of the buoy-release mechanisms toward their release positions at different speeds in a release direction.
28. 28. The method of Claim 27, comprising applying respectively different gear ratios between the interface and the buoy-release mechanisms.
29. 29. The method of any of Claims 21 to 28, comprising engaging an ROV torque tool with the interface and thereby applying the rotary drive.
30. 30. The method of any of Claims 21 to 29, comprising releasing at least one buoy with initial rotary drive and releasing at least one further buoy with continued rotary drive.