Securing buoyancy modules underwater

Abstract

A method of promoting clamping engagement of a buoyancy module with an elongate subsea element such as a riser comprises actuating a tensioning device to apply a clamping force to sections of the buoyancy module in response to hydrostatic pressure acting on the buoyancy module or on the tensioning device itself. The clamping force is applied via a fastening that holds together the sections of the buoyancy module, namely a strap that extends around those sections. The tensioning device is actuated by admitting water under hydrostatic pressure into an expansion chamber in a cylinder. This drives a piston along a stroke within the cylinder against a volume of gas in a compression chamber in the cylinder. Movement of the piston thereby draws together the sections of the buoyancy module around the element as the tensioning device contracts.

Description

[0001] Securing buoyancy modules underwater

[0002] This invention relates to the challenges of securing buoyancy modules that are clamped to an underlying structure such as a subsea riser, including steel lazy-wave risers (SLWRs) and dynamic umbilical risers.

[0003] Buoyancy modules, also known as buoyancy units or buoys, are commonly used to modify or tailor the buoyancy of elongate subsea elements such as risers, umbilicals, conduits or cables to which such modules are attached. For example, a longitudinally- spaced series of buoyancy modules can be attached to a hogbend portion of a riser to impart a wave configuration to the riser.

[0004] Conventionally, buoyancy modules are attached successively to an elongate element as the element is overboarded or launched progressively from an installation vessel into water. With reference to Figure 1a of the drawings, each buoyancy module 10 is typically assembled from two or more sections 12 of a positively-buoyant material such as a syntactic foam. The sections 12 are brought together in mutual opposition around the element 14 to form an annular body of the buoyancy module 10. Thus assembled, the buoyancy module 10 is then clamped to the element 14, for example by operating clamping mechanisms that act directly between the parts of the module 10 or, as shown here, by tensioning bands or straps 16 that encircle the module 10. For example, a fastener 18 that joins opposed ends of a strap 16 can apply tension to the strap 16.

[0005] The prior art includes various proposals for clamps to secure buoyancy modules with straps or otherwise. For example, US 8573888 discloses a buoyancy module and mounting apparatus for rigid pipelines, enhancing resistance to slippage by means of resilient coils that clamp onto the pipe surface.

[0006] US 2020 / 0340304 discloses a retention strap assembly for securing buoyancy modules of riser systems. The assembly comprises an elongate strap member, a tensioning buckle and multiple attachment points for auxiliary equipment. The aim is to allow efficient tensioning without mechanical fasteners, enhancing operational speed and reducing wear on the buoyancy modules. US 11225839 discloses an attachment device to fasten a buoyancy module securely to a riser. A split annular collar and a separable flange part allow for installation and adjustment.

[0007] US 9428969 discloses a modular kit for assembling a clamp to secure an elongate member underwater. The kit includes separate clamp bodies and a strap, enabling adaptation to various diameters. The design allows for circumferential spacing and inward movement of the clamp bodies upon tightening.

[0008] Another clamp device disclosed in US 2009 / 0272855 incorporates a resilient inner layer and a tensile reinforcement layer. Its design aims to accommodate flexure and variations in riser diameter, thereby reducing the risk of slippage and fatigue in the tensioning band.

[0009] US 2008 / 0251668 discloses a clamp device that comprises a multi-segment body with an elastomer hinge and a tensioning band to enhance flexibility and load distribution, and to ease installation.

[0010] As more distant background, US 10712214 and GB 2561196 describe techniques for monitoring the buoyancy or other parameters of buoyancy modules.

[0011] Resistance to slippage of a buoyancy module 10 from its desired longitudinal position on an elongate element 14 depends upon friction between the buoyancy module 10 and the outer coating of the element 14. Frictional resistance depends, in turn, on the radially inward clamping force that the buoyancy module 10 applies to the element 14. Where straps 16 are used, an effective level of clamping force and hence friction is inferred by applying a sufficient degree of tension to the straps 16 before the buoyancy module 10 is carried into the water on the underlying element 14.

[0012] A tensioning operation performed on the straps 16 aboard an installation vessel is necessarily performed at atmospheric pressure of, nominally, one bar. However, once launched into water, an elongate element 14 such as a riser can carry attached buoyancy modules 10 to a great depth of, for example, 1000m or more. At such depths, the buoyancy modules 10 are subject to massive hydrostatic pressure that squeezes their sections 12 of buoyant material, hence reducing the volume and thickness of the annular body assembled from those sections 12. It should be noted that all external surfaces of the sections 12 will be exposed to hydrostatic pressure at depth, including their concave inner faces facing toward the element 14. Thus, as the sections 12 will not only be squeezed inwardly toward the element 14, hydrostatic pressure does nothing to increase the clamping force exerted by the buoyancy module 10 upon the element 14. To the contrary, the resulting shrinkage of the annular body, shown in Figure 1b, reduces the tension in the straps 16 and the clamping force that the buoyancy module 10 exerts on the element 14. The consequent reduction of friction acting between the buoyancy module 10 and the element 14 risks slippage or dislodgement of the buoyancy module 10 along the element 14.

[0013] As best, slippage of a buoyancy module 10 along an elongate element 14 will result in the module 10 moving to an incorrect position on the element 14. This could distort the desired shape of a buoyancy-supported element 14 once installed in the water column. More seriously, there is a risk of buoyant upthrust causing runaway slippage of a buoyancy module 10 along an element 14, especially where the underlying portion of the element 14 is in a generally upright orientation. In that case, the module 10 can accelerate upwardly in the water column before reaching the surface with such force as to risk serious injury to personnel, expensive damage to installation equipment and costly delay to an installation project.

[0014] In this respect, reference is made to Figures 2a and 2b in which an installation vessel 20 is shown installing an elongate element 14, in the form of a subsea dynamic umbilical riser, via a moonpool extending through the hull of the vessel 20. In this example, the upper end of the element 14 is coupled to a floating production, storage and offloading (FPSO) vessel 22 but the element 14 could instead terminate at another structure such as a floating platform or a spar buoy.

[0015] Figure 2a shows the vessel 20 attaching a series of buoyancy modules 10 to the element 14 as the element 14 is launched progressively into the water. The element 14 then pulls the buoyancy modules 10 beneath the surface 24. As more of the length of the element 14 is launched into the water, the buoyancy modules 10 are carried deeper in the water column until eventually, as shown in Figure 2b, the buoyancy modules 10 reach such a depth as to risk slippage along the element 14 due to hydrostatic squeeze. It will be apparent from Figure 2b that some of the buoyancy modules 10 have become dislodged and have slid back up an upright portion of the element 14 due to their buoyant upthrust. Once those buoyancy modules 10 start to slide upwardly, they can accelerate to a dangerous velocity on their way to the surface 24 and will then either collide with the hull of the vessel 20 or fly up from the water within the moonpool of the vessel 20. In either case, the risks of injury and damage are self-evident.

[0016] Against this background, the invention provides a method of promoting clamping engagement of a buoyancy module with an elongate subsea element, the method comprising actuating a tensioning device to apply a clamping force to sections of the buoyancy module in response to hydrostatic pressure acting on the buoyancy module, preferably in response to hydrostatic pressure acting on the tensioning device fitted to the buoyancy module. The tensioning device suitably contracts from an extended state when actuated.

[0017] The clamping force of the tensioning device may be applied via a fastening that holds together the sections of the buoyancy module. Such a fastening can be a strap that extends around the sections of the buoyancy module. However, it may also be possible to apply the clamping force directly to at least one of the sections of the buoyancy module.

[0018] The tensioning device can be actuated by admitting water under hydrostatic pressure into a cylinder of the tensioning device to drive a piston along a stroke within the cylinder, the piston thereby drawing together the sections of the buoyancy module around the element. The piston can act on a fastening such as a strap, in which case the piston can be connected to an end of the strap and the cylinder can be connected to an opposite end of the strap. The piston can be driven against a volume of gas trapped in the cylinder, hence compressing the gas in response to the admission of water.

[0019] Where the gas is air, the method may be preceded by opening a relief port that communicates with a compression chamber within the cylinder to equalise the compression chamber with ambient atmospheric pressure. The relief port can then be closed before the tensioning device is submerged with the buoyancy module. The method can also be preceded by tensioning the fastening when the tensioning device is extended, with the piston then being at a starting end of the stroke. The method can also be preceded by opening a flood port that communicates with an expansion chamber within the cylinder before submerging the tensioning device with the buoyancy module. Water can then be admitted into the expansion chamber via the flood port.

[0020] The tensioning device can be actuated in response to a loss of tension in the fastening, that loss of tension being caused by hydrostatic pressure acting on the buoyancy module. For example, tension in the fastening, such as a strap, can be sensed and the tensioning device can be actuated in response to a reduction in the sensed tension.

[0021] The method can involve maintaining a watertight seal between the piston and the cylinder. Additionally or alternatively, the piston can be locked or maintained in at least one predefined locked position within the cylinder. The piston may be moved from a first predefined locked position to a second predefined locked position as it moves along its stroke within the cylinder.

[0022] The inventive concept embraces a buoyancy module comprising two or more sections and a tensioning device that is arranged to apply a clamping force to the sections in response to hydrostatic pressure acting on the buoyancy module. Similarly, the inventive concept embraces a tensioning device arranged to apply a clamping force to sections of a buoyancy module in response to hydrostatic pressure acting on the buoyancy module.

[0023] The tensioning device can act via a fastening such as a strap that holds the sections together or can act directly on at least one of the sections. The tensioning device may be arranged to contract from an extended state when actuated in response to hydrostatic pressure and / or a loss of tension in a fastening, in the latter case being responsive to a tension sensor on the fastening.

[0024] The tensioning device may comprise a cylinder containing a piston that divides the interior of the cylinder into a gas-filled compression chamber and a floodable expansion chamber. During installation of buoyancy modules on elongate subsea elements such as rigid or flexible pipeline risers, umbilical risers, conduits or cables, there is a risk that hydrostatic squeeze of the buoyancy module will cause clamping straps to lose tension at depth.

[0025] To mitigate that risk, a passive tensioning device of the invention, which may be described as a hydrostatic bolt, is inserted in line with a strap that clamps a buoyancy module to an elongate element. By contracting or shortening under the same high hydrostatic pressure as is experienced by the buoyancy module to which the device is fitted, the device counteracts loss of tension in the strap.

[0026] The tensioning device of the invention employs a simple mechanical solution comprising a passive hydrostatically-actuated self-tensioning cylinder that works on the principle of differential pressure. The device can be incorporated in-line in addition to, or as a part of, a strap around a buoyancy module.

[0027] The tensioning device is set at the surface by fully extending or withdrawing a shaft of a piston that is slidable within the cylinder, hence setting the piston toward a starting end of its stroke. Next, the pressure of air in a compression chamber on a forward side of the piston is equalised with atmospheric pressure. The compression chamber is then sealed tight, for example by inserting a relief plug into a relief port, to lock air in the compression chamber at atmospheric pressure of nominally one bar.

[0028] When in that extended state, the tensioning device is installed in line with a strap placed around a buoyancy module aboard the lay vessel. The strap is then tensioned in the usual way to a required tension, immediately before the element is overboarded with the buoyancy module thereby clamped to it.

[0029] As laying of the elongate element continues, the buoyancy module is carried deeper in the water column and so experiences squeeze under increasing hydrostatic pressure. The resulting reduction in circumference or radial thickness of the buoyancy module would normally cause the strap to lose some tension. However, as the buoyancy module goes deeper, the increase in water pressure causes the tensioning device to contract, thus pulling on attached opposed ends of the strap to remove any slack in the strap. This maintains enough tension in the strap to keep the buoyancy module clamped to the underlying element, thereby mitigating any risk of the buoyancy module becoming loose and floating back to the surface. The device contracts by virtue of water under hydrostatic pressure being allowed to enter an expansion chamber on a rearward side of the piston, opposed to the compression chamber in the cylinder. The piston is therefore forced along the cylinder, hence retracting the shaft, because the air inside the compression chamber is at a substantially lower pressure than the incoming water. As the incoming water is under increasing hydrostatic pressure with greater depth, the piston will be forced further along the cylinder as the buoyancy module goes deeper. The compression chamber shortens with further compression of the air within.

[0030] In some instances, the tensioning device may comprise a first sealing mechanism arranged to maintain a watertight seal between the piston and the cylinder. The sealing mechanism may suitably correspond to a sealing disc that is located on the piston, for example on a face of the piston that opposes a seat against which the piston rests within the cylinder at the start of its stroke.

[0031] The tensioning device may additionally or alternatively comprise a locking mechanism arranged to lock the piston in at least one predefined locked position along the stroke of the piston within the cylinder. In some examples, the locking mechanism may suitably correspond to a ratchet mechanism comprising a plurality of ratchet teeth located along the shaft of the piston, and a ratchet lock located within the cylinder.

[0032] In some instances, the tensioning device comprises a second sealing mechanism arranged to maintain a watertight seal around a portion of a shaft of the piston. Specifically, the second sealing mechanism may be located within an aperture via and through which the shaft of the piston extends into the main body of the cylinder.

[0033] In summary, a method of promoting clamping engagement of a buoyancy module with an elongate subsea element such as a riser comprises actuating a tensioning device to apply a clamping force to sections of the buoyancy module in response to hydrostatic pressure acting on the buoyancy module or, therefore, on the tensioning device itself.

[0034] The clamping force may be applied via a fastening that holds together the sections of the buoyancy module, for example a strap that extends around those sections. The tensioning device can be actuated by admitting water under hydrostatic pressure into an expansion chamber defined within a cylinder. This drives a piston along a stroke within the cylinder against a volume of gas in a compression chamber in the cylinder, opposed to the expansion chamber across the piston. Movement of the piston thereby draws together the sections of the buoyancy module around the element as the tensioning device contracts.

[0035] To put the invention into context, reference has already been made to Figures 1a to 2b of the accompanying drawings, in which:

[0036] Figures 1a and 1b are schematic cross-sectional views of a buoyancy module of the prior art, clamped around a subsea riser by tension in a surrounding banding strap; and

[0037] Figures 2a and 2b are schematic side views of a dynamic umbilical riser of the prior art being installed underwater by a vessel.

[0038] In order that the invention may be more readily understood, reference will now be made, by way of example, to the remainder of the accompanying drawings in which:

[0039] Figures 3a and 3b correspond to Figures 1a and 1b but show a banding strap of the buoyancy module fitted with a tensioning device of the invention;

[0040] Figure 4 is a sectional side view of the tensioning device shown in Figures 3a and 3b;

[0041] Figures 5a and 5b are sectional side views showing operation of the tensioning device shown in Figure 4;

[0042] Figures 6a and 6b correspond to Figures 3a and 3b but show a variant of the invention; and

[0043] Figure 7a and 7b are sectional side views showing operation of a variant of the tensioning device shown in Figure 4.

[0044] Figures 3a and 3b show a strap assembly 26, comprising a tensioning device 28 of the invention disposed in line with a fastening in the form of a strap 16, encircling the annular body of a conventional buoyancy module 10. Once the strap 16 is tensioned, the strap assembly 26 clamps the two opposed sections 12 of the body onto an elongate element 14, exemplified here as a riser pipe. When the buoyancy module 10 is assembled around the elongate element 14 on the working deck of an installation vessel and hence at atmospheric pressure, opposed ends of the strap 16 are joined by the tensioning device 28 to complete the strap assembly 26. At that stage, the tensioning device 28 is in the fully extended state shown in Figure 3a and therefore has become inextensible, enabling the strap assembly 26 to be tensioned to clamp the buoyancy module 10 to the element 14. Consequently, the circumferential length of the strap assembly 26 substantially matches the outer circumference of the encircled buoyancy module 10. The buoyancy module 10 is then overboarded from the vessel during installation of the element 14 into a body of water.

[0045] When the buoyancy module 10 has been conveyed to depth and is therefore squeezed by hydrostatic pressure, the tensioning device 28 contracts to pull the ends of the strap 16 toward each other as shown in Figure 3b. In this way, the circumferential length of the strap assembly 26 reduces to compensate for shrinkage of the body of the buoyancy module 10. This maintains tension in the strap assembly 26 and so maintains the clamping force that enables frictional engagement of the buoyancy module 10 with the underlying element 14.

[0046] Figure 4 shows the tensioning device 28 in isolation. The device 28 comprises an elongate cylinder 30 that accommodates a piston 32 slidable within the cylinder 30 and thereby serves as an actuator. A series of piston rings 34 ensures that the piston 32 seals against the tubular side wall of the cylinder 30.

[0047] The cylinder 30 has closed ends, save for an aperture 36 in one end that accommodates a shaft 38. The shaft 38 is fixed to the piston 32 to move longitudinally with the piston 32 and protrudes from the cylinder 30 through the aperture 36. An annular shaft seal 40 within the aperture 36 forms a sliding seal against the shaft 38.

[0048] The piston 32 divides the interior of the cylinder 30 into a compression chamber 42 on one side of the piston 32 and an expansion chamber 44 on the other side of the piston 32. The chambers 42, 44 are sealed by the walls of the cylinder 30, save for a relief port 46 that communicates with the compression chamber 42 and a flood port 48 that communicates with the expansion chamber 44. The relief port 46 and the flood port 48 are shown here as penetrating the side wall of the cylinder 30 but could instead penetrate respective end walls of the cylinder 30. The relief port 46 can be closed selectively by inserting a removable relief plug 50 and the flood port 48 can be closed selectively by inserting a removable flood plug 52.

[0049] The tensioning device 28 has external formations for attaching the device 28 to the opposed ends of a strap 16 to form a strap assembly 26 like that shown in Figures 3a and 3b. In this example, the shaft 38 terminates at its outer end in a swivel padeye 54 and the end wall of the cylinder 30 at the opposite end of the device 28 has a fixed padeye 56. As shown in Figures 5a and 5b, the padeyes 54, 56 receive respective shackles 58 that are received, in turn, in respective loops 60 formed in the ends of the strap 16.

[0050] Figure 5a shows the shaft 38 of the tensioning device 28 fully extended, with the piston 32 bearing against a seat 62 that projects into the expansion chamber 44 from the end wall of the cylinder 30, around the aperture 36. This corresponds to the situation when the buoyancy module 10 is still aboard an installation vessel as shown in Figure 3a, where the strap assembly 26 is ready to be tensioned to clamp the sections 12 of the buoyancy module 10 around an elongate element 14.

[0051] The relief plug 50 is removed to open the relief port 46, hence allowing air to enter the compression chamber 42. This equalises the pressure of the air in the compression chamber 42 with ambient atmospheric pressure and reduces resistance to movement of the piston 32 as the piston 32 is drawn to the start of its stroke, bearing against the seat 62, under tension in the strap assembly 26. The relief plug 50 is then replaced, as shown in Figure 5a, to close the relief port 46 and to seal the compression chamber 42 with air trapped therein.

[0052] Figure 5a also shows the flood plug 52 removed to open the flood port 48. This can be done to allow air to exit the expansion chamber 44 as the piston 32 moves onto the seat 62, hence also reducing resistance to movement of the piston 32. In any event, the flood port 48 is opened to allow water to enter the expansion chamber 42 when the tensioning device 28 is submerged, as shown in Figure 5b corresponding to the situation shown in Figure 3b.

[0053] As hydrostatic pressure is greater than the ambient atmospheric pressure, water under hydrostatic pressure enters the expansion chamber 44 through the flood port 48 as shown in Figure 5b. Being substantially incompressible, the incoming water drives the piston 32 along the cylinder 30 as the expansion chamber 44 expands, causing the piston 32 to compress the air trapped in the compression chamber 42. Consequently, the piston 32 draws the shaft 38 into the cylinder 30, shortening the overall length of the tensioning device 28 and hence drawing together the ends of the strap 16 to compensate for hydrostatic shrinkage of the body of the buoyancy module 10.

[0054] The tensioning device 28 shown in Figures 3a to 5b is elegantly simple, operating passively to solve the shrinkage problem by using the same hydrostatic pressure that causes that problem. However, the broad principle of the invention could be realised in other ways, for example as shown by the tensioning device 64 of Figures 6a and 6b.

[0055] The tensioning device 64 shown Figures 6a and 6b comprises an actuator 66 that could be powered by means other than hydrostatic pressure, for example hydraulically, and whose operation could be controlled by sensing hydrostatic pressure. A power and / or control unit 68, which can include or be connected to a pressure sensor, is therefore shown here coupled to the actuator 66. The unit 68 could instead be located remotely from the actuator 66, and its power and control functions could be performed by separate units.

[0056] The unit 68 could also respond to sensor inputs other than, or in addition to, hydrostatic pressure, for example tension inputs. In this respect, a tension sensor 70 such as a strain gauge is shown optionally mounted on the strap 16. The tension sensor 70 can communicate with the unit 68 via a wired or wireless connection.

[0057] Many other variations are possible within the inventive concept. For example, valves on the relief port 46 and / or the flood port 48 of the tensioning device 28 could be opened and closed to perform the functions of the relief plug 50 and the flood plug 52 respectively. More generally, such valves could be used to control ingress or egress of air into or from the compression chamber 42 and / or water into or from the expansion chamber 44. For example, air could be expelled from the compression chamber 42 under the restrictive control of a valve on the relief port 46 as the piston 32 advances within the cylinder 30. Also, the flood plug 52 is optional as, in principle, it would be possible to leave the expansion chamber 44 open. Similarly, the relief port 46 is optional as, in principle, it would be possible to leave the compression chamber 42 closed if it always contains a volume of air or other gas whose pressure is below hydrostatic pressure.

[0058] In some examples, the tensioning device 28 may be modified to incorporate one or more additional sealing mechanisms, and / or an additional locking mechanism in association with the piston 32.

[0059] One such example is illustrated in Figures 7a and 7b; as the tensioning device 28 shown in these figures is very similar to that shown and described above in relation to Figures 5a and 5b, corresponding reference numerals have been used to indicate corresponding features that are present in both sets of figures. In the example of Figures 7a and 7b, a first additional sealing mechanism is positioned on an end 32a of the piston 32 proximate to the seat 62 against which the piston 32 bears at the beginning of its stroke. This first additional sealing mechanism helps to achieve a tight seal between the piston 32 and the inner walls of the cylinder 30; as well as between the piston 32 and the seat 62 prior to the beginning of the piston’s stroke. This minimises leakage of water and / or air around the piston 32 (e.g., between the compression chamber 42 and the expansion chamber 44).

[0060] In the example shown in Figures 7a and 7b, the first additional sealing mechanism is implemented in the form of a sealing disc 72. The sealing disc 72 extends across the end 32a of the piston 32, surrounding the shaft 38, and terminates in raised edges 72a that are in contact with the walls of the cylinder 30. A watertight seal is thereby achieved which minimises undesirable transfer of fluid (air and / or water) between the compression chamber 42 and the expansion chamber 44.

[0061] A second additional sealing mechanism is provided in the example shown in Figures 7a and 7b. This second sealing mechanism takes the form of a seal 74 that is located within the aperture 36 through which the shaft 38 extends to join the piston 32.

[0062] In some examples, the shaft 38 of the tensioning device 28, along with the corresponding portion of the cylinder 30 and aperture 36, can be modified to introduce an additional locking mechanism. This locking mechanism can be used to lock the piston 32 and shaft 38 in one or more predefined locked positions along the path taken by the piston 32 as it is moved along its stroke within the cylinder 30. In the example implementations of Figures 7a and 7b, the locking mechanism takes the form of a ratchet mechanism, which comprises a plurality of angled ratchet teeth 76 and a corresponding ratchet lock 78 arranged to engage with the ratchet teeth 76. The configuration of, and engagement between, the ratchet teeth 76 and the ratchet lock 78 allows substantially unidirectional movement of the shaft 38 (in the direction of the stroke of the piston 32), and prevents the piston 32 from travelling backwards within the cylinder 30. As such, the piston 32 can be maintained in at least one desired specific position within the cylinder 30 as it is moved along its stroke. The engagement provided between the ratchet teeth 76 and the ratchet lock 78 therefore locks the piston 32 and shaft 38 in place. Movement of the piston 32 over time (as a result of differential pressure, e.g., between the compression chamber 42 and the expansion chamber 44) is thereby reduced. This in turn ensures that the tensioning device 28 is able to maintain a desired position, and hence a desired amount of tensioning, over a prolonged period of time.

[0063] Whilst the tensioning devices described above act on a strap that encircles a buoyancy module, it would be possible instead for a tensioning device of the invention to act on another fastening such as a clasp or other device that holds together sections of a buoyancy module. Indeed, in principle, a tensioning device of the invention could act directly on at least one of those sections. By contracting in response to increasing hydrostatic pressure, the tensioning device will force the sections together to clamp around an elongate element even if the sections shrink under that pressure.

Claims

Claims1. A method of promoting clamping engagement of a buoyancy module with an elongate subsea element, the method comprising actuating a tensioning device to apply a clamping force to sections of the buoyancy module in response to hydrostatic pressure acting on the buoyancy module.

2. The method of Claim 1, wherein the tensioning device contracts from an extended state when actuated.

3. The method of Claim 1 or Claim 2, comprising applying the clamping force of the tensioning device via a fastening that holds together the sections of the buoyancy module.

4. The method of Claim 3, wherein the fastening comprises a strap that extends around the sections of the buoyancy module.

5. The method of Claim 3 or Claim 4, comprising actuating the tensioning device in response to a loss of tension in the fastening, that loss of tension being caused by hydrostatic pressure acting on the buoyancy module.

6. The method of Claim 5, comprising sensing tension in the fastening and actuating the tensioning device in response to a reduction in the sensed tension.

7. The method of Claim 1 or Claim 2, comprising applying the clamping force directly to at least one of the sections of the buoyancy module.

8. The method of any preceding claim, comprising actuating the tensioning device in response to hydrostatic pressure acting on the tensioning device.

9. The method of Claim 8, comprising actuating the tensioning device by admitting water under hydrostatic pressure into a cylinder of the tensioning device to drive a piston along a stroke within the cylinder, the piston thereby drawing together the sections of the buoyancy module around the element.

10. The method of Claim 9, comprising driving the piston against a volume of gas trapped in the cylinder, hence compressing the gas in response to said admission of water.

11. The method of Claim 10, wherein the gas is air, preceded by opening a relief port that communicates with a compression chamber within the cylinder to equalise the compression chamber with ambient atmospheric pressure before closing the relief port and then submerging the tensioning device with the buoyancy module.

12. The method of any of Claims 9 to 11 when dependent on Claim 3, wherein the piston acts on the fastening.

13. The method of Claim 12 when dependent on Claim 4, wherein the piston is connected to an end of the strap and the cylinder is connected to an opposite end of the strap.

14. The method of Claim 12 or Claim 13, preceded by tensioning the fastening when the tensioning device is extended, with the piston at a starting end of the stroke.

15. The method of any of Claims 9 to 14, preceded by opening a flood port that communicates with an expansion chamber within the cylinder before submerging the tensioning device with the buoyancy module and then admitting the water into the expansion chamber.

16. The method of any of Claims 9 to 15, comprising maintaining a watertight seal between the piston and the cylinder.

17. The method of any of Claims 9 to 16, comprising locking the piston in at least one predefined locked position within the cylinder.

18. A buoyancy module comprising two or more sections and a tensioning device arranged to apply a clamping force to the sections in response to hydrostatic pressure acting on the buoyancy module.

19. The buoyancy module of Claim 18, wherein the tensioning device is arranged to contract from an extended state when actuated.

20. The buoyancy module of Claim 18 or Claim 19, wherein the tensioning device acts via a fastening that holds the sections together.

21. The buoyancy module of Claim 20, wherein the fastening comprises a strap that extends around the sections.

22. The buoyancy module of Claim 20 or Claim 21, wherein the tensioning device is responsive to a tension sensor on the fastening.

23. The buoyancy module of Claim 18 or Claim 19, wherein the tensioning device acts directly on at least one of the sections.

24. The buoyancy module of any of Claims 18 to 23, wherein the tensioning device comprises a cylinder containing a piston that divides the cylinder into a gas-filled compression chamber and a floodable expansion chamber.

25. The buoyancy module of Claim 24 when dependent on Claim 20, wherein the piston acts on the fastening.

26. The buoyancy module of Claim 25 when dependent on Claim 21 , wherein the piston is connected to an end of the strap and the cylinder is connected to an opposite end of the strap.

27. The buoyancy module of any of Claims 24 to 26, wherein the tensioning device comprises a first sealing mechanism arranged to maintain a watertight seal between the piston and the cylinder.

28. The buoyancy module of Claim 27, wherein the first sealing mechanism is a sealing disc located on the piston.

29. The buoyancy module of any of Claims 24 to 28, wherein the tensioning device comprises a locking mechanism arranged to lock the piston in at least one predefined locked position within the cylinder.

30. The buoyancy module of Claim 29, wherein the locking mechanism is a ratchet mechanism comprising a plurality of ratchet teeth located along a shaft of the piston, and a ratchet lock located within the cylinder.

31. The buoyancy module of any of Claims 27 to 30, wherein the tensioning device comprises a second sealing mechanism arranged to maintain a watertight seal around a portion of a shaft of the piston, wherein the second sealing mechanism is located within an aperture through which the piston extends into the cylinder.

32. A tensioning device arranged to apply a clamping force to sections of a buoyancy module in response to hydrostatic pressure acting on the buoyancy module.

33. The device of Claim 32, being arranged to contract from an extended state in response to hydrostatic pressure.

34. The device of Claim 32 or Claim 33, in combination with a fastening that is arranged to hold the sections together.

35. The device of Claim 34, wherein the fastening comprises a strap for extending around the sections.

36. The device of any of Claims 32 to 35, comprising a cylinder containing a piston that divides the cylinder into a gas-filled compression chamber and a floodable expansion chamber.

37. The device of Claim 36 when dependent on Claim 34, wherein the piston acts on the fastening.

38. The device of Claim 37 when dependent on Claim 35, wherein the piston is connected to an end of the strap and the cylinder is connected to an opposite end of the strap.

39. The device of any of Claims 35 to 38, comprising a first sealing mechanism arranged to maintain a watertight seal between the piston and the cylinder.

40. The buoyancy module of Claim 39, wherein the first sealing mechanism is a sealing disc located on the piston.

41. The buoyancy module of any of Claims 36 to 40, wherein the tensioning device comprises a locking mechanism arranged to lock the piston in at least one predefined locked position within the cylinder.

42. The buoyancy module of Claim 41 , wherein the locking mechanism is a ratchet mechanism comprising a plurality of ratchet teeth located along a shaft of the piston, and a ratchet lock located within the cylinder.

43. The buoyancy module of any of Claims 39 to 42, wherein the tensioning device comprises a second sealing mechanism arranged to maintain a watertight seal around a portion of a shaft of the piston, wherein the second sealing mechanism is located within an aperture through which the piston extends into the cylinder.

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