Construction of buoyant elements comprising packed macrospheres

A method of filling a chamber with buoyant macrospheres places a mass of the spheres into a mould cavity. In the mould cavity, packing of the spheres is optimized to form an optimally-packed mass, followed by fixing the spheres in the optimally-packed mass to form a block. The block is then transferred from the mould cavity into the chamber while the spheres of the block remain fixed in the optimally-packed mass. This method enables the production of a buoyant element comprising an envelope defining an internal chamber that contains a mass of buoyant macrospheres each with an external diameter of at least 5 mm, packed with a packing factor of at least 50%.

This invention relates to the provision of buoyancy in subsea applications, using packed macrospheres to produce buoyancy modules or other buoyant elements.

The art of subsea engineering requires long-lasting buoyant elements that will resist being crushed under hydrostatic pressure in use. For many years, this need has been met by syntactic foams having a binary composite structure that comprises hollow rigid microspheres, micro-balloons or beads embedded in a rigid matrix. For example, buoyant elements of syntactic foam may be attached to or incorporated into a structure or apparatus used in the subsea oil and gas industry, such as a flowline, a riser, a pipeline bundle or an ROV.

Usually, the microspheres used in syntactic foams are of glass or ceramics, with a typical outer diameter of substantially less than 1 mm, say 10 μm to 200 μm. The matrix of such foams is usually of epoxy resin, polyester or wax. The microspheres have a specific gravity that is low enough and a volume fraction that is high enough, in bulk, to confer substantial positive buoyancy on a body of syntactic foam into which they are incorporated.

It is also known to use macrospheres in syntactic foams, those macrospheres having a typical outer diameter in a range from about 5 mm to about 50 mm but usually greater than 10 mm. Macrospheres are commonly made of glass, glass fibre or ceramics, like microspheres. In principle, however, metals and polymers such as epoxy resins may also be used to make macrospheres.

Syntactic foam is just one example of the use of a granular mass of buoyant spheres or beads to provide buoyancy. For example, buoyant spheres can be supported in a liquid, providing what is known in the art as ‘liquid buoyancy’, or surrounded by a gas. Buoyant spheres can be placed in a rigid buoyancy tank or in a bag.

Buoyant spheres typically have a rigid spherical wall or shell surrounding a hollow interior that contains a gas such as air or nitrogen. It is also possible for the interior of a buoyant sphere to contain a solid material that is light enough for the product to have positive buoyancy in seawater. For example, some macrospheres comprise a spherical core of lightweight polymer foam, around which a rigid shell of epoxy resin is applied as a coating. The shell of a macrosphere may include reinforcing elements such as glass fibres.

It is not essential that buoyant spheres or beads are perfectly spherical. However, substantial sphericity is an advantage to maximise resistance to crushing, to minimise material usage and to simplify packing.

By way of example, buoyant spheres of glass are sold in bulk by 3M™ as ‘Glass Bubbles’, in various grades. Being of glass, these spheres have the benefit of being virtually insoluble in seawater.

The exemplary dimensions of microspheres and macrospheres set out above are given for ease of understanding and are not intended to be limiting. In the context of the invention, the main distinction between microspheres and macrospheres is that the packing density or packing factor is more significant for macrospheres than for microspheres.

Specifically, macrospheres in a bulk mass will pack or nest against each other in various arrays, so that their positions tend to be determined to a substantial extent by the positions of adjoining macrospheres or arrays in the mass. This affects the packing factor, expressed as a percentage of the internal volume of an envelope or container that is occupied by macrospheres, which will depend upon how tightly the macrospheres are packed together. Conversely, microspheres are so fine-grained or even powdery in bulk that they become suspended in the bulk matrix. This means that their positions in the matrix are less dependent upon the positions of neighbouring microspheres.

In syntactic foams comprising a mixture of microspheres and macrospheres, the macrospheres pack against each other to leave interstitial voids between them; the microspheres and the matrix together fill those voids. However, it is possible for a syntactic foam to comprise only macrospheres embedded in a matrix that fills the interstitial voids but does not contain any microspheres.

It is also possible for a buoyant element to comprise macrospheres with substantially no matrix or other materials in the interstitial voids between them. Eliminating the matrix in this way may be desirable to reduce cost and to obviate creep or ageing of the matrix with time and subsea use. Creep that reduces the volume of the matrix is undesirable not only because it reduces buoyancy but also because it causes an unpredictable drift of buoyancy over time spent underwater.

Where there is no matrix, the macrospheres are held together as a granular bulk material in an external envelope that can be fastened to, or built into, a subsea structure or apparatus. The envelope preferably holds the macrospheres in a tightly-packed configuration to maximise the packing factor. For example, the envelope may apply inward compression to the mass of macrospheres to hold them together.

An envelope that contains macrospheres could be rigid and sealed to exclude seawater from its interior, so that buoyant upthrust will arise from displacement of seawater corresponding to the external volume of the envelope. In that case, the function of the packed macrospheres is to support the rigid envelope that surrounds them, and to preserve useful secondary buoyancy in the event that the envelope is ever flooded.

Preferably, however, the envelope has openings, holes, perforations or porosity so as to admit seawater into its interior, flooding the voids between the macrospheres. This means that the envelope does not need to resist hydrostatic pressure and so can be either rigid or flexible. As an example of this approach, JP 4983003 discloses a rigid tank containing macrospheres and makes provision for seawater to circulate around the macrospheres in the tank.

Where the interior of an envelope surrounding a mass of macrospheres is flooded, buoyant upthrust will arise from displacement of seawater corresponding to the aggregate external volume of the macrospheres within the envelope. Upthrust forces are transferred from the macrospheres to the surrounding envelope and from there to the subsea structure or apparatus to which the envelope is attached, or into which the envelope is incorporated.

The packing factor for macrospheres is important because it determines how many macrospheres, each of a particular external volume, can be incorporated within an envelope having a given internal volume. This therefore determines the aggregate buoyancy, or at least the secondary buoyancy, of a buoyant element comprising an envelope of a given size. Dense packing of macrospheres also improves the stability, and hence the load-bearing ability, of a mass of such macrospheres, as each macrosphere is held against movement by virtue of simultaneous contact with multiple other macrospheres in a three-dimensional array.

FIGS. 1 to 4illustrate various lattice systems, being examples of different ways in which substantially identical macrospheres10can pack together in a mass. The macrospheres are drawn as identical and may have substantially homogeneous size or may be of different sizes. Other lattice systems are possible in a packed mass, especially if the macrospheres in the mass vary in size. It will be noted that in the absence of an interstitial matrix, neighbouring macrospheres can come into direct contact with each other as shown.

FIG. 1shows a primitive cubic array12comprising eight macrospheres10. It will be apparent that the macrospheres10surround a large central void. If flooded by seawater in use, such large interstitial voids will significantly reduce the overall buoyancy of a given mass of macrospheres10packed loosely in this way. It will also be apparent that successive layers of the macrospheres10can slide past each other readily, which reduces the stability and hence the load-bearing ability of the mass.

In the body-centred cubic array14shown inFIG. 2, a central macrosphere10partially fills the void between eight other macrospheres10that are in a slightly enlarged cubic array. It will be apparent that these nine macrospheres10fit into a cuboidal volume only slightly larger than that of the primitive cubic array ofFIG. 1. The packing factor is correspondingly higher, which increases the buoyancy of a given volume of macrospheres packed in this way. Also, successive layers of the macrospheres interlock to increase the stability of the mass.

The face-centred cubic array16shown inFIG. 3and the hexagonal close-packed array18shown inFIG. 4further increase the packing factor and further improve the stability of a mass of macrospheres10packed in those ways.

Filling a volume with buoyant spheres can be achieved simply by pouring a mass of such spheres into that volume. For example, WO 94/04865 shows how buoyant spheres may be poured into a rigid housing. However, as the packing factor is not optimised, buoyancy is lost because large voids may remain between many of the spheres. This is also a problem for the aforementioned JP 4983003.

It will be apparent that the number of buoyancy-providing macrospheres that can be poured into a rigid and complex envelope is limited by the ways in which the macrospheres will pack together. It can be demonstrated that the volume occupied by a mass of substantially identical macrospheres will range from 34% to 74% of the internal volume of the envelope. The lower end of that range corresponds to the most loosely-packed diamond cubic configuration, which is not shown inFIGS. 1 to 4. The upper end of that range corresponds to the most closely-packed configurations shown inFIGS. 3 and 4.

In practice, the internal configuration of a poured mass of macrospheres is not homogenous or consistent throughout the mass. There will be a mixture of different lattice systems within such a mass, including many in less stable arrangements such as primitive cubic arrays.

Those skilled in the art know that shaking or vibrating a rigid envelope filled with a poured mass of macrospheres will settle the macrospheres into more stable arrays under the influence of gravity. This rearrangement of macrospheres helps to increase the packing factor. They also know that additional macrospheres should be poured into the envelope during vibration to compensate for shrinkage of the mass as the packing factor increases. For example, vibration of a poured mass may increase the packing factor from around the 52% that characterises a wholly primitive-cubic configuration shown inFIG. 1toward the 68% that characterises a wholly body-centered cubic configuration shown inFIG. 2. In practice, an overall packing factor of 60%±5% is typically achievable by vibrating an envelope that contains a poured mass of macrospheres.

Unfortunately, it is not always possible to vibrate an envelope effectively due to its size. For example, the envelope of a buoyant element may be defined by a large structure such as a tubular buoyancy member for a bundled flowline, which may be greater than 100 m in length. Consequently, pouring a mass of macrospheres into such an envelope will result in a low packing factor and correspondingly low aggregate buoyancy for a given size of envelope. This requires the structure to be made larger to achieve a desired degree of buoyancy, which makes an already large structure considerably more costly and more challenging to handle.

Another problem is that where there is no matrix or other binding material in the interstitial voids between packed macrospheres, the macrospheres are free to reorganize themselves after vibration has ceased. Consequently, when the envelope of a buoyant element is moved after being vibrated, the packing factor of the macrospheres within is likely to deteriorate. This is particularly problematic if the envelope is flexible.

A low packing factor is a significant problem where the unoccupied space within the envelope will be filled by seawater, as this increases the equivalent density of the buoyant element and hence reduces its buoyancy. Consequently, the envelope must be made larger than is ideal if it is to hold enough macrospheres to provide a desired degree of buoyancy.

Where buoyant spheres are held inside a rigid watertight envelope as disclosed in WO 94/04865, a low packing factor arising from large interstitial voids is less damaging to buoyancy because the voids will contain air or another gas, typically nitrogen. However, a rigid envelope containing gas cannot withstand high hydrostatic pressure, especially if the spheres within the envelope are not optimally packed. Strengthening the envelope to withstand such pressure would increase its weight and hence reduce the aggregate buoyancy of a buoyant element comprising such an envelope.

In JP 4826215, the envelope is a rubber bag. Here, again, filling is not optimal and the bag can collapse under high hydrostatic pressure. In the alternative of liquid buoyancy as disclosed in GB 2517511, buoyant spheres are submerged in a liquid. Again, optimal packing is required to maximise buoyancy because the liquid is denser than the spheres: the surrounding envelope should contain as many of the spheres and as little of the liquid as possible.

U.S. Pat. No. 3,703,012 teaches that the shape of the envelope of a buoyant element may be designed for optimum packing with buoyant spheres. However, such idealised envelope shapes cannot necessarily be used in practice. Nor is such a solution useful for flexible envelopes such as bags.

U.S. Pat. No. 3,773,475 describes filling and bonding of spheres inside a rigid envelope to produce a structural member. The spheres are individually pressurised internally and are heated to expand, distort into non-spherical shapes and bond together within the envelope. This solution is not practical where the envelope is too large to be heated or is of a material that would be damaged if it was heated to a temperature sufficient to soften the spheres.

WO 2014/145027 teaches preparing a syntactic wax or oil by mixing microspheres into a matrix of wax or oil. The matrix is kept at a temperature that is high enough to be in a liquid or pliable state for insertion into a chamber such as a hollow structural member. The wax or oil is then allowed to cool to ambient temperature so that it solidifies to create a buoyant element of a solid syntactic material. However, there is no teaching of optimising packing of buoyant macrospheres; as noted above, microspheres suspended in a matrix do not pack together in any meaningful way. Also, the mechanical strength of the solid syntactic material may be low, and the matrix is prone to creep and degradation with use and time.

WO 2003/074598 relates to a method of manufacturing low-density syntactic foam containing microspheres. A mixture of microspheres and liquid phase binder are placed in a mould. The microspheres are naturally buoyant in the binder and so float to the upper surface of the mould and self-pack in a layer. Excess liquid phase binder is then drained to leave a packed layer of microspheres that subsequently hardens into a close packed syntactic foam layer. As already noted, microspheres that are suspended in a matrix do not pack together optimally.

JP 2007126060 discloses a flexible buoy for use on mooring cables. The buoy comprises a flexible outer shell that houses a mass of spherical buoyant bodies. A through-hole in the outer shell allows seawater to flood the buoy in use. This equalises hydrostatic pressure and so allows the outer shell to be kept thin. There is no provision to optimise the packing density of the spherical buoyant bodies.

EP 2845792 describes a buoyancy module for use in subsea applications. The buoyancy module contains a buoyant fluid that comprises a base fluid, a plurality of microspheres, and an activator that acts as a setting agent for the buoyant fluid.

US 2013/251957 teaches a buoyancy module that includes hollow cylindrical tubes surrounded by a mixture of macrospheres and syntactic foam.

Against this background, the invention provides a method of filling a chamber with buoyant spheres, such as macrospheres having an external diameter of at least 5 mm. The method comprises: placing a mass of the spheres into a mould cavity; in the mould cavity, optimising packing of the spheres to form an optimally-packed mass and then fixing the spheres in the optimally-packed mass to form a block; and transferring the block from the mould cavity into the chamber while the spheres of the block remain fixed in the optimally-packed mass.

A block may be stored before being placed into the chamber. Preferably the chamber is substantially filled with one of more of the blocks.

The spheres are preferably fixed by holding them together in the mould cavity with a matrix material that may embed the spheres. The matrix material may be introduced into the mould cavity in a liquid phase and then transformed in the mould cavity into a solid phase, for example by freezing the matrix material. More generally, optimising packing of the spheres may take place at a first temperature and fixing the spheres may take place at a second temperature lower than the first temperature. Preferably, optimising packing of the spheres begins or takes place before introducing the matrix material into the mould cavity.

The spheres of the block may be unfixed after the block is placed in the chamber. This may be achieved by removing a matrix material of the block from the chamber, for example by liquefying the matrix material and draining the liquefied matrix material from the chamber. Conversely, voids in the chamber may be flooded in use to surround the spheres with water.

The unfixed spheres may be constrained to maintain substantially optimal packing of the spheres in the chamber, for example with at least one barrier placed in or against the block.

The chamber may be defined by a flexible or rigid envelope. Preferably the mould cavity is shaped to match the chamber. For example, the mould cavity may be tubular to produce a cylindrical block and the chamber may be defined by the interior of a pipe into which one or more of those blocks are inserted longitudinally through an open end.

The spheres may be packed around or beside an insert in the mould cavity, and may be constrained with at least one barrier supported by the insert.

The inventive concept embraces a buoyant element comprising an envelope defining an internal chamber that contains a mass of buoyant macrospheres each with an external diameter of at least 5 mm, packed with a packing factor of at least 50%.

The macrospheres are suitably substantially homogeneous in size throughout the mass. Voids between the macrospheres in the mass are preferably in fluid communication with one or more openings that penetrate the envelope, which may be flexible or rigid and could be defined by a pipe.

At least one barrier may subdivide the chamber and constrain movement of the macrospheres within the chamber. The macrospheres may be packed around or beside at least one insert, which may conveniently support at least one such barrier.

Thus, preferred embodiments of the invention solve the problems of the prior art by exploiting a phase transition of a matrix material, such as water, from liquid to solid. Specifically, macrospheres are poured into a mould cavity whose shape is preferably adapted to the desired shape of a buoyancy chamber, such as may be defined by an envelope of a buoyancy module. The packing factor of the mass of macrospheres is increased, typically by vibrating the mass. Next, the mould is closed and water is introduced into the mould cavity and frozen solid to mould or cast a block.

The resulting block of ice containing packed macrospheres is demoulded and optionally stored in a cold room or refrigerated container, which may be moved to another location. The block is then installed in a receiving chamber that may be defined by an envelope such as a tube or pipe into which the block is pushed, before the ice is melted to revert to liquid water that flows out of the envelope. This leaves the macrospheres in the receiving chamber, retaining the increased packing factor achieved in the mould cavity without necessarily having any ice or other matrix material left between them.

The invention is especially suitable for use with a bundled flowline comprising a hollow polymer or composite buoyancy pipe filled with buoyant spheres. The pipe may have holes to allow melt water or other liquefied matrix materials such as molten hydrocarbons to flow out of the pipe.

In summary, embodiments of the invention may involve firstly optimising packing of buoyant spheres or beads in a rigid container or chamber, for example by vibration. The container may already contain a liquid or a liquid may be introduced into the container after vibration. Then the phase of the liquid is changed to a solid to fix the buoyant spheres in an optimally-packed arrangement in which the container is preferably more than 60% or even more than 70% full of spheres. A refilling circuit may communicate with the container to continue adding spheres to the container as the packing factor increases. Vibration may continue while the liquid is being solidified.

The resulting solid block is transferred directly or indirectly from the container into a final container, chamber or envelope, for example via a rigid support. The envelope can be rigid or flexible, and preferably allows water to enter voids between the buoyant spheres in use. Such voids may be created by causing or allowing the solidified liquid to revert to the liquid phase by melting in the final container or envelope, whereupon the liquid can drain away.

Thus, these embodiments provide a method for optimising buoyancy, comprising: filling a container with buoyant spheres, preferably macrospheres; filling the container with a fluid at a first temperature; packing the spheres and adding as many spheres as required for optimum packing; closing the container; applying a second temperature, lower than the first temperature, so that the fluid solidifies; and at this second temperature, opening the container and transferring solid blocks containing the spheres to be placed into a final container, such as an envelope of a buoyancy module. The envelope may be flexible, such as a bag, or relatively rigid, such as a pipe.

The first temperature may be ambient temperature, for example 5° C. to 30° C., if the filling fluid is water. Alternatively, the first temperature may be a higher temperature, for example greater than 35° C., if the filling fluid is a molten hydrocarbon such as a wax, paraffin or vegetal oil. The second temperature may be below 0° C. if the filling fluid is water or may be below, say, 20° C. if the filling fluid is a hydrocarbon such as wax, paraffin or a vegetal oil.

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:

FIG. 5is a schematic sectional side view of a mould for use in accordance with the invention, in the process of being filled with poured macrospheres;

FIG. 6corresponds toFIG. 5but shows the mould now filled with macrospheres loosely packed in a primitive cubic array;

FIG. 7corresponds toFIG. 6but shows the mould now being vibrated to settle the macrospheres into a more densely-packed body-centred cubic array, as additional macrospheres are poured in to fill the space thus created in the mould tool;

FIG. 8corresponds toFIG. 7but shows the mould still being vibrated and now filled with macrospheres packed densely in a body-centred cubic array;

FIG. 9corresponds toFIG. 8but shows voids between and around the densely-packed macrospheres filled with a matrix formed of a liquid or a fluidised solid that has been injected or poured into the mould;

FIG. 10corresponds toFIG. 9but shows the matrix in the voids between and around the densely-packed macrospheres now solidified to form a solid block comprising the packed macrospheres embedded in the solid matrix, which block is being removed from the mould;

FIG. 11is a schematic sectional side view of a storage facility for holding blocks like that shown inFIG. 10;

FIG. 12is a schematic sectional side view of a block like those shown inFIGS. 10 and 11placed within a close-fitting flexible envelope in accordance with the invention;

FIG. 13corresponds toFIG. 12but shows the densely-packed macrospheres left in the envelope after the matrix component of the block has been removed from within the envelope by liquefaction and drainage;

FIG. 14is a schematic sectional side view of solid blocks like those shown inFIGS. 10 and 11being inserted into a close-fitting rigid envelope in accordance with the invention;

FIG. 15is a schematic sectional side view showing the envelope now closed around the blocks within;

FIG. 16corresponds toFIG. 15but shows the densely-packed macrospheres left in the envelope after the matrix component of the blocks has been removed from within the envelope by liquefaction and drainage;

FIG. 17corresponds toFIG. 10but shows a variant in which macrospheres have been packed around an insert in the mould to form a solid block that incorporates the insert;

FIG. 18corresponds toFIG. 16but shows a mass of macrospheres surrounding inserts as shown in the variant ofFIG. 17, placed within a close-fitting rigid envelope;

FIG. 19is a schematic side view of a variant of the inserts shown inFIGS. 17 and 18, modified to retain the macrospheres with a packing factor similar to that of the blocks; and

FIG. 20corresponds toFIG. 18but shows a mass of macrospheres surrounding inserts as shown in the variant ofFIG. 19, again placed within a close-fitting rigid envelope.

Referring next, then, toFIGS. 5 to 10, this sequence of drawings features a mould20for use in accordance with the invention. Features in common between these drawings will be described first before the drawings are discussed in turn to describe a method of the invention.

The mould20comprises a hollow body22that defines an internal mould cavity24. The body22comprises a closure26that is movable, or removable, to allow access to the mould cavity24for demoulding its solidified contents as a solid block28after use of the mould20, as is shown inFIG. 10.

The size and shape of the mould cavity24is preferably chosen to produce a block28that will suit the size and shape of an envelope of a buoyant element into which the block will be placed, as will be explained later with reference toFIGS. 12 to 16. For example, the mould cavity24may be tubular with a circular section to produce a cylindrical block28that matches the internal size and shape of a pipe that will serve as an envelope. Alternatively the mould cavity24may be generally cuboidal if correspondingly-shaped blocks28will better suit the shape of the envelope in question.

An intake opening30, optionally communicating with an external hopper32, penetrates an upper wall of the body22to receive bulk macrospheres10that are poured from a source34into the mould cavity24. These schematic drawings are not to scale; the macrospheres10shown in the drawings are greatly enlarged relative to the mould cavity24for ease of illustration.

A fluid inlet36also penetrates a wall of the body22. As shown inFIG. 9, the fluid inlet36introduces a matrix material38into the mould cavity24from a reservoir40under the control of an inlet valve42. The matrix material38is initially in a fluid form and is preferably initially in the liquid phase, such as water or a molten wax such as paraffin wax. The matrix material38may be poured into the mould cavity24under gravity and/or be injected into the mould cavity24under pressure.

While it remains fluid, the matrix material38floods and fills voids between and around the macrospheres10that were poured previously into the mould cavity24. The matrix material38then solidifies to fix the macrospheres10relative to each other and to form a solid block28that can be removed from the mould cavity24as shown inFIG. 10.

In this example, transformation of the matrix material38from the liquid phase to the solid phase may be driven by reducing the temperature of the matrix material38to freeze it.

Where phase change of the matrix material38is driven or promoted by a change in temperature, an optional temperature-management system comprises heat-transfer elements44for cooling and/or heating the mould cavity24. For this purpose, the elements44are in thermal contact with walls of the body22that surround the mould cavity24, for example by being embedded in the walls as shown inFIGS. 5 to 10.

A vibratory apparatus acts on the mould20to increase the packing factor of microspheres10in the mould cavity24. The vibratory apparatus is exemplified here as a shaker table46that supports the mould20.

Having been discussed collectively so far,FIGS. 5 to 10will now be discussed individually and in turn.

For ease of illustration,FIGS. 5 and 6represent the low packing factor of poured macrospheres10by, artificially, showing all of the macrospheres10in the mould cavity24packed in a primitive cubic array like that ofFIG. 1. It will, of course, be appreciated that in reality there will be a random mixture of different lattice systems in the mass of macrospheres10.

FIG. 5shows macrospheres10being poured in bulk under gravity from the source34into the external hopper32, from which the macrospheres10fall into the mould cavity24through the intake opening30. As a mass of them fills the mould cavity24, the macrospheres10will fall randomly into a mixture of different lattice systems. The result is that the packing factor of the mass of macrospheres10is initially substantially lower than desired and hence is sub-optimal.

FIG. 6shows the mould cavity24now full of a poured mass of macrospheres10. Like those ofFIG. 5, the macrospheres10in that mass are packed with the sub-optimally low packing factor that results from pouring.

FIG. 7shows the shaker table46now activated to vibrate the mould20. This causes the macrospheres10in the mould cavity24to settle together into more stable lattice systems, hence increasing the packing factor of the mass. The overall volume of the mass of macrospheres10therefore decreases as shown inFIG. 7, allowing further macrospheres10flowing from the source34to fall from the hopper32through the intake opening30and into the mould cavity24.

Eventually the mould cavity24is filled by a mass of macrospheres10as shown inFIG. 8, now packed with a higher packing factor that is considered optimal. In this respect, it should be noted that optimisation does not require that the packing factor is perfect. In practice, a shaken mass of macrospheres10cannot achieve an ideal packing factor but can merely approach it. Optimisation by vibration is therefore a process of improvement rather than perfection.

For ease of illustration,FIGS. 7 and 8represent the higher packing factor of the poured macrospheres10by, artificially, showing all of the macrospheres10in the mould cavity24packed in a body-centred cubic array like that ofFIG. 2. Again, it will be appreciated that in reality there will still be a mixture of different lattice systems in the mass of macrospheres10. However, the overall packing factor of the mass will be higher than before the mass was vibrated, as shown inFIG. 6.

Turning next toFIG. 9, a matrix material38, exemplified here by liquid water, is poured into the mould cavity24to flood and fill the voids between and around the macrospheres10in the mould cavity24, which remain in the optimally-packed configuration shown inFIG. 8. When the inlet valve42is opened, the matrix material38flows from the reservoir40and enters the mould cavity24through the fluid inlet36that penetrates a wall of the body22of the mould20.

In this example, if the matrix material38is liquid water, the phase of the matrix material38is changed from liquid to solid by lowering the temperature within the mould cavity24to below 0° C. This freezes the water to hold the macrospheres10embedded in a solid block of ice. Conveniently, the necessary reduction in temperature can be achieved by passing a refrigerant fluid through pipes embedded in walls of the body22that serve as the heat-transfer elements44. The skilled reader will understand the ancillary compressor and evaporator arrangements that would be necessary to effect heat transfer in this example, which therefore need no elaboration.

If the matrix material38is a wax that is solid at typical ambient temperatures, the heat-transfer elements44could instead be heating elements to keep the wax in a flowable molten state until the voids between and around the macrospheres10have been flooded. Then, the heat-transfer elements44can be switched off to allow the wax matrix material38to cool and solidify as it approaches ambient temperature. Examples of heating elements are pipes for carrying hot water or steam, or electric resistance elements. Some heat-transfer elements44could be switchable to a cooling mode to accelerate or control cooling and solidification of the wax, for example pipes that can carry both hot and cold water.

FIG. 10shows the closure26removed from the remainder of the body22to allow demoulding of the solidified contents of the mould cavity24as a solid block28.

Although omitted fromFIG. 10for simplicity, the mould20could include a pusher such as a ram to push the block28out of the mould cavity24; similarly, a puller could be used instead or additionally to pull the block28out of the mould cavity24.

Once removed from the mould cavity24, the block28may be stored temporarily with other blocks28in a storage facility48as shown inFIG. 11. If the block28comprises ice that will melt at ambient temperature, the storage facility48must be refrigerated and preferably insulated to maintain the integrity of the blocks28.

Next, one or more of the blocks28are placed within a close-fitting submersible envelope to produce a buoyancy module in which the macrospheres10will be held substantially in an optimised closely-packed arrangement for use. In this respect,FIGS. 12 and 13show a single block28placed into a flexible envelope50, whereasFIGS. 14 to 16show multiple blocks28placed into a rigid envelope52such as a pipe of polymer, composites or thin-walled steel. It would of course be possible instead to place a single block28into a rigid envelope52or to place multiple blocks28into a flexible envelope50.

InFIG. 12, a buoyancy module54comprises a single block28disposed within a close-fitting flexible envelope50such as a bag made of a reinforced flexible polymer.

The envelope50is penetrated by holes56for drainage and for equalisation of hydrostatic pressure. The envelope50may be substantially inextensible to confine the mass of densely-packed macrospheres10of the block28or may contract resiliently around the macrospheres10to apply inward pressure to that mass.

Initially, the ice or wax forming the matrix material38of the block28is at a temperature low enough for the ice or wax to remain solid. That temperature will be below 0° C. for ice or a typical ambient temperature for wax. The temperature of the block28is then raised, or allowed to rise, until the block28reaches a melting temperature of above 0° C. for ice or typically above about 40° C. for wax. The ice or wax then melts and runs out of the envelope50through the holes56. This leaves the macrospheres10packed densely in the envelope50as shown inFIG. 13.

Turning next toFIG. 14, a pipe that serves as a rigid envelope52is shown being loaded with multiple cylindrical blocks28that fit closely in series into the interior of the pipe. A ram58exemplifies a pusher that is arranged to push the blocks28successively into the envelope52through an open end60, like pistons; the other end62of the envelope52may be open or may be closed as shown.

When the envelope52is full, both of its ends60,62are closed as shown inFIG. 15to create a buoyancy module64. Next, the temperature of the blocks28is raised, or allowed to rise, until the ice or wax of the blocks28reaches an appropriate melting temperature. The ice or wax then melts and runs out of the envelope52through holes56that penetrate the envelope52for drainage and for equalisation of hydrostatic pressure. This leaves the macrospheres10packed densely in the envelope52of the buoyancy module64, as shown inFIG. 16.

The holes56that penetrate the envelope52are shown oversized in the drawings for ease of illustration. In reality, the holes56should of course be smaller than the macrospheres10to stop the macrospheres10spilling out of the envelope52.

Moving on now toFIGS. 17 and 18, these show how macrospheres10may be packed around or beside an insert66to form a solid block68that incorporates the insert66. The insert66may be hollow, such as a polymer tube, or solid, such as a rod or other body of syntactic foam. The insert66may, for example, be elongate and may extend along a central longitudinal axis of the block68as shown. In this example, an elongate insert66is embedded in the block68by being surrounded with an annular mass of macrospheres10.

The insert66reduces the number of macrospheres10that are required to form the block68, hence reducing the loss of macrospheres10if a surrounding envelope50,52is breached and a spillage results. The insert66also helps to maintain a favourable packing factor when the matrix material38of the block68has been liquefied and drained out of the envelope50,52. For this purpose, the insert66may be shaped or textured to engage or retain the adjoining macrospheres10.

As in the preceding embodiment, the packing factor of the macrospheres10shown inFIGS. 17 and 18has been optimised by vibrating the mould20after pouring the macrospheres10into the mould cavity24around the insert66. Also, as before, the optimised packing factor has been fixed by solidifying a liquid matrix material38such as water or molten wax around and between the macrospheres10to form the block68, as shown inFIG. 17.

The block68is then ready for storage or for insertion into a surrounding envelope50,52. In this respect,FIG. 18shows a buoyancy module70containing a mass of optimally-packed macrospheres10surrounding a series of inserts66within a pipe that serves as a rigid envelope52. The inserts66and the surrounding macrospheres10shown inFIG. 18were part of multiple blocks68previously inserted into the envelope52. The matrix material38of the blocks68was then liquefied and drained out of the envelope52through the holes56to leave the inserts66and the macrospheres10behind.

Turning finally toFIGS. 19 and 20, these drawings show the possibility of equipping a block72of macrospheres10with one or more restraints or barriers74that hold the macrospheres10together in a densely-packed arrangement. In this example, the barriers74are end plates at respective ends of the block72, supported by respective ends of the insert66. However, it would be possible instead to support such barriers74using other joining members as spacers, whether or not an insert66is present.

When a series of blocks72equipped with barriers74are inserted into an envelope52to make a buoyancy module76as shown inFIG. 20, the barriers74create a series of longitudinally-spaced transverse partitions that define smaller packed compartments within the envelope52. This is useful in the case of very long envelopes52, such as pipes that could be several kilometres long. In such applications, the matrix38of the first blocks72to be inserted could start to melt before the ends60,62of the envelope52are closed to hold the macrospheres10within.

Thus, the partitions created by the successive barriers74help to ensure that most of the mass of macrospheres10in the envelope52will remain near-optimally packed, even in the case of partial failure of blocks72or leakage of macrospheres10at one end of the envelope52. Thus, the barriers74cooperate with the envelope52to contain the macrospheres10but do not prevent the interior of the envelope52flooding with seawater when the buoyancy module76is in use.

A rigid outer envelope52could be filled with a succession of floodable inner envelopes, each containing optimally-packed macrospheres10, either with or without a matrix38still fixing the macrospheres10. If the matrix38has been removed, the inner envelope can fix the macrospheres10by constraining them in an optimally packed arrangement.

It will be apparent that end walls of such inner envelopes would serve as barriers to perform the retaining function of the barriers74ofFIGS. 19 and 20, similarly forming a series of transverse partitions within the outer envelope52. The inner envelopes would be shaped to fit closely within the interior of the outer envelope52and could be either rigid or flexible like the envelopes50shown inFIGS. 12 and 13.

Barriers to retain the macrospheres10in a block may be moulded into the block in the mould20or may be placed around the block after moulding.

Several variants of the invention have been described above. Many other variations are possible within the inventive concept. For example, vibration of the mould20may take place continuously or intermittently throughout the operation of pouring macrospheres10into the mould cavity24. Vibration of the mould20may continue until the mould cavity24is full of macrospheres10, while the matrix material38is being introduced into the mould cavity24and even after the matrix material38has filled the mould cavity24. Similarly, steps to solidify the matrix material38, such as heat transfer to effect cooling, could be initiated while the mould cavity24is still being filled. However, to avoid displacing optimally-packed macrospheres10, it is preferred that vibration of the mould20to optimise the packing factor is completed before introducing the matrix material38into the mould cavity24.

The matrix material38could initially be another flowable fluid material such as a gel, a paste or a suspension, or a fluidised solid such as a blown powder. Other than freezing the matrix material38by reducing its temperature, transformation of the matrix material38from the liquid phase to the solid phase may be driven by other processes depending upon the nature of the material. Such processes may include baking or sintering the matrix material38by the application of heat and/or pressure, or curing or otherwise setting the matrix material38. Heat-transfer elements44arranged to heat the mould cavity24may be used to promote solidification in any manner appropriate to the matrix material38that is chosen.

Subsequent liquefaction of the solidified matrix material38can also be achieved in various ways other than melting, including disintegration or dissolution in a solvent such as water, or by chemical attack. For example, the matrix material38could be a solid that dissolves in seawater, such as common salt.

In many applications of the invention, it is preferred that the macrospheres10are not bound or attached together within the surrounding envelope50,52. This allows macrospheres10to be removed from the envelope50,52if it is ever necessary to adjust buoyancy. In other applications of the invention, however, removal of macrospheres10may not be necessary. In that case, it is not essential for the matrix material38to be removed entirely or even partially from between and around the macrospheres10before a buoyancy module54,60of the invention is used.

By fixing the macrospheres10in an optimally-packed arrangement, the invention maximises the buoyant upthrust of the buoyancy module54,60such that any reduction in buoyancy due to degradation of the matrix material38is less significant. Thus, in principle, the matrix material38could be a polymer like those used in syntactic foams. However, it is preferred to avoid the use of such costly and dense polymers so as to reduce the cost and to improve the buoyancy of the buoyancy module54,60.

The macrospheres10need not be fixed in a fully embedding solid matrix material38but could instead adhere to adjoining, contacting macrospheres10to become fixed into a solid but porous mass. In such examples, a matrix material interposed between the macrospheres10at their points of mutual contact will form a discontinuous matrix containing floodable voids.

For example, macrospheres10could be coated with an adhesive layer that does not hinder the flow of macrospheres10into or within the mould cavity24and that is activated only after the packing factor has been optimised by vibration. Activation of such an adhesive layer could, for example, be achieved by heating, cooling, pressure and/or exposure to water or other activating fluids. After fixing the optimally-packed macrospheres10for transfer into an envelope50,52, the adhesive could be soluble in seawater to free the macrospheres10within a buoyancy module54,60when the module54,60is submerged and flooded in use.

It is also possible for the macrospheres10to be moistened with, or otherwise coated by, a thin layer of water or other liquid such as a molten wax. Again, such a liquid layer would not hinder the flow of macrospheres10into or within the mould cavity24.

However, such a layer will effect adhesion between adjoining macrospheres10when it is solidified into ice or solid wax by cooling.

Depending upon the matrix material38, the macrospheres10may be heated or cooled before and/or during their residence in the mould cavity24. For example, pre-cooling macrospheres10to below the freezing temperature of the matrix material38may be helpful to promote solidification of the matrix material38when it is injected subsequently into the mould cavity24. Where the matrix material38is ice, ice may form on pre-cooled macrospheres10to hold them together on introduction of water into the mould cavity24in liquid or vapour form, noting that water vapour will condense on cold surfaces.

Pre-heating macrospheres10to above the freezing temperature of the matrix material38may be helpful to delay solidification of the matrix material38when it is injected subsequently into the mould cavity24, especially if the matrix material38is a hydrocarbon such as a wax.