Patent ID: 12195144

DETAILED DESCRIPTION OF THE INVENTION

Referring firstly toFIGS.1and2, these schematic drawings show that buoys10of the invention may have various outer shapes. The buoys10shown here have conventional shapes, exemplified by a cylindrical buoy10inFIG.1, which is rotationally symmetrical about a central longitudinal axis12, and a cuboidal buoy10inFIG.2. The invention also facilitates the manufacture of buoys with other shapes, such as the part-annular buoyancy elements shown inFIG.5.

Fixings or formations for attaching the buoys10to other subsea elements or structures, such as chains or pipelines, have been omitted from these simplified drawings.

Each buoy10comprises an external shell14that is a pressure-resistant continuous hollow outer shell of a rigid material, preferably of metal and more preferably of aluminium. The shell14is formed integrally with, and supported by, a foraminous internal structure16that is shown schematically inFIG.2.

The shell14and the internal structure16are formed together by an additive manufacturing process, such that the shell14surrounds the internal structure16completely and continuously within an internal space17that is hollow, sealed and fully enclosed by the external shell14. The shell14thereby resists hydrostatic pressure and water ingress while being supported against collapse by the internal structure16.

The internal structure16comprises a rigid structural frame18, also preferably of metal such as steel or aluminium, and more preferably of aluminium, that is formed integrally with the shell14during the additive manufacturing process. The frame18defines multiple internal voids or cavities20.

Thus, in a preferred embodiment the shell14and the frame18are both formed of aluminium. In other embodiments different materials may be used for the shell11and the frame18to achieve a desired balance between structural strength and density. For example, the frame18may be of aluminium and the shell14of steel.

The cavities20may be discrete spaces that are separated or isolated from each other as shown here, or may instead be conjoined, interconnected or intercommunicating with each other. Thus, the frame18may surround each cavity20continuously, in the manner of a matrix in which the cavities20are embedded like individual bubbles, pores or spheres in a rigid or syntactic foam. Alternatively, the cavities20may surround or contain structural members21that define the frame18, in the manner of a skeleton or a spaceframe comprising multiple trusses33.

The cavities20contain a gas, such as air, nitrogen or a noble gas, which may be at atmospheric or elevated pressure. Where the cavities20contain a gas at elevated pressure, that pressure may suitably be selected to counterbalance, at least partially but not necessarily fully, the hydrostatic pressure expected at the operational depth.

The cavities20together define an aggregate gas-filled volume that offsets the weight in seawater of the shell14and the frame18. Thus, the volume of water displaced by the shell14must weigh substantially more than the weight in water of the shell14and the frame18, plus the weight of gas trapped within the cavities20. That excess displacement defines the net positive buoyancy or upthrust that will be provided by the buoy10when it is submerged fully in seawater in use.

Throughout the appendant drawings, the frame18and the cavities20are not shown to scale. In practice, the cavities20may be smaller, more numerous and closer together than is shown. Thus, the webs or members21of the frame18occupying the interstices between, or extending through, the cavities20will be thinner and hence lighter than is shown. It will of course be appreciated that the frame18should be as light as possible and that as much as possible of the internal space within the shell14should be devoted to the cavities20, This ensures that the buoys10can be as compact and inexpensive as possible for a given level of upthrust, consistent with maintaining the necessary degree of resistance to hydrostatic pressure.

Turning next toFIGS.3and4, these drawings illustrate advantages of the invention, as exemplified inFIG.4, over an equivalent prior art arrangement as shown inFIG.3. Like numerals are used for like features.

FIG.3shows a detail of a buoy22in which cavities20are defined by a closely-packed mass of spheres24as known in prior art such as the aforementioned WO 2016/128884. The spheres24may be classified as microspheres or macrospheres, depending upon their diameter. They are held together in a surrounding housing, which is not shown, to transfer the upthrust of their collective buoyancy to that housing.

It will be noted that interstitial spaces26are left around and between the spheres24, no matter how closely-packed the spheres24may be. The interstitial spaces26are typically flooded with seawater in use and so do not contribute to the net upthrust of the buoy22. Each individual sphere26is therefore subjected to hydrostatic pressure while the surrounding housing is not. The spheres24therefore need thick and heavy walls to resist collapse. Similarly, there are double wall thicknesses between adjacent cavities20defined by neighbouring spheres24. This increases the amount of negatively-buoyant material within the housing and therefore further reduces the net upthrust of the buoy22for a given size and hence external displacement.

If the spheres24are instead contained within a sealed rigid housing or shell14like that shown inFIGS.1and2, the interstitial spaces26could be instead filled with a gas such as air so as to contribute to the net upthrust of the buoy22. However, in that case, the intercommunicating interstitial spaces26would all flood if the shell14is ever breached and therefore there would be a risk of a catastrophic and sudden loss of buoyancy in the event of even minor damage to the buoy22. Also, a mass of spheres24presents an uneven or undulating outer surface that provides ineffective support to a surrounding shell14.

By contrast, the internal structure16of the invention shown inFIG.4illustrates that, by virtue of additive manufacturing, ellipsoidal or spherical cavities20of a similar size to those shown inFIG.3may be defined with less negatively-buoyant material in the continuous matrix between them that serves as the frame18. In particular, there is no need for a double wall thickness between adjacent cavities20because a single wall thickness is instead shared between those cavities20. Nor are there any floodable interstitial spaces between the cavities20and consequently there is no need or risk of flooding. Also, as will be explained, the cavities20can be sized, positioned and distributed in the matrix to provide optimal support for the surrounding shell14that is shown inFIGS.1and2.

FIG.5shows a further advantage of making buoys of the invention in a single additive manufacturing process, which is to produce a seamless structure in one operation with any desired outer shape. In this respect,FIG.5shows part-annular buoyancy elements28of the invention being assembled together around a subsea pipeline30to form an annular buoyancy module.

The buoyancy elements28shown inFIG.5have interlocking formations32that engage with their opposed counterparts to hold the buoyancy elements28together around the pipeline30. Conveniently, the interlocking formations32may be defined by shaping the shell14during additive manufacturing rather than being attached to, or removed from, the shell14in a subsequent operation. This avoids lines of weakness by removing interfaces between the interlocking formations32and the remainder of the shell14. The interlocking formations32may also contain cavities20that contribute to net upthrust.

The remaining drawings,FIGS.6to12, show various arrangements for the frame18and cavities20of foraminous internal structures of the invention, formed integrally with the shell14during additive manufacture of a buoy10. The buoys10shown in these drawings may have various external shapes, including cylindrical and cuboidal shapes like the buoys10shown inFIGS.1and2.FIGS.6to12illustrate characteristics of the internal structure that may be applied to buoys10of any external shape. In this respect, is to be understood that foraminous does not necessarily mean porous and that the cavities20may either communicate with each other or be closed, or there may be a combination of such cavities20.

InFIG.6, the shell14of a buoy10is integral with an internal structure whose frame18is in the form of a lattice. In this example, the lattice frame18comprises members21that are arranged in a regular triangulated array. The members of the frame18shown here may represent rods or struts19that define the trusses33extending across the interior of the shell14between conjoined cavities20. Alternatively, the members of the frame18may be continuous walls or webs that surround and enclose separate cavities20, for example spaces or cells of a tetrahedral, octahedral or other polyhedral shape, which could be discrete and sealed from each other.

The buoy10shown inFIG.7illustrates a variant of the arrangement shown inFIG.6, in which the regular array of the members of the frame18is replaced by a fractal array. The cavities20in the simplified fractal structure of the frame18shown here are arranged in two tiers in rows extending parallel to the adjacent outer surface of the shell14, namely an inner tier of cavities20A and an outer tier of cavities20B disposed between the inner tier and the shell14. Thus, the members of the frame18between the cavities20B of the outer tier support the shell14directly and also lie outside the cavities20A of the inner tier.

The cavities20B of the outer tier are smaller than the cavities20A of the inner tier. Consequently, the members of the frame18between the cavities20B are correspondingly closer together than the members of the frame18between the cavities20A. This distributes or concentrates more of the material of the frame18closer to the shell14and therefore optimises support for the shell14against hydrostatic pressure acting inwardly against the shell14.

As the members of the frame18in the inner tier are spaced further apart than those in the outer tier, a greater weight per unit volume of the frame18in the outer tier is offset to some extent by a lesser weight per unit volume of the frame18in the inner tier.

It will be apparent that, in this example, there is a self-similar or substantially fractal relationship between the cavities20B of the outer tier and the cavities20A of the inner tier. The cavities20B of the outer tier are arranged in smaller triangular arrays that, apart from their size, otherwise correspond to the triangular array of the cavities20A of the inner tier. The side of each cavity20B is about half of the length of the corresponding side of each cavity20A.

FIG.8and the enlarged detail view ofFIG.9also illustrate a buoy10in which the internal structure defined by the members of the frame18comprises a fractal array. Again, the effect is to concentrate the material of the frame18closer to the shell14, maximising the number of members of the frame18that connect to the shell14while minimising their mutual spacing. This maximises support for the shell14against inward hydrostatic pressure without a commensurate increase in the overall weight of the frame18.

In the example shown inFIGS.8and9, the cavities20are generally cuboidal. Also, there are intermediate tiers of cavities20C,20D interposed between the larger cavities20A of the innermost tier and the smaller cavities20B of the outermost tier. As before, the tiers extend in rows parallel to the adjacent outer surface of the shell14.

Again, there is a self-similar or substantially fractal relationship between the cavities20A,20B,20C,20D of the tiers in outward succession. In this case, the side of each cavity20B,20C,20D is about half of the length of the corresponding side of each cavity20A,20B,20C of the tier immediately within.

Turning next toFIG.10, this shows how cavities20within a buoy10may also be disposed in curved fractal arrays. In this example, the frame18is a matrix that extends continuously between and around the discrete cavities20. Here, the cavities20are spheroidal or ellipsoidal and are disposed mainly in circular arrays or rings that are concentric with a circular-section shell14of the buoy10. The arrays are tiered in hyperbolic relation such that the cavities20become smaller from tier to tier, and the successive tiers therefore become narrower, in a radially outward direction.

Specifically, the buoy10shown inFIG.10comprises a central cavity20surrounded by an inner tier of cavities20A in a heptagonal array and an outer tier of cavities20B of differing diameters, the arrangement being akin to a Poincaré hyperbolic disk. Further tiers of yet smaller cavities could of course be provided within the shell14outside the outer tier of cavities20B.

It will be apparent fromFIG.10that, again, the material of the frame18is concentrated closer to the shell14. The number of members of the frame18that connect to the shell14is maximised and their mutual spacing is minimised to provide optimal support for the shell14against hydrostatic pressure.

FIG.10also shows the optional feature of an anti-corrosion coating, layer or jacket34, for example of a polymer material, that extends continuously around the outside of the shell14to protect the buoy10from corrosion due to exposure to seawater.

FIG.11shows how the internal structure of a buoy10may be formed from a frame18of angularly-spaced, radially-extending members being ribs or spokes36that define discrete cavities20between them. This example also adopts a fractal arrangement because each spoke has a Y-shape, being split or bifurcated at an intermediate radius from a radially inner root portion36A into a splayed pair of arms36B that join the circular-section shell14of the buoy10. The root portions36A form an inner tier of the fractal arrangement and the arms36B form an outer tier of the fractal arrangement.

Thus, inFIG.11, the number of members of the frame18that connect to the shell14is again maximised and their mutual spacing is minimised to provide optimal support for the shell14against hydrostatic pressure. It would of course be possible for each arm36B of each spoke36to divide again to form further tiers of the fractal structure.

FIG.11also shows the optional feature of sacrificial anodes38to protect the buoy from seawater corrosion. The anodes38are positioned in electrically conductive contact with the outside of the shell14, and may for example be distributed angularly around the shell14as shown. Anodes38may be provided instead of, or in addition to, an anti-corrosion coating like that shown inFIG.10.

It is also possible to determine the structure of a buoy10using a topology optimisation algorithm, which typically results in an irregular structure that achieves improved performance in terms of weight and buoyancy relative to the buoys10described above having frames18with regular and/or fractal arrays.

An example of a buoy10having such an optimised structure can be seen inFIG.12, which shows the internal structure of an upper half of the buoy10. The frame18, the physical characteristics of which have been determined using a topology optimisation algorithm, has cavities20that are irregular in size, shape and spacing, which are defined by correspondingly irregular members of the frame18. The shell14remains continuous to enclose the internal volume of the buoy10, but varies in thickness in accordance with the local level of support provided by the members of the frame18where they connect to the shell14, thereby achieving weight savings in areas where the underlying support provided by the frame18is locally enhanced.

To reduce the computing power and time required to determine the overall structure, in this example the buoy10has a pair of orthogonal, radial axes of symmetry40dividing the frame18into quadrants42having similar, albeit mirrored structures. Accordingly, the lower half of the buoy10that is not visible inFIG.12is a mirror-image of the visible upper half. The embodiment shown inFIG.12therefore has a degree of regularly to the extent that the quadrants42are similar, while each individual quadrant42remains irregular. It will be appreciated that the structure could be entirely irregular, however.

Many other variations are possible within the inventive concept. For example, it may be possible for the cavities within a buoy to be filled with a fluid other than gas, such as a liquid like kerosene that is less dense than water, or another material such as a foam that is that is less dense than water.

Members of the frame of the internal structure of a buoy may be of variable or differing thicknesses. For example, the members of the frame in an inner tier may be thicker than members of the frame in an outer tier of a fractal arrangement.