Buoyancy device for very deep water and production method thereof

Abstract

A buoyancy device (1) comprises a support structure 2, which can be connected to an underwater application (3) and one or more buoyancy spheres (4) having a specific weight of less than 500 kg/m.sup.3 connected to the support structure (2) and having a light metal spherical shell (5) defining a spherical inner volume (6) and which has an outer diameter (d) greater than 0.5 cm, and a radial thickness (t) greater than 0.08 mm, wherein the spherical shell (5) is obtained in one piece in nano-crystalline metal with an average grain size of less than 1000 nanometers.

Claims

1. A buoyancy device, comprising: a support structure which can be connected to one of an underwater installation and an underwater vehicle, one or more buoyancy spheres connected to the support structure, said buoyancy spheres having a specific weight of less than 500 kg/m.sup.3, and a metal spherical shell defining a spherical inner volume and which has an outer diameter greater than 0.5 cm, and a radial thickness greater than 0.08 mm, wherein the spherical shell is obtained in one piece in nano-crystalline metal with an average grain size of less than 1000 nanometers.

2. The buoyancy device according to claim 1, wherein the spherical shell is obtained by deposition of metal nano-particles along a predetermined spherical geometry.

3. The buoyancy device according to claim 2, wherein the spherical shell is obtained by deposition of electrodeposition of aluminum or aluminum alloy.

4. The buoyancy device according to claim 1, wherein the nano-crystalline metal of the spherical shell has a particle size substantially without an amorphous phase.

5. The buoyancy device according to claim 1, wherein the outer diameter of the spherical shell ranges between 0.5 cm and 10.16 cm, and the radial thickness of the spherical shell ranges from 0.08 mm to 5 mm.

6. The buoyancy device according to claim 1, wherein the support structure comprises a polymeric matrix which houses a plurality of said buoyancy spheres.

7. The buoyancy device according to claim 1, wherein the support structure comprises at least one flexible net forming seats which receive the buoyancy spheres.

8. The buoyancy device according to claim 1, wherein the support structure comprises at least one grid-shaped rigid frame which connects seats which receive the buoyancy spheres together.

9. The buoyancy device according to claim 1, wherein the support structure comprises grouping seats, each of which receives a plurality of said buoyancy spheres.

10. The buoyancy device according to claim 8, wherein said seats form cavities with a substantially spherical curvature.

11. The buoyancy device (1) according to claim 8, wherein the seats can be reversibly opened and accessed for the replacement of the buoyancy spheres.

12. The buoyancy device according to claim 1, comprising a plurality of said support structures which are configured as modules which are reversibly connectable together.

13. The buoyancy device according to claim 12, wherein said modules are stackable and have one of an egg-box and ball-grid-box shape.

14. The buoyancy device according to claim 1, wherein the buoyancy spheres comprise smaller buoyancy spheres and larger buoyancy spheres of different dimensions than the smaller buoyancy spheres, and the smaller buoyancy spheres and the larger buoyancy spheres are positioned in the support structure so that the smaller buoyancy spheres fill interspaces between the larger buoyancy spheres.

15. The buoyancy device according to claim 1, wherein the buoyancy spheres are externally coated by a protective layer suitable to attenuate impacts.

16. A method of producing a buoyancy device, comprising: producing one or more buoyancy spheres having a specific weight of less than 500 kg/m.sup.3, and a metal spherical shell defining a spherical inner volume and which has an outer diameter greater than 0.5 cm and a radial thickness greater than 0.08 mm, connecting said one or more buoyancy spheres to a support structure for a connection to underwater installations or underwater vehicles, obtaining the spherical shell in one piece by deposition of metal nano-particles along a predetermined spherical geometry.

17. A buoyancy device, comprising: a support structure with can be connected to one of an underwater installation and an underwater vehicle, one or more buoyancy spheres connected to the support structure, said buoyancy spheres having a specific gravity of less than 500 kg/m.sup.3, and a metal spherical shell defining a spherical inner volume and which has an outer diameter greater than 0.5 cm and a radial thickness greater than 0.08 mm, wherein the spherical shell is obtained in one piece in a metal alloy having: an elastic module E greater than 68 GPa, and a yield stress y greater than 680 MPa, and a density of less than 3000 Kg/m.sup.3.

Description

(1) In order to better understand the invention and appreciate its advantages, some non-limitative examples of embodiments will be described below with reference to the figures, in which:

(2) FIG. 1 shows a buoyancy device according to a possible embodiment of the invention,

(3) FIG. 2 is a section view taken along a diametric plane of a buoyancy sphere of the buoyancy device according to the invention,

(4) FIGS. 3, 4 and 5 show embodiments of the buoyancy device, in which the buoyancy spheres are individually received in a support net,

(5) FIG. 6 shows an embodiment of the buoyancy device, in which a plurality of buoyancy spheres are received and grouped in a grouping seat of a support net,

(6) FIGS. 7, 8 show embodiments of the device, in which the buoyancy spheres are individually connected to a three-dimensional and modular frame or grid,

(7) FIGS. 9, 10 show embodiments of the buoyancy device, in which the buoyancy spheres are individually received in the seats of a module having an egg-box shape of a modular support structure,

(8) FIG. 11 shows embodiments of the buoyancy device, in which the buoyancy spheres are individually received in the seats of a module having a ball-grid box shape of a modular support structure,

(9) FIG. 12 shows a chart, which indicates the ratio between thickness of the spherical shell and outer diameter (OD) of the buoyancy spheres for different levels of geometric imperfection of the spherical shell.

(10) With reference to the figures, a buoyancy device is indicated as a whole by reference numeral 1 and comprises a support structure 2, which can be connected (e.g. by means of a fastening band 17) to an underwater application, e.g. a riser 3, one or more buoyancy spheres 4 connected to the support structure 2 and having a metal spherical shell 5, which delimits a spherical inner volume 6 (not necessarily completely void). The buoyancy spheres 4 each has an outer diameter greater than 0.5 cm, a radial thickness t of the spherical shell 5 greater than 0.08 mm, and a specific weight lower than 500 kg/m.sup.3. The spherical shell is obtained in one piece (without mechanical joints and without weld seams or gluing) in nano-crystalline metal with an average grain size of less than 1000 nanometers, preferably in the range from 10 nm to 800 nm, and even more preferably in the range from 10 nm to 200 nm.

(11) According to an embodiment, the spherical shell 5 is obtained by deposition of metal nano-particles along a predetermined spherical geometry.

(12) The spherical geometry may be dictated by a substrate 9 of predetermined spherical shape, on which the nano-particles are deposited. In the case in which this substrate 9 defines the shape of a spherical inner surface of the spherical shell 5 to be constructed and remains therein, the spherical shell would constitute a supporting layer of a multilayer spherical wall 8 having a base layer 9 (substrate) with a deposition surface 10 on which the spherical shell 5 is formed, e.g. by means of electrodeposition.

(13) In alternative embodiments, the spherical shell 5 may be constructed by means of the deposition of nano-particles on substrate systems or outer spherical shapes, on substrate or spherical shapes, which are either subsequently or sequentially removed from the spherical shell 5, or by means of the deposition of particles, e.g. nano-powders in the absence of a support spherical substrate (3D printing principle).

(14) In a preferred embodiment, the spherical shell 5 is made of aluminum or aluminum alloy, e.g. aluminum-manganese alloy (AlMn).

(15) In an embodiment, the nano-crystalline metal of the spherical shell 5 has a granulometry substantially without an amorphous phase, and preferably also substantially unimodal. The choice of configuring the spherical shell 5 in nano-crystalline metal without an amorphous phase reduces the onset of at least some fragility phenomena which can be related precisely to the presence of the amorphous phase in the metal.

(16) The support structure 2 may comprise a polymeric matrix 11 (epoxy resin, polyester or other polymers) or a syntactic foam, as described with reference to the prior art, in which one or more buoyancy spheres 4 (FIG. 1) are either mixed or inserted with or without sphere-matrix adhesion or received. Alternatively or additionally, the support structure 2 may comprise one or more flexible nets 12 (FIGS. 3-6) or one or more grid-shaped rigid frames 13 (FIGS. 7, 8), which either form or connect individual seats 14 and/or grouping seats 15 to one another configured to receive the buoyancy spheres 4 either individually (FIG. 3) or in groups (FIG. 6) or in clusters (FIG. 5). For example, such seats 14, 15 may be spherical or semi-spherical caps (FIGS. 7, 8, 9), connected to one another in either fixed or modular manner by means of rods 16. Furthermore, the seats 14, 15 may be reversibly opened and accessible for replacement and maintenance operations of the buoyancy spheres 4.

(17) According to an embodiment, the buoyancy device comprises a plurality of such support structures 2 configured as reversibly connectible modules, and preferably mutually stackable. FIGS. 9, 10, 11 show examples of construction of single modules of the support structure 2 having an egg-box and ball-grid-box shape, e.g. made of plastic, aluminum or stainless steel.

(18) The buoyancy spheres 4 may comprise buoyancy spheres 4 of different size positioned in the support structure 2 (syntactic foam, frame, net, cage housing) so that the smaller buoyancy spheres 4 fill the interspaces between the larger buoyancy spheres 4, thus compacting the buoyancy device 1 and concentrating the buoyancy in smaller spaces. The buoyancy spheres 4 may be externally coated by a protection layer 18 of material adapted to attenuate impacts and/or to dissipate the impact energy, e.g. soft rubber, polymeric foams.

(19) According to an embodiment, the buoyancy sphere 4 and the buoyancy device 1 are manufactured by the following steps: providing a hollow inner sphere 9 (substrate which will form the future base layer 9 of the multilayer spherical wall 8) with an outer diameter corresponding to the inner diameter of the spherical shell 5 to be obtained. In the embodiments considered here and deemed most appropriate for underwater applications at depths greater than 3000 meters (e.g. about 4500 m-5500 m), the inner sphere 9 may have an outer diameter in the range from of an inch to 4 inches or, for particular applications, in the range from 4 inches to 20 inches (1 inch=2.54 cm) and can be made of a chosen material (e.g. plastic) with manufacturing tolerances compatible with the final precision requirements of the buoyancy spheres 4. The inner sphere 9 does not perform any structural function in the buoyancy sphere 4 and is preferably hollow or alternatively either full or partially full, e.g. with a very low density polymeric foam.

(20) In an embodiment, the plastic inner sphere 9 is made by means of roto-molding, by introducing polymeric powders in a revolving heated hollow mold, which melts and distributes the polymeric resin uniformly about the spherical inner wall and then cools the module to solidify and extract the inner sphere 9.

(21) The inner sphere may be constructed by two or more parts. Preparing a deposition surface 10 for the electrodeposition. The plastic inner sphere 9 is not electrically conductive and could require a metallization of the deposition surface on which to construct the spherical shell 5. Such a metallization may be performed, for example, by means of an electroless plating process, in which the plastic material is etched using oxidizing solutions which make the surface adapted to form hydrogen bonds ready for the subsequent deposition of metals, such as, for example, nickel or copper solution.

(22) Alternatively, the metallization of the inner sphere 9 may be performed by means of vacuum spraying, flame spraying or arc spraying.

(23) Metals which can be used for metallization are, for example, Ni, Cu, Zn, Al, Ag.

(24) The step of preparing by means of metallization can be avoided by making the inner sphere 9 directly of a suitable material as substrate for the later construction of the spherical shell 5. Electrodepositing the spherical shell 5 on the deposition surface 10 of the inner sphere 9 in ionic liquid solution, applying either pulsed current (PC) or direct current (DC), and using a 99.9% pure aluminum surface (sheet) as anode and the substrate material, e.g. 99.9% pure copper, as cathode. Other metals forming the alloy, e.g. Mn, may be provided in form of ions present in the ionic solution. Controlling the outer sphericity of the buoyancy sphere 4, by means of optical measurement, Optionally, coating the outside of the buoyancy sphere 4 by means of an anti-shock protection layer, e.g. made of soft polymeric material. Connecting one or more buoyancy spheres 4 to a support structure 2 to complete the buoyancy device 1.

(25) In an embodiment of the buoyancy device 1 for 4000 m of depth, the buoyancy spheres 4 have outer diameters comprised between of an inch and 4 inches (1 inch=2.54 cm) and thickness from 0.08 mm to 5 mm as a function of the outer diameter.

(26) The sphericity tolerances may be referred to the critical arc model, which is known and widely disclosed in literature and will not be repeated here for the sake of conciseness, and may be in the order of up to 10% of sphericity tolerances and up to 10% of thickness tolerances (along the critical arc) in any point of the buoyancy sphere 4.

(27) An outer working pressure is of 410 bar and requires a maximum dimensioning pressure of the buoyancy spheres 4 of 600 bar, considering an exemplary safety factor of 1.5 applied to the working pressure. In the example, the modulus of elasticity of the nano-structured metal material (AlMn aluminum alloy) of the spherical shell 5 is of 70 GPa. Thus, the modulus of elasticity E and also the yield stress limit .sub.y of the metal alloy of the spherical shell 5 are much higher than the yield stress values of the aluminum alloys used in the prior art for particular applications (e.g. Al 7075-T6.sub.y=570 MPa, Al 7068-T6511=680 MPa), while the specific weight (density) of the metal alloy of the spherical shell 5 remains lower than 3000 kg/m.sup.3, preferably lower than 2820 kg/m.sup.3.

(28) FIG. 12 indicates an example of the ratio between thickness of the spherical shell and outer diameter (OD) of the buoyancy spheres 4 of the buoyancy device 1 for different levels of geometric imperfection of the spherical shell 5. The boundary conditions for the actual use of the buoyancy spheres 4 shown in the chart are: hydrostatic working pressure 400 bar; buckling strength at an outer pressure of 600 bar; material: Aluminum alloy.

(29) The chart in FIG. 12 shows the enormous influence of the geometric imperfection control on the maximum achievable working load and consequently on the possibility of lightening the buoyancy spheres (by reducing the thickness t thereof) and of increasing buoyancy efficiency at very great depths.

(30) The chart further indicates exemplary and preferred ranges, diameters and diameter/thickness ratios of the buoyancy spheres 4 according to the invention.

(31) The buoyancy device 1 according to the invention has many advantages, in particular: improved mechanical features, in particular with reference to strength/specific weight ratio, buckling strength, and resistance to fatigue of the buoyancy elements (considering typical stresses in the range from 10.sup.3 to 10.sup.6 cycles), shapes suited to numerous applications (risers, ROV, midwater arch etc.) both with buoyancy spheres 4 inserted in a polymeric matrix, or with buoyancy spheres 4 inserted in a liquid, semi-liquid or gelified matrix, e.g. for use with insulation systems in riser towers, or with spheres directly exposed to contact with water. low relative density which allows to reach a seabed deeper than 3000 m, with particular advantages about 4000 m with relative density (of the single sphere) of about 0.25-0.30.

(32) Obviously, a person skilled in art may make further changes and variants to the buoyancy device 1 and to the production method according to the present invention, all of which without departing from the scope of protection of the invention, as defined in the following claims.