WO2013083152A1

WO2013083152A1 - Composite pressure vessel and method for manufacturing the same

Info

Publication number: WO2013083152A1
Application number: PCT/EP2011/071788
Authority: WO
Inventors: Francesco Nettis; Vanni Neri TOMASELLI
Original assignee: Blue Wave Co S.A.
Priority date: 2011-12-05
Filing date: 2011-12-05
Publication date: 2013-06-13
Also published as: EP2788664A1

Abstract

A pressure vessel is produced for storing and transporting CNG. The vessel comprises a body defining an internal volume in which the CNG can be stored, and an inlet for loading the CNG into the vessel. The body of the vessel comprises a structural shell made entirely of a fibre-reinforced filament-wound composite material, which is provided with fibres to carry mechanical loads and a matrix to keep the fibres together and to make the composite impermeable to the CNG. Because the vessel is not provided with any liner, the CNG is in direct contact with an inner side of the structural shell.

Description

Composite pressure vessel and method for manufacturing the same Field of the invention The invention relates to pressure vessels for storing and transporting compressed gas, and, in particular, compressed natural gas (CNG). In particular, it relates to CNG pressure vessels involving composite materials.

The present invention also relates to a method of manufacturing the above vessels.

Background art

Increased capacity and efficiency requests in the field of CNG transportation, and the common use of steel-based cylinders therefor, has led to the development of steel- based cylinders with a thicker structure, which usually results in a heavy device or a device with a lower mass ratio of transported gas to containment system. This effect can be mitigated by introducing composite materials in the vessels, which are lighter and can be designed to withstand the same stresses as those supported by steel vessels.

Some existing solutions therefore already use composite structures in order to reduce the weight of the device. It is known to cover cylindrical steel vessels with resin-coated reinforcement fibres which are hoop-wrapped only around the cylindrical portion of the body of the vessel. These hybrid vessels use less steel compared to equivalent vessels made entirely of steel - i.e. vessels having the same internal volume and that can withstand the same nominal pressure, and which can therefore transport the same quantity of CNG.

It is also known to fully wrap a metallic liner (i.e. a relatively thin layer of metal with negligible structural contribution to the overall strength of vessel) with a fibre-reinforced composite material; in this case it is necessary to coat with the composite the whole body of the vessel. The resin-coated fibres are therefore wrapped around the cylinder of the vessel (hoop wrapping) and also around its dome-shaped ends (or caps) (helical wrapping). These vessels are much lighter than the ones which have an internal structural layer of steel and have been hoop wrapped only around the cylindrical body. In certain applications, it is also possible to provide non-metallic liners to composite vessels, in order to further reduce their weight. However, all of the above vessels, despite achieving increasing degrees of lightness, still present various types of mechanical interfaces or discontinuities. Interfaces and joints, as known from mechanical theory, can be a potential point of weakness for a structure. Technical problem to be solved

In light of the presence of mechanical interfaces, even the lightest, fully composite- wrapped vessels, with non-metallic liners, can be said to present the user with certain disadvantages. In these vessels, the interface is given by the transition between the wrapping material and the non-metallic liner.

Interface weakness also affects composite vessels with metallic liners. Here, the potential weakness in the material arises because of the presence of an interface between very non-homogeneous materials in the walls of the vessel.

Composite over-wrapped vessels with an inner structural steel layer may also present interface weaknesses, albeit in a more limited manner because the steel and the composite tend to share the structural load, and are therefore likely to mechanically deform at the same or similar rate.

Examples of potential interface weaknesses for structures containing interfaces are: galvanic corrosion of materials when they have a different electrochemical affinity; and difference in thermal expansion of materials, which can result in localised stresses at the interface.

In addition, there can be problems connected with the dynamics of the interface itself. As the vessels undergo expansion due to the loading of pressurized gas into the vessels, the two interfaced materials, one on one side of the interface and the other on the other side of the interface, will react differently, each according to its own stress/strain reactive characteristics. For example, a cylindrical pressure vessel made of carbon fibre-based structural composite material can expand up to and above 1.5% circumferentially when the nominal pressure is reached in the vessel. A similar vessel made of glass fibre-based structural composite material can expand up to and above 4% circumferentially. Since liners have no structural bearing, they tend to follow the expansion of the structural materials. Depending on the material chosen for the liner, the liner can therefore yield or suffer from increased porosity under the strain imposed by the circumferential expansion of the structural composite. This situation can create internal residual or additional stresses that can cause internal fractures typically in the liners, but also potentially in the outer structural material.

Further, due to the cyclical nature of the loading and unloading processes of the pressure vessels - the vessels are reused many times - liners can also undergo mechanical hystheresis once their yield strength has been overcome. This can lead to a rapid plastic deterioration of the liners.

Further still, cycling can cause general structural fatigue at the interface even when the nominal yield strength has not been overcome.

An additional disadvantage concerns the manufacturability of the liners. Because of all or many of the above-identified problems or disadvantages, specialized know-how is required to manufacture suitable liners. First of all, appropriate liner materials with appropriate properties, such as impermeability to raw untreated gases (such as CNG) have to be selected. Further, forming the liners into their final shape for forming a suitable internal wall of the vessel requires special skills.

A method of forming a complete metallic liner for a pressure vessel of the kind related to the present invention can be to weld together several sheets of metal along welding lines (see Figure 1). However, because metallic liners are relatively thin, and need to satisfy certain impermeability characteristics even after the welding, the welding of these metal sheets requires special technical expertise and skills, and is therefore expensive and time consuming, especially considering that weldings have to be inspected and tested for specific compliance with international Standards prior to their qualification and use. Further still, for the above-identified reasons, the manufacturing of metal-lined composite pressure vessels is recognised to be an operation, which requires constant human supervision, and is thus not readily adaptable for automation. When composite pressure vessels are produced by filament winding, liners need to withstand winding stresses while the fibres of the composite material are wound around them. This increases the otherwise minimal, or simply self supporting mechanical requirements for liners of filament-wound composite pressure vessels. The present invention therefore aims to overcome or alleviate at least one of the disadvantages of the known pressure vessels.

In particular, the present invention seeks to provide pressure vessels for CNG, which are light in weight.

The present invention also aims to eliminate or at least alleviate any one or more of the disadvantages that may arise from the presence of an interface between a structural outer layer of composite material and its liner in a CNG pressure vessel. The present invention also aims to improve the lightness of composite material CNG pressure vessels, while still alleviating or eliminating problems connected with the presence of interfaces or discontinuities in the body of the vessels.

The present invention also aims to ease the manufacturability of filament-wound composite over-wrapped CNG pressure vessels.

The present invention also aims to reduce the incidence of liner-related mechanical problems that can affect composite CNG pressure vessels. Summary of the invention

According to a first aspect of the present invention, there is provided a pressure vessel for storing and transporting CNG comprising a body defining an internal volume in which the CNG can be stored and an inlet for loading the CNG into the vessel, the body of the vessel comprising a structural shell made entirely and solely of a fibre- reinforced filament-wound composite material comprising fibres and a CNG impermeable matrix.

The term CNG means compressed natural gas, be it well stream fluids, i.e. gas and liquid hydrocarbons received untreated from the source, or treated compressed natural gas - which will have fewer impurities.

CNG fluids can include various potential component parts in a variable mixture of ratios, some in their gas phase and others in a liquid phase, or a mix of both. Those component parts will typically comprise one or more of the following compounds: C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂, C₆H₁₄, C₇H₁₆, C₈H₁₈, C₉+ hydrocarbons, C0₂ and H₂S, plus potentially toluene, diesel and octane in a liquid state.

It is preferred that in use, the CNG is in direct contact with an inner side of the structural shell.

Preferably the structural shell comprises a cylinder section and two terminations, one at either end of the cylinder section, all being made of the fibre-reinforced filament-wound composite material. Preferably in use, the CNG will be in direct contact with internal parts of said cylinder section and terminations.

Preferably the terminations are dome-like terminations.

Preferably the dome-like terminations have a geodesic shape in respect of helical wrapping of fibres around the vessel.

Preferably the CNG is in contact with the whole of the internal surface of the fibre- reinforced filament-wound composite material. The pressure vessel typically will have a nominal external diameter in the range of 1 m to 6m. It may, for example, have a nominal diameter of about 2 metres.

Preferably the nominal thickness of the fibre-reinforced filament-wound composite material shell is between 20 and 800 mm. Preferably the fibres of the composite material comprise at least one of carbon fibres, glass fibres or Kevlar®.

Preferably the resin of the composite material comprises at least one of a polyester resin, a vinylester resin, an epoxy resin, a phenolic resin, a high-purity dicyclopentadiene resin, a bismaleimide resin and a polyimide resin.

The present invention also provides a method of manufacturing a composite pressure vessel for storage and transportation of CNG comprising the steps of providing a disposable mandrel and winding filament fibres around the disposable mandrel to form the shape of a pressure vessel, the shape including an inlet/outlet. The inlet/outlet is typically an aperture in an end thereof. There may be two apertures, one in each end. The ends are typically opposing ends. The method typically involves the step of removing the disposable mandrel through the inlet/outlet after the composite is cured.

Preferably the method comprises the further the step of aggregating the filament fibres to form a tape before winding them around the disposable mandrel.

Preferably the method comprises the further the step of impregnating the filament fibres with a resin before winding the fibres around the disposable mandrel.

Preferably the impregnation of the fibres takes place after the fibres have been formed into a tape and by immersing the tape into a batch of resin, such as in a bath of resin.

Preferably the method comprises the further the step of curing the composite while it is around the disposable mandrel, at least to a sufficient extent for it to be self-supporting. Preferably the method comprises the further the step of curing the composite and removing the disposable mandrel once the composite has been cured at least to a sufficient extent for it to be self-supporting.

Preferably the mandrel comprises ice, and the removal of the mandrel may then comprise melting the ice. Preferably the mandrel comprises compacted sand, and the removal of the mandrel then may comprise shaking the sand out of the vessel. The mandrel may comprise a scaffold, and the removal of the mandrel may then comprise collapsing the scaffold.

The mandrel may comprise a structure formed from a disolvable chemical compound (such as one that is desolvvable in water) and the removal of the mandrel may then comprise the dissolution of the structure to a liquid state.

Preferably the method forms a vessel as defined above.

Advantages of the invention

An advantage of the present invention is that lighter pressure vessels can be produced in comparison to those achievable, of a comparable size, based upon the prior art teachings. A further advantage of the present invention is that the produced pressure vessels can be less prone to interface problems, or they might even be virtually immune from known interface- related problems.

The present invention can also eliminate galvanic or thermal-related expansion stresses at interfaces between the outer later of composite material and the inner liner in certain types of CNG pressure vessels.

The present invention can also potentially eliminate or reduce overstressing or overstretching of liners of CNG pressure vessels due to the loading of pressurized CNG into the vessels.

The present invention can also potentially eliminate or reduce fatigue related liner fracture in CNG pressure vessels.

Brief description of the drawings The present invention will now be described purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic representation of a cross section of a CNG pressure vessel according to the prior art;

Figure 2 is a schematic representation of a method of manufacturing a composite pressure vessel according to the present invention;

Figure 3 is a schematic representation of the wrapping of fibres around a support for producing a pressure vessel according to the present invention; and

Figure 4 is a schematic representation of a cross section of a CNG pressure vessel according to the present invention.

Detailed description of the invention

Figure 1 shows a cross section of a so-called "type 3" vessel 10 for storage and transportation of CNG, taken along the longitudinal direction of the vessel. A type 3 vessel denotes a vessel that has an external structural layer 2 made of a composite, fibre reinforced, material and an inner metallic liner. The external composite material provides the structural strength of the vessel, while the inner liner provides an impermeable layer for containment of the CNG. The liner is made of a metal which is highly chemically resistant to the CNG. The CNG stored in this type of vessel is usually untreated and therefore contains aggressive and corrosive impurities.

The purpose of the liner is not just to provide a layer suitable for contact with the CNG, but also that of providing a substrate over which the composite material can be formed. A way of forming said composite material is by winding its fibres around the liner. The liner, therefore, is designed so as to be able to withstand fibre winding stresses.

One method of forming such a resistant liner is that of line welding several sheets of metal together. As shown in Figure 1 , six similar sheets of metal 13, 14, 15 are welded together along six lines 22, 23, 24 (seen in end-section) to form the liner. Only some of the sheets and lines are numbered in the Figure - this is to void overcrowding of the Figure. The metal sheets will first be bent to the part circumferential shape and then welded in the axial direction in order to "close" the circumference as the sixe sheets are hoined together. Several bent metal sheets may be required, according to the vessel length, for the hoop (cylindrical) section, and they can be welded together in the circumferential direction for then forming the whole hoop section. Further welding between the hoop section and the end domes is then performed. Those end domes can be formed through a spin forming process, such as those that are well known in the industry.

It should be highlighted that this method produces a large number of welds, each of which are potentially a point of failure since they are usually weaker than the sheet metal itself, in terms of their strength/durability properties, due to the structural changes to the material that can occur during or after the thermal shock of the welding process. In light of this, manufacturing a suitable liner requires know-how, materials, time and suitable equipment, such as welding equipment and specialist sheet-clamping equipment, and this is all adding to the costs involved.

The present invention, on the other hand, provides that no liner should be present inside the vessels.

In order to manufacture a liner-free vessel suitable for containment of CNG, the method illustrated in Figure 2 is provided. In that method, a plurality of reels 31 , 32, 33, 34 is provided, each housing a reel of a selected fibre, for example a carbon fibre, or an aramid fibre, or Kevlar®. In one embodiment, the reels accommodate individual filaments of the selected fibre. In other embodiments, yarns of fibres can be reeled, or the fibres can be bundled into tows, ropes or cords, or braids. Alternatively, the fibres may be woven into ribbons or narrow sheets of material (a fabric of fibre), including flat-fibres or webbing.

The single fibres, yarns, tows, ropes or narrow ribbons of fibre(s) are then fed to a tape machine 35. The tape machine arranges those multiple "fibres", which are effectively one-dimensional, into a single tape 37. The tape will still be relatively narrow, but it will now be in a wider, or two-dimensional, form, i.e. it will be wider that the individual "fibres" that come off the reels 31 , 32, 33, 34.

The tape can be treated as being effectively a substantially parallel arrangement of "fibres", the fibres extending largely side by side, i.e. transversally or perpendicularly to the direction of travel, along the length of the tape.

The tape 27 is then immersed into a resin, such as a batch of resin in a bath 38. Suitable resins can be, for example, polyester resins, vinylester resins, epoxy resins, dicyclopentadiene resins, phenolic resins, bismaleimide resins and polyimide resins. Such resins can generally be classified as either a thermoplastic resin or a thermosetting resin, according to their behaviour when heated and cooled. Thermoplastic resins can be re-heated and softened after they have been cured, while thermosetting resins, once cured, cannot be re-heated to soften them without causing permanent damage, i.e. they will not melt at normal manufacturing temperatures. On the other hand, thermosetting resins allow higher stiffness and higher general mechanical properties to be provided, along with their generally lower viscosity before curing of the resin (these advantages typically allow a better or faster winding/manufacturing process and a better impregnation of the composite fibers). It is observed, therefore, that thermosetting resins and thermoplastic resins can both be suitable for this method, provided that the resin of choice is formulated to be chemically CNG resistant and substantially, or virtually completely, impermeable to the component parts of CNG at the desired operating pressures of the vessels. Those component parts will comprise one or more of the following compounds: C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂, C₆H₁₄, C₇H₁₆, C₈H₁₈, C₉+ hydrocarbons, C0₂ and H₂S, plus potentially toluene, diesel and octane in a liquid state.

The resin impregnated tape 39 is then fed to a mechanical head 40 which is responsible for winding the impregnated tape (now shown in the drawings by reference sign 41) around the mandrel 45. Various methods can be used for winding the impregnated tape around the mandrel. A simple way, however, is that of employing a mechanical head 40 which moves back and forth one-dimensionally, i.e. along a line parallel to the mandrel, the head delivering the tape 41 as the mandrel 45 rotates. This arrangement is schematically shown in Figures 2 and 3. Preferably the winding process involves helical and hoop winding. Helical winding goes around the geodesic heads (ends) of the mandrel and around the openings. Hoop winding goes on the cylindrical section only in a circumferential direction. The hoop section may be arranged such that it accommodates approximately double the fiber amount of the heads, e.g. in terms of the thickness of the winding.

In the formation of the prior art vessel of Figure 1 , the mandrel would take the form of the liner. However, in accordance with this embodiment of the present invention, the mandrel instead takes the form of a disposable mandrel, i.e. a mandrel that can be eliminated or removed from the inside area of the vessel once the composite layer or composite laminate has been created thereon. That mandrel thus needs to be able to withstand the winding stresses during the winding of the tape, plus the lamination stresses as the layers build up - lamination refers to the process of growing the thickness of the composite wrap by gradually stacking layers of tape over one another by winding the tape continually, back and forth over the mandrel so as to pass over other previously wound layers of tape. The winding thus forms multiple helical or coiled (i.e. hooped) layers of tape so as to provide a substantially uniform or flat surface.

In the embodiment of Figure 3, the disposable mandrel 47 is made of compacted sand. Other disposable mandrels are also anticipated to be useable here, such as expandable stent-like arrangements, or braids, or balloons, or other solid arrangements such as ice or clay shells, plus other collapsible structures.

Figure 3 only shows the wound tape in loose-wind form, and prior to completing the full number of windings required, and this is for illustrating the principle of the winding only - in practice it would be wound down tight against the mandrel 47, and the tape windings would be overlayered many times so as to form the vessel's form.

Once the over-layer of fibre-reinforced composite material has been fabricated over the disposable mandrel 47, so as to have formed the desired material thickness, the sand can be removed from the centre of the vessel. This can be achieved, for example, by applying vibrations to the finished component, e.g. by means of an electromechanical shaker. The vibrations produced by the shaker will break up the compacted sand, which will then be able to be removed from the inside of the vessel, e.g. by tipping or washing it out of an aperture in an end thereof. The aperture will be formed (or left) at the end of the vessel during the winding process. Indeed, an aperture would usually be left or formed at both ends thereof. Such apertures not only allow the sand to be removed, but they also will ultimately allow CNG to be loaded into and unloaded from the vessel, during use. The ends of the vessel are formed over dome-shaped ends of the mandrel, but only to a sufficient degree so as to leave thereat the aperture(s).

Figure 3 shows the general trajectory of the resin-impregnated tape 51 as it is wound around the disposable mandrel. The mandrel 47 has the shape of the internal volume of the CNG pressure vessel, in this case a cylinder with two dome-shaped ends or terminations.

The resin-impregnated tape 51 is supplied onto the mandrel 47 starting from an origin or first free end 50. In Figure 3, the origin 50 of the tape is located close to the left dome of the disposable mandrel 47.

While the mandrel spins around its longitudinal axis, the mechanical tape delivery head moves longitudinally (parallel to that axis) so as to create hoops or circles 53 (a coil or helix) of fibres around the mandrel 47. Those hoops or circles maintain a substantially constant angle relative to the axis along that cylindrical part of the mandrel.

When the head reaches the first (right) end of the mandrel 47, the wrapping characteristics change. For example, as it reaches the right end of the mandrel 47, it slows down, and so does the rotation of the mandrel 47. The angle of the hoops or circles therefore may be changed. Further, as the winding slows down, torsional forces are generated within the mandrel, and these are in addition to the winding forces already being generated (the winding forces tend to compress the mandrel from outside in). The torsional forces result from the momentum of the mandrel, and can be considerable if rotational speeds vary rapidly (remember that the mandrel can be very large when fabricating a CNG transportation vessels, e.g. a vessel that potentially has a diameter of at least 2m, and potentially even a diameter of 6m or more). Such additional torsional forces, however, would also have occurred in arrangements where the mandrel takes the form of a liner, and as such have been an existing problem with the known over-liner winding techniques, whereby it has been something that has contributed to the need to make those liners stronger, and thus heavier, than ultimately desirable, so as to prevent them from deforming out of shape as a result of the wrapping. However, when using the disposable mandrel of the present invention, such heavier mandrels do not cause a problem for the final product since the disposable mandrel will not remain within the finished vessel.

Another advantage of having a filled structure/mandrel or a structure with radial components is potentially a better behavioural response to torsional winding forces than a traditional liner, which is usually a thin-walled approximation and having a smaller material section.

This aspect of the present invention therefore offers a considerable advantage over the prior art.

With the compacted sand solution for the mandrel, there should be little at issue in relation to handling the additional torsional forces either. That is because the compacted sand can be made to have significant robustness to such loadings. Other solutions for the mandrel can also offer such advantages. For example, a solid mandrel formed from a destructible, dissolvable or meltable material (including compacted sand, ice, clay, gypsum and many other granular or dusty, yet compactable, materials) can be designed such that it cannot easily twist out of shape. Likewise, a fracturable clay liner, or collapsible structures like balloons, or braid or stent-like arrangements, or other collapsible scaffolds, can be made such that they are strong enough to survive such forces, but yet still being removable through the aperture(s). Such scaffolds can even support arrangements such as compacted sand pads or ice pads, thereby making the mandrel significantly lighter than a solid mandrel.

Returning to the process of winding the composite, once the fibre has reached the first end of the mandrel, it is wound around the dome of the mandrel at that end, albeit leaving the aperture part of the vessel, at that end, uncovered, and then it is returned towards the other end of the mandrel through a further coiled path 54 generally diagonally across the body of the mandrel 47. This path will typically have a different angle to the preceding coil, although that is optional, and it may even be sufficiently angulated so as not to form a full loop around the cylindrical section of the vessel. Preferably it loops around less than half of the vessel's circumference, and even more preferably it loops around even less than one third or perhaps one quarter of the vessel's circumference before inverting back towards line of passage back towards the first end again, usually at the same angle as the first-described passage towards that first end. The speed of rotation of the mandrel, and the speed of movement of the head 40, will be controlled - reduced - at the second end of the mandrel too. Further, the tape is wound around the dome of the mandrel at that second end much like that at the first end, again leaving an aperture for the vessel at that second end. When the winding operation is concluded, the tape is cut and a second free end of the tape 52 is formed. This is accommodated on the layer of fibres already wound around the mandrel.

The vessel is now ready for curing (or "cooking").

At the end of the curing, the disposable mandrel can be removed in an appropriate manner, such as by the vibration technique for compacted sand, or by dismantling the scaffold, etc (typically the aperture will be an 18-inch (45cm) or larger (e.g. 24 inch - 60cm) aperture, whereby an operator can access the inside of the finished vessel, for example by climbing into it for achieving a complete evacuation of the mandrel).

The above process has been illustrated herein only schematically.

It should be noted that several layers of tape will need to be wound around the mandrel until a desired thickness, such as one of several mm or cm, is eventually obtained. Preferred thicknesses for the finished article will be between 20 and 800mm, but the actual thickness that is desirable for a given vessel will depend upon the target pressure containment capacity, and also the diameter of the finished vessel. Conventional hoop-stress analysis can be used for determining these desired dimensions since the strength of the fibres, and the angles of winding, are all known.

Multiple axis filament winding machines can also be used to implement a method of the present invention. For example, 2-axes or 3-axes filament winding machines can be used. These are machines whose fibre delivery head can move respectively in a plane or in a 2- or 3-dimensional space. It is even known that winding machine heads can be shifting and turning with up to 5-axes.

Further, the number of axes around which the mandrel can spin could be two, three or more, instead of just the one (longitudinal) axis as described above.

The machine used will depend on the design of the desired vessel.

An example of a finished product obtained with the manufacturing process illustrated above can be seen in Figure 4. In this Figure, the vessel 64 is made entirely and solely of a structural portion of fibre-reinforced composite material 62. It was wound around a disposable mandrel, presenting an inner surface 63 directly into contact with the CNG. The structural composite itself therefore is capable of containing the CNG within the vessel - no liner or internal coating is needed (although a coating may beneficially be applied if desired, from inside of the vessel).

Embodiments

Example 1

A composite pressure vessel made of carbon or graphite fibres having a strength of 3200 MPa or higher and a Young Modulus of 230 GPa or higher, with 12000, 24000 or 48000 filaments per yarn, and a thermosetting resin (epoxy-based or high-purity poly- dicyclopentadiene-based resin), the vessel being obtained by over-wrapping a disposable mandrel made of clay of generally cylindrical shape having a diameter of around 2 metres, and a length of around 5 metres.

Example 2 A composite pressure vessel made of carbon or graphite fibres having a strength of 3200 MPa or higher and a Young Modulus of 230 GPa or higher, with 12000, 24000 or 48000 filaments per yarn, and a thermosetting resin (epoxy-based or high-purity poly- dicyclopentadiene-based resin), the vessel being obtained by over-wrapping an disposable mandrel made of ice, having diameter of greater than 2 metres, and a length of approximately twice the diameter. Example 3

A composite pressure vessel made of carbon or graphite fibres having a strength of 3200 MPa or higher and a Young Modulus of 230 GPa or higher, with 12000, 24000 or 48000 filaments per yarn, and a thermosetting resin (epoxy-based or high-purity poly- dicyclopentadiene-based resin), the vessel being obtained by over-wrapping a mandrel made of a chemically etchable material having a diameter greater than 2 metres. Example 4

A composite pressure vessel made of E-glass or S-glass fibres having a strength of 1500 Mpa or higher and a Young Modulus of 65 GPa or higher and a thermosetting resin (epoxy-based or high-purity poly-dicyclopentadiene-based resin), the vessel being obtained by over-wrapping a spherical mandrel of diameter grater than 2 metres which is made of modules which can be mechanically disassembled, and the single components or modules can be pulled out of the vessel, once disassembled, through the vessel's inlet/outlet aperture. Example 5

A composite pressure vessel made of E-glass or S-glass fibres having a strength of 1500 MPa or higher and a Young Modulus of 65 GPa or higher and a thermosetting resin (epoxy-based or high-purity poly-dicyclopentadiene-based resin), the vessel being obtained by over-wrapping filaments of the fibres on a disposable mandrel made of gypsum.

The pressure vessels described herein can carry a variety of gases, such as raw gas straight from a bore well, including raw natural gas, e.g. when compressed - raw CNG or RCNG, or H₂, or C0₂ or processed natural gas (methane), or raw or part processed natural gas, e.g. with C0₂ allowances of up to 14% molar, H₂S allowances of up to 1 ,000 ppm, or H₂ and C0₂ gas impurities, or other impurities or corrosive species. The preferred use, however, is CNG transportation, be that raw CNG, part processed CNG or clean CNG - processed to a standard deliverable to the end user, e.g. commercial, industrial or residential. CNG can include various potential component parts in a variable mixture of ratios, some in their gas phase and others in a liquid phase, or a mix of both. Those component parts will typically comprise one or more of the following compounds: C₂H₆, C₃H₈, C₄H₁₀, C₅H₁₂, C₆H₁₄, C₇H₁₆, C₈H₁₈, C₉+ hydrocarbons, C0₂ and H₂S, plus potentially toluene, diesel and octane in a liquid state, and other impurities/species.

These and other features of the present invention may be used independently or in combination, within the scope of the claims and/or the present disclosure.

The invention has been described above purely by way of example. Many other alternative embodiments are available to the skilled person within the scope of the appended claims.

Claims

CLAIMS:

1. A pressure vessel for storing and transporting CNG comprising a body defining an internal volume in which the CNG can be stored and an inlet for loading the CNG into the vessel, the body of the vessel comprising a structural shell made entirely and solely of a fibre-reinforced filament-wound composite material comprising fibres and a CNG impermeable matrix, wherein, in use, the CNG is in direct contact with an inner side of the structural shell.

2. A pressure vessel according to claim 1 , wherein the structural shell comprises a cylinder section and two terminations, one at either end of the cylinder section, all being made of the fibre-reinforced filament-wound composite material, and wherein, in use, the CNG will be in direct contact with internal parts of said cylinder section and terminations.

3. A pressure vessel according to claim 2, wherein the terminations are dome-like terminations.

4. A pressure vessel according to claim 3, wherein the dome-like terminations have a geodesic shape in respect of helical wrapping of fibres around the vessel.

5. A pressure vessel according to any one of the preceding claims, wherein the CNG is in contact with the whole of the internal surface of the fibre-reinforced filament- wound composite material.

6. A pressure vessel according to any one of the preceding claims, having a nominal external diameter in the range of 1 m to 6m.

7. A pressure vessel according to any one of the preceding claims, having a nominal diameter of about 2 metres.

8. A pressure vessel according to any one of the preceding claims, wherein the nominal thickness of the fibre-reinforced filament-wound composite material shell is between 20 and 800mm.

8. A pressure vessel according to any one of the preceding claims, wherein the fibres of the composite material comprise at least one of carbon fibres, glass fibres or Kevlar®.

9. A pressure vessel according to any one of the preceding claims, wherein the resin of the composite material comprises at least one of a polyester resin, a vinylester resin, an epoxy resin, a phenolic resin, a bismaleimide resin and a polyimide resin.

10. A method of manufacturing a composite pressure vessel for storage and transportation of CNG comprising the steps of:

a) providing a disposable mandrel;

b) winding filament fibres around the disposable mandrel to form the shape of a pressure vessel, the shape including an inlet/outlet; and

c) removing the disposable mandrel through the inlet/outlet.

1 1. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to claim 10, further comprising the step of aggregating the filament fibres to form a tape before winding them around the disposable mandrel.

12. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to claim 10 or 11 , further comprising the step of impregnating the filament fibres with a resin before winding the fibres around the disposable mandrel.

13. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to claim 12, wherein the impregnation of the fibres takes place after the fibres have been formed into a tape and by immersing the tape into a batch of resin.

14. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to any one of claims 10 to 13, further comprising the step of curing the composite while it is around the disposable mandrel, at least to a sufficient extent for it to be self supporting.

15. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to any one of claims 10 to 14, further comprising the step of curing the composite and removing the disposable mandrel once the composite has been cured at least to a sufficient extent for it to be self supporting.

16. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to any one of claims 10 to 15, wherein the mandrel comprises ice, and wherein the removal of the mandrel comprises melting the ice.

17. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to any one of claims 10 to 15, wherein the mandrel comprises compacted sand, and wherein the removal of the mandrel comprises shaking the sand out of the vessel.

18. A method of manufacturing a composite pressure vessel for storage and transportation of CNG according to any one of claims 10 to 17, wherein the mandrel comprises a scaffold, and wherein the removal of the mandrel comprises collapsing the scaffold.

19. A method of manufacturing a composite pressure vessel for storage and transportation of CNG substantially as hereinbefore described with reference to Figures 2 to 4.

20. A pressure vessel for storing and transporting CNG substantially as hereinbefore described with reference to Figures 2 to 4.

21. A ship comprising a plurality of pressure vessels according to any one of claims 1 to 9 or 20.