LARGE DIAMETER CYLINDRICAL PRESSURE VESSEL

Brian E. Spencer, PhD

Zachary B. Spencer

FIELD

This invention relates to metal cylinder pressure vessels greater than 3 meters in diameter wherein the diameter is reduced or tapered to accept end domes that are 2 meters in diameter or less.

BACKGROUND

The detrimental effects of the burning of fossil fuels on the environment are becoming more and more of a concern and have spurred great interest in alternative energy sources. While progress is being made with solar, wind, nuclear, geothermal, and other energy sources, it is quite clear that the widespread availability of economical alternate energy sources, in particular for high energy use applications, remains an elusive target. In the meantime, fossil fuels are forecast to dominate the energy market for the foreseeable future. Among the fossil fuels, natural gas is the cleanest burning and therefore the clear choice for energy production. There is, therefore, a movement afoot to supplement or supplant, as much as possible, other fossil fuels such as coal and petroleum with natural gas as the world becomes more conscious of the environmental repercussions of burning fossil fuels. Unfortunately, much of world's natural gas deposits exist in remote, difficult to access regions of the planet. Terrain and geopolitical factors render it extremely difficult to reliably and economically extract the natural gas from these regions. The use of pipelines and overland transport has been evaluated, in some instances attempted, and found to be uneconomical. Interestingly, a large portion of the earth's remote natural gas reserves is located in relatively close proximity to the oceans and other bodies of water having ready access to the oceans. This quite naturally raised the possibility of transporting natural gas by ship. At present, marine transport of natural gas from the remote locations as a compressed natural gas, CNG, is rapidly becoming the de facto method for obtaining natural gas from remote sources.

The most common method of transporting CNG by ship is in cylindrical pressure vessels having one or two domed ends. Those currently in use are up to about 18 meters long and a maximum of about 2 meters in diameter. While it would be beneficial to increase both the length and the diameter of these pressure vessels - pressure vessels 30 or more meters in length and 6 or more meters in diameter have been suggested - a major problem exists in that the fabrication of the domes for cylinders over about 2 meters (7 feet) in diameter has proven to be extremely difficult and expensive, to the point in fact that it is currently considered cost prohibitive.

What is needed, then, are large diameter cylindrical pressure vessels in which the need for large end domes, i.e. of the same diameter as the pressure vessel, is eliminated. The current invention provides such vessels.

SUMMARY

Thus, in one aspect, this invention is directed to a pressure vessel comprising: a hollow cylinder having a proximal end, a distal end and a side wall that defines an inside diameter of the cylinder, the diameter being 3 meters or more;

a frustum, wherein:

the base of the frustum is coupled to the proximal end of the hollow cylinder, or the base of one frustum is coupled to the proximal end of the hollow cylinder and the base of a second frustum is coupled to the distal end of the hollow cylinder;

at the truncated apex of the frustum, a smaller internal diameter of the frustum is two meters or less; and,

the surface of the frustum proceeds from the point of coupling of the base of the frustum to the proximal end of the cylinder toward the truncated apex of the frustum at an angle of 15° to 60° with respect to the cylinder side wall.

Instead of a 2m diameter, in other aspects of the invention, for cylinders with an inside diameter larger than 4m, the smaller inside diameter of the frustum may be 3m or less.

In an aspect of this invention, the frustum is made of the same metal as the cylinder or of a different metal from that of the cylinder.

In an aspect of this invention, the pressure vessel further comprises a dome coupled to the truncated apex of the frustum.

In an aspect of this invention, the dome comprises the same metal as the cylinder, the same metal as the frustum, a different metal from that of either the cylinder or the frustum, or of a polymeric composite.

In an aspect of this invention, the dome comprises a polar opening to which a boss is coupled. In an aspect of this invention, the boss is comprised of a the same metal as the cylinder, the same metal as the frustum, the same metal as the dome, a different metal from that of the cylinder, the frustum and the dome, or of a polymeric composite.

In an aspect of this invention, the boss comprises a one-piece polymeric composite construct.

In an aspect of this invention, the pressure vessel further comprises a boss coupled to the truncated apex of the frustum.

In an aspect of this invention, the boss is made of a same metal as the frustum, a different metal from the frustum or a polymeric composite.

In an aspect of this invention, the boss comprises a one-piece polymeric composite construct.

In an aspect of this invention, the vessel is used for the containment and transport of compressed natural gas.

In an aspect of this invention, the compressed natural gas comprises compressed raw natural gas.

The vessel may be filled with compressed natural gas. The compressed natural gas may be at a pressure of about 250bar.

An aspect of this invention is a ship comprising the pressure vessel as defined above.

DETAILED DESCRIPTION

Brief description of the figures

These figures are provided for illustrative purposes only and are not intended nor should they be construed as limiting this invention in any manner whatsoever.

Figure 1 shows a frustum 1 of cone 2.

Figure 2 shows a pressure vessel with a cylindrical center section and one frustum end section.

Figure 3 shows an arrangement of pressure vessels according to the present invention arranged in a hull of a ship.

Discussion

It is understood that, with regard to this description and the appended claims, reference to any aspect of this invention made in the singular includes the plural and vice versa unless it is expressly stated or unambiguously clear from the context that such is not intended. For instance, reference to a "truncated apex conical end section" is to be construed as referring to one such end section or two such end sections on a cylindrical pressure vessel.

As used herein, any term of approximation such as, without limitation, near, about, approximately, substantially, essentially and the like, mean that the word or phrase modified by the term of approximation need not be exactly that which is written but may vary from that written description to some extent. The extent to which the description may vary will depend on how great a change can be instituted and have one of ordinary skill in the art recognize the modified version as still having the properties, characteristics and capabilities of the word or phrase unmodified by the term of approximation. In general, but with the preceding discussion in mind, a numerical value herein that is modified by a word of approximation may vary from the stated value by ±10%, unless expressly stated otherwise.

The terms "proximal" and "distal" simply refer to the opposite ends of a construct and are used as a method of orienting an object with relation to another object such as the orientation of a frustum of this invention with the cylindrical main body of a pressure vessel. In general, which end is designated as proximal and which as distal is purely arbitrary unless the context unambiguously expresses otherwise.

As used herein, the use of "preferred," "preferably," or "more preferred," and the like refers to preferences as they existed at the time of filing of this patent application.

As used herein, a "pressure vessel" refers to any closed container designed to hold fluids at a pressure substantially different from ambient pressure. In particular at present, it refers to such containers used to hold and transport compressed natural gas, CNG. Pressure vessels may take a variety of shapes but most often seen in actual use are spherical, oblate spheroidal, toroidal and cylindrical center section vessels usually with domed end sections at either or both ends.

As used herein, a "frustum" has its normal meaning: a truncated cone in which the plane cutting off the apex of the cone to form the frustum is parallel to the base of the cone. A frustum of a cone is shown is Fig.1 .

As used herein, a "fluid" refers to a gas, a liquid or a mixture of gas and liquid. For example, without limitation, natural gas as it is extracted from the ground and transported to a processing center is often a mixture of the gas with liquid contaminants. Such mixture would constitute a fluid for the purposes of this invention.

As used herein, a "polymeric composite" has the meaning that would be ascribed to it by those skilled in the art. In brief, it refers to a polymer, usually designated as the "matrix" or "matrix polymer" in polymeric composites, which matrix has been infiltrated with a fibrous filler material. Put another way, a polymeric composite comprise a fibrous material that has been impregnated with a matrix polymer.

As used herein, a "boss" refers to a device as such would be understood by those skilled in the art. In brief, a "boss" is a device used to connect a pressure vessel with external piping through which the pressure vessel is filled with or emptied of a fluid.

The current most common pressure vessel for the containment and transport of large quantities of compressed fluids is the all metal cylindrical pressure vessel, made usually of steel or aluminum, although other metals have been used. These vessels consist of a hollow cylinder central section and one two domed end section, which are fabricated separately and then welded to the end of the cylinder. In this manner, quite large pressure vessels can be constructed at least with regard to length wherein virtually any dimension is possible, although possibly not practical. Currently, the largest pressure vessels are in the vicinity of 18 meters in length. The diameter of metal pressure vessels is another story entirely. While it would be extremely valuable to be able to increase the diameter of metal vessels instead of or concomitant with increasing their length, it has been empirically found that fabrication of domes in excess of about two meters in diameter presents challenging mechanical problems that render domes of such size economically impractical. The present invention solves the problem by interposing a frustum between the end of the cylindrical center section of a metal pressure vessel and its dome end(s). The frustum can be constructed of the same metal as the cylindrical portion of the pressure vessel or it may be constructed of a different metal, the only proviso being that, if the metals are different, they are compatible. Those skilled in the art know or can readily ascertain which metals are compatible with regard to use in a pressure vessel those that are not.

With the effective diameter of a pressure vessel reduced by introduction of a frustum into the vessel design, any conventional dome presently in use can be coupled to the truncated apex of the frustum to form the basic shell of what may be a very large pressure vessel both in length and diameter. By "basic shell" is meant simply that, as those skilled in the art immediately recognize, additional paraphernalia will be added to the pressure vessel to render it complete, such as, without limitation, a boss may be added to a polar opening in the dome to serve as the connection between the pressure vessel and external sources of fluids and destinations to which a fluid contained in the pressure vessel may be transferred. A pressure vessel of this invention is shown in Fig. 2. In an alternate embodiment, the frustum tapers down to a final diameter at the truncated apex that is just sufficient for coupling to a boss, thereby eliminating the need for a domed end section entirely.

In a further alternate embodiment, the dome has an arc-angle defined so as to tangentially align with the angle of the frustum. This avoids an inflection point between the dome and the frustum, thus removing a bending hoop stress when the vessel is under pressure.

Figure 2 shows a pressure vessel 10 with a cylindrical center section 20, a frustum end section 30, a truncated apex 40 and a dome 50 having a polar opening 60 for connection of a boss. In general, a "polar opening" refers to a hole in the dome, usually circular in shape, the perimeter of which is radially equidistant from centerline 70 of vessel 10. The sides of the frustum angle toward centerline 70 at angle a between 15 ^s and 60 ^s . The angle, as shown is generally constant.

If desired, opposite end 80 of pressure vessel 10 can also comprise a frustum, a dome, etc. The contour of the other end may be the mirror image of frustum 30, dome 50 and polar opening 60 and angle a shown in Fig. 2 or it may be different, i.e., the angle of the frustum side may be different. It would generally want to be within the previously stated limits, however. The dome may have a different contour and the polar opening may be of a different diameter. Often a bottom opening is smaller than a top. For example, the pressure vessel may comprise a manhole at one end - usually a top end - for entering and/or inspecting the interior of the vessel. The manhole may be a 24 inch (60cm) manhole, or equivalent, for allowing internal inspection, e.g. by a person climbing into the vessel. The manhole may have closing means for allowing sealed closing of the opening thereof. The manhole is preferably provided according to ASME (American Society of Mechanical Engineers) standards.

With regard to the dome that is coupled to the truncated apex of the frustum, it may (1 ) be fabricated of the same metal as cylindrical center section 20 and frustum 30 or, (2) if frustum 30 is made of a different metal than cylindrical center section 20, of the same metal as frustum 30 or (3) of a different metal than either cylindrical center section 20 and frustum 30 or (4) of a different material entirely, such as, without limitation, a polymeric composite or a ceramic.

If the dome is metal, which, as above, can be the same as any other metal used in the fabrication of the pressure vessel or may be an entirely different metal, it can be coupled to the truncated apex using any of the techniques known to those skilled in the art, such as, without limitation, welding or fastening with bolts and the like. At present, welding is the preferred means of coupling a metal dome to a frustum of this invention.

On the other hand, the dome may be a polymeric composite. In this scenario, it may be coupled to the truncated apex of the frustum in the matter disclosed in our co- pending patent application entitled "Type II pressure vessel with composite dome", the entire contentes of which are incorporated herein by way of reference. For example, however, coupling the circular cross-section base to the proximal end of the cylindrical center section may comprise:

forming the composite dome with a cylindrical base extension that terminates with the circular cross-section base, wherein:

the cylindrical base extension has an outside diameter that permits it to be inserted into the hollow cylindrical center section;

forming a semicircular groove circumferentially around an outer surface of the cylindrical base extension, the groove having a preselected diameter;

forming a semicircular groove circumferentially around an inner surface of the cylindrical center section, the groove having the same diameter as the cylindrical base extension groove;

forming an additional groove circumferentially around one of the outer surface of the cylindrical base extension or the inner surface of the cylindrical center section, the groove being disposed between the semicircular groove in that surface and the circular cross-section base or the proximal end of the cylindrical center section;

inserting a seal into the additional groove;

inserting the cylindrical base extension into the proximal end of the cylindrical center section until the semicircular grooves align to form a circular cross-section circumferential cavity;

forming a circular cross-section lumen that extends from the outer surface of the cylindrical center section until it intersects the circumferential cavity, the lumen having a diameter substantially the same as that of the cavity;

inserting a flexible wire having a diameter incrementally smaller than the diameter of the lumen and the cavity through the lumen and into the cavity; and

advancing the wire through the cavity until it traverses substantially the entire circumference of the cavity. The seal might selected from the group consisting of an o-ring seal, a lip seal, a cup seal, a V-seal, a bore seal and a face seal. The flexible wire might be selected from the group consisting of steel, titanium, aluminum and a nickel-based alloy.

Alternatively the method may comprise:

forming the composite dome with a dome flange extending circumferentially outward from the circular cross-section base, the flange having a plurality of through- holes disposed around the flange circumference;

forming the cylindrical center section with a center section flange extending circumferentially outward from the proximal end of the cylindrical center section, the center section flange comprising a plurality of through-holes disposed around the flange circumference; wherein:

the dome flange comprises a surface that can be placed contiguous to a surface of the center section flange; and

when the flanges are contiguous, the through-holes align;

forming a circumferential cavity in the proximal end of the center section, the cavity being disposed within the center section thickness;

inserting a seal in the circumferential cavity;

placing the dome flange contiguous with the center section flange such that the through-holes in the dome flange and those center section flange align; and

coupling the dome flange to the center section flange with fasteners disposed through the aligned through-holes.

The fasteners may comprise nut and bolt assemblies.

In a further alternative, the coupling of the composite dome to the cylindrical metal tube may comprise:

forming the composite dome with a dome flange that extends circumferentially outward from the circular cross-section base, the dome flange having a distal surface that is parallel to the dome base and a proximal surface that tapers from the origin of the flange at an outer surface of the dome toward an outer edge of the flange;

forming the cylindrical center section with a center section flange extending circumferentially outward from and parallel to the proximal end of the cylindrical center section, wherein the center section flange is longer than the dome flange;

forming a circumferential cavity in either the dome flange or the center section flange

inserting a seal in the cavity; placing the dome flange contiguous to the center section flange;

providing a separate metal annular ring with a cross-section having a proximal surface that is tapered counter to the taper in the dome flange surface and is of substantially the same length as proximal surface taper in the dome flange, a lower surface that extends outward from the point at which the tapered surface begins parallel to the center section flange surface and an upper surface that extends outward from the point where the tapered surface ends parallel to the lower flange surface;

placing the dome flange contiguous to the center section flange

placing the metal annular ring over the dome such that its lower surface is contiguous to the center section flange surface and its tapered surface is contiguous to the dome flange tapered surface; and,

coupling the annular ring to the center section flange.

Coupling the annular ring to the center section flange may comprise welding the two together.

Coupling the annular ring to the center section flange may comprise forming through-holes in the annular ring and in the center section flange wherein the though- holes align when the annular ring and the center section flange are contiguous and coupling the annular ring to the center section flange using fasteners inserted into the through-holes.

The fasteners may comprise nut and bolt assemblies.

Coupling the annular ring to the center section flange may even comprise using a plurality of C-clamps.

The composite dome typically comprises a fibrous material embedded in a polymeric matrix.

The fibrous material may be selected from the group consisting of glass fiber, carbon fiber, aramid fiber, ultra-high molecular weight polyethylene fiber and combinations thereof.

The polymeric matrix may comprises a dicyclopentadiene polymer formulation.

Preferably the dicyclopentadiene polymer formulation comprises at least 92% pure dicyclopentadiene.

Regardless of whether the dome is metal, polymeric composite or some other material such as a ceramic, a metal boss may be fitted to polar opening 60. Similarly to the frustum, the boss may be fabricated of the same metal as the cylinder, the same metal as the frustum, or a different metal from that of either the cylinder or the frustum. In the alternative, the boss may be a single-piece polymeric composite boss as such is disclosed in co-pending patent application entitled "Pressure Vessel With Composite Boss". For example, the one-piece composite boss comprises:

a hollow elongate cylinder having a proximal end, a distal end, an outer surface and an inner surface, the inner surface defining the diameter of the hollow portion of the elongate cylinder.

A portion of the outer surface of the cylinder can be contiguous with a thickness of a wall of the pressure vessel that defines a circular opening in the pressure vessel.

The proximal end of the cylinder can terminate exterior to the pressure vessel in a proximal end surface.

The proximal end surface can comprise a plurality of peripherally disposed threaded holes.

The distal end of the cylinder can terminate in a flange having a flange surface that is contiguous with an inner surface of the pressure vessel, a flange diameter that is larger than the diameter of the circular opening in the pressure vessel and a flange thickness at the point where the flange surface meets the diameter of the circular opening, that is sufficient to withstand a pressure exerted by a compressed fluid contained in the pressure vessel.

In an aspect of this invention, surfaces of the boss that would otherwise come in contact with the compressed fluid are separated from the compressed fluid by a layer of material that is substantially impenetrable by the compressed fluid at the operating pressure of the pressure vessel.

In an aspect of this invention, the layer of material is also substantially inert to the compressed fluid.

In an aspect of this invention, the layer of material comprises a metal, a ceramic or a polymer.

In an aspect of this invention, the shape of the pressure vessel comprises a sphere, an oblate spheroid, a torus or an elongate hollow cylinder with one or two domed end sections.

In an aspect of this invention, the pressure vessel is made entirely of a metal of sufficient thickness to withstand the pressure exerted by the compressed fluid contained therein.

In an aspect of this invention, the hollow cylinder with one or two domed end section comprises a thin metal liner that is hoop-wrapped with a polymeric composite and the one or two domed end sections comprise a metal, which may be the same as or different than the metal of the cylinder liner, at a sufficient thickness to withstand the pressure exerted by the compressed fluid contained in the pressure vessel.

In an aspect of this invention, the hollow cylindrical and the one or two domed end sections comprise a thin metal liner, wherein:

the hollow cylinder is hoop-wrapped with a polymeric composite and the cylinder and domed end sections are isotensoidally-wrapped with a polymeric composite, which may be the same as, or different than the polymeric composite of the hoop wrap.

In an aspect of this invention, the hollow cylindrical and the one or two domed end sections comprise a polymeric liner that is hoop-wrapped, isotensoidally wrapped or a combination of hoop- and isotensoidally- wrapped with a polymeric composite.

In an aspect of this invention, the pressure vessel further comprises a shear ply positioned between surfaces of the boss and surfaces of the polymeric composite wrap at locations where boss surfaces would otherwise be in direct contact with wrap surfaces.

In an aspect of this invention, the diameter of the flange extends at least to an inflection point in the one or two domed end section contours.

In an aspect of this invention, the polymeric composite comprises a thermoset polymer matrix.

In an aspect of this invention, the thermoset polymer matrix is selected from the group consisting of epoxy resins, polyester resins, vinyl ester resins, polyimide resins, dicyclopentadiene resins and combinations thereof.

In an aspect of this invention, the thermoset polymer matrix is formed from a prepolymer formulation that comprises dicyclopentadiene, which is at least 92% pure.

In an aspect of this invention, the polymeric composite comprises a fibrous material.

In an aspect of this invention, the fibrous material is selected from the group consisting of metal fibers, ceramic fibers, natural fibers, glass fibers, carbon fibers, aramid fibers, ultra-high molecular weight polyethylene fibers and combinations thereof.

In an aspect of this invention, the fibrous material is selected from the group consisting of glass fibers and carbon fibers.

In an aspect of this invention, the pressure vessel further comprises metallic inserts having a threaded outer surface that mates with the threaded holes in the proximal end surface of the boss and a threaded inner surface sized to mate with threads of an external pipe coupling device. In an aspect of this invention, the compressed fluid comprises compressed natural gas.

In an aspect of this invention, the compressed natural gas comprises compressed raw natural gas.

The use of composite domes and bosses can improve the economics of transport of compressed fluids in the pressure vessels of this invention by reducing, in some cases substantially, the weight of the pressure vessel, thereby increasing the contained compressed fluid to tare weight ratio and concomitantly the value of the fluid.

With regard to composite domes and composite bosses, each may comprise a polymeric matrix containing a fibrous or filamentous material, which is responsible for the overall strength on the composite. The polymeric matrix can be any polymer known or found to have properties consistent with use with pressure vessels such as those of this invention.

While thermoplastic polymers, thermoplastic elastomers, thermoset resins and combinations thereof can be used, presently preferred are thermoset polymers, which can exhibit significantly better mechanical properties, chemical resistance, thermal stability and overall durability than the other types of polymers. An advantage of most thermoset resins is that their precursor monomers or prepolymers tend to have relatively low viscosities under ambient conditions of pressure and temperature and therefore can be introduced into or combined with fibers and filaments quite easily. As used herein, "ambient temperature" simply refers to the temperature in the environs where application and curing of the prepolymer is to occur, wherein the environs is not heated specifically to achieve a suitable application and curing temperature. Generally, ambient temperature is from about 55 °F to about 100 °F, although the presently preferred dicyclopentadiene prepolymer formulation of this invention may be used at ambient temperatures both above and, particularly, well below this range. This avoids the need for special temperature- controlled environments, an exceedingly beneficial objective particularly when fabricating very large pressure vessels such as those described earlier. Another advantage is that thermoset polymers can usually be chemically cured isothermally, that is, at the same temperature at which they are combined with the fibers/filaments, which can be room temperature.

Suitable thermoset resins include, without limitation, epoxy resins, polyester resins, vinyl ester resins, polyimide resins, dicyclopentadiene resins and combinations thereof. Presently preferred are dicyclopentadiene resins. With regard to dicyclopentadiene resins, it is presently preferred that the dicyclopentadiene monomer in the prepolymer formulation that will be used for the fabrication of the boss or dome is at least 92%.

As used herein, a "dicyclopentadiene prepolymer formulation" refers to a blend of at least 92% pure dicyclopentadiene with one or more reactive ethylene monomer(s), a polymerization initiator or curing agent plus any other desirable additives prior to curing. After curing, the polymer matrix obtained from the dicyclopentadiene prepolymer formulation is termed a "dicyclopentadiene polymer."

In general, any type of fibrous or filamentous material that is known or found to have the requisite strength requirements for use as a pressure vessel composite may be employed. Such materials include, without limitation, natural (silk, hemp, flax, etc.), metal, ceramic, basalt and synthetic polymer fibers and filaments. Presently preferred materials include glass fibers, commonly known as fiberglass, carbon fibers, aramid fibers, which go mostly notably under the trade name Kevlar ^® and ultra-high molecular weight polyethylene, such as Spectra ^® (Honeywell Corporation) and Dyneeva ^® (Royal DSM N.V.).

A pressure vessel of this invention can be used to contain and transport any type of fluid so long as the metal of the vessel, the metal or other substance from which the frustum is fabricated, the metal or other material from which the dome is fabricated and the metal or other material from which the boss is fabricated, are selected so as to be impermeable and inert to the compressed fluid to be contained in the pressure vessel. A presently preferred use of a pressure vessel of this invention is the containment and transport of natural gas, often referred to as "compressed natural gas" or simply "CNG."

CNG may be contained and transported in the vessels of this invention both as a purified gas and as "raw gas." Raw gas refers to natural gas as it comes, unprocessed, directly from the well. It contains, of course, the natural gas (methane) itself but also may contain liquids such as condensate, natural gasoline and liquefied petroleum gas. Water may also be present as may other gases, either in the gaseous state or dissolved in the water, such as nitrogen, carbon dioxide, hydrogen sulfide and helium. Some of these may be reactive in their own right or may be reactive when dissolved in water, such as carbon dioxide and hydrogen sulfide which produces an acid when dissolved in water.

A plurality of the pressure vessels can be arranged on a ship, for example in one or more modules or compartments. See, for example, Figure 3, where the vessels 10 are all arranged vertically. The vessels may be in a regular array within the ship or the modules or compartments, as shown. The arrangements can be chosen or designed to fit appropriately in the ship's hull.

For external inspection-ability reasons it is preferred that the distance between the vessels be at least 380mm, or more preferably at least 600 mm. These distances also allow space for vessel expansion when loaded with the pressurized gas - the vessels may expand by 2% or more in volume when loaded (and changes in the ambient temperature can also cause the vessel to change their volume).

Preferably the distance between the outer vessels of an array and the walls or boundaries of the ship or modules or compartments, or between adjacent outer vessels of neighbouring modules or compartments, such as where no physical wall separates neighbouring modules or compartments, will be at least 600mm, or more preferably at least 1 m, again for external inspection-ability reasons, and/or to allow for vessel expansion.

The pressure vessels can be interconnected for loading and offloading operations.

Each pressure vessel row (or column) in an array can be interconnected with a piping system intended for loading and offloading operations.

The piping for loading or offloading is preferably connected at the bottom of each vessel via a further opening, preferably a 12 inch (30cm) opening, and the connections are to main headers of the ship's loading and offloading infrastructure, such as through motorized valves.

Preferably the vessels are mounted within the ship in a vertical orientation. Alternatively they can be horizontal. However, when the vessels are vertically mounted, they are less critical in following dynamic loads resulting from the ship motion. Moreover the vertical arrangement allows an easier replacement of single vessels when necessary - they can be lifted out without the need to first remove other vessels from above, assuming vessels are not stacked above one another - they are preferably designed to be long, e.g. in excess of 15m, whereby vertical stacking is unlikely.

This vertical, and long, non-stacked, configuration can also potentially allow a fast installation time.

Mounting the vessels in vertical positions also allows condensed liquids to fall under the influence of gravity to the bottom, thereby being off-loadable from the vessels using, e.g. the 12 inch opening at the bottom of each vessel.

Offloading of the gas will advantageously also be from the bottom of the vessel. With the majority of the piping installed towards the bottom of the modules, the center of gravity of the whole arrangement will be also in a low position, which is recommended or preferred, especially for improving stability at sea, or during gas transportation.

Modules or compartments containing pressure vessels are preferably kept in a controlled environment with nitrogen gas being between the vessels and the modules' walls, thus reducing fire hazard. Alternatively, the engine exhaust gas could be used for this inerting function thanks to its composition being rich in C0 ₂ .

The pressure vessels discussed above 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.

Storage/transportation pressures can be anything up to say 400bar, but usually up to 300 bar, and normally in excess of 100 bar.

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.The transported CNG will typically be at a pressure in excess of 60bar, and potentially in excess of 10Obar, 150 bar, 200 bar or 250 bar, and potentially peaking at 300 bar or 350 bar.

The present invention has been described above purely by way of example. Modifications in detail may be made to the invention within the scope of the claims appended hereto.