WO2013083160A1

WO2013083160A1 - System for containing and transporting compressed natural gas in inspectable cylindrical containers, combined in modules

Info

Publication number: WO2013083160A1
Application number: PCT/EP2011/071796
Authority: WO
Inventors: Francesco Nettis; Daniele D'AMELJ; Gianfranco NISO; Paolo REDONDI; Amedeo SILVAGNI; 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: CN104114929A; KR20140111668A

Abstract

A system for containing and transporting natural and compressed gas by ship, with cylindrical containers arranged vertically with parallel axes characterized by the fact that the said containers (100, 200) have equal heights but at least two different diameters, wherein two of the said diameters fall between 1 m and 6 m, the containers being combined in standardized modules (40), where the said cylindrical containers (100, 200) have a diameter respectively between 3 m and 6 m for a larger size of the containers (100) and between 1 and 2.5 m for a smaller size of the containers (200).

Description

SYSTEM FOR CONTAINING AND TRANSPORTING COMPRESSED NATURAL

GAS IN INSPECTABLE CYLINDRICAL CONTAINERS, COMBINED IN MODULES

[0001 ] The subject of this invention is a new system for containing and transporting natural gas, known as CNG, by means of cylindrical containers named pressure vessels which are designed for containing gas with or without liquids at a pressure that is substantially different from that of the environment, e.g. at a pressure in excess of 10Obar, or 150 bar or 200 bar or 250 bar, and up to perhaps 300 bar or 350 bar.

Field of application

[0002] Fuel gas has conventionally been transported at sea principally in the form of LNG, liquefied natural gas, or in the form of LPG, liquefied petroleum gas. The former is composed for the most part of methane in a liquid state and is conserved at a pressure close to atmospheric pressure and at a temperature close to -170°C. The latter is composed of butane, propane and other hydrocarbons and it is conserved at a moderate pressure and at a temperature close to -48 °C. CNG, an acronym for compressed natural gas, was only introduced recently and its composition is similar to LNG but it is conserved in a gaseous state and at high pressures: typically 250 barg at ambient temperature, i.e. at around 15^, or at lower temperatures, i.e. at around -30°C, thereby having a reduced pressure of about 130 barg (or up to 160 barg). CNG may additionally contain a liquid fraction, but the majority of the stored fluid is gaseous,

[0003] It is obvious that great advantages derive from the compression and reduction of the volume of the gas, especially in relation to minimising transportation costs. The increase of the working pressure, however, has rendered new container and new transport systems necessary, especially in light of legal requirements associated with safety because of the working pressure and/or temperature of these compressed/liquefied forms. From a strictly economic point of view, since CNG is less densely compacted than LNG and LPG it becomes fundamental for companies operating in the CNG transport sector to search for optimum systems a) for transporting the maximum volume of gas conservable and transportable in safety on a ship from a loading place to an unloading place, b) for streamlining the loading- unloading procedures, and c) for simplifying the maintenance operations on the containers, and any valves therefor.

The state of the art

[0004] Conventionally, the systems most commonly used for transporting CNG by sea use a plurality of cylindrical containers of various diameters and lengths, in steel or composite materials, named "pressure vessels" or PVs. They are specially designed to support high pressures and to be placed inside the hull of ships designed for the purpose.

Depending on the envisaged stresses, the materials used, and the production and handling costs, the most commonly used cylindrical containers have a diameter of 100 cm when made of steel, and these are otherwise known as "pipes" or "bottles". They can sometimes reach diameters of 300 cm if made of polymer and composite materials.

The typical length of these containers is typically constant, but that length is linked to their arrangement inside the ships: it can be slightly less than the width of the hull, or a part of it, such that they fit within the hull, laterally, when the cylinders are arranged horizontally; or it can be equal to the height of the hull, or part of it, if they are instead arranged vertically, e.g. on one or more levels.

As a rule, the cylindrical containers are placed side by side in parallel arrays, and they are fixed to a plurality of spacer supports specially integrated within the hulls.

[0005] An alternative system envisages long flexible pipes with an ample section, 15 cm for example, wound in high-diameter coils with dimensions which are sometimes equal to the width of the ship itself, arranged with a vertical axis.

[0006] Systems for storing gas and transporting it by ship, in the form of CNG or LNG, i.e. either compressed or in the liquid state, in cylindrical containers arranged vertically or horizontally, and in parallel, sometimes with solutions integrated in the hull, are therefore known. Solutions using high-diameter coils are also known. Further, in the case of gas not subjected to high pressures, systems which envisage the construction of large tanks, in various forms and dimensions, inside the ship have also been known for some time. Some particular solutions have been studied to optimize the volume of gas transported, e.g. by optimizing the arrangement of the containers or of the coils inside the ship. Other studies have been for improving and simplifying the loading and unloading operations, or for optimizing valves, manifolds and safety systems. Yet further studies have been for optimizing the sections of the containers, or for improving the materials used. For example, the following prior art documents relate to this area of technology: D1 : US 20030106324 (Bishop)

D2: US6339996 (Campbell)

D3: US 5839383 (Stenning)

D4: US4446804 (Olaf Kristiansen)

D5: US4182254 (Campbell)

D6: US3076423 (Leathard)

D7: W09716678 (Stenning)

D8: CA2636100 (Fawley)

D9: US6779565 (Fawley) [0007] D1 describes the conditions for improving the efficiency of CNG transport by ship by optimizing the volume of gas on the basis of the pressure and temperature variables. In particular, it presupposes that given a mass of gas to transport it is desirable to reduce the temperature and compression factor in order to reduce its volume, and as a consequence there can be a reduction in the transportation costs. Dimensioning is associated with the form and performance of the material used for containers and so the steels most suitable for improving safety are described. The containers are preferably steel pipes, even if other materials such as nickel alloys or composites are suitable. The said pipes are arranged horizontally inside the ships, with their interaxes staggered to optimize space and they are blocked by an original modular support system, or alternatively by using crossed metal bands. The system envisages hulls designed for the purpose, or modular containment structures.

Among the alternatives a description is provided of a dedicated reinforced concrete hull, provided longitudinally at the bottom with cylindrical compartments intended for ballast, with a smaller diameter and with interaxes staggered compared with the cylindrical containers above containing pipes. [0008] D2 describes a system for transporting CNG at ambient temperature, or just below it, with tanks made of a composite material, lighter compared with traditional steel tanks. The new tanks have suitable characteristics for transport both by ship and by truck, or modular for containers.

In one preferential transport configuration the cylindrical containers are arranged vertically and placed side by side with staggered interaxes for greater efficiency during transport. [0009] D3 describes a method for the storage and transport of fluids, particularly CNG, preferably by ship. The method is based on the superimposition of circular coil containers internally housing a continuous pipe rolled spirally, filled with gas and maintained at a controlled pressure. This method makes it possible to transport a higher quantity of gas, compared with solutions D1 and D2, and it simplifies the manifold and valve system at the same time. The possible arrangements of containers in the form of coils inside the ship include the description of a configuration with a vertical axis, with a hexagonal grid with semi-hexagonal bulkheads which ensures particularly high efficiency as regards the useful volume of gas transported.

[0010] D4 illustrates a method for transporting and storing oil or gas under high pressure, with a particular filling and emptying system facilitated by a suitable liquid; water for example. As well as the methodology that is the subject of the invention, a description is given of the containers and of their arrangement inside the ship which is of a certain usefulness for defining the state of the art: numerous cylindrical containers, or tanks, are arranged side by side, vertically, and have a diameter of 2 metres, for example, and a height of 22.5 metres, a volume of about 80 cu.m and a calculated working pressure of 100 bar. In an alternative configuration the tanks can be arranged horizontally.

[001 1 ] D5 describes a tank with cells for transporting or storing liquid gas under pressure. The overall dimensions of the structure are similar to the sum of a multitude of cylinders arranged side by side with the avoidance of the loss of space wasted between the circular sections. The result is obtained by welding a series of flat partitions arranged longitudinally in an orthogonal manner with interaxes of 3 - 4 metres, preferably 3.5 metres, and closing the sides with intermodal lobes along the entire length of the multiple tank to obtain a plurality of tunnels side by side with an essentially square section.

The cell-type tank succeeds in optimizing the usable storage volume while also apportioning the pressures and minimizing leaks due to any external breakage.

[0012] D7 describes a system for transporting CNG by ship formed by a plurality of vertical cylindrical containers grouped in units known as cells, each formed by a number from 3 to 30 containers, with a manifold and valve control system dedicated to each unit.

[0013] D9 describes a metal pressure vessel faced with a composite material providing reinforcement. State of the art closest to the invention

[0014] D6 describes a method for the transport by ship of gas in the liquid state at a low temperature and at a pressure close to atmospheric pressure which envisages the subdivision of the hull into six main compartments, each of which has a circular tank with a vertical axis, with a diameter equal to the maximum available width, and another eight circular containers with a lower diameter, called wing tanks, which occupy the unused space between a main tank and the one beside it.

Each of the wing tanks is closed by watertight bulkheads.

Of particular interest, even if not referring to the transport of CNG and to the needs linked to it, is the insertion of containers with a smaller diameter than that of the main cylindrical tanks into a delimited space in order to exploit the wasted volume. [0015] D8 describes a method for transporting compressed gases using individual containers known as racks which support and space out a plurality of cylindrical containers of the pressure vessel type that is faced with a composite material, with the cylindrical containers being arranged longitudinally, with parallel axes. The pressure vessels are connected with one another and the entire rack is then fed, monitored and handled individually. [0016] Given the above, it is clear that systems of storage and transport of CNG by sea is known. In particular, a system comprising cylindrical containers of the pressure vessel or pipe type, arranged in parallel and side by side inside the hull, with a structure inside the hull to hold the cylindrical containers and space them out, and with a connection system with a circuit, pressure gauges, and filling and discharge valves for managing and controlling the pressure and temperature both of the individual containers (pipes) and of a group of them; plus with the cylindrical containers being of the type with a diameter of 1 m if made of steel, or greater (up to 3 m for example) if made of composite materials, are known. Also known are particular systems for reinforcing cylindrical metal containers by means of facing them with a composite material. Furthermore, the cylindrical containers can be arranged in parallel with their axes staggered to reduce the volume of the unexploited space. Furthermore, systems in which the said cylindrical containers are grouped to reduce the complexity of the connections and any monitoring thereof, are known.

[0017] It is also known to provide a system of storage and transport of non-CNG gas by sea formed as follows: - cylindrical tanks with a diameter substantially equal to the width of the hull, separated by watertight bulkheads for safety in the event of damage to the hull;

- and in which the unused space between the said cylindrical tanks and the walls of the hull is occupied by cylindrical tanks of the same height but with a lower diameter.

Drawbacks

[0018] The solutions proposed and known for the storage and transport of CNG by ship have several drawbacks. In all cases, unexploited empty spaces inside the hulls are known: these spaces are the parts not occupied by the containers, and thus not used for transporting gas as a consequence of that.

Some authors, for example, as in D2, D3 and D7, suggest arranging the cylindrical containers, such as pipes or coils, with staggered interaxes to optimize the available volumes. Others, as in D1 , suggest increasing the diameter of the said containers to reduce costs and the complexity of the connections, although that requires compatibly with the design aspects linked to the strength of the material. Yet others, as in D6, insert tanks for uncompressed gas of reduced dimensions in the empty spaces left between the bigger tanks. Others again, as in D7 and D8, propose reducing the complexity of the connections by grouping the cylindrical containers in groups. Finally, D9 suggests using facings made of composite materials in order to increase the strength of the pressure vessels on a like for like thickness basis.

In principle, it is reasonable to affirm that the design of containers for CNG is conditioned by the different production processes available for the materials used, i.e. predominantly steels and composites. Further, their suitability is governed by the thicknesses of those materials, which depend on the storage conditions to be withstood, especially pressure and temperature. A further design constraint is the requirement for there to be compatibility with the anchoring systems inside the ship. In the case of steel cylinders, for example, the widespread use of containers with diameters from about 1 m to 1 .5 m is known and it is therefore very difficult to introduce innovative solutions which, at the same time, maximize the net volume used inside the ship, and which are economically advantageous and meet the safety standards and requirements laid down by international standards, such as ASME and IMO.

It is also known, for example, that producing cylindrical steel containers with diameters above 1 m for CNG entails considerable design and production difficulties, especially given the need for the designs to have compliance with the restrictive international Standards.

The main objective of companies in the sector is therefore the reduction of the unexploited spaces, e.g. by means of steel containers with greater diameters than conventional ones, but without jeopardizing the functionality and safety of the means of transport.

[0019] A second problem found regards the ability to inspect containers with small diameters. In particular, conventional steel cylinders with 1 -metre diameters, otherwise known as "pipes" or "bottles", are not provided with a means for internal inspection, so that operators are unable to get inside to check the state of the vessel, or to carry out any necessary maintenance work, such as periodical painting. The only diagnostic system applied conventionally for this container type consists of digital scanning using very costly apparatuses and complex software. Furthermore, most of these operations are carried out with the ship dry-docked, or on land, and only after removing the container in question. It follows that it would be considerably more advantageous for a qualified operator to be able to inspect the inside of the container directly on board without dry-docking the ship and without long waiting times for disconnections and handling operations. This is an aspect that is only possible by increasing the diameter of the steel containers and providing them with appropriate means for allowing the inspection, e.g. trapdoors or manholes.

[0020] A third problem found is closely linked to the number of containers transported, and to the safety systems implemented. In general, controlling the pressure or temperature requires complex and delicate constraints as regards the connection between containers or groups of containers with numerous circuits, manifolds, sensors, valves and circuit-breakers. It follows that if the number of containers is reduced, the entire connection system is simplified.

[0021 ] A further problem closely linked to the number of containers transported regards the probability of the failure of components, valves and manifolds, for example. It follows that if the number of containers is reduced, the risk factor is also reduced.

[0022] Having considered all of this, the need of companies in the sector to identify innovative solutions which are capable of overcoming at least the problems set out above is reasonable.

Statements of the invention [0023] The present invention seeks to mitigate one or more of the above issues by way of the invention defined in the annexed claims, with the solution of the problems illustrated by means of a system for containing and transporting gas, in the form of CNG, using cylindrical containers of the inspectable pressure vessel type, with different diameters, 3 m and 1 .5 m for example, where the said cylindrical containers are combined in modules and are standardized and optimized for transport by ship. The present invention also provides a system for containing and transporting natural and compressed gas by ship, the system comprising an arrangement of pressure vessels within a module, the arrangement comprising two different sizes of pressure vessel, one having a first diameter and the other having a second, smaller, diameter, wherein the vessels include at least four of the larger vessels, arranged in a uniform, square, array, and at least one smaller vessel, the smaller vessel being arranged in the space defined in the middle of the array of four larger vessel, wherein the four larger pressure vessels are arranged with a common gap therebetween, that gap being at least 380mm, and likewise, the gap between the smaller vessel, and each of those four larger vessels, is at least 380mm.

These systems preferably are such that the diameters of the two different sizes of pressure vessel meet the following requirements:

r- D D

V2— = e +—

2 2

where

"D" is the outside diameter of the four larger pressure vessels;

"d" is the outside diameter of the one smaller pressure vessel;

"e" is the distance between the outside diameter of a larger pressure vessel and the nearest vertex of a square defined by four imaginary or real boundary lines each formed to touch two of the four larger vessels; and

"g" is the minimum distance between each of the neighbouring larger pressure vessels. The modules preferably have a square base, 10 m per side for example, and a variable height depending on the dimensions of the ship's cargo hold.

They can be complete with clamps, circuits, valves and safety devices. They can be arranged side by side in a repeatable manner depending on the space to fill.

In the case of a standard module with a 10 x 10 m base, it is possible to integrate 13 cylindrical pressure vessels equipped with manholes, of the same type and height but with different diameters, for example 9 containers with a 3-metre diameter, and another 4 containers with 1 .5-metre diameters inserted in the central empty spaces between the vessels with the higher diameter. It is possible in this way to achieve an exploitation factor above 73% for the total volume of the module and an extremely low total number of containers compared with conventional solutions.

All the containers can be fitted with an inspection manhole that can be accessed directly by an operator inside the hull without any need to remove or move the containers themselves for maintenance operations and for restoring the internal lining.

[0024] Various considerable advantages are achieved as a result of the notable creative contribution, the effect of which constitutes immediate and far from negligible technical progress.

[0025] The present invention can increase the efficiency factor regarding the volume of CNG stored and transported in a ship due to a considerable reduction of the percentage of unused space.

[0026] The present invention can offer the possibility of inspecting and maintaining the said pressure vessels directly by the operators by means of an access trapdoor or manhole. This advantage makes it possible to carry out inspections and obtain the certifications required without any need to dock or dry-dock the ship to be inspected.

[0027] The present invention can simplify the connections as a result of the extremely low number of containers, with the consequent savings in time and cost during manufacture and installation and maintenance.

[0028] The present invention can by simplifying the connections, especially by reducing the numbers of valves and safety devices, create a considerable reduction in the accident and malfunction risk factor. [0029] The present invention can also simplify the connections with regards to any integrated management, e.g. by means of logical control units with a dedicated processor and software, of all the parameters involved in the connections themselves such as pressure, temperature, safety and filling, etc.

[0030] These and other advantages of the present invention will become apparent from the description below of a solution for a preferred embodiment, described by way of example only with the aid of the attached drawings.

Contents of the drawings Fig. 1 is a schematic plan view of a conventional configuration of cylindrical steel containers with a diameter of 1 m, known as pipes, arranged vertically on a module with a 10 x 10 m base.

Fig. 2 is a schematic plan view of a configuration of cylindrical steel containers, of the inspectable pressure vessel type, with diameters of 3 m and 1 .5 m, arranged vertically and combined on a module with a 10 x 10 m base.

Fig. 3 is a schematic side elevation, viewed at line A-A from the configuration shown in Fig. 2.

Fig. 4 is a schematic side elevation, viewed at line B-B of the configuration shown in Fig. 2.

Fig. 5 is a schematic plan view of a more basic combination.

Figure 6 schematically shows a section through a ship hull showing two modules arranged side by side; and

Figure 7 schematically shows a more detailed view of the top-side pipework.

Practical execution of the invention

[0031 ] The subject of this invention is a new containment and transportation system for gas in CNG form by means of cylindrical containers of the inspectable pressure vessel type, combined in modules that are standardized and optimized for transport by ship. It is known that the internal hull of a ship intended for transporting goods has an essentially box form due principally to the presence of transverse watertight bulkheads to prevent the possibility of flooding and sinking. In practice, the available volume is usually comparable with parallelepipeds with a substantially square or almost rectangular base and arranged to extend vertically.

The invention envisages the advantageous occupation of these spaces with a combination of inspectable cylindrical pressure vessels with the same height but different diameters (100, 200).

These cylindrical pressure vessels are combined in standardized modules, with a square base for example, and have diameters between 1 m and 6 m with, in particular, diameters between 3 m and 6 m for the larger containers (100) and between 1 m and 2.5 m for the smaller containers (200) that are intended to occupy the empty spaces.

In the conventional case of a hull which, for example but without limitation, has a useful internal width of about 20 m along its entire length with the exception of the bows and stern, and which also has internal partitions every 10 m, in both the longitudinal and transverse directions, an advantageous organization of the space can be noted as it is subdivided into a multitude of modular compartments, for example watertight cells (40), with a 10 x 10 m base and a height that is substantially equal to the height of the hull.

It is possible to advantageously occupy these compartments with a combination of 13 inspectable cylindrical pressure vessels arranged vertically, as shown in Figures 2 to 4, each appropriately fixed and spaced in accordance with the relevant regulations. For example, it is preferred that the distance between vessels within the compartments 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 and the walls or boundaries of the compartments, or between adjacent outer vessels of neighbouring compartments (such as where no physical wall separates neighbouring compartments) will be at least 600mm, or more preferably at least 1 m.

These gaps or spacings permit external inspection, and also allow for vessel expansion.

In the illustrated embodiment, nine of the vessels (100) have a 3-metre diameter and they occupy most of the available volume. Then another four of the vessels (200) have a 1 .5 m diameter, and they occupy the center of the empty spaces between the greater diameter vessels.

The said cylindrical containers (100, 200) are made of conventional single-layer steel or, in an alternative application, the said containers (100, 200) have a multilayer structure where one layer of steel with a reduced section is, for example and without limitation, faced with a composite reinforcing material.

In one case, the said cells (40) can also constitute independent modules provided with a suitable frame for ensuring the immovability with regard to the same of each of the cylindrical containers (100, 200) and where each module can be removed with regard to the ship's hold in such a way that it can easily be taken out itself, rather than just the pressure vessels therefrom.

In a non-exclusive preferential configuration, the said modules integrate the connections such as manifolds, valves and safety systems and are managed operatively in an integrated manner by means of logical control units with dedicated processors and software.

[0032] This combined arrangement of Figure 2 permits an improved efficiency factor compared to the prior art arrangement of Figure 1 .

In the illustrated arrangement of Figure 2, as described further above, an efficiency factor of 73.87% is achieved in the exploitation of the volume of use for transporting CNG. This is compared with a factor of 54.36% for a conventional arrangement of cylinders with a 1 -metre diameter only, arranged and spaced out in the same compartment, as in Fig. 1 . In particular, sixty four containers are installed in the conventional solution while only thirteen containers are installed in the combined one of the present invention, i.e. with two different diameters of 3m and 1 .5m. [0033] The optimal combined arrangement can be obtained with a simple mathematical system of equations that consider the different radius values of the two types of the pressure vessels (variable to suit the available space) and the space between them typically fixed by regulation). As showed in the Fig 5, other arrangements are also possible, with Figure 5 being a basic configuration, with just a single small vessel, and four large vessels, which can be expanded as appropriate with further small vessels in the gaps and large vessels in a continuing array. Figure 5 illustrates the relevant dimensions to be considered when performing the mathematical calculation.

"D" is the outside diameter of the bigger pressure vessel,

"d" is the outside diameter of the smaller pressure vessel.

"e" is the distance between the outside diameter and the nearest vertex of the square.

"g" is the minimum distance between two pressure vessels.

"L" is the space available, and the vessels need to fit into that space. The two equations that govern the calculations are:

where "g" is usually a fixed value, for instance ABS rules express a minimum distance of 0.380m between the external surface of two different pressure vessels.

With this system of equations it is possible to determine the value of the outside diameter of one pressure vessel when fixing the other one, be that the large vessel or the small one. [0033] The containers, when provided with a large diameter, e.g. more than 1 m, such as 3 m and 1 .5 m for example, have the advantage that they can be inspected internally by a person by means of a manhole, a flanged trapdoor for example, which manhole can measure at least 18 or 24 inches in diameter if circular or with an equivalent area if in a different form. The manhole can be located at the top of each of the said cylindrical containers, whereby it is easily accessible by an operator in order to check the state of conservation of the internal surface of the vessel thereof, which can be damaged mainly by corrosive processes. Further, the manhole permits, where necessary, for personnel to carry out maintenance such as, for example, internal painting and restoration work, but also operations for inspecting and checking the structural soundness of the container by means of non-destructive tests (NDT), and internal lining work in general. [0034] These containers are also shown to be provided with an opening located in their bottom end. This bottom opening can have a diameter of e.g. 12 inches, and it can instead be used for filling and discharging fluids, including any condensate liquids, the drainage of which is facilitated by gravity. Each pressure vessel is interconnected with a piping system intended for loading and offloading operations from the bottom of each vessel, such as through the preferably 12 inch (30cm) opening, to main headers, such as through motorized valves. Figures 6 and 7 illustrate such connections in a schematic manner. The main headers can comprise various different pressure levels, for example three of them (high - e.g. 250 bar, medium - e.g. 150 bar and low - e.g. 90 bar), plus one blow down header and one nitrogen header for inert purposes.

[0035] The combination of vessels with different diameters side by side is modular and easy to multiply, such as, for example but without limitation, in the case of modules with a 10 x 10 m base inside a hull with a usable width of about 30 m. It is also flexible as it always combines the different diameters in a volumetrically advantageous manner. [0036] The cylindrical containers described in this way can comply with the Standards in force governing containers for gas transportation, such as compressed natural gas, or other gases compressed at high pressure. Those Standards can include, for example, ASME or API, and associated industrial Standards.

The complexity of managing heavy gauges when machining high-pressure steel containers with large diameters is abundantly compensated by the considerable advantages already described.

[0037] The configuration with individually fluid-tight modules, also makes it possible to create and maintain an inert atmosphere using an appropriate gas (e.g. nitrogen), that would prevent the creation of a potentially explosive mixture in the event of accidental leakage of natural gas. This gas can surround the vessels, e.g. within the compartment(s). Alternatively, the engine exhaust gas could be used for this inerting function thanks to its composition being rich in C0₂. [0038] The vertical arrangement of the cylindrical containers permits a better response to the dynamic loads of the ship during navigation as compared with horizontal installation. Moreover the vertical arrangement allows an easier replacement of single vessels in the module or compartment 40 when necessary - they can be lifted out without the need to first remove other vessels from above, unless compartments are stacked above one another.

In some configurations, the vessels, or at least some of them (or all of them in a given compartment), can be provided to be longer than the hull depth, so as to extend as singular columns from just above the base of the hull to the top of the arrangement of vessels - a position located above the hull's sides - see Figure 6. This configuration can also potentially allow a fast installation time since there will be no stacking.

Mounting the vessels 10 (Fig. 6) 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 7 at the bottom of each vessel 10, prior to an offload of the CNG. After all, offloading of the gas will advantageously also be from the bottom of the vessel 10. With the majority of the piping 60 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.

By maximizing the size of the individual larger vessels, such as by making them, for example, up to 6 meter in diameter and up to 30 meter in length, for the same total volume contained the total number of vessels may be reduced, which in turn allows to reduce connection and inter-piping complexity, and hence reduces the number of possible leakage points, which usually occur in weaker locations such as weldings, joints and manifolds. Preferred arrangements call for diameters of at least 2m.

One dedicated vessel in each compartment, or one of a plurality of compartments, can be set aside for liquid storage (such as condensate) using the same concept of interconnection used for the loading or offloading of the stored gas. The compartments are thus potentially all connected together to allow a distribution of such liquid from other compartments to the dedicated vessel (or compartment) - a ship will typically feature multiple compartments. To achieve this it is possible to arrange the valves and fluid flow management system to allow any condensate to be redistributed within the vessels, such as into the smaller vessels, i.e. after it has collected at the bottoms of the vessels during transportation. The smaller vessels would generally be large enough to collect that liquid component - typically impurities found in the natural gas - wheras collecting it instead in a larger one of the vessels, would compromise a larger volume of CNG useable storage.

In and out gas storage piping may advantageously be linked with at least one of metering, heating, and/or blow down systems and scavenging systems through valve-connected manifolds. They may preferably be remotely activated by a Distributed Control System (DCS).

Piping diameters are preferably as follows:

18 inch, for the three main headers (low, medium and high pressure) dedicated to

CNG loading/offloading.

24 inch, for the blow-down CNG line.

6 inch, for the pipe feeding the module with the inert gas.

10 inch, for the blow-down inert gas line.

10 inch, for the pipe dedicated to possible liquid loading/offloading. All modules are preferably equipped with adequate firefighting systems, as foreseen by international codes, standards and rules.

[0039] In a further preferential solution, the said system can also be applied for transporting other gaseous hydrocarbons, other than CNG, or other gases, such as nitrogen, oxygen, carbon dioxide and hydrogen. The preferred use, however, is CNG transportation.

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. 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.

The transported CNG will typically be at a pressure in excess of 60bar, and potentially in excess of 100bar, 150 bar, 200 bar or 250 bar, and potentially peaking at 300 bar or 350 bar. [0040] The arrangement of the fluid filling and discharge system (with the associated system of valves and interconnections between the cylindrical containers and between the modules) underneath the containers, contributes to the stability of navigation and lowers the height of the centre of gravity.

Key

(40) modular cell

(100) cylindrical containers with diameters between 3 m and 6 m, 3 m for example

(101 ) manhole/flanged trapdoor

(200) cylindrical containers with diameters between 1 m and 2.5 m, 1 .5 m for example.

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.

Claims

1 . System for containing and transporting natural and compressed gas by ship, with cylindrical containers arranged vertically with parallel axes characterized by the fact that the said containers (100, 200) have equal heights but at least two different diameters, wherein two of the said diameters fall between 1 m and 6 m, the containers being combined in standardized modules (40), where the said cylindrical containers (100, 200) have a diameter respectively between 3 m and 6 m for a larger size of the containers (100) and between 1 and 2.5 m for a smaller size of the containers (200).

2. System for containing and transporting natural and compressed gas by ship, according to Claim 1 , characterized by the fact that the combination of inspectable cylindrical containers in parallel with one another in a module is composed of nine cylindrical containers (100) with a first, larger, diameter, preferably of about 3 m, which occupy most of the volume, and four cylindrical containers (200) with a second, smaller diameter, preferably of about 1 .5 m, which occupy the central empty spaces between the nine larger cylindrical containers (100).

3. System for containing and transporting natural and compressed gas by ship, according to Claim 1 or Claim 2, characterized by the fact that the height of the combined cylindrical containers (100, 200) is equal to or greater than the useful height of the hold.

4. System for containing and transporting natural and compressed gas by ship, according to any one of the previous claims, characterized by the fact that the said containers can be inspected by means of a manhole (101 ) fitted to the tops of the cylindrical containers (100, 200).

5. System for containing and transporting natural and compressed gas by ship, according to any one of the previous claims, characterized by the fact that the said cylindrical containers (100, 200) are made of steel.

6. System for containing and transporting natural and compressed gas by ship, according to any one of Claims 1 to 4, characterized by the fact that the said cylindrical containers (100, 200) are made of steel with a cylindrical main portion with an insufficient strength to withstand, alone, the hoop stresses from the natural and compressed gas when at an internal/external pressure difference in excess of 10Obar, that cylindrical main portion being wrapped with a composite material for reinforcement thereof so as to be able to withstand such pressure differences.

7. System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the modules include connections including manifolds, valves and safety systems.

8. System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the modules are managed in operational terms in an integrated manner by means of logical control units with dedicated processors and software.

9. System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the said containers (100, 200) are combined in modules with a four sided base.

10. System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the said containers (100, 200) are combined in modules with a width of 10 m and length of 10 m, and the diameters of the cylindrical containers (100, 200) are two different diameters, one being about 3m and the other being about 1 .5 m.

1 1 . System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the said modules are independent water- or fluid-tight cells and have an atmosphere rendered substantially inert.

12. System for containing and transporting natural and compressed gas by ship, according to any one of the preceding Claims, characterized by the fact that the said modules are provided with a frame suitable for ensuring that each of the cylindrical containers (100, 200) is immovable with regard to the same, and where each module is removable from the ship's hold in such a way that it can be taken out of the ship's hull.

13. A system for containing and transporting natural and compressed gas by ship, the system comprising an arrangement of pressure vessels within a module, the arrangement comprising two different sizes of pressure vessel, one having a first diameter and the other having a second, smaller, diameter, wherein the vessels include at least four of the larger vessels, arranged in a uniform, square, array, and at least one smaller vessel, the smaller vessel being arranged in the space defined in the middle of the array of four larger vessel, wherein the four larger pressure vessels are arranged with a common gap therebetween, that gap being at least 380mm, and likewise, the gap between the smaller vessel, and each of those four larger vessels, is at least 380mm.

14. The system of claim 13, wherein the diameters of the two different sizes of pressure vessel meet the following requirements:

2 2

where

"D" is the outside diameter of the four larger pressure vessels;

"d" is the outside diameter of the one smaller pressure vessel;

"g" is the minimum distance between each of the neighbouring larger pressure vessels.

15. The system of claim 13 or claim 14, also being in accordance with any one of claims 1 to 12.

16. A ship comprising a system according to any one of the preceding claims.