Integrated storage/offloading facility for an LNG production plant

An LNG production plant positioned at a production location adjacent to a body of water is described. The LNG production plant includes a plurality of spaced-apart facilities including a first facility and a second facility, each facility provided with plant equipment related to a pre-determined function associated with the production of LNG, where the first facility is an onshore facility and the second facility is an integrated storage/offloading facility arranged on a gravity-based structure having a base that rests on the seabed at a selected location within the body of water.

FIELD

The present invention relates to an integrated storage/offloading facility for a liquefied natural gas (“LNG”) production plant.

BACKGROUND

Large volumes of natural gas (i.e., primarily methane) are located in remote areas of the world. This gas has significant value if it can be economically transported to market. Natural gas (“NG”) is routinely transported from an onshore LNG production plant to another location in its liquid state as liquefied natural gas (“LNG”) by way of loading the LNG in the cryogenic storage tanks of purpose built large ocean going vessels known as “LNG Carriers”. Liquefaction of the natural gas makes it more economical to transport as LNG occupies only about 1/600th of the volume than the same amount of natural gas does in its gaseous state. Prior to liquefaction, raw natural gas that has been sourced from a wellhead is subjected to a series of gas pre-treatment processes including acid gas removal and dehydration to remove contaminants. After liquefaction, LNG is typically stored in cryogenic storage tanks at the LNG production plant either at or slightly above atmospheric pressure at a temperature of around −160 degrees Celsius.

Gas pre-treatment, liquefaction and storage are typically undertaken at a fixed onshore LNG production plant associated with a jetty that is built in sufficiently deepwater to allow berthing of the LNG Carriers. To ship liquefied natural gas (LNG) by sea, a way to transfer LNG between the cryogenic storage tanks of the onshore LNG production plant and the cryogenic storage tanks of the LNG Carrier is required. Traditionally, the transfer means has taken the form of an insulated pipe that is laid on an elevated supporting trestle structure between the onshore LNG production plant and the jetty so that the insulated pipe remains at all times above the water line. These prior art transfer facilities include a vapour return line to return boil-off gas to the onshore LNG production plant. After LNG have been loaded into the cryogenic storage tanks of the LNG Carrier vessel for marine transport LNG is regasified before distribution to end users through a pipeline or other distribution network at a temperature and pressure that meets the delivery requirements of the end users.

The cost of LNG storage and offloading facilities has continued to increase through the years and is now a very significant component of the total installed cost for an LNG project. Efforts to reduce this cost have largely been focused on storage tank size optimization and seeking to leverage the economics of scale via increased LNG train capacity size and improvement in LNG berth utilization.

There remains a need to explore alternative designs for LNG storage and offloading facilities.

SUMMARY

According to a first aspect of the present invention there is provided an LNG production plant positioned at a production location adjacent to a body of water, the LNG production plant comprising a plurality of spaced-apart facilities including a first facility and a second facility, each facility provided with plant equipment related to a pre-determined function associated with the production of LNG, wherein the first facility is an onshore facility and the second facility is an integrated storage/offloading facility arranged on a gravity-based structure having a base that rests on the seabed at a selected location within the body of water, wherein the first facility is a liquefaction facility for receiving a stream of pre-treated gas from a gas processing facility and liquefying the pre-treated gas to produce a product stream of LNG, and, the integrated storage/offloading facility includes a first cryogenic storage tank operatively associated with the liquefaction facility for receiving and storing the product stream of LNG from the liquefaction facility via a cryogenic pipeline, and, an LNG transfer facility for transferring LNG from the first cryogenic storage tank to a second cryogenic storage tank onboard an LNG Carrier on an as-needs basis.

In one form, the selected location has a water depth of at least 14 meters, at least 15 meters, or at least 16 meters. In one form, the integrated storage/offloading facility is a breakwater for an LNG Carrier. In one form, the cryogenic pipeline is a cryogenic subsea pipeline or a cryogenic pipeline on a trestle.

In one form, the first facility is a gas processing facility for receiving raw hydrocarbons from a producing well and treating the raw hydrocarbons to remove contaminants therefrom to produce a stream of treated gas as a source of feed to a liquefaction facility for receiving the stream of treated gas from a gas processing module and liquefying the natural gas to produce LNG, and wherein the liquefaction facility is located on the gravity-based structure with the integrated storage/offloading facility.

In one form, the integrated storage/offloading facility is transportable from a construction location to an assembly location by towing or on floating barges. In one form, the integrated storage/offloading facility is transportable from an assembly location to the production location by towing or on floating barges. In one form, commissioning of the integrated storage/offloading facility is done at an onshore construction location or an onshore assembly location prior to transportation of integrated storage/offloading facility to the production location. In one form, the integrated storage/offloading facility includes a ballast storage compartment, and the integrated storage/offloading facility is settled into the selected location by way of addition of a ballasting material to the ballast storage compartment. In one form, the ballast storage compartment is arranged around the periphery of the integrated storage/offloading facility or arranged toward the base of the integrated storage/offloading facility for ballasting. In one form, the ballast storage compartment is at least partially filled with one or both of a solid ballasting material or a liquid ballasting material. In one form, the solid ballasting material is iron ore or sand. In one form, the liquid ballasting material is one or more of: water, condensate, monoethylene glycol (MEG), methanol, diesel, demineralised water, diesel, or, LPG.

In one form, integrated storage/offloading facility includes a boil-off gas reliquefaction facility. In one form, the integrated storage/offloading facility has at least one lateral side which has a length of a sufficient size to allow an LNG Carrier to be moored along alongside the gravity-based structure without overhang of the LNG Carrier beyond an end of the gravity-based structure. In one form, the integrated storage/offloading facility has a lee side, whereby, in use, the LNG Carrier approaches the integrated storage/offloading facility from the lee side of integrated storage/offloading facility. In one form, the integrated storage/offloading facility has a longitudinal axis aligned substantially parallel to the direction of a predominant current for bi-directional berthing of an LNG Carrier.

In one form, the integrated storage/offloading facility comprises a plurality of similarly-sized sub-facilities, which are integrated at a construction location, the production location, or at an independent assembly location. In one form, the sub-facilities are constructed at a plurality of construction locations and towed to a common assembly location for integration. In one form, the sub-facilities are assembled to form the integrated storage/offloading facility at the assembly location and testing or commissioning of the sub-facilities is conducted a construction or assembly location before transportation of the integrated storage/offloading facility to the production location.

In one form, the integrated storage/offloading facility is movable from a first production location to a second production location. In one form, the integrated storage/offloading facility includes a boil-off gas reliquefaction facility for liquefying at least a portion of the boil off gas that is generated either during the transfer of the LNG through the pipeline to the first cryogenic storage tank of the integrated storage/offloading facility or during the transfer of the LNG from the first cryogenic storage tank to the second cryogenic storage tank of the LNG Carrier. In one form, a portion of boil off gas is a source of fuel for a first power generation system which forms part of the integrated storage/offloading facility or a second power generation facility onboard the LNG Carrier. In one form, a first portion of the LNG produced by the liquefaction facility is transferred directly into a second cryogenic storage tank onboard an LNG Carrier and a second portion of the LNG produced by the liquefaction facility is stored in the first cryogenic storage tank of the integrated storage/offloading facility. In one form, the integrated storage/offloading facility has a multilateral footprint when viewed in plan view. In one form, the footprint is triangular, rectangular, square, pentagonal or hexagonal whereby, in use, a first LNG Carrier berths at a first lateral side of the integrated storage/offloading facility while a second LNG Carrier berths at a second lateral side of the integrated storage/offloading facility.

In one form, the LNG production plant further comprises a breakwater facility positioned adjacent to the integrated storage/offloading facility at the selected location. In one form, a first breakwater facility is located towards a first end of the integrated storage/offloading facility and a second breakwater facility is located towards a second end of the integrated storage/offloading facility.

It is to be noted that the drawings illustrate only preferred embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments. Like reference numerals refer to like parts. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all drawings are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely.

DETAILED DESCRIPTION

Particular embodiments of the present invention are now described. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs.

Throughout this specification, the term “integrated storage/offloading facility” refers to a storage facility that is located together with an offloading facility, for example on top of or inside of a gravity based structure.

Using the process and system of the present invention, an LNG production plant is positioned at a production location adjacent to a body of water, the LNG production plant comprising a plurality of spaced-apart facilities including a first facility and a second facility, each facility provided with plant equipment related to a pre-determined function associated with the production of LNG, wherein the first facility is an onshore facility and the second facility is arranged on a gravity-based structure having a base that rests on the seabed at a selected location within the body of water. More specifically, embodiments of the present invention relate to an LNG production plant including at least the following facilities:a) a gas processing facility for receiving raw hydrocarbons from a producing well and treating the raw hydrocarbons to remove contaminants therefrom to produce a stream of treated gas;b) a liquefaction facility for receiving the stream of treated gas from a gas processing facility and liquefying the natural gas to produce a product stream of LNG;c) a storage facility operatively associated with a transfer means for receiving the product stream of LNG from the liquefaction facility for receiving and storing LNG in a first cryogenic storage tank; and,d) an offloading facility including LNG transfer facilities to transfer the LNG from the first cryogenic storage tank of the storage facility to a second cryogenic storage tank onboard an LNG Carrier on an as-needs basis.

A first embodiment is now described with reference toFIGS. 1 to 3in which an LNG production plant10is positioned at a production location12adjacent to a body of water14. The production plant includes a first facility in the form of an onshore liquefaction facility16for receiving a stream of pre-treated gas18from a gas processing facility20and liquefying the pre-treated gas stream to produce a product stream of LNG22. The gas processing facility includes an acid gas removal facility24and a dehydration and mercury removal facility26of the kind that is known in the art. Liquefaction is achieved onshore each liquefaction facility using any liquefaction process well established in the art which typically involve compression, expansion and cooling. Such prior art liquefaction processes include processes based on a nitrogen cycle, the APCI C3/MR™ or Split MR™ or AP-X™ processes, the Phillips Optimized Cascade Process, the Linde Mixed Fluid Cascade process or the Shell Double Mixed Refrigerant or Parallel Mixed Refrigerant process.

In the embodiment illustrated inFIG. 1, the storage facility and the offloading facility have been combined to provide a second facility in the form of an integrated storage/offloading facility28arranged at a selected location30in the body of water14. The integrated storage/offloading facility28has a first cryogenic storage tank32operatively associated with the liquefaction facility16for receiving LNG from the onshore liquefaction facility via a cryogenic pipeline34and storing LNG in the first cryogenic storage tank. The first storage tank32may be one of a plurality of first storage tanks with two such first storage tanks illustrated inFIG. 1by way of example only. The integrated storage/offloading28further includes an LNG transfer facility36for transferring LNG from the first cryogenic storage tank32to a second cryogenic storage38tank onboard an LNG Carrier40. The first cryogenic storage tank may be a double containment, full containment, prismatic or membrane systems with a primary tank constructed from, by way of example, stainless steel, aluminum, and/or 9%-nickel steel. The first cryogenic storage tank may include pre-tensioned concrete to provide structural resistance to the stored LNG, boil off gas pressure loads and to external hazards.

In the embodiment illustrated inFIG. 2, the integrated storage/offloading facility28is a gravity based structure with a base42of the integrated storage/offloading facility28resting on the seabed44at the selected location30within the body of water14to maximise the stability of the integrated storage/offloading28. By way of example, the gravity based structure is constructed using lightweight or semi-lightweight concrete (having a density of less than about 2000 kg/m3). Alternatively or additionally, the gravity based structure may be constructed of steel or a hybrid comprising a combination of steel and concrete or a composite material. Advantageously, the integrated storage/offloading facility is able to be constructed and commissioned at a construction location such as a shipyard or another location where trained and efficient labour force is available and floated in before being positioned at the selected location30to act as a breakwater for the LNG Carrier to reduce environmental loads (illustrated by way of an arrow inFIG. 2) on the LNG Carrier40.

Referring toFIG. 3, the integrated storage/offloading28is transportable from a construction location46to the production location12or from the assembly location48to the production location12by towing or on floating barges. The construction location may be one of a plurality of constructions locations with three shown inFIG. 3by way of example only. Advantageously, testing or pre-commissioning of the integrated storage/offloading28can be conducted before transportation of the integrated storage/offloading28to the production location14. This feature not only allows the facility to be deployed where required but is also advantageous when maintenance or upgrading is required. The integrated storage/offloading facility may be re-deployed at a different location at a later time to suit LNG supply and demand, for example, due to changes in the capacity of the production plant or towards the end of a gas field life. Thus with reference toFIG. 3, the integrated storage/offloading facility can be moved from a first production location50to a second production location52.

To allow sufficient water depth for an LNG Carrier40to berth alongside the integrated storage/offloading facility28, the selected location30has a water depth as measured from the waterline54to the seabed44of at least 14 meters, at least 15 meters, or at least 16 meters. The integrated storage/offloading facility28includes a ballast storage compartment56, preferably arranged around the periphery of the integrated storage/offloading facility or arranged toward the base of the facility, for ballasting. For flexibility to adjust the level of ballasting to suit the seabed conditions at a given selected location30, the ballast storage compartment may be one of a plurality of ballast storage compartments with three ballast storage compartments shown inFIG. 2by way of example only. The integrated storage/offloading facility28is towed from the construction or assembly location (46or48, respectively) to the production location12and then arranged at the selected location30where settling is achieved by the addition of a ballasting material to the ballast storage compartment56until the base42of the integrated storage/offloading facility28rests on the seabed44to secure the position of the integrated storage/offloading facility28. This provides the integrated storage/offloading facility with greater stability. The amount of ballasting material required to secure the integrated storage/offloading facility to the seabed at the selected location depends on a number of relevant factors including but not limited to the shear strength of the underlying clay or silt material found at the bottom of the body of water at the selected location. If required, the the integrated storage/offloading facility28may include a piling system58to anchor the integrated storage/offloading facility28into the seabed44. The ballasting material may be a solid ballasting material or a liquid ballasting material. By way of example, one or both of iron ore and sand may be used as the solid ballast material. In one embodiment of the present invention, the liquid ballasting material is water, condensate, monoethylene glycol (MEG), methanol, diesel, demineralised water, LPG or combinations thereof. The liquid ballasting material may be stored in a non-cryogenic storage tank.

In use, an LNG Carrier40comes in to berth at the integrated storage/offloading facility28to receive a cargo of LNG. The integrated storage/offloading facility28is designed so that the LNG Carrier40may approach the integrated storage/offloading facility from either direction depending on the prevailing weather conditions. A side of the integrated storage/offloading facility that is facing away from the prevailing weather conditions is referred to as the “lee side”. Preferably, the integrated storage/offloading facility28has a lee side60, whereby, in use, the LNG Carrier40approaches the integrated storage/offloading facility28from the lee side60of the integrated storage/offloading facility28. Depending on the size of the LNG Carrier, the bow62or the stern64of the LNG Carrier40may extend beyond an end66of the integrated storage/offloading facility28when the LNG Carrier40is berthed alongside the integrated storage/offloading facility28. This overhang of the bow or stern of the LNG Carrier beyond an end of the integrated storage/offloading facility may expose the LNG Carrier to adverse environmental conditions. To minimize this effect, the integrated storage/offloading facility28preferably has at least one lateral side68which has a length of a sufficient size to allow an LNG carrier to be moored along alongside the integrated storage/offloading facility28without overhang of the LNG Carrier40beyond an end of the integrated storage/offloading facility. The integrated storage/offloading facility28can be fitted with fendering equipment (not shown) to absorb a substantial portion of a load generated by impact of the LNG Carrier with the integrated storage/offloading facility during transfer of LNG from the first cryogenic tank32to the second cryogenic tank38.

The integrated storage/offloading facility28may comprise a plurality of similarly-sized sub-modules82, which can be integrated at the production location12, at a construction location46, or at an independent assembly location48. The sub-modules may be constructed at separate construction locations and towed to a common assembly location. This option is particularly attractive if there is a restriction on the space available at the dry dock or “graving dock” or restrictions on the towable or installable size of a given facility or sub-facility. Advantageously, once the sub-modules have been assembled to form the integrated storage/offloading facility at the assembly location, testing or pre-commissioning of the integrated storage/offloading facility can be conducted before transportation of the integrated storage/offloading facility to the production location. It is particularly advantageous when such pre-commissioning can be done at an assembly location onshore prior to transportation of the facility to a production location offshore or near shore.

As set out above, the first cryogenic storage tank32which forms a part of the integrated storage/offloading facility28is operatively associated with the liquefaction facility16and receives a product stream of LNG22from the liquefaction facility16via a cryogenic pipeline34. In the embodiment illustrated inFIG. 2, the cryogenic pipeline is a subsea cryogenic pipeline which is a preferred option when the selected location30is located more than 500 meters from the shoreline29. However, a cryogenic pipeline arranged on a trestle may be used as an alternative, particularly when the selected location30is less than 500 meters from the shoreline29. When the cryogenic pipeline is a subsea cryogenic pipeline, it may take the form of a dual-wall pipe-in-pipe or triple-wall pipe-in-pipe-in-pipe system. Using a dual-wall pipe-in-pipe system, the cryogenic subsea pipeline includes an inner pipe for carrying LNG and an outer jacket around the inner pipe defining an annular space there between with a layer of insulation in the annular space. Using a triple-wall pipe-in-pipe-in-pipe system, an intermediate pipe is located between an inner pipe and an outer pipe to protect the inner pipe from damage and to insulate the inner pipe to reduce heat leak and minimise LNG vaporization. Using either system, the inner pipe is preferably constructed from a pipeline material having sufficient ductility and toughness to be usable at cryogenic temperatures, for example, aluminium, high nickel content steels or austenitic stainless steels. One example of a suitable pipeline material is 36% nickel steel (known in the art commercially as INVAR) which allows the cryogenic subsea pipelie it to be restrained at both ends and used for LNG service without the use of an expansion joint. Alternatively, the cryogenic subsea pipeline may include at least one expansion joint84to compensate for thermal expansion and contraction. One example of an expansion joint is a bellows type expansion joint in which contraction is taken up by a longitudinal bellows or corrugations in the inner pipe. The bellows is constructed out of a material that is relatively thinner than the material of the LNG pipeline so the bellows is free to expand and contract axially with respect to the LNG pipeline.

In the embodiment illustrated inFIG. 2, the cryogenic subsea pipeline34includes an elongate open frame86adapted to be laid underwater is used for supporting the pipeline and resisting subsea forces on the pipeline. At least one pipe anchor88is attached to the pipeline34and to the frame86to transfer axial forces in pipeline34to the frame86. At least one steel or concrete ground anchor90is attached to the frame86for transmitting axial forces in the frame86to the sea bed44.

The LNG transfer facility36located on the integrated storage/offloading facility28includes a fixed or swivel joint loading arm above the water surface, preferably fitted with an emergency release system at one end of the loading arm. Between transfer operations, the LNG transfer facility may be kept cold by re-circulation of a small quantity of LNG. The LNG transfer facility may include an emergency safety system to allow loading to be stopped if required in a quick, safe, and controlled manner by closing an isolation valve on the LNG transfer lines or shutting down the cargo pumps associated with the cryogenic storage tank38onboard the LNG carrier40. The emergency safety system is designed to allow LNG transfer to be restarted with minimum delay after corrective action has been taken.

In a preferred embodiment, the integrated storage/offloading facility28includes a boil-off gas reliquefaction facility92for liquefying at least a portion of the boil off gas that is generated either during the transfer of the LNG through the cryogenic subsea pipeline to the first cryogenic storage tank32of the integrated storage/offloading facility28or during the transfer of the LNG from the first cryogenic storage tank32to the second cryogenic storage tank38of the LNG Carrier. The reliquefied boil-off gas may be returned for storage in the first cryogenic storage tank. Boil off gas is generated due to one or more of the following: a) cooling down of the interior surfaces of the second cryogenic storage tank onboard the LNG Carrier; b) heat leaking in from the environment through the exterior surfaces of the second cryogenic storage tank onboard the LNG Carrier; c) heat from the cryogenic pumps used to transfer the LNG from the first cryogenic storage tank to the second cryogenic storage tank; and d) heat ingress from the LNG transfer facility transfer hoses or loading arms; e) flashing off due to a temperature increase during the transfer operation, and, f) flashing due to pressure drop during LNG transfer from liquefaction to storage. The inclusion of a boil-off gas reliquefaction facility as an integral part of the integrated storage/offloading facility overcomes the need for the cryogenic subsea pipeline to include a vapour return line. Alternatively or additionally, a portion of the boil off gas may be used as a source of fuel for a first power generation system which forms part of the integrated storage/offloading facility or a second power generation facility onboard the LNG Carrier. In addition to this, the first cryogenic storage tank of the integrated storage/offloading facility can be operated at a higher pressure compared with the second cryogenic storage tank of the LNG carrier by way of using reinforced membrane tank technology to minimize boil-off gas generation. Alternatively, the boil-off gas may be compressed and transferred via a subsea pipeline to an onshore gas processing plant or recycled back to the integrated storage/offloading facility28.

A second embodiment of the present invention is now described with reference toFIG. 4which allows continuous or semi-continuous loading of LNG without the need to use a cryogenic pipeline34. In this embodiment, the LNG production plant10, positioned at a production location12adjacent to a body of water14, includes an onshore gas processing plant20with the liquefaction facility being integrated into the integrated storage/offloading facility28arranged at a selected location30in the body of water14. In this embodiment, a gas pipeline94replaces the cryogenic pipeline34, the gas pipeline being cheaper to construct, lay, and maintain. Advantageously, a portion of the LNG produced by the liquefaction facility16can be transferred directly into the second cryogenic storage38tank onboard an LNG Carrier40, whenever an LNG Carrier is berthing at the offloading facility28, reducing the need to store the LNG in the first cryogenic storage tank32of the offloading facility28. In the embodiment illustrated inFIG. 4, the integrated storage/offloading facility28has a multilateral footprint when viewed in plan view. This footprint provides for continuous or semicontinuous production whereby a first LNG Carrier70berths at a first lateral side72of the integrated storage/offloading facility28while a second LNG Carrier74berths at a second lateral side76of the integrated storage/offloading facility28. In the embodiment shown inFIG. 4, the integrated storage/offloading facility28has a triangular footprint with the first and second lateral sides (72and76, respectively) each representing a lee side60based on the prevailing weather conditions indicated by the arrow inFIG. 4. However, the multilateral footprint could equally be rectangular, square, pentagonal or hexagonal. In the embodiment illustrated inFIG. 5, the footprint of the integrated storage/offloading facility28is rectangular such that the integrated storage/offloading facility28has a longitudinal axis80aligned substantially parallel to the direction of a predominant current for bi-directional berthing of an LNG Carrier40. Preferably each of the first and second lateral sides (72and76, respectively) has a length of a sufficient size to allow an LNG carrier to be moored along alongside the integrated storage/offloading facility28without overhang of the LNG Carrier40beyond an end of the integrated storage/offloading facility.

A third embodiment is now described with reference toFIG. 6andFIG. 7which illustrate continuous production. In the embodiment illustrated inFIG. 6, the lee side60has a length that is less than the length of the LNG Carrier40. When the LNG carrier40is moored along alongside the integrated storage/offloading facility28, one or both of the stern62or the bow64of the LNG Carrier40extends beyond the length of the lee side60of the integrated storage/offloading facility28. In order to provide breakwater protection for the LNG Carrier40, a breakwater facility100is floated in and positioned adjacent to the integrated storage/offloading facility28at the selected location30. InFIG. 6, the breakwater facility100is arranged to provide a breakwater to the integrated storage/offloading facility28and the LNG Carrier40with only one breakwater facility100being shown. In embodiment illustrated inFIG. 7, two such breakwater facilities are shown. InFIG. 6, the liquefaction facility16is the breakwater facility100. InFIG. 7, the liquefaction facility16is onshore. A first breakwater facility102is located towards a first end104of the integrated storage/offloading facility28with a second breakwater facility106being located towards a second end108of the integrated storage/offloading facility28. In this embodiment, the first cryogenic storage tank32operatively associated with the liquefaction facility16is integrated with the breakwater facility100with the boil-off gas reliquefaction facility92being integrated with the second breakwater facility.

Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications can be made without departing from the basic inventive concepts. For example, an LNG Carrier may be used as the offloading facility. By way of further example, the liquefaction facility may be integrated with the offloading facility. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims.

It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word “comprise” or variations such as “comprises” or “comprising” is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.