EP2045506A1

EP2045506A1 - Offshore system for vaporizing a liquefied hydrocarbon stream, method of providing such a system, and method of vaporizing a liquefied hydrocarbon stream

Info

Publication number: EP2045506A1
Application number: EP07117933A
Authority: EP
Inventors: Aart Alfons Geurtsen; Ewoud Van Haaften
Original assignee: Shell Internationale Research Maatschappij BV
Current assignee: Shell Internationale Research Maatschappij BV
Priority date: 2007-10-04
Filing date: 2007-10-05
Publication date: 2009-04-08

Abstract

An offshore system and method for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream is provided. A structure provided with a storage tank (4) for a load of liquefied hydrocarbon, an offloading system (6) in communication with the storage tank (4) for receiving a stream of the liquefied hydrocarbon (8) from an auxiliary carrier (10), vaporization equipment (7, 12, 13, 14, 15) in communication with the storage tank (4), and a ballast storage area (2,2a). The ballast storage area (2, 2a) is charged with a ballast material having a specific gravity equal to or higher than that of water. The amount of ballast material being charged is sufficient to keep the carrier grounded on the bottom of a body of water (41) in a condition that the storage tank (4) does not contain any liquefied hydrocarbon. The structure is selected to be an existing liquefied hydrocarbon carrier (1).

Description

Aspects of the present invention relate to an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, and a method of providing such an offshore system.
In another aspect, the present invention relates to a method of vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream.
Several methods and systems for liquefying a natural gas stream and vaporizing a liquefied natural gas (LNG) stream are known. It is desirable to liquefy a natural gas stream for a number of reasons. As an example, natural gas can be stored and transported over long distances more readily as a liquid than in gaseous form, because it occupies a smaller volume and typically does not need to be stored at a high pressure.
At an import site, the liquid is typically vaporized as one step in obtaining a gaseous stream that is ready for consumption by end users.
US 2005/0115248 proposes an offshore gravity base structure that may receive, store, and process liquefied natural gas (LNG) from a carrier. The structure has a system of ballast storage areas, docking equipment to allow direct mooring of carriers, transfer equipment to offload LNG from a carrier, and LNG tanks for storage of offloaded LNG. Vaporization equipment that may be used to vaporize LNG to natural gas, may be disposed on the structure as well. A portion of the structure is comprised of lightweight concrete. Such structures may be constructed on-shore and then towed to an appropriate site and positioned on the bottom of a body of water. A ballast material may be used to fill the ballast storage areas, in order to maintain the structure on the bottom of the body of water.
One concern regarding this proposed offshore gravity base structure is the time it takes to construct it, as well as the capital investment associated with acquiring the offshore gravity base structure.
The present invention provides a method of providing an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising:

providing a structure provided with a storage tank for a load of liquefied hydrocarbon, an offloading system in communication with the storage tank for receiving a stream of the liquefied hydrocarbon from an auxiliary carrier; vaporization equipment in communication with the storage tank; and a ballast storage area;
charging the ballast storage area with a ballast material, whereby the amount of ballast being charged is sufficient to keep the carrier grounded on the bottom of a body of water in a condition that the storage tank does not contain any liquefied hydrocarbon;

The invention further provides an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising a structure provided with:

a storage tank for storing a load of liquefied hydrocarbon;
an offloading system in communication with the storage tank for receiving a stream of the liquefied hydrocarbon from an auxiliary carrier;
vaporization equipment in communication with the storage tank; and
a ballast storage area at least partially filled with a ballast material, whereby the amount of ballast material in the ballast storage area is sufficient to keep the carrier grounded on the bottom of a body of water in a condition that the storage tank does not contain any liquefied hydrocarbon;

The invention further provides a method of vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising

providing an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, as defined above; and
pumping a working stream of the liquefied hydrocarbon from the storage tank to the vaporization equipment;
vaporizing at least the working stream of the liquefied hydrocarbon to produce a vaporized hydrocarbon stream.

Embodiments of the present invention will now be described by way of example only, and with reference to the accompanying non-limiting schematic drawings in which:

Figure 1 shows a schematic cross section of an existing LNG carrier;
Figure 2 shows a schematic diagram setting forth an offshore method and system for vaporizing a liquefied hydrocarbon stream;
Figure 3 shows a schematic cross section of an existing LNG carrier grounded on a foundation; and
Figure 4 shows a schematic cross section of an auxiliary LNG carrier moored to a grounded existing LNG carrier.

For the purpose of this description, a single reference number will be assigned to a line as well as a stream carried in that line. Same reference numbers refer to similar components, streams or lines.
Described are offshore systems for vaporizing a liquefied hydrocarbon stream, that employ a structure based on an existing liquefied hydrocarbon carrier, and methods of providing such systems. Such systems may be used for vaporizing a liquefied hydrocarbon stream. Suitably, the liquefied hydrocarbon carrier is an existing liquefied natural gas carrier.
It is proposed to use the existing liquefied hydrocarbon carrier as a gravity base structure (GBS), for the purpose of providing an offshore re-gasification system. The liquefied hydrocarbons may be supplied by offloading an auxiliary liquefied hydrocarbon carrier.
In various embodiments of the invention, it is proposed to install process equipment on an existing LNG carrier and to ground the vessel in shallow water, to form an offshore system for vaporization of a liquefied hydrocarbon, sometimes referred to as an offshore re-gasification system, in the form of a grounded LNG re-gasification vessel. This may be done by pumping a ballast material, which may be water (sweet water or sea water) or any suitable dense material, in the ballast tanks.
Process equipment may include vaporization (re-gasification) equipment.
Preferably, a membrane type LNG carrier could be used for conversion into the offshore re-gasification system, because more deck space is available for the process equipment.
The liquefied hydrocarbon streams may be any suitable liquefied hydrocarbon-containing stream, but they are in practice usually substantially comprised of a liquefied natural gas stream obtained from liquefying natural gas obtained from natural gas reservoirs or petroleum reservoirs. As an alternative the natural gas may also be obtained from any other source, including a synthetic source such as a Fischer-Tropsch process.
By selecting and using an existing liquefied hydrocarbon carrier, the construction time is reduced significantly. This is particularly true when a carrier of opportunity is being used. An existing LNG carrier is usually already provided with suitable LNG tanks, such that construction LNG tanks would no longer be necessary. Typically, construction of LNG tanks can consume up to over a year.
Moreover, since the structure is an existing liquefied hydrocarbon carrier, it may have depreciated and thus have become more cost effective than constructing the structure new.
Another advantage of using an existing liquefied hydrocarbon carrier is that reuse at another site is easier than with conventional gravity base structures of the prior art, because the carrier is of its nature more suitable for being transported than typical GBS. By removing the ballast material, the carrier can be refloated.
There is no need to construct a jetty or a jacket. The grounded existing carrier may moreover serve as a breakwater for the auxiliary LNG carrier, which is expected to increase operability or which may reduce the need and/or requirements of an additional dedicated breakwater.
For the purpose of the present disclosure, an existing liquefied hydrocarbon carrier is defined as a vessel designed for receiving a load of the liquefied hydrocarbon at a loading site, shipping the load across a body of water from the loading site to an offloading site, and substantially offloading the load at the offloading site, and having been used as such at least once.
The auxiliary liquefied hydrocarbon carrier is used to ship the liquefied hydrocarbons from a loading site (typically associated with and/or in close vicinity to a liquefaction plant) to the grounded liquefied hydrocarbon carrier.
Preferably, the existing liquefied hydrocarbon carrier is selected to be an old existing liquefied hydrocarbon carrier, more preferably one that has been substantially written off or been decommissioned for its design purpose. Such a carrier will have depreciated substantially. It will be appreciated that the capital investment reduces as the value of the carrier is lower.
The existing liquefied hydrocarbon carrier may need to be converted to some extent in order to render it fully suitable as a grounded offshore re-gasification system. The extent of conversion depends on the equipment that was already provided on the carrier, and on the structural integrity of the carrier in view of the forces that it will undergo as a result of the grounding.
Typically, a storage tank for liquefied hydrocarbons, in the form of an LNG storage tank 4, including submerged cryogenic LNG pumps, and a ballast storage area are already provided on an existing LNG carrier 1, as is schematically shown in Figure 1. The ballast storage area may comprise a plurality of tanks, of which tanks 2 and 2a are shown in Figure 1. Other equipment may or may not be already present.
Figure 2 shows a schematic diagram setting forth an offshore method and system for vaporizing a liquefied hydrocarbon stream. The boundary of the offshore system is schematically indicated by means of a dashed line, whereby everything depicted inside the line may be on or in the converted existing LNG carrier 1. An offloading system 6 is provided, in communication with the LNG storage tank 4 for receiving a stream of the liquefied hydrocarbon 8 from an auxiliary carrier 10. The offloading system may comprise conventional LNG loading arms, such as hard arms, sufficiently large to bridge the gap between the existing LNG carrier 1 and the auxiliary carrier 10. A plurality of such arms may be provided, several of which to transfer LNG, one for a carrying a return stream 9 of vapour to the auxiliary carrier 10 and one or more spares for use in either mode.
A submerged pump 5, preferably a low-pressure cryogenic pump, is located inside each LNG storage tank 4, to transfer a working stream of the liquefied hydrocarbon, herein in the form of LNG, from the storage tank 4 to a recondenser 7 via line 3. The recondenser feeds the LNG to vaporization equipment. A bottom stream 12 from the recondenser 7 is fed to a suction inlet of a high-pressure LNG booster pump 13. The high-pressure outlet of the booster pump 13 feeds a pressurized LNG stream 14 into one or more vaporizers, here schematically indicated at 15. In some embodiments, the recondenser 7 may be bypassed, for instance by optionally connecting line 3 directly to line 12 via a bypass line 22.
Any suitable type of vaporizer may be employed, such as heat exchange vaporizers that may heat exchange the LNG against water from the body of water to convert the LNG into a natural gas stream 16. While air heated vaporizers are in principle possible, these often require a relatively large amount of deck space.
In some embodiments, the vaporizers may be open rack vaporizers. Water from the body of water may be obtained using any suitable water intake system, preferably a water intake system configured to reduce the amount of sea life and debris entering the heat exchange vaporizers.
A part of the vaporized natural gas stream 16 may be branched off to obtain a fuel gas stream 17 for use as a fuel gas. The remaining part may be dispatched as end stream 18 to a pipeline, optionally via optional post processing equipment 19, which may comprise one or more of metering equipment, inert injection such as nitrogen injection to reduce heating value.
The LNG storage tanks 4 may contain vapor in addition to liquefied natural gas, as a result of heat entering into the LNG storage tanks 4. This vaporized LNG is typically referred to as boil-off gas ("BOG"), and it may be drawn from the LNG storage tank 4 via line 11. The BOG in line 11 may be recompressed using compressor 20, before feeding it into the recondenser 7 via line 21. Compressor 20 may be of any suitable type, such as a centrifugal compressor. One or more spare compressors may be provided. The recondenser 7 may recondense at least a part of the BOG and provide sufficient pressure and surge volume at the suction of the high-pressure LNG booster pump 13. BOG may be recondensed by mixing it with a portion of cold LNG from the storage tanks 4.
An optional flare system 23 may be provided for disposal of BOG during commissioning and during emergency situations.
As the person skilled in the art readily understands how to vaporize a liquefied hydrocarbon stream, this will not be further discussed in more detail here.
Other equipment to be provided may include one or more of a nitrogen plant, power generation equipment, optionally fuelled at least in part by fuel gas 17, waste treatment equipment, fire fighting equipment, lifesaving equipment, utility and potable water system. Some of these may be already present on an existing LNG carrier.
Safety considerations may be applied when converting the existing LNG carrier. The vaporization equipment belongs to the most hazardous parts of the system, because of the large volumes of LNG and vapour that are involved. The vaporizers, BOG compressors, recondenser and pumps contain volumes of gas and LNG under high pressure and the distance from the accommodation should thus be maximized. Since the accommodation on exiting LNG carriers is typically located in the stern area of the carrier, the vaporization equipment should thus be located away from the stern. The nitrogen injection plant and the metering station will be located away from the stern as well.
Such equipment may even be located away from the ship, such as on shore.
Natural gas is exported from the offshore system through a riser. To maximize the distance from the living quarters and to minimize the length of these high-pressure gas lines, the riser should be located close to the vaporization equipment. In order to prevent the auxiliary LNG carrier from bumping into the risers during berthing, they are preferably located on the leeside.
The LNG loading arms should preferably be located amidships, so they line up with the auxiliary LNG carrier manifold. The power generating modules will be located in between the vaporizers and the deck house.
The main hazard from the flare is radiation. Flaring should not take place during normal operation, but only during commissioning (cool down of the tanks) and in case of emergencies. A safe location for the flare stack is far away from the living quarters and on the opposite site from where the auxiliary LNG carrier moors.
The auxiliary carrier 10, for instance in the form of an auxiliary LNG carrier, may moor alongside the offshore structure 1, and the LNG can be transferred through the offloading system 6, which may comprise conventional loading arms. After re-gasification, the gas may be sent to shore via any suitable means, typically through a subsea pipeline.
Preferably, the auxiliary LNG carrier has less draft than the LNG carrier that is used for conversion into the offshore re-gasification structure, in order to allow for sufficient under-keel clearance.
Preferably, the total available volume in the storage tanks for the liquefied hydrocarbons in the selected existing LNG carrier is equal to or exceeds that of the auxiliary carrier. This allows for fully offloading a full charge of the liquefied hydrocarbons without being restricted by vaporization rate.
If the available auxiliary carriers generally have a larger liquefied hydrocarbon load capacity than the existing hydrocarbon carrier used as the present GBS, then an additional liquefied hydrocarbon storage tank may be provided. Preferably, a transfer line is provided between the additional liquefied hydrocarbon storage tank to the vaporization equipment of the selected existing liquefied hydrocarbon carrier, e.g. via the hydrocarbon storage tank of the selected existing liquefied hydrocarbon carrier.
The additional hydrocarbon storage tank may be provided by providing an additional existing liquefied hydrocarbon carrier and mooring it permanently or quasi permanently to the grounded existing liquefied hydrocarbon carrier. Alternatively, or in addition to mooring, the additional existing liquefied hydrocarbon carrier may also be grounded.
Preferably, the additional liquefied hydrocarbon storage tank is installed while the offshore system is in operation vaporizing liquefied hydrocarbons, after commissioning of the offshore system. This way, the storage capacity can be expanded while it is already possible to deliver re-gasified product to end users.
Instead of using an additional existing liquefied hydrocarbon carrier for this purpose, the additional liquefied hydrocarbon storage tank may even be a dedicated structure constructed on site or onshore. The relatively long construction time is at least partly mitigated by a capability of production of re-vaporized product in the mean time.
The ballast storage area of the existing liquefied hydrocarbon carrier will be charged with a ballast material. Conveniently, the ballast material comprises water taken in from the body of water wherein the existing liquefied hydrocarbon carrier will be grounded or from any other source. This may be sweet water or sea water. The ballast material may be water or consists essentially of water. However, in various embodiments, the ballast material is selected have a density that is higher than that of water, in particular than that of the water contained in the body of water in which the existing liquefied hydrocarbon carrier is grounded. Density may be expressed in terms of specific gravity, usually defined as the relative density compared to that of distilled water.
However, no matter which ballast material and/or the density thereof is selected, the amount of ballast material being charged should be sufficient to keep the carrier grounded on the bottom of the body of water, even in a condition that the storage tank does not contain any liquefied hydrocarbon. Ultimately, the weight of the existing carrier including all the equipment, but excluding any liquefied hydrocarbon charge, together with the ballast should exceed the maximally expected upward force (buoyancy, taking into account high tide and waves) exercised by the body of water as a result of the submerged volume of the existing carrier. Hence, more ballast is needed as the existing vessel needs to be sunk deeper in order to find support from the sea bed.
For typical existing LNG carriers and typical water depths, the maximum tolerable tidal range is about 3 m, while the maximum tolerable wave height is about 5 m.
The required density of the ballast material depends on the relative volume of the available ballast storage area and on a fill factor. The ballast material may be a solid and/or a liquid ballast material. Sand may be used as a solid ballast material, or a material with a higher density than sand, e.g. iron ore.
Preferably, the ballast material is removable. The ballast material may be pumpable to facilitate charging and discharging (removing) the ballast material from the ballast storage areas. A mixture of a solid with water may be a suitable pumpable material. In some embodiments, the ballast material may be and/or behave like a thixotropic material. Non-Newtonian pseudoplastic fluids showing a time-dependent viscosity which decreases over time as the fluid is exposed to shear are considered to be thixotropic.
A suitable thixotropic material is commercially available in densities ranging from 640 to 7,040 kg/m³ under the name Ballast-Crete. It is stated to be a preblended combination of water and inorganic non-toxic granular and ground fines. During charging, the flowabiliy and waterretentivenes compares to that of mortar and thus it is pumpable. After charging, the material firms to a semi-solid mass, but it can be removed again using pressurized water.
The offshore system proposed herein may be operated in a way that is very similar to any known type of re-gasification GBS. One difference however is that the presently proposed structure will consist of essentially a converted ship, originally designed to float and not to stand on the bottom. This situation has been analyzed in theory and it has been concluded that this is nevertheless feasible.
Prior to said charging the ballast storage area, the existing liquefied hydrocarbon carrier may be positioned in a body of water having a depth that is about equal to the draft of the existing liquefied hydrocarbon carrier during normal sailing operations, preferably under conditions that it was originally designed for. This will provide sufficient freeboard to prevent greenwater.
Said positioning may be effectuated with the aggregate weights of ballast in the ballast storage area and liquefied hydrocarbon in the liquefied hydrocarbon storage tanks is lower than during normal sailing operations, to ensure a smaller draft during the positioning during positioning than the normal draft.
The water depth may typically be between 10 and 18 meters, preferably between 12 and 14 meters, depending on the selected existing carrier. If the water depth at a desired location is up to several meters deeper than desired, a foundation may be constructed to artificially raise the bottom of the water body locally.
Before installation, preparation of the seabed may be necessary depending on the type and/or conditions of sea bed soil present, for instance by providing a foundation. Foundations as such are known, for instance for supporting caisson breakwaters.
The foundation may provide a substantially flat, preferably horizontal, bed to place the existing liquefied hydrocarbon carrier on. It may further serve to spread the vertical load exercised by the weight of the offshore re-gasification system, in combination with the waves, over a larger footprint area of the sea bed. Moreover, the submerged volume of the offshore system may be reduced by means of the foundation, so that buoyancy forces may be reduced and less ballast material may be needed.
Figure 3 shows an example of such a foundation, in cross section. It comprises various components. A core layer 31 is placed on a sea bed 33. A layer of a geotextile 35 covers the core layer 31 and is buried partly below the surface of the sea bed 33 in a toe area 36 of the foundation. The carrier 1 is placed on the geotextile layer 35. The exposed parts of the foundation are covered by an armour layer 39. In order to spread the load of the heavy armour layers 39 on the geotextile 35, an underlayer 37 may be provided between the geotextile layer 35 and the armour layer 39.
The principal functions of the material in the core layer 31 are volume filling and offering support for the layers on top of it. There are many different materials that can fulfil these functions and the choice is therefore usually dictated by economics. Quarry run may be used, which is typically the cheapest grade of armour stone. For the present purpose, however, it is preferred to use gravel. Gravel forms a smooth bed to place the existing LNG carrier 1 on. This way peak loads on the hull and damaging of the coating will be prevented. In addition, the small particle size will result in a large contact area with the bottom of the vessel and thus in a large friction coefficient.
A height of the core layer 31 of 2 meters is chosen in order to secure a good spreading of the vertical load to the subsoil 33, but to keep costs for material and construction work limited. Standard gradients for the slopes 32 are 1 : 1,5 .
Geotextiles are permeable textiles made from artificial fibres and are placed on top of the core layer 31. They prevent the washing out of sand and gravel particles, while allowing the free passage of water. In addition, the geotextiles increase the stability of the soil body and prevent the coating on the bottom of the hull from being damaged during installation.
Geotextiles can be divided in woven and non-woven geotextiles. For both types the maximum standard available width is 5 meters. The length is limited by the available transport facilities and ease of handling onsite, but can generally be up to 200 meters. The thickness of most geotextiles lies between 0,2 and 10 mm. Large areas can be covered by overlapping sheets.
The geotextile layer 35 is provided to be able to withstand puncturing loads imposed during installation and during service. This resistance reduces over time however by oxidation. Durability of the geotextile might be influenced by temperature or UV radiation. A service lifetime of 5 years is well within the limits however, especially since the geotextile will be fully submerged and out of the main wave action.
To protect the foundation structure against wave action and current forces, an armour layer 39 is provided. This layer may consist of heavy armour units, either quarry stones (often granite or basalt) or specially shaped concrete blocks.
Concrete blocks can be applied in a single layer, because they show a very high degree of interlocking due to their shape. This makes them more resistant to wave forces, since a locally high wave force is distributed throughout several units. Armour stones show a much lower degree of interlocking, resulting in a higher chance that some of the stones will be moved due to the wave forces. Therefore, they may be preferably applied in a double layer.
Since the height of the foundation is limited, the amount of armour units that is needed is relatively small. Accordingly, the effort to fabricate concrete blocks of suitable size and shape is relatively high. Therefore it is preferred to use granite quarry stones for the armour layer. These are available EN standard grades. Another advantage of the use of armour stones is that they are easier to install than concrete blocks, which have to be positioned much more precisely.
It is expected that the person of oridinary skill in the art will know how to determine the best size for the armour stones. Armour stone gradings are standardized in EN 13383. A 300 - 1000 kg grading contains stones with nominal diameters varying from 0.49 to 0.72 m, and is considered suitable for application here. Typically, a median mass of the armour stones of 310 kg is deemed suitable. However, depending on the situation, stones from another grades may be selected instead.
If the offshore re-gasification system is placed perpendicular to the coast, armour stones of within this size range may have to be used on both sides of the foundation, since waves can come from either side. If the offshore re-gasification system is placed parallel to the coast, ocean waves cannot come from the shore side. In such a case, a smaller size range may be tolerated for armour stones on the shore side of the foundation. However, waves and currents may be induced on this side by the LNG carrier and the tugboats during berthing operations. Therefore, and for practical reasons during construction, similar sized stones may be selected on both sides anyway.
A minimum of 3 armour stones is typically recommended on the berm 34 (the horizontal section between the vertical wall of the ship and the beginning of the slope). Thus, the width of the berm will then be around 2 meters, given the preferred size range of the armor stones.
The thickness of the armour layer depends on median nominal diameter of the armour stones, the number of layers, and a layer coefficient that is dependent on the shape of the armour stones. For rough quarry stones, the coefficient may be assumed to be unity. The median nominal diameter for the stones from the 300 - 1000 kg grading may be around 0.60 meters. This results in a layer thickness of 1.2 meters.
To spread the load of the heavy armour stones on the geotextile layer 35, and to provide interlocking, an underlayer is recommended. The weight of the units in the underlayer should typically be between 1/15 and 1/10 of the weight of the armour units, corresponding to a 10 - 60 kg grading as defined in EN 13383. Two layers may be applied, resulting in an underlayer thickness of around 0.5 meters.
Toe structure 36 is provided to provide a stable footing to the armour layer 39 and for scour protection. This toe may be made by extending the underlayer and the armour layer to an essentially horizontal section beyond slope 32. To maximize the under keel clearance of the LNG carrier, if moored next to the off-shore gasification structure 1, a trench may be excavated in the toe area 36 in order to at least partly burry the geotextile layer 35, the underlayer 37 and the armour layer 39, as depicted in figure 3. The toe width may allow at least three stones to be placed and will thus be around 2 meters.
Depending on the situation, the entire foundation may be sinked at least partially into a trench excavated in the sea bed, e.g. when the water is so shallow that no water depth can be sacrificed.
Depending for instance on slope 32 gradient, an optional roundhead may be provided to terminate the foundation in a more stable manner. The stability number of the armour blocks is lower on the heads of the foundation than on the trunk, because the armour stones are less supported from neighbouring stones due to the curvature. To account for this, the gradient for the roundheads is reduced from the slope 32 gradient 1 : 1,5 to 1 : 2.
Especially under the knots of these standing waves local scouring can occur. Therefore, additional protection, such as the placement of gabion mattresses might be desired.
In particular when a foundation is provided, the auxiliary carrier 10 might not be able to moor side-by-side to the existing LNG carrier 1 within a proximity of 7 meters or so. Special fender systems exist that are capable of bridging larger gaps, including triple fenders or monopole fender systems. An advantage of triple fenders is their low cost, an advantage of monopoles is that impact forces during berthing of the auxiliary carrier on the existing LNG carrier would be eliminated.
Figure 4 shows a schematic cross section of an auxiliary carrier 10 moored to the existing carrier 1 separated by a triple fender system 42. The existing carrier 1 is grounded, by means of a sufficient amount of ballast material, on the sea bed 33, partially submerged by the body of water 41.
Although the method according to the present invention is applicable to various liquefied hydrocarbon streams, it is particularly suitable for liquefied natural gas streams.
Usually natural gas is comprised substantially of methane. Preferably the feed stream comprises at least 60 mol% methane, more preferably at least 80 mol% methane.
The described use of an existing liquefied hydrocarbon carrier for creating a gravity base structure for an offshore vaporization system may likewise be applied for an offshore hydrocarbon liquefaction system. The person of skill in the art will understand that instead of, or in addition of vaporization equipment the process equipment would have to include liquefaction equipment. These could for instance be used for the development of marginal, possibly remote, gas fields
The person skilled in the art will understand that the present invention can be carried out in many various ways without departing from the scope of the appended claims.

Claims

A method of providing an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising:
- providing a structure provided with a storage tank for a load of liquefied hydrocarbon, an offloading system in communication with the storage tank for receiving a stream of the liquefied hydrocarbon from an auxiliary carrier; vaporization equipment in communication with the storage tank; and a ballast storage area;

- charging the ballast storage area with a ballast material, whereby the amount of ballast material being charged is sufficient to keep the carrier grounded on the bottom of a body of water in a condition that the storage tank does not contain any liquefied hydrocarbon;
wherein the structure is selected to be an existing liquefied hydrocarbon carrier.
The method of claim 1, wherein the selected existing liquefied hydrocarbon carrier is converted to provide a converted liquefied hydrocarbon carrier.
The method of claim 1 or claim 2, wherein the ballast is pumpable.
The method of any one of the previous claims,
wherein the ballast material comprises water, preferably water taken in from said body of water.
The method of claim 4, wherein the ballast material consists essentially of said water.
The method of any one of claims 1 to 4, wherein the ballast material has a density that is higher than that of water, preferably higher than that of the water contained in said body of water.
The method of claim 6, wherein the ballast comprises a thixotropic material.
The method of any one of the previous claims, further comprising, prior to said charging the ballast storage area, positioning the existing liquefied hydrocarbon carrier in a body of water having a depth that is about equal to the draft of the selected existing liquefied hydrocarbon carrier during normal sailing operations.
The method of any one of the previous claims, further comprising creating a foundation on the bottom of said body of water, and, prior to said charging the ballast storage area, positioning the existing liquefied hydrocarbon carrier in said body of water over the foundation.
The method of any one of the previous claims,
wherein providing said structure comprises removing ballast material from the ballast storage area to refloat the structure.
The method of any one of the previous claims, further comprising
- commissioning of the offshore system; and

- providing at least an additional liquefied hydrocarbon storage tank while the offshore system is in operation.
An offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising a structure provided with:
- a storage tank for storing a load of liquefied hydrocarbon;

- an offloading system in communication with the storage tank for receiving a stream of the liquefied hydrocarbon from an auxiliary carrier;

- vaporization equipment in communication with the storage tank; and

- a ballast storage area at least partially filled with a ballast material, whereby the amount of ballast material in the ballast storage area is sufficient to keep the carrier grounded on the bottom of a body of water in a condition that the storage tank does not contain any liquefied hydrocarbon;
wherein the structure is an existing liquefied hydrocarbon carrier.
A method of vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, comprising
- providing an offshore system for vaporizing a liquefied hydrocarbon stream, such as a liquefied natural gas stream, as defined in claim 12; and

- pumping a working stream of the liquefied hydrocarbon from the storage tank to the vaporization equipment;

- vaporizing at least the working stream of the liquefied hydrocarbon to produce a vaporized hydrocarbon stream.