WO2006088371A1

WO2006088371A1 - System and method for offshore offloading and regasification of LNG

Info

Publication number: WO2006088371A1
Application number: PCT/NO2006/000057
Authority: WO
Inventors: G. BREIVIK Kåre; Trygve G. Egge; Harald Kleppestø
Original assignee: Statoil Asa
Priority date: 2005-02-17
Filing date: 2006-02-13
Publication date: 2006-08-24
Also published as: NO20050856D0; NO20050856L

Abstract

A system is provided for offshore offloading and regasification of LNG from an LNG tanker (1), which system is connected to a natural gas receiving system and includes a plurality of mooring and offloading systems (5) and a plurality of regasification vessels (2), to which mooring and offloading system (5) an LNG tanker is moored, and a regasification vessel (2) is connected to the LNG tanker (1) for offloading regasified LNG into the receiving system, LNG being offloaded from the LNG tanker (1) and regasified via the regasification vessel (2). A method is also provided for offshore offloading and regasification of LNG from an LNG tanker (1) for delivery to a natural gas receiving system, including the steps of: providing a plurality of mooring and offloading systems (5) and a plurality of regasification vessels (2); connecting a regasification vessel (2) to the LNG tanker (1); mooring the LNG tanker (1) to the mooring and offloading system (5); connecting the regasification vessel (2) to the mooring and offloading system (5); offloading LNG onto the regasification vessel (2) from the LNG tanker (1); regasifying LNG in the regasification vessel (2); and offloading LNG into the receiving system from the regasification vessel (2). The system and the method permit the use of standard LNG tankers without any modification.

Description

System and method for offshore offloading and regasification of LNG

The present invention relates to a system and a method for offshore offloading and regasification of LNG as respectively disclosed in independent claims 1 and 12.

As is well known, liquefied natural gas is also referred to as LNG. The gas is cooled down to about -163⁰C in a cooling plant at the tanker loading site and transported by special tank ships to dedicated receiving terminals around the globe. During transport, the LNG remains at approximately atmospheric pressure. The load capacity of the LNG tankers has traditionally been about 145,000 m³, but today considerably larger ships are also being planned and built.

During recent years LNG has become an increasingly more sought-after energy resource. It is expected that natural gas will to an ever greater degree replace oil as an energy source.

The USA is a major consumer of LNG. The LNG tankers arrive a few central receiving terminals where the gas is unloaded in liquid form into large onshore storage tanks. The receiving terminals are connected to an extensive pipe distribution system which covers virtually the whole of the USA. Before LNG is fed into the pipe distribution network, it is heated and converted into gas. This process is referred to as regasification.

Today there is not sufficient receiving and regasification capacity in the USA to meet future needs. Therefore, plans are being drawn up for new receiving and gasification facilities in several places. However, receiving and gasification facilities are huge, costly installations, which tie up large areas of land in the form of safety zones. Moreover, in particular in the USA, there is a great fear that such installations may be potential terror targets. US public opinion is therefore very much against new plants. Consequently, extensive planning and protection of such land-based LNG receiving terminals will be required in the future.

If new LNG receiving terminals could be located offshore, it would be easier to build new plants in the USA. Preferably, the plants should be located far enough from the coast so as not to represent a safety hazard for populated areas on shore. In recent years a number of such solutions have been proposed. As far as the Applicant is aware, these solutions require both extensive adaptation of the LNG tankers and/or involvement of new technology that must be qualified.

It is considered a major advantage if offshore LNG receiving and regasification terminals could be designed to allow standard LNG tankers to be used without any modification. This would give considerable advantages and increased operational flexibility. Such terminals could also be realised at relatively lower costs, which is of course of great importance.

According to the invention there is therefore provided a system and a method for offshore offloading and regasification of LNG, as respectively disclosed in the characterising clauses of independent claims 1 and 12.

The invention could of course also be used in places other than the USA, on which particular attention was focussed in the above.

Advantageous embodiments of the invention are set forth in the dependent claims.

Embodiments of the invention are described in more detail below with reference to the attached drawings, wherein:

Figure 1 is a schematic diagram of a field arrangement according to the invention;

Figure 2 is an enlarged detail section of the field arrangement shown in Figure 1, with a regasification vessel firmly moored to a LNG tanker;

Figure 3 is a detail drawing of a regasification vessel according to the invention moored to an LNG vessel under dynamic positioning;

Figures 4 a and b are schematic diagrams of the structure of a regasification vessel according to the invention, from above and from the side respectively; and

Figure 4 c shows the disconnecting operation of an LNG hose from an LNG tanker.

According to the invention, the Applicant has developed an offshore LNG receiving and regasification solution, which is referred to as an LNG-ORT (LNG Offshore Regasification Terminal). An LNG-ORT is essentially based on prior art technology. The system has also been developed with a view to obtaining maximum operational flexibility.

A typical LNG-ORT field arrangement may include one or two mooring and gasification systems, or two mooring systems and one regasification vessel which serves both locations.

With reference to Figure 1, an LNG-ORT field arrangement is shown, including two LNG tankers 1, two regasification vessels 2, mooring lines 3, offshore mooring and offloading systems 5 including gas hoses 4, mooring lines 6 for LNG tankers 1, mooring bases 7, pipelines 8, offshore receiving platform 9, swivel 10 and buoyancy and mooring buoys 11.

Although the illustrated exemplary embodiment comprises an offshore platform 9, this is not a requirement, as an optimal solution is field and project contingent and must be assessed on a case-to-case basis, as will be apparent to the skilled person in the light of the present description.

If an offshore platform 9 is available, the metering station for the gas which is pumped into the pipe network 8 running to shore will typically be placed on the platform 9. However, it is also fully possible to equip the regasification vessels 2 with all necessary equipment to be able to operate as a complete and integrated regasification station.

When LNG tanker 1 arrives, the regasification vessel 2 will be disconnected from the gas hose 4 and can assist the LNG tanker 1 during the mooring operation. Similarly, the regasification vessel 2 will be able to provide assistance in connection with the departure of the LNG tanker 1 and during necessary maintenance operations on the field-based equipment etc. In other words, the regasification ship 2 has a multi-use function in the field. This is described in more detail below.

Referring to Figure 2, a regasification vessel 2 firmly moored to an LNG tanker 1 is shown in more detail. In periods of good weather conditions, it would be natural for the regasification vessel 2 to remain firmly moored to the tanker 1, optionally with an extra cable (not shown) from the bow to the field-installed mooring and offloading system 5. Appropriate, robust fender arrangements (not shown) will then advantageously be installed between the two vessels 1, 2. Under such operating conditions, the regasification vessel 2 will also be able to ensure that the tanker 1 remains in the correct position and heading relative to the mooring system 5.

Referring to Figure 3, the regasification vessel 2 is shown under dynamic positioning (DP). As can be seen in more detail from Figure 3, the illustrated regasification vessel 2 advantageously includes a base 14 for a swivelling hose boom 13, and three regasification modules 15. The base 14 may advantageously comprise a metering station.

The regasification vessel 2 will thus have great flexibility to increase the regasification capacity in order to satisfy the existing needs by increasing the number of LNG hoses 12 and the capacity of the regasification plant.

When wave height increases, it is not safe to have the vessels 1, 2 firmly moored alongside each other. In such situations, with typical significant wave heights (Hs) of between 4 and 5 metres, the regasification vessel 2 will be able to switch quickly to dynamic positioning (DP) and maintain a distance of 10 - 15 metres from the tanker 1. The cable connections 3 will then slacken and function as an extra backup in case the DP system should fail. Dynamic positioning can thus be used when the weather conditions make it necessary on account of large relative motions between the large LNG tanker 1 and the relatively much smaller regasification vessel 2.

The hose boom 13 of the regasification ship 2 is so arranged that three LNG hoses 12 between the two vessels 1, 2 hang freely both when the regasification vessel 2 is firmly moored to the tanker and when it operates in DP.

The hose boom 13 will, when necessary, be capable of being operated dynamically to dampen the motions of the LNG hoses 12 in marginal weather conditions.

When the offloading operation has been completed, the LNG hose 12 is emptied according to the standard, known procedure and disconnected from a manifold on the LNG tanker 1. By means of the swivelling hose boom 13, the hose 12 is efficiently brought back into its storage position on board the regasification vessel 2. Then the regasification vessel 2 can simply be disconnected from the gas hose 4, which is temporarily placed on the seabed, and assist the LNG tanker 1 in the disconnecting operation as required. Referring to Figures 4 a and b, the structure of the regasification vessel 2 is shown in a schematic form, from above and from the side respectively. Further, with reference to Figure 4c, the disconnecting operation of the LNG hose 12 from the tanker 1 manifold is shown in a simplified version from in front, and where the hose boom 13 is shown respectively in solid and stippled lines to indicate the connected and disconnected positions.

From Figures 4 a and b it can be seen that the regasification vessel 2 advantageously comprises bow and stern mounted positioning propellers 19, a moonpool 16, a coupling unit 17 for the gas hose 4 and a hydroacoustic reference unit 18. A reference system is available which at any given time shows the position and heading of the ships relative to each other (not shown).

The regasification vessel 2 has, as described above, a multi-use function in the field. Among the tasks attended to are the following:

coordination of all operations (control and command centre); assistance to the tankers 1 in connection with mooring and departure; regasification of LNG; optional base for metering station; power generation; maintenance operations in the field; support for positioning of the tanker 1 in periods of good weather/no wind in order to prevent the tanker 1 from drifting over the bottom- anchored offloading and mooring system 5; and standby, including fire standby.

The multi-use function will to a large extent contribute to a reduction of operating costs and an improvement in the efficiency of operations. It is also important to note that the regasification vessel 2 has the size and capacity to allow large regasification units 15 to be installed thereon. In the case of other solutions, it is likely that this would be a major challenge technically and/or commercially.

To meet the capacity requirement, the swivelling hose boom 13 will be arranged for the installation and operation of several parallel LNG hoses 12, as shown in Figure 3. When the operation has been completed, the hoses 12 are emptied and disconnected from the tanker 1 manifolds. At the same time, the boom 13 is swivelled back towards its storage position on board the regasification vessel 2. The hoses 12 will accompany the boom and lie extended along an arranged storage path or storage means on board the regasification vessel 2.

The solution is based on experience from buoy loading operations in the North Sea, but has not previously been used with LNG.

It should also be noted that LNG hoses 12 of large dimensions (typically more than 12 inches) as of today are not qualified for dynamic applications. However, hoses 12 of smaller diameters, typically up to 12 inches, are qualified. By using several parallel LNG hoses 12, it is thus possible to make use of qualified technology in connection with LNG-ORT.

As the skilled person will be aware, contact between LNG and water will trigger a phenomenon called RPT (Rapid Phase Transition), which involves the LNG expanding 611 times in the course of a very short time (milliseconds) in the same way as dynamite or other explosive. To prevent this phenomenon from occurring, it is therefore important that the hose 12 is prevented from becoming loose or worn during offloading.

As a person of skill in the art will also know, a water curtain is always used during offloading and loading of LNG to prevent the ship side from being cooled quickly by possible leaks from the loading manifold. The regasification vessel 2 may advantageously also help in the establishment of such a protection function. The water curtain can also be used to cause any LNG that leaks out to be "heated" up before it hits the sea surface. In this way, steps have been taken to at least partly prevent RPT.

It can also be mentioned that a shut-off system on the seabed for the hose 4 advantageously prevents water penetration into the hose 4 itself to prevent formation of hydrates. A person of skill in the art will know that this is a problem related to gas, but which will not occur in connection with oil.

Furthermore, it can be mentioned that although the illustrated and described regasification (or ancillary) vessel 2 is of a conventional single hull type, other forms of vessel, such as multihull, are also conceivable. Lastly, it can also be mentioned that maintenance of regasification vessels and equipment normally takes place out in the field. In connection with large maintenance operations, the regasification vessel 2 can easily be disconnected and move to shore. This is both more cost-efficient and safer than doing the work offshore.

Claims

P a t e n t c l a i m s

1.

A system for offshore offloading and regasification of LNG from an LNG tanker (1), s which system is connected to a natural gas receiving system, characterised by including a plurality of mooring and offloading systems (5) and a plurality of regasification vessels (2), to which mooring and offloading system (5) an LNG tanker (1) is moored, and a regasification vessel (2) is connected to the LNG tanker (1) for offloading regasified LNG into the receiving system, LNG being offloaded from the LNG tanker o (1) and regasified via the regasification vessel (2), the LNG tanker (1) being anchored to the seabed via the mooring and offloading system (5) and the regasification vessel (2) being selectively moored to the LNG tanker (1) or operated in dynamic positioning (DP) during the offloading operation.

s 2.

A system according to claim 1, characterised in that the receiving system is a land-based pipe distribution system.

3. 0 A system according to claim 1 or 2, characterised in that the mooring and offloading system (5) includes a gas hose (4) for connection to a regasification vessel (2) at one end and is connected to a mooring base (7) at the other end, the mooring base (7) being connected to the land-based pipe distribution system via a pipeline (8).

₅ 4.

A system according to claim 3, characterised in that the mooring and offloading system (5) further includes a buoyancy and mooring buoy (11) anchored to the mooring base (7) for mooring the LNG tanker via a mooring line (6).

0 5.

A system according to any one of claims 2 to 4, characterised in that the mooring and offloading system (5) is connected to the land-based pipe distribution system via an offshore receiving platform (9), preferably with a metering station for the gas provided on the receiving platform (9). ₅ 6.

A system according to any one of the preceding claims, characterised by two mooring and offloading systems (5) and two regasification vessels (2), two mooring and offloading systems (5) and one regasification vessel (2), or one mooring and offloading system (5) and one regasification vessel (2).

7.

A system according to any one of the preceding claims, characterised in that the regasification vessel (2) includes a plurality of regasification modules (15), a plurality of LNG hoses (12), a swivelling hose boom (13) for the plurality of hoses (12), and a coupling unit (17) for the hose (4).

A system according to claim 7, characterised in that the regasification vessel (2) further includes a moonpool (16) for passage of the hose (4), a base (14) for the hose boom (13), a hydrodynamic reference unit (18) and a plurality of bow and stern mounted positioning propellers (19).

9. A system according to claim 7 or 8, characterised in that the hose boom (13) can be swivelled into a storage position in which the plurality of LNG hoses (12) lie extended along a storage path or storage means on board the regasification vessel (2).

10. A system according to any one of claims 7 to 9, characterised by a plurality of, preferably three, parallel LNG hoses (12).

11.

A system according to any one of claims 7 to 10, characterised in that the regasification vessel (2), selectively and quickly, can be moored firmly to the LNG tanker (1) with taut mooring lines (3) or operate in dynamic positioning (DP) with slack mooring lines (3) as backup, the hose boom (13) advantageously being capable, when necessary, of being operated dynamically to dampen the motions of the plurality of LNG hoses (12). 12.

A method for offshore offloading and regasification of LNG from an LNG tanker (1) for delivery to a natural gas receiving system, characterised by the steps, not necessarily in said order, of - providing a plurality of mooring and offloading systems (5) and a plurality of regasification vessels (2); selectively mooring the regasification vessel (2) to the LNG tanker (1) or operating the regasification vessel (2) in dynamic positioning (DP); anchoring the LNG tanker (1) to the bottom via the mooring and offloading system (5); connecting a regasification vessel (2) to the LNG tanker (1) via a plurality of LNG hoses (12); connecting the regasification vessel (2) to a gas hose (4) included in the mooring and offloading system (5) and connected to the receiving system; - offloading LNG onto the regasification vessel (2) from the LNG tanker (1) via the plurality of LNG hoses (12); regasifying LNG in the regasification vessel (2); and offloading regasified LNG into the receiving system from the regasification vessel (2) via the gas hose (4).

13.

A method according to claim 12, characterised by mooring the LNG tanker (1) to a mooring base (7) included in the mooring and offloading system (5) via a mooring line

(6).

14.

A method according to claim 12 or 13, characterised by connecting a plurality of LNG hoses (12) to the LNG tanker, which LNG hoses (12) are connected to a plurality of regasification modules (15) on the regasification vessel (2).

15.

A method according to claim 14, characterised by the steps, not necessarily in said order, of offloading LNG from the LNG tanker (1) onto the regasification vessel (2) via the plurality LNG hoses (12); regasifying LNG in the plurality of regasification modules (15); and transferring the regasified LNG to the receiving system. 16.

A method according to claim 14 or 15, characterised by the steps of: disconnecting the plurality of LNG hoses (12) from the LNG tanker (1); - swivelling a hose boom (13) for the plurality of LNG hoses (12) into a storage position so that the plurality of LNG hoses (12) lie extended along a storage path or storage means on board the regasification vessel (2); disconnecting the gas hose (4) from the regasification vessel (2).

17.

A method according to claim 16, characterised by using a shut-off system on the seabed for the gas hose (4) after disconnection from the regasification vessel (2) to prevent water penetration into the gas hose (4).

18.

A method according to any one of claims 12 to 17, characterised by multi-use of the regasification vessel (2).

19. A method according to claim 18, characterised in that the multi-use includes one or more of the following steps: coordination of all operations (control and command centre): assistance to the LNG tanker (1) in connection with mooring and departure; metering of gas; - power generation; field maintenance operations; support for positioning of LNG tanker (1); and standby, including fire standby.