WO2002076819A1

WO2002076819A1 - Vessel and unloading system

Info

Publication number: WO2002076819A1
Application number: PCT/NO2002/000112
Authority: WO
Inventors: Rolf Emblem; Øystein Bruno LARSEN
Original assignee: Leif Høegh & Co. Asa
Priority date: 2001-03-23
Filing date: 2002-03-18
Publication date: 2002-10-03
Also published as: CN1257085C; NO20011524L; NO20011524D0; JP2004525816A; KR20030090686A; CN1511099A; EP1383676B1; ATE316489T1; DK1383676T3; DE60208877T2; DE60208877D1; ES2257529T3; PT1383676E; EP1383676A1

Abstract

A system for transferring fluid from a vessel to a shore-based facility, characterized in that in the system is included at least one vessel and at least one buoy structure which is established in the unloading position. The vessels travel in continuous shuttle traffic between the loading station and the buoy structure and are organised in such a manner that at least one of the vessels is periodically connected to one of the buoy structures with the result that the fluid is conveyed from the vessel through the buoy and directly ashore via a pipeline system for further distribution of the fluid. The invention further comprises vessels with storage tanks for storing natural gas in its liquid state, a vaporizer for regasifying the liquefied natural gas on arrival at the unloading point and a buoy structure arranged in the bottom area of the vessel's hull for transferring natural gas from the vessel to a submerged pipeline system.

Description

Vessel and unloading system

The invention relates to a special vessel together with a system for unloading fluid from a vessel to a shore-based infrastructure.

In connection with the transport of fluid such as natural gas from the field to the area in which the natural gas has to be unloaded, a system is often employed involving a number of special vessels and a series of fairly complicated operational steps. A known vessel which is commonly employed for this purpose is an LNG carrier, which is equipped with special tanks suitable for storing natural gas in its liquid state. When loading on board the LNG carrier in the field, at least one additional vessel is often employed for receiving fluid directly from the well and treating the fluid before it is transferred to the LNG carrier. According to common practice, on arrival at the unloading point the LNG carrier will transfer the load to an intermediate storage unit where the liquefied natural gas is converted to a gaseous state before being transferred to the end user.

These systems, requiring the use of several different special vessels and the performance of related complicated operations, are shown to be complex and extremely costly. It is an object of the present invention to attempt to reduce the number of special vessels which require to be included in such a system, and to improve the characteristics of these special vessels in relation to the operations which have to be carried out.

The following special vessels and systems are known from the patent literature:

In US 6089022 an unloading system is described involving an LNG carrier which is equipped with spherical tanks for storing liquefied natural gas

(LNG) and vaporizers for regasifying the liquefied natural gas. On arrival at the unloading point, the carrier is moored in such a manner that the vessel is located at a distance from the mooring structure by mooring lines which extend from the bow area to the mooring structure. The liquefied natural gas is regasified before being transported in a pipeline system which transfers the natural gas from the carrier to a shore-based installation which in the publication represents the end user.

In the arrangement in US 6089022 a single pipeline is employed for unloading the natural gas from the vessel to shore. In figure 1 of the publication this pipeline is illustrated arranged in such a manner that it extends from the bow area, through the mooring structure, from where there is provided yet another pipeline which transfers the natural gas to shore. In the publication there is no indication of any possibility of using a buoy structure or alternative technical solutions, which can be mounted in the vessel's hull for unloading natural gas.

The technical solution according to the present invention indicates the use of a submerged buoy structure which has to be brought into abutment in a recess in the vessel's hull for transferring natural gas to shore. Compared to the technique disclosed in US 6089022, by means of the buoy structure a substantially simplified solution is achieved which provides advantages both with regard to manning and the equipment situation, since mooring and fluid transfer are implemented by means of one and the same structure.

US 5564957 discloses a vessel in which a buoy structure is provided for installation in a recess in the bow portion. The buoy structure is intended for use as a combined mooring and transfer structure for transport of a liquid medium to and from the vessel.

The technique disclosed in this publication differs from the invention in that there is no vaporizer provided on board the vessel, nor is there any indication in the publication that the medium has to be transferred directly to a shore- based installation.

In US 6094937 a processing plant is described for converting natural gas to a liquid state (LNG) and a shuttle system for transporting LNG from the field. In this shuttle system two buoys and four carriers may be employed for transporting LNG, in order to maintain almost continuous production in the field.

The invention has some features in common with the system disclosed in US 6094937 in that use is made of several carriers and several buoy structures for fluid as a part of a transport system. However, there is a basic difference between the systems, since the known system is used in connection with loading, while the system according to the invention has to be used for unloading. This is manifested by the fact that a liquefier is provided on board the carrier in US 6094937, while a vaporizer is provided in the LNG tanker according to the invention. In US 6094937 there is no suggestion that this loading/transport system will be capable of use in connection with unloading. Nor will a vessel like that in US 6094937 be able to be used for unloading natural gas to a shore-based infrastructure without the need for considerable structural modifications to the vessel.

The existing technical solutions disclosed in these known publications demonstrate an inadequacy in relation to the complex requirements which have initiated the present invention.

It is therefore an object of the present invention to provide a system for unloading fluid, and especially natural gas, wherein the carrier vessel has storage tanks as well as a vaporizer on board, and is arranged to receive a buoy structure which has both a mooring function and a transfer function.

It is a further object of the present invention that the vessel with its special functions should be able to form part of a shuttle system which in a preferred embodiment includes a plurality of vessels and a plurality of the buoy structures concerned. The object of this system is to achieve the continuous supply of natural gas from the unloading point to the infrastructure.

In connection with the invention, reference should be made to the independent patent claims. Furthermore, the related dependent patent claims indicate embodiments of the invention.

With the vessel according to the invention, a number of the operations required when using the previously known systems are made superfluous. By means of the invention a number of the operations associated with mooring, connection of pipelines, transfer of liquid cargo from one vessel to another will be made superfluous and/or simplified. In addition, when using the system according to the invention the number of crew members will be reduced, and thereby also the operating costs, due to the fact that both the number of operations performed becomes less and more functions are concentrated on one vessel. The design and production of a multi-functional vessel of this kind can be an extremely costly and time-consuming process. It has therefore been an object of the present invention to provide the vessel in a cost-effective manner, thus enabling the vessel to be offered at a competitive price. In a preferred embodiment of the invention the vessel is provided in a simple and inexpensive manner using a standard LNG carrier as the basis. The carrier with its storage tanks for liquefied natural gas is equipped with one or more vaporizers preferably in the deck area, and a receiving recess is constructed in the hull for receiving the buoy structure, thus making the vessel suitable for performing several types of operation. This special vessel is described as a "Shuttle and Regas Vessel (SRV)", and the vessel may of course also be used as an ordinary LNG carrier.

In a preferred embodiment, the vessel according to the invention is equipped with spherical tanks for storing liquefied natural gas, but other types of storage tanks may also be suitable, such as, for example, membrane tanks. These different types of storage tanks are well-known both from the patent literature and in practical use. Thus it will be up to a person skilled in the art to select the type of tank which is most suitable in the individual case. The principles and equipment to be used in connection with the regasification of the liquefied natural gas also represent known per se technology. Sea water may be utilised as a heat exchange medium in the vaporization process, but also other media such as, for example, propane either alone or together with sea water, and a water-glycol mixture may be suitable vaporization media.

The buoy structure which is to form part of the system may be designed in many ways. From the patent literature several examples are known of submerged buoy structures which can be connected to a vessel, thus enabling the vessel to rotate freely around the buoy. However, with regard to the buoy structure's mode of operation, rapid connection and disconnection of the buoy are a requirement in order that the unloading of fluid can start almost immediately without delays due to time-consuming mooring procedures.

When the buoy structure is not in use, it is in a submerged condition. On arrival at the unloading area, the vessel will pick up an auxiliary buoy, which has a line attached to the buoy structure. The line is used to convey the buoy structure to the surface, whereupon the buoy structure is brought into abutment in the vessel's recess.

Around its circumference the buoy structure is equipped with mooring lines which extend down to the mooring points on the seabed. Risers are provided up to the middle of the buoy structure and a swivel structure is arranged above the buoy structure. By means of this arrangement fluid can be passed through the buoy structure via the riser to the submerged pipelines, while at the same time the vessel rotates around the buoy structure. The submerged pipelines transport fluid away from the vessel towards the shore-based infrastructure.

The invention proposes a system for achieving an efficient unloading of fluid to a shore-based infrastructure. According to a preferred embodiment of the invention, two buoy structures and two or more carrier vessels are included in the system. The number of vessels included in the system depends on the distance to the loading point. One and the same vessel will alternate between different operations such as loading from a loading station, for example in the field, transporting LNG from the loading station to the unloading point and regasifying liquefied natural gas with subsequent transfer to a receiving system on shore. When the unloading of fluid through the buoy structure takes place almost continuously by having at least one vessel connected to one of the buoy structures at all times, while the other vessels are on the way to or from the loading stations, optimal efficiency is achieved in the utilisation of the system. On arrival at the unloading point, the buoy structure will be inserted in the receiving recess in the carrier vessel. Liquefied natural gas will be passed from the storage tanks to the vaporizer where the fluid is regasified and passed directly through the buoy structure to submerged pipelines which convey the natural gas to a shore-based infrastructure. The shore-based infrastructure may take many forms, as long as the infrastructure is suitable for receiving the regasified fluid which is sent ashore from the carrier and the fluid is further distributed to the end users. For example, the infrastructure may comprise a pipeline network which conveys the natural gas directly to the end user or the infrastructure may comprise a depot which is connected to appropriate transport means for further transport of natural gas to the consumers, etc.

The invention will now be explained in more detail with reference to the figures in which: fig. 1 is a side view of a carrier vessel with the necessary equipment. Fig. 2 is a perspective view of one of the vessels coupled up to one of the buoy structures.

Fig. 3 is a perspective view of the connection of the pipelines with the infrastructure.

Fig. 4 is a general view of the organisation of the vessels in order to obtain an efficient unloading of fluid.

Fig. 5 illustrates the vessel connected to both buoy structures simultaneously.

Fig. 1 is a simplified schematic view of the retrofitted LNG carrier "Shuttle and Regas Vessel (RSV)" according to the invention. The vessel 1 is illustrated provided with a plurality of tanks 2 for storing liquefied natural gas. The figure shows how in the vessel's hull there is provided a conical recess 5 which forms a receiving arrangement for the buoy structure 7 (not illustrated in the drawing). The vessel 1 is further provided with at least one vaporizer 4. In order to improve the vessel's manoeuvrability, the vessel is also equipped with thrusters 6 in the vessel's bow portion and stern portion respectively.

Fig. 2 illustrates an arrangement of buoy structures 7 which are connected to a submerged pipeline system. The submerged pipeline system comprises a riser 8 which is attached to each of the buoy structures 7, and furthermore a pipeline 9 is connected to the end of the riser 8. The pipelines 9 are connected to an on-shore infrastructure, illustrated here in the figure by the pipeline system 10. The buoy structures 7 are equipped with mooring lines 1 1 extending from the mooring point on the seabed to an attachment on the circumference of the buoy structure.

Figure 2 illustrates two vessels 1 where one of the vessels 1 is connected to one of the buoy structures 7, while the other vessel T illustrates the voyage to/from the loading point. On connection with the buoy structure 7, the liquefied natural gas is transferred to the vaporizer 4, where the fluid is regasified before being transferred through the buoy structure 7 via the submerged pipelines 9 which are connected to yet another pipeline 10 for bringing natural gas ashore to the infrastructure.

Figure 3 illustrates the same situation as in figure 2, but in addition it also illustrates the pipeline system's connection to the infrastructure, which in this case is composed of a shore-based pipeline system 12. Fig. 4 illustrates the principle for the organisation of the connection of the various vessels to the buoy structures as well as transport to and from the loading station where LNG is loaded on board the vessel. In this example four vessels are included in the system. In fig. 5 both the buoy structures are connected to the vessels 1 and 1' simultaneously. In the situation illustrated in the figure the vessel 1 is in the process of completing the unloading of natural gas, while the vessel 1' has just received the buoy structure 7 in the recess 5. By means of this arrangement a uniform transition will be achieved in the unloading process from vessel 1 to vessel 1', and a continuous flow of natural gas into the submerged pipeline system will thereby be maintained.

Claims

PATENT CLAIMS

1. A system for transferring fluid from a vessel to a shore-based facility, characterized in that in the system there are included at least one vessel and at least one buoy structure which is established in the unloading position, where the vessels preferably travel in continuous shuttle traffic between the loading station and the buoy structure and are organised in such a manner that at least one vessel is periodically connected to one of the buoy structures for transferring the fluid from the vessel through the buoy and directly ashore via a pipeline system for further distribution of the fluid.

2. A system according to claim 1, characterized in that two or more vessels and two buoy structures are included in the system.

3. A system according to claim 1, characterized in that the vessel is a retrofitted LNG tanker.

4. A system according to claim 1, characterized in that the fluid is natural gas.

5. A system for unloading from a vessel, characterized in that the vessel is equipped with a recess in the bottom of the hull, where the recess is designed for receiving a corresponding submerged buoy structure connected to a pipeline system which transfers fluid from storage in the vessel to the shore-based infrastructure.

6. A system for unloading from a vessel according to claim 2, characterized in that the vessel is equipped with a vaporizer.

7. A system for unloading from a vessel provided with storage tanks for storing natural gas in a liquid state (LNG), characterized in that the system comprises

- a vaporizer provided on the vessel for vaporizing the liquefied natural gas into a gaseous state on arrival at the unloading point,

- a buoy structure provided in the bottom area of the vessel's hull for transferring natural gas from the vessel to a submerged pipeline system,

- transport of natural gas in the submerged pipeline system which transfers the natural gas to the shore-based infrastructure.

8. A vessel, characterized in that it comprises

- storage tanks for storing natural gas in its liquid state,

- a vaporizer for regasifying the liquefied natural gas on arrival at the unloading point,

- a buoy structure arranged in the bottom area of the vessel's hull for transferring natural gas from the vessel to a submerged pipeline system.