GB2367049A

GB2367049A - Ship to ship LNG transfer system

Info

Publication number: GB2367049A
Application number: GB0023157A
Authority: GB
Inventors: B Evans
Original assignee: Ocean Technologies Ltd
Current assignee: Ocean Technologies Ltd
Priority date: 2000-09-21
Filing date: 2000-09-21
Publication date: 2002-03-27
Also published as: GB0023157D0

Abstract

A cryogenic fluid transfer system allows the transportation of cryogenic fluid and associated vapour between a discharging facility 1 and a receiving vessel 4, e.g. an FPLSO (Floating Production, Liquefaction, Storage, and Offloading vessel) and a cater vessel, whilst accommodating any relative movements. Rigid pipelines 3 supported by frames (pipehangers) 19, 32 suspended from a cable system, and flexible or articulated conduits 2, 21, 22, 23 are releasably connected to form a continuous passage between the discharging facility 1 and receiving vessel 4. The cable system is tensioned between a releasably mounted cable clamping device (guidehead) 15 and a constant tension device 20 mounted on the discharging facility 1. The guidehead 15 incorporates means for locking and unlocking to the upper cable 14, and is permanently attached to the lower cable 30. A guidehead receiver 17 mounted on the receiving vessel 4 has locking means for locating the guidehead 15. The cable system is established by deploying and attaching the upper cable 14 to a winch 18 mounted on the receiving vessel 4.

Description

SHIP TO SHIP LNG TRANSFER SYSTEM This invention relates to the development of offshore stranded oil & gas fields, in particular the

transfer of LNG between floating vessels.

There are many remote offshore oil & gas fields where the transfer of gas by pipeline would be uneconomic due to the long length of pipelines required. In the past, gas would be flared where it was associated with crude oil production or alternatively re-injected, but there is currently a worldwide ban on gas flaring, and gas re-injection into the reservoir is not always an option.

Therefore such fields remained undeveloped and considered as stranded fields.

Hence, there is a need to produce LNG offshore by production and liquefaction of natural gas, requiring the transfer of the LNG between floating vessels, an operation hitherto unprecedented.

Current ship to ship transfer systems for other fluids exist, but are not suitable for LNG, which must be maintained at a temperature of-163C. Concepts for ship to ship LNG transfer systems currently being developed involve the use of heavy lifting cranes, and complex systems including hydraulics, position control, ship movement compensation, and a large number of parts. Therefore, current concepts are very heavy and expensive, operator unfriendly, difficult to maintain, and prone to failure. Further disadvantages of current concepts are that the discharging vessel, (usually a Floating Production, Liquefaction, Storage & Offloading Ship) requires significant strengthening, and the operating envelope for transfer of LNG to the gas carrier vessel (receiving vessel) is limited.

We have now devised a ship to ship LNG transfer system which alleviates the above mentioned problems.

According to the present invention there is provided a cryogenic fluid transfer system comprising flexible or articulated conduits suitable for transporting cryogenic fluids and associated vapours, releasable connections for said conduits fitted to a discharging facility and to a receiving vessel, a cable system mounted on the discharging facility and adjustably connected thereto between a constant tension device and a releasably mounted cable clamping device (guidehead), the cable system supporting pipelines from cable suspended frames (pipehangers), said pipelines having releasable connections at each end to which said conduits can be connected, said guidehead being shaped accordingly to fit with a receiving device (guidehead receiver) mounted on the receiving vessel, the guidehead receiver having locking means for locating the guidehead, and a means for initial deployment of the cable system between discharging facility and receiving vessel.

Preferably, the cable system is arranged with an upper and lower cable.

Preferably, the guidehead incorporates means for remotely locking and unlocking the upper cable to it.

Preferably, the lower cable is permanently attached to the guidehead.

Preferably, the constant tension device is a double drum constant tension winch allowing synchronous and independent operation of the two winch drums.

Preferably the pipelines are formed of one continuous length thereby obviating the need for additional flanges & the associated risk of leaks.

Preferably, the pipelines are of a concentric arrangement such that the cryogenic fluid pipeline is located within a larger diameter pipeline and associated vapour is returned through the annulus.

However, in an alternative embodiment, separate pipelines are provided for cryogenic fluid flow and for associated vapour return flow.

Preferably the flexible or articulated conduits will be made of a material suitable for bending and stretching in all degrees of freedom. However, in an alternative embodiment swivels may be used to aid flexibility of movement.

Preferably, the pipehangers incorporate a releasable locking device for clamping/unclamping them to the cable.

Preferably, the means of initial deployment of the cable system between discharging facility and receiving vessel is achieved by attachment of the upper cable to a messenger line which is passed from the discharging facility to the receiving vessel, the messenger line being subsequently attached to a winching device mounted on the receiving vessel.

Also, in accordance with this invention there is provided a method of transferring cryogenic fluids between a discharging facility and a receiving vessel using a cryogenic fluid transfer system comprising flexible or articulated conduits suitable for transporting cryogenic fluids and associated vapours, releasable connections for said conduits fitted to a discharging facility and to a receiving vessel, a cable system mounted on the discharging facility and adjustably connected thereto between a constant tension device and a releasably mounted cable clamping device (guidehead), the cable system supporting pipelines from cable suspended frames (pipehangers), said pipelines having releasable connections at each end to which said conduits can be connected, said guidehead being shaped accordingly to fit with a receiving device (guidehead receiver) mounted on the receiving vessel, the guidehead receiver having locking means for locating the guidehead, and a means for initial deployment of the cable system between discharging facility and receiving vessel, the method comprising moving and suspending the cable system, pipehangers, and supported pipelines between discharging facility and receiving vessel, applying constant tension to the cable system, and connecting said conduits by one end to each pipeline and the other end into the discharging facility and receiving vessel respectively, so forming a continuous passage for flow of cryogenic fluid and any associated vapour between discharging facility and receiving vessel, the cable system being moved between discharging facility and receiving vessel by deploying and attaching the upper cable to a winch mounted on the receiving vessel, locking the upper cable to said guidehead, the lower cable being permanently attached to the guidehead, and winding in the guidehead to lock with said guidehead receiver.

In preferred use the guidehead is releasably locked into the guidehead receiver on the receiving vessel, thereby suspending the cable system and pipelines between the discharging facility and receiving vessel.

Preferably, the locking mechanism in the guidehead automatically releases the upper cable upon the guidehead becoming locked in the guidehead receiver. Thus, the guidehead receiver is preferably arranged internally to activate release of the upper cable from the guidehead. This ensures that the upper cable is not over tensioned whilst being deployed.

Preferably, the releasable connections fitted to the discharging facility and receiving vessel are remotely activated quick release couplings with double ball valves. This allows immediate and safe disconnection of the flexible or articulated conduits in the event that the receiving vessel exceeds its operating envelope.

Preferably, the system is used for transporting LNG and its associated vapour between floating LNG discharge and receiving vessels.

Preferably a plurality of releasable connections on the LNG discharging vessel are positioned to allow connection of the system, such that LNG can be circulated through the pipelines on the discharging vessel. This enables the system to be cooled prior to offloading onto the receiving vessel and reduces offloading duration.

A specific embodiment of the invention will now be described by way of example only and with reference to the accompanying drawings in which: Figure 1 shows an isometric view of the system with the pipelines being cooled prior to connection between the two vessels.

Figure 2 shows initial contact set up between the vessels.

Figure 3 illustrates establishment of the cable suspension system between the two vessels.

Figure 4 shows the system set up for LNG transfer between the two vessels.

Figure 5 shows in cross section, the central pipehanger with upper cable clamping mechanism and pipeline support saddles.

Figure 6 shows in side view, one of the outer pipehangers with a cable deflector and pipeline support saddle.

Figure 7 shows in cross section, an alternative embodiment with the pipelines arranged concentrically, and held by a pipeline support saddle.

Referring to Figure 1, there is shown the LNG transfer system set up on the LNG discharging vessel, 1, so that the flexible or articulated conduits, 2,21, 22 and 23, and the pipelines, 3 (2 shown), can be cooled prior to connection to the LNG receiving vessel, 4. The LNG is pumped in the direction of arrows, 5 and 6, and LNG vapour is returned in the direction shown by arrows, 7 and 8. In this way the system is cooled by the LNG flow and this enables transfer duration between vessels to be shortened. In the case of there being infrequent offloading then this step may be omitted and the system connected to the receiving vessel, 4, without prior cooling.

The flexible or articulated conduits, 2,21, 22 and 23, are connected to the discharging vessel, 1, with quick release couplings, 9 and 24 (4 shown), and to the pipelines, 3, with releasable couplings, 10 and 28 (4 shown). A mechanical handling system (not shown) may be necessary to lift and align the cold flexible or articulated conduits, 2,21, 22 and 23 into place. The pipelines, 3, are held by saddles, 29, attached to cable supported pipehangers, 19 and 32.

The cables consist of a pulling line, 14, and a lower cable, 30. The pulling line, 14, is secured at one end to a double drum constant tension winch, 20, said winch, 20, allowing separate and synchronous operation of its drums, and at the other end is releasably secured to a guidehead, 15.

The guidehead, 15, being releasably secured to a stowing lock, 16. The stowing lock, 16, is supported by a frame (not shown) attached to the discharging vessel, 1. The lower cable, 30, is secured to the guidehead, 15, and tensioned between said guidehead, 15, and the double drum constant tension winch, 20. The double drum constant tension winch, 20, is mounted to the discharging vessel, 1, on supports (not shown). On arrival of the receiving vessel, 4, a messenger line, 12, which is attached to the pulling line, 14, is passed from the discharging vessel, 1. The position of the messenger line, 12, is indicated by a marker buoy, 11.

Referring to Figure 2, there is shown initial contact established between the two vessels. The receiving vessel, 4, is operated using dynamic positioning (DP) and takes up a position defined by an operating envelope at a defined distance (typically 60m but could be more or less) from the discharging vessel, 1. The messenger line, 12, shown in Figure 1 is passed to the receiving vessel, 4, pulled through the guidehead receiver, 17, and attached to a winch, 18. A hawser, 13, may be used to loosely attach the two vessels as an added security against drift separation.

The winch, 18, is used to wind in the messenger line, 12. The pulling line, 14, is released from the guidehead, 15, which at this stage is held by the stowing lock, 16. Winch, 18, is used to wind in the pulling line, 14, until it is securely held on the winch drum, 18. As it is reeled in, the pulling line, 14, passes through the guidehead receiver, 17, the guidehead, 15, and the pipehangers, 19 and 32, and is paid out under constant tension from the double drum constant tension winch, 20. The lower cable, 30, remains stationary, tensioned between the guidehead, 15, and the double drum constant tension winch, 20.

Referring to Figure 3, there is shown the cable system set up between the two vessels. Contact is established as previously described and shown in Figure 2. Then the pulling line, 14, is secured to the guidehead, 15, which is released from the stowing lock, 16. Winch, 18 is then used to wind in the pulling line, 14, carrying with it the attached guidehead, 15, and lower cable, 30.

During this process the double drum constant tension winch, 20, winds out with both drums at constant tension, and the cables, 14 and 30, pass through the pipehangers, 19 and 32.

As the guidehead, 15, enters the guidehead receiver, 17, on the receiving vessel, 4, it is automatically locked in position, and simultaneously releases the pulling line, 14.

The guidehead, 15, guidehead, receiver, 17, assembly may be fitted with an hydraulic motor (not shown) for rotating the assembly if entanglement of cables, 14 and 30, should occur during deployment.

Referring to Figure 4, there is shown the system set up for LNG transfer between the two vessels. The guidehead, 15, is locked into the guide receiver, 17, and releases the pulling line, 14, as described previously. The flexible or articulated conduits, 2,21, 22 and 23, are purged of fluid and disconnected from the discharging ship, 1. The central pipehanger, 19, is now releasably attached using a clamping mechanism, 31, (Figure 5) to the pulling cable, 14, in order that the pipelines, 3, can be winched across. The other pipehangers, 32, are not clamped, thereby allowing thermal contraction and expansion of the pipelines, 3, to take place outwards. Winch, 18, is then used to wind in the pulling line, 14, at constant tension, until the pipelines, 3, are in the desired position. Winch, 18, is then braked and locked in position. The flexible or articulated conduits, 2,21, 22 and 23, are connected to allow flow of LNG between vessels.

Two flexible or articulated conduits, 2 and 21, are connected to the quick release couplings, 9 and 24, on the discharging vessel, 1, and to the releasable couplings, 10, at one end of the pipelines, 3. The other two flexible or articulated conduits, 22 and 23, are connected to the quick release couplings, 26 and 27, on the receiving vessel, 4, and to the releasable couplings, 28, at the other end of the pipelines, 3.

LNG is pumped between vessels in the directions shown by arrows, 5 and 6. LNG vapour is returned between vessels in the directions of arrows, 7 and 8. Recovery of the system to the discharging vessel, 1, is achieved by the reversal of the above operations.

Referring to Figures 5 and 6, there is shown a cross sectional view of the central pipehanger, 19, and an end view of one of the outer pipehangers, 32. The pipehangers, 19 and 32, consist of a main U-shaped frame, 33, connected to saddles, 29, and cable pulleys, 34. The saddles, 29, are used to support the pipelines, 3. The central pipehanger, 19, also has cable clamps, 31, attached, which are used to lock the pulling line, 14, to the pipehanger, 19. The outer pipehangers, 32, also have cable deflectors, 35, attached, in order to assist smooth operation of the cable system.

Referring to Figure 7 there is shown an alternative embodiment, whereby the pipelines, 3, are replaced by a single pipeline consisting of concentric pipes, 37 and 38, for flow of LNG and return flow of LNG vapour. The inner pipe, 37, carries the LNG, and the outer pipe, 38, carries the LNG vapour. The pipes, 37 and 38, are supported with a saddle, 36.

If at any time during the transfer process (including system deployment and retrieval) the catenary deflection of the pulling line, 14, is considered excessive, then the winch, 20, can increase the tension, but the receiving vessel, 4, must produce equivalent reverse thrust.

In the event of an emergency separation being required the quick release couplings, 9 and 24, enable the system to be disconnected quickly. This could be necessary if the two vessels move outside of the pre-determined safe operational distance. In this event alarms would sound, and the flexible or articulated conduits, 2 and 21, would be automatically released from the quick release couplings, 9 and 24. The fluids would be safely contained in the system due to double ball valves (not shown) located in the quick release couplings, 9 and 24.

Alternatively in another embodiment, the system could be disconnected at the releasable connections, 10. This would require that the connections, 10, were of the quick release type incorporating double ball valves.

Should transverse motion between the ships be considered as probable to cause relative movement outside of the pre-determined operating envelope then means, for example horizontal sheaves (not shown) mounted on the discharging vessel, 1, would be provided for safe retrieval of the system.

Claims

CLAIMS 1. A cryogenic fluid transfer system comprising flexible or articulated conduits suitable for transporting cryogenic fluids and associated vapours, releasable connections for said conduits fitted to a discharging facility and to a receiving vessel, a cable system mounted on the discharging facility and adjustably connected thereto between a constant tension device and a releasably mounted cable clamping device (guidehead), the cable system supporting pipelines from cable suspended frames (pipehangers), said pipelines having releasable connections at each end to which said conduits can be connected, said guidehead being shaped accordingly to fit with a receiving device (guidehead receiver) mounted on the receiving vessel, the guidehead receiver having locking means for locating the guidehead, and a means for initial deployment of the cable system between discharging facility and receiving vessel.
2. A cryogenic fluid transfer system as claimed in Claim 1 wherein the cryogenic fluid is LNG.
3. A cryogenic fluid transfer system as claimed in Claim 1 or Claim 2, wherein the cryogenic fluid is transferred between floating discharge and receiving vessels.
4. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the cable system is arranged with an upper and lower cable.
5. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the guidehead incorporates means for remotely locking and unlocking the upper cable to it.
6. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the lower cable is permanently attached to the guidehead.
7. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the constant tension device is a double drum constant tension winch, said winch allowing synchronous and independent operation of the two winch drums.
8. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the pipelines are formed of one continuous length thereby obviating the need for additional flanges.
9. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the pipelines are of a concentric arrangement such that the cryogenic fluid pipeline is located within a larger diameter pipeline and associated vapour is returned through the annulus.
10. A cryogenic fluid transfer system as claimed in any of claims 1 to 8, wherein separate pipelines are provided for cryogenic fluid flow and for associated vapour return flow.
11. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the flexible or articulated conduits are made of a material suitable for bending and stretching in all degrees of freedom.
12. A cryogenic fluid transfer system as claimed in any preceding claim, wherein swivels are used to aid flexibility of movement of the flexible or articulated conduits.
13. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the pipehangers incorporate a releasable locking device for clamping/unclamping them to the cable.
14. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the pipehangers have cable pulleys attached.
15. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the pipehangers are connected to pipeline support saddles.
16. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the outer pipehangers are equipped with cable deflectors.
17. A cryogenic fluid transfer system as claimed in any preceding claim, wherein the means of initial deployment of the cable system between discharging facility and receiving vessel is achieved by attachment of the upper cable to a messenger line which is passed from the discharging facility to the receiving vessel, the messenger line being subsequently attached to a winching device mounted on the receiving vessel.
18. A cryogenic fluid transfer system substantially as herein described and illustrated in the accompanying drawings.
19. A method of transferring cryogenic fluids between a discharging facility and a receiving vessel using a cryogenic fluid transfer system comprising flexible or articulated conduits suitable for transporting cryogenic fluids and associated vapours, releasable connections for said conduits fitted to a discharging facility and to a receiving vessel, a cable system mounted on the discharging facility and adjustably connected thereto between a constant tension device and a releasably mounted cable clamping device (guidehead), the cable system supporting pipelines from cable suspended frames (pipehangers), said pipelines having releasable connections at each end to which said conduits can be connected, said guidehead being shaped accordingly to fit with a receiving device (guidehead receiver) mounted on the receiving vessel, the guidehead receiver having locking means for locating the guidehead, and a means for initial deployment of the cable system between discharging facility and receiving vessel, the method comprising moving and suspending the cable system, pipehangers, and supported pipelines between discharging facility and receiving vessel, applying constant tension to the cable system, and connecting said conduits by one end to each pipeline and the other end into the discharging facility and receiving vessel respectively, so forming a continuous passage for flow of cryogenic fluid and any associated vapour between discharging facility and receiving vessel, the cable system being moved between discharging facility and receiving vessel by deploying and attaching the upper cable to a winch mounted on the receiving vessel, locking the upper cable to said guidehead, the lower cable being permanently attached to the guidehead, and winding in the guidehead to lock with said guidehead receiver.
20. A method as claimed in Claim 19 wherein the guidehead is releasably locked into the guidehead receiver, thereby suspending the cable system and pipelines between the discharging facility and receiving vessel.
21. A method as claimed in Claim 19 or Claim 20, wherein the locking mechanism in the guidehead automatically releases the upper cable upon the guidehead becoming locked in the guidehead receiver.
22. A method as claimed in any of claims 19 to 21, wherein the guidehead receiver is arranged internally to activate release of the upper cable from the guidehead.
23. A method as claimed in any of claims 19 to 22, wherein the guidehead and guidehead receiver assembly is fitted with a hydraulic motor such that the assembly can be rotated should cable entanglement occur during deployment.
24. A method as claimed in any of claims 19 to 23, wherein the releasable connections fitted to the discharging facility and/or receiving vessel are remotely activated quick release couplings with double ball valves, such that immediate and safe disconnection of the flexible or articulated conduits can be achieved in the event that the receiving vessel exceeds its operating envelope.
25. A method as claimed in any of claims 19 to 23, wherein the releasable connections fitted to the pipelines are remotely activated quick release couplings with double ball valves, such that immediate and safe disconnection of the flexible or articulated conduits can be achieved in the event that the receiving vessel exceeds its operating envelope.
26. A method as claimed in any of claims 19 to 25, wherein horizontal sheaves are mounted to the discharging vessel, the cables passing therethrough such that safe retrieval of the cable system can be achieved in the event that the receiving vessel exceeds its transverse operating envelope.
27. A method as claimed in any of claims 19 to 26, wherein a plurality of releasable connections on the discharging facility are positioned to allow connection of the system, such that cryogenic fluid can be circulated through the pipelines on the discharging facility, thereby enabling the system to be cooled prior to deployment and offloading onto the receiving vessel.
28. A method of transferring cryogenic fluids between a discharging facility and a receiving vessel, the method being substantially as herein described and illustrated in the accompanying drawings.