Disconnectable mooring system and LNG transfer system and method

This application is based upon provisional application Ser. No. 60/362,876 filed on Mar. 8, 2002, the priority of which is claimed.

1. Field of the Invention

This invention relates generally to Floating Production Storage and Offloading vessels (FPSO's) where a turret is rotatably supported on the vessel and where the turret is fixed to the seabed by anchor legs so that the vessel can weathervane about the turret. In particular, this invention relates to stern offloading of a permanently moored LNG (Liquefied Natural Gas) liquefaction process vessel wherein pipelines on the seafloor bring natural gas to the permanently moored vessel. The gas is conducted from the seafloor to the vessel through flexible pipes (risers). An LNG plant on board the permanently moored vessel liquefies the gas, then large quantities of liquid gas are stored on board. Periodically, LNG carriers (shuttle tankers) arrive and load the liquefied gas for transport to distant ports. Temporary mooring connection and offloading to an LNG carrier vessel in offshore sea conditions requires highly reliable and safe equipment. Historically, most LNG loading operations are performed in mild environments in protected waters and at dockside. Performing this operation offshore between floating vessels is not known to have been done to date.

2. Description of the Prior Art

European patent application EP0947464A1 describes a fluid transfer system comprising a counterweighted transfer boom with coaxial fluid ducts mounted on the stern of a LNG storage vessel. The LNG storage vessel is also known by the term LNG process vessel and also LNG/FPSO. The drawings of this application show an LNG shuttle tanker (or “carrier”) moored to the LNG storage vessel by means of a hawser. A disadvantage of this system concerns the method of mooring the LNG shuttle tanker with a flexible hawser to the LNG storage vessel. The relatively short reach of the fluid transfer boom makes it necessary to bring the LNG shuttle tanker very close to the storage vessel. This creates danger of collision of the two vessels, because the hawser does not restrict the forward motion of the LNG carrier. Therefore, a tugboat is required at all times at the LNG carrier stern to attempt prevention of collision.

FIGS. 1 and 2 of the drawings illustrate a prior art LNG transfer system 3 , such as the FMC Technologies BTT system, with piping and flexible joint swivels connecting the stern S of LNG/FPSO vessel 1 to the bow B of LNG carrier 2 . Hawser 80 endures the mooring force to hold LNG carrier 2 to the stern S of LNG/FPSO vessel 1 . The hawser mooring system of FIGS. 1 and 2 can be disadvantageous in rough seas in that there is no restraint to prevent LNG carrier 2 from surging forward and colliding with LNG/FPSO vessel 1 . Piping pantograph 5 is flexible and allows limited lateral motion of pantograph fluid connector 6 (see motion arrows “L” in FIG. 2 ), such as within a circle of 12 meters radius. As LNG carrier 2 sways laterally, crane boom 4 rotates automatically to follow the wide excursions of the bow B of LNG carrier 2 while connected to LNG carrier manifold 7 . The system of FIGS. 1 and 2 requires a large separation distance, such as 60 to 100 meters to reduce the chance of collision of the vessels. The large separation distance requires a long and expensive crane boom 4 .

U.S. Pat. No. 4,568,295 shows a yoke mooring system between a buoy and a moored vessel.

3. Identification of Objects of the Invention

A primary object of the present invention is to provide a disconnectable mooring device in combination with a fluid transfer arrangement to connect an LNG carrier (shuttle tanker) to an LNG process vessel (LNG/FPSO) that is intended for frequent connection and disconnection of the LNG carrier vessel in an offshore environment of at least Hs 2 meters significant wave height that causes relative motion between the two vessels.

Another object of the invention is to provide a disconnectable mooring system that temporarily moors an LNG carrier to an LNG process and storage vessel.

The objects identified above, as well as other features and advantages of the invention, are provided by a system which includes a disconnectable mooring arrangement and LNG transfer system between an LNG storage vessel and an LNG carrier. The mooring arrangement is characterized by a yoke with one end pivotable about horizontal axes of one of the vessels, e.g., the LNG storage vessel, with the yoke having an opposite end with a plug coupling arrangement. The plug coupling is arranged and designed to be pulled into a receptacle on the LNG carrier for selective coupling thereto. Liquid Natural Gas from the LNG storage holds is transferred to the LNG carrier by means of a fluid conduit and pantograph arrangement carried by a tower mounted boom which can be positioned to establish couplings on the LNG carrier.

FIGS. 1 and 2 are prior art illustrations described above which illustrate an LNG carrier (shuttle vessel) moored to the stern of an LNG process vessel (LNG/FPSO for example) by means of a hawser and with a tower mounted boom and pantograph arrangement providing a fluid flowpath for LNG conduits from the process vessel to the carrier;

FIG. 3 is a side view of the mooring arrangement and LNG transfer structure according to the invention with a plug coupling at the end of the yoke secured within a female receptacle at the bow of the LNG carrier and shows a temporary mooring hawser in a slack position with a pickup line floating on the sea;

FIG. 4 is a top view of the arrangement of FIG. 3 ; and

FIG. 5 is a side view of the mooring arrangement and LNG transfer structure with the plug coupling at the end of the yoke disconnected from the bow of the LNG carrier with the system illustrating the LNG conduits mounted on the pantograph disconnected from an LNG carrier bow coupling and a hawser connected between the LNG/FPSO and the LNG carrier.

The illustrations of a preferred embodiment of the arrangements of FIGS. 3-5 according to the invention are described in connection with reference numbers as follows:

As illustrated in FIGS. 3 and 4 , the LNG shuttle tanker 2 (LNG carrier) is temporarily moored to the stern of an LNG process vessel 1 (such as an LNG/FPSO) by means of mooring yoke arms 27 of yoke 3 . The yoke arms 27 are suspended from a yoke support structure or frame 19 by vertical links 23 and two two axis universal joints 18 . Two three axis universal joints 17 couple links 23 to yoke arms 27 . A loading pipe 16 is secured to the inward ends of arms 27 . The outward ends of arms 27 are coupled by a three axis universal joint to a shock absorbing cone 25 which is arranged and designed to be pulled up into a hydraulic yoke connector 11 on an extension 28 of the bow of LNG carrier 2 .

The yoke 3 , comprising arms 27 , LNG loading pipe 16 , and pivotable cone 25 , is designed and arranged so that the center of gravity of the yoke is along a line 26 that is located a small distance outward of the connection of links 23 to arms 27 so that the yoke is generally balanced, but the outward end of the yoke rotates toward the sea when the cone 25 is disconnected from connector 11 . Although not essential, cone 25 is constructed to provide flotation so that when the LNG carrier 25 is disconnected, the yoke 3 tips about the connection to links 23 with the outer end sloping toward the sea surface and with cone 25 providing flotation. Male shock absorbing connector cone 25 is connected to the ends 27 of yoke arms 27 by a three axis universal joint arrangement with a roll axis bearing joint 8 , pitch axis joint 9 and vertical axis joint 10 thereof illustrated in FIGS. 3 and 5 . A ball joint and hitch arrangement can alternatively be provided for connecting the arms 27 to the shock absorbing cone connector 25 .

In operation, starting with the view of FIG. 3 , a mooring hawser 20 is wound about mooring winch 22 on LNG process vessel 1 and extends over sheaves 29 with a pickup line 21 floating on the sea surface. When the vessel 2 is disconnected, the yoke 3 outer end with cone 25 floats on the sea surface.

When a vessel arrives for connection, as shown in FIG. 5 , the hawser 20 is picked up and connected to a mooring bitt 14 on extension 28 of the LNG carrier vessel 2 . The boom 4 and pantograph pipe arrangement 6 have been moved upwardly to provide clearance for the LNG carrier vessel 2 to be hauled into a position in proximity with the yoke 3 . A pull-in cable or line 12 from pull-in winch 13 on extension 28 is passed through a passage of hydraulic yoke connector 11 mounted in extension 28 of the bow of LNG carrier 2 . Bow thrusters 24 of the LNG carrier 2 provide position control for LNG carrier 2 . The end of the pull-in cable 12 is secured to the cone 25 . From the position of FIG. 5 , the cone 25 is pulled into the connector 11 . When pull-in is complete, the outer end of yoke 3 is fastened to the LNG carrier vessel 2 by the cone 25 being secured in hydraulic connector 11 . Next, boom 4 is moved into position over the bow of the vessel 2 , and the double pantograph pipe arrangement 6 is lowered into position for fluid coupling of coupling head 30 with LNG pipe quick connect/disconnect unit 5 . The hawser 20 is disconnected from vessel 2 and the LNG carrier vessel is moored to the LNG process vessel 1 by means of frame 19 and mooring yoke 3 as shown in FIGS. 3 and 4 .

When connected as illustrated in FIG. 3 , an LNG fluid flow path is established from the LNG process vessel 1 via a crane turntable 7 , boom 4 , double pantograph pipe arrangement 6 , LNG pipe connector 5 and an LNG loading pipe 16 . An LNG vapor return pipe 15 from the LNG carrier 2 to the LNG carrier is also provided.