AU2014224155A1 - System and method for station keeping of a floating lng vessel within a station keeping envelope - Google Patents

Abstract

- 21 A system for offshore production of LNG from an FLNG vessel is described. The FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel 5 within a station keeping envelope at a production location. The system includes a computer processor for receiving a set of real-time monitored location data indicative of the longitude and latitude of the FLNG vessel, wherein the computer processer is programmed with a mathematical algorithm to: (i) compare the set of real-time monitored location data to a set of stored station keeping set points held in a data storage means; (ii) 10 generate a station control correction signal when the set of real-time monitored location data indicates that the FLNG vessel is not positioned at a desired stored station keeping set point; and, (iii) transmit the station control correction signal to the dynamic positioning control system.

Description

- 1 SYSTEM AND METHOD FOR STATION KEEPING OF A FLOATING LNG VESSEL WITHIN A STATION KEEPING ENVELOPE 5 FIELD OF THE INVENTION The present invention generally relate to a system for offshore production of LNG from an FLNG vessel, which system is connected to a natural gas receiving system with the FLNG vessel being located at a station keeping point. The present invention relates particularly to an FLNG vessel being operated in dynamic positioning mode for station control of the 10 FLNG vessel within a station keeping envelope at a production location. BACKGROUND TO THE INVENTION Liquefied natural gas is commonly referred to by the acronym 'LNG'. During recent years LNG has become an increasingly more sought-after energy resource. It is expected that 15 natural gas will to an ever greater degree replace oil as an energy source. It is known to cool natural gas down to about -1630C to produce LNG at dedicated onshore export terminals. It is also known to load LNG into purpose built LNG tankers to transport the LNG at approximately atmospheric pressure to dedicated receiving terminals 20 around the globe. It has been proposed for some time, that floating offshore structures, such as floating liquefaction vessels (referred to in the art as FLNG vessels'), could be used to liquefy natural gas although no such vessel has been put into production at this time. 25 It has been proposed that an FLNG vessel will be permanently moored to the seabed at a desired production location using a 'spread mooring system'. A spread mooring system relies on attaching heavy mooring lines or chains to the hull of the FLNG vessel and anchoring the chains to the seabed to ensure that weathervaning cannot occur. However, a spread mooring system is only an option in relatively benign locations where the 30 prevailing weather is known to be highly directional. Such locations are not common. Alternatively, it has been proposed that an FLNG vessel will be permanently moored to the seabed at a desired production location using a single point mooring system connecting it to the seafloor via a series of mooring lines (typically chains or wires). The -2 mooring lines extend below sea level to the ocean floor and can cost in the order of one hundred million US dollars. A single point mooring system is placed within or adjacent to the FLNG vessel. The single point mooring system is designed to receive a stream of hydrocarbons delivered to the single point mooring through one or more production risers 5 connected to wells on the sea floor. In addition to this, well risers, umbilicals and other subsea services necessary to the operation of the FLNG vessel and its associated feed gas architecture pass through the single point mooring system. In addition to performing this function, prior art single point mooring systems are designed and sized to moor the FLNG vessel at or near a preset longitude and latitude whilst allowing the FLNG vessel to 10 freely weathervane around the single point mooring. Such single point mooring turrets are designed and sized such that the FLNG vessel can remain moored and weathervane around the single point mooring system whilst withstanding the forces of up to a 10000 year storm so that FLNG vessel remains fixed to the single point mooring at all times during the producing life of the FLNG vessel. Consequently, the proposed FLNG vessel 15 are designed to have no means for self-propulsion with the result that it operates more like a barge than a ship. Using the single mooring systems currently proposed for use for FLNG vessels, the proposed FLNG vessel is held on a station keeping point by the suitably sized single point 20 mooring system and the orientation or 'heading' of the FLNG vessel is primarily dependent on the weather conditions, current direction, wind direction, and wave direction. Such single point mooring systems are extremely large, extremely complex and extremely expensive, costing in the order of 500 to 900 million US dollars. If there is a desire to hold the FLNG vessel at a heading that differs from the weathervaning heading, 25 the FLNG vessel must be fitted with thrusters that are located aft of the single point mooring system so as to cause the FLNG vessel to be rotated around the single point mooring system, either alone or in combination with a separate self-propelled vessel such as a tug boat that is used to apply a local pushing or pulling force to the hull of the FLNG vessel to provide heading control. 30 There remains a need for an alternative system for station keeping control of an FLNG vessel.

-3 SUMMARY OF THE INVENTION According to a second aspect of the present invention there is provided a system for offshore production of LNG from an FLNG vessel, which system is connected to a natural gas receiving system, the system characterised in that the FLNG vessel is operated in 5 dynamic positioning mode for station control of the FLNG vessel within a station keeping envelope at a production location. In one form, the system comprises: a floating LNG vessel having a hull and a deck; 10 a topsides hydrocarbon processing facility installed at or above the deck of the hull of the FLNG vessel; an FLNG vessel cargo containment system comprising one or more insulated FLNG vessel cryogenic storage tanks installed within the hull of the FLNG vessel; and, a dynamic positioning control system operatively associated with a system of 15 thrusters onboard the FLNG vessel wherein the dynamic positioning control system maintains the FLNG vessel at a desired station keeping point. In one form, the system includes a computer processor for receiving a set of real-time monitored location data indicative of the longitude and latitude of the FLNG vessel, 20 wherein the computer processer is programmed with a mathematical algorithm to: (i) compare the set of real-time monitored location data to a set of stored station keeping set points held in a data storage means; (ii) generate a station control correction signal when the set of real-time monitored location data indicates that the FLNG vessel is not positioned at a desired stored station 25 keeping set point; and, (iii) transmit the station control correction signal to the dynamic positioning control system. In one form, the set of real-time monitored location data is a set of global positioning data 30 sourced from an external data supplier. In one form, the system comprises a set of location sensors for generating part or all of the set of real-time monitored location data. In one form, the set of location sensors can include one or more of the following: a global positioning system; a position reference -4 sensor; a motion sensor; a compass; a sonar device, a laser range finder; an accelerometer, or combinations thereof. In one form, the desired station keeping point is a marine riser location. In one form, the 5 desired station keeping point is a hydrocarbon production turret location. In one form, the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with a hydrocarbon production turret. In one form, the FLNG vessel is operated in dynamic 10 positioning mode for station control of the FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with a seawater intake riser. In one form, the FLNG vessel is rotatably connected to the hydrocarbon production turret, wherein, during production operations, the hydrocarbon production turret is in fluid communication with a source of natural gas via a marine riser, the hydrocarbon production turret being tethered to the 15 seabed by a turret tethering system. In one form, the hydrocarbon production turret is positioned within or adjacent to the hull of the FLNG vessel. 20 In one form, the system includes a flexible tubular jumper member extending from a near surface buoyancy element to a hydrocarbon production turret, and, a fluid connection means for interconnecting the marine riser to the flexible tubular jumper member, the fluid connection means being located at or within the near-surface buoyancy element, the near-surface buoyancy element being secured to the seabed using a plurality of tethers 25 secured to a tether foundation. In one form, the marine riser is a vertical or near-vertical marine riser. In one form, the system comprises: a marine riser extending from the seabed towards the ocean surface for delivery of 30 a stream of wellhead gas; a near-surface buoyancy module connected to the upper end of the marine riser, the near-surface buoyancy module in fluid communication with a near-surface or at surface hydrocarbon production swivel; -5 a flexible tubular jumper member having a lower end in fluid communication with the hydrocarbon production swivel and an upper end in fluid communication with the topsides hydrocarbon processing facility onboard the FLNG vessel; and, wherein the dynamic positioning control system maintains the FLNG vessel at a 5 station keeping point during LNG production operations. In one form, the topsides hydrocarbon processing facility includes at least a liquefaction facility arranged to receive an inlet stream of dry sweet natural gas and generate an outlet stream of LNG. In one form, the dynamic positioning control system is located on the 10 FLNG vessel. According to a second aspect of the present invention there is provided a method for offshore production of LNG from an FLNG vessel using the system of any one of the preceding claims, which system is connected to a natural gas receiving system, the 15 system characterised in that the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel within a station keeping envelope at a production location. In one form of the method, the system includes a computer processor for receiving a set 20 of real-time monitored location data indicative of the longitude and latitude of the FLNG vessel, wherein the computer processer is programmed with a mathematical algorithm to: (i) compare the set of real-time monitored location data to a set of stored station keeping set points held in a data storage means; (ii) generate a station control correction signal when the set of real-time monitored 25 location data indicates that the FLNG vessel is not positioned at a desired stored station keeping set point; and, (iii) transmit the station control correction signal to the dynamic positioning control system. 30 The more important features of the invention have thus been outlined in order that the more detailed description that follows may be better understood and in order that the present contribution to the art may better be appreciated. Additional features of the invention will be described hereinafter and will form the subject matter of the claims that follow.

-6 BRIEF DESCRIPTION OF THE DRAWINGS In order to facilitate a more detailed understanding of the nature of the invention several embodiments of the present invention will now be described in detail, by way of example only, with reference to the accompanying drawings, in which: 5 Figure 1 is a schematic top view of one embodiment of the present invention showing an FLNG vessel with a turret within the hull and a topsides hydrocarbon processing facility including a liquefaction facility and a gas pre-treatment facility on or above the deck, showing an LNG tanker arranged side by side with the FLNG vessel for offloading a cargo of LNG; 10 Figure 2 is a schematic side view of the embodiment of Figure 1 with the LNG tanker omitted for clarity; Figure 3 is a schematic top view of one embodiment of the present invention showing an FLNG vessel with a turret outside of the hull and a topsides hydrocarbon processing facility including a liquefaction facility with an off-board gas pre-treatment 15 facility, the FLNG vessel including a dedicated propulsion system, showing an LNG tanker arranged bow to stern with the FLNG vessel for tandem offloading a cargo of LNG; and, Figure 4 is a schematic side view of the embodiment of Figure 3 with the LNG tanker omitted for clarity; Figure 5 is a schematic top view of one embodiment of the present invention 20 showing an FLNG vessel with a circular footprint with a topsides hydrocarbon processing facility on or above the deck including a liquefaction facility, an off-board gas pre treatment facility on a fixed structure; and a system of azimuthal thrusters arranged around the circumference of the hull of the FLNG vessel; and, Figure 6 is a schematic view of one embodiment of the system showing the 25 computer processor and storage means. It is to be noted that the drawings illustrate only preferred embodiments of the invention and are therefore not to be considered limiting of the invention's scope as it may admit to other equally effective embodiments. Like reference numerals refer to like parts. The 30 components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention. Moreover, all drawings are intended to convey concepts, where relative sizes, shapes and other detailed attributes may be illustrated schematically rather than literally or precisely.

-7 DETAILED DESCRIPTION OF PARTICULAR EMBODIMENTS Particular embodiments of the present invention are now described. The terminology used herein is for the purpose of describing particular embodiments only, and is not intended to limit the scope of the present invention. Unless defined otherwise, all technical and 5 scientific terms used herein have the same meanings as commonly understood by one of ordinary skill in the art to which this invention belongs. The term 'natural gas' refers to a gas that is primarily methane gas with small amounts of ethane, propane, butane, and a percentage of heavier components. The acronym 'LNG' 10 is used throughout this specification and the claims to refer to liquefied natural gas. The acronym 'LPG' is used throughout this specification and the claims to refer to liquefied petroleum gas. The acronym FLNG' is used throughout this specification and the claims to refer to 'floating liquefied natural gas'. Thus the term FLNG vessel' means a 15 floating liquefied natural gas vessel which receives a source of natural gas and produces LNG onboard the vessel. The term 'LNG tanker' is used to refer to a vessel that receives a cargo of LNG and transports that cargo of LNG to a location that is remote from the location where the cargo was received. The acronym 'DP' is used throughout this specification and the claims to refer to dynamic positioning. 20 The term 'environmental conditions' is used to refer to the combined effect of the magnitude of weather conditions including wind direction, wind velocity, wave direction, and, wave height, and also includes other metocean conditions such as current direction and current velocity, air temperature, air pressure and the like. 25 The term 'single point mooring system' is used to refer to a system that serves two primary functions. The first primary function of the single point mooring system is that of mooring a vessel at or near a desired station keeping point whilst allowing the vessel to weathervane around it. The second primary function is that of receiving a stream of 30 hydrocarbons delivered to the single point mooring system through one or more production risers connected to wells on the sea floor. In addition to this, well risers, umbilicals and other subsea services necessary to the operation of the FLNG vessel and its associated feed gas architecture pass through the single point mooring system.

-8 The term 'hydrocarbon production turret' is used throughout this specification and the claims to refer to a device that serves the single primary function of receiving a stream of hydrocarbons delivered to the turret through one or more production risers connected to wells on the sea floor. In addition to this, well risers, umbilicals and other subsea services 5 necessary to the operation of the FLNG vessel and its associated feed gas architecture pass through the turret. The turret includes a swivel to accommodate changes in the heading of the FLNG vessel. In contrast to a single point mooring system, a hydrocarbon production turret (as defined in this specification and the claims) is not designed and sized to serve the primary function of mooring a vessel at or near a desired station keeping 10 point. As such, a hydrocarbon production turret may assist in positioning the vessel at or near a desired station keeping point but this is not one of its primary functions. Before describing the system of the present invention in detail, embodiments of an FLNG vessel suitable to be included in the system (10) and method of the present invention are 15 first described with reference to Figures 1 to 4. The FLNG vessel (12) has a hull (14) and a deck (16). In order to facilitate offshore production of LNG by the FLNG vessel, the FLNG vessel has a topsides hydrocarbon processing facility (18) installed on or above the deck of the hull of the FLNG vessel and an FLNG vessel cargo containment system (20) comprising a plurality of insulated FLNG vessel cryogenic storage tanks (22) installed 20 within the hull. The topsides hydrocarbon processing facility consists of a plurality of interconnected systems which allow the FLNG vessel to produce sales-quality LNG (optionally LPG and condensate) in a standalone fashion in relative close proximity to a hydrocarbon reservoir. The topsides hydrocarbon processing facility is designed and sized so that the FLNG vessel has an anticipated production capacity in the range of 0.5 25 and 7 million tonnes of LNG per annum, preferably in the range of 1 to 4 million tonnes of LNG per annum. The topsides hydrocarbon processing facility includes at least a liquefaction facility (24) arranged to receive an inlet stream (26) of dry sweet natural gas and generate an outlet stream of LNG (28). The liquefaction facility includes one or more cryogenic heat exchangers (30) arranged in series or parallel. Each cryogenic heat 30 exchanger is a spiral wound heat exchanger or a braised aluminium heat exchanger. The liquefaction facility may include a spiral wound heat exchanger being used in parallel or in series with a braised aluminium heat exchanger. The liquefaction facilities operate using a cycle selected from the list comprising: a nitrogen cycle; a single mixed refrigerant cycle; a dual mixed refrigerant cycle; a cascade refrigerant cycle; a hybrid liquefaction cycle, a -9 carbon dioxide and nitrogen liquefaction cycle, or another natural gas liquefaction cycle. Such liquefaction cycles are well known in the LNG production arts and need not be described here as the selection of liquefaction cycle does not form part of the present invention. 5 Referring to Figures 1 and 2, the topsides hydrocarbon processing facility (18) includes a gas pre-treatment facility (32). The gas pre-treatment facility includes an acid gas removal facility (34) for receiving a stream of sour natural gas (36) and producing a stream of sweet natural gas (38) and a dehydration facility (40) for receiving a stream of wet natural 10 gas (42) and producing a stream of dry natural gas (44). The topsides hydrocarbon processing facility may further include a pre-cooling facility (46) wherein the inlet stream of dry sweet natural gas (26) fed to the liquefaction facility (24) is a stream of pre-cooled dry sweet gas (48) produced by the pre-cooling facility. Additionally, the gas pre-treatment facility may include a wellhead gas separator (50) for removing liquids and solids from an 15 inlet stream of hydrocarbon reservoir fluids (52) to produce a stream of wet sour natural gas (54). The gas pre-treatment facility may further include a condensate removal facility (56) for removing a stream of condensate (58) comprising pentane, propane, and butane which can be further processed to produce LPG or stored for sale as condensate. The gas pre-treatment facility includes a mercury removal facility (60) for removal of mercury 20 upstream of the liquefaction facility. Various kinds of suitable gas pre-treatment facilities are well known in the art and are not described in detail here as the type and kind of gas pre-treatment facilities do not form part of the present invention. In the embodiment illustrated in Figure 1 and 2, the topsides hydrocarbon processing 25 facility (18) includes the gas pre-treatment facility (32) and the liquefaction facility (24) onboard the FLNG vessel. In alternative embodiments illustrated in Figures 3, 4 and 5, the liquefaction facility is located onboard the FLNG vessel as part of the topsides hydrocarbon processing facility, while the gas pre-treatment facility is an off-board gas pre-treatment facility (62). In the embodiment illustrated in Figure 3, the off-board gas 30 pre-treatment facility is arranged on a floating structure (68). The floating structure can be a floating gas pre-treatment vessel, a semi-submersible platform, a tender-assisted self erecting structure, a tension-leg platform, a normally unmanned platform, a satellite platform, or a spar. If desired, the floating structure (68) can be provided with a second dynamic positioning control system that communicates with the dynamic positioning - 10 control system of the FLNG vessel (described in detail below) to assist in maintaining safe separation distance at all times during operations. In the embodiment illustrated in Figure 5, the off-board gas pre-treatment facility is arranged on a fixed structure (64) at a gas production location (66) outside of the station keeping envelope of the FLNG vessel. The 5 fixed structure can be a fixed platform, a tension-leg platform, a fixed jacket structure or a gravity based structure, depending on such relevant factors as the contours and depth of the sea bed at the gas production location. The outlet stream of LNG (28) of the liquefaction facility (24) of the FLNG vessel (12) may 10 be directed to flow into the FLNG vessel cargo containment system (20). Alternatively, if an LNG tanker (70) having an LNG tanker cargo containment system (74) comprising a plurality of LNG tanker cryogenic storage tanks (74) is available to receive a cargo of LNG, the outlet stream of LNG of the liquefaction facility of the FLNG vessel may be directed to flow into the LNG tanker cargo containment system. Each of the FLNG vessel 15 insulated cryogenic storage tanks (22) can be a membrane storage tank maintained at ambient pressure or a prismatic type containment system or a Moss-style tank, or combination thereof. The insulation on the LNG storage tanks allows some of the LNG to warm over time and return to its gaseous form (a process referred to in the art as "boil off"). The storage tanks are operated in such a manner that removal of the boil off gas 20 allows the remaining LNG to be maintained at a constant cold temperature, typically 1630C in its liquid form. The plurality of FLNG vessel cryogenic storage tanks can each be interconnected, but are preferably independent of each other. The FLNG vessel cargo containment system has a storage capacity in the range of 90,000m 3 - 300,000m 3 , depending on a number of relevant factors including the production capacity of the 25 topsides LNG production facilities. A plurality of additional systems (generally designated by reference numeral 76) may also be built into and/or onto the FLNG vessel hull. The plurality of additional systems may include: the electrical utility systems, the cargo containment systems and associated 30 pumps; fans or other equipment associated with the topsides hydrocarbon processing facility; the lighting systems; the accommodation unit; the communications systems; the air supply systems; the water systems; and, the waste treatment systems, and cranes or lifting systems. In order to accommodate the topsides hydrocarbon processing facility and the plurality of additional systems, the FLNG vessel may be a steel single-hulled or -11 double-hulled vessel having a length in the range of 200 to 600 meters and a width (or "beam") in the range of 40 to 90 meters. By comparison, a prior art LNG tanker in operation at this time has a maximum hull length or around 350 meters and a maximum width of 55 meters. Depending on the level of complexity of the topsides hydrocarbon 5 processing facility and the anticipated production capacity of the FLNG vessel, it is likely that the FLNG vessel will be larger or much larger in size than a prior art LNG tanker that is used to receive and transport LNG cargoes. Various embodiments of a system for offshore production of LNG from an FLNG vessel, 10 which system is connected to a source of natural gas, are now described in detail. The system characterised in that the FLNG vessel is located at a station keeping point (100) and the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel within a station keeping envelope at a production location. The system includes a dynamic positioning system operatively associated with a system of thrusters 15 onboard the FLNG vessel wherein the dynamic positioning system maintains the FLNG vessel at the desired station keeping point (100). The DP control system may be located on the FLNG vessel itself or operated from a remote DP operation location (106). When the hull of the FLNG vessel has a rectangular or 'ship-shaped' footprint (as 20 illustrated in the embodiments shown in Figures 1 to 4), the FLNG vessel has a bow (108) and stern (110), and, the system of thrusters (104) can include one or more bow thrusters (112) and one or more stern thrusters (114). The system of thrusters can include one or more tunnel or pod thrusters (116). Each tunnel or pod thruster has an adjustable thruster output. Alternatively or additionally, the system of thrusters can include one or more 25 azimuthal thrusters (118), each azimuthal thruster having an adjustable thruster output and an adjustable thruster angle. The system of thrusters can comprise one or more tunnel or pod thrusters and one or more azimuthal thrusters. In the embodiment illustrated in Figure 1, the hull (14) of the FLNG vessel (12) has a 30 rectangular footprint and is provided with a tunnel thruster (116) at the bow (108) and three azimuthal thrusters (118) at the stern (110). In the embodiment illustrated in Figure 3, the hull of the FLNG vessel has a rectangular footprint and the system of thrusters includes a tunnel thruster (116) and an azimuthal thruster (118) at the stern (110) and two azimuthal thrusters (118) at the bow (108). Using these embodiments, the DP control - 12 system of the present invention achieves heading control of the FLNG vessel by adjusting one or both of (i) the output and the angle of at least one of the plurality of azimuthal thrusters; and (ii) the output of the tunnel thruster. Pod thrusters could equally be used in the place of the tunnel thrusters in this embodiment. 5 In the embodiment illustrated in Figure 5, the system of thrusters comprises a plurality of azimuthal thrusters. Referring to Figure 5, the hull (14) of the FLNG vessel (12) has a circular footprint with six azimuthal thrusters arranged around the circumference of the hull, by way of illustration only. It is to be understood that that the number of azimuthal 10 thrusters can vary. Using this system of thrusters, the DP control system of the present invention achieves station keeping control of the FLNG vessel by adjusting one or both of the output and the angle of at least one of the plurality of azimuthal thrusters. Referring to Figure 6, the system (10) comprises a computer processor (120) for receiving 15 a set of real-time monitored location data (220) indicative of the longitude and latitude of the FLNG vessel, wherein the computer processer is programmed with a mathematical algorithm to: (i) compare the set of real-time monitored location data to a set of stored station keeping set points (222) held in a data storage means (224); 20 (ii) generate a station control correction signal (226) when the set of real-time monitored location data indicates that the FLNG vessel is not positioned at a desired stored station keeping set point; and, (iii) transmit the station control correction signal to the dynamic positioning system (102) to optimize the station keeping of the FLNG vessel in response to the real-time 25 monitored environmental data. The computer processor can be monitored directly onboard the FLNG vessel. Alternatively or additionally, the computer processor can have source or executable instructions to communicate with a network (130) to form an executive dashboard (132) 30 enabling a remote user (134) to view the set of real-time monitored environmental data 24 hours a day, 7 days as week. The real-time monitored environmental data can be stored to provide a measure of the cumulative load hours experienced by the FLNG vessel during over the operating life of the FLNG vessel. The cumulative load hours can be used a guideline to inform a maintenance schedule for the FLNG vessel.

-13 In one embodiment, the set of real-time monitored location data is a set of global positioning data (228) sourced from an external data supplier. Alternatively or additionally, the system (10) includes a set of location sensors (230) for generating part or all of the set of real-time monitored location data. The set of location sensors can include one or more 5 of the following: a global positioning system; a position reference sensor; a motion sensor; a compass; a sonar device, a laser range finder; an accelerometer, or combinations thereof. Referring to the embodiments illustrated in Figures 2 and 4, the FLNG vessel is rotatably 10 connected to a hydrocarbon production turret (232), wherein, during production operations, the hydrocarbon production turret is in fluid communication with a source of natural gas via a marine riser (234), the hydrocarbon production turret being tethered to the seabed (236) by a turret tethering system (238). In the embodiment illustrated in Figure 2, the hydrocarbon production turret is positioned within the hull of the FLNG 15 vessel and the marine riser is a flexible or steel catenary riser. The desired station keeping point in this embodiment is a marine riser location (240) or a hydrocarbon production turret location (242). In the embodiment illustrated in Figure 4, the hydrocarbon production turret (232) is 20 positioned adjacent to the FLNG vessel (12). The system includes a flexible tubular jumper member (244) extending from a near-surface buoyancy element (246) to the hydrocarbon production turret and a fluid connection means (248) for interconnecting the marine riser (234) to the flexible tubular jumper member, the fluid connection means being located at or within the near-surface buoyancy element. The term 'near-surface buoyancy 25 element' refers to a buoyancy element that floats near the surface of the ocean but beneath the surface of the ocean. In either case, the buoyancy module locates the upper end of the marine riser below a zone affected by wind and waves. By way of example, the buoyancy module may be positioned between 30 and 100m meters below the surface of the water so as to keep the marine riser vertical whilst maintaining tension on the marine 30 riser at all times. In this way, overall motion of the marine riser is reduced, leading to decreased wear and metal fatigue. The marine riser is a vertical or near-vertical marine riser and the marine riser is vertically tensioned by the near-surface buoyancy module and the vertical tension is created by near-surface buoyancy module is reacted by the riser foundation that can be either a driven pile, suction pile or a gravity base structure. In this - 14 embodiment, the near-surface buoyancy element is secured to the seabed (236) using an optional plurality of tethers (250) secured to a tether foundation (252) on the seabed. The tether foundation (252) may be one of a plurality of tether foundations. The marine riser is secured to a riser foundation (254) that is separated from the tether foundation. 5 For each of the embodiments described above, the FLNG vessel can be operated in dynamic positioning mode for station control of the FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with the hydrocarbon production turret. The FLNG vessel can analogously be operated in dynamic positioning mode for station control of the 10 FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with a seawater intake riser (260). Because the DP control system can assist with both station keeping and heading control, the system of the present invention allows for a significant reduction in the forces acting 15 on the hydrocarbon production turret in the embodiments illustrated in Figures 2 and 4, thereby enabling a smaller and/or simpler turret to be deployed instead of the larger and more complex single point mooring systems of the prior art. Additionally, as the hydrocarbon production turret is not required to be sized to accommodate the loads associated with a plurality of prior art mooring chains designed for the station keeping 20 needs of a prior art FLNG vessel, the turret in this embodiment is far smaller and far less expensive than prior art turrets. The DP control system facilitates a cheaper and more rapid deployment (or re-deployment) of the FLNG vessel from a first location to a second location because the FLNG vessel can be easily connected, disconnected or re connected to the smaller and less complex hydrocarbon production turret capable of being 25 used with the system of the present invention. Advantageously, an independent offshore installation vessel is not required to connect, disconnect or re-connect the FLNG vessel to the turret, providing additional savings in cost and time. In addition to station keeping, the DP control system also facilitates cyclone evasion by minimising the risk and time taken to disconnect the FLNG vessel from the larger and more complex mechanical turrets of the 30 prior art. The system of thrusters (104) is sufficient to achieve station keeping for the FLNG vessel (12) at the station keeping point (100) and in this way, the system of thrusters operate in combination with the DP control system to serve the function of a propulsion system for - 15 moving the FLNG vessel from a first location to a second location within a station keeping envelope (162). The system may optionally include a dedicated FLNG vessel propulsion system (164) in the form of a main propulsion engine (166) and a propeller (168). The main propulsion engine can be any ship propulsion system known in the art, such as a 5 dual fuel gas turbine system, a dual fuel diesel motor system, a dual fuel diesel-electric system, a steam turbine system, a direct drive diesel motor system, and, a diesel-engine powered electric motor system. The propeller can be a variable pitch propeller or screw fixed propellers. In the embodiment illustrated in Figure 3 and Figure 4, the FLNG vessel includes the dedicated propulsion system for moving the FLNG vessel from a first location 10 to a second location within the station keeping envelope. In the embodiment illustrated in Figure 1 and Figure 2, the FLNG vessel does not include a dedicated propulsion system, relying instead on a system of thrusters to operated under control of the DP control system to move the FLNG vessel from the first location to the second location within the station keeping envelope. 15 The DP control system can be used to help in guiding the FLNG vessel as it moves from a first location to a more benign location in anticipation of a major storm, such as a gale, a cyclone, or a hurricane. Maintenance of the FLNG vessel may be conducted whilst it is being maintained within a station keeping envelope. However, when large maintenance 20 operations require that the FLNG vessel is disconnected and moved to a maintenance location, the FLNG vessel can use its system of thrusters alone or in combination with an optional onboard propulsion system or in combination with a towing vessel with additional guidance from the DP control system to achieve this. 25 The system (10) includes a power generation and distribution system (170) for sharing power between the dynamic positioning control system (102) and the topsides hydrocarbon processing facility (18). The power generation and distribution system can also provide power to a plurality of additional systems (76). When the FLNG vessel is provided with a dedicated propulsion system (164), the power generation and distribution 30 system is configured share power between the dynamic positioning control system, the topsides hydrocarbon processing facility, and the dedicated propulsion system. The power requirements for the dynamic positioning control system (102) are characterised by long periods of low power consumption (less than about 10 MW) and -16 short periods of very high power consumption (in the range of about 20 MW to 50 MW) during relatively brief extreme sea and atmospheric condition. In the embodiment illustrated in Figure 5, the power generation and distribution system (170) is configured to charge a battery bank (172) for the dynamic positioning control system (102) when one or 5 both of the dedicated propulsion system (164) and the topsides hydrocarbon processing facility (18) is experiencing an off-peak load condition. As power needs increase for the dynamic positioning control system in severe sea and atmospheric conditions, the power distribution system redistributes load by reducing loads from less critical equipment such as that used in the topsides hydrocarbon processing facility. 10 Now that several embodiments of the invention have been described in detail, it will be apparent to persons skilled in the relevant art that numerous variations and modifications have been enabled by the foregoing disclosure. By way of example, the FLNG vessel may be operated in dynamic positioning mode for station keeping in addition to heading 15 control. By way of further example, the DP control system may be located on the FLNG vessel itself or operated from a remote DP operation location. All such modifications and variations are considered to be within the scope of the present invention, the nature of which is to be determined from the foregoing description and the appended claims. 20 It will be clearly understood that, although a number of prior art publications are referred to herein, this reference does not constitute an admission that any of these documents forms part of the common general knowledge in the art, in Australia or in any other country. In the summary of the invention, the description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word 25 "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the invention.

Claims (18)

1. A system for offshore production of LNG from an FLNG vessel, which system is connected to a natural gas receiving system, the system characterised in that the FLNG 5 vessel is operated in dynamic positioning mode for station control of the FLNG vessel within a station keeping envelope at a production location.

2. The system of claim 1 comprising: a floating LNG vessel having a hull and a deck; 10 a topsides hydrocarbon processing facility installed at or above the deck of the hull of the FLNG vessel; an FLNG vessel cargo containment system comprising one or more insulated FLNG vessel cryogenic storage tanks installed within the hull of the FLNG vessel; and, a dynamic positioning control system operatively associated with a system of 15 thrusters onboard the FLNG vessel wherein the dynamic positioning control system maintains the FLNG vessel at a desired station keeping point.

3. The system of claim 1 or 2 wherein the system includes a computer processor for receiving a set of real-time monitored location data indicative of the longitude and latitude 20 of the FLNG vessel, wherein the computer processer is programmed with a mathematical algorithm to: (i) compare the set of real-time monitored location data to a set of stored station keeping set points held in a data storage means; (ii) generate a station control correction signal when the set of real-time monitored 25 location data indicates that the FLNG vessel is not positioned at a desired stored station keeping set point; and, (iii) transmit the station control correction signal to the dynamic positioning control system. 30

4. The system of claim 3 wherein the set of real-time monitored location data is a set of global positioning data sourced from an external data supplier.

5. The system of claim 3 or 4 wherein the system comprises a set of location sensors for generating part or all of the set of real-time monitored location data. - 18

6. The system of claim 5 wherein the set of location sensors can include one or more of the following: a global positioning system; a position reference sensor; a motion sensor; a compass; a sonar device, a laser range finder; an accelerometer, or combinations thereof. 5

7. The system of any one of claims 1 to 6 wherein the desired station keeping point is a marine riser location.

8. The system of any one of claims 1 to 6 wherein the desired station keeping point is 10 a hydrocarbon production turret location.

9. The system of any one of claims 1 to 8 wherein the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with a hydrocarbon production turret. 15

10. The system of any one of claims 1 to 8 wherein the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel whilst the FLNG vessel connects, disconnects or reconnects with a seawater intake riser. 20

11. The system of any one of claims 1 to 10 wherein the FLNG vessel is rotatably connected to the hydrocarbon production turret, wherein, during production operations, the hydrocarbon production turret is in fluid communication with a source of natural gas via a marine riser, the hydrocarbon production turret being tethered to the seabed by a turret tethering system. 25

12. The system of claim 11 wherein the hydrocarbon production turret is positioned within or adjacent to the hull of the FLNG vessel.

13. The system of any one of claims 1 to 10 wherein the system includes a flexible 30 tubular jumper member extending from a near-surface buoyancy element to a hydrocarbon production turret, and, a fluid connection means for interconnecting the marine riser to the flexible tubular jumper member, the fluid connection means being located at or within the near-surface buoyancy element, the near-surface buoyancy -19 element being secured to the seabed using a plurality of tethers secured to a tether foundation.

14. The system of any one of claims 11 to 13 wherein the marine riser is a vertical or 5 near-vertical marine riser.

15. The system of any one of claims 1 to 10 comprising: a marine riser extending from the seabed towards the ocean surface for delivery of a stream of wellhead gas; 10 a near-surface buoyancy module connected to the upper end of the marine riser, the near-surface buoyancy module in fluid communication with a near-surface or at surface hydrocarbon production swivel; a flexible tubular jumper member having a lower end in fluid communication with the hydrocarbon production swivel and an upper end in fluid communication with the 15 topsides hydrocarbon processing facility onboard the FLNG vessel; and, wherein the dynamic positioning control system maintains the FLNG vessel at a station keeping point during LNG production operations. 15. The system of any one of claims 2 to 14 wherein the topsides hydrocarbon 20 processing facility includes at least a liquefaction facility arranged to receive an inlet stream of dry sweet natural gas and generate an outlet stream of LNG.

16. The system of any one of the preceding claims wherein the dynamic positioning control system is located on the FLNG vessel. 25

17. A method for offshore production of LNG from an FLNG vessel using the system of any one of the preceding claims, which system is connected to a natural gas receiving system, the system characterised in that the FLNG vessel is operated in dynamic positioning mode for station control of the FLNG vessel within a station keeping envelope 30 at a production location. - 20

18. The method of claim 17 wherein the system includes a computer processor for receiving a set of real-time monitored location data indicative of the longitude and latitude of the FLNG vessel, wherein the computer processer is programmed with a mathematical algorithm to: 5 (i) compare the set of real-time monitored location data to a set of stored station keeping set points held in a data storage means; (ii) generate a station control correction signal when the set of real-time monitored location data indicates that the FLNG vessel is not positioned at a desired stored station keeping set point; and, 10 (iii) transmit the station control correction signal to the dynamic positioning control system.