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FIELD OF THE INVENTION 
     The present invention relates to a system for the development of an offshore hydrocarbon field comprising
         at least one floating hydrocarbon production unit that is moored to the seabed and connected to a hydrocarbon reservoir via risers and a wellhead on the seabed, the unit receiving and processing the hydrocarbon mixture received from the reservoir via the riser, the floating hydrocarbon processing unit being provided with process equipment to separate from the received hydrocarbon mixture the oil, water and gas and provided with storage tank for storage of the separated oil,   a gas export riser connected to the floating hydrocarbon processing unit to export the separated gas from the hydrocarbon processing unit, and   a floating gas processing unit for processing and liquefying the gas received so that it can be transported to shore.       

     The present invention also relates to a method of loading, treating and distributing a hydrocarbon mixture collected from a hydrocarbon reservoir, comprising the steps of:
         receiving via a riser an hydrocarbon mixture on at least one floating production unit that is moored to the seabed,   processing the oil and the gas on the floating production unit provided with process equipment to separate from the received hydrocarbon mixture the oil, water and gas,   storing the separated oil in the storage tanks provided on the floating production unit,   exporting the separated gas from the floating production unit,   liquefying the separated gas on floating gas processing unit, and   transporting the liquefied gas to shore,       

     BACKGROUND OF THE INVENTION 
     In offshore production of hydrocarbons (oil and/or gas) it is known to supply natural gas from an underground source to a field installation for gas treatment, the gas after possible purification being transferred in compressed form from the field installed on to a LNG tanker, where at least a part of the gas is liquefied before being transferred to storage tanks on board the tanker. For a further description of this system reference may e.g. be made to U.S. Pat. No. 6,003,603 
     U.S. Pat. No. 6,434,948 describes a system for production, storage and export of liquid natural gas (LNG) from an Floating Production Storage and Offloading (FPSO) vessel with an LNG liquefaction plant to an LNG tank. The LNG tank is provided with a buffer storage capacity for temporary storage of the continuously produced LNG during the absence of an LNG tank vessel. 
     In these known systems, nothing is mentioned on how to deal with the remaining gas produced after oil processing. 
     The present solution permits to eliminate expensive and non environmentally friendly solutions such as gas re-injection or flaring to get rid of the associated gas produced on the field development. The present invention also proposes a solution which would enable an offshore hydrocarbon production unit and an offshore liquefaction unit to work in conjunction, with no interdependence between their activities which increases the system uptime and reliability. 
     SUMMARY OF THE INVENTION 
     The invention provides a system for the development of an offshore hydrocarbon field comprising:
         at least one floating hydrocarbon processing unit that is moored to the seabed and connected to a hydrocarbon reservoir via risers and a wellhead on the seabed, the unit receiving and processing the hydrocarbon mixture received from the reservoir via the riser, the floating hydrocarbon processing unit being provided with process equipment to separate from the received hydrocarbon mixture the oil, water and gas and provided with storage tanks for storage of the separated oil,   a gas storage buffer reservoir placed at a certain distance from and isolated from the hydrocarbon reservoir,   a gas export riser connected to the floating hydrocarbon processing unit and the gas buffer reservoir to export the separated gas from the hydrocarbon processing unit and inject the gas into the gas buffer reservoir for temporary storage of the produced gas, and   a gas riser connected with one end to the gas buffer reservoir and with another end connected to a floating gas processing unit that processes and liquefies the gas received from the gas buffer reservoir so that the gas can be transported to shore as LNG.       

     The invention also provides a method of loading, treating and distributing an hydrocarbon mixture collected from an hydrocarbon reservoir, comprising the steps of:
         receiving via a riser an hydrocarbon mixture on at least one floating production unit that is moored to the seabed,   processing the oil and the gas on the floating production unit provided with process equipment to separate from the received hydrocarbon mixture the oil, water and gas,   storing the separated oil in the storage tanks provided on the floating production unit,   exporting the separated gas from the floating production unit to at least one storage buffer reservoir placed at a certain distance from and isolated from the hydrocarbon reservoir,   extracting the gas via a gas riser from the gas buffer reservoir to a floating gas processing unit,   processing and liquefying the gas received from the gas buffer reservoir, and   transporting the liquefied gas to shore.       

     The use of a storage buffer reservoir placed at a certain distance from and isolated from the hydrocarbon reservoir is proposed according to the present invention as it provides a temporary storage for gas in case of production interruption on either unit. In a case where the floating hydrocarbon production unit is installed ahead of the floating gas processing unit, the separated gas can be re-injected into the gas storage buffer reservoir as a temporary outlet until the floating gas processing unit is in operation. The buffer gas reservoir has the advantage to control the gas flow between a floating hydrocarbon production unit and a floating gas processing unit working in conjunction in an associated gas field development. 
     By means of the method according to the invention there is also a possibility to unlock additional gas reserves that were previously considered stranded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be further described below in connection with exemplary embodiments with reference to the accompanying drawings, wherein: 
         FIG. 1  shows a view of a system for the development of an offshore hydrocarbon field according to the present invention, 
         FIG. 2  shows a schematic view of a similar system with two floating hydrocarbon production units, 
         FIG. 3  shows another schematic view of a similar system as the one shown in  FIG. 2 , the floating gas processing unit being moored to a disconnectable buoy, 
         FIG. 4  shows a schematic view of a system according to the present invention where different connection points for the floating gas processing unit are shown, and 
         FIG. 5  shows a schematic view of a system according to the present invention where the system comprises two gas buffer reservoirs and where the floating gas processing unit is dynamically positioned and provided with a particular offshore loading system. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 1  shows a view of a system for the development of an offshore hydrocarbon field according to the present invention. The system shown in  FIG. 1  comprises a floating production unit  1  that is moored to the seabed (not shown) and connected via risers  2  to several wellheads  3  on the seabed, the unit  1  receiving and processing the hydrocarbon mixture received from the reservoir via the risers  2 , (see arrows  4 ). 
     The floating hydrocarbon processing unit  1  is provided with process equipment to separate from the received hydrocarbon mixture the oil, water and gas and is also provided with storage tanks for storage of the separate oil. Once the oil, water and gas are separated on the floating production unit  1 , one gas export riser  5  connected to the floating unit  1  exports the separated gas (see arrows  6 ) from the hydrocarbon processing unit and the gas is then injected into a gas buffer reservoir  7  for temporary storage of the produced gas. As clearly shown in  FIG. 1  the gas storage buffer reservoir  7  is placed at a certain distance from and isolated from the hydrocarbon reservoirs  3 . 
     The gas storage buffer reservoir  7  is also connected to a gas riser  8  that transports the gas extracted from the gas buffer reservoir  7  to a floating gas processing unit  9  (see arrow  10 ). The floating gas processing unit is provided with a liquefaction plant so that the gas received from the gas buffer reservoir  7  can be processed to be transported to shore as liquefied gas. 
       FIG. 2  shows a schematic view of a similar system with two floating production units  1 . Each unit  1  is anchored to the seabed via a mooring system comprising mooring lines  11  connected to chain sections  22  at the seabed  21 . Buoyancy elements  23  are attached to the mooring lines  11  to facilitate the mooring. This mooring system is ideal in deep water environments, however according to the invention this is just one embodiment to possible mooring systems for the floating production units  1 , any type of mooring systems, permanent mooring system, disconnectable mooring system could also be convenient. A number of risers  2  extend between the seabed  21  and a turret  20  locate in the unit hull, the risers  2  at the bottom being connected to respective well heads  3 , installed on two different production wells. In this case the risers comprise two risers  2  that bring an hydrocarbon mixture to the turret on the floating production unit  1 . The floating production units  1  are provided with process equipment to separate, from the received hydrocarbon mixture, the oil, water and gas and are also provided with storage tanks  12  for storage of the separate oil. Oil shuttle tankers  16  are used to transport the produced oil from the floating production units  1 . Between the floating production unit  1  and the oil shutter tanker  16 , a transferring means  18  is arranged to transport the oil to tanks  25  on board the oil tanker  16 . The oil shuttle tanker  16  is moored to the floating production unit  1  by means of a mooring line  26 . 
     As already shown in  FIG. 1 , a gas export riser  5  connected to the floating unit  1  exports the separated gas (see arrows  6 ) from the hydrocarbon processing unit  1  and the gas is then injected into a gas buffer reservoir  7  for temporary storage of the produced gas. Each gas export riser  5  from each floating hydrocarbon processing unit  1  is connected to the other before reaching the gas buffer reservoir  7 . The gas buffer reservoir  7  can be a salt cavern, a depleted reservoir or any other type of underground reservoir placed at a certain distance from and isolated from the hydrocarbon reservoir. 
     In  FIG. 2  one a gas riser  8  is shown, connected, on one side, to the gas buffer reservoir  7  and to the other side to a floating gas processing unit  9 . In this case the gas processing unit  9  is a LNG carrier provided with a gas liquefaction plant  13 , with some storage tanks  14 . The floating gas processing unit is hence able to process, liquefy and store the liquefied gas. In the embodiment shown on  FIG. 2 , the floating gas processing unit  9  is moored to a LNG carrier  15  by means of a mooring line  27 . Between the floating gas processing unit  9  and the LNG carrier  15 , a liquefied gas transferring means  28  is arranged to transport the liquefied gas to tanks  29  on board the LNG carrier  15 . The LNG carrier  15  is then used to transport the liquefied gas from the gas processing unit  9  to shore. 
       FIG. 3  shows another schematic view of a similar system as the one shown in  FIG. 2 , the floating gas processing unit  9  being moored to a disconnectable buoy  30 . As shown in  FIG. 2  there are two floating hydrocarbon processing units  1 , each moored to oil shutter tankers  16 . In this case, the gas buffer reservoir  7  is connected to two gas export risers  5 , each having an independent connection to the gas buffer reservoir  7  and each connected to one floating hydrocarbon processing unit  1 . In this case as well the gas processing unit  9  is a LNG carrier provided with a gas liquefaction plant  13 , with some storage tanks  14 . The floating gas processing unit  9  is hence able to process and store the liquefied gas. In  FIG. 3  one a gas riser  8  is shown, connected, on one side, to the gas buffer reservoir  7  and to the other side to a buoy loading system comprising a disconnectable submerged buoy received in a submerged downwardly open receiving space at the bottom of the floating gas processing unit  9 , so that the floating gas processing unit  9  is able to turn on the anchored central buoy member under the influence of wind, waves and water currents. The buoy  30  could be of a Submerged Turret Production type of buoy, a Submerged Turret Loading type of buoy which are well known. 
     By means of the STL/STP technique there is achieved that one is able to carry out loading/unloading as well as offshore production of liquefied gas in nearly all weathers, a connection as well as a disconnection between vessel  9  and buoy  30  being able to be carried out in a simple and quick manner, also under very difficult weather conditions with high waves. Further, the buoy  30  may remain connected to the vessel  9  in all weathers, a quick disconnection being able to be carried out if a weather limitation should be exceeded. 
     The LNG carrier  15  is weathervaning moored and can be disconnected and reconnected to the mooring system and the gas riser  8 , so to store, transport and offload the produced liquefied gas to shore. 
       FIG. 4  shows a schematic view of a system according to the present invention where different connection points for the floating gas processing unit  9  are shown. In this embodiment, as shown in  FIG. 3 , there are two floating hydrocarbon processing units  1 , each moored to oil shutter tankers  16 . In this case, the gas buffer reservoir  7  is connected to two gas export risers  5 , each connected to one floating hydrocarbon processing unit  1 . In this case as well the gas processing unit  9  is also a LNG carrier provided with a gas liquefaction plant  13 , with some storage tanks  14 . The floating gas processing unit  9  is hence able to process and store the liquefied gas In this embodiment there are two gas risers  8  shown, connected, on one side, to the gas buffer reservoir  7  and to the other side to a buoy loading system comprising a disconnectable submerged buoy  30 ,  31 . The buoy  30  as shown in  FIG. 3  is received in a submerged downwardly open receiving space at the bottom of the floating gas processing unit  9 , so that the floating gas processing unit  9  is able to turn on the anchored central buoy member under the influence of wind, waves and water currents. Another floating gas processing unit could connect to the other submerged buoy  31 , to process and store gas drawn from the gas buffer reservoir  7  via the other gas riser  8 ′. 
     In  FIGS. 2 to 4  the floating gas processing unit  9  is moored to the seabed  21  with a similar mooring system as the one described in  FIG. 2  for the floating hydrocarbon processing unit  1 . 
       FIG. 5  shows a schematic view of a system according to the present invention where the system comprises two gas buffer reservoirs  7  and  7 ′ and where there are two floating gas processing units  9  which are dynamically positioned and provided with a particular offshore loading system. In this embodiment, the floating gas processing unit  9  is provided with a flexible loading system for a dynamically positioned vessel (DP) comprising a base  32  firmly attached to the seabed  21  and forming an interface between the buffer well head  36  and a flexible flowline  33  and having a substantially vertical outlet to which the flexible flowline is connected. The flowline  33  is positively buoyant such that it rises substantially vertically from the base  32  to the surface of the sea, its length being substantially greater than the depth of the sea such that the upper portion floats on the sea surface, a storage reel  34  rotatably mounted on the floating gas processing unit to which the upper end of the flowline  33  can be releasably connected such that the flowline  33  may be reeled in or out depending on the position of the vessel relative to the base. 
     In this case, as shown in  FIG. 4 , two floating gas processing units  9  cam work at the same time, each drawing gas from one gas buffer reservoir  7  and  7 ′. Alternatively, the gas processing unit  9  can first liquefy some gas from one gas buffer reservoir  7 , then disconnect and reconnect to the other gas buffer reservoir  7 ′. 
     According to the invention, the floating gas processing unit  9  is disconnectable from a mooring system (when moored to the seabed) and a first gas riser  8  that is connected to a first gas buffer reservoir  7  and can be connected to a second mooring system (when moored to the seabed) and a second gas riser  8 ′ that is connected to a second buffer reservoir  7 ′, for receiving and liquefying of temporary stored gas. 
     In the shown embodiments, offloading form a production unit to a tanker is done in a tandem configuration, however according to the present invention the transfer between two vessels is not only limited to tandem offloading configuration, but can also be side-by-side. 
     As will be apparent to those skilled in the art in the light of the foregoing disclosure, many alterations and modifications are possible in the practice of this invention without departing from the spirit or scope thereof. Accordingly, the scope of the invention is to be construed in accordance with the substance defined by the following claims.

Summary:
A system for developing offshore hydrocarbon fields includes a floating hydrocarbon processing unit moored to the seabed and connected to a hydrocarbon reservoir via a riser and a wellhead on the seabed, the unit receiving and processing the hydrocarbon mixture received from the reservoir via the riser, and having equipment to separate the hydrocarbon mixture into oil, water and gas and store them in storage tanks. A gas storage buffer reservoir is placed at a distance from and isolated from the hydrocarbon reservoir. A gas export riser is connected to the floating hydrocarbon processing unit and the gas buffer reservoir to export separated gas and inject it into the gas buffer reservoir. A gas riser has one end connected to the gas buffer reservoir and another end connected to the floating gas processing unit for processing gas received from the gas buffer reservoir for enabling transportation to shore.