Offshore nitrogen production and injection

Abstract

Nitrogen is produced above an offshore hydrocarbon reservoir ( 12 ) and injected into the reservoir, by a complex ( 40 ) mounted on a previously used vessel hull ( 16 ). The vessel weathervanes and the complex includes equipment that takes in ordinary air from a bow end ( 32 ) of the vessel, cools the air using sea water pumped into the equipment, and liquefies the oxygen component of air so pure nitrogen remains. The complex also includes injection equipment that heats and pressurizes the pure nitrogen to inject it into the reservoir. The complex is mounted above pipes ( 24 ) on the deck ( 22 ) of the vessel hull, on spacers ( 62 ) so mounting of equipment can be accomplished rapidly and without major alterations to the hull. A riser bundle ( 150 ) that extends from the hull to the sea floor, may include both a production riser ( 152 ) that carries crude hydrocarbons up to the vessel for storage in hull tanks ( 20 ), and an injection riser ( 160 ) that carries pressured nitrogen. The lower end of one of the risers is connected to a sea floor pipeline ( 162 ) that extends at least 100 meters away from the lower end of the other riser.

1 . Apparatus for injecting gas into an undersea hydrocarbon reservoir that lies under the sea floor of a sea, comprising: 
 a seaworthy vessel hull that has a deck and that is moored to the sea floor;    a nitrogen production complex that has an air intake for receiving environmental air, equipment for cooling the air and separating out nitrogen in the air from other components in the air, and equipment for pressurizing the nitrogen to inject it into the reservoir;    most of the volume of said nitrogen production complex being mounted on top of said deck of said vessel hull, with at least 80% of the volume inside the vessel hull being left unoccupied by the separation system to provide buoyancy for the separation system on top of the vessel hull.    
   
   
       2 . The apparatus described in  claim 1  including: 
 hydrocarbon processing equipment mounted on said vessel hull;    a riser bundle that extends from said vessel to the sea floor and that includes a crude oil riser and an injection riser;    said crude oil riser that has a crude riser lower end connected to a first production location in said reservoir to carry crude hydrocarbons up to said vessel hull, and a crude riser upper end connected to said hydrocarbon processing equipment;    an injection riser that has an upper end connected to said equipment for receiving pressurized nitrogen and a lower end connected to a second injection location in said reservoir to inject nitrogen therein;    said second injection location being horizontally spaced by at least 100 meters from said first injection location.    
   
   
       3 . The apparatus described in  claim 2 , wherein said vessel hull is moored by catenary lines, and including a turret rotatably mounted about a primarily vertical axis on said hull and a fluid swivel having a stationary part connected to said turret and a rotatable part connected to said vessel hull, including: 
 a riser bundle that extends between said swivel and a platform on the sea floor, and which includes both said crude riser and said injection riser, and at least one pipe extending along the sea floor by at least 100 meters to one of said locations.    
   
   
       4 . The apparatus described in  claim 2  wherein: 
 said nitrogen production complex is energized by some of the crude hydrocarbons carried up to said vessel hull through said crude oil riser and processed by said hydrocarbon processing equipment, and said vessel hull has tanks that contain hydrocarbons processed by said hydrocarbon processing equipment.    
   
   
       5 . The apparatus described in  claim 1  wherein the mooring of said vessel hull to the sea floor allows said vessel hull to weathervane so a bow end of said vessel lies upwind of a stern of said vessel, and wherein; 
 said air intake lies closer to said bow end than to a midpoint of said vessel that lies halfway between the vessel bow and stern, to thereby avoid the intake of vapors originating on the vessel.    
   
   
       6 . The apparatus described in  claim 1  wherein: 
 said vessel hull is a previously used hull with a deck and with a plurality of on-deck pipes lying closely over said deck, and including a plurality of spacers that mount said equipment of said complex above said on-deck pipes.    
   
   
       7 . Apparatus for injecting gas into an undersea hydrocarbon reservoir to pressurize it, comprising: 
 a seaworthy vessel hull that has a bow and stern;    a separation system that has an air intake for receiving environmental air, equipment for cooling the air and separating out nitrogen in the air from other components in the air, and equipment for pressurizing the separated-out nitrogen to pump it into the reservoir;    a mooring system that limits hull drift while allowing the hull to weathervane so its stern points in changing directions away from its bow with changing winds, waves and currents;    said air intake is closer to the bow than to the stern of the hull.    
   
   
       8 . The apparatus described in  claim 7  wherein: 
 said vessel hull has a deck;    at least 90% of the volume of said equipment for cooling and separating and said equipment for pressurizing, lies on top of said deck.    
   
   
       9 . A method for pressurizing an undersea hydrocarbon reservoir comprising; 
 mounting a separation system on a deck of a vessel hull and mooring the hull to the sea floor;    drawing ambient air into the separation system and operating the separation system including cooling the drawn-in air using sea water pumped in from the sea as a heat sink and further cooling the air and separating out the nitrogen from the oxygen;    operating the separation system to heat the separated-out nitrogen, and pump the separated-out and heated nitrogen into a first location in the undersea hydrocarbon reservoir.    
   
   
       10 . The method described in  claim 9  including: 
 producing hydrocarbons from said undersea reservoir, from a second location that is spaced at least 100 meters from said first location, and using some of said produced hydrocarbons to energize said separation system.    
   
   
       11 . The method described in  claim 9  wherein: 
 said step of mounting a separation system comprises mounting at least 90% of said volume of said separation system above the deck of said vessel hull.