Abstract:
Systems and methods are provided for delivering pressurized liquefied natural gas to an import terminal equipped with containers and vaporization facilities suitable for conventional LNG.

Description:
This application claims the benefit of U.S. Provisional Application No. 60/306,986, filed Jul. 20, 2001. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to systems and methods for delivering pressurized liquefied natural gas to an import terminal that contains storage tanks and vaporization facilities suitable for conventional liquefied natural gas at atmospheric pressure. The pressurized liquefied natural gas cargo, or any fraction thereof, is converted into conventional liquefied natural gas and sent to storage tanks suitable for conventional liquefied natural gas. Any of the cargo not converted to conventional liquefied natural gas can be compressed and warmed to pipeline specifications. This gas can then pass into a sendout pipeline. 
     BACKGROUND OF THE INVENTION 
     Various terms are defined in the following specification. For convenience, a Glossary of terms is provided herein, immediately preceding the claims. 
     Large volumes of natural gas (i.e. primarily methane) are produced in remote areas of the world. This gas has significant value if it can be economically transported to market. Where the production area is in reasonable proximity to a market and the terrain between the two locations permits, the gas is typically transported through submerged and/or land-based pipelines. However, when gas is produced in locations where laying a pipeline is infeasible or economically prohibitive, other techniques must be used for getting this gas to market. 
     A commonly used technique for non-pipeline transport of gas involves liquefying the gas at or near the production site and then transporting the liquefied natural gas to market in specially-designed storage tanks aboard transport vessels. The natural gas is cooled and condensed to a liquid state to produce liquefied natural gas at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.) (“LNG”), thereby significantly increasing the amount of gas which can be stored in a particular storage tank. Once an LNG transport vessel reaches its destination, the LNG is typically off-loaded into other storage tanks from which the LNG can then be revaporized as needed and transported as a gas to end users through pipelines or the like. 
     U.S. Pat. No. 6,085,528 (the “PLNG Patent”), having corresponding International Publication Number WO 98/59085, and entitled “Improved System for Processing, Storing, and Transporting Liquefied Natural Gas”, describes containers and transportation vessels for storage and marine transportation of pressurized liquefied natural gas (PLNG) at a pressure in the broad range of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature in the broad range of about −123° C. (−190° F.) to about −62° C. (−80° F.). Containers described in the PLNG Patent are constructed from ultra-high strength, low alloy steels containing less than 9 wt % nickel and having tensile strengths greater than 830 MPa (120 ksi) and adequate toughness for containing PLNG. U.S. patent application Ser. No. 09/495,831 (the “PLNG Patent Application”), having corresponding International Publication Number WO 00/57102, and entitled “Improved System and Methods for Producing and Storing Liquefied Natural Gas”, also describes containers for storage and transport of PLNG. Containers described in the PLNG Patent Application comprise a load-bearing vessel made from a composite material and a substantially impermeable, non-load-bearing liner in contact with the vessel. Any container suitable for storing PLNG shall be referred to hereinafter as a PLNG Container. Any container suitable for storing LNG that is not also suitable for storing PLNG shall be referred to hereinafter as an LNG Container. The PLNG Patent and the PLNG Patent Application are hereby incorporated herein by reference. 
     PLNG may be unloaded at an import terminal into pressurized PLNG Containers, e.g., by using some of the displaced vapors to maintain a minimum required pressure in the PLNG Containers on the transport ship. However, it may be desirable to deliver PLNG to a conventional LNG import terminal that is equipped with conventional LNG Containers but is not equipped with PLNG Containers 
     In spite of the aforementioned advances in technology, to our knowledge, systems and methods for delivering PLNG to an import terminal equipped with LNG Containers and vaporization facilities suitable for LNG, do not currently exist. It would be advantageous to have such systems and methods. 
     Therefore, an object of this invention is to provide such systems and methods. Other objects of this invention will be made apparent by the following description of the invention. 
     SUMMARY OF THE INVENTION 
     Consistent with the above-stated objects of the present invention, systems and methods for delivering PLNG to an import terminal equipped with LNG Containers and vaporization facilities suitable for LNG are provided. A system according to the present invention comprises: (a) pressurized liquefied natural gas at a pressure of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature of about −123° C. (−190° F.) to about −62° C. (−80° F.) stored in one or more PLNG Containers having adequate strength and toughness to contain said pressurized liquefied natural gas at said pressure and temperature conditions; (b) one or more LNG Containers suitable for storing liquefied natural gas at substantially atmospheric pressure and at a temperature of about −162° C. (−260° F.); (c) means for removing and reducing the pressure of at least a portion of said pressurized liquefied natural gas from said one or more PLNG Containers, which removed pressurized liquefied natural gas comprises a substantially gaseous portion and a substantially liquid portion; (d) separation equipment suitable for separating said substantially gaseous portion and said substantially-liquid portion; (e) pressurization equipment suitable for pressurizing said substantially gaseous portion to a desired pressure; (f) gas delivery equipment suitable for delivering said pressurized substantially gaseous portion to a gaseous portion destination; (g) depressurization equipment suitable for reducing the pressure of said substantially liquid portion to substantially atmospheric pressure in one or more steps; and (h) liquid delivery equipment suitable for delivering said substantially atmospheric pressure liquid portion to said one or more LNG Containers. In one embodiment, the means for reducing the pressure of at least a portion of the pressurized liquefied natural gas consists essentially of expansion. A method according to the present invention comprises the steps: (a) storing pressurized liquefied natural gas at a pressure of about 1035 kPa (150 psia) to about 7590 kPa (1100 psia) and at a temperature of about −123° C. (−190° F.) to about −62° C. (−80° F.) in one or more PLNG Containers having adequate strength and toughness to contain said pressurized liquefied natural gas at said pressure and temperature conditions; (b) removing and reducing the pressure of at least a portion of said pressurized liquefied natural gas from said one or more PLNG Containers, which removed pressurized liquefied natural gas comprises a substantially gaseous portion and a substantially liquid portion; (c) separating said substantially gaseous portion and said substantially liquid portion; (d) pressurizing said substantially gaseous portion to a desired pressure; (e) delivering said pressurized substantially gaseous portion to a gaseous portion destination; (f) reducing the pressure of said substantially liquid portion to substantially atmospheric pressure in one or more steps; and (g) delivering said substantially atmospheric pressure liquid portion to one or more LNG Containers suitable for storing liquefied natural gas at substantially atmospheric pressure and at a temperature of about −162° C. (−260° F.). In the process of removing PLNG from said PLNG Containers, displacement vapor may be used to maintain pressure and prevent auto-refrigeration of the remaining cargo. In one embodiment, reducing the pressure of at least a portion of the pressurized liquefied natural gas consists essentially of expanding the pressurized liquefied natural gas. 
     All of, or a portion of, the PLNG is let down through one or more liquid expanders and/or control valve, such as Joule-Thompson valves, in series to the LNG Containers. Resulting flash vapors are collected from flash vessels downstream of the expanders and control valves and fed to a compression system designed to recompress the vapors to pipeline delivery pressure. Displacement vapors for unloading the PLNG Containers on the transport ship can be withdrawn, as needed, from the vapors being recompressed to the sales gas pipeline. 
     In one embodiment, the predominantly isenthalpic and/or isentropic expansion and partial vaporization of the pressurized cryogenic liquid streams can provide substantially all the refrigeration needed for cooling the remaining (unvaporized) liquid. The end result is a conventional LNG product that has been cooled to its bubble point temperature at essentially atmospheric pressure. This liquid can then be stored in existing conventional LNG import terminal facilities, including LNG Containers, and eventually revaporized for use. If only a portion of the PLNG is let down in pressure, the remaining PLNG can be unloaded and vaporized by any available method, for example without limiting this invention, by the methods described in U.S. Pat. No. 6,112,528. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The advantages of the present invention will be better understood by referring to the following detailed description and the attached drawing in which: 
     FIG. 1 is a schematic, flow diagram of a system according to the present invention. 
    
    
     While the invention will be described in connection with its preferred embodiments, it will be understood that the invention is not limited thereto. On the contrary, the invention is intended to cover all alternatives, modifications, and equivalents which may be included within the spirit and scope of the present disclosure, as defined by the appended claims. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Stored PLNG is pressurized out of a PLNG Container and depressurized through one or more depressurization stages in series to substantially atmospheric pressure, using a combination of liquid expanders and/or Joule-Thompson control valves, to produce conventional LNG. Vapors associated with the pressure letdown are recovered from separator vessels and compressed to sales gas pressure. A portion of the vapors can be used to displace PLNG being unloaded from PLNG Containers on the transport ship, if needed. 
     The LNG resulting from the multistage letdown process is sent to conventional LNG Containers. Subsequently, this LNG can be pumped up to sales gas pressure and vaporized in any type of conventional LNG vaporizer for delivery to the sales gas pipeline. 
     An example of a system  10  in accordance with this invention is illustrated by FIG.  1 . This invention is not limited to the example presented. Optimum system process arrangement will vary with gas composition and site specific economics. Many variations not specifically discussed herein, e.g., a system with only one stage, are considered within the scope of this invention. In this non-limiting example, PLNG having a standard regasified equivalent of 939 K std m 3 /hr (800 MSCFD) is being unloaded from PLNG Container  12  onboard a transport ship (not shown). The PLNG cargo is let down to conventional LNG storage pressure, i.e., substantially atmospheric pressure. In this example, approximately half of the stream is converted to LNG and stored in conventional LNG Containers. The other half is recovered as flash gas and compressed to sales. 
     In somewhat greater detail, PLNG feed product at about 30.4 bar (441 psia) and about −96° C. (−140° F.) is unloaded from PLNG Container  12  at a standard regasified equivalent rate of about 939 K std m 3 /hr (800 MSCFD) into liquid accumulator  14  through line  15 . Pressure is maintained in PLNG Container  12  by vapors entering through line  100 . These vapors can be obtained by taking a slipstream from the process or from any other acceptable source, as will be familiar to those skilled in the art. In this embodiment, the vapors volumetrically replace the PLNG in PLNG Container  12 . Liquid accumulator  14  provides a substantially stable feed rate to the rest of the process. Any vapors or gaseous feed product (an insignificant volume) at about 30.4 bar (441 psia) and about −96° C. (−140° F.) separates from liquid feed product within liquid accumulator  14  and flows through first valve  18  via line  17 . Any gaseous PLNG present exits first valve  18  at about 21.0 bar (305 psia) and −107° C. (−160° F.) and flows through line  19  to a first depressurization flash tank  16 . Liquid PLNG at about 30.4 bar (441 psia) and about −96° C. (−140° F.) flows from liquid accumulator  14  through line  21  to a first turboexpander  20  at a rate of about 643,500 kg/hr (1,419,000 lb/hr). First turboexpander  20  generates about 668 kW (895 horsepower) of recoverable energy while liquid and gaseous feed product exit first turboexpander  20  at about 20.7 bar (300 psia) and about −107° C. (−160° F.) at a rate of about 643,500 kg/hr (1,419,500 lb/hr) and flow to first depressurization flash tank  16  through line  23 . Gaseous feed product at about 20.7 bar (300 psia) and about −107° C. (−160° F.) at a rate of about 163.2 K std m 3 /hr (138.6 MSCFD) flows out of first depressurization flash tank  16  to a first mixer  26  through line  25 . 
     Liquid PLNG at about 20.7 bar (300 psia) and about −107° C. (−160° F.) flows out of first depressurization flash tank  16  through line  27  to a second turboexpander  28  at a rate of about 532,390 kg/hr (1,173,700 lb/hr). Second turboexpander  28  generates about 755 kW (1012 horsepower) of recoverable energy while liquid and gaseous feed product exit second turboexpander  28  at about 10.3 bar (150 psia) and about −123° C. (−190° F.) at a rate of about 532,390 kg/hr (1,173,700 lb/hr) and flow to second depressurization flash tank  30  through line  29 . Gaseous feed product at about 10.3 bar (150 psia) and about −123° C. (−190° F.) at a rate of about 136 K std m 3 /hr (115.5 MSCFD) flows out of second depressurization flash tank  30  to a second mixer  32  through line  31 . 
     Liquid PLNG at about 10.3 bar (150 psia) and about −123° C. (−190° F.) flows out of second depressurization flash tank  30  through line  33  to a third turboexpander  34  at a rate of about 493,800 kg/hr (969,700 lb/hr). Third turboexpander  34  generates about 794 kW (1064 horsepower) of recoverable energy while liquid and gaseous feed product exit third turboexpander  34  at about 3.1 bar (45 psia) and about −145° C. (−230° F.) at a rate of about 439,800 kg/hr (969,700 lb/hr) and flow to third depressurization flash tank  36  through line  35 . Gaseous feed product at about 3.1 bar (45 psia) and about −145° C. (−230° F.) at a rate of about 109.1 K std m 3 /hr (92.6 MSCFD) flows out of third depressurization flash tank  36  to a third mixer  38  through line  37 . 
     Liquid feed product at about 3.1 bar (45 psia) and about −145° C. (−230° F.) flows out of third depressurization flash tank  36  through line  39  to a fourth turboexpander  40  at a rate of about 365,700 kg/hr (806,200 lb/hr). Fourth turboexpander  40  generates about 301 kW (404 horsepower) of recoverable energy while liquid and gaseous feed product exit fourth turboexpander  40  at substantially atmospheric pressure and about −162° C. (−260° F.), i.e., as LNG, at a rate of about 365,700 kg/hr (806,200 lb/hr) and flow to fourth depressurization flash tank  42  through line  41 . About 328,600 kg/hr (724,400 lb/hr) of LNG is pumped out of fourth depressurization flash tank  42  by pump  46  through line  45  to LNG Containers (not shown). 
     Gaseous feed product at substantially atmospheric pressure and about −162° C. (−260° F.) at a rate of about 54.7 K std m 3 /hr (46.4 MSCFD) flows out of fourth depressurization flash tank  42  to a first compressor  44  through line  43 . Gaseous feed product exits first compressor  44  at about 3.5 bar (50 psia) and about −110° C. (−167° F.) at a rate of about 54.7 K std m 3 /hr (46.4 MSCFD) and flows through line  49  to third mixer  38  where it is mixed with gaseous feed product at about 3.1 bar (45 psia) and about −145° C. (−230° F.) at a rate of about 109.1 K std m 3 /hr (92.6 MSCFD) from third depressurization flash tank  36 . 
     Gaseous feed product flows out of third mixer  38  at about 3.1 bar (45 psia) and about −134° C. (−210° F.) at a rate of about 163.7 K std m 3 /hr (139 MSCFD) to a second compressor  52  through line  51 . Gaseous feed product exits second compressor  52  at about 11.0 bar (160 psia) and about −64° C. (−84° F.) at a rate of about 163.7 K std m 3 /hr (139 MSCFD) and flows through line  55  to second mixer  32  where it is mixed with gaseous feed product at about 10.3 bar (150 psia) and about −123° C. (−190° F.) at a rate of about 136 K std m 3 /hr (115.5 MSCFD) from second depressurization flash tank  30 . 
     Gaseous feed product flows out of second mixer  32  at about 10.3 bar (150 psia) and about −92° C. (−134° F.) at a rate of about 299.8 K std m 3 /hr (254.5 MSCFD) to a third compressor  58  through line  57 . Gaseous feed product exits third compressor  58  at about 21.7 bar (315 psia) and about −43° C. (−45° F.) at a rate of about 299.8 K std m 3 /hr (254.5 MSCFD) and flows through line  61  to first mixer  26  where it is mixed with gaseous feed product at about 20.7 bar (300 psia) and about −107° C. (−160° F.) at a rate of about 163.2 K std m 3 /hr (138.6 MSCFD) from first depressurization flash tank  16 . 
     Gaseous feed product flows out of first mixer  26  at about 20.7 bar (300 psia) and about −67° C. (−89° F.) at a rate of about 462.9 K std m 3 /hr (393.1 MSCFD) to a fourth compressor  64  through line  63 . Gaseous feed product exits fourth compressor  64  at about 69.0 bar (1000 psia) and about 23° C. (74° F.) at a rate of about 462.9 K std m 3 /hr (393.1 MSCFD) and flows through line  65  to sales. 
     In one embodiment, at least a part of the refrigeration for cooling is provided by expansion and partial vaporization of the pressurized cryogenic liquid streams. Advantageously, in one embodiment, substantially all of the refrigeration for cooling is provided by expansion and partial vaporization of the pressurized cryogenic liquid streams, without the need for refrigeration equipment that must be powered. 
     Various options are available with this invention. For example, without limiting this invention: (a) Conventional LNG reserve storage volumes can be maintained at any level desired while the LNG is pumped through to sales; (b) Power recovered from the liquid expanders (e.g., turboexpanders) can be used to generate electric power, or alternatively used directly to offset compression requirements; (c) Cryogenic vapors generated by depressurization of the PLNG can be fed directly to non-lube compressors containing alloy steels capable of processing the cryogenic temperatures involved, e.g., for minimizing horsepower requirements; (d) Cryogenic vapors generated by depressurization of the PLNG can be cross-exchanged to recover the refrigeration and preheat the compressor suction vapors to temperatures acceptable for commercial carbon steel alloys, if desired; (d) Joule-Thompson valves can be substituted at any point for turboexpanders to reduce the cost of the facilities, at the sacrifice of energy recovery and increasing the volume of vapor generated in the depressurization sequence. 
     Particular advantages of the present invention are that the feeding of cryogenic vapors directly to special alloy, non-lubricated compressors minimizes horsepower requirements for the compressor to sales. In addition, coupling the turboexpanders with the PLNG letdown allows for recovery of energy, e.g., for generation of electrical power, and for minimizing the volumes of vapor generated. 
     While the present invention has been described in terms of one or more preferred embodiments, it is to be understood that other modifications may be made without departing from the scope of the invention, which is set forth in the claims below. 
     GLOSSARY OF TERMS 
     bar: a unit of pressure equal to 105 newtons per square meter; 
     cryogenic temperature: any temperature of about −40° C. (−40° F.) and lower; 
     kg/hr: kilograms per hour 
     lb/hr: pounds per hour 
     LNG: liquefied natural gas at substantially atmospheric pressure and at temperatures of about −162° C. (−260° F.); 
     K std m 3 /hr: thousand standard cubic meters per hour; 
     kW: kilowatts, i.e., thousands of watts; 
     LNG Container: any container suitable for storing LNG that is not also suitable for storing PLNG; 
     MSCFD: million standard cubic feet per day; 
     PLNG: pressurized liquefied natural gas; 
     PLNG Container: any container suitable for storing PLNG;