Abstract:
An LNG gasifier configured to be installed on an offshore structure for vaporizing LNG includes a water passage into which water flows; an LNG passage provided inside the water passage and configured to pass the LNG and that allows heat exchange between the water and the LNG; and a bubble generating unit configured to generate bubbles of air into the water in the water passage. The water passage has an inlet port from where the water is taken in and a discharge port from where air and the water are discharged.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a liquefied-natural-gas (LNG) gasifier and a method of gasification. 
   2. Description of the Related Art 
   A liquefied natural gas (LNG), which is liquid methane, is a liquid having a boiling point of about −165° C. The LNG is transported from a liquefaction station by sea with an LNG ship provided with a cryogenic tank. The LNG is handled at on-shore LNG-receiving terminals (stations) near a port of various places to be supplied to customers. 
   Such LNG-receiving terminals are provided with, for example, an insulated tank to hold the LNG from a ship, a gasifier (vaporizer) to vaporize the LNG to convert into a natural gas (NG), in other words, a heat exchanger, and a controlling and measuring installation that regulates and measures an amount of the LNG supplied to pipeline of a customer. 
   To serve the LNG at an area without such LNG-receiving terminal, a floating-production-storage-and-offloading (FPSO) vessel is proposed. The FPSO is provided with a gasifier to convert the LNG into the NG at sea. The NG converted from the LNG at sea, for example, on a ship, is supplied through a pipeline to an on-shore pipeline for NG (Patent Literature 1). 
   An example of an LNG gasifier on a ship is shown in  FIG. 4 . As shown in  FIG. 4 , a conventional LNG gasifier includes a pipe  2  to feed the LNG from an LNG storage tank  1 . An outer surface of a pipe  2   a  is brought into contact with a heating medium such as seawater  3 . The pipe  2   a  is surrounded by a tubular shell  4 . A seawater pump  5  is provided inside the tubular shell  4  to send the seawater  3  through the shell  4 . A motor  6  is provided in a ship  7  to drive the seawater pump  5 . The NG obtained by vaporization is collected in a collection tank  8  and sent to shore by a pipe  9 . 
   Patent Literature 1: Japanese Patent Publication No. 2003-517545 
   In the conventional LNG gasifier disclosed in patent literature 1, the seawater pump  5  needs to be provided inside the tubular shell  4  to supply seawater, which is used as a heat source for vaporization. This requires provision of the motor  6  to drive the seawater pump  5  as well as maintenance of the seawater pump  5 . 
   On the other hand, when, for example an open-rack-type LNG gasifier as shown in  FIG. 5  that uses seawater for heat exchange is provided in the FPSO, seawater  103  is brought into a seawater trough  101  from a seawater supply port  102 . The LNG passing through a heat exchanging tube  104  is vaporized by means of the seawater  103  overflowing from the seawater trough  101 . Thus, the open rack type LNG gasifier needs a stable supply of seawater. However, a stable supply of seawater from the seawater trough  101  cannot be maintained due to swaying of the ship. 
   Providing the FPSO with another type of LNG gasifier such as an LNG gasifier that carries out heat exchange by supplying a gas from a burner to a water cistern requires maintenance of the burner and accompanying combustion facilities. Moreover, this results in a high fuel cost. 
   Providing the FPSO with still another type of LNG gasifier such as an LNG gasifier that carries out heat exchange by means of an intermediate heating medium requires use of combustible liquefied-petroleum gas (LPG) or chlorofluorocarbon substitute as the intermediate medium. This causes difficult handling, for example, in inspection and maintenance. 
   SUMMARY OF THE INVENTION 
   It is an object of the present invention to solve the above problems and to provide an LNG gasifier that is simple in structure and enables stable vaporization of LNG in the FPSO, and a method of gasification. 
   To solve the above problems, a first aspect of the invention of the present invention includes an LNG gasifier for vaporizing LNG that includes a seawater inlet passage that is provided in a main unit and into which seawater flows; a heat exchanging tube for causing heat exchange between the seawater and the LNG, the heat exchanging tube provided inside the seawater inlet passage; a bubbling device for supplying air into the seawater, the bubbling device provided near an inlet port of the seawater inlet passage; an air supplying device configured to supply external air to the bubbling device; and a discharge port for discharging bubbling air, which is generated in the bubbling device, outside the main unit, the discharge port configured to communicate with the seawater inlet passage. The bubbling air generated in the bubbling device brings the seawater inside the inlet passage from the seawater inlet port to vaporize the LNG supplied inside the heat exchanging tube. 
   In a second aspect of the invention according to the first aspect of the invention, the seawater inlet passage opens in a vertical axial direction. 
   In a third aspect of the invention according to the first aspect of the invention, the heat exchanging tube includes a spiral shaped tube. 
   In a fourth aspect of the invention according to the first aspect of the invention, the heat exchanging tube includes a flange joint so that the heat exchanging tube is configured to separate into multiple parts. 
   In a fifth aspect of the invention according to the first aspect of the invention, the seawater inlet port is located below the discharge port. 
   A sixth aspect of the invention according to the present invention includes a ship that includes the LNG gasifier according to any one of the first to the fifth aspect the invention. 
   A seventh aspect of the invention according to the present invention includes an offshore structure that includes the LNG gasifier according to any one of the first to the fifth aspect of the invention. 
   An eighth aspect of the invention according to the present invention includes a method of gasification of LNG that includes causing bubbles with air inside a seawater inlet passage provided in a main unit to take in seawater inside the seawater inlet passage; and supplying LNG into a heat exchanging tube provided inside the seawater inlet passage to vaporize the LNG. 
   According to the present invention, it is possible to realize stable gasification of an LNG on an FPSO and an LNG gasifier having a simple structure in which a seawater pump required in a conventional LNG gasifier for supplying a heat source is not required. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a cross-section of an LNG gasifier according to a first embodiment of the present invention; 
       FIG. 2  is a plan view of the LNG gasifier according to the first embodiment; 
       FIG. 3  is a schematic of a ship provided with the LNG gasifier according to a second embodiment of the present invention; 
       FIG. 4  is a schematic of an LNG gasifier according to a conventional technology; and 
       FIG. 5  is a schematic of another LNG gasifier according to a conventional technology. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Exemplary embodiments of the present invention are explained in detail below with reference to the accompanying drawings. The present invention is not limited to the embodiments. Structural elements described in the embodiments include all modifications and alternative constructions, which may occur to one skilled in the art. 
   An LNG gasifier according to a first embodiment of the present invention is explained with reference to the accompanying drawings. 
     FIG. 1  is a cross-section of the LNG gasifier according to the first embodiment.  FIG. 2  is a plan view of the LNG gasifier. 
   As shown in  FIG. 1  and  FIG. 2 , an LNG gasifier  10  for vaporizing LNG is provided in a main unit  11  immersed in sea  20 . Seawater  12 A is taken into a seawater inlet passage  13 . A heat exchanging tube  14  is provided along an axial direction of a passage inside the seawater inlet passage  13  to cause heat exchange between the seawater  12  and LNG. A bubbling device  16  provided near a seawater inlet port  13   a  of the seawater inlet passage  13  supplies air  15  to the seawater  12 . An air supplying device  17  continuously supplies the air  15  from outside to the bubbling device  16  through a passage  17   a.  Through a discharge port  18  communicating with the seawater inlet passage  13 , the air  15  in a form of bubble generated in the bubbling device  16  is discharged out of the main unit  11  together with the seawater  12 . The seawater  12  is forcibly brought inside the seawater inlet passage  13  through the seawater inlet port  13   a  collaterally with movement of the air  15  in bubbles generated in the bubbling device  16 . Thus, the LNG supplied inside the heat exchanging tube  14  from an LNG tank is vaporized to an NG. 
   According to the present embodiment, the seawater inlet passage  13  is formed in such a manner that the seawater inlet passage  13  runs through in a vertical axial direction inside the main unit  11  and has a port  13   b  that communicates with the outside. 
   The discharge port  18  is provided on a sidewall  11   a  of the main unit  11  in such a manner that the discharge port  18  communicates with the seawater inlet passage  13  so that the bubbling air  15  is swiftly discharged. 
   According to the present embodiment, the heat exchanging tube  14  is, for example, a trombone-shaped spiral tube so that heat exchange efficiency is enhanced. However, the present invention is not to be thus limited, and a tube of any shape having high heat exchange efficiency may be applied. 
   According to the present embodiment, the heat exchanging tube  14  includes a flange joint  19  and is separable into multiple parts. Thus, the heat exchanging tube  14  can be separated or connected when the heat exchanging tube  14  is to be inserted into or removed from the seawater inlet passage  13 , thereby making insertion or removal of the heat exchanging tube easier. 
   According to the present embodiment, the seawater inlet port  13   a  is located below the discharge port  18 , thereby increasing efficiency in supplying seawater and increasing the heat exchange efficiency. 
   According to the present embodiment, the air supplying device  17  supplies air to the bubbling device  16 , and an airlift force of the bubbling causes the seawater  12  to be brought inside the seawater inlet passage  13 . Then, LNG is supplied into the heat exchanging tube  14 . Thus, heat exchange is caused between the LNG and the seawater  12  to vaporize the LNG into NG. 
   According to the present embodiment, providing the LNG gasifier, for example, to a ship to be an FPSO, it is possible to realize a stable vaporization of LNG. Thus, an LNG gasifier that has a simple structure in which a seawater pump for supplying a heat source, which is required in a conventional LNG gasifier, is not required can be provided. 
   According to the present embodiment, the seawater is forcibly brought into the seawater inlet passage by supplying air to the bubbling device. Unlike the conventional technology, a heat source such as a burner is not needed, and use of an intermediate medium (LNG or chlorofluorocarbon substitute) is not required. Thus, inspection and maintenance can be easily carried out, and a stable supply of seawater can be maintained without being affected by swaying of the ship at sea. 
   Second Embodiment 
   A ship provided with an LNG gasifier according to a second embodiment of the present invention is explained with reference to the accompanying drawings. 
     FIG. 3  is a schematic of the ship provided with the LNG gasifier according to the second embodiment. 
   As shown in  FIG. 3 , the LNG gasifier  10  is arranged at a bow of a ship  30  according to the present embodiment. The LNG supplied from an LNG tank  31  via a pipe  32  is vaporized in the LNG gasifier  10  and supplied to an on-shore pipeline  35  via a pipeline  34 . 
   Thus, LNG can be stably vaporized and supplied as NG even to a place on the shore without an LNG receiving terminal. Moreover, the NG supplied can be directly supplied to the on-shore pipeline. 
   According to the present embodiment, it is possible to vaporize the LNG with a simple structure using the LNG gasifier arranged at the bow of the ship, and to directly supply the NG obtained by vaporization to the on-shore pipeline. 
   While in the present embodiment, the LNG gasifier shown in  FIG. 1  is provided on the ship, the present invention is not thus limited and the LNG gasifier may also be provided on a marine structure located offshore. 
   Moreover, while in the present embodiment, the seawater is used to vaporize the LNG, the present invention is not thus limited, and other liquid media, for example, water may be used. 
   INDUSTRIAL APPLICABILITY 
   The LNG gasifier and method for LNG gasification according to the present invention can be applied to ships or offshore structures that include an LNG gasifier.