Patent Publication Number: US-2005139595-A1

Title: Method of constructing a liquefied natural gas or liquefied petroleum gas terminal

Description:
The present invention relates to a method of constructing a liquefied natural gas or liquefied petroleum gas terminal, an external structure and a storage tank for implementing the method, and a terminal comprising a structure and a storage tank of this kind.  
      Liquefied natural gas (LNG) and liquefied petroleum gas (LPG) terminals for loading these liquids onto ships (of the methane tanker type in the case of LNG) or off loading these liquids from ships for regassification and landing via a pipeline are known in the art.  
      These terminals, which may be of the gravity type (in shallow water) or floating type (in deep water), typically comprise an external structure made of concrete and/or steel containing cylindrical, prismatic or spherical metal storage tanks or membrane storage tanks.  
      These large storage tanks are assembled inside the concrete structure when the latter is completed or on the point of being completed.  
      The construction time of these terminals is usually relatively long and a particular object of the present invention is to provide a solution that significantly reduces the construction time.  
      This object of the invention is achieved with a method of constructing an LNG or LPG terminal of the type comprising an external structure and at least one storage tank, which method is noteworthy in that it comprises the steps of: 
          constructing said external structure,     in parallel and independently, constructing said storage tank,     when the construction of said external structure and said storage tank is completed or on the point of being completed, floating said external structure and said storage tank,     floating said storage tank into said external structure, and     fixing said storage tank to the interior of said external structure.        

      This method uses flotation of the storage tank as the transport means, so that the storage tank may be floated into the external structure of the terminal in one piece, regardless of its size.  
      Using this method, the external structure and the storage tanks of the terminal may be constructed completely and simultaneously, which shortens the critical path and therefore the overall site time.  
      Moreover, the method of the invention envisages constructing the external structure and the storage tanks on separate specialist sites, which optimizes costs, quality and reliability.  
      According to other optional features of the method of the invention: 
          the method comprises the further step of modifying the relative conditions of flotation of said external structure and said storage tank to enable the fixing of said storage tank to the interior of said external structure,     said external structure remains floating,     said external structure is placed on the seabed,     the method comprises the additional step of sheltering and enclosing said storage tank inside said external structure,     the method further comprises the step of emptying out the seawater inside said external structure to isolate said storage tank from the marine environment,     the method further comprises the step of installing equipment on said storage tank during its construction.        

      The present invention also relates to an external terminal structure for implementing the above method that is remarkable in that it is conformed to be able to accommodate said storage tank once constructed.  
      According to other optional features of this external structure: 
          it has a U-shaped cross section,     it is of the gravity type,     it is formed of a material chosen from the group comprising concrete, steel, and concrete+steel,     it comprises closure means,     said closure means are chosen from the group comprising a sealed caisson and a door formed from a material chosen from the group comprising a metal alloy and concrete,     said closure means comprise sealing means adapted to be disposed between the walls of said external structure and those of said storage tank.        

      The present invention also relates to a storage tank for equipping the above external terminal structure that is noteworthy in that it floats and is adapted to be mounted inside said external structure once constructed.  
      According to other optional features of this storage tank: 
          it has a shape chosen from the group comprising parallelepiped, cylinder, prism and sphere shapes,     it is chosen from the group comprising double-shell and single-shell metal storage tanks with thermal insulation and interior membrane, as appropriate to the type of shell concerned,     it comprises equipment chosen from the group comprising process equipment, utilities and crew&#39;s quarters.        

      The present invention also relates to an LNG or LPG terminal noteworthy in that it comprises the above external structure and one or more of the above storage tanks. 
    
    
      Other features and advantages of the present invention will become apparent in the light of the following description and from examining the appended drawing, in which:  
       FIG. 1  is a sectional view of a ship alongside an LNG/LPG terminal of the invention,  
       FIG. 2  is a plan view of the entities shown in  FIG. 1 , and  
       FIG. 3  is a view analogous to that of  FIG. 2  including a variant of the terminal according to the invention. 
    
    
       FIGS. 1 and 2  show a ship  1  such as a methane tanker alongside a terminal  3  of the invention.  
      Pipes  5  connecting the ship  1  to the terminal  3  transport fluids such as LNG or LPG from one of these entities to the other.  
      In the example represented in  FIGS. 1 and 2 , the terminal  3  comprises a gravity type external structure  7  formed of concrete, for example, and resting on the seabed  9 .  
      Inside this external structure, which has a U-shaped section open at the top, as may be seen in  FIG. 1 , is a metal storage tank  11  of substantially parallelepiped shape complementary to the cavity defined by the external structure  7 .  
      The metal storage tank  11  rests on the bottom of the external structure  7  through appropriate mechanical means  13  (welds, nut and bolt fasteners, tie-rods, etc.).  
      The metal storage tank  11  has on top equipment  15  that may comprise process equipment, utilities (networks of pipes, etc.) and crew&#39;s quarters.  
      In the example represented in  FIGS. 1 and 2 , any seawater in the cavity defined by the external structure  7  has been emptied out, for example pumped out.  
      Where applicable, this cavity is isolated from the seawater by means that may comprise seals  17  disposed between the interior walls of the cavity and the storage tank  11  or a watertight steel or concrete caisson  19 .  
      The metal storage tank  11  is of the double-shell or single-shell type with thermal insulation and an interior membrane according to the type of shell selected.  
      Moreover, this storage tank is designed to float.  
      In the variant represented in  FIG. 3 , the single storage tank  11  is replaced by three substantially cylindrical storage tanks  11   a,    11   b,    11   c  disposed inside the cavity defined by the external structure  7 .  
      It will also be noted that in this variant the closure caisson  19  is sufficiently thick to receive the equipment  15 .  
      It will further be noted that the seals  17  are disposed between a mobile door  21  and the internal walls of the structure  7 .  
      The terminal  3  that has just been described is constructed as follows.  
      The following account refers to the variant represented in  FIGS. 1 and 2 , it being understood that the method of constructing the terminal represented in  FIG. 3  is similar in all respects.  
      The external structure  7  and the storage tank  11  are constructed in parallel and independently, where applicable on separate sites.  
      When the construction of the external structure  7  is completed, it is floated to the operating site, where it either remains floating or is ballasted so that it rests on the seabed  9 .  
      When the construction of the tank  11  is sufficiently far advanced for the storage tank to float, it is also floated and moved (for example by means of a tug) into the cavity defined by the external structure  7 .  
      When the storage tank is inside the cavity, it is placed on the bottom of the cavity by ballasting the storage tank and/or deballasting the external structure  7 , after which the storage tank is fixed to the bottom of the cavity using appropriate mechanical means  13 .  
      When the storage tank  11  is inside the structure  7 , the cavity defined by the structure may be isolated by means of the seals  17  and/or the caisson  19  and/or the door  21 .  
      The water between the storage tank and the internal walls of the structure  7  may then advantageously be emptied out (if necessary).  
      The connections may then be made between the installations on the external structure  7  and the equipment  15  on the storage tank  11 .  
      As is clear from the foregoing description, the construction of the external structure  7  and that of the storage tanks  11 ,  11   a,    11   b,    11   c  may be effected in parallel and totally independently, where applicable on separate specialist sites.  
      This shortens the critical path and therefore the overall time on site for the construction of the terminal  3 .  
      Furthermore, if separate specialist sites are used for the construction of the external structure  7  and the storage tanks, optimum costs, quality and reliability may be achieved.  
      It will also be noted that physically dissociating the external structure  7  and the storage tanks in the terminal of the invention circumvents constraints regarding the external structure  7  specific to the storage of liquids such as LNG or LPG.  
      It will further be noted that the solution consisting in emptying out the seawater in the cavity defined by the external structure  7  would seem to be preferable in that it enables easy maintenance of the storage tanks and the detection and repair of leaks, as well as reducing the number and/or the size of the mechanical fixing means  13 .  
      Of course, the present invention is not restricted to the embodiments described hereinabove by way of illustrative and nonlimiting example.  
      For example, the external structure  7  need not be of the gravity type, i.e. of the type resting on the seabed.  
      In very deep waters, applying the present invention to an FPSO type floating external structure  7  may be envisaged.  
      Furthermore, the storage tanks disposed inside the cavity defined by the external structure of the terminal could be of any type and any shape other than those described hereinabove.  
      For example, the storage tanks could be spherical or prismatic.