Abstract:
A floating terminal for loading/offloading cargo of ships such as methane tankers comprises an area for mooring ships and a submerged skirt to damp oscillation of the terminal to attenuate relative movement of the terminal and ships in the mooring area.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a floating terminal for loading/offloading ships such as methane tankers.  
         [0003]     2. Description of the Prior Art  
         [0004]     Floating terminals for offshore mooring of ships such as methane tankers are known in the art.  
         [0005]     These floating terminals are used to load/offload, and sometimes to store, the cargo of these ships (generally liquefied natural gas or liquefied petroleum gas) using loading/offloading arms.  
         [0006]     Floating terminals avoid these ships having to enter ports and therefore eliminate the problems of space and safety that are inherent to the size and the nature, respectively, of the cargo of these ships.  
         [0007]     To carry out loading/offloading operations, a ship must moor alongside the quay of a floating terminal, the loading/offloading manifolds or valves of the ship being generally disposed half way along its length.  
         [0008]     Until now, this kind of mooring could be envisaged only under relatively calm sea conditions, to limit relative movement of the floating terminal and the ship and thus to carry out the loading/offloading operations under acceptable safety conditions. Limitation of relative movement of the floating terminal and the ship is also necessary when the floating terminal is adapted to process hydrocarbons.  
         [0009]     As a result of this, a floating terminal has in fact been available only intermittently, which has wasted a lot of time and therefore seriously compromised the return on the investment in the system.  
         [0010]     An object of the present invention is to provide a floating terminal that in particular avoids this major drawback.  
       SUMMARY OF THE INVENTION  
       [0011]     This object of the invention is achieved with a floating terminal for loading/offloading cargo of ships such as methane tankers, the terminal comprising an area for mooring ships and submerged skirt means disposed to damp oscillation of the terminal in order to attenuate relative movement of the terminal and ships in the mooring area.  
         [0012]     Because of the skirts for damping oscillation of the floating terminal, relative movement of the terminal and the ship may be significantly reduced throughout mooring operations and during loading/offloading the cargo of the ships.  
         [0013]     Thus the floating terminal many be used even when sea conditions are bad, which makes the terminal more profitable than in the prior art.  
         [0014]     According to other features of the terminal of the invention: 
        the floating terminal comprises a submerged horizontal skirt;     the floating terminal comprises two submerged horizontal skirts extending over at least a portion of the length of its lower portion;     the floating terminal comprises a submerged vertical skirt;     the floating terminal comprises two submerged vertical skirts extending over at least a portion of the length of its lower portion;     the floating terminal comprises a skirt oriented at an angle between the horizontal and the vertical and extending over at least a portion of the length of its lower portion;     the floating terminal comprises means for orienting the floating terminal relative to the swell, the wind, and the current;     the orientation means comprise fixed positioning means;     the fixed positioning means comprise a catenary anchoring system;     the orientation means comprise mobile positioning means;     the mobile positioning means comprise a system chosen from the group comprising turret systems and articulated arm systems;     the orientation means comprise displacement means;     the displacement means are chosen from the group comprising thrusters and tugs;     the floating terminal comprises a swell damping chamber chosen from the group comprising chambers on the upstream side of the floating terminal relative to the direction of the swell and chambers on the downstream side in that direction;     the damping chamber is of the perforated wall type;     the floating terminal comprises means for storing the cargo;     the floating terminal comprises plant chosen from the group comprising lines and risers for importing/exporting the cargo, loading/offloading arms for the cargo, means for treating the cargo, crew quarters, maintenance areas, and control areas; and     the floating terminal is constructed from a material chosen from the group comprising steel, reinforced concrete, and prestressed concrete.        
 
         [0032]     Other features and advantages of the invention will become apparent on reading the following description and examining the appended drawing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0033]      FIGS. 1 and 2  are plan views of a ship moored to a terminal according to the invention connected to the sea bed by two variants of a positioning system.  
         [0034]     FIGS.  3  to  8  are views along the shorter side of six variants of a terminal according to the invention.  
         [0035]      FIG. 9  is a view along the longer side of a further variant of a terminal according to the invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Refer now to  FIG. 1 , which represents a floating terminal  1  essentially formed of a floating caisson  3  moored to the sea bed by fixed positioning means that may comprise a catenary anchorage system  5   a  to  5   d.    
         [0037]     The floating caisson  3  may be made of steel and/or reinforced concrete and/or prestressed concrete.  
         [0038]     It may support diverse plant, such as loading arms  7   a ,  7   b , means for processing the cargo offloaded from or to be loaded onto the ships, crew quarters, maintenance and control areas (not shown), etc.  
         [0039]      FIG. 1  also represents a ship  9  moored to the floating terminal  1  by any appropriate means such as hawsers  11   a  to  11   c.    
         [0040]     The ship  9  may be a methane tanker, for example, for transporting liquefied natural gas (LNG) or liquefied petroleum gas (LPG).  
         [0041]     In this case, the processing means supported by the floating caisson  3  may comprise plant for liquefying/gasifying the cargo of the ship  9 .  
         [0042]     The caisson  3  comprises storage means (not shown) for storing the cargo offloaded from or to be loaded onto the ship  9 .  
         [0043]     It will be noted that the orientation of the floating terminal  1  to the prevalent direction D of the swell protects the ship  9  from the swell.  
         [0044]     Refer now to  FIG. 2 , which represents a variant in which the floating terminal  1  is connected to the seabed by a turret  13  known in the art and about which the caisson may pivot in a horizontal plane.  
         [0045]     Instead, the turret  13  may be replaced by a soft yoke, i.e. an articulated arm mounted on a jacket (metal truss structure) anchorage system (this variant is not shown).  
         [0046]     These two variants constitute means for orienting the terminal  1  appropriately relative to the direction D of the swell and relative to the directions of the wind and the current.  
         [0047]     In this case, displacement means  14  such as thrusters or tugs may be envisaged for modifying the orientation of the caisson  3  by causing it to pivot about the vertical axis of the turret  13  or the soft yoke (articulated arm).  
         [0048]     It will be noted that  FIGS. 1 and 2  show loading/offloading arms  7   a ,  7   b  that are disposed substantially halfway along the length of the floating caisson  3 .  
         [0049]     This substantially corresponds to the position of the loading/offloading manifolds (valves) of methane tanker type ships, which are generally situated halfway along their length.  
         [0050]     Refer now to  FIG. 3 , in which it is seen that the immersed portion of the floating caisson  3  comprises horizontal skirts  15   a ,  15   b  extending over at least a portion of its length.  
         [0051]     The variant represented in  FIG. 4  differs from that from the  FIG. 3  variant in that the skirts  17   a ,  17   b  are substantially vertical.  
         [0052]     In the variant represented in  FIG. 5 , the caisson  3  is equipped with horizontal skirts  15   a ,  15   b  and with vertical skirts  17   a ,  17   b.    
         [0053]     In another variant, not shown, the floating caisson  3  could be provided with skirts oriented at an angle between the horizontal and the vertical.  
         [0054]     Refer now to  FIG. 6 , in which it is seen that the floating caisson  3  may be provided with a damping chamber  19  disposed on the side of the caisson on which the swell impinges, i.e. on the upstream side of the floating caisson  3  relative to the direction D of the swell.  
         [0055]     A damping chamber of this kind is known in the art, and in particular from French patent FR 2 693 216, and may typically comprise an exterior wall  21  provided with a plurality of regularly distributed orifices  23 , as shown in  FIG. 9 .  
         [0056]     This kind of wall is sometimes called a Jarlan wall.  
         [0057]     In the variant represented in  FIG. 7 , the damping chamber  19  is on the downstream side of the floating caisson  3  relative to the direction D of the swell.  
         [0058]     In the variant represented in  FIG. 8 , the floating caisson  3  comprises an upstream damping chamber  19   a  and a downstream damping chamber  19   b.    
         [0059]     The mode of operation and the advantages of the terminal according to the invention follow directly from the foregoing description.  
         [0060]     The ship  9  wishing to load or offload cargo moors alongside the quay of the floating terminal  3  on the side sheltered from the swell D (see  FIGS. 1 and 2 ).  
         [0061]     The horizontal skirts  15   a ,  15   b  damp vertical oscillation of the terminal.  
         [0062]     The vertical skirts  17   a ,  17   b  damp horizontal oscillation of the terminal.  
         [0063]     Furthermore, the combination of these skirts limits the coefficient of transmission of swell between the upstream and downstream sides of the terminal (the terms “upstream” and “downstream” being understood as meaning with respect to the direction of the swell).  
         [0064]     In the variant represented in  FIG. 2 , the orientation of the floating terminal  1  may be modified at will to optimize the protection from the swell D, from the wind and from the current. The combination of submerged skirts and means for controlling the orientation of the terminal is particularly advantageous.  
         [0065]     All this radically reduces relative movement of the terminal and the ship  9  when the ship is moving alongside and being moored to the caisson  3  and while loading/offloading the cargo of the ship by means of the arms  7   a ,  7   b.    
         [0066]     Thus the floating terminal may be used even if the sea conditions are bad, which makes this terminal more profitable than in the prior art.  
         [0067]     The cargo of the ship  9  may be stored and/or converted on the terminal  1  or be sent directly from ship to land or vice-versa by means of appropriate pipes.  
         [0068]     Of course, the present invention is not limited to the embodiment described and shown, which is provided by way of illustrative and nonlimiting example.