Abstract:
A liquid containment system for an ocean-going vessel. The liquid containment system includes a tank having converging upper walls. The converging upper walls of the tank contain a substantial portion of the liquid therebetween and reduce the free surface area associated with the liquid. The upper converging walls of the tank extend above the horizontal deck of the vessel, but still allow sufficient deck space for supporting various required equipment. In one embodiment, the liquid containment system is a prismatic membrane tank designed to receive and hold liquefied natural gas (LNG).

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to marine transportation of liquids. In another aspect, the invention concerns ocean-going vessels for transporting liquefied natural gas (LNG) over large distances. 
   2. Description of the Prior Art 
   Vessels designed to carry liquefied natural gas (LNG) are among the most expensive commercial cargo-carrying vessels in the world. This is primarily due to the relatively light weight of LNG (requiring a large volume for a given weight of cargo) and the extremely low temperature required to keep the LNG in its liquid state under the low pressures necessary to enable long at-sea transit of commercially viable LNG quantities. LNG is typically transported at or slightly above atmospheric pressure and at a temperature of approximately −260° F. (−160° C.). All LNG containment systems (i.e., tanks) must be constructed of materials which can withstand the extremely low temperatures and the wide temperature changes from ambient conditions to in-service conditions. Further, all tanks must provide effective temperature insulation to prevent heat inflow and unacceptable cooling of the vessel&#39;s basic hull structure. 
   Conventional tanks for carrying LNG aboard ocean-going vessels generally fit into one of the following two categories: (1) “independent tanks,” which are generally self-supporting and rely only upon foundations to transmit the gravitational and other forces of their weight and the weight of their contents to the surrounding hull structure; and (2) “membrane tanks,” which rely entirely upon the surrounding hull structure to maintain their shape and integrity and to absorb all of the hydrostatic forces imposed by their contents. Membrane tanks are generally constructed of either stainless steel or Invar (a high nickel content alloy with minimal thermal expansion characteristics). Membrane tank systems include load-bearing thermal insulation that can transmit the hydrostatic and hydrodynamic loads to the hull structure. 
   A large percentage of LNG tanker-ships in use today include several independent, free-standing spherical tanks lined up along the length of the ship. Each spherical tank is supported by a cylinder or circular ring that is in turn supported by the bottom of the ship&#39;s hull. Spherical tanks, while attractive from the standpoint of maximizing volume-to-surface ratio and equalizing stresses over the surface, have serious drawbacks as cargo tanks. For example, the shape of a spherical tank does not match the shape of the tanker-ship, thereby resulting in wasted space in the hull. This void space near the bottom of the hull forces the center of gravity of the ship upwardly, thereby destabilizing the ship. Spherical tanks typically extend above the deck of the ship, which can dramatically reduce the amount of horizontal deck space available to supporting mooring equipment and other equipment. In addition, the spheres themselves are not free-standing, and so free-standing spherical tank systems include a significant support system. This support system adds both to the cost and the weight of the overall containment system. 
   Prismatic tanks avoid some drawbacks of spherical tanks. A “prismatic” tank is a tank that is shaped to follow the contours of the ship&#39;s hull. At midship the tanks may be in the shape of rectangular solids, with six flat sides (four vertical sides, a top side, and a bottom side). They may also have flat sides that converge downwardly to better match the hull. Free-standing prismatic tanks make more efficient use of below-deck volume than do spherical tanks. However, prismatic tanks contribute significantly to weight and cost because they employ heavy plates and a considerable amount of bracing to keep the plates from distorting under load. Some conventional LNG tanker-ships employ prismatic membrane tanks. Prismatic membrane tanks offer the same space efficiency advantages as independent prismatic tanks, but are typically much lighter than free-standing tanks. 
   When LNG is carried in a tanker-ship, sloshing of the LNG can be problematic because it increases the hydrodynamic loads on the tank, decreases the stability of the ship, and promotes vaporization of the LNG. Sloshing is cause by the movement of the ship and the existence of free surface area of the LNG. Sloshing could be substantially eliminated if it were possible to completely fill the tank with LNG. However, conventional practice is to fill LNG tanks to a maximum of about 98.5% of their full capacity so as to allow for expansion. In addition, it is not economically feasible to fill LNG tanks to 100% capacity because doing so would require a significant decrease in the fill rate of the tank during filling of the final 1–2% of capacity. This decrease in flow rate is required in order to avoid rapid over pressurization of the tank and/or overfilling and leakage through the venting or other systems. The filling of conventional LNG tanks to less than 100% capacity leaves a void space between the surface of the LNG and the top of the tank The resulting free surface area of the LNG allows sloshing to occur and promotes vaporization of the LNG. One way to inhibit sloshing in LNG tanks is to equip the tank with internal baffles. However, the use of anti-sloshing baffles increases the material, construction, and maintenance costs of the tank. 
   OBJECTS AND SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide a high volume liquid containment system for an ocean-going vessel that minimizes sloshing of the liquid without using internal baffles. 
   A further object of the present invention is to provide a high volume liquid containment system for an ocean-going vessel that enhances the stability of the vessel. 
   A still further object of the present invention is to provide a high volume liquid containment system for an ocean-going vessel that minimizes free surface area of liquid contained therein. 
   A yet further object of the present invention is to provide a high volume liquid containment system for an ocean-going vessel that makes efficient use of the volume defined within the hull. 
   Another object of the present invention is to provide a high volume liquid containment system for an ocean-going vessel that maintains the center of gravity of the vessel as low as possible. 
   Still another object of the present invention is to provide a high volume liquid containment system for an ocean-going vessel that is capable of being filled to various levels below its full capacity without causing unacceptable sloshing of the liquid during transportation. 
   Yet another object of the present invention is to provide a tanker-ship having a large amount of horizontal deck space to support mooring equipment and other equipment. 
   Yet still another object of the present invention is to provide a high volume LNG tank that minimizes vaporization of LNG during transportation. 
   It should be understood that these objects are only exemplary. Further objects and advantages of the present invention will be readily apparent upon reading the following detailed description and viewing the drawings. 
   It should be noted that certain systems which do not accomplish all of the above-listed objects may still fall within, and are intended to be encompassed by, the scope of the appended claims. The present invention includes various aspects that are capable of accomplishing one or more of the above listed objects. 
   A first aspect of the present invention provides a ship comprising a non-spherical tank defining a total internal volume. The tank includes at least three upwardly converging walls defining therebetween at least about 10 percent of the total internal volume. 
   A second aspect of the present invention provides a ship comprising a prismatic tank and a deck. The prismatic tank includes a pair of laterally spaced upwardly converging side walls. The deck presents a substantially horizontal upper surface when the ship is upright. At least a portion of the tank extends above the upper surface of the deck. 
   A third aspect of the present invention provides a tanker-ship for transporting a liquid. The ship comprises a tank defining an internal volume for receiving and holding the liquid. The internal volume has a shape which presents a pair of laterally spaced, upwardly converging side faces. Each of the side faces presents an upper edge and lower edge. The internal volume also presents a top face that extends between the upper edges of the converging side faces. The upper and lower edges of the converging side faces are vertically spaced from one another by a minimum vertical distance that is at least 20 percent of the maximum lateral distance between the lower edges of the converging side faces. 
   A fourth aspect of the present invention provides a tanker-ship for transporting LNG. The ship comprises a plurality of individual tanks and a structural deck. Each of the tanks defines a respective total internal volume for receiving and holding a quantity of the LNG. Each of the tanks includes at least three converging walls defining therebetween an upper portion of the internal volume. The upper portion of the internal volume presents at least three substantially planar faces defined by the converging side walls. The upper portion of the total internal volume has a volume that is in the range of from about 20 to about 40 percent of the total internal volume. At least a portion of the total internal volume extends above a substantially horizontal upper surface of the deck. 

   
     BRIEF DESCRIPTION OF THE DRAWING FIGURES 
     A preferred embodiment of the present invention is described in detail below with reference to the attached drawing figures, wherein: 
       FIG. 1  is a side view of a tanker-ship constructed in accordance with the principles of the present invention, particularly illustrating the shape and orientation of a plurality of prismatic tanks received in and supported by the hull of the ship; 
       FIG. 2  is a top view of the tanker-ship shown in  FIG. 1 , particularly illustrating the arrangement of the mooring equipment supported on the deck of the ship; 
       FIG. 3  is a sectional view of the tanker-ship taken along line  3 — 3  in  FIG. 2 , particularly illustrating the upwardly converging side walls of the tank, the liquid disposed within the internal volume defined by the tank, and the extension of the internal volume above the upper surface of the deck; 
       FIG. 4   a  is an isometric view illustrating the shape of the internal volume defined by the tanks of  FIGS. 1–3 , particularly illustrating a broad lower portion of the internal volume and an upwardly narrowing upper portion of the internal volume, with the upper portion presenting two converging side faces and a sloped front face; 
       FIG. 4   b  is a side view of the internal volume shown  FIG. 4   a , particularly illustrating the slope of the front face, the vertical orientation of the rear face, and the length of the internal volume; 
       FIG. 4   c  is an end view of the internal volume shown in  FIG. 4   a , particularly illustrating the slope of the side faces, the width of the internal volume, and the relative heights of the upper and lower portions of the internal volume; 
       FIG. 4   d  is a top view of the internal volume shown in  FIG. 4   a , particularly illustrating the length and width of the internal volume; 
       FIG. 5   a  is an isometric view of an alternative internal volume that can be defined by a tank having an alternative configuration, particularly illustrating that the upper portion of the internal volume has four upwardly converging faces, as opposed to the three upwardly converging faces of the internal volume illustrated in  FIGS. 4   a–d;    
       FIG. 5   b  is a side view of the internal volume shown in  FIG. 5   a;    
       FIG. 5   c  is a end view of the internal volume shown in  FIG. 5   a;    
       FIG. 5   d  is a top view of the internal volume shown in  FIG. 5   a;    
       FIG. 6   a  is an isometric view of an alternative internal volume that can be defined by a tank having an alternative configuration, particularly illustrating that the upper portion of the internal volume has only two upwardly converging faces, as opposed to the three upwardly converging faces of the internal volume illustrated in  FIGS. 4   a–d;    
       FIG. 6   b  is a side view of the internal volume shown in  FIG. 6   a;    
       FIG. 6   c  is a end view of the internal volume shown in  FIG. 6   a ; and 
       FIG. 6   d  is a top view of the internal volume shown in  FIG. 6   a.    
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring initially to  FIGS. 1 and 2 , a tanker-ship  10  is illustrated as generally comprising a hull  12 , a structural deck  14 , and a plurality of tanks  16 . Tanker-ship  10  can be any type of ocean-going vessel designed to carry a load of liquid over large distances. Preferably, tanker-ship  10  is a liquefied natural gas carrier (LNGC) that is equipped to transport liquefied natural gas (LNG) at low temperatures (e.g., about −260° F.) and at approximately atmospheric pressure. Tanks  16  are received in and supported by hull  12 . Deck  14  extends across the top of hull  12  and presents a substantially planar, substantially horizontal, exposed upper surface  18 . It is preferred for a portion of tanks  16  to extend above upper deck surface  18 . 
   Tanker-ship  10  can also include mooring equipment supported on upper surface  18  of deck  14 . The mooring equipment generally includes a mooring winch  20 , a mooring line  22 , and a bit  24 . It is preferred for mooring winch  20  to be spaced from the sides of tanker-ship  10  in order to provide a greater length of the mooring line  22 . A longer mooring line provides for safer mooring of tanker-ship  10  because mooring line  22  is resilient and allows for some movement between tanker-ship  10  and the dock (not shown). Short mooring lines create a more rigid connection between the dock and tanker-ship  10 . Such a rigid connection can damage tanker-ship  10  and/or the dock if an outside force (e.g., wind and waves) urges relative movement between tanker-ship  10  and the dock. Thus, it is preferred for mooring winch  20  to be located at about the longitudinal center line of ship  10 , with bit  24  being located proximate the side of ship  10 . Preferably, at least one mooring winch  20  is located on the substantially horizontal upper surface  18  of deck  14  between the portions of adjacent tanks  16  that extend above upper deck surface  18 . In addition, a rear super structure  26  extends upwardly from deck  14  behind tanks  16 . Rear super structure  26  includes an aft bridge  28 , which should be sufficiently elevated above upper deck surface  18  so as to provide visibility over the portions of tanks  16  that extend above upper deck surface  18 . 
   Referring now to  FIG. 3 , tank  16  defines an internal volume  30  for receiving and holding a liquid  32 . As illustrated in  FIG. 3 , tank  16  is a prismatic tank that conforms generally to the shape of hull  12 . It is preferred for tank  16  to be a prismatic membrane tank configured to receive and hold LNG. Prismatic membrane tanks are well-known in the art and generally include a liquid-impermeable membrane (e.g., stainless steel or Invar) defining the internal volume of the tank and a load-bearing insulation system that transfers the hydrostatic and hydrodynamic forces of the liquid to the hull. While the present invention is particularly well-suited for LNG tanker-ships employing prismatic membrane tanks, it should be understood that the invention also provides advantages when used in non-LNG transportation and/or when used with independent (i.e., free-standing) tanks. 
   As shown in  FIG. 3 , which is a cross-sectional view taken orthogonally to the direction of elongation of tanker-ship  10 , upper deck surface  18  extends outwardly on opposite sides of tank  16 . The cross-section of  FIG. 3  is taken at a location which shows the minimum width of upper deck surface  18  along the longitudinal axis of tanker-ship  10  where tanks  16  are present. It is preferred for the cumulative width of upper deck surface  18  (i.e., the combined width of upper deck surface  18  on both sides of tank  16 ) to be at least about 25 percent as wide as the total width of the ship at all locations where a cross-section that is orthogonal to the direction of elongation of tanker-ship  10  and that extends through tanks  16  can be taken, more preferably the cumulative width of upper deck surface  18  is in the range of from about 35 to about 75 percent of the maximum width of the ship at such locations. This minimum width of upper deck surface  18  ensures that enough horizontal space will be provided for supporting various equipment (e.g., reliquefaction equipment and/or mooring equipment). As shown in  FIG. 2 , it is preferred for upper surface  18  of deck  14  to circumscribe tanks  16  at the locations where tanks  16  protrude upwardly from upper surface  18  . 
   Referring again to  FIG. 3 , internal volume  30  defined by tank  16  generally includes a relatively broad lower portion  34  and an upwardly narrowing upper portion  36 . The fact that upper portion  36  of internal volume  30  is narrower at the top than at the bottom reduces the free surface area  38  of liquid  32 . This reduction in free surface area  38 , reduces sloshing of liquid  32  within tank  16 . The reduction of sloshing can provide a more stable vessel without requiring internal baffles. In addition, when liquid  32  is LNG, the reduction in free surface area  38  and the reduction in sloshing can help to minimize vaporization of the LNG. Further, when tank  16  is a membrane tank, the reduction of the sloshing can help to prevent damage to the membrane. 
   Referring to  FIGS. 1–3 , upper portion  36  of internal volume  30  is defined between a pair of laterally spaced, upwardly converging side walls  40 , a front wall  42 , and a rear wall  44  of tank  16 . When used to describe the configuration of tanker-ship  10 , the term “laterally” shall denote a direction that is perpendicular to the axis of elongation of the ship  10 . A cap  46  of tank  16  is coupled to and extends laterally across the uppermost edges of side walls  40  to thereby define the top of internal volume  30 . Lower portion  34  of internal volume  30  has a fairly conventional configuration being defined by a pair of vertical or slightly downwardly converging side walls  48 , a pair of vertical end walls  50 , and extending base  52 . 
   Some conventional prismatic tanks included short, upwardly converging side walls at the top of the vertical sidewalls. However, the upwardly converging side walls of these conventional prismatic tanks do not extend nearly as far upward as side walls  40  of the inventive tank  16 . Therefore, such conventional tanks do not adequately minimize free surface area and do not allow a significant portion of the liquid to be contained between the converging side walls. With respect to inventive tank  16 , it is preferred for the volume of upper portion  36  to be at least about 10 percent of the total volume of internal volume  30 , more preferably at least about 15 percent of the total volume, still more preferably in the range of from about 20 to about 40 percent of the total volume, and most preferably in the range of from 25 to 35 percent of the total volume. It is also preferred for the volume of lower portion  34  to be in the range of from about 60 to about 90 percent of the total volume of internal volume  30 , most preferably in the range of from 75 to 85 percent of the total volume. 
   Referring now to  FIGS. 1–3  and  4   a–c , upper portion  36  of internal volume  30  presents a pair of upwardly converging side faces  54  that are defined by the inner surface of side walls  40 . Upper portion  36  also presents a front and rear faces  56 , 58  that are defined by the inner surfaces of front and rear walls  42 ,  44  respectively. In addition, upper portion  36  presents a top face  60  that is defined by the inner surface of cap  46 . In the description that follow, the shape of internal volume  30  is defined in detail. It should be understood that a description of the shape of internal volume  30  inherently describes the shape of tank  16  because each face of internal volume  30  is defined by an inner surface of tank  16 . 
   Referring to  FIGS. 4   a–d , it is preferred for the side, front, rear, and top faces  54 ,  56 ,  58 ,  60  to be substantially planar. Each of the side faces  54  presents a front edge  62 , a rear edge  64 , a top edge  66 , and a bottom edge  68 . Front face  56  extends between front edges  62 , rear face  58  extends between rear edges  64 , and top face  60  extends between top edges  66 . It should be understood that edges  62 ,  64 ,  66 ,  68  can be somewhat rounded. It is preferred for top edges  66  to extend substantially parallel to one another and for bottom edges  68  to extend substantially parallel to one another. However, when internal volume  30  is defined within a tank that is located near the front or rear of the ship, top edges  66  and bottom edges may need to be skewed to conform to the shape of the hull. Referring to  FIG. 4   c , it is preferred for side faces  54  to extend upwardly at an angle (Θ) that is at least about 20 degrees from horizontal, more preferably at an angle (Θ) in the range of from about 30 to about 60 degrees, and most preferably at an angle (Θ) in the range of 40 to 50 degrees. Referring to  FIG. 4   b , it is preferred for front face  56  to extend upwardly at an angle (Φ) that is at least about 10 degrees from horizontal, more preferably at an angle (Φ) in the range of from about 15 to about 60 degrees, and most preferably at an angle (Φ) in the range of 20 to 45 degrees. It is preferred for rear face  58  to extend substantially vertically and for top face  60  to extend substantially horizontally. 
   As shown in  FIGS. 4   a–d , internal volume  30  has a width (W), a length (L), a total height (H T ), a height of the upper portion (H U ), and a height of the lower portion (H L ). It is preferred for internal volume  30  to have a length (L) that is greater than its width (W), most preferably the ratio of length (L) to width (W) is in the range of 1.25:1 to 2:1. It is preferred for internal volume  30  to have a ratio of total height (H T ) to width (W) that is in the range of from about 0.5:1 to about 2:1, most preferably in the range of 0.75:1 to 1.5:1. It is preferred for the height of the upper portion (H U ) to be at least about 25 percent of the total height (H T ) of internal volume  30 , more preferably at least about 35 percent of the total height (H T ), still more preferably in the range of from about 40 to about 75 percent of the total height (H T ), and most preferably in the range of 50 to 60 percent of the total height (H T ). It is preferred for the height of the lower portion (H L ) to be in the range of from about 25 to about 75 percent of the total height (H T ), most preferably in the range of 40 to 60 percent of the total height (H T ). It is preferred for the height of the upper portion (H U ) to be at least about 20 percent of the maximum width (W) of internal volume  30 , more preferably the height of upper portion (H U ) is in the range of from about 30 to about 70 percent of the maximum width (W) of internal volume  30 , and most preferably in the range of 40 to 60 percent of the maximum width (W). 
   Referring to  FIGS. 3 and 4   a–d , it is preferred that side walls  40  do not converge into contact with one another to thereby form a point. Thus, it is preferred for side walls  40  to be spaced by a minimum distance (i.e., the width of top face  60 ) that is at least about 5 percent of the maximum width (W) of internal volume  30 , more preferably in the range of from about 5 to about 50 percent of the maximum width (W), and most preferably in the range of 10 to 25 percent of the maximum width (W). Converging side walls  40 , as well as front wall  42  causes the free surface area  38  of liquid  32  to be substantially less at the top of upper portion  36  than at the bottom of upper portion  36 . The free surface area at various vertical locations in the internal volume  30  can be defined by the area of a horizontal plane extending though internal volume  30  and bounded by the outer faces of internal volume  30 . It is preferred for the free surface area at the vertical location of upper edges  66  to be less than about 75 percent of the free surface area at the vertical location of bottom edges  68 , more preferably less than about 50 percent of the free surface area at bottom edges  68 , and most preferably less than 25 percent of the free surface area at bottom edges  68 . 
   Referring to  FIG. 3 , upper deck surface  18  preferably defines a substantially horizontal plane that intersects side walls  40  of tank  16 . It is preferred for at least about 2 percent of internal volume  30  to be disposed at a vertical elevation above upper deck surface  18 , most preferably 5 to 20 percent of internal volume  30  is disposed above the vertical elevation of upper deck surface  18 . 
   Referring now to  FIGS. 5   a–d , an alternatively configured internal volume  100  is illustrated. Internal volume  100  has a similar shape to internal volume  30 , described above with reference to  FIGS. 4   a–d , except that internal volume  100  includes a rear face  102  that is sloped rather than vertical. It is preferred for rear face  102  to have substantially the same slop as front face  56  of internal volume  30 , described above. Thus, upper portion  104  of internal volume  100  presents four upwardly converging faces. 
   Referring now to  FIGS. 6   a–d , an alternatively configured internal volume  200  is illustrated. Internal volume  200  has a similar shape to internal volume  30 , described above with reference to  FIGS. 4   a–d , except that internal volume  200  includes a front face  202  that is substantially vertical. Thus, upper portion  204  of internal volume  200  presents only two upwardly converging faces (i.e., the side faces). 
   The preferred forms of the invention described above are to be used as illustration only, and should not be used in a limiting sense to interpret the scope of the present invention. Obvious modifications to the exemplary embodiments, set forth above, could be readily made by those skilled in the art without departing from the spirit of the present invention. 
   The inventors hereby state their intent to rely on the Doctrine of Equivalents to determine and assess the reasonably fair scope of the present invention as it pertains to any apparatus not materially departing from but outside the literal scope of the invention as set forth in the following claims.