Patent Publication Number: US-10767815-B2

Title: Tank arrangement

Description:
RELATED APPLICATION 
     This application claims priority as a continuation application under 35 U.S.C. § 120 to PCT/FI2016/050306, which was filed as an International Application on May 10, 2016, designating the U.S., the entire content of which is hereby incorporated by reference in its entirety. 
    
    
     FIELD 
     The present disclosure relates to a tank arrangement for storing liquefied natural gas. 
     BACKGROUND INFORMATION 
     Natural gas, and mixtures of hydrocarbons that are volatile enough to make the mixture appear in gaseous form in room temperature, can constitute an advantageous alternative to fuel oil as the fuel of internal combustion engines. In sea-going vessels that use natural gas as fuel, the natural gas can be stored onboard in liquid form, giving rise to the commonly used acronym LNG (Liquefied Natural Gas). Natural gas can be kept in liquid form by maintaining its temperature below a boiling point, which is approximately −162 degrees centigrade. LNG can be stored at a pressure that is close to the atmospheric pressure, but large tanks used for storing LNG need to withstand significant hydrostatic pressures and a certain overpressure. For achieving good mechanical strength, LNG tanks can be constructed as cylindrical or spherical contain-ers. However, for practical reasons, large LNG tanks are sometimes designed as bilobe or multilobe tanks instead of cylindrical tanks. A bilobe tank includes two mating curved halves, for instance two spherical caps or two cylindrical segments. A multilobe tank includes at least three curved sections that are joined to each other. The sections can be partial cylinders or spheres. 
     Due to the low temperature desired for storing LNG, the dimensions of a tank that is in use and an empty tank that can be at a temperature of +30° C. can differ significantly from each other. This makes supporting of the LNG tanks challenging especially in ships, where it is desirable to firmly attach the tank to the hull of the ship. 
     SUMMARY 
     A tank arrangement is disclosed for storing liquefied natural gas, the arrangement comprising: a multilobe tank having a longitudinal axis defining a longitudinal direction and a lateral axis defining a lateral direction, the tank including at least three tank sections arranged in a row in the lateral direction, each tank section being supported against a support surface with a first support supporting the tank in vertical direction and allowing movement of the tank relative to the support surface in the longitudinal direction, and with a second support supporting the tank in the vertical direction, at least one of the second supports preventing movement of the tank relative to the support surface in the longitudinal direction; and one of the tank sections including a third support, which supports the tank in the vertical direction and allows movement of the tank relative to the support surface in the longitudinal direction and prevents movement in the lateral direction, wherein the first and second supports are configured to allow shrinkage and expansion of the tank in the lateral direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other features and advantages of the present disclosure will be appreciated from exemplary embodiments as disclosed herein with reference to the accompanying drawings, in which: 
         FIG. 1  shows a cross-sectional view of an exemplary ship having an LNG tank arrangement; 
         FIG. 2  shows a top view of the exemplary tank arrangement of  FIG. 1 ; 
         FIG. 3  shows a side view of the exemplary tank arrangement; 
         FIG. 4  shows a side view of the exemplary LNG tank with supports; 
         FIG. 5  shows an exemplary support for the tank; 
         FIG. 6  shows an end view of the tank; and 
         FIG. 7  shows a top view of the tank with supports. 
     
    
    
     DETAILED DESCRIPTION 
     An improved tank arrangement is disclosed for storing liquefied natural gas. 
     An exemplary arrangement according to the present disclosure includes a multilobe tank having a longitudinal axis defining a longitudinal direction and a lateral axis defining a lateral direction, the tank including at least three tank sections arranged in a row in the lateral direction, wherein each tank section is supported against a support surface with a first support allowing movement of the tank relative to the support surface in the longitudinal direction and with a second support, at least one of the second supports preventing movement of the tank relative to the support surface in the longitudinal direction. One of the tank sections is further provided with a third support, which allows movement of the tank relative to the support surface in the longitudinal direction and prevents movement in the lateral direction, the first and second supports being configured to allow shrinkage and expansion of the tank in the lateral direction. 
     With the exemplary tank arrangement according to the present disclosure, expansion and shrinkage of the tank is allowed, but the tank is fixed to a supporting surface both in the longitudinal direction and the lateral direction. 
     According to an exemplary embodiment of the present disclosure, all the second supports of the arrangement are configured to prevent movement of the tank relative to the support surface in the longitudinal direction. Longitudinal forces are thus carried by several supports. 
     According to an exemplary embodiment of the present disclosure, each of the first supports allows movement of the tank in the lateral direction. This ensures that the tank can shrink and expand both in the lateral and the longitudinal direction. Alternatively, the first support of the tank section that is supported by the third support is configured to prevent movement of the tank in the lateral direction and the first supports of other tank sections are configured to allow movement of the tank in the lateral direction. One of the first supports can fix the tank in the lateral direction, and the tank can still expand and shrink in the lateral direction. 
     The second supports can be configured in a similar way. Thus, each of the second supports can allow movement of the tank in the lateral direction. Alternatively, the second support of the tank section that is supported by the third support can be configured to prevent movement of the tank in the lateral direction and the second supports of other tank sections can be configured to allow movement of the tank in the lateral direction. 
     According to an exemplary embodiment of the present disclosure, the third support is arranged to support the centermost tank section. The tank thus shrinks symmetrically about the longitudinal center line of the tank and the center of gravity does not change. 
     According to an exemplary embodiment of the present disclosure, the tank section that is provided with the third support is further provided with a fourth support which allows movement of the tank relative to the support surface in the longitudinal direction and prevents movement in the lateral direction. The fourth support prevents spinning of the tank about a vertical axis. 
     According to an exemplary embodiment of the present disclosure, the first support and the second support are arranged between the third support and fourth support in the longitudinal direction. By placing the third support and the fourth support close to the ends of the tank, smaller lateral forces are needed for preventing the tank from spinning about a vertical axis. 
     An exemplary sea going vessel according to the present disclosure includes a tank arrangement as described herein. 
     According to an exemplary embodiment of the present disclosure, the longitudinal axis of the tank is parallel to the longitudinal axis of the vessel. 
       FIGS. 1 to 3  show an exemplary LNG tank arrangement of a ship  2 . The arrangement includes an LNG tank  1 . The LNG tank  1  is a container that is configured to store liquefied natural gas. Natural gas is kept in liquid form by maintaining its temperature below a boiling point, which is approximately −162 degrees centigrade. The LNG tank  1  is located in a tank hold  3 , which is located around the longitudinal center line of the ship  2 . The LNG tank  1  stores liquefied gas that is used as fuel in one or more engines of the ship  2 . 
     The LNG tank  1  can be a single shell structure. The space holding the LNG is formed by a shell  6  that is made of a cold resistant material. The expression “cold resistant material” refers to a material that can withstand the temperature of liquefied natural gas. Minimum design temperature of the material should be at most −165° C. The material can be, for instance, stainless steel. Suitable materials are, for instance, 9% nickel steel, low manganese steel, austenitic steels, such as types 304, 304L, 316, 316L, 321 and 347 and austenitic Fe—Ni alloy (36% nickel). An insulation layer  7  is arranged around the shell  6 . The insulation layer  7  can be made of, for instance, polyurethane. 
     The LNG tank  1  can be a multilobe tank. The expression “multilobe tank” refers here to a tank that includes at least three sections that have a curved cross-sectional profile and which are joined to each other such that the shell  6  of the tank  1  has an undulating shape at least on two sides. In the exemplary embodiments of the figures, the LNG tank  1  includes five sections  1   a ,  1   b ,  1   c ,  1   d ,  1   e  each having the shape of a partial cylinder. The longitudinal center lines of the sections  1   a ,  1   b ,  1   c ,  1   d ,  1   e  are parallel to each other. The centermost section  1   c  has a shape that is formed by cutting a segment from a horizontal cylinder by two vertical planes. The other sections  1   a ,  1   b ,  1   d ,  1   e  each have a shape that is formed by cutting a segment from a horizontal cylinder by one vertical plane. The exemplary sections  1   a ,  1   b ,  1   c ,  1   d ,  1   e  of the tank  1  are arranged in a row in a horizontal plane. The tank  1  has thus an undulating upper surface and an undulating lower surface. The outermost sections  1   a ,  1   e  are shorter than the three sections  1   b ,  1   c ,  1   d  in the middle of the LNG tank  1 . The ends of the sections  1   a ,  1   b ,  1   c ,  1   d ,  1   e  are closed by end caps  4   a ,  4   b ,  4   c ,  4   d ,  4   e ,  5   a ,  5   b ,  5   c ,  5   d ,  5   e . The end caps can have a shape of a spherical cap or part of a spherical cap. 
     The exemplary tank  1  has a longitudinal axis  19  defining a longitudinal direction and a lateral axis  20  defining a lateral direction, which can be best seen in  FIG. 7 . The tank  1  has a first end  27  and a second end  28 . The tank  1  is supported against a support surface  21 , which can be formed by the hull of a ship  2 , with supports that support the tank  1  in the vertical direction and also limit movements of the tank  1  in the horizontal plane. The support arrangement is described by referring in particular to  FIGS. 4 to 7 . 
     Each section  1   a ,  1   b ,  1   c ,  1   d ,  1   e  of the tank  1  can be supported against the support surface  21  with a first support  22  and a second support  23 . The first support  22  is arranged closer to the first end  27  and the second support  23  is arranged closer to the second end  28  of the tank  1 . The first supports  22  form a first row of supports and the second supports  23  form a second row of supports. Each of the first supports  22  and the second supports  23  has a curved upper surface that is configured to the shape of the lower surface of the corresponding tank section  1   a ,  1   b ,  1   c ,  1   d ,  1   e.    
     Due to thermal expansion, the dimensions of the tank  1  may vary significantly. For instance, when the exemplary tank  1  is in use, its temperature should be approximately −162° C. An empty tank  1  can have a temperature of +30° C. To allow expansion and shrinkage of the tank  1 , one row of supports is configured to allow movements of the tank  1  in the longitudinal direction. In the exemplary embodiments of the Figures, the first supports  22  allow movements of the tank  1  in the longitudinal direction. The first supports  22  also allow movements of the tank  1  in the lateral direction, which allows expansion of the tank  1  in that direction. However, one of the first supports  22  could also fix the tank  1  in the lateral direction, and the tank  1  could still shrink and expand. 
     The second supports  23  are configured to prevent movements of the tank  1  in the longitudinal direction. The position of the tank  1  can thus be fixed in the longitudinal direction, although the first end  27  and the second end  28  are allowed to move in the longitudinal direction outwards from the fixing points defined by the second supports  23 . It is not necessary that all the second supports  23  prevent movements of the tank  1  in the longitudinal direction, but it can be sufficient that one of the second supports  23  prevents longitudinal movements of the tank  1 . If only one of the second supports  23  prevents longitudinal movements, it is for example, the second support  23  supporting the centermost tank section  1   c . If two or more second supports  23  prevent longitudinal movements, they can be, for example, arranged symmetrically about the longitudinal center line of the tank  1 . All second supports  23 , or all except one of the second supports  23 , also can allow movements of the tank  1  in the lateral direction. The tank  1  can thus freely expand in the lateral direction. 
     One of the second supports  23  can fix the tank  1  in the lateral direction, but that is not necessary. If one of the second supports  23  and one of the first supports  22  fix the tank  1  in the lateral direction, they should support, for example, the same tank section. 
     The exemplary tank arrangement can be provided with a third support  24 . The third support  24  allows movements of the tank  1  in the longitudinal direction but prevents movements in the lateral direction. The third support  24  thus fixes the tank  1  in the lateral direction. Since the lateral position of the tank  1  is fixed by the third support  24 , all the first supports  22  and second supports  23  can allow movements of the tank  1  in the lateral direction. In the exemplary embodiment of the Figures, the third support  24  is arranged to support the centermost tank section  1   c . The position of the longitudinal center line of the tank  1  is thus stationary and the tank  1  expands and shrinks around the center line  1 . This can be advantageous especially in ships, since the position of the center of gravity of the tank  1  does not change. 
     In the exemplary embodiments of the Figures, the tank arrangement is further provided with a fourth support  25 . Also the fourth support  25  allows movements of the tank  1  in the longitudinal direction but prevents movements in the lateral direction. The fourth support  25  is arranged to support the same tank section as the third support  24 . The fourth support  25  ensures that the tank  1  does not spin about a vertical axis. The third support  24  is arranged close to the first end  27  of the tank  1  and the fourth support  25  is arranged close to the second end  28  of the tank  1 . The first support  22  and the second support  23  are thus between the third support  24  and the fourth support  25  in the longitudinal direction of the tank  1 . By placing the third support  24  and the fourth support  25  close to opposite ends  27 ,  28  of the tank  1 , smaller lateral support forces are needed to prevent spinning of the tank  1  about the vertical axis. 
       FIG. 5  shows an example of a support that can be used as the third support  24  or the fourth support  25 . The support  24 ,  25  can include a support element  30  that can be attached to the support surface  21 . A rail  29  is supported against the support element  30  so that the rail  29  can move in the longitudinal direction of the support element  30 . Lateral forces exerted on the rail  29  are carried by side supports  32  that are arranged on both sides of the rail  29 . A reinforcement plate  31  is attached to an upper surface of the rail  29 . The reinforcement plate  31  is attached to the bottom of one of the tank sections  1   a ,  1   b ,  1   c ,  1   d ,  1   e . The reinforcement plate  31  is welded to the shell  6  of the tank  1  and functions as an additional reinforcement member. 
     It will be appreciated by a person skilled in the art that the invention is not limited to the embodiments described above, but may vary within the scope of the appended claims. For instance, the tank can include only three sections, or more than five sections. 
     Thus, It will be appreciated by those skilled in the art that the present invention can be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The presently disclosed embodiments are therefore considered in all respects to be illustrative and not restricted. The scope of the invention is indicated by the appended claims rather than the foregoing description and all changes that come within the meaning and range and equivalence thereof are intended to be em-braced therein.