Abstract:
A structural block is fabricated with necessary components and structure for LNG storage and/or process. The structural block may be an LNG tank with the same configuration as that in an LNG carrier. The structural block may also be a regasification plant or a liquefaction plant to be used for LNG process. An existing vessel, e.g. an LNG carrier, is cut apart to form a forward section and an aft section. The forward section and the aft section are moved away from each other to form a space therebetween. The structural block is then placed into the space and jointed to the forward and aft sections, by welding for example, to form an integrated new vessel. The structural block provides the new vessel with increased LNG storage and transportation capabilities as well as regasification and/or liquefaction process facilities to meet the increased demand for LNG storage, transportation and processing.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a method for vessel construction and in particular, to a method for vessel modification to increase the storage and/or deck space capacity of existing vessels to serve similar or a different purposes. 
       BACKGROUND 
       [0002]    There are existing Liquid Natural Gas (LNG) Carriers in the market serving as LNG transportation vessels, and some of them are approaching the end of the charterer period, and/or available for life-extension program for continuous services as the LNG Carriers or other purposes. LNG Carriers are usually well-maintained vessel and in particularly the LNG storage tanks are still at good conditions. These existing carriers are mainly of 4 types of containment systems:
       1) IGC Independent Tank Type-B MOSS System (Spherical Dome)   2) IGC Independent Tank Type-B SPB System (IHI)   3) Membrane Type LNG Containment System (GTT)   4) IGC Independent Tank Type-C Pressurized System       
 
         [0007]    Similar to FPSO conversion from crude oil tanker, there are markets for these existing LNG carriers to be converted into offshore or near-shore oil and gas processing facilities, such as LNG export or import facilities, as they term as FLNG (Floating Liquefied Natural Gas) units and FSRU (Floating Storage &amp; Regasification Unit). 
         [0008]    However, some of these existing LNG carriers have their limitation to be redeployed for these purposes and thus there exists a need for technical solutions. Some of these limitations are related to lacking of available space and/or storage capacities on existing LNG carrier to better serve its new purposes. For example, MOSS Type LNG carriers has very limited deck space available for more gas processing facilities, and this imposes limitation for its suitability to be converted into FLNG; some of the earlier generation of LNG carriers are also lesser in LNG storage capacities, e.g. below 130,000 m 3  which are less ideal as FSRU receiving terminal. 
         [0009]    As for existing LNG carriers, these vessels have been optimized to have maximum storage and transportation efficiency. Existing LNG carriers have fully utilized the available space of the vessel for the LNG storage tanks, hence there is no enough space to further increase the storage capacity on a given vessel. In addition, such vessels do not have facilities required to process the LNG, e.g. the regasification process facility and/or liquefaction process facility, as these are conventionally carried out at the offshore or near shore terminals. These existing LNG carriers are also usually less efficient than the newer generation bigger LNG carriers, which are built with larger LNG storage capacities with higher transportation efficiencies. This also means that the older and smaller capacity yet still functional vessels are at competitive disadvantage even if they continue to function for LNG transportation as compare with the newer generation of LNG carriers. These existing LNG carriers may be converted to be used as FLNG or FSRU, however in some cases, limited due to storage space constraints. 
       SUMMARY OF THE INVENTION 
       [0010]    To meet higher LNG storage capacity requirement and deck space for additional gas processing facilities, an added structural section is proposed to be integrated with the existing LNG carriers, for additional storage and deck space requirement. 
         [0011]    Embodiments of the present invention provide solutions for existing LNG carriers to be reused for LNG storage and transportation beyond their original capacity. In addition, embodiments of the present invention provide solutions to enable existing LNG carriers to perform LNG process functions such as regasification and/or liquefaction. 
         [0012]    According to one embodiment, a structural block is prefabricated with necessary full set or partial set of the components and structure for LNG storage and/or treatment processes. The structural block may contain one or more LNG tank(s) with the same or different configuration or containment system as that of the existing LNG carrier. For example, where the existing LNG carrier is a MOSS type LNG carrier, the added LNG storage can be either MOSS, or other types, i.e. Membrane, SPB or Type-C tanks. The structural block may also contain a regasification plant or a liquefaction plant with related gas processing plant and function as FLNG or FSRU together with the LNG storage tanks on the vessel. The structural block can be part of second existing LNG carrier which may contain LNG tank with the same or different configuration or containment system as that of the first existing LNG carrier. In this case only LNG tank from second existing LNG carrier can utilize with new structural block but hull structure should match as per the first existing LNG carrier. 
         [0013]    An existing LNG carrier is cut apart to form a forward section and an aft section. The fabrication of the new structural block may be carried out at a shipyard, but during the fabrication of the structural block, the LNG vessel needs not be present at the shipyard. Alternatively, fabrication of the structural block may be carried out at a workshop or a fabrication area, which is independently operable from a shipyard used for cutting the LNG carrier. In either situation, this will allow the LNG carriers to continue to operate as LNG transportation vessel, during the structural block fabrication period. One or more structural blocks may be fabricated based on optimized working schedule, construction sequence and coordination with the LNG carrier work process. 
         [0014]    Upon near completion of the structural block, the existing LNG carrier will be dry-docked for cutting of the hull for vessel assembly. When the existing hull is separated into forward and aft sections, either one or both sections are moved away from each other to form a space there between. The new structural block is then placed into the space and jointed to the forward and aft sections, by welding for example, to form an integrated new vessel. The structural block provides the new vessel with increased LNG storage and space capacity, to meet the increased demand or new functions. In embodiments where the structural block includes a regasification plant and/or liquefaction plant, relevant LNG-process functions are added to the existing LNG carrier which originally possesses only the LNG storage and transportation capabilities. Embodiments of the present invention can provide the following advantages:
   1) The existing vessels are only needed during vessel assembly period in the yard, which minimizes the period for the vessel to be present in a shipyard. This will maximize the utililization of the LNG Carriers;   2) The new integrated vessel can still be functional as LNG Carrier, and at the same time capable of utilizing it as FLNG or FSRU. This makes the new units flexible for multiple deployment, for example LNG-RV (LNG Regasification Vessel) which can cross function as FSRU and as LNG Carriers.   3) The new integration vessel contains different LNG containment systems, which may give further advantage to allow deck-space, and/or flexibility in terms of cargo operation. For example, they may serve as pressurized storage tanks for better BOG (Boil-off-Gas) handling.   4) This may also allow the new structure block and vessel assembly to be done separately at most effective arrangement, and transported for final integration. The new structure block with LNG processes (Liqufaction and/or Regasification) with related gas processing modules maybe able to be fully tested and commissioned, as much as possible, prior to cutting of the vessel. This allows added advantage to reduce risk and unnecessary downtime of the LNG carriers.   
 
         [0019]    Other aspects and advantages of the present invention will become apparent from the following detailed description, illustrating by way of example the inventive concept of the present invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    These and other aspects of the present invention will be described in detail with reference to the accompanying drawings, in which: 
           [0021]      FIG. 1A  is a partial cross sectional side view of an existing LNG carrier; 
           [0022]      FIG. 1B  is a partial top view of  FIG. 1A ; 
           [0023]      FIG. 1C  is a block diagram showing a method of LNG carrier construction according to one embodiment of the present invention; 
           [0024]      FIG. 2A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to one embodiment of the present invention; 
           [0025]      FIG. 2B  is a partial top view of  FIG. 2A ; 
           [0026]      FIG. 2C  is a partial cross sectional side view of a LNG carrier constructed according to the method shown in  FIGS. 2A and 2B ; 
           [0027]      FIG. 2D  is a partial top view of  FIG. 2C ; 
           [0028]      FIG. 3A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to an alternative embodiment of the present invention; 
           [0029]      FIG. 3B  is a partial top view of  FIG. 3A ; 
           [0030]      FIG. 3C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 3A and 3B ; 
           [0031]      FIG. 3D  is a partial top view of  FIG. 3C ; 
           [0032]      FIG. 4A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to an alternative embodiment of the present invention; 
           [0033]      FIG. 4B  is a partial top view of  FIG. 4A ; 
           [0034]      FIG. 4C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 4A and 4B ; 
           [0035]      FIG. 4D  is a partial top view of  FIG. 4C ; 
           [0036]      FIG. 5A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to an alternative embodiment of the present invention; 
           [0037]      FIG. 5B  is a partial top view of  FIG. 5A ; 
           [0038]      FIG. 5C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 5A and 5B ; 
           [0039]      FIG. 5D  is a partial top view of  FIG. 5C ; 
           [0040]      FIG. 6A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to an alternative embodiment of the present invention; 
           [0041]      FIG. 6B  is a partial top view of  FIG. 6A ; 
           [0042]      FIG. 6C  is a partial cross sectional side view of a LNG carrier constructed according to the method shown in  FIGS. 6A and 6B ; 
           [0043]      FIG. 6D  is a partial top view of  FIG. 6C ; 
           [0044]      FIG. 7A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier of  FIG. 1A  according to a further embodiment of the present invention; 
           [0045]      FIG. 7B  is a partial top view of  FIG. 7A ; 
           [0046]      FIG. 7C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 7A and 7B ; 
           [0047]      FIG. 7D  is a partial top view of  FIG. 7C ; 
           [0048]      FIG. 8A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier according to a still further embodiment of the present invention; 
           [0049]      FIG. 8B  is a partial top view of  FIG. 8A ; 
           [0050]      FIG. 8C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 8A and 8B ; 
           [0051]      FIG. 8D  is a partial top view of  FIG. 8C ; 
           [0052]      FIG. 9A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier according to another further embodiment of the present invention; 
           [0053]      FIG. 9B  is a partial top view of  FIG. 9A ; 
           [0054]      FIG. 9C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 9A and 9B ; 
           [0055]      FIG. 9D  is a partial top view of  FIG. 9C ; 
           [0056]      FIG. 10A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier according to another further embodiment of the present invention; 
           [0057]      FIG. 10B  is a partial top view of  FIG. 10A ; 
           [0058]      FIG. 10C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 10A and 10B ; 
           [0059]      FIG. 10D  is a partial top view of  FIG. 10C . 
           [0060]      FIG. 11A  is a partial cross sectional side view showing construction of an LNG carrier using an existing LNG carrier according to another further embodiment of the present invention; 
           [0061]      FIG. 11B  is a partial top view of  FIG. 11A ; 
           [0062]      FIG. 11C  is a partial cross sectional side view of an LNG carrier constructed according to the method shown in  FIGS. 11A and 11B ; 
           [0063]      FIG. 11D  is a partial top view of  FIG. 11C . 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0064]      FIGS. 1A and 1B  show an existing LNG carrier  20  based on which a method of vessel construction according to one embodiment of the present invention may be applied. Existing LNG Carrier  20  includes a bow  201 , a stern  209  and a midship between bow  201  and stern  209 . The midship includes four segments  202 ,  204 ,  206  and  208 . Each segment has a corresponding first, second, third and fourth LNG tank  212 ,  214 ,  216 ,  218  built therein. Each individual LNG tank has its fixed storage capacity and hence the total storage and transportation capacity of the vessel is the sum capacity of all the individual LNG tanks. 
         [0065]    In a method  250  for constructing an LNG carrier according to one embodiment of the present invention, as shown in  FIG. 1C , a structural block is fabricated (block  252 ) or provided as a prefabricated unit from other sources. An existing LNG Carrier is cut apart into a forward section and an aft section (block  254 ). The structural block is placed into a space formed between the forward section and the aft section (block  256 ) and then the structural block is jointed to the forward and aft sections (block  258 ) to form an integrated new LNG carrier. 
         [0066]    Before an existing LNG carrier is cut, measurements and/or site inspections may be carried out to determine the characteristics of the existing LNG carrier, e.g. the dimensions, piping/cable layouts/configurations, hook-up points and connection interfaces, etc. Such information may be used for the fabrication of the structural block to ensure compatibility. In situations where such information is available, e.g. from previous measurements/inspections, it may be used directly for the fabrication of the structural block. 
         [0067]    Further details of this and other embodiments will be illustrated below in conjunction with drawings. 
         [0068]    As shown in  FIGS. 2A and 2B , a structural block  220 , e.g. an LNG storage tank, a regasification plant or both, is fabricated at a first site, e.g. a fabrication plant. During the fabrication of the structural block  220 , a second site e.g. in this case a shipyard dock can perform its normal operations without being affected by the structural block fabrication. When the structural block fabrication is completed, an existing LNG carrier e.g. existing LNG carrier  20  may be brought to the shipyard dock, and separated by e.g. cutting. Alternatively, the structural block may be fabricated at the same side as that used to cut the existing vessel, e.g. a shipyard dock or fabrication site, but the existing vessel needs not be present during the fabrication. When fabrication of the structural block is completed, the existing vessel can dry dock to undergo the cutting and vessel assembly process. 
         [0069]    In the embodiment shown in  FIGS. 2A and 2B , cutting is performed between stern  209  and the fourth LNG tank  218 , forming a forward section  232  which includes bow  201 , first LNG tank  212 , second LNG tank  214 , third LNG tank  216  and fourth LNG tank  218 , and an aft section  236  which is the stern  209 . A space  234  is formed between forward section  232  and aft section  236 . Structural block  220  is then placed in space  234  by floating or heave lift crane, with the forward section  232  jointed to front end  222  of structural block  220  and the aft section  236  jointed to rear end  226  of structural block  220 . Upon joining, the structural block  220  together with the forward section  232  and aft section  236 , form a new vessel  22  with the structural block  220  integrated to the existing LNG carrier, as shown in  FIGS. 2C and 2D . 
         [0070]    The LNG tank integrated into the structural block  220  may be the same type and configuration as that in the existing vessel. Such tank may be selected from a group consisting of a MOSS type tank, a Gaztransport &amp; Technigaz (GTT) tank, a self-supporting prismatic type B (SPB) tank, an independent type A tank and an independent type C tank. 
         [0071]    Building an FSRU or FLNG by utilizing an existing, smaller capacity LNG carrier, embodiments of the present invention provides new integrated vessels having lengthened hull with increased LNG storage and space capacity and/or added LNG process capability. Relevant industrial demand can be met with a much shorter delivery schedule than that required to build a new vessel from sketch, and with a greatly reduced CAPEX (Capital Expenditure). Embodiments of the present invention also enable reviving of existing, small-capacity LNG carriers which contributes to further cost-effective of the overall economics. 
         [0072]    According to another embodiment, an existing vessel may be separated into forward and aft sections at a different location, for integration of a structural block therebetween. As shown in  FIGS. 3A ,  3 B,  3 C and  3 D, an existing vessel  30  is cut apart into a forward section  332  and an aft section  336 . Cutting is performed between a third LNG tank  316  and a fourth LNG tank  318 , hence the forward section  332  includes the bow  301 , first, second and third LNG tanks  312 ,  314  and  316 . The aft section  336  includes fourth tank  318  and the stern  309 . A prefabricated structural block  320  e.g. a regasification plant, an LNG storage tank or a combination thereof, is then placed into the space  334  formed between forward section  332  and aft section  336 , and jointed to forward and aft sections  332 ,  336  to form a new integrated vessel  32  with the structural block  320  integrated to the existing LNG carrier, as shown in  FIGS. 3C and 3D . 
         [0073]    Alternatively, as shown in respective  FIGS. 4A to 4D ,  5 A to  5 D and  6 A to  6 D, an existing vessel  40 ,  50 ,  60  may be cut apart at amidship section (between a second LNG tank  414  and third LNG tank  416 ,  FIGS. 4A and 4B ), a forward section (between a first LNG tank  512  and second LNG tank  514 ,  FIGS. 5A and 5B ) or a forward bow section (between bow  601  and first LNG tank  612 ,  FIGS. 6A and 6B ). A prefabricated structural block  420 ,  520 ,  620  is then placed at the space  434 ,  534 ,  634  formed by the cutting of the existing vessel, joining with the cut-apart forward and aft sections of the existing vessel and form integrated new vessel  42  ( FIGS. 4C ,  4 D),  52  ( FIGS. 5C ,  5 D) or  62  ( FIGS. 6C ,  6 D). 
         [0074]    In a further embodiment, as shown in  FIGS. 7A to 7D , a prefabricated structural block  720  may include a liquefaction plant and/or one or more LNG storage tanks. An existing LNG carrier  70 , is cut apart into a forward section  732  and an aft section  736 , forming a space  734  therebetween. Structural block  720  is then placed in space  734  and joint to forward and aft sections  732 ,  736  to form a new LNG carrier  72  which has a liquefaction facility integrated into an existing LNG carrier. Similar to previous embodiments, cutting of existing vessel may be performed at various different locations of the hull, based on actual requirements, and integrate the structural block at these locations. In the case that no LNG storage tank is installed, one or more intermediate structural decks  752  can be installed inside of the new structural block to form deck space for machinery, gas/LNG processing plants, or other purposes. 
         [0075]    In a further embodiment shown in  FIGS. 8A to 8D , a prefabricated structural block  820  includes a new bow  821  and a regasification plant and/or one or more LNG storage tanks  826 . A first existing LNG carrier  80 , is cut apart into a forward section  831  and an aft section  836 . Forward section  831  is a bow of first existing LNG carrier  80 . After cutting the first existing LNG carrier  80 , the forward section  831  (in this case, the bow) is removed and the structural block  820  is joint to aft section  836  to form a new LNG carrier  82  which has a regasification facility integrated into first existing LNG carrier  80 , together with the new bow  821 . 
         [0076]    Structural block  820  may also include a turret  828  built on bow  821 . Turret  828  provides a non-rotating platform for supporting the mooring lines and flexible risers dedicated for gas export/import and associated control/service lines. 
         [0077]    Either one or both the bow  821 /turret  828  and the LNG storage tanks/regasification plant  826  may be obtained from a second existing LNG carrier/regasification plant, or built from sketch. 
         [0078]    In another further embodiment shown in  FIGS. 9A to 9D , a prefabricated structural block  920  includes a new bow  921  and a liquefaction plant and/or one or more LNG storage tanks  926 . A first existing LNG carrier  90 , is cut apart into a forward section  931  and an aft section  936 . Forward section  931  is a bow of first existing LNG carrier  90 . After cutting the first existing LNG carrier  90 , the forward section  931  (in this case, the bow) is removed and the structural block  920  is joint to aft section  936  to form a new LNG carrier  92  which has a liquefaction facility integrated into first existing LNG carrier  90 , together with the new bow  921 . 
         [0079]    Structural block  920  may also include a turret  928  built on new bow  921 . Turret  928  provides a non-rotating platform for supporting the mooring lines and flexible risers dedicated for gas export/import and associated control/service lines. 
         [0080]    Either one or both the bow  921 /turret  928  and the LNG storage tanks/liquefaction plant  926  may be obtained from a second existing LNG carrier/liquefaction plant, or built from sketch. 
         [0081]    In another further embodiment shown in  FIGS. 10A to 10D , a prefabricated structural block  1020  includes a stern  1029  and a regasification plant and/or one or more LNG storage tanks  1026 . A first existing LNG carrier  100 , is cut apart into a forward section  1032  and an aft section  1039 . Aft section  1039  is a stern of first existing LNG carrier  100 . After cutting the first existing LNG carrier  100 , the aft section  1039  (in this case, the stern) is removed and the structural block  1020  is joint to forward section  1032  to form a new LNG carrier  102  which has a regasification facility integrated into first existing LNG carrier  100 , together with the new stern  1029 . 
         [0082]    Either one or both the stern  1029  and the LNG storage tanks/regasification plant  1026  may be obtained from a second existing LNG carrier/regasification plant, or built from sketch. 
         [0083]    In another further embodiment shown in  FIGS. 11A to 11D , a prefabricated structural block  1120  includes a stern  1129  and, a liquefaction plant and/or one or more LNG storage tanks  1126 . A first existing LNG carrier  110  is cut apart into a forward section  1132  and an aft section  1139 . Aft section  1139  is a stern of first existing LNG carrier  110 . After cutting the first existing LNG carrier  110 , the aft section  1139  (in this case, the stern) is removed and the structural block  1120  is joint to forward section  1132  to form a new LNG carrier  112  which has a liquefaction facility integrated into first existing LNG carrier  110 , together with the new stern  1129 . 
         [0084]    Either one or both the stern  1129  and the LNG storage tanks/liquefaction plant  1126  may be obtained from a second existing LNG carrier/liquefaction plant, or built from sketch. 
         [0085]    Although embodiments of the present invention have been illustrated in conjunction with the accompanying drawings and described in the foregoing detailed description, it should be appreciated that the present invention is not limited to the embodiments disclosed. Therefore, the present invention should be understood to be capable of numerous rearrangements, modifications, alternatives and substitutions without departing from the spirit of the invention as set forth and recited by the following claims.