Abstract:
A method and apparatus for liquefying natural gas vapour is provided. Firstly, liquid natural gas is sub-cooled at a first heat exchanger using a liquid coolant such as liquid nitrogen. The sub-cooled liquid natural gas is then used to condense the natural gas vapour at a second heat exchanger.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to liquefaction of natural gas vapour, particularly during the transport and storage of liquefied natural gas (LNG). In particular, but not exclusively, the present invention relates to the handling of boil off gas in a bunker vessel used to refuel LNG-powered vessels. 
       BACKGROUND TO THE INVENTION 
       [0002]    Liquefied natural gas (LNG) is natural gas (typically methane—CH4) that has been liquefied, typically to make it more manageable during storage and/or transport. At atmospheric pressure, this means the temperature of the LNG is reduced to around −163 degrees centigrade or below. 
         [0003]    LNG carriers are vessels used to carry LNG across large distances, particularly to carry LNG from gas producing nations to gas consuming nations. The journey times of such vessels are significant, measured in days, weeks and even months. During such time, the LNG is stored at low temperature in order that it does not vaporise. 
         [0004]    Notwithstanding the efforts made to maintain this low temperature, there is in practice some evaporation of LNG during the vessel&#39;s journey. This evaporation creates boil-off gas (BOG) which must be handled by the vessel in some way. Indeed, this has been a factor in the continuing use of steam-turbine propulsion on LNG carriers. Carriers that use such a propulsion system are able to use BOG to drive turbines for propulsion and electricity generation. 
         [0005]    However, steam turbine propulsion is a relatively inefficient means to drive a large ship. As a result, a move towards slow speed diesel engine powered vessels has been apparent. The diesel engine in such a ship is unable to handle BOG and instead an independent LNG re-liquefaction plant is provided on the vessel. Typically nitrogen vapour is compressed and expanded in a “compander” in such a way as to lower its temperature so that it can be used as a coolant during the re-liquefaction process. The re-liquefaction plant is typically powered by the electricity generated on-board and has significant requirements in this regard, with ratings of 5 MW or above not uncommon. Furthermore, as well as high power requirements, the re-liquefaction plant is typically unable to handle high boil-off rates. Excess BOG that the LNG re-liquefaction plant is unable to handle is burnt in a gas combustion unit (GCU) and thereby wasted. 
         [0006]    Another type of propulsion system used in LNG carriers is a dual fuel diesel electric propulsion system which can operate off either diesel or LNG itself In such a vessel, the BOG can be used as fuel for the engines to provide propulsion and electrical load. If there is insufficient engine load it is necessary to burn the excess BOG in a GCU. 
         [0007]    There has been an increasing drive towards the use of LNG as a primary fuel in vessels of all types, driven at least partially by the relative cost of LNG in comparison to other fuels. As mentioned above, while this offers a potential method for handling BOG needed to provide propulsion and/or electricity, there remains the issue of handling such BOG when the required load does not match the BOG present. 
         [0008]    Simply burning the BOG in a GCU is both wasteful and polluting. Nor can the BOG simply be vented to the atmosphere, for similar reasons. Indeed, environmental regulations in many territories prohibit the handling of BOG in this way within close proximity to shore. Furthermore, using LNG as a primary fuel source creates its own challenges. For example, in many circumstances it may be desirable to re-fuel an LNG fuelled vessel using a bunker vessel. A bunker vessel is a smaller vessel designed to carry fuel to the carrier from shore. The bunker vessel must dock with the carrier and transfer fuel from its tanks to that of the carrier. 
         [0009]    As well as having to handle BOG during general operation, the bunker vessel must cope with the fact that the process of fuel transfer is likely to involve an increased production of BOG as the LNG is transferred between the vessels. Even if this were not the case the base level production of BOG would still need to be handled. However, as both vessels are stationary during this process, the BOG created cannot be used for propulsion and must be handled in some other manner. 
         [0010]    A re-liquefaction plant is impractical for a relatively small vessel such as a bunker vessel due to its large energy requirements and, as mentioned above, burning off excess BOG is both wasteful and environmentally unsound. There remains a need to find a method of handling BOG in bunker vessels that is both practical and effective. 
       SUMMARY OF THE INVENTION 
       [0011]    According to a first aspect of the present invention there is provided a method for condensing natural gas vapour to generate liquefied natural gas (LNG), comprising: 
         [0012]    providing a liquid coolant, wherein the liquid coolant has a boiling point less than that of natural gas; 
         [0013]    cooling LNG at a first heat exchanger using the liquid coolant to generate sub-cooled LNG; and 
         [0014]    condensing natural gas vapour at a second heat exchanger using the sub-cooled LNG to liquefy the natural gas vapour and thereby generate further LNG. 
         [0015]    According to a second aspect of the present invention, there is provided a system for condensing natural gas vapour to generate liquefied natural gas (LNG); comprising 
         [0016]    a first heat exchanger arranged to cool LNG using a liquid coolant to generate sub-cooled LNG, wherein the liquid coolant has a boiling point less than that of natural gas; and 
         [0017]    a second heat exchanger arranged to condense natural gas vapour using the sub-cooled LNG to liquefy the natural gas vapour and thereby generate further LNG. 
         [0018]    The present invention can provide an efficient method of condensing natural gas vapour to provide LNG. Existing LNG is cooled at a first heat exchanger to create sub-cooled LNG that can be used to condense the natural gas vapour. This is significantly more energy efficient than provision of a plant for generating LNG directly from the natural gas vapour. The use of a liquid coolant to first sub-cool existing available LNG reduces the need for energy intensive refrigeration cycles to generate a coolant to directly cool the natural gas vapour. 
         [0019]    The present invention finds particular utility in circumstances in which boil-off gas is present. Boil-off gas is natural gas vapour that has arisen through the evaporation of an LNG source. In such circumstances, the boil-off gas is likely not to be significantly warmer than the boiling temperature of natural gas, and furthermore LNG is likely to be available for sub-cooling. Thus, in a preferred embodiment, the natural gas vapour is boil-off gas. 
         [0020]    The method may comprise receiving the LNG for cooling at the first heat exchanger from at least one LNG storage tank and returning the sub-cooled LNG to the at least one storage tank after it is used at the second heat exchanger. In this manner, LNG may be re-used for subsequent condensing of natural gas vapour. After use at the second heat exchanger the sub-cooled LNG is likely to have warmed somewhat, but the method is preferably arranged such that it remains in liquid form for returning to the LNG storage tank. 
         [0021]    Preferably, the method may further comprise delivering the further LNG to the at least one storage tank. Accordingly, the LNG that is created through the method may subsequently be used for condensing further natural gas vapour in future cycles. Furthermore, the system is compact and efficient in that there is no requirement for separate tanks for the coolant used at the second heat exchanger and the generated LNG. 
         [0022]    As mentioned above, the liquid coolant used at the first generator has a boiling point less than that of the natural gas vapour. The natural gas vapour is preferably predominantly methane, more preferably at least 90 Mole % methane, at least 95 Mole % methane or over 97% Mole methane. Methane has a boiling point at atmospheric pressure of around −163 degrees centigrade. Preferably, the liquid coolant has a boiling point at atmospheric pressure of less than that of methane (i.e. less than −163 degrees centigrade), more preferably less than −170 degrees centigrade, and most preferably less than −190 degrees centigrade. In preferred embodiments, the liquid coolant is liquid nitrogen. The boiling point of nitrogen is around −193 degrees centigrade at atmospheric pressure. References to atmospheric pressure refer to a pressure equal to the understood unit of one atmosphere, rather than to prevailing climatic conditions. 
         [0023]    The system may comprise a storage facility for the liquid coolant. For example, the system may comprise at least one storage tank for storing the liquid coolant. The system may additionally or alternatively comprise a generator for generating the liquid coolant. Equally, the method may comprise a step of generating the liquid coolant. For example, where the system is provided on a maritime vessel, an initial supply of liquid coolant may be provided to the storage facility from an on-shore facility while the vessel is docked, but when the vessel is at sea additional liquid coolant may be provided as needed by the generator. 
         [0024]    According to a further aspect, a vessel may be provided comprising the system of the second aspect. The vessel may comprise a flow boom arranged to transfer LNG to a second vessel and to receive natural gas vapour from the second vessel. The method may further comprise transferring LNG from a first vessel to a second vessel, and receiving the natural gas vapour from the second vessel. In preferred embodiments, one or both vessels are maritime vessels. In particular, the vessel comprising the system (i.e. the first vessel) may be a bunker vessel while the second vessel may be an LNG fuelled vessel. Alternatively or additionally, the second vessel may be a carrier, particularly an LNG carrier. A bunker vessel is a vessel arranged to re-fuel other vessels while at sea. Preferably, one or both vessels are LNG powered vessels. 
     
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         [0025]    A preferred embodiment of the present invention will now be described with reference to the accompanying drawings, in which: 
           [0026]      FIG. 1  is a schematic diagram showing a bunker vessel comprising a system or handling LNG; 
           [0027]      FIG. 2  is a schematic diagram showing the connection between various elements of the system in more detail; and 
           [0028]      FIG. 3  is a schematic diagram illustrating the components of a re-condenser unit. 
       
    
    
     DETAILED DESCRIPTION 
       [0029]    Referring to  FIG. 1 , a schematic diagram is provided showing the principle elements of a LNG handling system aboard a bunker vessel  1  for re-fuelling larger ships. The system comprises two LNG storage tanks  10 , port and starboard manifolds  20 , a liquid nitrogen tank  30 , a gaseous nitrogen tank  40 , a liquid nitrogen generator  50 , a re-condenser unit  60 , engines  70  and a flow boom  80 .  FIG. 1  illustrates functional connections between the various illustrated elements of the system for the transfer of LNG (indicated as “L”, dotted lines), natural gas vapour (indicated as “V”, thick lines) and nitrogen (both liquid and gaseous indicated as “N”, thin lines). 
         [0030]    The port and starboard manifolds  20  are arranged to allow transfer of LNG, natural gas vapour, and nitrogen between the bunker vessel and an on-shore facility. LNG received from the port and starboard manifolds is stored in the storage tanks  10 . In this embodiment, the storage tanks are pressurised, C-class, storage tanks which may operate at up to 10 bar, but the skilled person will recognize that alternative tanks may be used. Indeed, the storage tanks may be any type of chamber or container suitable to act as a reservoir for LNG. Pressurised tanks, particularly pressurised “C” Type tanks, allow a wider range of operating temperature and pressure within the bunker vessel. The storage tanks store LNG at around −163 degrees centigrade. 
         [0031]    Liquid nitrogen received from the manifolds  20  is stored in liquid nitrogen tank  30 . Nitrogen that evaporates from liquid nitrogen tank  30  may be received in gas nitrogen tank  40  from where it is passed to the ship&#39;s systems for use in for purging of cargo/fuel lines, inerting tanks and hold spaces and so on. Alternatively or additionally, nitrogen may be vented to the atmosphere or re-used by liquid nitrogen generator  50 . The liquid nitrogen tank  30  and the gas nitrogen tank  40  may be any suitable container or chamber suitable to act as a reservoir for liquid nitrogen and gaseous nitrogen respectively. 
         [0032]    The liquid nitrogen generator  50  passes generated liquid nitrogen back to the liquid nitrogen tank  30 . The liquid nitrogen generator  50  may be a compression cooling system, and is preferably powered by the vessel&#39;s electrical systems. For example, the liquid nitrogen generator  50  may be arranged to filter and compress atmospheric air before carbon dioxide water and residual hydrocarbons are removed in an air purification unit. The air is then passed to a cold box where it is cooled and liquefied. The liquid air is distilled then in a distillation column to yield pure nitrogen gas which is condensed in a condenser to yield pure liquid nitrogen. 
         [0033]    The system comprises a flow boom  80  for transferring LNG to another vessel, such as an LNG carrier. The flow boom  80  is a transfer boom in this embodiment. The transfer boom  80  receives LNG from LNG storage tank  10  and may also receive natural gas vapour from the other vessel. Natural gas vapour received by the transfer boom is passed to the re-condenser unit  60 . 
         [0034]    The re-condenser unit  60  also receives natural gas vapour from the LNG storage tank  10 . The re-condenser unit  60  is arranged to re-liquefy the natural gas vapour to return it to an LNG state. Once re-liquefied the LNG may be passed back to the LNG storage tank  10 . 
         [0035]    The re-condenser unit  60  also receives liquid nitrogen from the liquid nitrogen tank  30  and LNG from the LNG tanks  10 . These liquids are used during the processes of cooling the natural gas vapour at the re-condenser unit, as will be explained in greater detail below with reference to  FIG. 3 . 
         [0036]    In the embodiment shown in  FIG. 1 , the engines  70  of the bunker vessel use the stored LNG as a fuel source. In practice, natural gas vapour is used for combustion in the engines. The natural gas vapour used for this purpose may be boil-off gas (BOG) spontaneously occurring in the LNG storage tanks  10 , or may be deliberately vaporised LNG from the LNG storage tanks. LNG from the storage tank may be vaporised by a forcing vaporizer, for example. 
         [0037]      FIG. 2  shows in more detail the connections between various elements of the system of  FIG. 1 . In particular,  FIG. 2  illustrates the LNG storage tanks  10  and the port and starboard manifolds  20 . As shown, a variety of valves are provided to control the flows and pressures of gas and liquid in the system.  FIG. 2  also illustrates various connection points to other elements of the system. In particular, connection points  201  are provided for LNG returning from the re-condenser unit  60 , while connection points  202  are provided for natural gas vapour passed to the re-condensing unit  60 . There is also provided connection point  210  for passing LNG to the re-condensing unit  60  for use during a re-liquefaction process implemented by the re-condensing unit  60  and a connection point  214  for receiving LNG from the re-condensing unit that has been used for this purpose. LNG that is extracted from the LNG storage tanks  10  but is not ultimately used in the engine  70  or the re-condensing unit  60  may be returned to the LNG storage tanks via connection points  215 . 
         [0038]    Connection point  203  is shown for passing LNG to the transfer boom  80 , while connection point  204  receives natural gas vapour from the transfer boom  80 . In this manner, LG can be transferred to another vessel, while excess boil-off as from the vessel can be retrieved for handling on the bunker vessel. 
         [0039]    Connection point  205  is provided for passing gas vapour to the engines  70 . There is also provided a connection point  206  for transferring natural gas vapour to a gas combustion unit (GCU). In an emergency situation, this GCU may be used to burn and thus dispose of natural gas vapour that is not otherwise handled by the system. There is also provided a vent  208  to vent natural gas vapour to the atmosphere where this is appropriate. 
         [0040]    Connection point  207  is provided for the receipt of nitrogen vapour for purging of lines, and inerting spaces as required. Excess vapour would be vented via the GCU. The vapour may be received from the gas nitrogen tank  40  which receives boil-off from the liquid nitrogen tank  30 . Connection point  210  is for supply of liquid nitrogen via the manifolds  20  from shore for charging the liquid nitrogen storage tank  30 . 
         [0041]    Each LNG storage tank is provided with a discharge pump  11  for pumping LNG to the transfer boom  80  and a LNG fuel pump  12  for pumping LNG to the engines  70 . Various connections allow the LNG pumped by either the discharge pump  11  or the LNG fuel pump  12  to be re-directed as appropriate. 
         [0042]    Each LNG storage tank  10  also comprises a first LNG inlet  13  and a second LNG inlet  14 . LNG can be received at the LNG storage tanks  10  from the re-condenser unit  60  via connection points  201  and from the port and starboard manifolds  20 . The LNG storage tanks also comprise a gas dome  15  above the storage tanks, where boil off gas (BOG) from the stored LNG is collected. The LNG storage tanks  10  comprise a gas outlet  17  for passing this natural gas vapour to other elements of the system. 
         [0043]    The LNG storage tanks  10  also comprise a return spray header  16 . The spray header returns a proportion of the LNG extracted from the LNG storage tank  10  as a spray applied to the surface of the stored LNG in the tank. This helps to maintain uniform temperature within the stored LNG and thereby reduces the rate of generation of boil off gas. 
         [0044]    The port and starboard manifolds  20  comprise a first LNG interface  21 . A second LNG interface  22 , a natural gas vapour interface  23  and a nitrogen interface  24 . When the bunker vessel is docked, LNG may be provided to the LNG storage tanks  10  via the LNG interfaces  21 ,  22  while natural gas vapour may be returned to shore via the gas vapour interface  23 . Liquid nitrogen may also be provided to the liquid nitrogen tank  30  via the nitrogen interface  24 . 
         [0045]      FIG. 2  also illustrates a forcing vaporiser  211  on the line between the LNG storage tanks and connection point  205  to the engines  70 . This is used to vaporise LNG to produce natural gas vapour which can be combusted by the engines  70  to generate power for the bunker vessel&#39;s propulsion and electrical systems. The system further comprises a compressor  212  for compressing gas passed through the vaporiser  211  before it reaches the engines  70 . A further compressor  213  may also be provided for natural gas vapour being returned to the port and starboard manifolds  20 . 
         [0046]      FIG. 3  illustrates the re-condensing unit  60  in more detail. In order to facilitate comparison with  FIG. 2 , various functionally equivalent connections portions are shown in  FIG. 3  using the same reference numerals as used in  FIG. 2 . For example,  FIG. 3  shows connection points  202  for providing natural gas vapour, particularly BOG, to the re-condensing unit  60 . Moreover, connection points  201  for receiving the re-liquefied LNG from the re-condensing unit  60  and returning this to the LNG storage tanks  10  are also shown. 
         [0047]      FIG. 3  also illustrates liquid nitrogen tank  30  and gaseous nitrogen tank  40 . The liquid nitrogen tank  30  is filled from the port and starboard manifolds via connection point  209  (also shown in  FIG. 2 ) and may also be filled from bunker vessel&#39;s liquid nitrogen generator  50  via connection point  301 . Nitrogen which evaporates from the liquid nitrogen tank  30  may be passed to the gaseous nitrogen tank  40 , from where it may be vented via connection point  302  or passed to consumers via connection point  303 . Consumers in this case may include systems aboard the vessel; for example, nitrogen vapour may be used for purging of cargo/fuel lines, inerting tanks and hold spaces and so on. 
         [0048]    The re-condensing unit  60  comprises a first heat exchanger  62 , a second heat exchanger  64  and a compressor  66 . The first heat exchanger  62  is an LNG sub-cooler and is coupled to the liquid nitrogen tank  30  and to LNG outlets of the LNG tanks  10 . LNG from the LNG tanks  10  is cooled using liquid nitrogen from the liquid nitrogen tank  30  to below its temperature in the LNG tanks  10 . The LNG cooled in this manner is referred to as “sub-cooled”. 
         [0049]    The second heat exchanger  64  is coupled to the first heat exchanger  62  so as to receive the sub-cooled LNG therefrom. The second heat exchanger  64  is also arranged to receive natural gas vapour. The natural gas vapour may originate either at the LNG tanks  10  or be received from another vessel via the transfer boom  80 . Typically, the natural gas vapour is BOG that has occurred by evaporation of LNG. 
         [0050]    The second heat exchanger  64  is a condenser arranged to cool the natural gas vapour using the sub-cooled LNG received from the first heat exchanger such that it is liquefied. The second heat exchanger thus generates LNG which is returned to the LNG tanks  10 . Furthermore, once it has passed through the second heat exchanger, the sub-cooled LNG is returned to the LNG tanks  10 . 
         [0051]    The re-condensing unit also comprises a compressor  66 . The compressor  66  is used to compress natural gas vapour prior to its injection into the second heat exchanger  64 . This is found to increase the efficiency of heat exchange at the second heat exchanger  64 . 
         [0052]    In use, the bunker vessel comprising the system illustrated in  FIG. 1  is docked with another vessel which it is to re-fuel. In particular examples, this other ship is an LNG fuelled vessel. 
         [0053]    The transfer boom  80  is used to transfer LNG from the LNG tanks  10  to the LNG carrier. During this process, natural gas vapour is displaced from the tanks aboard the LNG carrier, and boil-off gas is also generated from the LNG tanks  10  on the bunker vessel and at other points in the system. This natural gas vapour is directed towards the re-condensing unit, where it first encounters  60  the compressor  66 . The compressor  66  acts to increase the pressure within the natural gas vapour by compressing the vapour, and the compressed vapour is then passed to the second heat exchanger  64 . The second heat exchanger  64  transfers heat between sub-cooled LNG and the compressed vapour, thereby cooling the compressed vapour until it condenses (i.e. liquefies), and creating LNG. This LNG is then passed to the LNG storage tanks  10 . 
         [0054]    As mentioned above, the sub-cooled LNG used in the second heat exchanger  64  is also passed to the LNG storage tanks  10  after passing through the second heat exchanger  64 . Prior to this, the sub-cooled LNG is generated by cooling using liquid nitrogen at the first heat exchanger  62 . 
         [0055]    As described above, the preferred embodiment is designed for use upon a bunker vessel used to refuel another ship. However, it will be understood that variations may be proposed for alternative types of marine vessels, and indeed to shore-based transport and storage. 
         [0056]    Other variations and modifications will be apparent to the skilled person. Such variations and modifications may involve equivalent and other features which are already known and which may be used instead of, or in addition to, features described herein. Features that are described in the context of separate embodiments may be provided in combination in a single embodiment. Conversely, features which are described in the context of a single embodiment may also be provided separately or in any suitable sub-combination. It should be noted that the term “comprising” does not exclude other elements or steps, the term “a” or “an” does not exclude a plurality, a single feature may fulfill the functions of several features recited in the claims and reference signs in the claims shall not be construed as limiting the scope of the claims. It should also be noted that the Figures are not necessarily to scale; emphasis instead generally being placed upon illustrating the principles of the present disclosure.