Abstract:
The invention relates to a method and system for operating a LNG fuelled marine vessel. The marine vessel ( 1 ) comprises a LNG storage tank ( 3 ) and a LNG fuelled power plant ( 71 ). LNG is stored in the LNG storage tank, and in connection with a bunkering operation the marine vessel is supplied with LNG by connecting a source of LNG to a bunkering line ( 14 ) of the marine vessel and subsequently supplying the marine vessel with LNG through the bunkering line. In order to achieve a faster bunkering operation the bunkering line ( 14 ) is cooled down prior to the bunkering operation.

Description:
TECHNICAL FIELD 
       [0001]    Method for operating a LNG fuelled marine vessel, which marine vessel comprises a LNG storage tank and a LNG fuelled power plant, in which method LNG is stored in the LNG storage tank, and in which method in connection with a bunkering operation the marine vessel is supplied with LNG by connecting a source of LNG to a bunkering line of the marine vessel and subsequently supplying the marine vessel with LNG through the bunkering line according to the preamble of claim  1 . The invention also relates to an operating system according to the preamble of claim  6 . 
       BACKGROUND ART 
       [0002]    Marine vessels can be fuelled by LNG (Liquefied Natural Gas). Clearly, as also with LNG, marine vessels need to be refuelled or bunkered at certain intervals. The bunkering operation usually takes place in port, but may also take place at other locations, such as at a floating bunkering facility out at sea. 
         [0003]    The bunkering operation of a LNG fuelled marine vessel usually takes a long time. The reason for this is the temperature difference between LNG (normally about −162° C.) and the bunkering line (normally in ambient temperature, around +20° C.). This temperature difference causes the LNG to boil in the bunkering line, which leads to a two-phase flow of gas and liquid. The two-phase flow causes control problems and pressure pulses, which are harmful for the supply procedure and for the piping arrangements of the bunkering line. Consequently, in order to arrive at a functioning bunkering operation, the LNG flow rate has to be kept very low in the beginning of the operation in order to minimize, or at least in an attempt to keep the pressure pulses at an acceptable level. After the bunkering line starts to cool down, the flow rate may slowly be increased. The bunkering operation consequently takes a very long time. 
         [0004]    A typical practice and trend in marine vessel operation of today is shortened port times and high operating speeds, the latter with high energy consumption, which would necessitate a transfer of large amounts of fuel in a short time. 
         [0005]    Such fast transfer of fuel is not available in the prior art. 
       SUMMARY OF THE INVENTION 
       [0006]    An object of the present invention is to avoid the drawbacks of the prior art and to provide for an efficient transfer of fuel, particularly LNG, while maintaining a controlled fuel flow. This object is achieved by the method according to claim  1 . 
         [0007]    The basic idea of the invention is to cool down the bunkering line aboard the marine vessel before the bunkering operation, i.e. before the refuelling of the marine vessel. The bunkering line is advantageously cooled down to a temperature level corresponding to the temperature level of the LNG. This helps to avoid a two-phase flow and to provide for lower pressure pulses in the bunkering line, which as a result provides for a higher flow rate in principle from the beginning of the bunkering operation and consequently for a faster bunkering operation. 
         [0008]    The cooling down of the bunkering line is advantageously carried out by circulating LNG from the LNG storage tank through the bunkering line. In this way any auxiliary cooling means are not necessary. 
         [0009]    LNG is advantageously pumped from a lower part of the LNG storage tank to the bunkering line of the marine vessel, which in connection with both a pressurized LNG storage tank and a LNG storage tank under hydrostatic pressure ensures that only LNG is re-circulated to the bunkering line. This is preferably carried out before the marine vessel arrives at a bunkering facility. 
         [0010]    Advantageously the method also employs a purge connection for emptying the bunkering line of residual LNG. 
         [0011]    The main and advantageous features of the operating system for a LNG fuelled marine vessel are defined in claims  6  to  10 . 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]    In the following the invention will be described, by way of example only, with reference to the accompanying schematic drawings, in which 
           [0013]      FIG. 1  illustrates a first embodiment of the present invention, and 
           [0014]      FIG. 2  illustrates a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]      FIG. 1 , illustrating a first embodiment of the invention, shows a marine vessel  1  at a bunkering facility  2 , such as a port. The marine vessel  1  is indicated by a line schematically showing the side shell of the marine vessel. The side shell accommodates a bunker station  11  comprising an inlet pipe  12  with appropriate valve means. The inlet pipe  12  leads to a bunkering line  14 , provided with a first emergency shut down valve  13 , which in turn leads to a LNG storage tank  3  on the marine vessel  1 . The LNG storage tank  3  is connected by way of a discharge conduit  6  and a fuel feed line  7  to a power plant (indicated schematically by reference numeral  71 ) of the marine vessel  1 . A liquid level of the LNG in the LNG storage tank is indicated by reference sign L. 
         [0016]    The bunkering line  14 , in this embodiment, includes a two-way valve  15 , a first branch pipe  16 , which is connected to the two-way valve  15  and which opens directly into the LNG storage tank  3 , and a second branch pipe  17 , which is connected to the two-way valve  15  and which is provided with a nozzle assembly  18  arranged within the LNG storage tank  3 . In a normal bunkering or re-fuelling operation, the LNG is supplied through the branch pipe  16 . If the LNG storage tank  3  is empty, and thus at an elevated temperature, LNG can initially be supplied through the second branch pipe  17  so that it can be sprayed by means of the nozzle assembly  18  into the LNG storage tank  3  in order to cool down the LNG storage tank  3 . 
         [0017]    The inlet pipe  12  is provided with a purge connection  19  in order to empty the inlet pipe  12  and the bunkering line  14  of residual LNG. Purging can be carried out by gas, e.g. N 2 . 
         [0018]    In this embodiment the LNG storage tank  3  is a pressurized (generally at a pressure of about 5 bar) storage tank and is thus provided with a pressure build-up circuit  4  passing through a heat exchanger  5  in order to maintain a desired pressure in the LNG storage tank  3 . 
         [0019]    LNG is supplied from the LNG storage tank  3  to the fuel supply line  7  through the discharge conduit  6 , which has its inlet in the bottom part of the LNG storage tank  3 , below liquid level L. The discharge conduit  6  passes through the heat exchanger  5  in order for the LNG to be vaporized for further transfer to the fuel feed line  7  of the power plant  71  of the marine vessel  1 . 
         [0020]    Heat is provided to the heat exchanger  5  by means of a heating medium circuit  8 . 
         [0021]    Further, in this embodiment, a source of LNG is shown in the form of a LNG tanker truck  10  at the port side. The LNG tanker truck  10  is provided with an outlet pipe  101  with appropriate valve means, such as a second emergency shut down valve  102 . For establishing a flow connection between the outlet pipe  101  of the LNG tanker truck  10  and the inlet pipe  12  of the bunker station  11  of the marine vessel  1 , a bunker connection line  103 , e.g. a fuel hose, is provided. 
         [0022]    The source of LNG could also be e.g. a storage tank based on land or on a floating support arrangement. 
         [0023]    In  FIG. 1 , according to the present invention, the bunkering line  14  is also provided with a cooling means. The cooling means comprises the discharge conduit  6  of the LNG storage tank  3 , which is also branched off into a cooling line  9  which is in flow connection with the bunkering line  14  through the first emergency shut down valve  13 . The cooling line  9  is provided with a cooling line valve  91  and a pump means, in this embodiment a circulation pump  92 . This provides for circulating LNG from the LNG storage tank  3  to the bunkering line  14  in order to cool down the bunkering line  14 , advantageously to a temperature level corresponding to the temperature level of the LNG. The cooling means, i.e. the cooling line  9  with the cooling line valve  91  and the pump means, i.e. said circulation pump  92  may of course be arranged separately form the discharge conduit  6 . 
         [0024]    The cooling down of the bunkering line  14  is preferably done before the marine vessel arrives at the bunkering facility  2 . In this way bunkering can be started immediately at arrival and be carried out by supplying LNG from the source of LNG at a high flow rate more or less from the beginning, whereby the bunkering operation can be carried out within a short time. 
         [0025]    The emergency shut down valves discussed above are advantageous in view of safety criteria with respect to the characteristics of LNG. In addition, the pipes, conduits and lines are preferably double-walled or enclosed in ducts in order to avoid leakage to the surroundings. 
         [0026]    In summary, and in view of the above, the operation of the marine vessel can shortly be described as follows. 
         [0027]    When the marine vessel  1  is out at sea, the power plant  71 , or propulsion machinery, of the marine vessel  1  is driven by vaporized NG (Natural Gas) supplied from the LNG storage tank  3 . The LNG is supplied by way of the discharge conduit  6  through the heat exchanger  5 , where the LNG is vaporized, so that it is delivered in gas mode (NG) to the fuel feed line  7  (two fuel feed lines in this embodiment, corresponding e.g. to two internal combustion engines) of the propulsion machinery. 
         [0028]    When the marine vessel  1  arrives at e.g. its port of destination, in addition to possible unloading and/or loading operations, it needs to be re-fuelled. Re-fuelling is provided by the LNG tanker truck  10 , or e.g. another source of LNG. 
         [0029]    In order to be prepared for an efficient refuelling operation, preparatory measures are taken aboard the marine vessel  1  before it arrives in port. The cooling line valve  91  is opened and the circulation pump  92  is started in order to re-circulate LNG from the LNG storage tank  3  to the bunkering line  14  through the cooling line  9 , and back to the LNG storage tank  3  by way of the bunkering line  14 . The respective valve means in said bunkering line and cooling line are naturally positioned so that fluid communication is established. The circulation pump  92  is run and the LNG is re-circulated for a sufficient time so that the bunkering line  14  is cooled down and so that a desired temperature level (advantageously a temperature level corresponding to the temperature level of the LNG) in the bunkering line  14  is achieved, whereby the re-circulation of the LNG can be discontinued. 
         [0030]    When the marine vessel  1  then arrives in port, the bunkering line  14  is at an appropriate temperature to receive the LNG from the LNG tanker truck  10 . The outlet pipe  101  of the LNG tanker truck  10  may then be connected by the bunker connection line  103  to the inlet pipe  12  of the bunker station  11  of the marine vessel  1 , whereby the bunkering operation can be started and carried out in a short time. As soon as the bunker connection line  103 , i.e. the fuel hose, has been cooled down by LNG from the LNG tanker truck  10 , the LNG can be delivered at a high flow rate to the cooled down bunkering line  14 . It is therefore advantageous that the bunker connection line  103  is short. 
         [0031]    The respective valve means are naturally positioned so that fluid communication from the LNG tanker truck  10  to the bunkering line  14  is established. When bunkering is finished, the LNG tanker truck  10  can be disconnected and the respective valve means closed. 
         [0032]    If the source of LNG is e.g. a fixed storage tank based on land or on a floating support arrangement, the piping leading from the fixed storage tank to a connection point for the bunker connection line at the dock side could advantageously be cooled down in similar way as described above. This could be carried out by have a re-circulation of LNG provided from the fixed storage tank through the piping leading to the dock side connection point, to which the bunker line connection is to be connected. This operation would be carried out before the marine vessel arrives at port in a corresponding manner as the preparatory measures on the marine vessel. 
         [0033]    Consequently, this provides for a fast bunkering operation and a short port time. The marine vessel  1  is then ready for further operation. 
         [0034]    The LNG storage tank and the fuel supply arrangement, including components like pumps, heat exchangers, compressors, and valve means, are as such considered to be known by a person skilled in the art and are therefore not described in more detail in this connection. 
         [0035]    The different stages of operation described above can be suitable controlled by appropriate automation systems. This is considered to lie within the knowledge of a skilled person in the art and is therefore not discussed in detail in this connection. 
         [0036]      FIG. 2 , illustrating a second embodiment of the invention, shows a marine vessel  1  at a bunkering facility  2 , such as a port. The marine vessel  1  is indicated by a line schematically showing the side shell of the marine vessel. The side shell accommodates a bunker station  11  comprising an inlet pipe  12  with appropriate valve means. The inlet pipe  12  leads to a bunkering line  14 , provided with a first emergency shut down valve  13 , which in turn leads to a LNG storage tank  30  on the marine vessel  1 . 
         [0037]    The bunkering line  14  leads directly to the LNG storage tank  30 , which is connected to a separate fuel feed tank  40 , which is a heat insulated pressure vessel and which is of a considerably smaller size than the LNG storage tank  30 . The LNG storage tank  30  is under atmospheric pressure, i.e. in practice under the hydrostatic pressure caused by the LNG. A liquid level of the LNG in the LNG storage tank is indicated by reference sign L. 
         [0038]    The inlet pipe  12  is provided with a purge connection  19  in order to empty the inlet pipe  12  and bunkering line  14  of residual LNG. Purging can be carried out by gas, e.g. N 2 . 
         [0039]    In this embodiment the LNG storage tank  30  is arranged to deliver LNG to the fuel feed tank  40 . For this purpose the LNG storage tank  30  is provided with a first fuel supply line  31  comprising a pump means, in this embodiment a supply pump  32 , advantageously a cryogenic pump. LNG from the lower part (below the liquid level L) of the LNG storage tank  30  is pumped in liquefied form by the supply pump  32  by way of said first fuel supply line  31  to the fuel feed tank  40  simultaneously raising the pressure. The LNG storage tank  30  is also provided with a second fuel supply line  33 , which is arranged in the upper part (above the liquid level L) of the LNG storage tank  30  for capturing and transporting boil-off gas from the LNG storage tank  30  to the fuel feed tank  40 . The second fuel supply line  33  is provided with a compressor  34  (in this embodiment two compressors) for pumping the boil-off gas from the low pressure LNG storage tank  30  to the high pressure fuel feed tank  40 . The compressor  34  raises the pressure of the boil-off gas to a pressure level prevailing in the fuel feed tank  40 . A liquid level of the LNG in the fuel feed tank  40  is indicated by reference sign L′. 
         [0040]    The fuel feed tank  40  is provided with a pressure build-up circuit  41  extending from the lower part (below the liquid level L′) of the fuel feed tank  40  to the upper part (above the liquid level L′) of the fuel feed tank  40  and passing through a first heat exchanger  42  in order to maintain a desired pressure level in the fuel feed tank  40 . Heat is provided to the first heat exchanger  42  by means of a first heating medium circuit  81 . 
         [0041]    LNG is supplied from the fuel feed tank  40  to a fuel feed line  70  through a discharge conduit  60 . The discharge conduit  60  passes through a second heat exchanger  50  in order for the LNG to be vaporized for further transfer to the fuel feed line  70  of the power plant  71  of the marine vessel  1 . Heat is provided to the second heat exchanger  50  by means of a second heating medium circuit  82 . 
         [0042]    Further, in this embodiment, a source of LNG is shown in the form of a LNG tanker truck  10  at the port side. The LNG tanker truck  10  is provided with an outlet pipe  101  with appropriate valve means, such as a second emergency shut down valve  102 . For establishing a flow connection between the outlet pipe  101  of the LNG tanker truck  10  and the inlet pipe  12  of the bunker station  11  of the marine vessel  1 , a bunker connection line  103 , e.g. a fuel hose, is provided. 
         [0043]    The source of LNG could also be e.g. a storage tank based on land or on a floating support arrangement. 
         [0044]    In  FIG. 2 , according to the present invention, the bunkering line  14  is provided with a cooling means. The cooling means comprises the first fuel supply line  31  arranged in the LNG storage tank  30 , which is also branched of into a cooling line  90 , provided with a cooling line valve  91 , which is in flow connection with the bunkering line  14  through the first emergency shut down valve  13 . LNG is circulated in the cooling line  90  by the pump means, i.e. said supply pump  32 . This provides for circulating LNG from the LNG storage tank  30  to the bunkering line  14  in order to cool down the bunkering line  14 , advantageously to a temperature level corresponding to the temperature level of the LNG. The cooling means, i.e. the cooling line  90  with the cooling line valve  91  may of course be arranged separately form the first fuel supply line  31 , and in that case with a separate pump means. 
         [0045]    The cooling down of the bunkering line  14  is preferably done before the marine vessel arrives at the bunkering facility  2 . In this way bunkering can be started immediately at arrival and be carried out by supplying LNG from the source of LNG at a high flow rate more or less from the beginning, whereby the bunkering operation can be carried out within a short time. 
         [0046]    The emergency shut down valves discussed above are advantageous in view of safety criteria with respect to the characteristics of LNG. Further, the pipes, conduits and lines are preferably double-walled in order to avoid leakage to the surroundings. 
         [0047]    In summary, and in view of the above, the operation of the marine vessel can shortly be described as follows. 
         [0048]    When the marine vessel  1  is out at sea, the power plant  71 , or propulsion machinery, of the marine vessel  1  is driven by vaporized NG (Natural Gas) supplied from the LNG storage tank  30 . The LNG is supplied by way of the first fuel supply line  31 , provided with the supply pump  32 , to the fuel feed tank  40 , and further through the discharge line  60  and the heat exchanger  50 , where the LNG is vaporized, so that it is delivered in gas mode (NG) to the fuel feed line  70  (four lines in this embodiment, corresponding e.g. to four internal combustion engines) of the propulsion machinery. 
         [0049]    When the marine vessel  1  arrives at e.g. its port of destination, in addition to possible unloading and/or loading operations, it needs to be re-fuelled. Re-fuelling is provided by the LNG tanker truck  10 , or e.g. another source of LNG. 
         [0050]    In order to be prepared for an efficient refuelling operation, preparatory measures are taken aboard the marine vessel  1  before it arrives in port. The supply pump  32  is run and the cooling line valve  91  is opened in order to re-circulate LNG from the LNG storage tank  30  to the bunkering line  14  through the cooling line  90 , and back to the LNG storage tank  30  by way of the bunkering line  14 . The respective valve means in said bunkering line and cooling line are naturally positioned so that fluid communication is established. This re-circulation is kept up for a sufficient time so that a desired temperature level (advantageously a temperature level corresponding to the temperature level of the LNG) in the bunkering line  14  is achieved, whereby the re-circulation of the LNG can be discontinued. 
         [0051]    When the marine vessel  1  then arrives in port, the bunkering line  14  is at an appropriate temperature to receive the LNG from the LNG tanker truck  10 . The outlet pipe  101  of the LNG tanker truck  10  may then be connected by the bunker connection line  103 , i.e. the fuel hose, to the inlet pipe  12  of the bunker station  11  of the marine vessel  1 . As soon as the bunker connection line  103 , i.e. the fuel hose, has been cooled down by LNG from the LNG tanker truck  10 , the LNG can be delivered at a high flow rate to the cooled down bunkering line  14 . It is therefore advantageous that the bunker connection line  103  is short. 
         [0052]    The respective valve means are naturally positioned so that fluid communication from the LNG tanker truck  10  to the bunkering line  14  is established. When bunkering is finished, the LNG tanker truck  10  can be disconnected and the respective valve means closed. 
         [0053]    If the source of LNG is e.g. a fixed storage tank based on land or on a floating support arrangement, the piping leading from the fixed storage tank to a connection point for the bunker connection line at the dock side could advantageously be cooled down in similar way as described above. This could be carried out by having a re-circulation of LNG provided from the fixed storage tank through the piping leading to the dock side connection point, to which the bunker line connection is to be connected. This operation would be carried out before the marine vessel arrives at port in a corresponding manner as the preparatory measures on the marine vessel. 
         [0054]    Consequently, this provides for a fast bunkering operation and a short port time. The marine vessel  1  is then ready for further operation. 
         [0055]    The LNG storage tank, the fuel feed tank, and the fuel supply arrangement, including components like pumps, heat exchangers, compressors, and valve means, are as such considered to be known by a person skilled in the art and are therefore not described in more detail in this connection. 
         [0056]    The different stages of operation described above can be suitable controlled by appropriate automation systems. This is considered to lie within the knowledge of a skilled person in the art and is therefore not discussed in detail in this connection. 
         [0057]    The description and the drawings related thereto are intended to clarify the basic idea of the invention. The invention may vary in detail within the scope of the ensuing claims.