Abstract:
Articulated tug and barge arrangements and methods for transportation, storage, and regasification of liquefied natural gas (LNG) aboard barge units and ballasting the barge units, and LNG bunker barge systems and methods for LNG bunkering, are provided.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation-in-part of and claims priority to U.S. Nonprovisional patent application Ser. No. 13/167,687, filed Jun. 23, 2011, of which the benefit of priority is claimed and the contents of which are hereby incorporated by reference. 
     
    
     BACKGROUND 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to an articulated tug and barge arrangement, and more particularly, to the transportation, storage and regasification of liquefied natural gas (LNG) aboard a barge unit and to the ballasting of the barge unit. 
         [0004]    2. Description of Related Art 
         [0005]    Large volumes of natural gas are produced in many remote areas of the world. Natural gas in stranded gas reserves has significant value if it can be economically transported to a market with commercial demand. When the terrain and distance permit, natural gas is often transported by submerged and/or land-based pipeline. However, it is well appreciated that where the natural gas is produced in distant locations where a pipeline is infeasible or economically prohibitive, other techniques must be developed and used to transport this gas to market. 
         [0006]    Liquefaction of natural gas facilitates storage and transportation of the natural gas because liquefied natural gas or “LNG” takes up only about 1/600 of the volume that the same amount of natural gas does in the gaseous state. The most commonly used technique for transportation of such distant natural gas resources involves liquefying the natural gas at or near the production site and then transporting the liquefied natural gas or “LNG” to market in massive, specially designed tanker ships called LNG carriers. LNG carriers have cryogenic compartments for carrying LNG to a destination port, where the LNG is offloaded to the storage tanks of a land-based regasification facility, where it may be stored in a liquid state or regasified. It is well understood that this requires the building and maintaining of onshore storage and gasification facilities at a major financial and time-consuming expense. Furthermore, economic and regulatory events have pushed LNG to the forefront as a growing alternative bunker fuel source in the US marine fuel market. Newer environmental regulatory regimes are placing tighten emissions requirements on commercial maritime industry, requiring vessels to use drastically cleaner burning alternatives like LNG instead of the traditional maritime fuels: fuel oil and diesel. Plus, recent discoveries of abundant low cost, non-conventional shale gas deposits in the US and worldwide have created a substantial price gap between LNG and environmentally compliant diesel. Thus, utilizing LNG as a marine fuel is economically advantageous and environmentally friendly, making it a sound alternative to traditional marine bunker fuels. 
         [0007]    For safety and ecological reasons, it has also been proposed to offload LNG in its liquid state into floating storage and regasification units (FSRUs), acting as LNG import terminals, which are typically between 350 to 400 meters long by up to 70 meters wide. LNG carriers are typically berthed and unloaded alongside the FSRU, and the LNG is stored in the FSRU&#39;s storage tanks. It is equally understood that building and maintaining FSRUs is an expensive and time consuming process and that relocation of FSRUs is slow, expensive and not common due to their physical limitations. 
         [0008]    U.S. Pat. No. 7,047,899, issued to Laurilehto et al., discloses a pusher-barge system in which a tug unit is supplied power by a barge unit by energy generated by LNG boil off gas on the barge unit. U.S. Pat. No. 6,089,022, issued to Zednik et al., discloses a method for regasification onboard an LNG carrier before transferring the gas to an onshore facility. U.S. Pat. No. 7,293,600, issued to Nierenberg, discloses an LNG carrier with a heat exchanger partially submerged in surrounding seawater. Thus while it has been proposed that regasification take place onboard an LNG carrier, financial and accessibility limitations remain major disadvantages. 
         [0009]    It has also been proposed that when the regasification facility is located onboard an LNG carrier, the source of heat used to regasify the LNG may be through the intake and discharge of seawater in the vicinity of the LNG carrier. However, discharging of the chilled seawater into the vicinity of the LNG carrier can have an undesirable impact on the environment and certain regulations now preclude the use of such an open loop system. 
         [0010]    It is thus a principal object of the present invention to provide a new system and method for storing, transporting and regasifying LNG, which is more economically feasible, environmentally friendly and in accord with regulations. 
         [0011]    Methods and systems of the present invention achieve aforementioned objects and goals by effectively replacing the storage tanks of the loading and discharging ports, thereby eliminating or minimizing the cost of otherwise necessary infrastructure. 
         [0012]    Methods and systems of the present invention achieve aforementioned objects and goals by providing an efficient, closed loop means for regasifying LNG using the inherent heat in seawater in conjunction with ballasting operations. 
         [0013]    The Daewoo patent WO Pat. No. 2011/046315 discloses a self propelled ship for bunkering liquefied fuel gas to a liquefied fuel gas propulsion ship; however financial, logistical and accessibility limitations remain as major disadvantages. It is thus the principle object of the present invention to provide a new system and method for transporting, storing and bunkering LNG, which is more adaptable, economically feasible. 
         [0014]    To overcome the limitations of the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specification, embodiments of the present invention provide a cost effective method and simplified means for transportation and regasification of LNG as well as an economically and environmentally acceptable way of regasifying LNG and performing ballasting operations. 
         [0015]    In addition, floating power production facilities, otherwise known as power generation barges or floating power plants (“FPPs”), are considered highly suitable and economically effective means for providing transportable power in the form of electricity to hard-to-reach regions of the world. FPPs were initially conceived as a means of monetizing stranded offshore natural gas, while simultaneously permitting offshore oil production in the deep waters of the world. Interest is now being shown to generate electrical power offshore in order to reduce the need for lengthy permitting applications needed for land based power plants. 
         [0016]    Moreover, FPPs may often be the only suitable means for power generation due to geographic and topographic restrictions precluding land-based power systems. In the operation of FPPs, greater consideration is being given to emissions and different fuels, particularly as the cost of oil increases to higher levels. Natural gas is considered a highly efficient and clean fuel for the operation of FPPs, yet the cost of supplying natural gas to FPPs such as through the construction and operation of new natural gas pipelines may prove uneconomical, and thus a needs exists for a system for transporting, storing and regasifying LNG for supply to FPPs. 
         [0017]    Since FPPs are typically not designed to sit in waters that are aggravated by waves but rather are normally designed for services in protected inland waters such as rivers, lagoons or small ports, FPPs are often not accessible by conventional large LNG carriers. Accordingly a need therefore exists for an efficient and economically sound system for transporting, storing and regasifying LNG for supply to floating power production facilities. Embodiments of the present invention are particularly suited to transport, store and supply fuel to floating power production facilities and thus satisfy these and other objects. 
       SUMMARY 
       [0018]    The following present a simplified summary of the present disclosure in a simplified form as a prelude to the more detailed description that is presented herein. 
         [0019]    The present invention is directed to an articulated tug and barge arrangement, to a method for transporting and regasifying LNG (liquefied natural gas) aboard a barge unit, to a method for storing LNG, and to a method for regasification aboard and ballasting of a barge unit. 
         [0020]    In one aspect, the present invention relates to an articulated tug and barge arrangement for transporting and regasifying LNG. The articulated tug and barge arrangement comprises a tug unit and a barge unit for conveying and regasifying a load of LNG, the barge unit comprising at least one type C tank for conveying the load of LNG under pressure, a gas combustion unit operatively coupled to the type C tank for the management of over pressurized gas within the type C tank, and at least one regasification unit fluidly coupled to receive LNG from said type C tank for regassifying LNG aboard the barge unit. The tug unit is operatively coupled to the barge unit and has a propulsion system capable of propelling the barge unit such that the tug unit may propel the barge unit and thus the load of LNG from a supply terminal to a desired location such as a natural gas pipeline or an offshore floating power production facility. In preferred embodiments, the present invention relates to an articulated tug and barge arrangement and method for transporting and storing LNG aboard a barge unit and to regasifying LNG aboard the barge unit. 
         [0021]    In one embodiment of the present invention, the barge unit comprises at least one ballast tank having ballast water and the regasification unit is fluidly coupled to the at least one ballast tank such that the ballast water may be circulated from the ballast tank through a heat exchanger disposed within said regasification unit to warm and regasify the LNG. 
         [0022]    In another embodiment of the present invention, the barge unit further comprises at least one ballast tank for the containment of ballast water, and the regasification unit is fluidly coupled to a water inlet disposed along the exterior surface of said barge unit such that water may be withdrawn from the body of water surrounding said barge unit and circulated through a heat exchanger disposed within the regasification unit to warm and regasify the LNG; and the regasification unit is also fluidly coupled to the at least one ballast tank such that water may be flowed from the regasification unit into said ballast tank for ballasting operations of the barge unit. 
         [0023]    Preferably, the barge unit further comprises a means for heating the ballast water. For instance, a second heat exchanger and a third heat exchanger fluidly coupled to a boiler may be provided aboard the barge unit for heating the ballast water. The second heat exchanger may be configured such that water from the water inlet is heated by steam from a boiler prior to circulation of the water through the regasification unit. The third heat exchanger may be configured such that chilled water flowing from the regasification unit would be heated prior to circulation into the ballast tanks. 
         [0024]    In another embodiment, the propulsion system of the tug unit is powered by LNG. 
         [0025]    In yet another embodiment, the barge unit further comprises a nitrogen injection system fluidly coupled to the send-out piping that is fluidly coupled to the regasification unit aboard the barge unit for adjustment of the calorific value of the natural gas being transferred to off board facilities such as a natural gas pipeline or an offshore floating power production facility. 
         [0026]    In another aspect, the present invention relates to a method for transporting and regasifying liquefied natural gas (LNG) aboard an articulated tug and barge arrangement wherein said tug and barge arrangement comprises a tug unit operatively coupled to a barge unit having at least one type C tank, and the method comprises loading LNG directly from a liquefaction plant into the or each type C tank disposed within the barge unit, employing the tug unit to propel the barge unit from the liquefaction plant to an off board facility, regasifying the LNG aboard the barge unit, and transferring the natural gas to the off board facility. In embodiments of the present invention, the off board facility may be an offshore floating power production facility, a natural pipeline, and/or an onshore power production facility. 
         [0027]    In yet another aspect, the present invention relates to a method for transporting and regasifying liquefied natural gas (LNG) aboard an articulated tug and barge arrangement and fueling an offshore power production facility, wherein said tug and barge arrangement comprises at least one barge unit having at least one type C tank and a tug unit operatively coupled to the at least one barge unit, and the method comprises loading LNG directly from a liquefaction plant into the or each type C tank disposed within the barge unit, employing the tug unit to propel the barge unit from the liquefaction plant to a floating power production facility, regasifying the LNG aboard the barge unit, and supplying fuel to said floating power production facility for power generation. 
         [0028]    In another aspect, the present invention relates to a method of regasifying liquefied natural gas (LNG) aboard a barge unit and ballasting said barge unit, where the method comprises withdrawing seawater from the body of water surrounding said barge unit and flowing said seawater through a regasification unit which is positioned aboard said barge unit for the heating and regasification of LNG, flowing said seawater from the regasification unit into ballast tanks disposed within the barge unit for the ballasting down of the barge unit, flowing LNG from type C tanks disposed within the barge unit for storing LNG during transport through said regasification unit to heat and transform said LNG into natural gas within said regasification unit, and transferring natural gas from the regasification unit to off board facilities. Preferably, the method further comprises heating the seawater. 
         [0029]    In another aspect, the present invention relates to a method for regasifying liquefied natural gas (LNG) aboard a barge unit and ballasting said barge unit, said barge unit having at least one ballast tank for the containment of water and at least one type C tank for the containment of LNG, and wherein the barge unit has at least one regasification unit fluidly coupled to the at least one ballast tank and to the at least one type C tank, and the method comprises the steps of introducing water into the at least one ballast tank of the barge unit, directing said water from the ballast tank through at least one heat exchanger disposed within said regasification unit for the regasification of LNG to a gaseous state, unloading said natural gas from the barge unit, directing said water from the heat exchanger to at least one ballast tank, and allowing the barge unit to ballast while unloading natural gas from the barge unit. Preferably, the method comprises heating the ballast water. 
         [0030]    By regasifying LNG aboard the barge unit before it is off-loaded from the barge unit to desired facilities such as a natural gas pipeline or an offshore floating power production facility, the need for onshore LNG storage tanks is eliminated thereby allowing transportation and deployment of LNG to markets that would not otherwise be available due to restrictions such as size preventing the use of conventional LNG carriers. Moreover, by using ballast water as a primary heat exchange medium for the onboard regasification units, embodiments of the present invention provide an environmentally-friendly method and system, by safely and efficiently enabling regasification and unloading operations without discharge to surrounding seawater while providing necessary ballast water to the barge unit offsetting respective displacements. 
         [0031]    Embodiments of the present invention are directed toward an articulated tug and barge arrangement for transporting and storing liquefied natural gas (LNG) and bunkering liquefied natural gas to a liquefied natural gas fueled ship; the articulated tug and barge arrangement comprising: a tug unit comprising a propulsion system; a barge unit for conveying and storing and bunkering a load of LNG, the barge unit comprising at least one type C tank for conveying and storing the load of LNG under pressure, a gas combustion unit operatively coupled to the type C tank for the management of over pressurized boil-off gas within the type C tank, and an at least one liquid manifold fluidly coupled to bunker LNG from the type C tank to the liquefied natural gas fueled ship; wherein said tug unit is operatively coupled to said barge unit for propelling said barge unit from one location to another. 
         [0032]    In another embodiment, the barge unit further comprising an at least one regasification unit fluidly coupled to receive LNG from said type C tank for regasifying LNG aboard the barge unit. 
         [0033]    In another aspect of the articulated tug and barge arrangement, the barge unit further comprising at least one ballast tank having ballast water and at least one regasification unit fluidly coupled to the at least one ballast tank such that the ballast water may be pumped from the ballast tank through a heat exchanger disposed within a regasification unit to warm and regasify the LNG. 
         [0034]    In yet another embodiment of the articulated tug and barge arrangement of the present invention, the barge unit further comprises at least one ballast tank for the containment of ballast water and at least one regasification unit is fluidly coupled to a water inlet disposed along the exterior surface of said barge unit such that water may be withdrawn from the body of water surrounding said barge unit and circulated through a heat exchanger disposed within said regasification unit to warm and regasify the LNG; and wherein the regasification unit is fluidly coupled to the at least one ballast tank such that water may be flowed from said regasification unit into said ballast tank for ballasting operations. 
         [0035]    In yet another aspect, the articulated tug and barge arrangement further comprises an attachment means for operatively connecting the tug unit to the barge unit. 
         [0036]    In yet another aspect, the propulsion system of the tug unit is powered by LNG. 
         [0037]    In yet another aspect of the articulated tug and barge arrangement, wherein the barge unit further comprises a nitrogen injection system fluidly coupled to a regasification unit by high pressure piping for adjustment of the calorific value of natural gas unloaded from the barge unit. 
         [0038]    In another embodiment, the present invention relates to a tug and barge arrangement for transporting and storing and transferring liquefied natural gas (LNG) to an off board facility, the tug and barge arrangement comprising: a tug unit comprising a propulsion system; a barge unit for conveying and transferring a load of LNG to the off board facility for fueling said off board facility, the barge unit comprising at least one type C tank for transporting the load of LNG under pressure, a gas combustion unit operatively coupled to the type C tank for the management of over pressurized boil-off gas within the type C tank, and at least one liquid manifold fluidly coupled to supply LNG from said type C tank to the off board facility; wherein said tug unit is operatively coupled to said barge unit for propelling said barge unit from one location to another. In one aspect of the present invention, the off board facility is a ship fueled by LNG. In yet another aspect, the off board facility receives the liquefied natural gas as bunker fuel. In yet another aspect, the tug is operatively coupled to said barge unit in an articulated manner. In yet another aspect tug is operatively coupled to said barge unit in a push-barge manner. 
         [0039]    In another aspect, the present invention relates to a method for transporting and storing liquefied natural gas (LNG) aboard an articulated tug and barge arrangement wherein said tug and barge arrangement comprises a tug unit operatively coupled to a barge unit having at least one type C tank, said method comprising: loading LNG directly from a liquefaction plant into the or each type C tank disposed within the barge unit; employing the tug unit to propel the barge unit from the liquefaction plant to a destination of an off board facility; storing the LNG within the or each type C tank disposed within the barge unit at the destination of the off board facility; and transferring the LNG to the off board facility as fuel. In one embodiment, the off board facility is a ship fueled by LNG. In another embodiment, the off board facility is a ship fueled by natural gas. In yet another embodiment, the off board facility is a ship fueled by natural gas, said ship having a regasification for the regasification of LNG onboard the ship. In yet another embodiment, the off board facility is a floating power production facility. In one embodiment, the method, the step of storing the LNG within the or each type C tank lasts up to 21 days. In another embodiment, the method further comprises stacking two or more barge units at the destination of the off board facility for while storing the LNG within the or each type C tank disposed within respective barge units. In yet another embodiment, the method further comprises allowing the barge unit to receive ballast while transferring the LNG to the off board facility. 
         [0040]    In another aspect, the present invention relates to a method for transporting and bunkering liquefied natural gas (LNG) fuel to a ship, said method comprising: loading LNG directly from a liquefaction plant into an at least one type C tank disposed within a barge unit for storing and transporting said LNG; employing a tug unit to operatively couple said barge unit to propel the barge unit from the liquefaction plant to the ship; unloading LNG from the at least one type C tank disposed within the barge unit; and bunkering LNG from the barge unit to the ship for supplying said ship with LNG fuel. In one embodiment, the method further comprises stacking two or more barge units at a destination of the ship while storing the LNG within the or each type C tank disposed within respective barge units. 
         [0041]    In another aspect, the present invention relates to a method for transporting and bunkering liquefied natural gas (LNG) fuel to a ship, said method comprising loading LNG directly from a liquefaction plant into an at least one membrane tank having a boil off gas management system disposed within a barge unit for storing and transporting said LNG; employing a tug unit to operatively couple said barge unit to propel the barge unit from the liquefaction plant to the ship; unloading LNG from the at least one membrane tank disposed within the barge unit; and bunkering LNG from the barge unit to the ship for supplying said ship with LNG fuel. 
         [0042]    In one embodiment, the present invention relates to such method further comprising stacking two or more barge units at a destination of the ship while storing the LNG within the or each membrane tank disposed within respective barge units. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0043]    Illustrative embodiments of the present invention are described herein with reference to the accompanying drawings, in which: 
           [0044]      FIG. 1  is a top plan view of a general arrangement of one embodiment of an articulated tug and barge arrangement, illustrating the barge unit having four bi-lobe type C tanks. 
           [0045]      FIG. 2  is an elevation view of a general arrangement of one embodiment of an exemplary barge unit. 
           [0046]      FIG. 3  is a top plan view of a general arrangement of one embodiment of an exemplary barge unit. 
           [0047]      FIG. 4A  is a cross section of one embodiment of an exemplary barge unit having bi-lobe type C tank. 
           [0048]      FIG. 4B  is another cross section of one embodiment of an exemplary barge unit having bi-lobe type C tanks. 
           [0049]    FIGS.  5  through  6 - 2  show example operations of a transportation system employing articulated tug and barge arrangement and push-barge tug and barge arrangement according to the present invention. 
           [0050]      FIG. 5-2A  is an enlarged, top plan view of a general arrangement of one embodiment of the present invention illustrating elements of the barge unit as shown in the exemplary operations of the transportation system of  FIG. 5-2  according to the present invention. 
           [0051]      FIG. 7  shows example operations of a storage and fueling system employing articulated tug and barge units according to the present invention. 
           [0052]      FIG. 8  is a schematic of a ballast and regasification system according to an embodiment of the present invention. 
           [0053]      FIG. 9  is a schematic of a ballast and regasification system according to a preferred embodiment of the present invention. 
           [0054]      FIG. 10  is a top plan view of a general arrangement of one embodiment of an exemplary floating power barge and regasification vessel having LNG storage. 
           [0055]      FIG. 11  is a top plan view of a general arrangement of one embodiment of an exemplary floating power barge and regasification vessel having LNG storage. 
           [0056]      FIG. 12  is a top plan view of a general arrangement of one embodiment of an exemplary floating power barge and regasification vessel having LNG storage. 
       
    
    
     DETAILED DESCRIPTION 
       [0057]    Persons of ordinary skill in the art will realize that the following disclosure is illustrative only and not in any way limiting. Other embodiments of the disclosure will readily suggest themselves to such skilled persons having the benefit of this disclosure. 
         [0058]    The present disclosure is directed to embodiments of an articulated tug and barge arrangement and methods for transporting, storing and regasifying LNG (liquefied natural gas) aboard a barge unit and a method for regasifying aboard and ballasting down a barge unit. 
         [0059]    For natural gas to be transported by sea, natural gas is typically liquefied into liquid form. This is known as liquefied natural gas or LNG. LNG is typically stored at a liquefaction facility in storage tanks, at which point it may be transferred to an LNG carrier for transport. Upon arrival at a destination, the LNG cargo is typically transferred to storage tanks at a terminal facility. Thereafter, the LNG is regasified back into natural gas and is transferred to a natural gas pipeline for distribution to a gas network and to consumers. 
         [0060]    Referring initially to  FIG. 1 ,  FIG. 2 ,  FIG. 3 ,  FIG. 4A  and  FIG. 4B , the basic constructional details, principles of operation and arrangement of an exemplary articulated tug and barge arrangement  100  according to a preferred embodiment of the present invention will be discussed. 
         [0061]    In  FIG. 1 , an articulated tug and barge arrangement  100  according to a preferred embodiment of the present invention is provided. In  FIG. 1 , the articulated tug and barge arrangement  100  comprises a barge unit  102  operatively connected to a tug unit  104  having a propulsion system for propelling the barge unit  102 . The articulated tug and barge arrangement  100  is in the form of a single-degree-of-freedom system, in which the barge unit  102  has an aft notch  106  with a recess  108  for receiving a front end  110  of the tug unit  104  with a pin connection  109 , thereby allowing quick connect and release operation between the tug unit  104  and barge unit  102  and combining good economics of tugboat and barge operation with the speed and weather-ability of a ship. 
         [0062]    While the articulated tug and barge arrangement  100  in  FIG. 1  is according to a preferred embodiment of the present invention, it can be appreciated that other forms of barge arrangements may be used. 
         [0063]    The barge unit  102  comprises at least one type C tank  112  for conveying a load of LNG under pressure, a gas combustion unit  114  operatively coupled to the type C tank  112  for the management of over pressurized boil-off gas within the type C tank  112 , and at least one regasification unit  116  fluidly coupled to receive LNG from said type C tank  112  for regassifying LNG aboard the barge unit  102 . In one embodiment, the barge unit  102  comprises a dome  170  disposed atop a type C tank  112  for the collection of boil-off gas, and the dome  170  has a small boil-off gas vapor header fluidly coupled to a gas compressor, for allowing for pressure reduction within the type C tank  112 . This is an economic and effective means of controlling the cargo tank pressure while the barge is at a discharging location. 
         [0064]    As will be understood by those skilled in the art, it is common practice to transport LNG in LNG receptacles aboard an LNG carrier, typically ranging in capacity from 100,000 m 3  to 160,000 m 3 , and when the LNG carrier reaches its destination, the LNG is offloaded (at typical rates of 10,000-12,000 cubic meters per hour (m 3 /hr)) in its liquid state onto shore where it is stored and thereafter revaporized before sending it on to end users as a gas. It is well understood that this requires the building and maintaining of onshore storage and gasification facilities at a major financial and time-consuming expense. It has also been proposed to offload LNG in its liquid state into floating storage and regasification units (FSRUs), acting as LNG import terminals, which are typically between 350 to 400 meters long by up to 70 meters wide. LNG carriers are typically berthed and unloaded alongside the FRSU, and the LNG is stored in the FSRU&#39;s storage tanks. Due to the large size of the LNG carriers, the resulting change in draft as a result of unloading cargo is typically very small and negligible. It is equally understood that building and maintaining FSRUs is an expensive and time consuming process and that relocation of FSRUs is slow, expensive and not common due to their physical limitations. 
         [0065]    In accordance with the present invention, the articulated tug and barge arrangement  100  is relatively inexpensive to build and operate compared to the LNG carrier and FSRU. The preferred size of the barge unit  102  is up to approximately 30,000 m 3 . The most preferred length of the barge unit is approximately 177 meters. A type C tank  112  is a pressure vessel having a design pressure of at least 2 bar. Referring to  FIGS. 1-3 , in a preferred embodiment, the LNG is stored aboard the barge unit  102  in four type C tanks  112  under pressure, enabling long transport and storage of LNG aboard the barge unit  102 . The type C tank  112  eliminates the need for continuous management of boil off gas, as it can, for example, sufficiently contain the LNG cargo under pressure for up to approximately twenty-one (21) days. Thus, embodiments of the present invention eliminate the need for LNG storage tanks at the destination. 
         [0066]    Preferably, the type C tanks  112  are of bi-lobe design ( FIGS. 1 ,  3 ,  4 A,  4 B) simply supported on insulated structural saddles such that they are not influenced by any loads emanating from the vessel hull girder during operations. A different variety of materials may be used for the type C tanks  112 . The type C tanks  112  are preferably constructed of either 9% nickel steel or stainless steel AISI 304L and would be complete with swash bulkheads, stiffening rings, vapor dome, pump wells, ladders and other connections as can be appreciated by one skilled in the art. The capacity of each type C tank can vary in size but the preferred embodiment is approximately 7,500 m 3 . The type C tanks  112  are designed to store LNG at a temperature of approximately −163 degrees C. and at near atmospheric pressure. However, it can be appreciated that the type C tanks  112  will be capable of internal pressure calculated based upon the service profile due to a design heat input. The preferred dimensions of each type C tank  112  are approximately 25 meters in width and approximately 30 meters in length. Preferably, each type C tank is a single containment and is insulated with insulation such that boil-off is limited to a rate of approximately 0.24% of the tank capacity per day. 
         [0067]    Referring to  FIG. 1 , the four type C tanks  112  are fluidly coupled to the regasification unit  116  via a liquid suction header  122 , which allows the LNG to be pumped from the type C tanks  112  to the regasification unit  116  for transformation of the LNG into a gaseous state (i.e., natural gas) onboard the barge unit  102 . A liquid header  124  is connected to the type C tanks  112  and configured for receipt of LNG from a supply terminal via the loading manifold  126  disposed at each side (starboard and port) of the barge unit  102 . 
         [0068]    In a preferred embodiment, the liquid header  124  is fluidly connected to an upper manifold  126   a  and to a lower manifold  126   b  of each loading manifold  126  ( FIG. 4B ). The upper manifold  126   a  ( FIG. 4B ) is configured for receipt of LNG from the storage tanks  128  of a traditional supply facility  130  ( FIG. 6 ). The lower manifold  126   b  ( FIG. 4B ) is configured for the receipt of LNG directly from a liquefaction plant  132  ( FIG. 6 ), obviating the need for LNG storage tanks  128  at a supply facility  130 . In effect, the articulated tug and barge arrangement  100  is capable of serving as a transport and storage facility, and acting as an transport terminal ring  111 , by the barge unit  102  receiving LNG directly from the liquefaction plant  132 , transporting the LNG to the destined off board facilities  152  and employing regasification unit  116  and type C tanks  112  in embodiments of the present invention, as illustrated in  FIG. 5 . 
         [0069]    The heat exchanger  134  of the regasification unit  116  of the present invention may be a shell and tube heat exchanger, a printed circuit heat exchanger, a bent-tube fixed-tube-sheet exchanger, plate-type exchanger, spiral wound exchanger, falling-film exchanger, or other heat exchangers commonly known by those skilled in the art that meet the temperature, volume and heat absorption requirements for the LNG to be regasified. 
         [0070]    Considering space limitations onboard the barge unit  102  and cost comparison, the regasification unit  116  preferably uses saturated steam as the direct heating medium. The regasification unit  116  allows the LNG to be pressurized and regasified to the discharge pressure of approximately 30 to 120 bar. 
         [0071]    Preferably, one or more dual fuel generators  136  are disposed upon the aft deck of the barge unit  102  and are used to power the barge unit  102  operations, including providing power to the conventional pressure pumps  150  used for water/fluid pumping and circulation as described herein. For instance, suction pumps or single stage centrifugal pumps are frequently used for water/fluid pumping in maritime and industrial applications, and are well known to those skilled in the art. In a preferred embodiment, two cargo pumps of centrifugal design submersible type are disposed within each type C tank  112 , one within each lobe, for discharging LNG when necessary. In a preferred embodiment, two suction pumps of centrifugal design are disposed within each type C tank  112 , one within each lobe, for transferring LNG to the regasification unit  116 . 
         [0072]    A nitrogen injection system comprising a nitrogen generator  138  is preferably provided aboard the barge unit  102  to supply nitrogen gas for drying out and inerting the type C tanks  112  before LNG cargo loading or grade changing operations and after discharging cargo. A nitrogen generator is, in effect, an air compressor which pushes air through a permeable membrane and separates nitrogen from air, as would be known to one skilled in the art. The nitrogen generator  138  is fluidly coupled to the type C tanks  112  by an inert gas header  140 . Preferably, a deck storage tank  142  for LNG is provided aboard the barge unit  102  and is fluidly coupled to the type C tanks  112  for gassing-up operations, to remove nitrogen from the type C tanks  112  or to remove cargo vapors of the previous cargo. A vapor return line  144  provides a fluid connection between the type C cargo tanks  112  and the loading manifold  126  to maintain tank pressure during loading and discharge operations. As vapor is generated during cool down operations, the vapor return line  144  allows the vapor to be sent from the type C tanks  112  through the manifold  126  to the supply facility  130 . While it is preferable to employ a nitrogen generator  138  aboard the barge unit  102 , it can be appreciated that an alternative is to employ a nitrogen storage tank aboard the barge unit  102 . 
         [0073]    A vapor header  146  provides a fluid connection to the regasification unit  116  for send-out or unloading of the natural gas directly from the barge unit  102  at the high pressure vapor discharge manifold  148  to the desired off board facility  152 , obviating the need for LNG storage tanks at the destined off board facility  152 . The off board facility  152  may be a natural gas pipeline  154  supplying natural gas to consumers, as illustrated in  FIG. 6 . In effect, the articulated tug and barge arrangement  100  is capable of serving as a transport and storage facility, and acting as an import terminal, by the barge unit  102  employing regasification unit  112  and type C tanks  112  in embodiments of the present invention. 
         [0074]    Alternatively, referring to  FIG. 7 , the off board facility  152  may be a floating power production facility  156 , otherwise referred to as a power generation barge or floating power plant (“FPP”), or the off board facility may be a liquefied natural gas fueled ship  152  having an LNG fuel tank  174  as shown in  FIG. 5-2  and  FIG. 6-2 . Since the cost of storing and supplying LNG or natural gas to FPPs such as through the construction and operation of new natural gas pipelines may prove uneconomical, the articulated tug and barge arrangement  100  economically and feasibly achieves this result by serving as a transport, storage and supply facility to the FPP  156 , by the barge unit  102  employing regasification unit  112  and type C tanks  112  in embodiments of the present invention. The small size and efficient maneuverability of the articulated tug and barge arrangement  100  enables direct accessibility to FPPs  152  often moored in the protected inland waters such as rivers, lagoons or small ports, as illustrated in  FIG. 7 . Embodiments of the present invention are customizable to meet the fuel demand of the FPP  156 . Moreover, a plurality of barge units  102  may be moored at the FPP, enabling additional reserve storage of fuel for the FPP  156 . 
         [0075]    Referring to  FIG. 5 , in a preferred method of operation, LNG is loaded directly from a liquefaction plant  132  through the lower manifold  126   b  ( FIG. 4B ) into at least one type C tank  112  disposed within a barge unit  102 , obviating the need for LNG storage tanks  128  nearby the liquefaction plant  132 . The loading operation is indicated by arrow A. A tug unit  104  is employed to propel the barge unit  102  from the liquefaction plant  132  to an off board facility  152 , as indicated by arrows B. LNG is regasified into natural gas aboard the barge unit  102 , and the natural gas is transferred to the off board facility  152 , as indicated by arrow C, obviating the need for LNG storage tanks at the off board facility  152 . The tug unit  104  may thereafter be employed to propel the barge unit  102  from the off board facility  152  to the liquefaction plant  132 , as indicated by arrow D. Moreover, in a more preferred method according to another aspect of the invention, one tug unit  104  and three barge units  102  are provided, whereby a first barge unit  102  is undergoing a loading operation (arrow A) at the liquefaction plant  132  while a second barge unit  102  is being transported (arrows B) to an off board facility  152  while a third barge is regasifying LNG and undergoing unloading operations (arrow C) at the off board facility  152 , as may be appreciated by illustration in  FIG. 5 . In effect, the articulated tug and barge arrangement  100  is capable of serving as a transport and regasification facility and provides for a transport terminal ring  111  as exemplified in  FIG. 5 . 
         [0076]    In embodiments of the present invention, as examples, the off board facility  152  may be an offshore floating power production facility, a natural pipeline, and/or an onshore power production facility. 
         [0077]    In yet another embodiment of the present invention, a method is provided for transporting and storing LNG. Referring to  FIGS. 5 ,  6  and  7 , LNG is loaded directly into a type C tank  112  disposed within one or more barge units  102 , at least one tug unit  104  is employed to transport each barge unit  102  to an off board facility  152 , and LNG is thereafter stored in the type C tank  112  disposed within the barge unit  102  at the off board facility  152 . Since the barge unit  102  can, for example, sufficiently contain the LNG cargo under pressure for up to approximately twenty-one (21) days, in effect, the articulated tug and barge arrangement  100  replaces otherwise necessary storage facilities at the off board facility  152 , by employing type C tanks  112  in the articulated tug and barge arrangement  100  in embodiments of the present invention. Moreover, by employing a regasification unit  116  aboard the barge unit  102 , LNG may thereafter be regasified into natural gas aboard the barge unit  102  and offloaded from the barge unit  102  to the off board facility  152 . By way of example, the off board facility  152  may be an offshore floating power production facility ( FIG. 7 ), a natural pipeline ( FIG. 6 ), and/or an onshore power production facility. 
         [0078]    While it has been proposed to use seawater as a heat source for the regasification of LNG aboard an LNG carrier, conventional discharging of the chilled seawater to the surrounding water body can have undesirable impact on the environment. 
         [0079]    Referring to  FIGS. 4A ,  4 B, and  8 , in one preferred embodiment, the barge unit  102  comprises at least one water inlet  158  fluidly coupled to a regasification unit  116 , such that seawater may be received through the water inlet  158  and pumped through the heat exchanger  134  disposed within the regasification unit  116  to warm and liquefy the LNG. The barge unit  102  further comprises at least one ballast tank  118  fluidly coupled to the regasification unit  116  such that chilled seawater flowing from the regasification unit  116  may be circulated to the ballast tank  118 , as schematically depicted in  FIGS. 8 and 9 , obviating the discharge of the chilled water into the environment and thereby preventing or mitigating the impact on the environment. 
         [0080]    It may be appreciated that the water inlet  158  may be disposed along an external surface of the barge unit  102  or within a sea chest disposed within the barge unit  102 . 
         [0081]    Preferably, the barge unit  102  further comprises a strainer  160  configured such that seawater may be flowed through the strainer  160  prior to entering the heat exchanger  134  of the regasification unit  116 , as schematically depicted in  FIGS. 8 and 9 . 
         [0082]    In one preferred embodiment, the barge unit  102  comprises at least one set of upper ballast tanks  120   a ,  120   b  disposed within the barge unit  102  vertically above the draft water line of the barge unit  102 , wherein the regasification unit  116  is fluidly coupled to the set of upper ballast tanks  120   a ,  120   b . Providing ballast tanks  120   a ,  120   b  above the draft water line of the barge unit  102  mitigates the heat transfer between the chilled ballast water in ballast tanks  120   a ,  120   b  and the surrounding water body, thereby further preventing or mitigating the impact on the environment. Moreover, providing chilled water to the ballast tanks  118 ,  12   a ,  120   b  in effect cools down the outer containment system of the barge unit  102  and thereby mitigates boil-off within the type C tanks  112  within the barge unit  102 . 
         [0083]    In a preferred embodiment, the barge unit  102  receives ballast water at a water inlet  158  and via circulation through the heat exchanger  134  and into the ballast tanks  118 ,  120   a ,  120   b  at a typical rate (e.g., 2000 m 3 /hour) during the offloading of the regasified cargo (i.e., natural gas) such that positive displacement of the barge unit  102  achieved from ballasting down is offset by negative displacement of the barge unit  102  achieved from offloading the cargo. In a most preferred embodiment, the barge unit  102  receives ballast water via circulation through the heat exchanger  134  and into the ballast tanks  118 ,  120   a ,  120   b  at substantially the same rate as the offloading of the regasified cargo (i.e., natural gas) such that the positive displacement of the barge unit  102  achieved from ballasting down is substantially offset by the negative displacement of the barge unit  102  achieved from offloading the cargo. 
         [0084]    Moreover, since ballast tanks  118 ,  120   a ,  120   b  are fluidly coupled to the regasification unit  116 , ballast water may also be pumped from the ballast tanks  118 ,  120   a ,  120   b  and circulated through the heat exchanger  134  disposed within the regasification unit  116  to warm and liquefy the LNG and thereafter circulated back to the ballast tanks  118 ,  120   a ,  120   b , as schematically depicted in  FIGS. 8 and 9 , in a closed loop system, again obviating the discharge of the chilled water into the environment and thereby preventing or mitigating the impact on the environment. 
         [0085]    Referring to  FIG. 9 , in one preferred alternative embodiment, the chilled seawater flowing from the regasification unit  116  may be circulated through a heat exchanger  162  such as a steam heater in fluid connection with a heating element such as a boiler  164  to warm the chilled seawater. The warmed seawater from the heat exchanger  162  is flowed into ballast tanks  118 ,  120   a ,  120   b . The temperature of the ballast water entering and stored in the ballast tanks  118 ,  120   a ,  120   b  can thus be controlled. Moreover, the temperature of the seawater flowed into the ballast tanks  118 ,  120   a ,  120   b  may be raised to a point such that it is sanitized (e.g. &gt;165 degrees F.), thereby enabling compliance with ballast water exchange regulations. 
         [0086]    Referring to  FIG. 9 , in yet another preferred embodiment, the seawater received from the water inlet  158  may be pumped through a heat exchanger  166  such as a steam heater in fluid connection with a heating element such as a boiler  168  where the seawater is warmed. It may be appreciated that other heating mediums such as glycol and propane may alternatively be used to heat the seawater received from the inlet  158 . The warmed seawater flowing from the steam heat exchanger  166  may be flowed through the regasification unit  116  to warm and regasify the LNG. The steam heat exchanger  166  is preferably a conventional shell and tube heat exchanger and may provide either all or a portion of the heat required for the LNG regasification. In the event that the local seawater temperature is not sufficient to provide the amount of heat required for the desired level of regasification operations, this embodiment of the invention provides operational advantages. 
         [0087]    By using ballast water as a primary heat exchange medium for the onboard regasification units  116 , embodiments of the present invention safely and efficiently enable regasification, unloading and ballasting operations without discharge to surrounding seawater while providing necessary ballast water to the barge unit  102  thereby offsetting respective displacements. 
         [0088]    It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made without departing from the spirit and scope of the invention. While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should only be defined in accordance with the following claims and their equivalents. All patents and publications discussed herein are incorporated in their entirety by reference thereto.