Patent Publication Number: US-2019193817-A1

Title: Natural gas liquefaction vessel

Description:
BACKGROUND 
     1. Field of the Invention 
     Embodiments of the invention described herein pertain to the field of marine liquefaction of natural gas. More particularly, but not by way of limitation, one or more embodiments of the invention describe a natural gas liquefaction vessel. 
     2. Description of the Related Art 
     Natural gas is typically transported by pipeline from the location where it is produced to the location where it is consumed. However, large quantities of natural gas may sometimes be produced in an area or country where production far exceeds demand, and it may not be feasible to transport the gas by pipeline to the location of commercial demand, for example because the location of production and the location of demand are separated by an ocean or rain forest. Without an effective way to transport the natural gas to a location where there is a commercial demand, opportunities to monetize the gas may be lost. 
     Liquefaction of natural gas facilitates storage and transportation of the natural gas. Liquefied natural gas (“LNG”) takes up only about 1/600th of the volume that the same amount of natural gas does in its gaseous state. LNG is produced by cooling natural gas below its boiling point (−259° F. at atmospheric pressure). LNG may be stored in cryogenic containers slightly above atmospheric pressure. By raising the temperature of the LNG, it may be regasified back to its gaseous form. 
     The demand for natural gas has stimulated the transportation of LNG by special vessels. Natural gas produced in locations where it is abundant, may be liquefied and shipped overseas in this manner to locations where it is most needed. Typically, the natural gas is gathered through one or more pipelines to a land-based liquefaction facility. Land-based liquefaction facilities and the associated gathering pipelines are costly, may occupy large areas of land and take several years to permit and construct. Thus, land-based liquefaction facilities are not optimally suited to adapt to variation in the location of natural gas supplies or to liquefy small or stranded gas reserves. In addition, once natural gas is liquefied at a land based facility, the LNG must be stored in large land-based cryogenic storage tanks, transported through a special cryogenic pipeline to a terminal facility, and then loaded onto a vessel equipped with cryogenic compartments (such a vessel may be referred to as an LNG carrier or “LNGC”), which in combination may increase the overall expense of transporting the gas to its ultimate destination. 
     LNG projects are inherently capital intensive. The liquefaction plant is the largest cost component, accounting for approximately 50% of the total cost of the LNG value chain; hence, cost reduction of the liquefaction plant is an important issue. The capital cost of the liquefaction plant is dependent on several factors such as plant location, size, site conditions, and quality of feed gas. The thermodynamics of the liquefaction processes are well developed. Thus, improvements in the industry come from improvements in the liquefaction process and infrastructure that reduce cost. Of course, development costs are capitalized over the life of the facility. Therefore, efficiency in process and infrastructure may reduce the overall cost per ton of LNG over the life of the liquefaction plant. 
     One approach to reducing the cost of liquefying natural gas is to liquefy the gas on a floating unit or vessel. Suppliers have converted existing LNGCs to accommodate onboard liquefaction equipment. However. LNGCs that are available for conversion are typically older ships that lack the necessary space and deadweight for the additional liquefaction equipment. On these older carriers, ship designers conventionally maximize the cargo tank size due to the ship&#39;s original functionality as an LNGC, and therefore cargo space in the hull consumes a large fraction of vessel&#39;s deadweight. Deadweight tonnage is a measure of how much mass a ship is carrying or can safely carry; it does not include the weight of the ship. A vessel&#39;s deadweight is critical to its use, because if the equipment onboard the ship is too heavy, the ship may sit too deeply in the water or break under excessive longitudinal stresses. Sponsons are sometimes added to the sides of the LNGC to create more cargo space and increase deadweight, but often this does not provide enough deadweight to allow the addition of complex liquefaction equipment. In addition, old tonnage LNGC vessels are typically steam powered and are not able to generate the required 40-50 MW of power needed for liquefaction equipment. As a result of these size and power constraints, old tonnage vessels require costly rework to convert them to natural gas liquefaction vessels. Such rework is especially economically infeasible where the LNGC will be in service for a long duration, such as more than five years, due to the cost of powering the older vessel. 
     While newbuild vessels may be created using existing LNGC design plans, with the added requirement of natural gas liquefaction, most LNGC plans do not provide the necessary deadweight for the addition of heavy liquefaction equipment. 
     As is apparent from the aforementioned problems, current old tonnage vessels are not good candidates for conversion to liquefaction-capable LNGCs, as they suffer from the many shortcomings mentioned above, and newbuild vessels designed to maximize cargo space do not have the necessary deadweight for liquefaction equipment. Therefore, there is a need for an improved natural gas liquefaction vessel. 
     SUMMARY 
     Embodiments described herein generally relate to an apparatus and system for a natural gas liquefaction vessel. A natural gas liquefaction vessel is described. An illustrative embodiment of a natural gas liquefaction vessel includes the natural gas liquefaction vessel newbuilt on a Q-Max class hull, the Q-Max class hull including a cargo hold, a plurality of membrane-type cryogenic cargo tanks installed within the cargo hold, the plurality of cryogenic cargo tanks each about 41 m wide, at least one pair of ballast wing tanks, one each on a port side and a starboard side of the cargo hold and adjacent to at least one cryogenic cargo tank of the plurality of cryogenic cargo tanks, each ballast wing tanks about 7 m wide, a trunk deck above the cargo hold, the trunk deck extending over the plurality of cryogenic cargo tanks and over the at least one pair of ballast wing tanks, and a natural gas liquefaction plant on the trunk deck, the natural gas liquefaction plant including a gas loading and reception manifold at the bow of the natural gas liquefaction vessel fluidly coupled to the source of natural gas, an amine module, the amine module including at least one compressor and acid gas removal pre-treatment equipment, the amine module fluidly coupled to a dehydration module and mercury removal equipment, the dehydration module and mercury removal equipment fluidly coupled to a liquefaction module, the liquefaction module and a Boil Off Gas (BOG) module both fluidly coupled to the plurality of cryogenic cargo tanks, and wherein the about 41 m wide cryogenic cargo tanks and about 7 m wide ballast wing tanks form a saved deadweight; and wherein the saved deadweight is applied to the trunk deck and the natural gas liquefaction plant. 
     An illustrative embodiment of a liquefied natural gas (LNG) vessel system includes a natural gas liquefaction vessel, a natural gas liquefaction vessel including a cargo hold in a hull of a natural gas liquefaction vessel, a plurality of cryogenic cargo tanks in the cargo hold, at least one pair of ballast wing tanks, one each on a port side and a starboard side of the cargo hold and coupled to at least one cryogenic cargo tank of the plurality of cryogenic cargo tanks, a trunk deck above the cargo hold, the trunk deck extending over the plurality of cryogenic cargo tanks and over the at least one pair of ballast wing tanks, and a natural gas liquefaction plant on the trunk deck. 
     An illustrative embodiment of a liquefied natural gas (LNG) vessel system includes a natural gas liquefaction vessel, a natural gas liquefaction vessel including a natural gas liquefaction vessel system including a natural gas liquefaction vessel including ballast wing tanks on a port side and a starboard side of at least one cargo tank, an upper trunk deck extending over and supported by the ballast wing tanks, and a liquefaction facility on the extended upper trunk deck, and dual-fuel diesel generator sets that power the natural gas liquefaction vessel propulsion and the liquefaction facility. 
     In further embodiments, features from specific embodiments may be combined with features from other embodiments. For example, features from one embodiment may be combined with features from any of the other embodiments. In further embodiments, additional features may be added to the specific embodiments described herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Advantages of the present invention may become apparent to those skilled in the art with the benefit of the following detailed description and upon reference to the accompanying drawings in which: 
         FIG. 1  is a cross sectional view of a cargo hold of a natural gas liquefaction vessel of an illustrative embodiment. 
         FIG. 2  is a cross sectional view of a mid-ship section of a natural gas liquefaction vessel of an illustrative embodiment. 
         FIG. 3  is a top plan view of an upper trunk deck of a natural gas liquefaction vessel of an illustrative embodiment. 
         FIG. 4  is a profile view of a natural gas liquefaction vessel of an illustrative embodiment. 
         FIG. 5  is a flowchart of a liquefaction process onboard a liquefaction vessel of an illustrative embodiment. 
     
    
    
     While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof are shown by way of example in the drawings and may herein be described in detail. The drawings may not be to scale. It should be understood, however, that the embodiments described herein and shown in the drawings are not intended to limit the invention to the particular form disclosed, but on the contrary, the intention is to cover all modifications, equivalents and alternatives falling within the scope of the present invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     A natural gas liquefaction vessel is described. In the following exemplary description, numerous specific details are set forth in order to provide a more thorough understanding of embodiments of the invention. It will be apparent, however, to an artisan of ordinary skill that the present invention may be practiced without incorporating all aspects of the specific details described herein. In other instances, specific features, quantities, or measurements well known to those of ordinary skill in the art have not been described in detail so as not to obscure the invention. Readers should note that although examples of the invention are set forth herein, the claims, and the full scope of any equivalents, are what define the metes and bounds of the invention. 
     As used in this specification and the appended claims, the singular forms “a”, “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to a cargo tank includes one or more cargo tanks. 
     As used in this specification and the appended claims, “capacity” refers to the amount of a material that can be contained within a cargo tank and/or within a liquefaction vessel as cargo. 
     As used in this specification and the appended claims, “coupled” refers to either a direct connection or an indirect connection (e.g., at least one intervening connection) between one or more objects or components. The phrase “directly attached” means a direct connection between objects or components. 
     As used in this specification and the appended claims, “liquefaction equipment,” “liquefaction train(s),” “liquefaction plant” and “liquefaction facility” refers to one or more pieces of any type or combination of equipment used to convert natural gas to liquefied natural gas (LNG). Thus, for example, liquefaction equipment, facility, plant or train(s) means any of one or more of a series of linked equipment elements or modules used in the natural gas pretreatment and liquefaction process, for example, warm liquefaction module, cold liquefaction module, dehydration module, amine module, boil-off gas (BOG) module, cold box and utility module, or any similar equipment of different names that accomplish the same purpose. 
     As used in this specification and the appended claims “high pressure” means, with respect to gaseous natural gas, between a pressure of about 45 barg and about 100 barg. With respect to a conduit, pipe, hose and/or transfer member for transferring gaseous natural gas, “high pressure” means capable of maintaining, transferring and/or accommodating natural gas at a pressure of between about 45 barg and about 100 barg. 
     One or more embodiments provide a natural gas liquefaction vessel. While for illustration purposes the invention is described in terms of natural gas, nothing herein is intended to limit the invention to that embodiment. The invention may be equally applicable to other hydrocarbon gases which may be transported as liquids, for example liquefied petroleum gas, propane or butane. 
     A natural gas liquefaction vessel including an increased deadweight tonnage, as compared to a liquefied natural gas carrier (LNGC) of a comparably-sized ship, is achieved by reducing the LNGC&#39;s cargo capacity. This difference creates room on the port and starboard sides of cargo tanks to increase the size of the adjacent wing tanks. The increased size of the wing tanks occupy the space created by the reduced cargo tank size of the vessel and may support a larger upper trunk deck. The ballast wing tanks and smaller cargo tanks increase the deadweight available. With this approach, the larger upper trunk deck of the vessel is able to support an efficient floating liquefaction plant that improves the LNG value chain because it is capable of producing 2.0-3.0 MTPA in the footprint of a standard vessel hull, such as for example a Q-Max hull. 
     An illustrative embodiment of a natural gas liquefaction vessel includes an improved cargo hold size and deck structure that may allow a liquefaction facility to be placed onboard a floating vessel that is newbuilt on a conventional LNG carrier hull, such as a Q-Max or Q-Flex hull. Using a hull design known to shipyards and those of skill in the art provides the advantages of reducing development costs and increasing reliability in newbuilt vessels. Where the natural gas liquefaction vessel is a newbuild vessel, sufficient deadweight may be dedicated to an expanded upper deck to support an expanded liquefaction train and improved power system. Where the natural gas liquefaction vessel is created by conversion of a prior vessel, illustrative embodiments may provide more space and flexibility for complex liquefaction process arrangements, as well as more efficient power utilization onboard the liquefaction vessel for propulsion, to power the vessel and to power the liquefaction process onboard the vessel. In exemplary embodiments, the natural gas liquefaction vessel may also include wider ballast wing tanks on the port and starboard side of the cargo tanks, and an extended upper trunk deck over the ballast wing tanks, as compared to conventional liquefaction floating units and/or conventional liquefied natural gas carriers (LNGCs) converted into liquefaction vessels. 
     A liquefaction vessel of illustrative embodiments may be diesel-electric powered by dual-fuel diesel generator sets, for example. The dual-fuel diesel generator sets may provide power for both vessel propulsion and the liquefaction train. In some embodiments, the natural gas liquefaction facility onboard the vessel may be powered by gas engines or gas turbines. Illustrative embodiments may increase deadweight of the liquefaction vessel to provide additional options for liquefaction facility process arrangements, as compared to conventional liquefaction vessels and/or conventional LNGCs of similar hull size and class. 
       FIG. 1  and  FIG. 2  illustrate an exemplary embodiment of a liquefaction vessel. Liquefaction vessel  100  may be moored offshore on buoy or tied up along a jetty for extended periods of time, such as for 5 years or more. As shown in  FIG. 1 , liquefaction vessel  100  has hull  105  with a size of 345 m×55 m×27 m, in one non-limiting example. An LNGC hull of this size in the prior art would have a maximized cargo capacity of 266,000 m 3  based on the deadweight of the vessel. However, as illustrated in  FIG. 1 , in an exemplary LNGC with a 345 m length hull, liquefaction vessel  100  may include cargo tanks  110  having a reduced capacity of only 180,000 m 3 . By reducing the cargo capacity (tank size), deadweight for additional liquefaction equipment may become available. Other sized hulls may also be used for liquefaction vessel  100 , such as hulls with cargo tanks  110  having a capacity of between 145,000 m 3  and 256,000 m 3 , for example. Thus, capacity of cargo tanks  110  onboard liquefaction vessel  100  may be on average at least 15% less than the maximum deadweight capacity of cargo tanks of prior similar vessels. In an exemplary embodiment, the capacity of cargo tanks  110  may be about 30% less than the maximum deadweight capacity of a similarly sized prior art LNGC. In some embodiments of the invention, all cargo tanks  110  in the cargo hold of liquefaction vessel  100  are reduced in volume from a conventional vessel of similar size, and the reduction in volume is at least 15% on average. Cargo tanks  110  may be membrane or self-supporting prismatic type cargo tanks. In some embodiments, the LNG containment system for liquefaction vessel  100  cargo tanks  110  may be membrane-type tanks, in a two row/ten tank configuration to minimize sloshing and provide mid-span deck support for installed liquefaction equipment  350 , for example. 
     Deck Space and Deadweight Increases 
     In the following description and figures, a Q-Max class hull is shown as an example; the invention is not so limited. The invention may also be applied to a Q-Flex class hull, with the appropriate scaling. As shown in  FIG. 1  and  FIG. 2 , the width α of cargo tanks  110  may be truncated in liquefaction vessel  100  from those in a conventional Q-Max hull. In an embodiment, width α may be about 41 m. Shortening the width α of cargo tanks  110  to about 41 m, without modifying the height and length of cargo tanks  110 , reduces the capacity of cargo tanks  110  without altering the overall structure of the vessel. Using an existing hull form from existing shipyards increases reliability and reduces development cost. The largest cost component of the LNG value chain is the liquefaction plant. A Q-Max hull form, for example, is a well-known and reliable hull form, where the “Q” stands for Qatar and the “Max” is the maximum size of ship able to dock at the LNG terminals in Qatar. Existing shipyards know how to build a Q-Max hull. With the modification of one or more embodiments of the invention, reducing the capacity of cargo tanks  110  creates additional deadweight. For example, in a 345-meter hull, cargo tanks  110  may be reduced from a 266,000 m 3  capacity to 180,000 m 3  by using narrower cargo tanks  110 , thereby making available 40,000 tons of deadweight for other purposes, such as liquefaction equipment  350 . Thus, extra space  130  is created on port side  120  and starboard side  125  of liquefaction vessel  100  adjacent to cargo tanks  110 . 
     In a conventional LNGC vessel, such as a Q-Max vessel for example, ballast wing-tank size may be based on tank volume and damage requirements. In illustrative embodiments, ballast wing tanks  115  may be installed and/or extended to occupy extra space  130 . Ballast wing tanks  115 , extended in size to about, for example, 7 m each, are larger than in a conventional vessel. Ballast wing tanks  115  may provide improved stability and/or structural support that allows liquefaction vessel  100  to support upper trunk deck  135  and/or extended portions  140 . In a Q-Max class embodiment, upper trunk deck  135  may be about 27 m and extended portions  140  may be about 14 m. 
     In some embodiments, upper trunk deck  135  may be extended on port side  120  and starboard side  125  of the vessel, as illustrated by extended portions  140 . Upper trunk deck  135  may extend the full length of liquefaction vessel  100  and may be raised above upper deck  145  in one or more illustrative embodiments. Where an LNGC is a regasification vessel, upper deck  145  is conventionally used for regasification equipment. In such conventional configurations, regasification equipment may be forward on upper deck  145 . Upper deck  145  is also where liquefaction equipment is located on a conventional floating liquefaction unit. Side shell  150 , between upper deck  145  and extended portions  140  of upper trunk deck  135 , may be non-tight and exposed to weather. Extended portions  140  of upper trunk deck  135  may be between 17 m and 20 m on both port  120  and starboard sides  125  of liquefaction vessel  100  for a hull 345 m in length. In some embodiments, upper trunk deck  135  may be 14 m on each side and extend the length of liquefaction vessel  100  as shown in  FIG. 3  and  FIG. 4 . 
     Liquefaction Equipment 
     For illustration purposes only.  FIG. 3  shows a Q-Max class vessel with dual-mixed refrigerant (DMR) that may be capable of 3.0 MTPA (Metric Tons per Annum) or greater sendout. In some embodiments, liquefaction vessel  100  may be a Q-Flex class vessel with single-mixed refrigerant (SMR) for smaller configurations with less than 2.0 MTPA sendout.  FIG. 3  is a plan view of upper trunk deck  135 , which may provide space for and support liquefaction equipment  350 . 
     In an embodiment, the liquefaction equipment  350  may reside on upper trunk deck  135 . Upper trunk deck  135  may extend the length of the vessel on both the port side  120  and starboard side  125 . Together a series of exchangers comprise a single LNG train. Liquefaction equipment  350  may be a single DMR train or multiple SMR trains. 
     Liquefaction equipment  350  may convert gaseous natural gas to liquefied natural gas (LNG) by removing heat from the natural gas until the natural gas is below its boiling point. As shown in  FIG. 3 , liquefaction equipment  350  may be placed on extended portions  140  of upper trunk deck  135 , rather than on upper deck  145  as would be located in a conventional vessel of similar size. In some embodiments, liquefaction equipment  350  may also be placed on upper deck  145  (as a supplement to that on upper trunk deck  135 ) and/or extended portions  140 . 
     As shown in  FIG. 3 , warm liquefaction module  300 , cold liquefaction module  305 , dehydration module  310 , amine module  315 , boil-off gas (BOG) module  330 , cold box  325  and utility module  320  all may reside on upper trunk deck  135 , specifically on extended portions  140  of upper trunk deck  135  and supported by ballast wing tanks  115 . As illustrated in  FIG. 3 , upper trunk deck  135  extends substantially the length of liquefaction vessel  100  on both port  120  and starboard  125  sides. Extended portions  140  of upper trunk deck  135  may extend a full beam (between 17 m and 20 m) of natural gas liquefaction vessel  100  to both port side  120  and starboard side  125  and over ballast wing tanks  115  for the length of natural gas liquefaction vessel  100 . 
     Liquefaction equipment  350  may be provided by Black &amp; Veatch Corporation of Overland Park, Kans., United States, Air Products and Chemicals, Inc. of Allentown, Pa., Linde AG of Pullach, Germany, Axens-IFP of Rueil, France, Royal Dutch Shell plc of The Hague, the Netherlands or LNG Ltd. of Perth, Australia. In one example, liquefaction equipment  350  may be Black &amp; Veatch single mixed refrigerant (SMR), dual mixed refrigerant (DMR) or another liquefaction technology known to those of skill in the art. 
     As opposed to selecting liquefaction equipment  350  to have a reduced equipment count or a smaller, more compact footprint than land-based liquefaction—as is conventionally necessary on floating liquefaction units—liquefaction equipment  350  in one or more embodiments may be more complex. Liquefaction equipment  350  may provide gas pretreatment and/or improved processing equipment because of the available additional deadweight and space of illustrative embodiments. In conventional systems, it has previously been desirable to move natural gas pretreatment systems onshore due to the limited space onboard liquefactions vessels, for example as described in WO 2014/168843 to Excelerate Liquefaction Solutions, LLC and entitled, “SYSTEMS AND METHODS FOR FLOATING DOCKSIDE LIQUEFACTION OF NATURAL GAS.” Illustrative embodiments of the invention may obviate the need for onshore pretreatment facilities or for pretreatment on a production platform, since more deck space and/or deadweight is available onboard liquefaction vessel  100  than in prior configurations. 
     Liquefaction Process Onboard the Vessel 
     The process of liquefaction of natural gas is well known in the art. However, many variations in the process, and in liquefaction plants, have developed. The propane precooled mixed refrigerant and pure-component cascade cycle processes have dominated the market in the past. One or more embodiments of liquefaction vessel  100  may use a DMR or SMR process. The invention may be compatible with any other process known to those of skill in the art that are supported by and are compatible with the advantages of the one or more embodiments of the invention. 
       FIG. 5  is a flowchart of exemplary processes that may take place onboard liquefaction vessel  100 . Liquefaction vessel  100  may receive natural gas from wellheads under the sea, or through a high-pressure hard arm and pipelines on a dock.  FIG. 5  illustrates an example where natural gas from subsea wellheads  500  may be supplied to liquefaction vessel  100  via flowline and risers  505 , though the invention is not so limited. In either case, the process of liquefaction as shown in  FIG. 5  may be the same once natural gas reaches liquefaction vessel  100 . As shown in  FIG. 4 , liquefaction vessel  100  may utilize either an integrated turret such as a submerged turret loading-buoy or external turret  400  at the bow of liquefaction vessel  100 . In some embodiments, external turret  400  may permit weathervaning of liquefaction vessel  100 . 
     Natural gas moves from its source through pipes from gas loading and reception  510  to fiscal metering  515 , and then may enter amine module  315  for compression  520  and acid gas removal  525 . Dehydration and mercury (Hg) removal  530  may be performed in dehydration module  310 . Dehydration module  310  is coupled by pipes to cold box/heavies module  325 , where the step of liquefaction and heavies removal  535  may occur. In some embodiments using a dual-mixed refrigerant (DMR) process such as Black and Veatch referenced above, liquefaction may occur using warm liquefaction module  300  and cold liquefaction module  305 . In this approach, warm liquefaction module  300  first cools the gas from ambient (warm) temperature, then cold liquefaction module  305  liquefies it at about −160° C. The LNG resulting from the liquefaction module(s) moves on to end flash process  540 , where additional sub-cooling of the LNG may be accomplished by passing it through an expander (a compressor operating in reverse). End flash process  540  may be needed because once liquefaction is complete the LNG may still be at relatively high pressure. However, LNG may be stored on liquefaction vessel  100  at a pressure slightly above atmospheric pressure. Therefore, it is advantageous to reduce the pressure through an expander, such that the LNG may be conditioned for storage, and may also produce some additional power via a generator connected to the expander. As the pressure drops in this step, some of the LNG flashes into vapor and may be used as fuel gas. The LNG may then be transferred to LNG storage  545 . Both end flash process  540  and LNG storage  545  may generate boil off gas. Boil off gas (BOG) may be used for fuel gas, or reliquefied and returned. Boil off gas/fuel gas handling  555  may occur in BOG module  330 . Flare tower  415  may be located at the bow of liquefaction vessel  100  and may burn off LNG and flare it safely away from liquefaction vessel  100 . Cryogenic loading arms or hoses (not shown) may provide LNG offloading  550  from liquefaction vessel  100 . 
     Liquefaction vessel  100  may also need to process byproducts of the liquefaction process onboard the vessel. Dehydration may generate water. Produced water treatment  575  may be stored at produced water storage  580 , but is eventually discarded at produced water disposal  585  where it is released into the vessel hull. MEG (MonoEthylene Glycol) recovery and regeneration  590  may also produce water for produced water treatment  575  when it accepts natural gas from gas loading and reception  510  and returns the natural gas to wellheads  500 . Liquefaction vessel  100  may also provide chemical injection  595  to wellheads  500 . Finally, gas loading and reception  510  and liquefaction/heavies removal  535  may generate condensate, which may be processed by condensate stabilization  560 , and then transferred to condensate storage  565  and eventually offloaded at condensate offloading  570 . 
     Power Systems 
     Liquefaction vessel  100 &#39;s power system  410  may be electric powered by dual fuel generator sets, with either one or two propellers  405 . Electricity to power liquefaction vessel  100  and liquefaction process equipment may be generated using the dual-fuel generator sets, which may, for example, be one or more dual-fuel diesel generator sets providing diesel electric power. In some embodiments, power to liquefaction vessel  100  and liquefaction equipment  350  may be provided by gas engines or gas turbines. Liquefaction vessel  100  may employ self-propulsion during transit to and/or from the location of production and/or to move out of harm&#39;s way, such as during inclement weather. 
     One of skill in the art may appreciate that the cargo tank, cargo capacity, trunk deck extension and wing tank dimensions described herein may be modified proportionally based on the size of the vessel hull employed on liquefaction vessel  100 . 
     A natural gas liquefaction vessel has been described. Illustrative embodiments may provide an improved cargo containment and deck structure for a floating liquefaction unit or vessel. Illustrative embodiments may more efficiently utilize deadweight of the vessel while sacrificing only limited storage space. Illustrative embodiments may provide space for a more efficient power system onboard the vessel, utilizing dual fuel diesel power for vessel propulsion, power to the vessel and liquefaction processes. The liquefaction vessel of illustrative embodiments may offer additional liquefaction process arrangements and more economic options for long-term charters. Illustrative embodiments may be capable of producing 2.0-3.0 MTPA in the footprint of a standard vessel hull, such as for example a Q-Max hull. 
     Further modifications and alternative embodiments of various aspects of the invention may be apparent to those skilled in the art in view of this description. Accordingly, this description is to be construed as illustrative only and is for the purpose of teaching those skilled in the art the general manner of carrying out the invention. It is to be understood that the forms of the invention shown and described herein are to be taken as the presently preferred embodiments. Elements and materials may be substituted for those illustrated and described herein, parts and processes may be reversed, and certain features of the invention may be utilized independently, all as would be apparent to one skilled in the art after having the benefit of this description of the invention. Changes may be made in the elements described herein without departing from the scope and range of equivalents as described in the following claims. In addition, it is to be understood that features described herein independently may, in certain embodiments, be combined.