TECHNICAL FIELD

This specification discloses a barge for and method of water cooling an LNG production plant. The LNG plant may be onshore, offshore or nearshore.

BACKGROUND ART

The production of LNG requires a natural gas feed stream to be cooled to a temperature in the order of -161° C at 1 bar pressure. The cooling is affected by circulating a refrigerant stream against the feed stream to facilitate the rejection of heat from, and thus the cooling of, the natural gas stream. The refrigerant flows through a circuit which includes a compression stage, a cooling stage and an expansion stage. LNG plants use either air coolers or water coolers to reject heat from the refrigerant stream for example in aftercoolers/intercoolers and condensers.

Air coolers are designed with an approach temperature of 10-15° C. Cooling water mediums such as seawater or tempered water have improved approach temperatures of up to 3-5° C leading to more efficient and compact designs. Currently, about 50% of the worlds LNG Facilities are serviced by water cooled plants while the remaining are air cooled.

For nearshore and onshore locations, environmental restrictions often preclude the use of direct seawater. The use of direct seawater entails pumping water from the sea through land-based heat exchangers to cool the refrigerant stream and then return the heated seawater back to the sea. The circulation of seawater in this manner has adverse environmental impacts including killing fish eggs entrained in the water and increasing the temperature of the seawater.

Irrespective of whether an LNG production facility is air cooled or water cooled the corresponding cooling facility requires substantial infrastructure and associated costs.

The above described background is not intended to limit the application of embodiments of the disclosed barge and method.

SUMMARY OF THE DISCLOSURE

In one aspect there is disclosed a barge for cooling an LNG production plant, the barge comprising: a hull; a deck supported on the hull; and a water cooling facility on the barge, the water cooling facility is capable of being thermally coupled with the LNG production plant to provide cooling to the LNG production plant.

As explained in greater detail below the water cooling facility utilises water drawn from a body of water in which the barge resides. This water is used to provide ballast to the barge as well as cooling to the LNG production plant. The water may be used to directly cool the LNG production plant or indirectly cool the LNG production plant by cooling an intermediate fluid which in turn cools the LNG production plant. However, in either case the water cooling facility does not return water to the sea while being used to provide cooling. The disclosed barge can be used with an LNG production plant that is onshore; and offshore or nearshore irrespective of whether the offshore or nearshore plants are floating, anchored, on jackets/piles; or gravity based. In one embodiment the water cooling facility comprises one or more water cooling towers supported on the deck and through which the water is able to flow, and one or more tanks in the hull for holding a supply of cooling water.

In one embodiment the one or more tanks comprise one or more ballast tanks of the barge and the cooling water when in the one or more tanks provides ballast for the barge.

In one embodiment the one or more water cooling towers are mechanical draft towers.

In one embodiment the water cooling towers are induced draft towers.

In one embodiment barge comprises one or more drains formed in the deck arranged to enable the cooling water after passing through the one or more towers to flow into the one or more tanks.

In one embodiment the barge comprises a ballast monitoring system arranged to replenish the supply of cooling water in the tanks from a body of water in which the barge is located to maintain the ballast of the barge at a predetermined draft. In one embodiment the water cooling facility is arranged to form a part of a water cooling circuit through which water cooled by the water cooling facility circulates through the LNG production plant.

In one embodiment the water cooling facility comprises a heat exchanger capable of fluid connection to the LNG production plant to form a part of a closed loop intermediate cooling fluid circuit, wherein the water cooling facility is arranged to cool the intermediate cooling fluid.

In one embodiment the water cooling facility comprises:

a heat exchanger in each of the cooling towers, each heat exchanger capable of fluid connection to the LNG production plant to form a part of a closed loop intermediate cooling fluid circuit; and a circulation system for circulating the cooling water through the one or more cooling towers and the tanks and across the heat exchangers.

In one embodiment the barge comprises one or a combination of any two or more of: a storage facility; an offloading facility; a power plant; and utilities, to support production of LNG or the transfer of LNG at the LNG production plant.

In a second aspect there is disclosed a method of cooling an LNG production plant, the method comprising: thermally coupling a water cooling facility located on a barge to the LNG production plant to provide cooling to the LNG production plant.

As in the first aspect, the thermal coupling provided by the disclosed method may be by direct flow of water cooled by the water cooling facility through the LNG production plant, or indirectly by use of the water to cool an intermediate fluid which flows through the LNG production plant. In one embodiment the method comprises holding a supply of water in the one or more tanks in the barge wherein the supply of water provides ballast for the barge; and, circulating the water through the water cooling facility to provide cooling to the LNG production plant. In one embodiment circulating the supply of water through the water cooling facility comprises circulating the water through one or more water cooling towers supported on a deck of the barge and the one or more tanks.

In one embodiment circulating the supply of water includes draining the water through one or more drains formed in the deck into the one or more tanks after flowing through the one or more cooling towers.

In one embodiment thermally connecting the water cooling facility comprises circulating water through the water cooling facility and the onshore LNG production plant.

In one embodiment thermally connecting the water cooling facility comprises using the water cooling facility to cool an intermediate cooling fluid flowing in an intermediate cooling fluid circuit.

In a third aspect there is disclosed a LNG production plant comprising: one or more LNG trains; and a barge located in a body of water, the barge having an on-board water cooling facility in thermal communication with the one or more LNG trains to provide cooling to refrigerant or other process fluids used in the production of LNG by the one or more LNG trains.

In one embodiment of the LNG production plant the barge has a one or more ballast tanks for holding water ballast and wherein the water cooling facility is arranged to use the water ballast to reject heat from the refrigerant or other process fluids.

In a fourth aspect there is disclosed a method of producing LNG comprising: operating one or more LNG trains at an LNG production site; cooling a refrigerant stream or other fluid streams used by the LNG trains to produce LNG by thermal communication with a supply of water held in one or more tanks on an offshore barge.

BRIEF DESCRIPTION OF THE DRAWINGS

Notwithstanding any other forms which may fall within the scope of the barge and method as set forth in the Summary, specific embodiments will now be described, by way of example only, with reference to becoming drawings in which:

Figure 1 is a representation of a prior art onshore LNG production plant; Figure 2 is a schematic representation of a prior art LNG train used at the onshore LNG production plant shown in Figure 1;

Figure 3 is a schematic representation of an LNG production plant incorporating an embodiment of the disclosed barge arranged to provide cooling to the LNG production plant; Figure 4 is a representation of an LNG production train showing an effect of use of embodiments of the disclosed barge by way of the omission of air cooling banks in comparison to the prior art LNG train shown in Figure 2; Figure 5 is a schematic representation of an embodiment of the disclosed barge;

Figure 6 the schematic representation of the hull of the barge shown in Figure 5;

Figure 7 is a circuit diagram illustrating fluid flow for a first embodiment of the method for providing thermal communication between an embodiment of the barge and an LNG train the purposes of providing cooling to an associated LNG production plant;

Figure 8 is a schematic representation of a water cooling facility for a first embodiment of the barge and used in the circuit and method shown in Figure 7;

Figure 9 is a circuit diagram illustrating fluid flow for a second embodiment of the method for providing thermal communication between an embodiment of the barge and an LNG train the purposes of providing cooling to an associated LNG production plant; and Figure 10 is a schematic representation of a water cooling facility for a first embodiment of the barge and used in the circuit and method shown in Figure 9.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Figure 1 depicts a prior art an onshore LNG production plant 10. The production plant has two LNG trains 12, LNG storage tanks 14 and various utilities 16 which may include for example power generation facilities and a control centre. An LNG transport vessel 18 is berthed at a jetty adjacent the LNG production plant 10 to receive LNG via a pipeline 20 which is in communication with the storage tanks 14.

Figure 2 is a schematic representation of one of the LNG trains 12. The LNG train 12 comprises various modules including but not limited to a decontamination module 24, propane compression module 26, a propane condenser module 27, liquefaction module 28 and a MR compression module 29. Pluralities of banks of air coolers 30 are associated with various one of these modules. The air coolers 30 provide heat rejection for refrigerant passing through after coolers or intermediate coolers in the refrigerant circuit.

Figure 3 is a representation of an LNG production plant 10a having a substantially equivalent (and indeed as explained later slightly increased) production capacity as the prior art production plant 10. The LNG production plant 10a incorporates two LNG trains 12a and an embodiment a barge 32 in accordance with the present disclosure. The barge 32 may be in the form of a floating storage, offloading and utilities unit (FSOU).

A substantive difference between the LNG production plant 10 and the LNG production plant 10a is that the primary cooling for the LNG production plant 10a is provided by the barge 32. Therefore, and as represented in Figure 4, the LNG trains 12a of the production plant 10a do not incorporate the air coolers 30 of the LNG trains 12. This is represented by the overlaying of blank spaces 33 on the LNG train 12a at the equivalent location of the air coolers 30 of the LNG trains 12.

The barge 32 (shown in greater detail in Figure 5) is provided with a water cooling facility 34 which is thermally coupled to the LNG production plant 10a to provide cooling to the plant 10a. The barge 32 has a hull 38 supporting a deck 36. In this embodiment the barge 32 is also provided with the equivalent utilities 16 of the production plant 10.

As shown in Figure 6 the barge 32 also includes storage tanks 40 for holding LNG produced by the plant 10a and tanks 42 for holding other products used in or produced by the LNG production plant. The utilities 16 and the tanks 40, and 42 are of no significance to the water cooling function of the barge 32 and are described simply for completeness.

The water cooling facility 34 on the barge 32 comprises a plurality of cooling towers 44 which are supported on the deck 36 and one or more tanks 46 in the hull 38 for holding the supply of cooling water used by the cooling facility 34. The tanks 46 also act as ballast tanks for the barge 32.

Therefore, the water cooling the LNG plant is also used as ballast for the barge 32. The water ballast may be provided in a volume required the dual purpose of providing cooling to the LNG production plant as well as providing the required draft of the barge 32, this includes providing zero draft if the barge 32 is grounded.

Storage in the ballast tanks, instead of a in conventional bath below the fans of the cooling tower, minimises sloshing which would otherwise arise if a conventional water cooling facility were simply installed on the deck of a barge. This enables the cooling towers to function effectively in moderate to harsh metaocean conditions

Figures 7 and 8 show one form of the water cooling facility 34 in thermal communication with an LNG train 12a LNG production plant 10a. In this arrangement the cooling water held within the tank 46 flows directly through the LNG production plant 10a. The cooling circuit is depicted in Figure 7.

Cooled water from the tank 46 on the barge 32 is pumped to flow along the flow path 48 and through heat exchangers 50 (for example inter coolers or after coolers) of the LNG train 12a of the onshore LNG plant 10a. The cooled water absorbs heat from a refrigerant (not shown) also flowing through the heat exchangers 50. This results in the refrigerant being cooled and the cooled water being heated. The heated water flows along a flow path 52 back to the cooling tower 44 on the barge 32. The heated water is sprayed downwardly through the tower 44 via a plurality of spray heads 54.

As the heated water is travelling downwardly through the tower 44 air is mechanically forced through the tower. In the current illustrated embodiment this is by way of an upwardly drafted air flow in a counter current to the water flow. The draft is provided by an induction fan 56 mounted in an upper region of the tower 44 which draws cool air 58 from a bottom of the tower 44 above the deck 36. The air 58 absorbs heat from the water returning through the tower 44 thereby cooling the water which returns via drains 60 in the deck 36 to the underlying tanks 46. Heated air 62 flows out of the top of the cooling tower 44 into the atmosphere. In this way the cooled water in the ballast tanks 46 is in thermal communication with and provides cooling to the onshore LNG production plant 10a. The water in the ballast tank 46 is subject to evaporation by the flow of air upwardly through the tower 44. The water may be replenished from time to time as necessary from the body of water in which the barge 32 resides. This may be achieved by use of an automatic system which maintains a volume of water with the tanks 46 as needed to provide the required ballast for the barge 32 and to provide the required heat rejection in the LNG production plant 10a. While the flow circuit for the water circulating through the cooling facility 34 and the LNG production plant 10a is essentially a closed circuit with, barring a leak in the circuit, the only loss of water being by evaporation in the tower 44.

Figures 9 and 10 depicted an embodiment of the barge based water cooling system 34 in which the cooling water provides indirect cooling to the LNG production plant 10a by cooling an intermediate cooling fluid which flows through a closed intermediate cooling fluid circuit. The closed intermediate cooling fluid circuit comprises a heat exchanger 64 may be in the form of a radiator located within the tower 44, flow path 48 from one end of the radiator 64 to an inlet of the heat exchanger 50 and the return flow path 52 from the heat exchanger 50 to the heat exchanger 64. The cooling fluid which circulates through this closed-circuit does not physically contact the water flowing down inside the tower 44 into the tanks 46.

The water in the tanks 46 is circulated through the cooling facility 34 by a pump 66 which lifts the water through a conduit 68 to the sprayers 54 in an upper region of the tower 44. The water then flows across the heat exchanger 64 to provide conductive cooling to the cooling fluid in the closed circuit. Simultaneously air 58 is drafted upwardly through the tower 44 by a fan 56. This cools the water flowing through the tower 44 as well as provides a cooling effect to the intermediate cooling fluid flowing through the heat exchanger 64. The air is therefore heated and exits as heated air 62 from the top of the tower 44.

Providing a water cooling facility 34 on the barge 32 provide several commercially significant benefits in the overall capex for the construction a LNG production plant as well as ongoing costs of production. These include but are not limited to the following:

• The overall land footprint required for an air-cooled LNG train 12 is significantly reduced for example in the order of 25% to 30%. Accordingly, there is a reduction in earthworks required for construction of an onshore LNG production plant.

The smaller land requirement may also enable installation of an LNG production plant at a location which would otherwise not be viable due to lack of land.

The above described reduction in footprint, and associated weight, is also of significance in terms of nearshore or offshore LNG production or liquefaction as it enables the remainder of an LNG production plant to be on smaller separate vessels with shallower draft

requirements than a conventional complete nearshore/offshore LNG production plant.

In terms of equipment weight reduction, calculations indicate that a prior art 5MTPA air cooled LNG train which may have a weight in the order of 7200 tonnes can be reduced to a weight of approximately 5400 tonnes. For a modular LNG train this may also reduce the number of modules from five to two or three. Accordingly, the construction cost for the LNG train is reduced. Additionally, the cost of transporting the LNG train from a construction location to the production location decreases because there are fewer modules to transport and as the weight of the modules is reduced, potentially smaller and cheaper vessels may be used for the transport.

• As water has a substantially higher heat capacity than air, the heat rejection in the LNG plant is increased which enables increase production. Initial calculations indicate that the capacity of a nominal 5 MTPA LNG train can be increased to 5.3 M PTA

• Barges in the form of a floating storage, offloading and utilities (FSOU) units are relatively common and ordinarily have substantial free deck space. Barges also are constructed with ballast tanks to provide appropriate ballast while being sailed across the ocean and when moored and in use. Accordingly, modification of an existing barge/FSOU to incorporate a water cooling facility is relatively simple and cheap in comparison to the incorporation of the required banks of air coolers in an LNG train.

It follows from the above description that embodiments of the barge 32 facilitate a method of providing cooling or heat rejection for an LNG production plant by thermally coupling a water cooling facility located on the barge to the LNG production plant. The method involves using a supply of water held in tanks in the barge which is cooled by the water cooling facility to either directly or indirectly cool the LNG plant. This water also acts as ballast for the barge 32 and thus the tanks 46 can be considered as ballast tanks. The cooling water storage can also be integrated into the ballast water tanks rather than as separate storage tanks in the hull.

Directly cooling the LNG plant involves circulating a flow of water from the barge tanks 46, through the LNG plant 10a, through the cooling towers 44 on the barge 32 where the water is cooled and returned to the barge tanks 46 for flow back to the LNG plant.

Indirectly cooling the LNG plant 10a involves the use of an intermediate cooling fluid which is caused to flow in a closed-circuit loop between the LNG plant and the water cooling facility 34 on the barge 32. The closed-circuit loop containing the intermediate fluid includes a radiator 64 which is disposed in the water cooling facility 34 and in particular the cooling tower 44. Water from the barge tanks 46 is circulated through the tower 44 to flow across the radiator 64 against a current of air flowing upwardly through the tower 44. The water flowing across the radiator 64 cools the intermediate cooling fluid in the closed intermediate cooling fluid circuit.

Whilst a number of specific embodiments of the barge and method have been described, it should be appreciated that the barge and method may be embodied in many other forms. For example, the water cooling facility 34 is illustrated and described as having an induced draft by way of a fan 56 mounted in an upper region of cooling tower 44. However, air flow for cooling the water in the water cooling facility may also be generated by forced draft for example by a blower arranged to force air flow into the bottom of the tower 44. Also, other types of water cooling facilities may be provided on the barge 22. Additionally, while the barge 22 is described as providing cooling for an onshore LNG plant it can provide cooling or heat rejection for a separate offshore (including nearshore) LNG plant or liquefaction facility. Generally, an offshore LNG production facility (e.g. a FLNG vessel) will have a dedicated on-board cooling facility. Nevertheless, the presently disclosed barge and system are inherently capable of providing cooling to offshore or nearshore facilities whether floating or gravity based.

In the preceding description and claims which follow, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to preclude the presence or addition of further features in various embodiments of the disclosed barge, methods and LNG production plant.