US20080184735A1

US20080184735A1 - Refrigerant storage in lng production

Info

Publication number: US20080184735A1
Application number: US11/971,646
Authority: US
Inventors: Wim van Wijngaarden; Jeremy Duncan Stuart Joynson
Original assignee: Single Buoy Moorings Inc
Current assignee: Single Buoy Moorings Inc
Priority date: 2007-02-01
Filing date: 2008-01-09
Publication date: 2008-08-07
Also published as: WO2008093186A1

Abstract

A refrigerant tank (40) holds a hydrocarbon refrigerant (42) used by refrigeration equipment (14) to cool natural gas to about −163° C. to produce LNG (liquefied natural gas) (45). The refrigerant is stored on a floating structure (10) in a way that avoids dangers that would arise if the refrigerant were stored on deck with the refrigeration equipment. A refrigerant container (40) is stored in one of the LNG tanks (20) that holds LNG at about −163° C.

Description

CROSS-REFERENCE

Applicant claims priority from U.S. provisional patent application Ser. No. 60/898,858 filed Feb. 1, 2007.

BACKGROUND OF THE INVENTION

Natural gas is transported long distances by tankers that carry the natural gas cooled to about −163° C. so it is in a liquid state as LNG (liquefied natural gas) or as LPG (liquefied petroleum gas). The LNG has a volume that is less than 0.2% of the volume of natural gas in a gaseous state (at atmospheric pressure). A recent development in the LNG industry is to liquefy natural gas offshore on board a floating liquefaction structure such as a vessel or barge that is usually more than a kilometer from shore. The floating structure carries refrigeration equipment to cool the natural gas to a liquid state and carries insulated LNG tanks to store the LNG prior to offloading it to a tanker. The LNG tank or tanks lie in the vessel hull primarily below the deck of the vessel. Refrigeration equipment is generally located on the deck where its operation can be more easily monitored, along with electricity generators that power the refrigeration equipment. A similar arrangement but with less refrigeration capacity, is used on the tanker that carries LNG a long distance, to keep the LNG cold.
The refrigeration equipment generally uses a refrigerant that is compressed to heat it, with the compressed refrigerant then cooled to a moderate temperature by a coolant, and with the cooled and compressed refrigerant then expanded to drop it to a cold temperature. The cold refrigerant passes through a heat exchanger where it is used to cool and liquefy the natural gas. For high efficiency, a refrigerant is preferably used that changes from a gas to a liquid when compressed and cooled, and which then turns back into a gas when expanded.
Natural gas is cooled by a large temperature difference such as about 200° C. when converted to LNG, and high efficiency is obtained by cooling it in about three stages. In the first, or precooling stage, a high boiling point refrigerant that is readily available at a gas field can be used, such as propane whose boiling temperature is −42° C. and whose freezing temperature is −190° C. In the second stage, a readily available middle temperature boiling point refrigerant can be used such as ethane whose boiling temperature is −89° C. In the third or subcooling stage, a readily available low boiling temperature refrigerant can be used, such as methane whose boiling temperature is −164° C., and nitrogen whose boiling temperature is −196° C. The boiling temperature varies with pressure.
In a vessel with an LNG storage capacity on the order of magnitude of 200,000 cubic meters, that stores produced LNG for transport, the vessel also stores a few hundred cubic meters of refrigerant. The refrigerant previously has been stored on the vessel deck near the refrigeration equipment that also cools the LNG. The refrigeration equipment keeps the refrigerant cold so it can be stored as a liquid. Where the refrigerant is a hydrocarbon that is gaseous at environmental temperature (e.g. 10° C.), the refrigerant is flammable and poses a great danger to the crew in the event of a leak. A liquefied storage system or facility that avoided such great danger to the crew would be of value.

SUMMARY OF THE INVENTION

In accordance with one embodiment of the invention, a floating structure with refrigeration equipment for producing LNG (liquefied natural gas) by cooling, and with LNG tanks for storing the LNG, has at least one refrigerant container that is safely and conveniently stored. The refrigerant container (often three of them for three refrigerants) holds a plurality of cubic meters of refrigerant used by the refrigeration equipment. The refrigerant container is stored within one of the tanks that holds LNG. This keeps the refrigerant away from the deck of the floating structure where leaking refrigerant would be very dangerous, and avoids the need for separate refrigeration equipment to keep the refrigerant cold.
The refrigerant includes liquefied gases which are preferably liquefied hydrocarbon gases, such as ethane, propane, etc., whose freezing temperatures (−190° C. for propane) is substantially below the liquefaction temperature of LNG (−163° C.) which generally constitutes most or a major portion of natural gas. Where the LNG is stored at a temperature of approximately −160° C., both propane and methane can be stored in the LNG tank. A refrigerant container that holds a hydrocarbon refrigerant is vented directly into the LNG tank, because any vented vapor is a proper part of LNG.
The novel features of the invention are set forth with particularity in the appended claims. The invention will be best understood from the following description when read in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side elevation view of a floating structure that is used to cool natural gas to about −163° C. to liquefy it and to store the LNG (liquefied natural gas) for offloading to a tanker.

FIG. 2 is a plan view of the floating structure of FIG. 1.

FIG. 3 is a sectional view taken on line 3-3 of FIG. 2.

FIG. 4 is a schematic view of the refrigeration equipment of FIG. 1.

FIG. 5 is a sectional view similar to that of FIG. 3, but showing a variation of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a floating structure 10 of the invention which has a hull 12 that supports refrigeration equipment 14 that is used to cool produced natural gas to about −163° C. to turn it into LNG (liquefied natural gas). The hull holds LNG tanks 20 that hold the LNG until a shuttle tanker sails to the vicinity of the floating structure 10 and moors close by so LNG can be transferred to the tanker. The tanker then carries the LNG at least a few hundred kilometers to a location where the LNG is offloaded and heated to provide natural gas. Both the floating structure and tanker have LNG tanks that can store on the order of magnitude of 200,000 cubic meters of LNG. The floating structure of FIG. 1 has a turret 22 at its bow that allows the floating structure to weathervane while produced natural gas is received through a riser 24. The natural gas is usually cleaned of sand, water, and other impurities before it is liquefied by the refrigeration equipment.
The refrigeration equipment 14 cools natural gas from the temperature at which it emerges from an undersea (or under land) reservoir to a temperature of about −163° C. A temperature of −163° C. is about the liquefaction temperature of natural gas (at zero pressure). Assuming the shuttle tanker has a LNG storage capacity of 200,000 cubic meters and returns once a week, the refrigeration equipment on the floating structure is provided with a capacity to liquefy natural gas at a rate of about 30,000 cubic meters per day. Such refrigeration equipment requires a few hundred cubic meters of refrigerant to make up for any losses that may occur (e.g. during a plant shutdown). The refrigerator equipment 14 is stored on the deck 30 of the floating structure hull. This not only provides more room for the LNG tanks, but enables technicians to have easy access to the refrigeration equipment to assure proper operation and maintenance, and places the refrigeration equipment close to an electric power generator 32 that produces electricity that energizes the refrigeration equipment.
Previously, the refrigerant tanks, which have a capacity of a few hundred cubic meters, were also stored on the deck, close to the refrigeration equipment. As discussed above, there are advantages to the use of components of natural gas as the refrigerants because they are readily available from the natural gas that is being produced or stored. Three of such hydrocarbon components that are useful as refrigerants are methane (one carbon atom per molecule), ethane (2 carbon atoms) and propane (three carbon atoms). FIG. 4 illustrates a known type of three-stage LNG liquefaction system where natural gas passes through three stages 33, 34, 35 of refrigeration. Applicant uses propane, ethane and methane and/or nitrogen as refrigerants. Nitrogen is readily obtained from air as by selective absorption or liquefaction. The natural gas liquefaction system has heat transfer regions 36, 37 and 38 where the natural gas is progressively cooled. Compressors such as 39 compress and thereby heat the refrigerant, and an expansion valve such as 41 allows the compressed and heated refrigerant to expand and become cold.
A major problem with the use of such hydrocarbons as refrigerants is that they are all flammable, and the considerable quantities that are stored create a hazard for the crew. The refrigerants are preferably stored in a liquid state, which not only reduces the storage volume, but enables a quick start-up of the liquefaction process after a partial or complete depressurization of the refrigeration equipment as a result of a process upset condition. If such refrigerants are released into the atmosphere as a result of an accident, they quickly evaporate. In a gaseous state the refrigerants are heavier than air so they tend to disperse very slowly, and such hydrocarbon vapors lying on a deck pose a significant explosion risk.
In accordance with the present invention, applicant stores liquid hydrocarbon refrigerants that are awaiting use in refrigeration equipment, below the deck, in a refrigerant container that lies in a cold environment formed by the LNG tanks. The refrigerant container preferably lies within an LNG storage tank. FIG. 3 shows a refrigerant container 40 that holds a plurality of cubic meters, and usually a plurality of tens of cubic meters of a refrigerant 42. The LNG tanks 20 on the floating structure store a total volume of LNG of about 200,000 cubic meters, which is about 400 times as much volume as about 500 cubic meters of refrigerant that is stored in perhaps three containers.
Applicant places the refrigerant container 40 (which may hold one of perhaps three different refrigerants) within one of the LNG tanks 20 and within the LNG 45 therein. The advantages of this arrangement are that a significant fire or explosion hazard is removed from the deck and more space is left on the deck for other purposes. In addition, a separate refrigerant cooling system is not required to cool the refrigerant, as the large quantities of LNG in the tank can be relied on to keep the refrigerant cold. Also, insulation is not required around the refrigerant container. The LNG consists of multiple hydrocarbons, and the hydrocarbon refrigerants (methane, ethane and propane) are each part of the LNG. Thus, any hydrocarbon refrigerant that leaks from a refrigerant container into the LNG tank, becomes a proper part of the LNG. It is not necessary to insulate the refrigerant tanks that lie at the bottom of the LNG tanks to keep them liquid when LNG is offloaded, because even when LNG is offloaded from the floating structure or from a tanker, there is at least 2% of LNG left in the LNG tanks, as a matter of LNG tanker design. This will keep the LNG tanks 20 and refrigerant lying in tanks therein cold until the LNG tanks are filled again.
Although the refrigerant container 40 does not require insulation, it may be desirable to insulate it in order to avoid a quick evaporation of liquid refrigerant during an initial filling of the LNG tank when it previously has been empty. Such initial filling may occur after commissioning of the floating structure, when there is no LNG inside the LNG tanks. The insulation of the refrigerant tank may include a vacuum jacket. If the cold liquid refrigerant in the container 40 should be heated and turn into gas, such gaseous refrigerant can be vented through a vent valve or vent 44 into the LNG tank.
The hydrocarbon refrigerants in the refrigerant containers such as 40, may be removed by a submerged pump, or by flowing a gas such as pressured nitrogen down along a tube 50 to the top of the refrigerant container 40. This forces refrigerant up out of a pipe 52 with a pipe end at the bottom of the refrigerant tank. Applicant notes that propane, whose boiling temperature is −42° C., remains liquid down to a temperature of −190° C. and ethane and methane remain liquid at even lower temperatures. Therefore, all of the refrigerants remain liquid and will flow out of an LNG tank at the LNG storage temperature.
The refrigerants in the refrigerant tank 40 may also be completely removed by means of draining directly into the LNG tank 20, using a drain pipe with drain valve in the bottom of the refrigerant tank.
The refrigerant tank can be placed in LNG tanks of a variety of types, such as self supporting LNG tanks, reinforced prismatic LNG tanks and spherical LNG tanks. In some cases, the refrigerant tanks can be placed in special regions below deck, such as in region 60 in FIG. 5. This is done to store the refrigerant container 62 and the refrigerant therein at a higher temperature (e.g. at about −108° C.) than the stored LNG, but at a temperature that is far below (more than 40° C. below) that of the environment (which is assumed to have an average temperature of 10° C. although it can vary from tropical to arctic). In almost all cases where propane is used, the temperature of hydrocarbon refrigerants will be maintained below the boiling temperature of propane (−42° C.) to keep it liquid. The temperature of the refrigerant is at least 40° C. below average environmental temperature (10° C.) due to the refrigerant being thermally coupled to the cold liquefied gas. The cold gas has a temperature at least 40° C. below environmental temperature. The higher temperature of stored propane than that of LNG can be useful to provide propane for the refrigeration equipment that is close to the temperature of other propane that already lies in the refrigeration equipment, to avoid a temperatures shock when new propane is added. In FIG. 5 the special region or cavity 60 has a cold wall(s) 63 that uses the low temperature of LNG to keep the propane cold, but also has a warm walls 64 that are part of the LNG tank inner wall 66. The tank 12 has double walls including an outer wall 68 and the inner wall 66. The warm walls 64 where they face the refrigerant cavity 60, are isolated from the LNG so the temperature at the warm wall rises above the temperature of the LNG. A vent 56 vents any gas buildup into the LNG tank.
Thus, the invention provides a liquefied natural gas storage facility that includes a floating body with a hull and with at least one LNG tank in the hull. The facility includes refrigeration equipment and includes a container that stores a plurality of cubic meters of hydrocarbon refrigerant. Applicant stores the refrigerant so it is thermally coupled to the LNG so the LNG cools the refrigerant without the need for a separate cooling system. A refrigerant tank preferably lies in or is vented to an LNG tank, so any refrigerant that leaks from the refrigerant tank leaks into the LNG tank where it becomes a proper part of the LNG. The same system can be used to store other liquefied gas such as LPG (liquefied petroleum gas) which is liquid at atmospheric pressure and a temperature of −42° C., and to store liquefied gas that is stored at a pressure above atmospheric pressure. In the case of LPG, it should be stored at a temperature within 10° C. of its boiling temperature of −42° C. The refrigerant container is preferably thermally insulated, and may be a pressure vessel.
Although particular embodiments of the invention have been described and illustrated herein, it is recognized that modifications and variations may readily occur to those skilled in the art, and consequently, it is intended that the claims be interpreted to cover such modifications and equivalents.

Claims

1. A liquefied gas storage facility that includes a hull that floats in a sea, a deck on top of said hull, at least one insulated liquefied gas tank lying primarily in said hull and below said deck and containing cold liquefied gas, and refrigeration equipment that includes expansion, compression and heat transfer portions, wherein said refrigeration equipment includes at least one refrigerant container that holds refrigerant used in at least one of said refrigeration equipment portions, wherein:

said refrigerant container lies below said deck, in a region that is thermally coupled to liquefied gas that lies in said liquefied gas tank, and the refrigerant temperature is maintained at least 40° C. below the average environmental temperature due to the refrigerant being thermally coupled to the cold liquefied gas.

2. The facility described in claim 1 wherein:

said refrigerant container lies completely within said liquefied gas tank.

3. The facility described in claim 1 including:

walls forming a cavity with said refrigerant container lying in said cavity, said cavity walls including a cold wall that separates said cavity from the inside of said liquefied gas tank, and said cavity having a warm wall that separates said cavity from the warmer environment.

4. The facility described in claim 3 wherein:

said hull includes inner and outer hull walls with said inner hull wall forming one of said walls of said cavity.

5. The facility described in claim 1, wherein:

said cold liquefied gas is LNG (liquefied natural gas) at a temperature within 20° C. of −163° C., said average environmental temperature is within 25° C. of a temperature of +10° C., said refrigerant is a hydrocarbon and said refrigerant tank lies completely within said liquefied gas tank.

6. The facility described in claim 1 wherein:

said cold liquefied gas is LPG (liquefied petroleum gas) at a temperature within 10° C. of −42° C.

7. The facility described in claim 1 wherein:

said refrigerant container has a vent (44) that opens to said liquefied gas tank.

8. A liquefied gas storage facility which includes a hull that floats in a sea and that has a deck lying above the sea, at least one liquefied gas tank lying primarily in said hull, a quantity of LNG (liquefied natural gas) lying in said liquefied gas tank, refrigerant equipment on said hull which includes expansion, compression and heat transfer refrigeration portions and at least one refrigerant container and a quantity of refrigerant in said refrigerant container, wherein:

said refrigerant container lies within said liquefied gas tank.

9. The facility described in claim 8 wherein:

said refrigerant comprises primarily a hydrocarbon and said refrigerant container has a vent that opens to a region of said liquefied gas tank outside said refrigerant container.

10. The facility described in claim 8 wherein:

the temperature of said LNG is within 25° C. of −163° C. and said refrigerant has a temperature close to −163° C. than to the temperature of the environment.