Patent Grant
8959930
This application claims priority to Korean Patent Application No. 10-2007-0058942, filed Jun. 15, 2007, which is hereby incorporated by reference in its entirety.
The present invention relates to a method for treating boil-off gas in an LNG carrier having a reliquefaction apparatus, and more particularly, to a method and an apparatus for treating boil-off gas which can prevent waste of boil-off gas and save energy by storing in an LNG storage tank, instead of discharging and burning, surplus boil-off gas which has not been returned to the LNG storage tank through a reliquefaction plant among the total amount of boil-off gas generated in the LNG storage tank.
Generally, natural Gas (NG) is turned into a liquid (also called liquefied natural gas or LNG) in a liquefaction plant, transported over long distances by an LNG carrier, and regasified by passing a floating storage and regasification unit (FSRU) or an unloading terminal on land to be supplied to consumers.
As liquefaction of natural gas occurs at a cryogenic temperature of approximately −163° C. at ambient pressure, LNG is likely to be vaporized even when the temperature of the LNG is slightly higher than −163° C. at ambient pressure. Though an LNG storage tank of an LNG carrier is thermally insulated, as heat is continually transmitted from the outside to the LNG in the LNG storage tank, the LNG is continually vaporized and boil-off gas (BOG) is generated in the LNG storage tank during the transportation of LNG by the LNG carrier.
If boil-off gas is generated in an LNG storage tank as described above, the pressure of the LNG storage tank is increased and becomes dangerous.
Conventionally, if the pressure of an LNG storage tank is increased beyond a set pressure, boil-off gas was discharged to the outside of the LNG storage tank and used as a fuel for propulsion of an LNG carrier, so as to maintain the pressure of the LNG storage tank at a safe level. However, a steam turbine propulsion system driven by the steam generated in a boiler by burning the boil-off gas generated in an LNG storage tank has a problem of low propulsion efficiency.
Also, a dual fuel diesel electric propulsion system, which uses the boil-off gas generated in an LNG storage tank as a fuel for a diesel engine after compressing the boil-off gas, has higher propulsion efficiency than the steam turbine propulsion system, but has difficulty in maintenance due to complicated integration of a medium-speed diesel engine and an electric propulsion unit in the system. In addition, this system, which must supply boil-off gas as a fuel, is forced to employ a gas compression method which requires higher installation and operational costs than a liquid compression method.
Further, such a conventional method using boil-off gas as a fuel for propulsion fails to achieve the efficiency of a two-stroke slow-speed diesel engine, which is used in ordinary ships.
Furthermore, the conventional method has such another problem that, in case the amount of boil-off gas generated in an LNG storage tank exceeds the capacity of a propulsion system, additional equipment such as a gas combustion unit is needed to treat surplus boil-off gas.
On the other hand, there is another method of maintaining a pressure of an LNG storage tank at a safe level. If the pressure of the LNG storage tank is increased beyond a set pressure, boil-off gas is discharged to the outside of the LNG storage tank and reliquefied in a reliquefaction plant and then returned to the LNG storage tank.
As shown in
Though the reliquefaction plant is provided on the LNG carrier, a treatment capacity of the reliquefaction plant is limited, and in case an amount of boil-off gas greater than the treatment capacity of the reliquefaction plant is generated, surplus boil-off gas must be burned and wasted. To burn surplus boil-off gas, a conventional LNG carrier has a gas combustion unit (103), and the surplus boil-off gas is heated in a gas heater (105) to an appropriate temperature and then supplied to the gas combustion unit (103) to be burned and wasted.
As illustrated in
Since a treatment capacity of a reliquefaction plant is limited, surplus boil-off gas beyond a treatment capacity of a reliquefaction plant is generated for 3 to 4 days at the beginning of a loaded voyage of an LNG carrier. Such surplus boil-off gas, as stated above, is all burned and wasted. Accordingly, the prior art has a problem that large quantities of surplus boil-off gas, which amount to 55 tons (see oblique lines in
In a case of an LNG carrier having a capacity of 150,000 m3, the quantity of boil-off gas burnt as described above amounts to 1500 to 2000 tons per year, which cost about 700,000 USD. Further, burning of boil-off gas raises a problem of environmental pollution.
In addition, the prior art has such other problems that since a reliquefaction plant and a gas combustion unit (103) should be operated together at the beginning of a loaded voyage of an LNG carrier, additional equipment such as a gas combustion unit (103) or a gas heater (105) is needed for treating the surplus boil-off gas, and that a large amount of energy is consumed due to operation of the gas combustion unit (103).
Korean Patent Laid-Open Publication Nos. KR 2001-0014021, KR 2001-0014033, KR 2001-0083920, KR 2001-0082235, and KR 2004-0015294 disclose techniques of suppressing the generation of boil-off gas in an LNG storage tank by maintaining the pressure of the boil-off gas in the LNG storage tank at a high pressure of approximately 200 bar (gauge pressure) without installing a thermal insulation wall in the LNG storage tank. However, this LNG storage tank must have a significantly high thickness to store boil-off gas having a high pressure of approximately 200 bar, and consequently it has problems of increasing manufacturing costs and requiring additional equipment such as a high-pressure pump, to maintain the pressure of boil-off gas at approximately 200 bar.
As stated above, a method for treating boil-off gas in an LNG carrier according to the prior art, which maintains an internal pressure of an LNG storage tank at a constant level and allows generation of boil-off gas during transportation of a cryogenic liquid, has a problem of consuming a large amount of boil-off gas or having to install additional equipment such as a reliquefaction plant and a gas combustion unit.
In addition, unlike a case of transporting a cryogenic liquid at a low atmospheric pressure, a method of transporting a cryogenic liquid using a storage tank, such as a pressure tank, which can withstand a high pressure at a somewhat high temperature, does not need to treat or waste boil-off gas, but has problems that the size of the tank is limited and that high manufacturing costs are required.
The present invention has been made in consideration of the situations mentioned above, and is to provide a method and an apparatus for treating boil-off gas which can maintain an amount of boil-off gas discharged from an LNG storage tank at a constant level by storing in the LNG storage tank, instead of discharging and burning, surplus boil-off gas which has not been returned to the LNG storage tank through a reliquefaction plant among the total amount of boil-off gas generated in the LNG storage tank, and which can prevent waste of boil-off gas and save energy by allowing a pressure in the LNG storage tank to be increased.
The following is a description of the reference signs related to main parts in the drawings:
An embodiment of the present invention provides a method for treating boil-off gas generated in an LNG storage tank of an LNG carrier for transporting LNG in a cryogenic liquid state, the LNG carrier having a boil-off gas reliquefaction plant, wherein an amount of boil-off gas, which corresponds to a treatment capacity of the reliquefaction plant, among the total amount of boil-off gas generated during the voyage of the LNG carrier, is discharged from the LNG storage tank and reliquefied by the reliquefaction plant.
Another embodiment of the present invention provides a method for treating boil-off gas generated in an LNG storage tank of an LNG carrier for transporting LNG in a cryogenic liquid state, the LNG carrier having a boil-off gas reliquefaction plant, wherein an amount of boil-off gas which has been discharged from the LNG storage tank among the total amount of boil-off gas generated during the voyage of the LNG carrier is maintained at a constant level, and a pressure in the LNG storage tank is allowed to be increased due to the boil-off gas which has not been discharged from the LNG storage tank among the total amount of boil-off gas generated.
In addition, another embodiment of the present invention provides an apparatus for treating boil-off gas generated in an LNG storage tank of an LNG carrier for transporting LNG in a cryogenic liquid state, the apparatus comprising: a controller for controlling a BOG discharging means so as to discharge from the LNG storage tank an amount of the boil-off gas corresponding to a treatment capacity of the reliquefaction plant among the total amount of boil-off gas generated during the voyage of the LNG carrier.
Further, another embodiment of the present invention provides an LNG carrier for transporting LNG in a cryogenic liquid state with the LNG stored in an LNG storage tank, the LNG carrier comprising: a boil-off gas treating apparatus including a boil-off gas reliquefaction plant, and a controller for controlling a BOG discharging means so as to discharge from the LNG storage tank an amount of the boil-off gas corresponding to a treatment capacity of the reliquefaction plant among the total amount of boil-off gas generated during the voyage of the LNG carrier; and an LNG storage tank whose internal pressure is allowed to be increased due to the boil-off gas which has not been discharged from the LNG storage tank among the total amount of the boil-off gas generated during the voyage of the LNG carrier.
The present invention relates to a somewhat high-pressure (near ambient pressure) tank for transporting LNG in a cryogenic liquid state, characterized in that some degree of change in the internal pressure of the tank is allowed during the transportation of LNG.
One embodiment of the present invention provides, in an LNG carrier having boil-off gas treating means for treating boil-off gas generated in an LNG storage tank, an LNG carrier characterized in that the vapor pressure in the LNG storage tank and the temperature of the LNG are allowed to be increased during the transportation of the LNG in the LNG storage tank.
In general, the following methods are known as means for treating boil-off gas: using the boil-off gas generated from an LNG storage tank for a boiler (e.g. a steam turbine propulsion boiler); using the boil-off gas as a fuel of a gas engine such as a DFDE and MEGI; using the boil-off gas for a gas turbine; and reliquefying the boil-off gas and returning the reliquefied boil-off gas to the LNG storage tank (see Korean Patent Laid-Open Publication No. 2004-0046836, Korean Patent Registration Nos. 0489804 and 0441857, and Korean Utility Model Publication No. 2006-0000158). These methods has problems of generation of excessive boil-off gas exceeding a treatment capacity of a general boil-off gas treating means (e.g. after LNG is loaded), or waste of boil-off gas by a boil-off gas combustion means such as a gas combustion unit (GCU) when the boil-off gas cannot be treated by the boil-off gas treating means, e.g. when an LNG carrier enters or leaves port and when it passes through a canal.
The present invention has an advantage of eliminating such waste of boil-off gas by improving flexibility in boil-off gas treatment. The LNG carrier according to the present invention may not require a GCU, or may require a GCU for improving flexibility in treating or managing boil-off gas in an emergency.
The LNG carrier of the present invention is equipped with boil-off gas treating means such as a boiler, a reliquefaction apparatus, and a gas engine for treating the boil-off gas generated from an LNG storage tank by discharging the boil-off gas to the outside of the LNG storage tank.
Another embodiment of the present invention provides, in a method for controlling a safety valve provided at an upper portion of an LNG storage tank for an LNG carrier, a method for opening and closing the safety valve characterized in that the set pressure of the safety valve during the loading of LNG in the LNG storage tank differs from the set pressure of the safety valve during the voyage of the LNG carrier. The present invention also provides a safety valve, an LNG storage tank, and an LNG carrier having said feature.
Conventionally, the pressure in an LNG storage tank is safely managed by installing a safety valve at an upper portion of the LNG storage tank for an LNG carrier which transports LNG in a cryogenic liquid state. Some known methods of safely managing the pressure in an LNG storage tank are: safeguarding against a possible explosion of an LNG storage tank by means of a safety valve; and treating the boil-off gas generated from the LNG storage tank, after LNG is loaded, by the above-mentioned methods including using the boil-off gas for a boiler (e.g. a steam turbine propulsion boiler), using the boil-off gas as a fuel of a gas engine such as a DFDE and MEGI, using the boil-off gas for a gas turbine, and reliquefying the boil-off gas and returning the reliquefied boil-off gas to the LNG storage tank. These methods are forced to generate excessive boil-off gas which exceeds a treatment capacity of a general boil-off gas treating means (e.g. after LNG is loaded in an LNG carrier), or to waste boil-off gas by a boil-off gas combustion means such as a GCU when an LNG carrier enters or leaves port, and when it passes through a canal. The pressure in an LNG storage tank for an LNG carrier is maintained within a predetermined range by such methods.
In such an LNG carrier, when the set value of a safety valve is 0.25 bar, a maximum of about 98% of the full capacity of an LNG storage tank is loaded with LNG and the remaining about 2% is left as an empty space. If more than 98% of the full capacity of an LNG storage tank is loaded with LNG, when the pressure of the LNG storage tank reaches 0.25 bar, the LNG in the LNG storage tank may overflow from the dome at an upper portion thereof. As shown in another embodiment of the present invention, if the pressure of LNG in an the LNG storage tank is continually allowed to be increased after the LNG is loaded, even when a small amount of LNG is loaded, the LNG in the LNG storage tank may overflow due to the expansion of the LNG caused by an increase in the temperature of the LNG at the set pressure of the safety valve according to the present invention. For example, it has been found that, when the vapor pressure in an LNG storage tank is 0.7 bar, even if 97% of the full capacity of the LNG storage tank is loaded with LNG, the LNG in the LNG storage tank may overflow. It follows that the amount of LNG to be loaded should be reduced.
Accordingly, instead of uniformly fixing the set pressure of a safety valve provided at an upper portion of an LNG storage tank to a somewhat high pressure near ambient pressure, it is possible to reduce waste of boil-off gas or increase flexibility in treatment of boil-off gas without reducing an initial LNG load, by fixing the set pressure of a safety valve to a lower pressure, e.g. 0.25 bar, as in an existing LNG carrier, during loading of LNG, and then increasing the set pressure of the safety valve, as in another embodiment of the present invention, when the amount of LNG in the LNG storage tank is reduced by using some boil-off gas (e.g. using the boil-off gas as a fuel of a boiler or engine) after the LNG carrier starts voyage. The present invention, if applied to an LNG carrier equipped with boil-off gas treating means (e.g. a boiler, a reliquefaction apparatus, or a gas engine) for treating the boil-off gas generated from an LNG storage tank by discharging the boil-off gas to the outside of the LNG storage tank, has a great effect in eliminating waste of boil-off gas.
Consequently, in the present invention, the set pressure of a safety valve is increased after the amount of LNG in an LNG storage tank is reduced by discharging the boil-off gas generated in the LNG storage tank to the outside thereof: preferably the set pressure during the loading of LNG is set at 0.25 bar or lower; and the pressure during the voyage of the LNG carrier is set from higher than 0.25 bar to 2 bar, and more preferably, the set pressure during the voyage of the LNG carrier is set from higher than 0.25 bar to 0.7 bar. Here, the set pressure of a safety valve during the voyage of the LNG carrier may be increased gradually, e.g. from 0.4 bar to 0.7 bar, according to the amount of boil-off gas used according to the voyage conditions.
In the present invention, the expression “during the voyage of an LNG carrier” means when the volume of LNG in an LNG storage tank is somewhat reduced by use of some boil-off gas after the LNG carrier starts voyage with LNG loaded therein. For example, it is desirable to set the set pressure of a safety valve at 0.25 bar when the volume of LNG in an LNG storage tank is 98.5%, at 0.4 bar when the volume of LNG is 98.0%, 0.5 bar when the volume of LNG is 97.7%, and 0.7 bar when the volume of LNG is 97.1%.
Another embodiment of the present invention provides an LNG storage tank for an LNG carrier for transporting LNG in a cryogenic liquid state, characterized in that the set pressure of a safety valve provided at an upper portion of the LNG storage tank is set from higher than 0.25 bar to 2 bar, preferably from higher than 0.25 bar to 0.7 bar, and more preferably approximately 0.7 bar. The present invention also provides a method for setting a safety valve, an LNG storage tank, and an LNG carrier having said technical feature.
As this method has problems of great waste of boil-off gas and increase of manufacturing costs of an LNG carrier, the present invention solves said problems by increasing the set pressure value of a safety valve of an LNG storage tank, thereby allowing increases in the internal pressure of the LNG storage tank and in the temperature of the LNG in the LNG storage tank during the voyage of an LNG carrier from after loading of LNG to before unloading of LNG.
Another embodiment of the present invention provides an LNG storage tank for an LNG carrier for transporting LNG in a cryogenic liquid state, characterized in that the vapor pressure in the LNG storage tank is controlled within near-ambient pressure, and that the vapor pressure in the LNG storage tank and the pressure of the LNG in the LNG storage tank are allowed to be increased during the transportation of the LNG. The LNG storage tank is also characterized in that the vapor pressure in the LNG storage tank ranges from higher than 0.25 bar to 2 bar, preferably from higher than 0.25 bar to 0.7 bar, and more preferably, approximately 0.7 bar. In addition, the LNG storage tank is characterized in that the boil-off gas at an upper portion of the LNG storage tank is mixed with the LNG at a lower portion of the LNG storage tank so as to maintain a uniform temperature distribution in the LNG storage tank. On one hand, as more LNG is likely to be vaporized when the temperature of one part of the LNG storage tank is higher than the temperature of the other part thereof, it is desirable to maintain a uniform temperature distribution of the LNG or boil-off gas in the LNG storage tank. On the other hand, as the boil-off gas at an upper portion of the LNG storage tank has a smaller heat capacity than the LNG at a lower portion of the LNG storage tank, a local sharp increase in the temperature at an upper portion of the LNG storage tank due to the heat ingress from the outside into the LNG storage tank may result in a sharp increase in the internal pressure of the LNG storage tank. The sharp increase in the internal pressure of the LNG storage tank can be prevented by mixing the boil-off gas at an upper portion of the LNG storage tank with the LNG at a lower portion of the LNG storage tank.
Also, according to another embodiment of the present invention, the vapor pressure in an LNG storage tank for an LNG carrier can be controlled to match the pressure in an LNG storage tank for receiving the LNG at an LNG terminal. For example, the pressure in the LNG storage tank for an LNG carrier can match the pressure of the LNG storage tank for receiving the LNG by continually increasing the pressure in the LNG storage tank for an LNG carrier during the voyage of the LNG carrier, in case the pressure in the LNG storage tank to receive LNG therein at an LNG unloading terminal, an LNG-RV, or a FSRU is high (e.g. approximately 0.4 to 0.7 bar), and by reducing the waste of boil-off gas by using the flexibility in boil-off gas treatment according to the present invention, in case the pressure in the LNG storage thank for receiving LNG therein at an LNG unloading terminal is low (approximately 0.2 bar) as in the prior art.
In addition, another embodiment of the present invention provides a method for transporting LNG in a cryogenic liquid state having said technical feature, and an LNG carrier having said LNG storage tank.
In particular, another embodiment of the present invention provides a membrane LNG storage tank having a somewhat high pressure near ambient pressure to transport LNG in a cryogenic liquid state, characterized in that some degree of change in the pressure in the LNG storage is allowed during the transportation of LNG. The membrane tank according to the present invention is a cargo space of an LNG tank as defined in IGC Code (2000). More specifically, a membrane tank is a non-self-supporting tank having a thermal insulation wall formed in a body and having a membrane formed at an upper portion of the tank. In the present application, the membrane tank is used to include a semi-membrane tank.
Some examples of the membrane tank are GTT NO 96-2 and Mark III as described below, and tanks as described in Korean Patent Nos. 499710 and 644217.
A membrane tank can be designed to withstand the pressure up to 0.7 bar (gauge pressure) by reinforcing the tank, but it is generally prescribed that a membrane tank should be designed to have the pressure not exceeding 0.25 bar. All the existing membrane tanks comply with this regulation, and are managed so that the vapor pressure in the tank is 0.25 bar or lower, and that the temperature and pressure of the LNG are almost constant during the voyage. On the contrary, the present invention is characterized in that at the pressure of higher than 0.25 bar, preferably from higher than 0.25 bar to 2 bar or lower, and more preferably from higher than 0.25 bar to 0.7 bar or lower, the pressure in the tank and the temperature of the LNG are allowed to be increased. Also, the method for treating boil-off gas by using the LNG storage tank according to the present invention is characterized by maintaining a uniform temperature distribution in the LNG storage tank.
According to another embodiment of the present invention, the present invention provides a large LNG carrier, or an LNG carrier having an LNG storage capacity of preferably 100,000 m3 or more. In case of an LNG carrier having a large capacity, to manufacture an LNG storage tank into a high-pressure tank, the manufacturing costs are sharply increased due to an increase in the thickness of the tank. In case of manufacturing a tank having a relative pressure of approximately 1 bar, near atmospheric pressure, as in the present invention, the manufacturing costs are not sharply increased, and also the tank can transport LNG, substantially withstanding the pressure generated by boil-off gas and not treating the boil-off gas.
The LNG storage tank according to the present invention is applicable to an LNG carrier, an LNG floating storage and regasification unit (FSRU), an unloading terminal on land, and an LNG regasification vessel (LNG-RV), etc. The LNG storage tank has advantages of reducing waste of boil-off gas by allowing increase in the pressure and temperature in the LNG storage tank and solving a problem of treating boil-off gas, and of increasing flexibility in LNG treatment, such as transporting and storing LNG, because it is possible to store LNG in said all kinds of LNG storage tanks for a long time, taking into account LNG demand.
The embodiments of the present invention will be described mainly by putting an example of an LNG storage tank applicable to an LNG carrier.
With regard to temperature, LNG which generally contains many impurities has a lower boiling point than a pure methane liquid. The pure methane has a boiling point of about −161° C. at 0.06 bar, and LNG for transportation which contains impurities such as nitrogen, ethane, etc., has a boiling point of approximately −163° C. Based on pure methane, LNG in an LNG storage tank after being loaded into the LNG storage tank has a temperature of approximately −161° C. at 0.06 bar. If the vapor pressure in the LNG storage tank is controlled to be 0.25 bar, taking into account the transportation distance and the consumption of boil-off gas, the temperature of the LNG is increased to approximately −159° C.; if the vapor pressure in the LNG storage tank is controlled to be 0.7 bar, the temperature of the LNG is increased to approximately −155° C.; if the vapor pressure in the LNG storage tank is controlled to be 2 bar, the temperature of the LNG is increased up to approximately −146° C.
The LNG storage tank for an LNG carrier according to the present invention comprises a thermal insulation wall and is designed by taking into account the pressure increase caused by the generation of boil-off gas. That is, the LNG storage tank is designed to have sufficient strength to withstand the pressure increase caused by the generation of boil-off gas. Accordingly, the boil-off gas generated in the LNG storage tank (1) for an LNG carrier during the voyage of the LNG carrier is accumulated in the LNG storage tank (1).
For example, the LNG storage tank (1) for an LNG carrier according to the embodiment of the present invention preferably comprises a thermal insulation wall, and is designed to withstand the pressure from higher than 0.25 bar to 2 bar (gauge pressure), and more preferably, the pressure of 0.6 to 1.5 bar (gauge pressure). Taking into account the transportation distance of LNG and the current IGC Code, it is desirable to design the LNG storage tank to withstand the pressure from higher than 0.25 bar to 0.7 bar, particularly, approximately 0.7 bar. However, making the pressure too low is not desirable because the transportation distance of LNG becomes too short, and also making the pressure too high causes difficulty in manufacturing the LNG storage tank.
In addition, since the LNG storage tank (1) for an LNG carrier according to the present invention can be sufficiently embodied by designing the LNG storage tank (1) to have a great thickness during an initial design, or simply by suitably reinforcing an existing general LNG storage tank for an LNG carrier through addition of a stiffener thereto without making a big change in the design of the existing LNG storage tank, it is economical in view of manufacturing costs.
Various conventional LNG storage tanks for LNG carriers with a thermal insulation wall therein are known in the related art as described below. Accordingly, the thermal insulation wall is omitted from
An LNG storage tank installed in an LNG carrier can be classified into an independent-type tank and a membrane-type tank, and is described in detail below.
GTT NO 96-2 and GTT Mark III in Table 1 below was renamed from GT and TGZ, respectively, when the Gaz Transport (GT) Corporation and Technigaz (TGZ) corporation was incorporated into GTT (Gaztransport & Technigaz) Corporation in 1995.
GT type and TGZ type tanks are disclosed in U.S. Pat. No. 6,035,795, U.S. Pat. No. 6,378,722, and U.S. Pat. No. 5,586,513, US Patent Publication US 2003-0000949, and Korean Patent Laid-Open Publication Nos. KR 2000-0011347, and KR 2000-0011346. Korean Patent Nos. 499710 and 0644217 disclose thermal insulation walls embodied as other concepts.
The prior art discloses LNG storage tanks for LNG carriers having various types of thermal insulation walls, which are to suppress generation of boil-off gas as much as possible.
The present invention can be applied to conventional LNG storage tanks for LNG carriers having various types of thermal insulation functions as stated above. Most of these LNG storage tanks for LNG carriers are designed to withstand the pressure of 0.25 bar or lower, and consume the boil-off gas generated in the LNG storage tanks as a fuel for propulsion of the LNG carriers or reliquefy the boil-off gas to maintain the pressure in the LNG storage tank at 0.2 bar or lower, e.g. 0.1 bar, and burn part or all of the boil-off gas if the pressure in the LNG storage tank is increased beyond the value. In addition, these LNG storage tanks have a safety valve therein, and if the LNG storage tanks fail to control the pressure therein as stated above, boil-off gas is discharged to the outside of the LNG storage tanks through the safety valve (mostly, having a set pressure of 0.25 bar).
On the contrary, according to the present invention, in case the pressure at an upper portion, usually a dome, of the LNG storage tank shown in
In addition, the LNG storage tank according to the present invention is configured to reduce the pressure in the LNG storage tank by reducing the local increase in temperature and pressure of the LNG storage tank. The LNG storage tank maintains a uniform temperature distribution thereof by spraying the LNG, having a lower temperature, at a lower portion of the LNG storage tank, toward the boil-off gas, having a higher temperature, at an upper portion of the LNG storage tank, and by injecting the boil-off gas, having a higher temperature, at an upper portion of the LNG storage tank, toward the LNG, having a lower temperature, at a lower portion of the LNG storage tank.
In
Such reduction of generation of boil-off gas is particularly useful for gradually increasing the pressure in the LNG storage tank because the generation of boil-off gas in an LNG carrier without having boil-off gas treating means has direct connection with the increase in pressure in the LNG storage tank. In case of an LNG carrier having boil-off gas treating means, if the pressure in the LNG storage tank is increased, a certain amount of boil-off gas is discharged to the outside, thereby controlling the pressure in the LNG storage tank, and consequently, spray of LNG or injection of boil-off gas may not be needed during the voyage of the LNG carrier.
In addition, if LNG is loaded in a sub-cooled liquid state into an LNG carrier at a production terminal where LNG is produced, it is possible to further reduce the generation of boil-off gas (or the increase in pressure) during the transportation of LNG to a destination. The pressure in the LNG storage tank for an LNG carrier can be a negative pressure (0 bar or lower) after LNG is loaded in a sub-cooled liquid state at a production terminal. To prevent the pressure from being decreased to a negative pressure, a vapor region of the LNG storage tank may be filled with nitrogen.
A method for treating boil-off gas using such an LNG storage tank for an LNG carrier will be described below.
During the voyage of an LNG carrier, the LNG storage tank (1) for an LNG carrier according to the present invention allows a pressure increase in the LNG storage tank (1) without treating the boil-off gas generated in the LNG storage tank (1), thereby increasing the temperature in the LNG storage tank (1), and accumulating most of the heat influx as internal energy of LNG and NG in the LNG storage tank, and then treating the boil-off gas accumulated in the LNG storage tank (1) for an LNG carrier at an unloading terminal when the LNG carrier arrives at a destination.
The unloading terminal is installed with a plurality of LNG storage tanks (2) for an unloading terminal, a high-pressure compressor (3a), a low-pressure compressor (3b), a recondenser (4), a high-pressure pump (P), and a vaporizer (5).
As a large amount of boil-off gas is accumulated in the LNG storage tank (1) for an LNG carrier, the boil-off gas in the LNG storage tank (1) is generally compressed to 70-80 bar by the high-pressure compressor (3a) at unloading terminals and then supplied directly to consumers. Part of the boil-off gas accumulated in the LNG storage tank (1) for an LNG carrier may generally be compressed to approximately 8 bar by the low-pressure compressor (3b), then recondensed by passing through the recondenser (4), and then regasified by the vaporizer (5) so as to be supplied to consumers.
When LNG is unloaded from the LNG storage tank for an LNG carrier to be loaded into an LNG storage tanks for an unloading terminal, additional boil-off gas is generated due to inflow of LNG having a higher pressure into the LNG storage tanks for an unloading terminal because the pressure of the LNG storage tank for an LNG carrier is higher than that of the LNG storage tank for an unloading terminal. To minimize generation of additional boil-off gas, LNG can be supplied to consumers by transmitting the LNG from the LNG storage tank for an LNG carrier directly to an inlet of a high-pressure pump at an unloading terminal. The LNG storage tank for an LNG carrier according to the present invention, as the pressure in the LNG storage tank is high during the unloading of LNG, has an advantage of shortening an unloading time by 10 to 20% over conventional LNG storage tanks.
Instead of being supplied to the LNG storage tanks (2) for an unloading terminal at an unloading terminal, the LNG stored in the LNG storage tank (1) for an LNG carrier may be supplied to the recondenser (4) to recondense boil-off gas and then regasified by the vaporizer (5), thereby being supplied directly to consumers.
On the other hand, if a recondenser is not installed at an unloading terminal, LNG may be supplied directly to a suction port of the high-pressure pump (P).
As stated above, if the plurality of LNG storage tanks (2) for an unloading terminal are installed at an unloading terminal and LNG is evenly distributed from the LNG storage tank (1) for an LNG carrier to each of the plurality of LNG storage tanks (2) for an unloading terminal, the effect of generation of boil-off gas in the LNG storage tanks (2) for an unloading terminal can be minimized due to dispersion of the generation of boil-off gas to the plurality of the LNG storage tanks (2) for an unloading terminal. Since the amount of boil-off gas generated in the LNG storage tanks (2) for an unloading terminal is small, the boil-off gas is generally compressed by the low-pressure compressor (3b) to approximately 8 bar and then recondensed by passing through the recondenser (4), and then regasified by the vaporizer (5), to be supplied to consumers.
In addition, according to the present invention, as the LNG storage tank for an LNG carrier is operated at a higher pressure than an existing design pressure, a process of filling boil-off gas or NG vapor in the LNG storage tank for an LNG carrier is not required to maintain the pressure in the LNG storage tank for an LNG carrier during the unloading of LNG.
Further, if a conventional LNG storage tank for an LNG terminal or for a floating storage and regasification unit (FSRU) is modified, or a new LNG storage tank for an unloading terminal or for a floating storage and regasification unit (FSRU) is constructed such that the storage pressure of the LNG storage tank corresponds to the pressure of the LNG storage tank for an LNG carrier according to the present invention, no additional boil-off gas is generated during the unloading of LNG from the LNG carrier, and consequently an existing unloading technique can be applied.
According to the present invention, an LNG regasification vessel (LNG-RV) may have merits of both an LNG carrier and an LNG floating storage and regasification unit (FSRU) as stated above.
S or V mode is appropriate when the allowable pressure of an LNG storage tank at an unloading terminal is 0.4 bar or lower. S and V modes are applicable to an LNG carrier having boil-off gas treating means. S mode indicates the voyage of an LNG carrier in which the pressure in the LNG storage tank of the LNG carrier is allowed to be continually increased to 0.4 bar or lower, the same as the allowable pressure of the LNG storage tank of an LNG unloading terminal.
V mode is to enlarge the width of operation of the pressure in the LNG storage tank for an LNG carrier, and has an advantage of reducing waste of boil-off gas by storing the excessive boil-off gas exceeding the amount of boil-off gas consumed by boil-off gas treating means, in the LNG storage for an LNG carrier. For example, when an LNG carrier passes through a canal, boil-off gas is not consumed because propulsion means using the boil-off gas as a fuel, such as a DFDE, MEGI, and gas turbine, does not operate. Accordingly, the boil-off gas generated in the LNG storage tank for an LNG carrier can be stored therein, thereby being capable of increasing the pressure of the LNG storage tank for an LNG carrier to 0.7 to 1.5 bar or lower. After an LNG carrier passes through a canal, the propulsion means using boil-off gas as a fuel is fully operated, thereby being capable of increasing the consumption of boil-off gas and decreasing the pressure of the LNG storage tank for an LNG carrier to 0.4 bar or lower.
The pressure operation types of an LNG storage tank for an LNG carrier can vary depending on whether or not a flash gas treatment facility for treating a large amount of flash gas is installed at an LNG unloading terminal. In case a flash gas treatment facility for treating a large amount of flash gas is installed at an LNG unloading terminal, the pressure of the LNG storage tank for an LNG carrier is operated in an F mode; in case a flash gas treatment facility for treating a large amount of flash gas is not installed at an LNG unloading terminal, the pressure of the LNG storage tank for an LNG carrier is operated in an S or V mode.
The apparatus for reducing the pressure increase in the LNG storage tank for an LNG carrier as illustrated in
This apparatus comprises a boil-off gas suction port (31) provided at an upper portion of the LNG storage tank for an LNG carrier, a pipe (33) having one end connected to the boil-off gas suction port (31) and the other end connected to the lower portion of the LNG storage tank (1), and a compressor (35) provided at a portion of the pipe (33).
As illustrated in the left side of
As illustrated in the right side of
It is desirable that liquid suction prevention means should be provided at the boil-off gas suction port (31). One example of the liquid suction prevention means is a demister.
The apparatus for reducing the pressure increase in the LNG storage for an LNG carrier is configured to reduce the local increase in the temperature and pressure of the LNG storage tank, thereby reducing the pressure of the LNG storage tank. The generation of boil-off gas can be reduced by injecting the boil-off gas, having a higher temperature, at an upper portion of the LNG storage tank (1) for an LNG carrier toward a lower portion of the LNG storage tank (1) for an LNG carrier having a lower temperature, thereby maintaining a uniform temperature distribution of the LNG storage tank for an LNG carrier, that is, preventing the local increase in the temperature in the LNG storage tank.
In case of an LNG carrier provided with a safety relief valve (SRV) or safety valve of the LNG storage tank, the set pressure of the safety valve is initially set low so as to maximize the cargo loading, but can be increased during the voyage according to the LNG volume decrease due to the consumption of boil-off gas.
If the set pressure of the safety valve is increased during the voyage, the amount of boil-off gas generated from the LNG storage tank (1) is decreased, thereby being capable of minimizing the amount of boil-off gas discharged to the atmosphere or consumed in a combustion unit.
As the measured values such as the level of LNG in the LNG storage tank are frequently changed during the voyage, the present invention comprises a system for eliminating outside noise and fluctuation caused by dynamic movement of a ship through an appropriate data processing, a system for calculating an allowable set pressure of the safety valve of the LNG storage tank by calculating the actual volume of the LNG in the LNG storage tank (1) by using the processed data, and an apparatus for displaying the results.
The system for confirming a set pressure of the safety valve of the LNG storage tank according to the embodiment, as illustrated in the left side of
It is desirable to process the data in the data processing module (61) by using a method of least squares, a moving average, or a low-pass filtering.
In addition, the system for confirming the set pressure of the safety valve of the LNG storage tank further comprises an LNG volume calculating module (63) for calculating the volume of the LNG in the LNG storage tank (1) by calculating the data processed in the data processing module (61).
The system for confirming the set pressure of the safety valve of the LNG storage tank calculates an allowable set pressure of the safety valve of the LNG storage tank (1) from the volume of the LNG calculated by the LNG volume calculating module (63).
On the other hand, it is possible to calculate the current volume of the LNG in the LNG storage tank by measuring the flow rate of the fuel gas supplied from the LNG storage tank (1) to fuel gas propulsion means of an LNG carrier and comparing the initial load of LNG with the amount of the used boil-off gas to, and to reflect the volume of the LNG calculated from the flow rate of the fuel gas measured as described above in the volume of the LNG calculated by the LNG volume processing module (63).
The allowable set pressure of the safety valve of the LNG storage tank and the volume of the LNG in the LNG storage tank calculated as described above are displayed on a display panel (65).
A differential pressure flow meter is used for measuring the flow rate of the fuel gas of an LNG carrier. In the flow meter, the measurement range is limited, and a large measurement error can occur for the flow rate out of the measurement range. To change the measurement range, an orifice itself should be replaced, which is an annoying and dangerous job.
Conventionally, only one orifice is installed and consequently the measurement range is limited, but if two orifices having different measurement ranges are arranged in series, the effective measurement range can be expanded simply by selecting and using the proper measurement values of the orifices according to the flow rate.
That is to say, to measure a large range of the flow rate of fuel gas, the effective measurement range can be simply expanded by arranging at least two orifices in series, each orifice having a different measurement range, and selecting and using the appropriate measurement values of the orifices according to the flow rate. In
The effective measurement range can be simply expanded by installing the selector (75), which is selectable according to the measurement range as described above, between the differential pressure measurers (73) and the flow meter (77) and selecting and using the appropriate measurement values of the orifices according to the flow rate.
In a conventional system, the capacity of a fuel gas orifice is set near NBOG (natural boil-off gas). Accordingly, in case of an LNG carrier whose consumption of boil-off gas is small, the accuracy in measurements is low. To make up for this inaccuracy, the present invention provides a method of additionally installing small orifices in series.
This method can measure the level, or volume, of the LNG in the LNG storage tank from the amount of LNG consumed.
In addition, the prior art does not know the composition of boil-off gas, which is an additional factor of reducing the accuracy in measurements. To make up for this, the composition of boil-off gas may be considered by adding gas chromatography.
Further, if the measurement of the level of LNG in the LNG storage becomes accurate as described above, it can improve the efficiency of the boil-off gas management method and apparatus of the present invention which maintains the pressure of the LNG storage tank at a somewhat higher than the prior art. That is, accurate measurement of the volume of LNG in an LNG storage tank can facilitate changing the setting of a safety valve of the LNG storage tank into multiple settings, and reduce the consumption of boil-off gas.
An LNG carrier, which has fuel gas propulsion means using as a propulsion fuel the compressed boil-off gas by compressing the boil-off gas at an upper portion of the LNG storage tank for an LNG carrier, cannot use the fuel gas at all when passing through a canal such as the Suez Canal, and consequently there is a great possibility of local increase in the temperature and pressure of the LNG storage tank. A boil-off gas extracting apparatus may be additionally needed to solve this problem. That is, as illustrated in
To do this, a boil-off gas branch line (L2) for returning the boil-off gas to the LNG storage tank (1) is installed in the middle of a fuel gas supply line (L1) for supplying compressed boil-off gas to the fuel gas propulsion means after compressing the boil-off gas at an upper portion of the LNG storage tank (1) for an LNG carrier. In addition, a compressor (41) is installed in the middle of the fuel gas supply line (L1) upstream of a meeting point of the fuel gas supply line (L1) and the boil-off gas branch line (L2).
A buffer tank (43) is installed in the middle of the boil-off gas branch line (L2). As there is a big difference between the pressure of the boil-off gas passing through the compressor (41) and the pressure of the LNG storage tank (1), it is desirable to temporarily store the boil-off gas passing through the compressor (41) in the buffer tank (43) and control the pressure of the boil-off gas to match the pressure of the LNG storage tank (1) and then return the boil-off gas to the LNG storage tank (1).
It is desirable to operate an apparatus for reducing pressure increase in the LNG storage tank for an LNG carrier at an interval of about 10 minutes per 2 hours.
Some examples of the fuel gas propulsion means are a double fuel diesel electric propulsion system (DFDE), a gas injection engine, and a gas turbine.
An LNG carrier, to which a DFDE, a gas injection engine, or a gas turbine is applied, uses the concept of compressing boil-off gas by a boil-off gas compressor and then sending the compressed boil-off gas to an engine to burn the boil-off gas. However, an LNG carrier which is configured to eliminate or reduce the discharge of boil-off gas of an LNG storage tank, as in the present invention, if no or a small amount of fuel gas is consumed in fuel gas propulsion means, to prevent a severe pressure increase due to a local increase in temperature in an LNG storage tank, compresses boil-off gas and then return the compressed boil-off gas to a lower portion of the LNG storage tank through a boil-off gas branch line, instead of sending the compressed boil-off gas to a DFDE.
Another embodiment of the present invention provides a fuel gas supply system for gasifying the LNG of an LNG storage tank and supplying the gasified LNG as a fuel gas to fuel gas propulsion means. That is, in the prior art, the fuel gas propulsion means uses as a fuel, not only LNG but also boil-off gas by using a high-pressure compressor, but the present invention does not use boil-off gas at all.
Instead, a boil-off gas reliquefaction apparatus using cold energy of LNG can be added. That is, boil-off gas is compressed and exchanges heat with the LNG of the fuel gas supply line, thereby being cooled (by the recondenser; there is no N2 refrigerator). In this case, only 40-60% of NBOG is reliquefied, but there is no problem because the LNG carrier according to the present invention is configured to eliminate or reduce the discharge of boil-off gas from the LNG storage tank. Further, if necessary, a small boil-off gas reliquefaction plant having a capacity of approximately 1 ton/hour can be installed particularly for ballast voyage.
The LNG storage tank (1) for an LNG carrier used in the fuel gas supply system according to this embodiment is designed to have strength to withstand pressure increase due to boil-off gas so as to allow pressure increase due to boil-off gas generated in the LNG storage tank during the voyage of the LNG carrier.
The fuel gas supply system in
A first pump (52) is installed in the fuel gas supply line (L11) upstream of the heat exchanger (53), so as to supply LNG, which has been compressed to meet the flow rate and pressure demands of the fuel gas propulsion means, to the fuel gas propulsion means.
A boil-off gas liquefaction line (L12) passes through the heat exchanger (53) so as to extract boil-off gas from the upper portion of the LNG storage tank (1) and return the extracted boil-off gas to one side of the LNG storage tank (1).
LNG whose temperature is increased by exchanging heat with the boil-off gas in the heat exchanger (53) is supplied to the fuel gas propulsion means, and boil-off gas which has been liquefied by exchanging heat with the LNG is returned to the LNG storage tank (1).
A second pump (54) is installed in the fuel gas supply line (L11) downstream of the heat exchanger (53) so as to supply to the fuel gas propulsion means the LNG which has exchanged heat with the boil-off gas in the heat exchanger (53) and has been compressed to meet the flow rate and pressure demands of the fuel gas propulsion means.
A heater (55) is installed in the fuel gas supply line (L11) downstream of the second pump (54) so as to heat the LNG which has exchanged heat with the boil-off gas in the heat exchanger (53) to supply the LNG to the fuel gas propulsion means.
A boil-off gas compressor (56) and a cooler (57) are sequentially installed in the boil-off gas liquefaction line (L12) upstream of the heat exchanger (53) so as to compress and cool the boil-off gas extracted from the LNG storage tank and then exchange heat between the boil-off gas and LNG.
In case the fuel gas pressure demand of the fuel gas propulsion means is high (e.g. 250 bar), LNG is compressed to 27 bar by the first pump (52), the temperature of the LNG, while passing through the heat exchanger (53), is increased from approximately −163° C. to approximately −100° C., and the LNG in a liquid state is supplied to the second pump (54) and compressed to approximately 250 bar by the second pump (54) (as it is in a supercritical state, there is no division between liquid and gas states), then gasified, while being heated in the heater (55), and then supplied to the fuel gas propulsion means. In this case, though the temperature of LNG, while passing through the heat exchanger (53), is increased, LNG is not gasified because the pressure of LNG supplied to the heat exchanger is high.
On the other hand, in case the fuel gas pressure demand of the fuel gas propulsion means is low (e.g. 6 bar), LNG is compressed to 6 bar by the first pump (52), part of the LNG is gasified while passing through the heat exchanger (53), supplied to the heater (55) and heated in the heater (55), and then supplied to the fuel gas propulsion means. In this case, the second pump (54) is not necessary.
According to this fuel gas supply system of an LNG carrier, LNG is extracted from the LNG storage tank, the extracted LNG is compressed to meet the flow rate and pressure demands of the fuel gas propulsion means, and the compressed LNG is supplied to the fuel gas propulsion means, but the supply of LNG to the fuel gas propulsion means is done after heat exchange between the LNG and boil-off gas extracted from the LNG storage tank. Accordingly, the fuel gas supply system has advantages of simplifying the configuration, reducing the required power, and preventing a severe increase in pressure of the LNG storage tank due to accumulation of boil-off gas therein, in supplying a fuel gas from an LNG carrier to the fuel gas propulsion means.
According to the preferred embodiment of the present invention, a method for treating boil-off gas in an LNG carrier having a reliquefaction plant will be described in detail below with references to
As illustrated in
The reliquefaction plant comprises: a boil-off gas compression unit (110) for compressing the boil-off gas generated in the LNG storage tank (1); a condenser (120) for condensing the compressed boil-off gas by exchanging heat with a refrigerant; and a refrigerant system (130) for providing cold heat for condensing the boil-off gas in the condenser.
The boil-off gas compression unit (110) can include at least one boil-off gas compressor (111), and it may be good to install a precooler (107) upstream of the boil-off gas compression unit (110) and precool the boil-off gas discharged from the LNG storage tank (1) so as to stably reliquefy the boil-off gas.
It is desirable to provide a gas-liquid separator (109; or a buffer tank) downstream of the condenser (120) to temporarily store the reliquefied LNG so as to stably return to the LNG storage tank (1) the boil-off gas, or LNG, which has been compressed while passing through the condenser (120).
The refrigerant system (130) is to supply cold heat for liquefying the boil-off gas through the condenser (120), and uses as a working fluid a refrigerant having predetermined temperature and flow rate. This refrigerant system (130) comprises a refrigerant compressor, a heat exchanger, and an expansion means, as a kind of refrigeration cycle.
Conventionally, in case an amount of boil-off gas greater than a treatment capacity of a reliquefaction is generated, as illustrated in
However, the present invention, maintaining a pressure in the LNG storage tank (1) somewhat higher (approximately 108-109 kPa) than the prior art (approximately 106 kPa), reliquefies an amount of boil-off gas corresponding to 100 percent of treatment capacity of the reliquefaction plant and then returns the boil-off gas to the LNG storage tank (1), and leaves surplus boil-off gas beyond the treatment capacity of the reliquefaction plant in the LNG storage tank, instead of discharging it to the outside.
To carry out the boil-off gas treating method according to the present invention, the boil-off gas treating apparatus having an LNG carrier therein according to the present invention comprises a controller (not illustrated) for controlling a BOG discharging means such as a discharge valve (not illustrated) provided in each LNG storage tank (1) so that an amount of boil-off gas exactly corresponding to 100 percent of treatment capacity of the reliquefaction plant can be discharged to the outside of the LNG storage tank (10), while maintaining an internal pressure of the LNG storage tank (1) somewhat higher (approximately 108-109 kPs) than the prior art (approximately 106 kPa) by allowing the internal pressure of the LNG storage tank (1) to be increased compared to the beginning of the loaded voyage.
As shown in
As stated above, since the present invention limits the amount of boil-off gas discharged from the LNG storage tank (1) to fit the treatment capacity of the reliquefaction plant, and leaves the surplus boil-off gas beyond the treatment capacity of the reliquefaction plant in the LNG storage tank (1), instead of discharging it to the outside, all boil-off gas discharged is reliquefied and returned to the LNG storage tank (1).
Accordingly, the present invention can prevent waste of boil-off gas, compared to the prior art which burns and wastes all surplus boil-off gas beyond the treatment capacity of a reliquefaction plant, and does not need additional equipment such as a combustor, and can save energy.
In addition, the embodiments illustrated in
Through the method and the apparatus for treating boil-off gas in an LNG carrier having a reliquefaction plant according to the present invention has been shown and described herein with references to the drawings, it would be understood that various modifications and variations may occur to those skilled in the art, and thus the description and drawings herein should be interpreted by way of illustrative purpose without limiting the scope and spirit of the present invention.
As stated above, the present invention provides a boil-off gas treating method and apparatus which can prevent waste of boil-off gas and save energy by storing in an LNG storage tank, instead of discharging and burning, the surplus boil-off gas which has not been returned to the LNG storage tank through a reliquefaction plant among the total amount of boil-off gas generated in the LNG storage tank.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10-2007-0058942 | Jun 2007 | KR | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 3733838 | Delahunty | May 1973 | A |
| 3763658 | Gaumer et al. | Oct 1973 | A |
| 4249387 | Crowley | Feb 1981 | A |
| 5586513 | Jean et al. | Dec 1996 | A |
| 6035795 | Dhellemmes et al. | Mar 2000 | A |
| 6378722 | Dhellemmes | Apr 2002 | B1 |
| 6530241 | Pozivil | Mar 2003 | B2 |
| 20020083719 | Hughes et al. | Jul 2002 | A1 |
| 20030000949 | Dhellemmes | Jan 2003 | A1 |
| 20060053806 | Tassel | Mar 2006 | A1 |
| 20060156758 | An et al. | Jul 2006 | A1 |
| 20070068176 | Pozivil | Mar 2007 | A1 |
| Number | Date | Country |
|---|---|---|
| 54159720 | Dec 1979 | JP |
| 10-2000-0011346 | Feb 2000 | KR |
| 10-2000-0011347 | Feb 2000 | KR |
| 10-2001-0044709 | Jun 2001 | KR |
| 10-2001-0092381 | Oct 2001 | KR |
| 10-2003-0052347 | Jun 2003 | KR |
| 10-2003-0053893 | Jul 2003 | KR |
| 10-2003-0073974 | Sep 2003 | KR |
| 10-2004-0046835 | Jun 2004 | KR |
| 10-2004-0046836 | Jun 2004 | KR |
| 10-0499710 | Jun 2005 | KR |
| 10-2005-0089922 | Sep 2005 | KR |
| 10-0644217 | Nov 2006 | KR |
| 20-2006-0000158 | Dec 2006 | KR |
| Number | Date | Country | |
|---|---|---|---|
| 20080308175 A1 | Dec 2008 | US |
