KR20080085624A - Bog managing method - Google Patents

Abstract

A method for managing BOG(Boil-Off Gas) is provided to obtain efficiency by conserving the BOG without loss even when the BOG generated during the carrying of LNG is greater than a consumption amount. A method for managing BOG is related to a method of managing the BOG generated in a large LNG storage tank(1) for carrying an ultra low temperature LNG. The LNG storage tank includes an LNG pump, an LNG spray, a spray nozzle for the BOG, and a compressor for the BOG. The LNG storage tank has a heat insulating wall to stand highly pressure due to the generation of the BOG. The LNG storage tank is designed to have strength for withstanding pressure increment due to the generation of BOG. Therefore, the BOG generated inside of the LNG storage tank is accumulated inside of the LNG storage tank during the navigation of an LNG carrier. The LNG storage tank further includes a pressure increase suppressing device which is designed to eject the BOG into a bottom of the LNG storage tank after compressing the BOG in a top portion of the LNG storage tank.

Description

Management method of BOOP {BOG MANAGING METHOD}

1 is a view showing the concept of the absorption of heat intake in the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

2 is a view schematically showing an LNG storage tank for an LNG carrier according to a preferred embodiment of the present invention.

3 is a view schematically showing a configuration for treating the boil-off gas at the unloading terminal using the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

Figure 4 is a schematic diagram showing the waste of the boil-off gas of the LNG carrier ship based on the concept that the pressure of the conventional LNG storage tank is maintained at about the same state.

5 is a schematic diagram showing a pressure operating mode of the LNG storage tank during the loading operation of the LNG carrier according to the LNG storage tank pressure of the LNG unloading terminal.

6 is a schematic diagram showing a method of injecting the BOG in the upper portion of the LNG storage tank to the LNG in the lower portion.

7 is a schematic diagram showing the configuration of a system that receives the relevant data in real time during operation and displays in real time the maximum possible set pressure of the safety valve of the LNG storage tank through appropriate data processing and calculation.

8 shows an apparatus for measuring the flow rate of fuel gas in an LNG carrier according to the present invention.

9 shows a flow rate measuring device for fuel gas of a conventional LNG carrier.

FIG. 10 shows that the BOG is compressed and then supplied to the lower portion of the LNG storage tank according to one embodiment of the present invention.

11 is a schematic diagram of a fuel gas supply system of an LNG carrier according to an embodiment of the present invention.

<Explanation of symbols for main parts of the drawings>

1 LNG storage tank for LNG carrier 2 LNG storage tank for cargo terminal

3: compressor 4: recondenser

5: carburetor P: high pressure pump

11 LNG pump 13 LNG spray

21: injection nozzle for boil-off gas 23: compressor for boil-off gas

The present invention relates to an LNG carrier having a BOG treatment means for treating boil-off gas (BOG) generated in an LNG storage tank.

In general, natural gas (NG) is produced in the form of liquefied liquefied natural gas (Liquefied Natural Gas, hereinafter referred to as LNG) at the production site, and then transported over a long distance to the destination by LNG carriers. Thereafter, the gas is regasified and supplied to a consumer through an LNG Floating Storage and Regasification Unit (FSRU) or an offshore loading terminal.

When LNG is transported by an LNG Regasification Vessel (RV), LNG is regasified from the LNG Regasification Vessel itself and supplied directly to the consumer without having to go through an LNG floating storage or regasification unit or a land unloading terminal. .

The liquefaction temperature of natural gas is about -163 ° C at ambient pressure, so LNG is evaporated even if its temperature is slightly higher than -163 ° C at normal pressure. In the case of a conventional LNG carrier, for example, the LNG storage tank of the LNG carrier is insulated, but since the external heat is continuously transmitted to the LNG, LNG is transported while the LNG carrier is transporting the LNG. Boil-off gas is generated in the LNG storage tank by continuously evaporating in the LNG storage tank.

When the boil-off gas is generated in the LNG storage tank in this way, the pressure of the LNG storage tank rises and becomes dangerous.

Conventionally, in order to maintain the pressure of the LNG storage tank in a safe state, the boil-off gas generated in the LNG storage tank was used as a fuel for propulsion of the LNG carrier. That is, in the case of a LNG carrier that carries LNG in a low temperature liquid state of the related art, the basic concept is that the LNG temperature in the tank is maintained at about the same temperature and the same pressure at around -163 ° C. As a result, the generated BOG was discharged to the outside and treated.

A steam turbine propulsion method driven by steam generated by burning an evaporated gas generated in an LNG storage tank in a boiler has a problem of low propulsion efficiency.

In addition, the dual fuel diesel electric propulsion system, which compresses the boil-off gas generated in the LNG storage tank and uses it as a fuel for a diesel engine, is more efficient than the steam turbine propulsion method, but the medium speed engine and the electric The propulsion device is complex and there are many difficulties in the maintenance of the equipment. In addition, since this method requires the supply of boil-off gas as fuel, a gas compression method having a large installation cost and an operating cost is inevitably applied.

In this way, the method of using the boil-off gas as the propulsion fuel does not in any case fall short of the efficiency of the two-stroke low speed diesel engine used for the general ship.

On the other hand, there is also a way to re-liquefy the boil-off gas generated in the LNG storage tank to return to the LNG storage tank. However, this method of reliquefaction of the boil-off gas has a problem that the LNG carrier to install a boil-off gas reliquefaction device of a complex system.

In addition, when the amount of evaporation gas generated in the propulsion system that can be used as fuel or can be processed by the boil-off gas reliquefaction apparatus is generated, the excess boil-off gas must be incinerated and treated in a gas combustor, so that the treatment of the excess boil-off gas There is a problem in that additional equipment such as a gas burner is added.

For example, as shown in Figure 4, when looking at the LNG carriers based on the basic concept of maintaining the pressure of the conventional LNG storage tank in almost the same state, after the LNG is shipped (3-5 days after shipment) LNG As the storage tank is a little hot, as indicated by the solid upper line, a considerable amount of excess BOG occurs compared to the BOG generation (NBOG, natural BOG) in operation. The fuel consumption of fuel diesel electric propulsion system is more than. Therefore, the BOG corresponding to the hatched portion representing the difference with the lower dotted line showing the amount of BOG used in the boiler or engine is inevitably burned out through the gas combustion unit (GCU). In addition, even when LNG carriers pass through the canal (e.g., 5-6 days in FIG. 4), there is no BOG consumption in the boiler or engine (when waiting for the canal), or less (when passing the canal), so that the BOG more than the engine needs is burned. I have to throw it away. In addition, even when the LNG carrier is waiting to enter or enter the port while the consumption of the BOG occurs or a small amount, there is no choice but to burn the excess BOG.

The amount of BOG ll ride like this when converted to the amount reaches to 1500-2000 tons per year of LNG carriers at 150,000m ³ capacity is equivalent to 600 million won. Furthermore, the burning of BOG also causes environmental pollution.

On the other hand, unlike the low-pressure tank as described above, without forming a thermal insulation wall in the LNG storage tank to maintain a high pressure of about 200 bar (gauge pressure) in the LNG storage tank to suppress the generation of boil-off gas in the LNG storage tank The technique is disclosed in Korean Patent Publication No. KR2001-0014021, KR2001-0014033, KR2001-0083920, KR2001-0082235, KR2004-0015294 and the like. However, in order for the LNG storage tank to accommodate the high pressure of about 200 bar inside the LNG storage tank, the thickness of the LNG storage tank must be considerably thick. There is a problem in that separate equipment such as a high pressure compressor for maintaining is added. Unlike this technology, there is a technology known as a pressure tank, which stores a highly volatile liquid in a tank of high temperature at room temperature, so that there is no problem in treating the BOG, but there is a limitation that the size of the tank cannot be increased, and the manufacturing cost thereof There is an increasing problem.

As described above, the LNG tank of the LNG carrier conventionally is a way to maintain the pressure of the cryogenic liquid at a pressure near the normal pressure during transport and to allow the generation of BOG, the consumption of BOG is large or separate reliquefaction There was a problem that the device must be mounted. In addition, unlike the tank for transporting the cryogenic liquid at atmospheric pressure and low pressure, the method of transporting the tank to a tank capable of withstanding high pressure at a somewhat high temperature such as a pressure tank does not have a treatment of BOG, but the size of the tank is limited and manufactured. There is a problem that the cost is high.

Accordingly, the present invention is to solve such a problem of the prior art, and relates to a somewhat high pressure tank near normal pressure for carrying a liquefied gas in a cryogenic state, and it is possible to manufacture a large capacity tank without increasing the manufacturing cost of the tank. It is an object of the present invention to provide an LNG storage tank and a method of transporting LNG using the same or an evaporation gas treatment method using the same, which can reduce BOG waste.

In order to achieve the above object, the present invention relates to a somewhat high pressure tank near the normal pressure carrying a liquefied gas in a cryogenic state, characterized in that it allows a certain degree of pressure change in the tank during transportation.

According to one embodiment of the present invention, in an LNG carrier having a BOG treatment means for treating boil-off gas (BOG) generated in the LNG storage tank, the vapor pressure in the tank and the LNG during the transportation of LNG in the LNG storage tank An LNG carrier and its method are provided for allowing an increase in the temperature of the gas.

As a means for treating BOG, BOG, which is generally generated from LNG storage tanks, is used in boilers (for example, steam turbine propulsion boilers), as a fuel for gas engines such as DFDE or MEGI, in gas turbines, or reliquefaction Is known to return to the LNG storage tank (for example, Korean Patent Publication No. 2004-0046836, Korean Patent Registration 0489804, 0441857, Korean Utility Publication 2006-0000158, etc.). If excess BOG is generated (e.g. after loading LNG), or if it cannot be processed by processing means such as port entry / departure, canal passage, etc., the BOG may be removed by BOG combustion means such as GCU (Gas Combustion Unit). Waste was inevitable.

In the present invention, the flexibility of the BOG process is increased, such BOG waste is removed. The LNG carrier according to the present invention may not require a GCU, and in some cases, a GCU may be required for improving BOG treatment or BOG management flexibility in an emergency.

In the present invention, the LNG carrier is provided with means for discharging and treating BOG from the LNG tank (boiler, reliquefaction apparatus, gas engine, etc.).

According to another embodiment of the present invention, in the control method of the safety valve installed on the upper portion of the LNG storage tank installed in the LNG carrier for transporting the LNG, when the LNG carrier and the LNG carrier Provided is a method for opening and closing a safety valve, wherein the opening and closing pressure values of the safety valve are different at the time of operation. In the present invention, a safety valve, an LNG storage tank, and an LNG carrier are provided.

Conventionally, a safety valve is installed on an upper portion of a storage tank of an LNG carrier carrying a cryogenic liquefied natural gas state to safely manage the pressure inside the LNG storage tank. The safety valve ensures stability against explosion of the tank, and the BOG generated after loading LNG is used in boilers (for example, steam turbine propulsion boilers) as above, or as fuel for gas engines such as DFDE and MEGI. Known, used in gas turbines, or liquefied and returned to LNG storage tanks. However, in these methods, when the processing by the processing means is not possible, such as the generation of excess BOG (for example, after loading LNG) or the port entry / departure, the passage of the canal, etc., exceeding the throughput by the usual processing means, the GCU ( Waste of BOG by BOG combustion means such as Gas Combustion Unit is inevitable. In this way, the pressure of the LNG storage tank of the LNG carrier was kept constant within a predetermined range.

Such LNG carriers ship LNG to a volume of about 98% of the LNG storage tank when the safety valve is set at 0.25 bar, leaving the remaining 2% as free space. If more than 98% of the LNG is filled, LNG will overflow from the dome on top of the LNG when the LNG storage tank reaches 0.25 bar. However, when the LNG pressure is continuously allowed to increase after the LNG is shipped as in another embodiment of the present invention, the LNG expands due to the temperature rise of the LNG at the safety valve set pressure according to the present invention even when a small amount of LNG is loaded. As a result, LNG may overflow. For example, when the vapor pressure of the LNG tank is 0.7 bar, it was found that the overflow may occur even when the LNG load is about 97%. This is directly connected to the problem of decreasing LNG load.

Due to this problem, rather than fixedly opening and closing the pressure of the safety valve installed in the upper portion of the LNG storage tank at a relatively high pressure near the normal pressure when loading, it is fixed at a low pressure, such as 0.25 bar, as in conventional LNG carriers, If the amount of LNG in the LNG storage tank is reduced by slightly using BOG (for example, as a fuel in a boiler, engine, etc.), the initial load is decreased by increasing the opening / closing pressure value of the safety valve as in another embodiment of the present invention. This can reduce BOG waste or increase the flexibility of BOG processing. The present invention has a great effect in that there is no waste of BOG when applied to an LNG carrier equipped with means for discharging and processing BOG from an LNG tank (boiler, reliquefaction apparatus, gas engine, etc.).

Therefore, in the present invention, the opening and closing pressure value of the safety valve is raised after the evaporated gas generated in the LNG storage tank is discharged to the outside to reduce the amount of LNG loaded in the LNG storage tank, and preferably to load the LNG. The open / close pressure value at the time of setting is below 0.25 bar, the pressure value at the time of the LNG carrier operation is set at more than 0.25-2 bar, Especially preferably, the pressure value at the time of the LNG carrier is more than 0.25-0.7 bar Is set in. Here, the opening and closing pressure value of the safety valve when the LNG carrier is operating can be increased step by step, for example, 0.4 bar, 0.7 bar according to the amount of use of the boil-off gas according to the operating conditions.

Therefore, when the LNG carrier operates in the present invention means the case where the volume of LNG in the LNG storage tank is somewhat reduced after using the BOG to start the operation after loading the LNG in the LNG carrier. For example, when the LNG volume is 98.5%, the open / close pressure value of the safety valve is set to 0.25 bar, when the LNG volume is 98.0%, the open / close pressure value of the safety valve is set to 0.4 bar, and when the volume of LNG is 97.7% It is preferable to set the open / close pressure value of the safety valve to 0.5 bar, and to set the open / close pressure value of the safety valve to 0.7 bar when the volume of LNG is 97.1%.

According to another embodiment of the present invention, in a storage tank of an LNG carrier carrying in a cryogenic liquefied natural gas state, the opening and closing pressure value of the safety valve installed above the storage tank is more than 0.25 to 2 bar, preferably 0.25 It is characterized by setting to more than 0.7 bar, more preferably about 0.7 bar. In the present invention, there is provided a method for opening and closing a safety valve, an LNG storage tank, and an LNG carrier, characterized in that the above configuration.

Such a method has a problem that the loss of BOG and the manufacturing cost of the LNG carrier is increased, in the present invention, the pressure of the LNG storage tank safety valve is increased, so that the pressure inside the tank and the temperature of the LNG are operated after the LNG is shipped from loading to unloading. The above problem was solved by allowing the rise.

 According to another embodiment of the present invention, as a storage tank for LNG carriers carried in the state of cryogenic liquefied natural gas, the steam pressure in the tank is regulated within a pressure range near normal pressure, and the steam pressure and the steam pressure in the tank during transportation of LNG are controlled. An LNG storage tank for an LNG carrier is provided, which allows an increase in the temperature of the LNG. The vapor pressure in the tank is characterized in that more than 0.25 to 2 bar, preferably more than 0.25 to 0.7 bar, more preferably about 0.7 bar. In addition, in order to make the temperature distribution inside the LNG tank uniform, the LNG of the lower portion of the LNG storage tank and the boil-off gas of the upper portion of the LNG storage tank are mixed. Since evaporation of LNG tends to occur more locally at high temperatures in the LNG storage tank, it is desirable to maintain a uniform temperature of LNG or BOG in the LNG storage tank. In addition, from the viewpoint, the boil-off gas in the upper portion of the LNG storage tank has a smaller heat capacity than the LNG in the bottom of the tank, which may cause a sudden increase in pressure due to a rise in temperature due to external inflow heat. As a result, a sudden increase in pressure of the LNG tank can be suppressed.

In addition, according to another embodiment of the present invention, it is possible to adjust the steam pressure in the LNG tank of the LNG carrier according to the pressure of the tank unloaded from the LNG terminal. For example, when the tank pressure at the LNG terminal, LNG-RV, FSRU, etc. to be unloaded is high (for example, around 0.4-0.7 bar), the tank pressure of the LNG carrier is continuously increased and the tank pressure is lowered as before. In the case (about 0.2 bar), the flexibility of the BOG treatment according to the present invention can be used to match the pressure of the tank being unloaded while reducing the BOG waste.

In addition, according to another embodiment of the present invention, there is provided a method for transporting liquefied natural gas in a cryogenic state having the above characteristics and an LNG carrier provided with the tank.

In particular, according to another embodiment of the present invention, the present invention relates to a somewhat high-pressure membrane-type LNG tank near normal pressure that carries a liquefied gas in a cryogenic state, characterized in that it allows a certain degree of pressure change in the tank during transportation. . The membrane tank described in the present invention means a membrane tank defined in the IGC Code (2000) regarding the cargo hold of the LNG tank. Specifically, Membrane tanks are non-self-supporting tanks, which means that a heat insulating wall is formed on the hull and a thin membrane is formed thereon. This includes semi-membrane tanks.

In the following, the tanks described in GTT NO 96, Mark III, Korean Patent Nos. 499710 and 644217 are examples of membrane type tanks.

Such a membrane tank can be designed to withstand up to 0.7 bar (gauge pressure) by reinforcement of the tank, but is generally designed to not exceed 0.25 bar. All conventional membrane-type tanks are in compliance with this regulation and managed so that the vapor pressure inside the tank is 0.25 bar or less, and the temperature and pressure of LNG are almost constant during operation. In contrast, the present invention is characterized in that the management to allow a rise in the tank internal pressure and the temperature of the LNG at a pressure exceeding 0.25 bar, preferably more than 0.25 bar 2 bar or less, more preferably more than 0.25 bar 0.7 bar or less. In addition, the boil-off gas treatment method using the LNG storage tank of the present invention is characterized in that to maintain a uniform temperature distribution inside the LNG storage tank.

According to another embodiment of the present invention, the present invention relates to a large LNG carrier. Preferably it relates to an LNG carrier having an LNG storage capacity of 100,000 m ³ or more. In order to manufacture LNG tanks into high pressure tanks, large-capacity LNG carriers increase their manufacturing costs drastically by increasing the thickness of their tanks. Also, LNG can be transported without BOG treatment while substantially supporting the pressure caused by the generation of boil-off gas without significantly increasing.

Hereinafter, with reference to the accompanying drawings will be described in detail a preferred embodiment of the present invention.

The LNG storage tank of the present invention can be applied to LNG storage tanks such as LNG carriers, LNG floating storage and regasification apparatus (FSRU), land unloading terminal, LNG regasification vessel (RV). As the LNG transport tanks allow for an increase in pressure and temperature while solving the BOG problem, the waste of BOG can be reduced, and LNG can be stored for a long time in the various LNG tanks in consideration of LNG demand at the destination. It also has the advantage of greater flexibility in transportation and storage.

In the present embodiment, the LNG storage tank to be applied to the LNG carrier will be described by way of example.

1 is a view showing the concept of absorbing the heat inflow in the LNG storage tank for LNG carriers according to the present invention, conventionally inflow heat from outside by maintaining the pressure in the LNG storage tank for LNG carriers within a certain range While most contribute to the generation of boil-off gas and also process all the boil-off gas generated in the LNG carrier, in the present invention, by allowing the pressure in the LNG storage tank for LNG carriers to rise, the LNG in the tank due to the saturation temperature rise due to the pressure rise Since most of the incoming heat is absorbed by the sensible heat increase of natural gas (hereinafter referred to as NG), the generation of boil-off gas is greatly reduced. For example, when the pressure of the LNG storage tank for LNG carriers is 0.7 bar, the saturation temperature rises about 6 ° C compared to the initial 0.06 bar.

2 schematically shows an LNG storage tank for an LNG carrier according to a preferred embodiment of the present invention. In the case of LNG storage tank (1) for LNG carriers with heat insulation walls, when the LNG is normally loaded, the internal pressure is about 0.06 bar (gauge pressure) at the start, and the evaporation gas is generated during the operation of the LNG carrier. Pressure increases gradually. For example, after loading LNG at the LNG production site, the pressure inside the LNG storage tank 1 for LNG carriers becomes 0.06 bar, and when the LNG carriers start and operate for about 15 to 20 days and arrive at their destinations, the LNG carriers The pressure inside the LNG storage tank 1 can rise to 0.7 bar.

When this is described in relation to temperature, LNG generally contains various impurities and is lower than the boiling point of pure methane liquid. Pure methane has a boiling point of -161. ℃ at 0.06 bar, but LNG transported in LNG transport actually has a boiling point of around -163 ℃ because it contains some impurities such as nitrogen and ethane. In terms of pure methane, the LNG temperature in the tank is around -161 ℃ at 0.06 bar after LNG shipment, and if the steam pressure in the tank is controlled at 0.25 bar considering the transport distance and BOG consumption, the LNG temperature is around -159 ℃. If the steam pressure in the tank is controlled to 0.7 bar, the LNG temperature rises to around -155 ° C. If the steam pressure in the tank is controlled to 2 bar, the LNG temperature rises to around -146 ° C.

LNG storage tank for LNG carrier of the present invention is designed in consideration of the pressure rise caused by the generation of the boil-off gas while having a heat insulating wall, that is, it is designed to withstand the pressure rise caused by the generation of boil-off gas. . Therefore, the boil-off gas generated inside the LNG storage tank 1 for LNG carriers is accumulated in the LNG storage tank 1 for LNG carriers during the operation period of the LNG carrier.

For example, the LNG storage tank 1 for LNG carriers according to the embodiment of the present invention is preferably designed to withstand a pressure of more than 0.25 to 2 bar (gauge pressure) with a heat insulating wall, more preferably. It is designed to withstand a pressure of 0.6 to 1.5 bar (gauge pressure). Given the distance of LNG transport and the current IGC Code, it is desirable to be designed to withstand pressures greater than 0.25 bar to 0.7 bar, in particular around 0.7 bar. However, if the pressure is too low, the distance for transporting the LNG is too short, it is not preferable, if too high there is a problem that the production of the tank is not easy.

In addition, the LNG storage tank (1) for LNG carriers according to the present invention is designed to increase the thickness of the initial design, or to add a reinforcement only without adding a reinforcement to the existing LNG storage tanks for general LNG carriers appropriate reinforcement It is economical in terms of production cost because it can be realized sufficiently.

On the other hand, there are various known LNG storage tanks for LNG carriers according to the prior art having a heat insulation (heat dissipation) wall as described below. Therefore, in FIG. 1, illustration of the heat insulation wall is omitted.

First, the LNG storage tank installed inside the LNG carrier can be divided into an independent tank type and a membrane type. The details are as follows.

In Table 1, aka GTT NO 96-2 type and GTT Mark III type were renamed Gaz Transport (GT) and Technigaz (TGZ) in 1995 as GTT (Gaztransport & Technigaz), respectively, GT type was GTT NO 96 -2 type, TGZ type is renamed GTT Mark III type.

GT and TGZ tank structures described above are described in US Pat.

Korean Patent Nos. 499710 and 0644217 disclose a heat insulating wall as another concept.

LNG storage tanks for LNG carriers having various types of insulating walls have been previously disclosed, which are intended to suppress possible vaporization of LNG.

As described above, it is possible to apply the present invention to LNG storage tanks for LNG carriers having various types of thermal insulation functions. Most LNG storage tanks for LNG carriers are designed to withstand a pressure of 0.25 bar or less, and when the pressure reaches or exceeds the pressure of 0.2 bar or less, for example, 0.1 bar, the evaporation gas is consumed as part of the vaporized gas. Or burn everything to the GCU. In addition, a safety valve (safty valve) is installed in the LNG storage tank is discharged to the outside air through a safety valve (usually opening and closing pressure is 0.25 bar) when the above control fails.

On the contrary, in the present invention, a safety valve is installed to control the discharge of the pressure when the pressure rises due to the evaporation gas generated from the LNG storage tank in the upper portion and the normal dome portion of the LNG storage tank of FIG. 2 during the operation of the LNG carrier (not shown). In the present invention, the pressure value of the safety valve is set to more than 0.25 to 2 bar, preferably more than 0.25 to 0.7 bar, more preferably about 0.7 bar.

In addition, the LNG storage tank according to the present invention is configured to reduce the pressure of the LNG storage tank by reducing the local rise in temperature and pressure, thereby reducing the relatively low temperature LNG at the bottom of the LNG storage tank for LNG carriers. LNG carrier for LNG carriers by injecting into the upper part of the LNG storage tank for relatively high temperature LNG carriers and injecting relatively high temperature boil-off gas of the upper part of the LNG carrier tank for LNG carriers to the lower part of the LNG storage tank for LNG carriers that are relatively low temperature. Maintain a uniform temperature distribution in the storage tank.

In FIG. 2, a pump 11 for LNG and an injection nozzle 21 for boil-off gas are provided at a lower portion of the LNG storage tank 1 for an LNG carrier, and a LNG storage tank 1 for an LNG carrier is provided at an upper portion of the LNG storage tank 1 for an LNG carrier. A spray 13 and a compressor 23 for the boil-off gas are provided. Here, the LNG pump 11 and the boil-off compressor 23 can be freely installed at the upper and lower parts. The relatively low temperature LNG in the lower portion of the LNG storage tank 1 for LNG carriers is supplied to the upper LNG spray 13 by the LNG pump 11 so that the LNG storage tank 1 for the LNG carrier is relatively hot. And the relatively high temperature boil-off gas of the upper portion of the LNG storage tank 1 for LNG carriers is supplied to the lower boil-off gas injection nozzle 21 by the boil-off compressor 23 for relatively low temperature. The amount of generation of the boil-off gas can be reduced by spraying the lower portion of the LNG storage tank 1 for LNG carriers and maintaining the temperature distribution of the LNG storage tank 1 for LNG carriers uniformly.

Reducing the amount of boil-off gas is especially useful for LNG carriers without BOG treatment, in order to increase the pressure slowly, since BOG generation is directly related to the pressure increase in the tank.In the case of LNG carriers with BOG treatment means, When the pressure rises, a certain amount of BOG may be discharged to adjust the pressure of the boil-off gas in the tank, so the LNG injection or BOG injection may not be necessary during the operation of the LNG carrier.

In addition, if the LNG is shipped to the LNG carrier in a supercooled state at the production terminal that produces LNG, it is possible to further reduce the boil-off gas (pressure rise) generated during transportation. After loading under supercooling at the production terminal, the LNG storage tank for LNG carriers may be underpressure (0 bar or less), which can be filled with nitrogen to prevent this.

Referring to the method of processing the boil-off gas using the LNG storage tank for LNG carriers as described above are as follows.

When the LNG carrier operates, the LNG storage tank 1 for LNG carrier according to the present invention does not process the boil-off gas, thereby allowing a pressure increase in the tank thereby allowing most of the heat inflow by the internal temperature rise of the tank. Accumulated by the elevated thermal energy of LNG and NG, and when the LNG carrier arrives at the destination, the boil-off terminal processes the boil-off gas accumulated in the LNG storage tank 1 for LNG carrier.

Figure 3 schematically shows a configuration for processing the boil-off gas at the unloading terminal using the LNG storage tank for LNG carriers according to a preferred embodiment of the present invention.

A plurality of LNG storage tanks 2, a high pressure compressor 3a, a low pressure compressor 3b, a recondenser 4, a high pressure pump P and a vaporizer 5 are provided in the cargo terminal.

Since the boil-off gas accumulated in the LNG storage tank 1 for LNG carriers is large, it is usually compressed to 70-80 bar by the high-pressure compressor 3a at the unloading terminal and then supplied directly to the consumer. On the other hand, a part of the boil-off gas accumulated in the LNG storage tank (1) for LNG carriers is usually compressed to around 8 bar by the low pressure compressor (3b) and then recondensed through the recondenser (4) and in the vaporizer (5) It may be vaporized again and supplied to the consumer.

When unloading LNG from LNG storage tank for LNG carrier to LNG storage tank for cargo terminal at the loading terminal, the pressure of LNG storage tank for LNG carrier is greater than the pressure of LNG storage tank for cargo terminal. When high pressure LNG is introduced into the gas, additional boil-off gas is generated.To minimize this, the LNG is directly connected to the inlet of the high-pressure pump at the unloading terminal from the LNG storage tank for the LNG carrier. There is this. LNG storage tank for LNG carrier according to the present invention has the advantage that the unloading time is shortened by 10-20% compared to the conventional LNG carrier because the pressure in the LNG tank is high when unloading.

The LNG stored in the LNG storage tank (1) for LNG carriers is not supplied to the LNG storage tank (2) for the loading terminal of the cargo terminal, but is supplied to the recondenser (4) to recondense the boil-off gas and then vaporize in the vaporizer (5). Can be supplied directly to the consumer.

On the other hand, when the recondenser is not provided in the unloading terminal, LNG can also be supplied directly to the inlet port of the high pressure pump P.

As described above, when a plurality of storage tanks for the cargo terminal 2 are installed in the cargo terminal, the LNG is equalized to the plurality of cargo storage tanks 2 for the cargo terminal from the LNG storage tank 1 for the LNG carrier. When distributed and unloaded, the generation of the boil-off gas is distributed to the plurality of LNG storage tanks 2 of the loading terminal to minimize the influence of the generation of the boil-off gas in the respective LNG storage tanks 2. Since the amount of boil-off gas generated in the storage tank 2 for the unloading terminal is small, it is usually compressed to around 8 bar by the low pressure compressor 3b and then recondensed through the recondenser 4 and vaporized again in the vaporizer 5. And is supplied to the consumer.

In addition, according to the present invention, since the LNG storage tank for LNG carriers is operated above the existing design pressure, to fill the boil-off gas or NG in the LNG storage tanks for LNG carriers to maintain the pressure in the LNG storage tanks for LNG carriers when the LNG unloading The process becomes unnecessary.

In addition, the LNG storage tank for LNG terminal or LNG storage tank for LNG floating storage and regasification unit (FSRU) can be adapted or new LNG terminal LNG so that the storage pressure corresponds to the storage tank pressure for LNG carrier of the present invention. When the LNG storage tank for the storage tank or LNG floating storage and regasification unit (FSRU) is constructed, there is no problem even if the existing unloading method is applied as it does not generate additional boil-off gas during LNG unloading.

According to the present invention, in the case of the LNG floating storage and regasification apparatus (FSRU), since the management flexibility of the boil-off gas is increased, the installation of the recondensation apparatus may be unnecessary.

According to the present invention, the LNG regasification vessel (RV) may have all of the advantages of the aforementioned LNG carrier and the LNG floating storage and regasification apparatus (FSRU).

Figure 5 shows the operating mode of the pressure of the LNG storage tank during the loading operation of the LNG carrier in accordance with the pressure of the LNG storage tank of the LNG unloading terminal, F mode is the allowable pressure of the LNG storage tank of the unloading terminal is 0.7 bar to 1.5 bar or less, for example In this case, the pressure in the LNG storage tank of the LNG carrier is operated while continuously increasing to 0.7 bar to 1.5 bar or less equal to the allowable pressure of the LNG storage tank of the LNG unloading terminal. This case is particularly useful in LNG carriers that do not have a BOG treatment.

S mode or V mode is suitable when the allowable pressure of the LNG storage tank of the LNG unloading terminal is 0.4 bar or less, for example. Both modes are applicable to LNG carriers with BOG treatment. The S mode is to operate while constantly increasing the pressure in the LNG storage tank of the LNG carrier in small increments. In other words, the S mode is to operate while continuously increasing the pressure in the LNG storage tank of the LNG carrier to 0.4 bar or less equal to the allowable pressure of the LNG storage tank of the LNG unloading terminal.

The V mode expands the operating range of pressure in the LNG storage tank of the LNG carrier. The BOG which exceeds the BOG consumption by the BOG treatment means can be stored in the LNG storage tank of the LNG carrier to reduce BOG waste. have. For example, when LNG carriers pass through the canal, the propulsion means for fueling LNG gas such as DFDE, MEGI, gas turbine, etc. does not work, so there is no consumption of BOG, so BOG generated in LNG storage tank of LNG carrier Can be raised inside the LNG carrier to raise the pressure of the LNG storage tank to 0.7 bar to 1.5 bar or less, and when the LNG carrier passes through the canal, the propulsion means using LNG gas as fuel is operated to the maximum. Increasing the consumption of BOG can also reduce the pressure of LNG storage tanks on LNG carriers below 0.4 bar.

On the other hand, depending on whether or not a flash gas processing facility that can process a large amount of flash gas is installed in the LNG unloading terminal, the pressure operation form of the LNG storage tank of the LNG carrier can be different. If a flash gas processing facility is installed that can handle a large amount of flash gas at the LNG unloading terminal, the LNG storage tank of the LNG carrier can be operated in F mode, and a large amount of flash gas can be processed at the LNG unloading terminal. If no flash gas treatment plant is installed, the LNG storage tank of the LNG carrier is operated in S mode or V mode.

6 is a schematic diagram showing an apparatus for reducing the pressure rise of the LNG storage tank of the LNG carrier by injecting the BOG of the upper portion of the LNG tank with the lower LNG.

The pressure rise reduction device of the LNG storage tank of the LNG carrier illustrated in FIG. 6 is configured to compress the boil-off gas of the upper portion of the LNG storage tank 1 of the LNG carrier and then spray it into the LNG of the lower portion of the LNG storage tank 1. It is.

This apparatus is connected to the boil-off gas inlet 31 provided at the upper portion of the LNG storage tank 1 of the LNG carrier, and one end thereof is connected to the boil-off gas inlet 31 and the other end is connected to the lower portion of the LNG storage tank 1 ( 33 and a compressor 35 provided in the middle of the pipe 33.

As illustrated on the left side of FIG. 6, the pipe 33 may be installed inside the LNG storage tank 1. When the pipe 33 is installed inside the LNG storage tank 1, the compressor 35 is preferably a submersible compressor installed under the pipe 33. The submersible compressor is sealed.

As illustrated on the right side of FIG. 6, the pipe 33 may be installed outside the LNG storage tank 1. When the pipe 33 is installed outside the LNG storage tank 1, the compressor 35 is a general compressor installed in the pipe 33. The general compressor refers to the one that is not sealed.

On the other hand, it is preferable that the liquid suction prevention means is provided in the boil-off gas inlet 31. The liquid suction prevention means includes a demister.

The pressure rise reduction device of the LNG storage tank of the LNG carrier is configured to reduce the pressure of the LNG storage tank by reducing the local rise in temperature and pressure, and relatively high temperature of the upper portion of the LNG storage tank (1) for the LNG carrier. The boil-off gas is injected into the lower portion of the LNG storage tank 1 for LNG carriers at a relatively low temperature to maintain a uniform temperature distribution of the LNG storage tank for LNG carriers, that is, the local temperature in the LNG storage tank. By preventing the increase, the amount of generated evaporated gas can be reduced.

7 is a diagram showing the configuration of a system that receives the relevant data in real time during operation and displays the current allowable maximum set pressure of the safety valve of the LNG storage tank in real time through appropriate data processing and calculation, through which the LNG storage tank safely The safety valve can be adjusted.

In the case of LNG carriers equipped with safety valves (SRV, Safety Relief Valve or Safety Valve) of the LNG storage tank (1), the set pressure of the safety valve is initially set to maximize the cargo load, but evaporation occurs during operation. The set pressure of the safety valve can be increased to meet the reduced cargo load due to the consumption of boil-off gas (BOG).

Increasing the set pressure of the safety valve during operation reduces the amount of boil-off gas generated in the LNG storage tank (1) can minimize the amount of air released or consumed by the combustion device.

Since the measured values such as the LNG level in the LNG storage tank 1 change frequently during operation, a system for removing dynamic behavior or external noise of the vessel through appropriate data processing and the actual data in the LNG storage tank 1 using the processed data. It consists of a system for calculating the allowable set pressure of safety valves in LNG storage tanks by calculating the volume of LNG and finally displaying the result.

The right side of FIG. 7 illustrates the relevant data measured for calculating the LNG volume in the LNG storage tank 1. The LNG level in the LNG storage tank is measured by the existing level gauge (not shown), the temperature of the LNG storage tank is measured by the existing temperature sensor (not shown), and the pressure of the LNG storage tank is the conventional pressure. It is selected by a sensor (not shown), the trim of the LNG carrier is measured by a conventional trim sensor (not shown), the list of LNG carriers by a conventional list sensor (not shown) It is measured. Here, the trim of the LNG carrier indicates the front and rear inclination of the LNG carrier, and the list of the LNG carrier indicates the left and right inclination of the LNG carrier.

The safety valve set pressure checking system of the LNG storage tank according to the present embodiment includes a data processing module 61 for processing measurement data illustrated on the right side of FIG. 7, as illustrated on the left side of FIG. 7.

In the data processing module 61, it is preferable to process the data using the least square method, the moving average method, or the low band filtering method.

The safety valve set pressure checking system of the LNG storage tank further includes an LNG volume calculation module 63 that calculates the volume of the LNG in the LNG storage tank 1 by calculating the data processed by the data processing module 61. .

In the safety valve set pressure checking system of the LNG storage tank, the allowable set pressure of the safety valve of the LNG storage tank 1 is calculated from the LNG volume calculated in the LNG volume calculation module 63.

Meanwhile, by measuring the flow rate of the fuel gas supplied from the LNG storage tank 1 to the fuel gas propulsion means of the LNG carrier, the LNG volume in the current LNG storage tank is calculated by comparing the initial LNG load with the amount of used boil-off gas. In addition, the LNG volume calculated from the flow rate of the fuel gas thus measured may be reflected in the LNG volume calculated by the LNG volume calculation module 63.

The calculated LNG volume in the LNG storage tank and the allowable set pressure of the safety valve of the LNG storage tank calculated in this way are displayed on the display panel 65.

8 shows an apparatus for measuring the flow rate of fuel gas in an LNG carrier according to the present invention.

Differential pressure flow measurement device is used to measure the flow rate of fuel gas in LNG carriers. Due to the characteristics of the device, the measurement range is limited and a large measurement error occurs for the flow rate beyond the measurement range. If you want to change the measuring range, you have to replace the orifice itself, which is not only cumbersome but also dangerous.

In the past, only one orifice was installed to limit the measuring range. However, the two measuring ranges can be arranged in series with different orifices to select and use an appropriate orifice measured value according to the flow rate.

That is, two or more orifices having different measurement ranges are arranged in series for measuring the flow rate of fuel gas in a wide range, and the effective measurement range can be easily expanded by selecting and using the appropriate orifice measured values according to the flow rate. In FIG. 8, orifices 71 and 71 ′ having different measuring ranges are provided in series in the middle of the fuel supply line piping 70 for supplying fuel gas from the LNG storage tank of the LNG carrier to the fuel gas propulsion means. The differential pressure measuring unit 73 is connected to the fuel supply line pipe 70 before and after each of the orifices 71 and 71 '. These differential pressure measuring units 73 are selectively connected to the flow rate measuring unit 77 through the selector 75 selectable according to the measurement range.

Thus, the selector 75 which can be selected according to the measurement range is installed between the differential pressure measuring units 73 and the flow rate measuring unit 77 to select and use the measured value of the appropriate orifice according to the flow rate. You can zoom in.

In conventional systems, the capacity of the orifice of fuel gas is adjusted to nBOG, so the measurement accuracy is low for LNG carriers with high BOG consumption. In order to compensate for this, the present invention is a method of additionally installing a small capacity orifice in series.

This method can measure the level, or volume, of LNG in the LNG storage tank from the LNG consumption in measuring the LNG level in the LNG storage tank.

Furthermore, it is known that the composition of BOG, which is an additional factor that lowers the accuracy of measurement in the related art, may be considered by adding gas chromatography to compensate for this.

In addition, if the LNG level in the LNG storage tank is accurately measured as described above, the efficiency of the BOG management method and apparatus of the present invention, which maintains the pressure of the LNG tank at a somewhat higher pressure than before, increases. In other words, knowing the exact amount of the LNG volume in the LNG tank, it is easy to change the setting of the safety valve of the LNG tank to multiple, it is possible to reduce the consumption of BOG.

On the contrary, FIG. 9 shows a flow rate measuring device for fuel gas of a conventional LNG carrier. In the past, only one orifice 71 for measuring the flow rate of differential pressure fuel gas is installed, and a valid measurement value can be obtained only in a specific measurement range. There is a disadvantage.

FIG. 10 shows the compression of BOG and feeding it to the bottom of the LNG tank in accordance with one embodiment of the present invention.

LNG carriers having fuel gas propulsion means for compressing the boil-off gas in the upper portion of the LNG storage tank of the LNG carrier and using it as a propellant fuel cannot use fuel gas at all when passing through a canal such as a Suez canal. The temperature and pressure of the storage tank are likely to rise locally. In order to solve this problem, a separate BOG extraction device may be required. That is, as shown in FIG. 10, the BOG is pulled out slightly and pressurized by the BOG compressor (about 3-5 bar) to the lower portion of the LNG storage tank 1.

To this end, the boil-off gas for returning the boil-off gas to the LNG storage tank 1 in the middle of the fuel gas supply line L1 which compresses the boil-off gas on the upper portion of the LNG storage tank 1 of the LNG carrier and supplies it to the fuel gas propulsion means. Branch line L2 is provided. Moreover, the compressor 41 is provided in the middle of the fuel gas supply line L1 of the upstream of the point which meets the boil-off gas branch line L2.

The buffer tank 43 is provided in the middle of the boil-off gas branch line L2. Since the pressure of the boil-off gas passing through the compressor 41 and the pressure of the LNG storage tank 1 differ, the boil-off gas passing through the compressor 41 is temporarily stored in the buffer tank 43 and the pressure is stored in the LNG storage tank ( After adjusting to the pressure of 1), it is preferable to return to the LNG storage tank (1).

It is preferable that the operation of the pressure rise reduction device of the LNG storage tank of the LNG carrier operates intermittently for about 10 minutes in 2 hours.

Fuel gas propulsion means include dual fuel diesel electric propulsion systems (DFDE), gas injection engines, gas turbines, and the like.

LNG carriers using DFDE, gas injection engines, gas turbines, etc., apply BOG compressors to compress the BOG and send it to the engine for combustion, but according to the present invention, it is configured to eliminate or reduce the emission of BOG in the LNG storage tank. In case of LNG carrier, when BOG consumes little or no fuel gas, BOG can be compressed and then sent to DFDE to prevent excessive pressure rise due to local temperature rise inside LNG storage tank. Return to the bottom of the LNG storage tank.

In another embodiment of the present invention, a fuel gas supply system for vaporizing LNG in an LNG storage tank and supplying the fuel gas as a fuel gas to a fuel gas propulsion means is provided. That is, in the prior art, the fuel gas propulsion means used BOG as fuel by using a high pressure compressor in addition to LNG of liquid, but in the present invention, BOG is not used at all.

Instead, BOG reliquefaction can be added using the energy of cold LNG. That is, the BOG is compressed and then exchanged with LNG in the fuel gas supply line for cooling (without the N2 refrigeration unit as a recondenser). In this case, only about 40-60% of the NBOG is liquefied, but according to the present invention there is no problem because the LNG carrier is configured to eliminate or reduce the discharge of BOG in the LNG storage tank. Furthermore, if necessary, about 1 ton / hour small BOG reliquefaction apparatus may be installed, especially for ballast voyages.

The LNG storage tank 1 of the LNG carrier used in the fuel gas supply system of this embodiment can withstand the pressure increase by the boil-off gas to allow the pressure rise by the boil-off gas generated inside during the operation of the LNG carrier. It is designed to have strength.

The fuel gas supply system illustrated in FIG. 11 is provided with a fuel gas supply line L11 which extracts LNG from the LNG storage tank 1 of the LNG carrier and supplies the fuel gas propulsion means to the fuel gas supply line L11. The heat exchanger 53 which heat-exchanges LNG with the boil-off gas which draws out from the LNG storage tank 1 is provided in the middle.

The fuel gas supply line L11 upstream of the heat exchanger 53 is provided with a primary pump 52 for compressing LNG according to the required flow rate and pressure of the fuel gas propulsion means and supplying it to the fuel gas propulsion means.

The heat exchanger 53 passes through a boil-off gas liquefaction line L12 which extracts boil-off gas from the top of the LNG storage tank 1 and returns it to one side of the LNG storage tank 1.

In the heat exchanger 53, the LNG is heated up by heat exchange with the boil-off gas and supplied to the fuel gas propulsion means, and the boil-off gas is liquefied by heat-exchange with the LNG and returned to the LNG storage tank 1.

The fuel gas supply line L11 downstream of the heat exchanger 53 compresses LNG exchanged with the boil-off gas in the heat exchanger 53 in accordance with the required flow rate and pressure of the fuel gas propulsion means to supply the fuel gas propulsion means. The secondary pump 54 is provided.

The fuel gas supply line L11 downstream of the secondary pump 54 is provided with a heater 55 for heating the LNG heat exchanged in the heat exchanger 53 to supply the fuel gas propulsion means.

In the boil-off gas liquefaction line (L2) upstream of the heat exchanger (53), a boil-off gas compressor (56) and a cooler (57) are installed in order to compress and cool the boil-off gas from the LNG storage tank and then heat-exchange with the LNG. It is.

If the pressure of the fuel gas required by the fuel gas propulsion means is high (e.g. 250 bar), the primary pump 52 compresses the LNG, for example 27 bar directly and then the LNG passes through the heat exchanger 53. The temperature is raised from about -163 ° C to about -100 ° C and then supplied to the secondary pump 54 in the liquid state and compressed to about 250 bar in the secondary pump 54 (the supercritical state, so liquid and gas separation is None) Next, it is vaporized while being heated in the heater 55 and supplied to the fuel gas propulsion means. In this case, since the pressure of LNG supplied to the heat exchanger 53 is high, even if the temperature of LNG rises through the heat exchanger 53, LNG will not vaporize.

On the other hand, when the pressure of the fuel gas required by the fuel gas propulsion means is low (for example, 6 bar), the primary pump 52 compresses the LNG, for example, 6 bars, and then LNG is heat exchanger 53. After passing through some vaporization is supplied to the heater 55 is heated in the heater 55 is supplied to the fuel gas propulsion means. In this case, the secondary pump 54 is not necessary.

According to the fuel gas supply system of the LNG carrier, the LNG is extracted from the LNG storage tank, compressed to meet the required flow rate and pressure of the fuel gas propulsion means, and supplied to the fuel gas propulsion means, but the LNG is extracted from the LNG storage tank. Since the heat exchanger supplies the fuel gas from the LNG carrier to the fuel gas propulsion means, the configuration is simple and power consumption can be reduced while preventing excessive pressure increase due to the accumulation of boil-off gas in the LNG storage tank.

While the invention has been described above with reference to specific embodiments, various modifications, changes or modifications may be made in the art within the spirit and scope of the appended claims, and thus, the foregoing description and drawings It should be construed as illustrating the present invention, not limiting the technical spirit of the present invention.

As described above, according to the present invention, an LNG carrier having a BOG treatment means allows an increase in the vapor pressure in the tank and the temperature of the LNG during transportation of LNG in the LNG storage tank, thereby reducing the waste of BOG or Increased flexibility

In particular, according to the present invention, even if the amount of boil-off gas generated during transportation of LNG is more than the consumption amount can be preserved without loss of the boil-off gas, bringing economical efficiency and efficiency. For example, in the case of an LNG carrier equipped with an engine for treating boil-off gas as shown in FIG. 4, an excess occurrence that occurs several days after shipment of LNG occurs during passage of a canal or during arrival of a canal or during arrival of a port or when entering a port The BOG more than the engine consumption to be burned off in the past using a GCU in most cases, but by applying the technology of the present invention can reduce the waste of such BOG.

In addition, when a gas / liquid combined gas injection engine is used in an LNG carrier, fuel can be supplied using a liquid pump instead of an evaporative gas compressor, thereby greatly reducing installation and operating costs.

Claims (1)

A method for managing boil-off gas (BOG) from large LNG storage tanks carrying cryogenic LNG.

Images