KR101637415B1 - Pressure control method and system for liquid cargo tank - Google Patents

Abstract

A method and system for controlling the pressure of a liquid storage tank installed in an offshore structure is disclosed. A method of controlling a pressure of a liquid storage tank according to the present invention is a method for controlling a pressure of a liquid storage tank by controlling a pressure in a tank interior space of the liquid storage tank, And a pressure equilibrium step of keeping the inner pressure of the inner space of the tank and the inner pressure of the primary heat insulating space evenly communicated by communicating between the primary heat insulating spaces.

Description

[0001] PRESSURE CONTROL METHOD AND SYSTEM FOR LIQUID CARGO TANK [0002]

The present invention relates to a method and system for controlling the pressure of a liquid storage tank, and more particularly, to a method and system for controlling the pressure of a liquid storage tank by providing an equalizing pipe connecting the inside of a liquid storage tank and a primary insulation space, And more particularly, to a method and system for controlling the pressure of a liquid storage tank that can maintain the pressure of the inside of the storage tank and the primary heat insulating space evenly.

Recently, the consumption of liquefied gas such as Liquefied Natural Gas (LNG) and Liquefied Petroleum Gas (LPG) has been rapidly increasing worldwide.

In particular, Liquefied Natural Gas (hereinafter referred to as "LNG") is an eco-friendly fuel with little emission of air pollutants during combustion, and is increasingly used in various fields.

Natural gas is a fossil fuel containing methane as a main component and a small amount of ethane, propane, and the like, and has recently been regarded as a low-pollution energy source in various technical fields.

Natural gas is transported in a gaseous state via land or sea gas piping, or stored in a LNG transport in the form of liquefied natural gas, LNG, and transported to a remote consumer. LNG is obtained by cooling natural gas at a cryogenic temperature (below about -163 ° C), and its volume is reduced to about 1/600 of that of natural gas, making it well suited for long distance transportation through the sea.

In recent years, the development of offshore plants capable of solving the problems of land acquisition, long-term construction, and opposition to local residents has been actively conducted. For example, LNG FPSO (Floating, Production , Storage and Offloading (LNG), Floating Storage and Regasification Unit (FSRU), Floating Storage Power Plant (FSPP) and Barge Mounted Power Plant (BMPP).

In order to store cryogenic liquefied natural gas, a storage tank (often called a 'cargo hold') having a special structure is installed in such a ship or offshore plant. LNG storage tanks for storing liquefied natural gas are generally made of primary and secondary The sealing wall and the primary and secondary insulating walls are alternately stacked and formed.

However, due to the temperature difference between the inside and the outside of the storage tank, the heat penetration into the storage tank is inevitable. Since the liquefaction temperature of natural gas is a cryogenic temperature of about -163 ° C at normal pressure, LNG is evaporated even if its temperature is slightly higher than -163 ° C at normal pressure. Although the LNG storage tank is adiabatically treated, since the external heat is continuously transferred to the LNG, the LNG is continuously vaporized in the LNG storage tank, and the boil-off gas (BOG) is generated in the LNG storage tank do.

Evaporative gases should be treated in an appropriate manner to make the pressure inside the storage tank higher than the operating pressure.

Generally, the evaporation gas generated during ship operation is used as fuel in various engines, boilers, gas combus- tion units (GCU), etc., or it is re-liquefied by the liquefaction facility and returned to the storage tank. Thereby preventing the pressure from rising.

In order to suppress the generation of evaporative gas from the liquefied natural gas stored in the storage tank, the boiling point of the liquefied natural gas is increased by raising the pressure of the storage tank to a certain pressure by using natural gas vaporized by the heat introduced into the storage tank, Has recently been proposed.

1, a typical membrane type LNG storage tank is provided with a secondary insulation wall 17, a secondary sealing wall 13, a primary insulation wall 15, and a primary And the sealing wall 11 are sequentially laminated. Specifically, in order to prevent leakage of the stored LNG, a primary sealing wall 11 that is in direct contact with the LNG installed and stored inside the hull 10, and a secondary sealing wall 11 A primary heat insulating wall 15 interposed between the primary sealing wall 11 and the secondary sealing wall 13 and a secondary insulating wall 15 interposed between the primary sealing wall 11 and the secondary sealing wall 13, And a secondary heat insulating wall 17 interposed between the sealing wall 13 and the hull 10.

However, when the pressure inside the storage tank is maintained at a high level, the LNG in the storage tank in an emergency where the crack (C) is generated in the primary sealing wall 11 and the LNG leaks flows into the inner space of the primary heat insulating wall 15 There is a problem that the LNG entering the inner space of the primary heat insulating wall 15 overflows to the upper part of the deck through the nitrogen line 21 or 43 or the like due to the pressure difference with the inside of the storage tank, There is a risk that it may lead to accidents or human accidents.

The primary insulation wall 15 and the secondary insulation wall 17 may be made of an air-permeable thermal insulation material, and in order to vaporize and discharge the leaked LNG when air enters the storage tank from the outside or LNG leaks from the storage tank The inert gas is circulated through the inner space of the primary heat insulating wall 15 and the inner space of the secondary heat insulating wall 17. [

(Hereinafter referred to as a "secondary heat insulating space 17") of the primary heat insulating wall 15 (hereinafter referred to as a "primary heat insulating space 15") and an internal space of the secondary heat insulating wall 17 Is maintained at a pressure higher than the atmospheric pressure and lower than the internal pressure of the storage tank by an inert gas such as nitrogen gas.

However, when the pressure inside the storage tank is kept high to suppress the generation of the evaporative gas, since the difference between the internal pressure of the storage tank and the internal pressure of the primary heat insulating space 15 is large, The LNG liquid in the primary heat insulating space 15 can be higher than the LNG liquid in the inner space of the storage tank when cracks C are formed in the car sealing wall 11 and LNG leaks.

Accordingly, when the internal pressure of the nitrogen supply line 21 for supplying nitrogen to the primary heat insulating space 15 or the internal pressure of the primary heat insulating space 15 becomes excessively high, the LNG flows through the nitrogen discharge line 43 for discharging nitrogen There is a risk of flooding over the deck.

The present invention solves this problem. When the LNG leaks into the primary heat insulating space (15) while maintaining the internal pressure of the storage tank higher than the atmospheric pressure, the internal pressure of the primary heat insulating space (15) It is proposed to raise the pressure equal to the internal pressure of the storage tank so as to establish a pressure equilibrium state so as to secure safety even in an emergency.

According to one aspect of the present invention, there is provided a method of controlling a pressure of a liquid storage tank installed in an offshore structure, comprising the steps of: providing a primary sealing wall provided in contact with a liquid stored in the primary sealing wall; A manufacturing step of manufacturing the liquid storage tank so as to include a wall, a primary heat insulating space formed between the primary sealing wall and the secondary sealing wall, and a secondary heat insulating space formed between the secondary sealing wall and the hull, Wow; A supply step of supplying liquid to the liquid storage tank; The liquid storage tank communicates between the tank internal space of the liquid storage tank and the primary heat insulating space so that when the liquid stored in the liquid storage tank breaks into the primary heat insulating space, A pressure balancing step of keeping the pressure and the internal pressure of the primary heat insulating space evenly maintained; A pressure control method of a liquid storage tank is provided.

In the pressure balancing step, the inner space of the tank and the primary heat insulating space can communicate with each other through the pressure balanced pipe.

Wherein the pressure equalizing pipe is separated from the internal space of the tank and the primary heat insulating space when the primary sealing wall is normal without breakage, and when the primary sealing wall is broken, And can be installed so as to communicate the car heat-insulating space.

The pressure balancing step may include blocking an on-off valve installed in a nitrogen discharge line connected to the primary heat-insulating space to discharge nitrogen from the primary heat-insulating space.

The pressure balancing step may include blocking an open / close valve installed in a nitrogen supply line connected to the primary heat insulating space to supply nitrogen to the primary heat insulating space.

And a pressure increasing step of increasing the internal pressure of the internal space of the tank without discharging the evaporated gas generated by evaporation of the liquid to the outside of the liquid storage tank after the supplying step.

In addition, after the supplying step, the discharging step may further include discharging the evaporation gas to the outside of the liquid storage tank.

The discharging step may be performed when the internal pressure of the liquid storage tank reaches a predetermined pressure value.

According to another aspect of the present invention, there is provided a pressure control system for a liquid storage tank installed in an offshore structure, the liquid storage tank comprising: a primary sealing wall installed in contact with a liquid stored therein; And a secondary insulation space formed between the primary sealing wall and the secondary sealing wall and a secondary insulation space formed between the secondary sealing wall and the hull, In order to maintain the internal pressure of the tank internal space of the liquid storage tank and the internal pressure of the primary heat insulating space evenly when the liquid sealed in the liquid sealing tank is broken and the liquid stored in the liquid sealing tank breaks into the primary heat insulating space And a pressure equalizing pipe communicating between the tank internal space and the primary heat insulating space.

Wherein the pressure equalizing pipe is separated from the internal space of the tank and the primary heat insulating space when the primary sealing wall is normal without breakage, and when the primary sealing wall is broken, And can be installed so as to communicate the car heat-insulating space.

The pressure equalizing pipe may be detachably connected to a first connection line extending from the inner space of the tank and a second connection line extending from the primary insulation space. The pressure control system of the liquid storage tank of the present invention may include an on-off valve installed in each of the first and second connection lines.

The pressure equalizing pipe may be fixedly connected to a first connecting line extending from the space inside the tank and a second connecting line extending from the primary insulating space. At least one of the first connection line, the second connection line, and the pressure equalizing pipe may include an on-off valve.

The pressure control system of the liquid storage tank of the present invention may include a nitrogen discharge line connected to the primary heat insulating space for discharging nitrogen from the primary heat insulating space and provided with an on-off valve.

The pressure control system of the liquid storage tank of the present invention may include a nitrogen supply line connected to the primary heat insulating space for supplying nitrogen to the primary heat insulating space and provided with an on-off valve.

The liquid storage tank may include a gas dome formed at an upper portion thereof and provided with piping for evaporation gas and a safety valve installed in the gas dome and opened when the internal pressure of the liquid storage tank reaches a predetermined pressure .

The safety valve may be set to open when the internal pressure of the liquid storage tank reaches a selected open pressure value at 2 bar or less.

In the method and system for controlling the pressure of the liquid storage tank according to the embodiment of the present invention, a pressure equalizing pipe connecting the inside of the liquid storage tank and the primary insulation space is provided, The pressure inside the tank and the primary insulation space can be maintained evenly.

The boiling point of the liquefied natural gas is increased by keeping the pressure of the liquid storage tank at a predetermined pressure or higher than the atmospheric pressure by the evaporation gas generated from the liquid cargo itself to suppress the generation of evaporative gas in the tank, It is possible to prevent the phenomenon that the level of the liquefied natural gas flowing into the primary insulation space becomes higher than the level of the liquid storage tank due to the pressure difference even if the tank is broken or the like by controlling the pressure inside the primary insulation space have.

Since the level of the liquid storage tank and the level of the primary insulation space are maintained evenly by the equal pressure, it is possible to prevent the liquefied natural gas from overflowing to the upper part of the deck, and the damage spreads due to the damage of the hull, And to ensure the safety of the ship.

Brief Description of the Drawings Fig. 1 is a schematic side cross-sectional view of a liquid storage tank for explaining a state in which a crack is generated in a primary sealing wall in a state where an internal pressure of a liquid storage tank is kept higher than an atmospheric pressure,
2 is a schematic plan view of a liquid storage tank for illustrating a pressure control system of a liquid storage tank according to an embodiment of the present invention, and
3 is a schematic side cross-sectional view of a liquid storage tank for illustrating a state in which the liquid level of liquefied natural gas is maintained evenly by the pressure control system of the liquid storage tank according to an embodiment of the present invention.

In order to fully understand the present invention, operational advantages of the present invention, and objects achieved by the practice of the present invention, reference should be made to the accompanying drawings and the accompanying drawings which illustrate preferred embodiments of the present invention.

Hereinafter, the present invention will be described in detail with reference to the preferred embodiments of the present invention with reference to the accompanying drawings. Like reference symbols in the drawings denote like elements.

The present invention can be applied to ships such as LNG RV (Regasification Vessel), LNG FSRU (Floating Storage and Regasification), and the like, in addition to an LNG carrier that carries LNG if it is an offshore structure provided with a storage tank capable of storing liquefied gas at a cryogenic temperature. LNG Floating and Regasification Unit (LNG), LNG FPSO (Floating, Production, Storage and Off-loading), FSPP (Floating Storage Power Plant) and BMPP (Barge Mounted Power Plant).

LNG RV is a LNG regeneration facility installed on a liquefied natural gas carrier capable of self-propulsion and floating, and LNG FSRU stores liquefied natural gas unloaded from LNG transit offshore at sea, LNG FRU is a structure that supplies liquefied natural gas to the demand of the land by vaporizing liquefied natural gas at sea while using LNG FRU in cooperation with a separate storage tank while omitting its storage function. LNG FPSO Is a structure used to purify the mined natural gas from the sea, directly liquefy it, store it in the storage tank, and transfer the LNG stored in this storage tank to the LNG transport if necessary. The FSPP is a structure used to produce electricity at sea by loading LNG storage tanks and power generation facilities on floating hulls at sea, and BMPP is a structure used to produce electricity at sea by installing power generation facilities on barges .

In the present specification, the term "offshore structure" includes not only liquefied gas carrier such as LNG carriers, LNG RV, LNG FPSO, LNG FSRU, LNG FRU, FSPP, BMPP and the like.

In the following description, a case is described in which LNG is stored in a liquid storage tank, for example. However, the liquid storage tank to which the present invention is applied is not only for accommodating cryogenic liquefied natural gas, Or may be for receiving liquid cargo.

A pressure control method and system according to an embodiment of the present invention is applied to a liquid storage tank that maintains the internal pressure of a liquid storage tank in which liquid such as LNG is stored at a pressure higher than atmospheric pressure.

Conventionally, the pressure in the liquid storage tank is maintained within a certain range of about 0.25 bar to about 0.25 bar, so that the influx of heat from outside contributes most to the generation of evaporative gas, and all of the generated evaporative gas is discharged to the outside of the liquid storage tank .

In the liquid storage tank 1 to which the pressure control method and system according to the embodiment of the present invention is applied, the pressure in the liquid storage tank 1 is allowed to rise, The increase in the sensible heat of the liquid in the tank due to the rise, for example, the liquefied natural gas (LNG) and the natural gas (NG), absorbs most of the input heat so as to greatly reduce the generation of the evaporative gas.

For example, when the internal pressure of the storage tank storing LNG is 0.7 bar, the saturation temperature rises about 6 ° C compared to the initial 0.06 bar.

For example, when the LNG storage tank for an LNG carrier with an insulating wall is taken as an example, when the LNG is normally loaded, the pressure inside the storage tank is about 0.06 bar (gauge pressure) The pressure inside the storage tank gradually increases as it occurs. For example, after loading LNG from LNG production site, the internal pressure of the LNG storage tank for LNG carrier is approximately 0.06 bar. After the LNG carrier starts to operate for about 15-20 days and arrives at the destination, Lt; / RTI > can rise to approximately 0.7 bar.

Describing this in terms of temperature, LNG generally contains various impurities and is generally lower than the boiling point of pure methane liquid. Pure methane has a boiling point of about -161 ° C at 0.06 bar. In fact, LNG transported in LNG transport contains a little impurity such as nitrogen and ethane, and boiling point is around -163 ℃. Based on pure methane, the LNG temperature in the tank is about -161 ° C at 0.06 bar after LNG shipment. When the vapor pressure in the tank is controlled to 0.25 bar in consideration of the transport distance and the BOG consumption amount, the LNG temperature is -159 ° C When the steam pressure in the tank is controlled to 0.7 bar, the LNG temperature is about -155 ° C. If the steam pressure in the tank is controlled to 2 bar, the LNG temperature rises to about -146 ° C.

The liquid storage tank 1 to which the pressure control method and system according to the embodiment of the present invention is applied is designed in consideration of the pressure rise due to the generation of the evaporative gas while having the heat insulating wall, So as to have a strength enough to withstand the pressure increase caused by the pressure increase. Accordingly, the evaporated gas generated inside the liquid storage tank during use of the liquid storage tank to which the present invention is applied is accumulated in the liquid storage tank without being discharged to the outside.

For example, when the pressure control method and system according to the embodiment of the present invention is applied to a liquid storage tank 1, that is, an LNG storage tank, installed in an LNG carrier, the liquid storage tank 1 preferably has a heat insulating wall And is designed to withstand a pressure of 2 bar (gauge pressure), more preferably designed to withstand pressures from 0.25 bar to 0.7 bar (gauge pressure). Considering the distance of LNG transport and the current IGC Code, it is preferred that the liquid storage tank 1 be designed to withstand pressures in excess of 0.25 bar to 0.7 bar, in particular 0.7 bar. However, if the pressure is too low, the distance to transport the LNG becomes too short, which is not preferable, and if the pressure is too high, the tank can not be easily manufactured.

The liquid storage tank 1 to which the embodiment of the present invention is applied can be designed so as to withstand the pressure in the initial design (for example, designed to have a large thickness) It is economical in terms of production cost since it can be realized sufficiently by merely adding a reinforcing material and proper reinforcement.

2, a liquid storage tank 1 to which an embodiment of the present invention is applied is a membrane-type storage tank, and includes a secondary insulation wall 17, a secondary sealing wall 13, A heat insulating wall 15 and a primary sealing wall 11 are sequentially laminated. Specifically, in order to prevent leakage of stored liquid such as LNG, a primary sealing wall 11 that is in direct contact with the LNG installed and stored inside the hull 10, And a secondary sealing wall 13 provided between the wall 11 and a primary heat insulating wall 15 interposed between the primary sealing wall 11 and the secondary sealing wall 13 for heat insulation, And a secondary insulation wall 17 interposed between the secondary sealing wall 13 and the hull 10.

The primary insulation wall 15 and the secondary insulation wall 17 may be made of an air-permeable thermal insulation material, and in order to vaporize and discharge the leaked LNG when air enters the storage tank from the outside or LNG leaks from the storage tank The inert gas is circulated through the inner space of the primary heat insulating wall 15 and the inner space of the secondary heat insulating wall 17. [

Generally, the inner space of the primary heat insulating wall 15 (hereinafter referred to as the 'primary heat insulating space 15') and the inner space of the secondary heat insulating wall 17 (hereinafter referred to as the 'secondary heat insulating space 17' Quot;) is maintained at a pressure higher than the atmospheric pressure and lower than the internal pressure of the liquid storage tank 1, for example, by supplying nitrogen gas.

The nitrogen supply lines 21 and 23 are connected to the primary heat insulating space 15 and the secondary heat insulating space 17 in order to supply nitrogen gas as an inert gas to the primary heat insulating space 15 and the secondary heat insulating space 17. [ Respectively. The respective nitrogen supply lines 21 and 23 are provided with open / close valves 22 and 24 to control the supply of nitrogen.

On the other hand, in order to prevent the pressure in the primary heat insulating space 15 and the secondary heat insulating space 17 from rising excessively, or to circulate nitrogen in the primary heat insulating space 15 and the secondary heat insulating space 17 Nitrogen discharge lines 43 and 47 are provided for the primary insulation space 15 and the secondary insulation space 17, respectively, for discharging to the outside. Each of the nitrogen discharge lines 43 and 47 is provided with safety valves 44 and 48 which are opened when the pressures of the primary heat insulating space 15 and the secondary heat insulating space 17 reach a predetermined pressure.

The safety valve 42 opened when the internal pressure of the liquid storage tank 1 reaches a predetermined pressure is connected to an evaporation gas discharge line 41 extending from the gas dome 30 provided at the upper portion of the liquid storage tank 1 ). The gas dome 30 is generally formed in a liquid storage tank for storing LNG or the like so that various piping related to the evaporation gas can be installed.

The evaporation gas discharged from the liquid storage tank 1 through the evaporation gas discharge line 41 at the time of opening the safety valve 42 is continuously discharged to the vent mast 50 through the vent line 51 .

The vent mast 50 may be utilized as a discharge port of nitrogen from the primary heat insulating space 15 and the secondary heat insulating space 17 as well as being used as a discharge port of evaporation gas (i.e., NG). The nitrogen gas can be discharged to the outside through the nitrogen discharge lines 43 and 47 and the safety valves 44 and 48 in accordance with the internal pressure of the primary heat insulating space 15 and the secondary heat insulating space 17, Nitrogen gas passing through the safety valves 44 and 48 can be discharged to the vent mast 50 through the vent line 53 and then discharged.

An open / close valve (45) is provided in the nitrogen discharge line (43) extending from the primary heat insulating space (15). When the internal space of the liquid storage tank 1, that is, the tank internal space 2 and the primary heat insulating space 15 are communicated by the pressure equalizing pipe 35 for pressure balance as described later, The open / close valve 45 provided in the nitrogen discharge line 43 must be closed to allow the internal pressure of the fuel tank 15 to rise.

2 shows only the safety valve 44 and the vent mast 50 are connected through the vent line 53. The safety valve 48 and the vent mast 50 are connected to each other through a vent line .

2, in order to control the emergency pressure of the liquid storage tank 1 provided in an offshore structure, the inner space of the tank 2, which is the inner space of the liquid storage tank 1, And a pressure equalizing pipe 35 is provided between the primary heat insulating space 15 and the primary heat insulating space 15 to maintain the pressure in the tank internal space 2 and the primary heat insulating space 15 equal.

The gas dome 30 is provided with the pressure equalizing pipe 35 in addition to the evaporation gas discharge line 41 in which the safety valve 42 is opened when the pressure in the tank internal space 2 is equal to or greater than a predetermined value, A second connection line 33 for connecting the pressure equalizing pipe 35 to the primary heat insulating space 15 may be installed.

Closing valves 32 and 34 are provided in the first connection line 31 and the second connection line 33 and the tank internal space 2 and the primary heat insulating space 15 are connected to each other through the pressure balance pipe 35, It is necessary to close all of the opening and closing valves 32 and 34 and to communicate the tank internal space 2 and the primary heat insulating space 15 through the pressure equalizing pipe 35 Only the on-off valves 32 and 34 are opened.

2, both of the opening / closing valves 32 and 34 provided in the first connecting line 31 and the second connecting line 33 are closed, and then the pressure equalizing pipe 35 is connected to the first and second 2 connection lines 31, 33, respectively. If the pressure balance pipe 35 is separated, malfunction of the valve can be prevented originally.

According to another embodiment of the present invention, although not shown, the pressure equalizing pipe 35 may be fixedly connected to the first and second connection lines 31 and 33. In this case, only one of the two on-off valves 32 and 34 may be provided, and the on-off valve may be provided on the pressure equalizing pipe 35 instead of the first and second connecting lines 31 and 33 .

Although only the communication between the tank internal space 2 of the liquid storage tank and the primary heat insulating space 15 by the pressure equalizing pipe 35 has been described so far according to the present invention, Or the secondary heat insulating space 17 in a manner similar to that of the secondary heat insulating space 35 or may be embodied so as to communicate between the primary heat insulating space 15 and the secondary heat insulating space 17 .

Hereinafter, the operation of the pressure control system according to the embodiment of the present invention will be described with reference to FIGS. 2 and 3. FIG.

When the primary sealing wall 11 of the liquid storage tank 1 is not broken, the communication between the tank internal space 2 of the liquid storage tank through the pressure equalizing pipe 35 and the primary heat insulating space 15 Is blocked. At this time, at least one of the open / close valves 32, 34 provided in the first and second connection lines 31, 33 may be closed or all of the open / close valves 32, 34 may be closed, The balance pipe 35 may be completely separated from the first and second connection lines 31 and 33. [

3, when the membrane of the liquid storage tank 1, that is, the primary sealing wall 11 is broken and a crack C is generated, the liquid inside the liquid storage tank is discharged through the pressure equalizing pipe 35, And communicates between the tank internal space (2) and the primary heat insulating space (15).

The liquefied natural gas leaks from the tank internal space 2 to the primary heat insulating space 15 when the primary sealing wall 11 of the liquid storage tank 1 is damaged. As described above, the internal pressure of the liquid storage tank 1 is set to a predetermined pressure value, for example, set to a pressure value selected from a range of about 2 bar or less, preferably from 0.25 bar to 0.7 bar or less, Since the internal pressure of the space 15 is set to a pressure value slightly higher than the atmospheric pressure, for example, 0.25 bar or less, the LNG liquid level in the primary heat insulating space 15 is lowered to the liquid storage tank 1) It can be overflowed to the top of the deck while being higher than the internal LNG liquid level.

In the present embodiment, in this emergency, the pressure balance pipe 35 is used to communicate the tank internal space 2 and the primary heat insulating space 15, and at the same time, to open and close the nitrogen discharge line 43 extending from the primary heat- The valve 45 is closed. The internal pressure of the primary heat insulating space 15 is reduced to the inside space 2 of the tank of the liquid storage tank by closing the opening and closing valve 45 and making the tank internal space 2 and the primary heat insulating space 15 communicate with each other, And it is prevented that the LNG liquid level in the primary heat insulating space 15 becomes higher than the LNG liquid level in the liquid storage tank 1.

When the internal pressure of the nitrogen supply line 21 for supplying nitrogen into the primary heat insulating space 15 is lower than the internal pressure of the internal space 2 of the tank, the opening / closing valve 22 ).

If the internal pressure of the liquid storage tank 1 and the internal pressure of the primary heat insulating space 15 are maintained evenly through the pressure equalizing pipe 35 in an emergency, even if the LNG leaks due to breakage of the primary sealing wall 11 The liquefied natural gas liquid level inside the tank and the primary insulation space is maintained evenly, so that liquefied natural gas does not leak to the top of the deck.

The internal pressure of the liquid storage tank 1 to which the pressure regulating method and system according to the embodiment of the present invention is applied can be set differently depending on the amount of stored liquid. That is, the safety valve 42 of the liquid storage tank 1 may be configured to have two or more set values according to the amount of liquid stored in the liquid storage tank.

When the opening pressure setting value of the safety valve 42 of the liquid storage tank 1 is 0.25 bar, the LNG is shipped to a volume of 98% of the liquid storage tank 1 when the LNG is shipped, and the remaining 2% Leave. If more than 98% of the LNG is filled with LNG, the LNG may flow over from the gas dome 30 when the internal pressure of the liquid storage tank reaches 0.25 bar. However, if the internal pressure rise of the liquid storage tank is allowed to continue after the LNG shipment, even if the LNG is shipped at 98% or less, there is a possibility of overflow due to the LNG expansion due to the temperature increase of the LNG. For example, when the steam pressure of the LNG tank is 0.7 bar, the overflow phenomenon may occur even when the LNG load is 97% or so.

Due to such a problem, rather than keeping the opening pressure value of the safety valve 42 installed at the upper part of the liquid storage tank fixed at one value, it is required to maintain the opening pressure at a relatively low pressure, for example, 0.25 bar And when the amount of LNG stored in the liquid storage tank 1 decreases as time passes after the shipment and the BOG is used (for example, as a fuel for a boiler or an engine), the value of the opening pressure of the safety valve 42 is increased It is possible to reduce the waste of BOG without increasing the initial shipment volume and to increase the flexibility of BOG treatment.

Therefore, the opening pressure value of the safety valve 42 is set at, for example, 0.25 bar or less at the time of loading, and the amount of LNG stored in the liquid storage tank as the evaporated gas generated in the liquid storage tank 1 is discharged to the outside Is set to a value within a range from more than 0.25 bar to 2 bar, preferably from 0.25 bar to 0.7 bar or less. Here, the opening pressure value of the safety valve 42 varies from 0.25 bar to 0.4 bar, and from 0.4 bar to 0.7 bar (for example, depending on the amount of liquid loading inside the liquid storage tank) It can be gradually increased.

For example, when the volume of the LNG stored in the liquid storage tank 1 is 98.5%, the opening pressure of the safety valve 42 is set to 0.25 bar, and when the volume of the LNG is 98.0%, the opening value of the safety valve is set to 0.4 bar, the opening pressure of the safety valve is adjusted to 0.5 bar when the volume of the LNG is 97.6%, and the opening pressure of the safety valve is adjusted to 0.7 bar when the volume of the LNG is 97.2%.

As described above, in the liquid storage tank provided in the offshore structure, since the evaporated gas (BOG) generated from the liquid cargo stored in the liquid storage tank during normal operation, for example, LNG, is not discharged, Up to a selected set pressure in the range of about 2 bar or less, preferably in the range of 0.25 bar to 0.7 bar or less, and the safety valve is opened when the internal pressure reaches the set pressure.

The internal pressure of the liquid storage tank and the internal pressure of the primary heat insulating space are measured through the pressure equalizing pipe 35 connecting the tank internal space 2 and the primary heat insulating space 15 of the liquid storage tank, By controlling the internal pressure equally, the liquid level of the liquefied natural gas becomes higher than the inside of the tank due to the pressure difference, thereby preventing the deck from overflowing. In order to prevent malfunctions, it is also possible to separate the pressure balanced pipe from the gas dome during normal operation.

With this system, the safety of the tank is restrained by suppressing the generation of excessive evaporation gas during normal operation of the offshore structure, so that the liquefied natural gas leaks to the upper part of the deck even in an emergency, thereby preventing the damage from spreading due to hull damage or human accidents And the safety of the ship can be ensured.

It will be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit and scope of the invention. It is.

1: Liquid storage tank
2: Inside tank space (of liquid storage tank)
10: Hull
11: primary sealing wall
13: Secondary sealing wall
15: Primary insulation wall (or primary insulation space)
17: Secondary insulation wall (or secondary insulation space)
21, 23: nitrogen supply line
22, 24: opening / closing valve (installed in the nitrogen supply line)
30: Gas dome
31: first connection line
32: opening / closing valve (installed in the first connecting line)
33: second connection line
34: opening / closing valve (installed in the second connecting line)
35: Pressure balanced pipe
41: Evaporative gas discharge line
42: Safety valve (of liquid storage tank)
43: Nitrogen discharge line (extending from the primary insulation space)
44: Safety valve (of primary insulation)
45: opening / closing valve (of nitrogen discharge line)
47: Nitrogen discharge line (extending from the secondary insulation space)
48: Safety valve (of secondary insulation)
50: Bent mast
51, 53: vent line

Claims (16)

A method of controlling a pressure of a liquid storage tank installed in an offshore structure,
A secondary sealing wall provided between the primary sealing wall and the hull, a primary heat insulating space formed between the primary sealing wall and the secondary sealing wall, And a secondary insulation space formed between the secondary sealing wall and the hull;
A supply step of supplying liquid to the liquid storage tank;
The liquid storage tank communicates between the tank internal space of the liquid storage tank and the primary heat insulating space so that when the liquid stored in the liquid storage tank breaks into the primary heat insulating space, A pressure balancing step of keeping the pressure and the internal pressure of the primary heat insulating space evenly maintained;
/ RTI >
In the pressure balancing step, the inner space of the tank is communicated with the primary heat insulating space through a pressure balance pipe,
Wherein the pressure equalizing pipe is separated from the internal space of the tank and the primary heat insulating space when the primary sealing wall is normal without breakage, and when the primary sealing wall is broken, Wherein the liquid storage tank is provided so as to communicate with the car heat-insulating space. delete delete The method according to claim 1,
Wherein the pressure equilibration step includes blocking an on-off valve installed in a nitrogen discharge line connected to the primary heat-insulating space to discharge nitrogen from the primary heat-insulating space. The method according to claim 1,
Wherein the pressure equilibration step includes blocking an on-off valve installed in a nitrogen supply line connected to the primary heat-insulating space to supply nitrogen to the primary heat-insulating space. The method according to claim 1,
Further comprising a pressure increasing step of increasing the internal pressure of the internal space of the tank without discharging the evaporation gas generated by evaporation of the liquid after the supplying step to the outside of the liquid storage tank . The method according to claim 1,
Further comprising the step of discharging the evaporated gas to the outside of the liquid storage tank after the supplying step. The method of claim 7,
Wherein the discharging step is performed when the internal pressure of the liquid storage tank reaches a predetermined pressure value. A pressure control system for a liquid storage tank installed in an offshore structure,
Wherein the liquid storage tank comprises: a primary sealing wall provided so as to be in contact with a liquid stored therein; a secondary sealing wall provided between the primary sealing wall and the hull; a first sealing wall formed between the primary sealing wall and the secondary sealing wall; And a secondary insulation space formed between the secondary sealing wall and the hull,
When the primary sealing wall is broken and the liquid stored in the liquid storage tank enters the primary heat insulating space, the internal pressure of the internal space of the tank of the liquid storage tank and the internal pressure of the primary heat insulating space are kept equal And a pressure equalizing pipe communicating between the internal space of the tank and the primary heat insulating space so as to allow the tank to communicate with the tank,
Wherein the pressure equalizing pipe is detachably connected to a first connection line extending from the inner space of the tank and a second connection line extending from the primary heat insulating space, To prevent malfunctions,
Wherein the first and second connection lines are each provided with an on-off valve. delete delete delete The method of claim 9,
And a nitrogen discharge line connected to the primary heat insulating space for discharging nitrogen from the primary heat insulating space and provided with an on-off valve. The method of claim 9,
And a nitrogen supply line connected to the primary heat insulating space for supplying nitrogen to the primary heat insulating space and provided with an on-off valve. The method of claim 9,
Wherein the liquid storage tank comprises a gas dome formed at an upper portion thereof and provided with piping for evaporation gas and a safety valve installed in the gas dome and opened when the internal pressure of the liquid storage tank reaches a predetermined pressure, Pressure control system in storage tank. 16. The method of claim 15,
Wherein the safety valve is set to open when the internal pressure of the liquid storage tank reaches a selected open pressure value at 2 bar or less.

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