KR20160131471A - Pressure control method and system for liquid cargo tank - Google Patents

Abstract

Disclosed are a method and a system for controlling a pressure of a liquid storage tank installed on an offshore structure. According to the present invention, the method for controlling a pressure of a liquid storage tank comprises: a manufacturing step of manufacturing the liquid storage tank to comprise a first sealing wall installed to come in contact with liquid stored therein, a second sealing wall installed between the first sealing wall and a ship hull, a first insulation space formed between the first sealing wall and the second sealing wall, and a second insulation space formed between the second sealing wall and the ship hull; and a supply step of supplying liquid to the liquid storage tank; and a pressure increasing step of increasing an internal pressure of the first insulation space for an emergency where the first sealing wall is damaged to allow the liquid stored in the liquid storage tank to enter the first insulation space.

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, To a liquid storage tank pressure control method and system that can prevent overflowing liquid from overflowing onto the deck.

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) The pressure in the storage tank is raised to approximately the same as the internal pressure of the storage tank to establish a pressure balance state.

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; A pressure rising step of raising the internal pressure of the primary heat insulating space when the primary sealing wall is broken and liquid stored in the liquid storage tank enters the primary heat insulating space; A pressure control method of a liquid storage tank is provided.

In the pressure increasing step, the opening pressure value set for the safety valve in the primary heat insulating space can be increased.

The safety valve of the primary insulation space may be set to have a selected opening pressure value within a range of 0 to 2 bar, and preferably set to have a selected opening pressure value within a range of 0.01 to 0.7 bar .

When the primary sealing wall of the liquid storage tank is not damaged, the primary heat insulating space can be maintained at a pressure of 0 to 0.25 bar, and preferably a pressure of 0.01 to 0.04 bar.

In the emergency, the internal pressure of the primary heat insulating space can be raised to a pressure equal to or lower than the set pressure of the liquid storage tank by 0.02 bar, and preferably to a pressure as low as 0.01 bar.

Wherein the internal pressure of the liquid storage tank is maintained at 0.25 bar or less when the liquid is shipped to the liquid storage tank and the internal pressure of the liquid storage tank after the liquid is shipped to the liquid storage tank is greater than 0.25 to less than 0.7 bar And can be kept within a range.

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 heat insulating space formed between the secondary sealing wall and the hull, wherein the secondary heat insulating space is formed between the primary sealing wall and the secondary sealing wall, And a safety valve capable of adjusting the set pressure so as to raise the internal pressure of the primary heat insulating space when the liquid stored in the liquid storage tank breaks into the primary heat insulating space due to the breakage of the primary sealing wall, A pressure control system for a storage tank is provided.

The safety valve of the primary heat insulating space may be set to have a selected opening pressure value within a range of 0.01 to 0.7 bar.

In the method and system for controlling the pressure of the liquid storage tank according to the embodiment of the present invention, the internal pressure of the primary insulation space in an emergency can be increased to maintain a pressure substantially equal to the internal pressure of the liquid storage tank .

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 kept substantially equal at approximately equal pressure, the phenomenon that the liquefied natural gas floats on top of the deck can be prevented, And the safety of the ship can be ensured.

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 explaining a pressure control system of a liquid storage tank according to a first embodiment of the present invention;
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, and
4 is a schematic plan view of a liquid storage tank for explaining a pressure control system of a liquid storage tank according to a second 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.

Taking as an example the case of an LNG storage tank for an LNG carrier (or LNG RV in operation) with an insulating wall, the pressure inside the storage tank is about 0.06 bar (gauge pressure) at the beginning of shipment when LNG is normally loaded, The pressure inside the storage tank gradually increases as evaporation gas is generated during the operation period of the carrier. 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.

On the other hand, in the case of LNG storage tanks installed in plants such as LNG FSRU, LNG FRU, FSPP and BMPP (or LNG RV during regasification work), immediately after the LNG is supplied from an LNG carrier (such as a shuttle line) As the storage period in the plant increases, the pressure in the storage tank gradually increases to about 0.7 bar as evaporation gas is generated.

The temperature is described as follows. LNG contains several impurities and is generally lower than the boiling point of a 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 this pressure is too low, the distance (or storage period) for transporting the LNG becomes too short, which is not preferable. If the pressure is too high, the tank is not 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, the liquid storage tank 1 to which the first embodiment of the present invention is applied is a membrane-type storage tank in which a secondary heat insulating wall 17, a secondary sealing wall 13, A primary heat insulating wall 15 and a primary sealing wall 11 in this order. 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. For example, the primary heat insulating space 15 is maintained at a pressure of about 0.01 to 0.04 bar.

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 for connecting between the primary heat insulating space 15 and the primary heat insulating space 15 is provided to maintain the pressure in the tank internal space 2 and the primary heat insulating space 15 substantially 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.

In the first embodiment shown in Fig. 2, all of the opening / closing valves 32 and 34 provided in the first connecting line 31 and the second connecting line 33 are closed, And the second connection line (31, 33). If the pressure balance pipe 35 is separated, malfunction of the valve can be prevented originally.

Although not shown, according to an alternative embodiment of the present invention, the pressure equalizing pipe 35 may be fixedly connected to the first and second connecting lines 31, 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 first 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. [

When the primary sealing wall 11 of the liquid storage tank 1 is not damaged, the primary heat insulating space 15 has a pressure of about 0 to 0.25 bar (gauge pressure), preferably about 0.01 to 0.04 bar ≪ / RTI > That is, the safety valve 44 of the normal-time primary heat-insulating space 15 is set to a selected opening pressure value within a range of approximately 0 to 0.25 bar (gauge pressure), preferably approximately 0.01 to 0.04 bar.

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 value slightly higher than the atmospheric pressure, for example, a pressure value of 0.25 bar or less, preferably 0.01 to 0.04 bar or so, The LNG liquid level in the space 15 becomes higher than the LNG liquid level in the liquid storage tank 1 and can flow over to the upper portion of the deck.

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 the LNG liquid level in the primary heat insulating space 15 is prevented from being higher than the LNG liquid level in the liquid storage tank 1.

The internal pressure of the primary heat-insulating space 15 in the emergency state is, for example, equal to or lower than the set pressure of the liquid storage tank by 0.02 bar (i.e., the difference from the set pressure of the liquid storage tank is 0 to 0.02 bar) And can be elevated to a pressure preferably lower than the set pressure of the liquid storage tank by 0.01 bar. That is, since the internal pressure of the liquid storage tank 1 is set to a pressure value selected from a predetermined pressure value, for example, approximately 2 bar or less, preferably 0.25 bar to 0.7 bar or less, The safety valve 44 of the valve 15 may be set to a selected open pressure value within a range of approximately 0.25 to 2 bar, preferably approximately 0.25 to 0.7 bar.

As a result, the safety valve 44 of the primary heat insulating space 15 can be set to have a selected opening pressure value within a range of approximately 0 to 2 bar, preferably approximately 0.01 to 0.7 bar.

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 tank internal space 2, the opening / closing valve 22 provided on the nitrogen supply line 21 Closing.

If the internal pressure of the liquid storage tank 1 and the internal pressure of the primary heat insulating space 15 become substantially equal through the pressure balancing pipe 35 in an emergency, that is, if the internal pressure of the primary heat insulating space 15 becomes equal to the above- Various valves and the like can be controlled so as to maintain the internal pressure of the primary heat insulating space 15 in such a state that the open / close valves 32 and 34 are closed.

Thus, according to the first embodiment of the present invention, even if the LNG leaks due to the breakage of the primary sealing wall 11, the liquid natural gas level inside the tank and the primary heat insulating space is maintained evenly, Liquefied natural gas will not leak.

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 excessive evaporation gas generation during normal operation of offshore structures, and the leakage of liquefied natural gas to the upper part of the deck even in an emergency prevents the spread of damage due to hull damage or human accidents And the safety of the ship can be ensured.

4, a liquid storage tank 1 to which a second embodiment of the present invention is applied is a membrane-type storage tank, which includes a secondary insulation wall 17, a secondary sealing wall 13, a primary 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. For example, the primary heat insulating space 15 is maintained at a pressure of about 0.01 to 0.04 bar.

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.

Unlike the first embodiment described above, in the second embodiment, the nitrogen discharge line 43 extending from the primary heat insulating space 15 may not have a separate on-off valve.

4 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 .

According to the second embodiment of the present invention, unlike the first embodiment shown in Fig. 2, between the tank internal space 2, which is the internal space of the liquid storage tank 1, and the primary heat insulating space 15 An equalizing pipe 35 (see FIG.

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

When the primary sealing wall 11 of the liquid storage tank 1 is not damaged, the primary heat insulating space 15 has a pressure of about 0 to 0.25 bar (gauge pressure), preferably about 0.01 to 0.04 bar ≪ / RTI > That is, the safety valve 44 of the normal-time primary heat-insulating space 15 is set to a selected opening pressure value within a range of approximately 0 to 0.25 bar (gauge pressure), preferably approximately 0.01 to 0.04 bar.

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 internal pressure of the primary heat insulating space 15 is set to, for example, To a pressure equal to or lower than 0.02 bar (i.e., a difference from the set pressure of the liquid storage tank of 0 to 0.02 bar), preferably to a pressure of 0.01 bar lower than the set pressure of the liquid storage tank. That is, since the internal pressure of the liquid storage tank 1 is set to a pressure value selected from a predetermined pressure value, for example, approximately 2 bar or less, preferably 0.25 bar to 0.7 bar or less, The safety valve 44 of the valve 15 may be set to a selected open pressure value within a range of approximately 0.25 to 2 bar, preferably approximately 0.25 to 0.7 bar.

As a result, the safety valve 44 of the primary heat insulating space 15 can be set to have a selected opening pressure value within a range of approximately 0 to 2 bar, preferably approximately 0.01 to 0.7 bar.

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 value slightly higher than the atmospheric pressure, for example, a pressure value of 0.25 bar or less, preferably 0.01 to 0.04 bar or so, The LNG liquid level in the space 15 becomes higher than the LNG liquid level in the liquid storage tank 1 and can flow over to the upper portion of the deck.

In this embodiment, the set value of the safety valve 44 provided in the nitrogen discharge line 43 extending from the primary heat insulating space is adjusted to a higher value in such an emergency. That is, by setting the set pressure value of the safety valve 44 to, for example, equal to or lower than the set pressure of the liquid storage tank by 0.02 bar (i.e., the difference from the set pressure of the liquid storage tank is 0 to 0.02 bar) , Preferably to a pressure of about 0.01 bar below the set pressure of the liquid storage tank.

As the opening pressure value of the safety valve 44 rises while the supply of nitrogen through the nitrogen supply line 21 is maintained, the internal pressure of the primary heat insulating space 15 is lowered to the inner tank space 2 of the liquid storage tank And the LNG liquid level in the primary heat insulating space 15 is prevented from becoming higher than the LNG liquid level in the liquid storage tank 1.

Thus, according to the second embodiment of the present invention, even if the LNG leaks due to breakage of the primary sealing wall 11, since the liquid natural gas liquid level inside the tank and the primary heat insulating space is maintained substantially uniform, The liquefied natural gas will not leak.

In this specification, the expression "the internal pressure of the liquid storage tank and the internal pressure of the primary insulation space is equal or approximately equal" means that the internal pressure of the liquid storage tank and the internal pressure of the primary insulation space are completely identical But also to mean that the difference between the internal pressure of the primary insulation space and the internal pressure of the liquid storage tank is between 0 and 0.02 bar.

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.

By controlling the opening pressure value of the safety valve 44 of the primary heat insulating space 15 in an emergency including breakage of the tank, the internal pressure of the liquid storage tank and the internal pressure of the primary heat insulating space are controlled approximately equally, The pressure differential causes the liquid level of the liquefied natural gas to rise above the inside of the tank, preventing it from overflowing into the deck.

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 (11)

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;
A pressure rising step of allowing the internal pressure of the primary heat insulating space to rise when the primary sealing wall is broken so that liquid stored in the liquid storage tank enters the primary heat insulating space;
Wherein the pressure of the liquid storage tank is controlled by the control unit. The method according to claim 1,
And increasing the opening pressure value set for the safety valve in the primary heat insulating space in the pressure increasing step. The method of claim 2,
Wherein the safety valve of the primary insulation space is set to have a selected open pressure value within a range of 0 to 2 bar. The method of claim 2,
Wherein the safety valve of the primary heat insulating space is set to have a selected opening pressure value within a range of 0.01 to 0.7 bar. The method according to claim 1,
Wherein when the primary sealing wall of the liquid storage tank is not damaged, the primary insulation space is maintained at a pressure of 0 to 0.25 bar. The method according to claim 1,
Wherein the primary heat insulating space is maintained at a pressure of 0.01 to 0.04 bar when the primary sealing wall of the liquid storage tank is not broken. The method according to claim 1,
Wherein in the emergency, the internal pressure of the primary heat insulating space can rise to a pressure equal to or lower than the set pressure of the liquid storage tank by 0.02 bar. The method according to claim 1,
Wherein in the emergency, the internal pressure of the primary heat insulating space can rise to a pressure lower than the set pressure of the liquid storage tank by 0.01 bar. The method according to claim 1,
Wherein the internal pressure of the liquid storage tank is maintained at 0.25 bar or less when the liquid is shipped to the liquid storage tank and the internal pressure of the liquid storage tank after the liquid is shipped to the liquid storage tank is greater than 0.25 to less than 0.7 bar Wherein the pressure in the reservoir tank is maintained within a predetermined range. 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,
And a safety valve capable of adjusting the set pressure so as to allow an increase in the internal pressure of the primary heat insulating space when the liquid stored in the liquid storage tank breaks through the primary sealing wall and enters the primary heat insulating space , A pressure control system for liquid storage tanks. The method of claim 10,
Wherein the safety valve of the primary insulation space is set to have a selected opening pressure value within a range of 0.01 to 0.7 bar.

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