RU2770514C1 - Underwater liquefied natural gas storage - Google Patents

Abstract

FIELD: offshore structures.

SUBSTANCE: invention relates to offshore structures, namely to offshore liquefied natural gas (LNG) storage facilities designed for underwater accumulation and storage of liquefied natural gas (LNG) in the waters of the Arctic Ocean. Underwater storage of liquefied natural gas contains a double-walled body in the form of a hemisphere with a flat bottom with the formation of a sealed volume, fixed by means of submerged piles on the seabed soil with a gap between the bottom of the body and the seabed. In the lower part of the housing there are branch pipes for supplying liquefied natural gas and a trough sump for pumping sea water or liquefied natural gas, and an agitator and a cooling system are installed in the cavity of the housing, made in the form of a coil with a coil height increasing from the periphery of the housing to the center, and communicating with lines for supplying liquid air and draining it. The domed part of the body communicates with the line for supplying nitrogen and removing the mixture of nitrogen with boil-off gas, on which an adjustable valve is installed, while the inner wall of the body is made of cryogenic steel, lined on the outside with layers of thermal insulation and polyurethane foam, and the outer wall is made of reinforced concrete, lined on the outside a layer of waterproofing in the form of a polymer coating.

EFFECT: complex one-time provision of conditions for increasing the bearing capacity of the storage under the influence of external hydrostatic pressure, to prevent the occurrence of "rollover", to reduce the intensity of the formation of boil-off gas inside the tank and maintain cryogenic temperature, to prevent swelling of the soil and the formation of ice lenses, as well as to create negative buoyancy structures.

1 cl, 5 dwg

Description

SUBSTANCE: invention relates to offshore structures, namely to offshore liquefied natural gas (LNG) storage facilities designed for underwater accumulation and storage of liquefied natural gas (LNG) in the waters of the Arctic Ocean.

An underwater reservoir is known that operates on the principle of water flooding product displacement, when water enters the storage tank or is displaced from it as the volume of stored oil changes. Such tanks can be located completely under water or rise above the surface of the water (US 3889477).

The disadvantage of such an oil storage is the presence of a zone of contact between oil and water and a high probability of their mixing, which can contribute to the growth of bacteria in this area and a gradual decrease in the useful capacity of the storage during operation. There is a risk of leakage of the product from the tank and pollution of the water area.

It is impossible to use such a design solution for LNG storage, since the possibility of direct contact of positive temperature water with cryogenic LNG is excluded.

Known installation for the production and storage of cryogenic liquid, including many underwater cells and the surface part, which is installed LNG plant and means of transferring the product to the surface tanker (US 7553107).

The disadvantage of this design is its dependence on the surface module, without which this structure cannot exist independently, which leads to the impossibility of using such a structure for fully underwater field development, as well as at great depths in the conditions of the Arctic shelf, where the navigation period is limited to 3-4 months along with the existence of severe ice conditions.

In addition, since the description of the invention lacks information on measures to prevent the occurrence of the “rollover” phenomenon, the reliability of operation of such a structure will not meet safety conditions.

Also known is a system of underwater storage of liquefied gases, containing two containers fastened together: a spherical one, inside which a flexible membrane is located, and a cubic one. The system works on the principle of communicating vessels: when LNG is pumped into a spherical container, gravity acts on the membrane and displaces the insulating liquid (isopentane) into the second reservoir (US 3727418).

Isopentane, getting into the second container, acts on the membrane and displaces water from the container into the environment with a volume equal to the changed volume of isopentane in the second container.

The disadvantage of this design is the difficulty of attaching a spherical tank to a cubic one, a large amount of isopentane for such a design, the cubic shape of the second tank leads to a large overturning moment due to the action of underwater currents. Such a design will not be able to ensure reliability and stability during the operation of the structure, in addition, it also does not provide for the prevention of the “rollover” phenomenon.

Known "Offshore LNG Terminal", which is a terminal for receiving, liquefying, storing, regasifying and delivering natural gas. This structure incorporates a reinforced concrete support base with a cryogenic tank located on it, while to create the necessary buoyancy of the structure around the tank, ballast compartments are used, into which sea water is pumped (US3766583).

The disadvantage of this technical solution is that the LNG cryogenic tank is not built into the body structure, but is only based on its upper part, which makes it unprotected from environmental influences. In addition, open ballast tanks, which are located above the water level, the absence of double sides and decks in the structure increase ice and wave loads on the structure during operation.

Known "Offshore cryogenic tank", containing a base, a cylindrical vertical wall, the inner part of which is divided by a horizontal partition (deck) into the upper tank - cryogenic tank and the lower tank - ballast compartment, roof and deck (GB1464260). To create the necessary buoyancy of the structure, water is taken in or emptied from the ballast compartment when filling or removing liquid from the internal tank, respectively.

The disadvantage of this technical solution is the lack of ice resistance of the structure, the cryogenic tank is located on the support base above the current waterline, which increases the applicate of the center of gravity of this structure, while the water level in the ballast compartment must always be controlled and maintained using ballast pumps, depending on the load ( filling) of the cryogenic tank.

In addition, the cylindrical shape of the structure creates a significant overturning moment during the operation of the structure, the structure has restrictions on the location on the seabed in depth, and needs to be connected to the environment to remove the boil-off gas that occurs during storage.

An underwater oil storage facility is known, made in the form of a flat bottom of a round shape and a body in the form of a hemisphere, rigidly connected to the bottom, with the formation of a sealed volume for storing oil products, while the body is connected by a pipeline to the gas collector, the bottom is made protruding beyond the body, and the body contains, at least one branch pipe for pumping oil products into the body and at least one pipe for draining oil products, while either steel or concrete or reinforced concrete is used as the material of the body and bottom, and the tank bottom is fixed to the bottom using anchor devices (RU133818).

The disadvantage of the known oil storage is: the high value of positive buoyancy due to the presence of the air mixture inside the tank. In addition, the immersion of the storage is carried out with the help of weights and, after fixing the structure, they are removed by floating cranes. A large mass of weights complicates the process of their transportation to the field, leads to the need to use large capacities of winches or floating cranes to lift them from the seabed, and increases the load on the anchors.

Of the known solutions, the closest to the proposed technical essence and the achieved result is a gravity-type floating storage of liquefied natural gas for operation in the coastal waters of the northern seas with difficult ice conditions, containing a polygonal ice-resistant reinforced concrete caisson, a double bottom, double sides, internal bulkheads and decks, a vertical cylindrical structure with a domed roof and a built-in isothermal LNG tank with multilayer insulation (RU 163720).

The described technical solution has the following disadvantages.

Since the bottom of the storage is completely adjacent to the seabed, i.e. there is direct contact with the seabed, then during storage there will be a "cooling" swelling of the bottom soil.

Once installed on the bottom, the protruding above-water part (isothermal tank) will be exposed to wind, which will limit the use of the storage facility only at shallow depths in coastal areas.

The interaction of the storage body with ice will lead to the need to increase the thickness of the walls of the caisson, which will lead to an increase in the metal consumption of the structure and an increase in its cost and labor intensity of manufacture.

In addition, the polygonality of the ice-resistant caisson of the reservoir will cause it to warp under ice conditions.

The description of the solution also lacks information about the implementation of the regasification process, as well as measures to prevent the “rollover” phenomenon.

The technical problem to be solved by the present invention is to increase the reliability and safety of operation of an underwater LNG storage tank under water.

This problem is solved by the fact that the underwater storage of liquefied natural gas contains a double-walled body in the form of a hemisphere with a flat bottom with the formation of a sealed volume, fixed by means of submerged piles on the seabed soil with a gap between the bottom of the body and the seabed, in the lower part of the body there are branch pipes for supplying liquefied natural gas and a tray sump for pumping out sea water or liquefied natural gas, and an agitator and a cooling system are installed in the cavity of the housing, made in the form of a coil with a coil height increasing from the periphery of the housing to the center, communicating with the lines for supplying liquid air and draining it, the domed part of the housing communicates with the nitrogen supply line and the removal of the nitrogen-boiling gas mixture, on which an adjustable valve is installed, while the inner wall of the housing is made of cryogenic steel, lined on the outside with successive layers of thermal insulation and polyurethane foam, and the outer wall is made of reinforced concrete, lined outside with a layer of waterproofing in the form of a polymer coating.

The achieved technical result consists in a complex one-time provision of conditions for increasing the storage capacity of the storage under the influence of external hydrostatic pressure, to prevent the occurrence of a "rollover", to reduce the intensity of the formation of boil-off gas inside the tank and maintain cryogenic temperature, to prevent swelling of the soil and the formation of ice lenses, and also to create negative buoyancy of the structure.

The proposed technical solution is illustrated by drawings, where:

in fig. 1 shows a general view of the proposed underwater LNG storage facility;

in fig. 2 is a sectional view of an underwater LNG storage facility in accordance with the present invention;

in fig. 3 shows the process of submerging an underwater LNG storage facility under water and installing it using suction devices;

in fig. 4 shows an example of the design position of an underwater LNG storage facility with a slope towards the flume sump 1:100;

in fig. 5 is a plan view of an underwater LNG storage facility;

List of abbreviations used in the description of the claimed invention:

LW - liquid air;

LNG - liquefied natural gas.

The proposed underwater storage of liquefied natural gas contains a double-walled body 1 (see Fig. 1, 2) in the form of a hemisphere with a flat bottom 2 with the formation of a sealed volume. The underwater storage is fixed by means of submersible piles 3 on the soil of the seabed 4. Submersible piles 3, embedded in the eyes 5 (see Fig. 5) of the flat bottom 2 in the form of a tripod, are immersed in the soil using suction devices 6 (see Fig. 3 ) with a gap 7 between the bottom of the hull 2 and the seabed 4. In the lower part of the double-walled hull 1 there are branch pipes 8 for supplying LNG and a trough sump 9 for pumping out sea water or LNG. A stirrer 10 with a drive (not shown in the figure), a cooling system 11, shut-off valves (not shown in the figure), instrumentation (not shown in the figure), which must be remotely controlled and integrated into the supervisory control and data acquisition system. The cooling system 11 is made in the form of a coil with a height of coils that increase from the periphery of the housing to the center. This improves the efficiency of the cooling system 11, which communicates with the supply lines 12 of liquid air and its discharge 13. The domed part of the housing 14 communicates with the nitrogen supply line and the outlet 15 of the mixture of nitrogen with boil-off gas 16, on which an adjustable valve 17 is installed, connected by a transmitter pressure 18. Next to the underwater LNG storage there is a gas collector 19, installed with the help of piles 20 on the soil of the seabed 4. The inner wall of the body 21 is made of cryogenic steel, lined on the outside successively with layers of thermal insulation 22 and polyurethane foam 23 by means of pins or pins welded to the body (not shown in Fig.), and the outer wall 24 is made of reinforced concrete, lined on the outside with a layer of waterproofing 25 in the form of a polymer coating. The outer wall 24 is embedded in the annular chute of the flat bottom 2 and is a monolithic structure.

The wall thickness of reinforced concrete should be several times greater than the wall thickness of cryogenic steel. This is due to the fact that the hydrostatic pressure of water on the outer wall 24 is much greater than the excess pressure of the gas mixture 16 above the LNG surface and the hydrostatic pressure of the LNG column on the inner wall of the housing 21. For example, the thickness of the outer wall 24 can be from 20 mm to 300 mm in in accordance with the increase in the depth of the seabed up to 500 m. At the same time, the wall thickness of the inner wall 21 must also increase proportionally from 10 mm to 50 mm to store the bulk product with a volume of up to 60,000 m ₃ . The use of larger underwater LNG storage facilities complicates and increases the cost of construction, transportation and submersion. The exact dimensions and wall thicknesses of the structural elements of the underwater LNG storage facility are selected according to the calculation of the strength and stability of the shells under the joint action of the main loads. The level of reliability of the design of an underwater LNG storage facility and the safety factor are determined depending on its carrying capacity.

The process of immersion and operation of the proposed invention is as follows:

An underwater LNG storage facility is positively buoyant before sinking to the seabed, and in order to sink it underwater, the Archimedean force must be overcome. For this purpose, a controlled immersion is used by gradually pumping a certain, calculated volume of sea water into the LNG underwater storage facility to impart negative buoyancy to the structure. Diving should be carried out slowly in order to prevent mechanical damage to the underwater LNG storage when installed on the seabed. containers (not shown in the figure) attached to the storage for its descent.

An underwater LNG storage facility with submersible piles 3 embedded in lugs 5 in the form of a tripod is submerged and installed in places drilled before lowering to the seabed 4. At the same time, these piles 3 are immersed in the ground by pumping sea water out of them with suction devices 6 through pre-prepared holes in the lugs 5. Moreover, the piles are driven into the seabed 4 in such a way that a gap 7 remains between the flat bottom 2 and the seabed 4 and a slight negative slope is created towards the flume sump 9. Submersible piles 3 protect the structure from the effects of underwater currents and prevent ascent to the surface water areas during its emptying. The gap 7 between the flat bottom 2 and the seabed 4 is necessary to prevent swelling of the soil and the formation of ice lenses, which also simplifies the preparation of the surface of the seabed 4. The slope towards the flume sump 9 is needed to completely empty the underwater storage from sea water or LNG. The location of piles 3 and their number is chosen due to the greater stability of such basing. The dimensions of these submerged piles are chosen so that when the subsea LNG storage is completely emptied, the position of the system remains stable.

Next, the process of immersion and piping of the gas collector 19 with the domed part of the body 14 is carried out. The design of the gas collector 19 is a sealed high-pressure steel cylinder in which the gas mixture 16 is sequentially compressed and expanded when emptying and filling the underwater LNG storage. Accordingly, there is gaseous nitrogen inside, since, unlike air, it does not create an explosive mixture. The nitrogen gas is under such an overpressure that when all the bulk product leaves the underwater LNG storage, the gas overpressure in the gas collector 19 would become equal to zero. The line for supplying gaseous nitrogen and removing 15 a mixture of gaseous nitrogen with boil-off gas 16 serves to transport the gas mixture to the gas collector and back when filling and emptying the underwater LNG storage.

Then the underwater LNG storage is completely emptied of water by pumping it out. This is possible because the gaseous nitrogen from the gas collector 19 fills the interior of the double-walled LNG body 1 and thereby prevents the formation of a vacuum. When filling the underwater LNG storage with a stream of liquefied natural gas using a cryogenic centrifugal pump (not shown in the figure) through the supply pipeline (not shown in the figure) connected to the branch pipe 8, the gas mixture 16 is compressed above the LNG mirror. Liquefied natural gas as it fills the volume, like a piston, moves up and compresses the gas mixture. In this case, nitrogen is mixed with hydrocarbons and an explosion-proof gas mixture 16 is formed. As soon as the excess gas pressure reaches a predetermined value, which is recorded by pressure sensors 18 installed in the cavity of the housing 1, the valve 17 is opened, and the gas mixture 16 enters the gas collector 19, which has high bearing capacity. To prevent the occurrence of "rollover", a mixing device is used - a mixer 10 with a drive. To reduce the formation of boil-off gas inside the tank and maintain the cryogenic temperature, a cooling system 11 is used, through the pipes of which liquid air circulates in the cavity of the housing 1 and it comes from an underwater LNG plant (not shown in the figure). When the liquid air temperature rises above -163°C, it is sent to a special container (not shown in the figure) for further use as a refrigerant in the liquefaction of natural gas. Thermal insulation of the pipes of the liquid air supply line 12 and its discharge 13, leading to the underwater LNG storage tank and leaving it.

Monolithic reinforced concrete dome of the body 14 with a flat bottom 2 provides the bearing capacity of the storage when exposed to external hydrostatic pressure. The sealing of the internal space, which is created by a monolithic structure made of reinforced concrete, inside of which there is a layer of polyurethane foam 23, and outside it is lined with a layer of waterproofing 25 in the form of a polymer coating, prevents LNG and its vapors from entering the water area, protects reinforcement from corrosion, protects the underwater LNG storage from moisture through the pores of concrete, ensures the environmental safety of the operation of the structure. The use of pre-calculated layers of thermal insulation 22 reduces the rate of LNG evaporation.

A gas carrier (not shown in Fig.) arriving at an underwater LNG storage facility must contain a container with liquid nitrogen, which is pumped into the gas collector 19. Liquid nitrogen is converted into gaseous nitrogen under the action of heat flows, creating an overpressure inside.

The supply of LNG to an underwater tanker occurs under the overpressure of the gas mixture 16 from the gas collector 19, which enters the underwater LNG storage and displaces the LNG. Emptying from LNG is carried out through a tray sump 9.

Underwater LNG storage facilities of the proposed design can be used to store LNG underwater at different depths, taking into account the pre-calculated tank wall thickness.

The volume of storage tanks should correspond, at a minimum, to the displacement of a shuttle tanker intended for the export of accumulated products. In turn, during the period of absence of the tanker in the underwater park, an appropriate amount of LNG must be accumulated in order for the tanker to be fully loaded.

The proposed reservoir can be built as a single one or as part of a group of similar reservoirs, forming an underwater LNG tank farm (in the case of a large field).

Thus, the proposed design of an underwater LNG storage facility provides an increase in the reliability and safety of operation of an underwater LNG storage tank under water.

Claims (1)

Underwater storage of liquefied natural gas, characterized in that it contains a double-walled body in the form of a hemisphere with a flat bottom with the formation of a sealed volume, fixed by means of submerged piles on the seabed soil with a gap between the bottom of the body and the seabed, in the lower part of the body there are nozzles for supply liquefied natural gas and a trough sump for pumping out sea water or liquefied natural gas, and an agitator and a cooling system are installed in the cavity of the housing, made in the form of a coil with a coil height increasing from the periphery of the housing to the center, and communicating with the lines for supplying liquid air and draining it , the domed part of the housing communicates with the line for supplying nitrogen and removing the mixture of nitrogen with boil-off gas, on which an adjustable valve is installed, while the inner wall of the housing is made of cryogenic steel, lined on the outside with layers of thermal insulation and polyurethane foam, and the outer wall is made of reinforced concrete, lined withoutside with a layer of waterproofing in the form of a polymer coating.

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