JP4316638B2 - Liquefied natural gas carrier and sea transportation method of liquefied natural gas - Google Patents

Description

  This invention relates to marine transportation of liquefied natural gas (LNG).

Since LNG emits less nitrogen oxide and sulfurous acid gas during combustion, demand for LNG is increasing year by year as clean energy. LNG is a natural gas cooled to about -160 ° C and liquefied, and the tank of the LNG carrier that transports this gas at sea uses low-temperature materials to withstand a wide range of temperature changes. It has a structure that takes stress into consideration. Further, since the LNG carrier is responsible for mission speed and mass transportation, 20 is normal to and a voyage speed before and after the knots, the hull will tend to increase in size, which exceeds the tank capacity 200,000M ³ NOW Is planned.

  In a conventional LNG carrier, LNG tanks mounted on the LNG carrier are roughly divided into two types, one is a spherical independent tank system, and the other is a membrane system (for example, US Pat. Nos. 5,697,312 and 7,137,345).

  In the spherical independent tank system, a spherical tank made of an aluminum alloy is installed in the hold through a skirt-like support structure extending downward from its equator. In this tank, the weight of the liquid loaded therein and the dynamic force acting on the liquid loaded by the ship's sway are all carried by the tank itself and transmitted to the hull via the skirt. The heat insulating material for the tank is, of course, provided on the outer surface of the tank.

  Since the shape of the hold is almost box-shaped, if a spherical tank is housed there, it is inevitable that a useless space is created around the spherical tank. For this reason, the spherical independent tank system has a drawback that the tank volume is small for the size of the hull.

  On the other hand, the membrane tank is provided with a heat insulating material inside the double hull structure of the hull, and its surface is liquid-tightly covered with a membrane. In this type of tank, the liquid pressure of LNG is transmitted to the hull structure via a heat insulating material. The membrane is made of stainless steel or a nickel alloy (Invar) with a small thermal expansion coefficient.

  Since this membrane-type tank can be made in a shape along the hold, the tank space can be increased and the volumetric efficiency is good. However, since the tank has an irregular shape in the thin part of the front part of the hull, there is a drawback that it takes time for construction work. In addition, the shape of the membrane tank is basically a square, and it is difficult to make the shape far away from it. For this reason, the shape of the hull of the portion where the foremost tank is accommodated has a U-shaped cross section at most, and in order to reduce the wave-making resistance at high speed, it cannot be made a very thin shape such as a V shape.

  Membrane-type tanks are called sloshing when they encounter stormy weather when the load is half-loaded, and the liquid load in the tank swells violently due to the shaking of the hull, and the impact damages the membrane and the heat insulating material. Sometimes. In a spherical tank, the tank wall is curved, so that it can receive impacts, and since the heat shield is outside the tank, sloshing is hardly a problem. Therefore, with respect to the membrane tank, it is required to keep the tank at a full load or close to it so that the LNG of the cargo does not wave.

  In both sphere and membrane types, heat input to the tank is inevitable even if heat protection is provided in the LNG tank, so boil-off gas (BOG) is generated from the load LNG, and the tank internal pressure gradually increases. The liquid temperature of LNG also rises. Therefore, in the LNG ship, the generated boil-off gas is sucked by a compressor and sent to a combustion device such as a boiler for combustion. The energy generated by the combustion is used for ship propulsion and the like. The boil-off rate per day varies depending on the thermal insulation performance of the tank, but is generally in the range of 0.10 to 0.25%. As described above, since boil-off occurs in the LNG ship, the amount of liquid loaded in the tank gradually decreases as the number of days passes. For this reason, in a membrane tank, sloshing tends to occur as the number of days elapses after leaving the loading area.

  It is an object of the present invention to provide an LNG tanker that is excellent in tank volumetric efficiency and easy to construct. Another object is to reduce the sloshing of the membrane tank.

  The present invention provides an LNG carrier in which a tank for loading LNG is composed of a spherical independent tank disposed in the forefront and a plurality of subsequent membrane tanks. In this case, most of the LNG tank is constituted by a membrane tank, so that the volumetric efficiency is excellent and a large tank volume can be obtained for the size of the ship.

  Since the foremost tank is located in the thin part of the front part of the hull, when this is used as a membrane tank, the tank shape becomes irregular, which makes the construction of heat insulation and membrane very troublesome. In this invention, the foremost tank is not a membrane but a spherical independent tank, so it is only necessary to assemble the spherical tank on the ground and mount it on the ship, making the construction work extremely easy and shortening the construction period. Can do. Further, in the case of a spherical tank, the hull cross-sectional shape can be accommodated in a thin shape like a V-shape, so that it is possible to make the bow portion fine and to have a shape suitable for high speed.

  Further, according to the present invention, the tank for loading LNG is composed of two types of tanks, a membrane tank and a spherical independent tank, and the LNG in the independent tank is transferred to the membrane tank during transportation. A method for sea transport of LNG is provided. According to this method, the amount of liquid lost as boil-off gas during transportation can be compensated, and the membrane tank can be held in a full state, and the membrane tank can be released from the sloshing problem.

  As shown in FIGS. 1 and 2, this LNG tanker is connected in order of the bow portion 10, the tank compartment 12, the engine room 14, and the stern part 16 from the front. A chamber 20 is provided. The tank compartment 12 is divided into a plurality of compartments by a horizontal partition wall 22, a spherical independent tank 24 is accommodated in the foremost compartment 23, and a membrane tank 26 is provided in each of the second to fifth compartments. Is formed.

  FIG. 3 is a cross-sectional view of the membrane tank. The hull has a double hull structure on the bottom and the ship side. Heat insulation 30 is applied to the surface of the inner shell 28, and the heat insulation surface is liquid-tight with the membrane 32. The membrane tank 26 is formed by covering with. Each membrane tank 26 has a head protruding upward through the upper deck 34 to increase the tank capacity, and the tank head is covered with a trunk deck 36.

  FIG. 4 is a cross-sectional view of the foremost compartment with a spherical independent tank. The spherical tank 24 is assembled on the ground with a skirt 40 extending downward from its equator, and incorporated into the ship. The lower end of the skirt 40 is supported by the hull double bottom. The upper part of the spherical tank 24 protrudes above the upper deck 34, and this protruding part is covered with a dome-shaped tank cover 42.

  For comparison with FIG. 4, FIG. 5 shows a case where a membrane tank is provided in the foremost tank section 23 instead of a spherical independent tank. Since the hull of the foremost tank section is thin, the membrane tank 26 formed therein becomes narrower as it goes forward as indicated by the chain line 44. For this reason, the tank has an irregular shape, and the construction is complicated and time-consuming. On the other hand, since the spherical tank 24 can be assembled on the ground and incorporated into the hull, the construction is easier and the construction period is shorter than the case where a membrane tank having a complicated structure is made by inboard work.

  Further, as can be seen from FIG. 5, the membrane tank is structurally nearly square, and the cross section of the hull that accommodates the membrane tank cannot have a very thin (fine) shape. On the other hand, in FIG. 4, since the tank is spherical and the tank is at a slightly higher position, the hull cross-sectional shape can be made into a V shape and can be accommodated in a very thin ship shape. For this reason, when the foremost tank is not a membrane tank but a spherical independent tank, it is possible to reduce the propulsion resistance of the ship in a high speed range by making the bow part fine.

  This LNG tanker is full of LNG in all tanks regardless of the type of tank in the loading area. During voyage toward the landing site, heat enters the LNG tanks from the outside, so boil-off gas is continuously generated and accumulated in the tanks. If this gas is left unattended, the temperature and internal pressure in the tank gradually increase, which is dangerous. Therefore, in the conventional LNG tanker, as shown in FIG. 6, the boil-off gas (BOG) accumulated in each tank is sucked out of the tank by the compressor 46 to keep the temperature and pressure in each LNG tank appropriately. To. The natural gas sucked out of the tank by the compressor is heated and then burned by the boiler 48. The steam generated in the boiler drives the turbine and is used as ship propulsion and ship power.

  Thus, the BOG generated in each tank is discharged to the outside of the tank, so that the amount of liquid in each tank is gradually reduced. Over the long voyage, the decrease in liquid volume becomes significant, and sloshing is a concern during stormy weather in membrane tanks. In the LNG tanker of the present invention, the liquid load (LNG) in the spherical independent tank 24 is sent out by a pump 50 with a tank and transferred to another tank through a pipe 52, so that the liquid amount in the membrane tank 26 is almost full. Or let me recover to a state close to that. By doing so, even when stormy weather is encountered, sloshing is unlikely to occur, and damage to the membrane tank due to sloshing can be prevented. On the other hand, the amount of liquid in the spherical independent tank 24 is gradually reduced by the transfer, but as described above, the sloshing problem does not occur in the spherical independent tank.

It is a side view of the LNG tanker by this invention. It is a top view of the LNG tanker shown in FIG. It is sectional drawing of the membrane-type tank part cut | disconnected by the 3-3 line of FIG. It is sectional drawing of the spherical tank part cut | disconnected by the 4-4 line | wire of FIG. It is a cross-sectional view of the foremost tank in the conventional membrane type LNG tanker. It is a piping diagram for performing the liquid load transfer between tanks in the LNG tanker shown in FIG.

Explanation of symbols

22 Foremost tank compartment 24 Spherical independent tank 26 Membrane tank

Claims (2)

  A liquefied natural gas carrier ship in which a tank for loading liquefied natural gas is composed of a spherical independent tank disposed in the forefront and a plurality of membrane tanks following the tank.   The tank for loading liquefied natural gas is composed of two types of tanks, a membrane tank and a spherical independent tank. The spherical independent tank is made up to compensate for the liquid volume of the membrane tank lost as boil-off gas during transportation. A method of transporting liquefied natural gas by sea, characterized in that the liquefied natural gas is transferred to the membrane tank.

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