KR20140031888A - Sloshing preventing device and sloshing preventing method - Google Patents

Abstract

The sloshing prevention device 20 of a simple or simple structure prevents the sloshing phenomenon in the liquid storage tank 10 of the membrane type of the liquid cargo transport ship or the floating marine facility 1. The anti-sloshing device comprises a plurality of floats 24 arranged in series in the longitudinal or transverse direction of the transport ship or facility; And a vertical pawl 23 that supports the float and guides the float in the vertical direction, resisting horizontal external forces acting on the float. The float has a weight to ensure adequate draft depth D. The float floats on the free liquid level LL in the tank for dividing the liquid near the free liquid surface and the free liquid surface, as a result of which the natural frequency of the liquid vibration in the tank moves in the high frequency band.

Description

SLOSHING PREVENTING DEVICE AND SLOSHING PREVENTING METHOD}

The present invention relates to a sloshing prevention device and a sloshing prevention method, and more particularly, a membrane type liquid storage tank that prevents sloshing from occurring in a tank of a liquid cargo ship or a floating marine facility. An apparatus and a method for preventing sloshing for the same.

Long-distance sea transport of liquefied natural gas Liquefied natural gas carriers (hereinafter referred to as "LNG ships") are known. Liquid natural gas, ie liquefied natural gas, produced by concentration of gas into liquid at cryogenic temperatures (-162 ° C.) is significantly reduced in volume compared to natural gas in the gas phase, and therefore liquefied natural gas can be transported efficiently. . LNG carriers have LNG storage tanks that have a specialized configuration to withstand such cryogenic temperatures (-162 ° C). As a conventional LNG storage tank method, a spherical tank method, a rectangular membrane method, a square SPB method and the like are known. Currently, LNG storage tanks of the spherical tank type and the square membrane type are mainstream.

The LNG tank structure of the old tank type is advantageous in structural strength, but due to internal volume efficiency, the hull size tends to be enlarged. Membrane-type LNG storage tanks, on the other hand, are advantageous in terms of construction costs, route options, etc., since the hull size can be reduced for specific transport capacities with equal load capacity. Therefore, along with the trend of larger LNG storage tanks with increasing LNG demand and transport volume, membrane tank type LNG storage tanks tend to be used in LNG ship design in recent years.

As a weak point of membrane tank type LNG storage tank, the sloshing phenomenon of the liquid in the tank which occurs in partial load state is known. Sloshing is a phenomenon in which a liquid cargo vibrates violently due to excitation due to the movement of the tank. The sloshing phenomenon leads to a problem of excessive impact pressure acting on the inner wall of the tank, and causes problems such as wave load on the support structure of the tank, influence on the movement of the transport ship, splashing or splashing of liquid, and the like. Meanwhile, as a natural gas production facility that can replace a fixed platform, a floating LNG (FLNG) facility, such as a floating production, storage and offloading system, has been attracting attention recently. . The FPSO facility receives natural gas from the borehole at sea, liquefies after gas separation and pretreatment, and stores and ships it as LNG. Floating facilities in FPSO installations are anchored at sea and, therefore, unlike normal ships, no adverse weather evacuation measures can be taken. In addition, a partial load condition of the LNG storage tank necessarily occurs during the LNG production process and during the LNG transfer to the ship. Therefore, in the FPSO floating facility equipped with the LNG storage tank of the membrane type, sloshing phenomenon is particularly concerned.

Japanese Patent Laid-Open No. 2009-18608 (Patent Document 1) discloses an LNG carrier having both an independent spherical tank and a membrane type tank. Japanese Patent Laid-Open No. 2011-505298 (Patent Document 2) discloses a sloshing prevention technique relating to a large LNG carrier having a membrane type LNG storage tank, wherein the LNG storage tank is divided by a bulkhead. The LNG vessel disclosed in Patent Document 1 attempts to properly transfer liquefied natural gas in a spherical tank to a membrane type tank to keep the membrane type tank in a fully loaded condition to prevent sloshing from occurring. will be. The LNG storage tank disclosed in Patent Document 2 is intended to prevent the occurrence of a sloshing phenomenon by completely dividing an area in the tank to reduce the volume of the individual internal space.

This sloshing phenomenon also occurs in ballast water in ballast tanks. Japanese Patent Laid-Open No. 53-44237 (Patent Document 3) discloses a technique for preventing the sloshing phenomenon of ballast water, and the sloshing preventing device is a floating type that divides the free liquid surface of the ballast water. With a bulkhead, a reduced area of free liquid level is formed on each side of the bulkhead. Further, Japanese Utility Model Publication No. 53-1792 (Patent Document 4) discloses a sloshing prevention device configured to greatly reduce the area of a free liquid surface by a large number of flat floating bodies floating in the free liquid surface of ballast water. It is starting.

Patent Document 1: Japanese Patent Publication No. 2009-18608 Patent Document 2: Japanese Patent Laid-Open No. 2011-505298 Patent Document 3: Japanese Patent Laid-Open No. 53-44237 Patent Document 4: Japanese Utility Model Publication No. 53-1792

The LNG vessel disclosed in Patent Document 1 is configured to properly supply the liquid in the spherical tank into the membrane type tank when the liquid amount of the membrane type tank is reduced, so that the filled state of the membrane type tank is always maintained. Therefore, the LNG carrier of patent document 1 must use a heterogeneous tank at all times, and since the facilities which transfer the liquefied natural gas between an old tank and a membrane type tank are installed in a hull, the structure of an LNG carrier becomes complicated in general. .

The sloshing prevention device disclosed in Patent Document 2 is configured to completely separate the space in the membrane type tank by the bulkhead. However, the inner surface of the tank is covered with a thin metal alloy with a thickness of 1 mm or less. When considering the structural stability, strength and durability of freestanding or upright bulkheads ranging from 25m to 40m in height, the joining structure to the bulkhead and the tank inner surface, and the solid support structure to support the bulkhead, Dividing the tank of the membrane type is accompanied by disadvantages of construction cost, complexity of the hull structure, difficulty in the design and drying of the hull and the like.

The floating bulkhead disclosed in Patent Document 3 has steel members fixed to the ballast tanks for guiding and maintaining the bulkhead, so that the free liquid level of the ballast number is divided by the bulkhead, and thereby the angle of the bulkhead The free liquid level on the side has a reduced area. This configuration relates to the prevention of sloshing of the ballast tanks. However, the inner surface of the tank of the membrane type is formed by a thin metal alloy having a thickness of 1 mm or less, so that the inner surface of the tank does not have sufficient strength to support the bulkhead, and it is highly desirable to install such steel on the inner surface of the tank. It is difficult. In addition, both ends of the bulkhead are movably supported vertically by the inner wall surface of the tank, and the bulkhead extends over the entire length or the entire width of the tank. Therefore, in the case of a large tank of large capacity, it is difficult to secure the strength and durability of the bulkhead because the distance between the bulkhead's fulcrums (the overall length of the bulkhead) is increased. In addition, when sloshing occurs, the level difference of the free liquid level also occurs in the longitudinal direction of the bulkhead, so the bulkhead must be designed to have a height higher than the level difference. In addition, this liquid level difference can cause the bulkhead to be generally inclined. Thus, the steel for guiding and retaining may obstruct or restrain the bulkhead, resulting in the bulkhead being unable to move freely vertically.

The sloshing prevention means disclosed in Patent Document 4 is configured to generally suppress and hinder the operation of the free liquid level of the ballast water by a plurality of plate floats, so that the tank has many guide members for supporting many plate floats. Must be installed in This configuration may be applicable with regard to the anti-sloshing of ballast tanks. However, a membrane type tank for transporting liquefied natural gas is difficult to install many guide members and supporting structures because the inner surface of the tank is formed by a thin metal alloy having a thickness of 1 mm or less. In addition, the sloshing prevention means disclosed in Patent Document 4 can only reduce the area of free water surface, and cannot directly regulate or control the behavior or vibration of water under the water surface.

It is an object of the present invention to provide a simple or simple structure of an anti-sloshing device which effectively prevents the sloshing phenomenon of liquid stored in a membrane type liquid storage tank.

It is also an object of the present invention to provide a sloshing prevention method that can effectively prevent the sloshing phenomenon of the liquid stored in the liquid storage tank of the membrane type through a simple or simple configuration.

In order to achieve the above object, the present invention provides a sloshing prevention device which is installed in a liquid cargo tank or a membrane type liquid storage tank of a floating marine facility and prevents a sloshing phenomenon occurring in the tank. Or a plurality of floats arranged in series in the longitudinal or transverse direction of the facility; And a plurality of vertical poles for supporting the float and guiding the float in a vertical direction, resisting a horizontal external force acting on the float, wherein each float maintains a predetermined draft depth. Having a weight for floating on a free liquid surface within the float, the float floating on the free liquid surface for dividing the free liquid surface and the liquid in the vicinity of the free liquid surface, and the liquid on each side of the float floating on the free liquid surface. An anti-slogging device is provided which is continuous with each other through the lower area.

In addition, the present invention is a sloshing prevention method for preventing sloshing phenomenon occurring in a liquid cargo tank or a liquid storage tank of the membrane type of a floating marine facility, in the longitudinal or transverse direction of the transport vessel or facility in series Arranging a plurality of floats, the floats being supported against a horizontal external force acting on the floats and vertically moved in response to a change in liquid level; The liquid on each side of the float is divided so that the liquid on each side of the float is contiguous with each other through the lower region of the float, so that each float floats on the float level of the float while the liquid remains on the free liquid level. It includes a step of floating, thereby providing a sloshing prevention method for moving the natural frequency of the liquid vibration in the tank to a high frequency band to prevent the occurrence of sloshing phenomenon.

According to the above arrangement of the present invention, the liquid stored in the tank is divided only by the liquid level and the liquid near the liquid level by the floating body, so that the liquid in the tank is totally continuous in the lower region of the floating body. Each float moves vertically and independently in response to the vertical movement of the liquid surface. The liquid vibration in the tank is damped by the vertical movement of the float, and the natural frequency of the liquid vibration moves in the high frequency band by the free liquid level division. According to the present invention, this movement of the natural frequency prevents liquid vibrations in the tank from synchronizing with ocean waves and hull motions, thereby preventing sloshing from occurring or suppressing sloshing. In addition, although the float divides the space in the tank in the form of a U-tube, according to the inventors' studies, no harmful U-tube vibration occurs. The rows of floats are arranged in the longitudinal direction of the hull (front and rear, longitudinal or rolling axial direction) of the hull to prevent sloshing due to the rolling motion of the hull, while It is arranged in the transverse direction of the hull (horizontal direction, width direction or pitching axial direction) to prevent sloshing.

According to the inventor's research, the anti-sloshing effect obtained by dividing the liquid level and the liquid below the liquid level is substantially the same as or equivalent to the anti-sloshing effect obtained by dividing all the liquid in the tank by the bulkhead. Turned out. That is, according to the present invention, the liquid in the tank is not divided entirely by the bulkhead, and the structural stability, strength and durability of the freestanding or upright bulkhead, the joining structure between the bulkhead and the tank inner surface, and the bulkhead are supported. It is possible to provide a sloshing prevention mechanism in the tank by floating the heat of the float in the tank without considering the problem of a rigid support structure or the like. In addition, in the present invention, the use of heterogeneous tanks is not required, and the transfer of LNG between tanks is not required. According to the present invention, only the free liquid surface and the liquid near the liquid surface are divided by the heat of the floating body, and it is not necessary to suppress and limit the behavior of the free liquid surface over a wide area. Therefore, the sloshing prevention device according to the present invention can effectively prevent the sloshing phenomenon of the membrane type liquid storage tank by the sloshing prevention mechanism having a simple or simple structure.

Further, according to the present invention, the liquid level is divided by a plurality of floats, and the horizontal distance between the fulcrums of the float is greatly shortened. Therefore, the strength of the floating body can be secured relatively easily. In addition, since the level difference of the free liquid level caused along the row of the float in the float column direction can be almost compensated by the difference in the level of the float, the height of the float can be reduced.

According to the sloshing prevention device of the present invention, the occurrence of sloshing phenomenon of the liquid stored in the liquid storage tank of the membrane type can be effectively prevented by a simple or simple structure.

According to the sloshing prevention method of the present invention, the occurrence of liquid sloshing phenomenon stored in the membrane type liquid storage tank can be effectively prevented through a simple or simple configuration.

1 is a longitudinal sectional view schematically showing an LNG carrier having a sloshing prevention device according to an embodiment of the present invention.
FIG. 2A is a cross-sectional view of the LNG storage tank of the II line of FIG. 1, and FIG. 2B is a cross-sectional view showing the limitation of the loading range applied to the LNG storage tank.
3 is a plan view and a partially enlarged plan view showing the configuration of an LNG storage tank having a square or rectangular cross section with an anti-sloshing device.
4A is a cross-sectional view of the II-II line of FIG. 3, and FIG. 4B is a cross-sectional view of the III-III line of FIG.
5 is a schematic cross-sectional view showing the structure and cross-sectional shape of the floating body.
FIG. 6 is a graph showing the relationship between the excitation frequency of waves causing the vibration in the transverse direction (the width direction of the hull) and the maximum wavelength width per degree of rolling angle occurring in the LNG storage tank. FIG. The numerical analysis results, in which the level ratio is estimated at 63%, are shown in FIG.
Fig. 7 is a graph showing the relationship between the wave frequency causing the horizontal vibration and the maximum wavelength width per degree of rolling angle occurring in the tank, wherein the ratio of liquid level in the tank is estimated to be 30%. Is shown in FIG. 7.
8 is a graph showing the relationship between the primary natural frequency of liquid motion in an LNG storage tank and the liquid level in the tank.
9 is a sectional view showing an example of a method for liquid level division by a float.
10 is a graph showing the change in natural frequency associated with the difference in free face number.
FIG. 11 is a graph for explaining the anti-sloshing effect of the float shown in FIG. 9A.
12 is a graph showing the relationship between the maximum wavelength width and the height of the vibration center.

 Preferably, the floats are arranged spaced apart from each other, and a gap in which liquid is flowable is formed between adjacent floats. In addition, a gap is formed between the float and the inner wall of the tank to allow liquid to flow. The flow of liquid in the gap acts to damp the vibrations of the liquid in the tank. Such a damping effect allows the occurrence of sloshing to be more effectively prevented.

Preferably, the immersed depth of the float is set to at least H × 0.05, preferably at least H × 0.1, or the distance between the bottom of the tank and the bottom of the float is h × 0.5 when h is H × 0.5. Is set to 0.80 or less, where "H" represents the overall height of the tank and "h" represents the liquid height. For example, when h is in the range of H × 0.3 to H × 0.7 (or in the range of H × 0.4 to H × 0.6), the distance between the bottom of the tank and the bottom of the float is set to h × 0.80 or less, The draft depth of the floating body is set to h × 0.20 or more.

According to a preferred embodiment of the present invention, a vertical pole extends through the float. Upper and lower base portions supporting the upper and lower ends of the pawl are fixed to the tank ceiling and bottom surfaces. The base portions define the range of vertical movement of the float and prevent the float from colliding with the ceiling or bottom of the tank. In addition, the bases shorten the distance between the supporting points of the pole to a distance less than the overall height of the inner region of the tank, thereby improving the strength and rigidity of the pole. Preferably, the distance between the lower surface of the upper base portion and the ceiling surface that suppresses the rise of the floating body is set to be H × 0.3 or less. More preferably, the distance between the upper surface of the lower base portion and the bottom surface of the tank which suppresses the drop of the floating body is set to be H × 0.1 or less.

Preferably, the free liquid surface of the liquid is equally divided in the transverse direction of the hull (the width direction of the hull) by the floating bodies arranged in the front-rear direction (the longitudinal direction of the hull). Each floating body is spaced apart from each other in the front-rear direction of the hull and is vertically movable by a plurality of vertical poles. For example, the floats are arranged in alignment on the center line of the tank extending in the front-rear direction of the hull, or arranged in substantially parallel rows.

In a preferred embodiment of the present invention, the float consists of a hollow polyhedron consisting of a vertical plane and a horizontal plane, provided with an internal cavity for securing buoyancy. Preferably, the float has a divider part extending vertically and side projections extending laterally from the divider part. The lateral protrusions function to dampen liquid vibrations and to suppress the vertical movement of the float itself.

Optionally, the float has buoyancy adjustment means for adjusting the draft depth of the float. Preferably, the buoyancy adjusting means is a buoyancy reducing means for introducing the liquid in the tank into the inner cavity of the float, or a weight adjusting means for adjusting the weight of the float. It is possible to provide a buoyancy control means which can variably control the draft depth in relation to the liquid level in the tank.

In a preferred embodiment of the invention, the cross section of the tank cut by the vertical cut plane is rectangular or square. Generally, such a cross-section tank is disadvantageous compared to an octagonal cross-section tank in view of anti-sloshing. Therefore, tanks of octagonal cross section have been generally used in spite of the fact that volume efficiency is poor in the past. However, both the improvement of the anti-slogging function and the improvement of the volume efficiency can be achieved by adopting the anti-sloshing mechanism of the above configuration in the tank having a rectangular or square cross section.

Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the accompanying drawings.

1 is a longitudinal cross-sectional view schematically showing the overall configuration of an LNG carrier (liquefied natural gas transport ship).

An LNG carrier with an anti-sloshing device 20 according to an embodiment of the invention is shown in FIG. 1. The LNG carrier 1 has a bow 2, a tank compartment 3, an engine room 4 and a stern 5. In the upper part of the engine room 4, a living area 6 and a steering chamber 7 are arranged. The tank partition area 3 is divided into compartments by a bulkhead 8 extending in the front-rear direction (width direction of the hull). In each compartment, the LNG storage tank 10 of the membrane system provided with the sloshing prevention apparatus 20 is arrange | positioned. The LNG carrier 1 shown in FIG. 1 can also be considered as LNG-FPSO. In this case, the LNG carrier 1 is moored at the sea level WL in the fixed position.

FIG. 2A is a cross sectional view of the LNG storage tank 10 taken along the I-I line of FIG. 1, the hull being shown by an imaginary line (dotted dashed line).

The LNG storage tank 10 (hereinafter referred to as "tank 10") has a structure in which the surface of the heat radiation material 11 mounted inside the hull is completely covered with a metal thin film membrane 12 having a thickness of 1 mm or less. The cross section of the tank 10 cut | disconnected by the vertical cut surface (I-I line) of the ship body direction is octagonal. As the heat dissipating material 11, boxes made of plywood, each of which includes foamed pearlite, polyurethane-based insulating material and the like, are generally used. As the metal thin film membrane 12, an invar alloy material (36% nickel) having a thickness of about 0.7 mm, a SUS3041 membrane, or the like is generally used. Tank 10 constitutes a large membrane type LNG storage tank having a width of 30 to 40m.

FIG. 2B is a cross sectional view showing a loading limit applied to such a membrane type LNG storage tank. FIG.

The tank 10 is provided with an LNG accommodating region 15 capable of accommodating liquefied natural gas (LNG). The free liquid level LL of the liquefied natural gas can be set at any level in the range of the total height H of the LNG receiving region 15 from the spatial viewpoint inside the tank. However, when a sloshing phenomenon occurs in the liquid (liquefied natural gas) in the LNG receiving region 15, an excessively high liquid pressure acts on the metal thin film membrane 12 by violent collision of the liquid, and as a result, the tank 10 Destruction of the structure may be caused by excessive liquid pressure. For example, it is known that the heat dissipation material 11 made of a wooden cold box collapses due to the liquid pressure when sloshing occurs, so that the metal thin film membrane 12 is broken. When the cryogenic liquefied natural gas flows out of the ship due to breakage or damage of the metal thin film membrane 12, the liquid vaporizes immediately. This can lead to a fire accident. Therefore, this situation needs to be prevented.

Under these circumstances, the loading restrictions of membrane type LNG storage tanks are prescribed by the Society Rules, etc., and the liquid level LL is limited to the ranges k1 and k3 corresponding to the height h1 or h3, and the liquid level LL is of the height h2. Partial loading conditions that appear in the range (range k2) are not allowed. Regulations such as the Rules of Classification Society may be changed in the future according to the revision of the rules, but according to the loading conditions prescribed by the Rules of Classification Society, height h1 is the total height H × 0.1, and height h1 + h2 is the total height H × 0.7 Should be That is, the loadable height range of the membrane type LNG storage tank is limited to the range k3 of height H × 0.7 or more, or the range k1 of height H × 0.1 or less. However, in the production process or the transport process of the LNG-FPSO at sea, the partial loading state in which the liquid level LL is located in the range of the height h2 (that is, the range k2 of Hx0.1 to Hx0.7) necessarily occurs. In addition, with regard to large LNG carriers, so-called two ports loading, ie, long-distance sea transport forms carrying large quantities of liquefied natural gas, in which liquefied natural gas is continuously loaded at several ports, may be requested or required. In such a mode of transport, a partial load condition with liquid level LL in the range k2 of height h2 may occur excessively. However, since sloshing can occur due to partial load conditions, the partial loading conditions of LNG-FPSO are difficult to allow in the production process or the transfer process. In addition, the LNG ship transport mode, in which the partial load state of the LNG ship is caused by two port loads or the like, is not allowed under the above limitation on the loaded state.

The LNG carrier 1 of this embodiment is equipped with the sloshing prevention apparatus 20 which prevents the sloshing which arises in the partial loading state. Sloshing is a type of vibration phenomenon and is caused by resonance due to the synchronization of the natural frequency of liquid motion (vibration of liquefied natural gas) in the tank 10 with the frequency (vibration frequency) of the ocean blue wave vibrating the tank 10. do. Such synchronization occurs when the natural frequency of the liquid motion in the tank 10 is synchronized with the natural frequency of the rolling motion of the hull, and therefore care needs to be taken for this synchronization.

As shown in FIG. 2A, the anti-sloshing device 20 comprises a pair of top base portion 21 and bottom base portion 22, a vertical pole or column 23, and a float 24. The pawl 23 extends vertically between the base portions 21, 22. The lower base portion 21 is provided in an upright position on the bottom surface 13 of the tank, and the upper base portion 22 is suspended on the ceiling surface 14 of the tank. The pawl 23 constitutes guide means for guiding the float 24 vertically. The bases 21, 22 constitute a stopper or vertical motion limiting means that define the float 24 vertical movement range. The base portions 21, 22 prevent the float 24 from colliding with the bottom surface 13 or the ceiling surface 14. In addition, the base portions 21 and 22 function as rigid supports of the poles which firmly fix the lower end and the upper end of the pawl 23 to the bottom surface 13 or the ceiling surface 14, respectively. The distance j2 between the supporting points of the pawl 23 is determined by the setting of the heights j1, j3 of the base portions 21, 22, and the stiffness and strength of the pawl 23 is determined by the distance j2. Directly related. At the time of sloshing, a relatively large liquid pressure acts on the floating body 24 with a horizontal external force, and therefore a relatively large horizontal load acts on the pawl 23. However, the heights j1, j3 of the base parts 21, 22 can be increased to reduce the distance j2 between the support points, thereby improving the stiffness and strength of the pawl 23. .

The height j1 is substantially equal to the distance between the bottom face 13 of the tank and the bottom or bottom face of the float 24 in the lowest position. The height j3 is substantially equal to the distance between the ceiling surface 14 of the tank and the top or top surface of the float 24 in the maximum raised position. The heights j1, j2, j3 correspond to the heights h1, h2, h3 and the ranges k1, k2, k3, respectively. For example, the heights j1, j2, j3 are set to substantially the same value as the heights h1, h2, h3. Optionally, the dimensions are set to j1 ≦ h1 and j3 ≦ h3 to ensure a range of sufficient vertical movement of the float 24.

3 is a plan view and a partially enlarged plan view conceptually showing the configuration of the tank 10, and FIG. 4 includes cross-sectional views of lines II-II and III-III of FIG. 3, wherein the tank 10 has a rectangular or square cross section. (Cross section of line II-II).

In general, LNG storage tanks are designed with an octagonal cross-section, in which the width of the bottom and top areas is gradually reduced, mainly taking into account anti-sloshing protection. However, as shown in FIGS. 3 and 4, the tank 10 has a rectangular or square cross section cut along a vertical cutting plane extending in the transverse direction. According to the anti-sloshing effect of the anti-sloshing device 20, the octagonal cross section is not necessarily adopted as the cross section of the LNG storage tank, and a rectangular or square cross section may be adopted as the cross section of the tank. When designing tanks with equal heights and widths, tanks of rectangular or square cross section are advantageous for improved volume efficiency compared to tanks of octagonal cross section.

As shown in Figs. 3 and 4, each tank 10 is spaced apart from each other a plurality of floats 24 (in this example, three) arranged in series in the front and rear direction of the hull (the longitudinal direction of the hull) Floating body). The liquid level LL is divided into two parts arranged in the transverse direction by the floating body 24. A gap or gap 25 is formed between the adjacent floating bodies 24 so that liquefied natural gas can flow. In addition, a gap or gap 26 is formed between the floating body 24 and the tank inner wall surface 16, so that liquefied natural gas can flow. For example, the ratio (ie, G / E) of the width G and the total length E of the floating body 24 is set to 1/100 to 1/10.

The vertical pawls 23 are arranged in alignment with the centerline X-X of the tank 10 extending in the longitudinal or longitudinal direction of the hull. Each float 24 is vertically movably supported by a plurality of vertical poles 23 (in this embodiment, a pair of poles 23) extending through the float 24. Each float 24 is a metallic hollow member having a hermetically sealed and hermetic structure which always floats on the liquid level LL by the buoyancy of the float 24. The draft depth D of the float 24 depends on the weight and buoyancy of the float 24. For example, in a case where it is difficult to adequately secure the draft depth D of the floating body 24, as a means for adjusting the buoyancy of the floating body, holes or openings for introducing the liquid LNG into the floating body are not provided. It may be formed at the bottom of the fluid, or otherwise may allow a relatively high specific gravity of a large number of liquid or solid materials or the like to be additionally received in the float 24.

5 is a sectional view and a perspective view schematically showing the structure of the floating body 24. 5A and 5B show a float 24 having an inverted T-shaped cross section as shown in FIGS. In FIG. 5C there is shown a float 24 having an I-shaped cross section without side projections. Also shown in FIG. 5D is a float 24 having a cross section, while FIG. 5E shows a float 24 having a modified inverted T-shaped cross section. Also shown in FIG. 5F is a float 24 having an inverted Y-shaped cross section, having a pair of left and right projections 29 hanging on the lower edges of the float.

Float 24 having an inverted T-shaped cross section as shown in FIGS. 5A and 5B has side projections extending from the bottom of the float. The float 24 has a plurality of sleeves 28 each having a rectangular or square cross section. Each sleeve 28 extends vertically through the float 24. Each vertical pawl 23 extends through each sleeve 28. For example, the pawl 23 is a metal tube made of a stainless metal plate having a rectangular cross section of thickness 5 cm and external dimension 80 cm x 40 cm. According to a simple structural design, this metal tube can have sufficient structural strength to ensure the function of the pawl 23. However, in view of the damping effect of the float 24 in relation to liquid vibration, the cross-sectional dimension of the pawl 23 can be reduced by performing a more detailed liquid motion simulation. The sleeve 28 has a rectangular or square cross section similar to the outline of the pawl 23. A predetermined gap is secured between the outer surface of the pawl 23 and the inner surface of the sleeve 28. The plurality of pawls 23 guide the vertical movement of the float 24 in a state of maintaining the posture of the float. As a variant of the floater 24 shown in FIGS. 5A and 5B, there is a float 24 having an inverted Y-shaped cross section shown in FIG. 5F. The protrusion 29 acts to impede fluid flow near the bottom surface of the float 24.

The float 24 shown in FIG. 5C is a hollow panel in the form of a box or housing without side projections, having a cavity 27 therein and having a rectangular cross section. The floating body 24 having such a cross section divides the liquid level LL and the liquid near the liquid level LL to effectively prevent sloshing generation.

As shown in Figs. 5A, 5D and 5E, a cross section known as a waveless configuration, i.e., a cross section which cannot be subjected to the wave exciting forces of the moving motion, may be employed, As a result, when the sloshing occurs, the effect of preventing the sloshing of the floating body 24 may be further improved by suppressing the moving motion of the floating body 24.

As waveless configurations other than those shown in Figs. 5A, 5D and 5E, the configuration of a float having a curved bottom surface, the configuration of a float having a triangular bottom surface, and the like can be considered. In some cases, the configuration with side protrusions as shown in FIGS. 5A, 5D and 5E serves to dampen liquid motion, which is advantageous for preventing sloshing.

The liquid vibration frequency, which requires the prevention of sloshing, depends on the shape and size of the tank 10, the structural characteristics of the support structure of the tank 10, the wave characteristics of the sea area on the course, and the like. Therefore, the dimension of each part of the floating body 24 cannot be determined reliably. For example, in the following description, the sloshing phenomenon is estimated to prevent the sloshing phenomenon for the tank 10 of FPSO 40 m wide and 40 m high in the North Atlantic in winter. The dimensions of the float are studied under conditions such that the frequency of liquid vibrations in the tank 10 is at least 0.15 Hz. When the dimensions of the float are studied to inhibit the vertical movement of the float and substantially prevent sloshing at any liquid height, the following dimensions are obtained:

With respect to the cross section of the float 24 as shown in FIGS. 5A and 5D, T = 8m, T1 = 4.8m, B = 3.3m, B1 = 7m,

T = 8m, T1 = 4.8m, T2 = 1.6m, B = 3.3m, B1 = 7.6m, B2 = 2m in relation to the cross section of the float 24m as shown in FIG. 5E.

These combinations of dimensions are exemplary only, and needless to say that various combinations of other parameters exhibiting equivalent anti-slashing performance are naturally predicted. Moreover, also about the cross-sectional shape of the floating body 24, the rectangular cross section (recessed cross section which opened downward, the cross section (II type | mold) opened), the reverse Y type cross section, the X type cross section, etc. can be employ | adopted.

The floating body 24 does not divide the liquid in the tank 10 as a whole, but only the liquid level LL and the liquid in the vicinity thereof. Thus, the liquid on each side of the floater 24 is continuous with each other through the lower region of the floater 24. The floating body 24 acts in response to the behavior of the liquid level LL and suppresses liquid vibration in the tank 10. The division of the liquid level LL causes the natural frequency of the liquid motion to move in the high frequency band. This provides the same effect as using the bulkhead to completely divide the area in the tank. Floating body 24 divides the space in the tank in the form of a U-tube, which may be concerned with the generation of U-tube mode vibration in which the left and right liquid columns alternately rise. However, such U-tube mode vibration does not occur excessively, and as described later, harmful U-tube vibration does not occur.

6 and 7 are graphs showing the relationship between the excitation frequency of waves per degree of rolling angle and the maximum wavelength width (μ) per degree of rolling angle generated in the tank. The maximum wavelength width (mu) is the amount of edge rise (maximum value) at the time of vibration with respect to the horizontal liquid level. The relationship between the excitation frequency and the maximum wavelength width is the result of a numerical analysis of a 40 m wide and 40 m high LNG storage tank. FIG. 6 shows numerical analysis results obtained when the liquid height ratio h / H is set to 63% (h = about 25m). FIG. 7 shows another numerical analysis result obtained when the liquid height ratio h / H is set to 30% (h = 12m). The center C of the rolling motion is set to the center of the tank (Xc = 20m, Zc = 20m).

The high incidence band α of the excitation frequency is indicated in Figs. 6 and 7, where the probability of ocean waves occurring in the North Atlantic in winter is high. In general, the ocean waves occurring in the North Atlantic in winter generally fall in the band α frequency (about 0.11 to about 0.14 Hz). 6 and 7 show natural frequencies of the rolling motion of the hull of the LNG carrier 1. In this embodiment, the natural frequency of the rolling motion of the hull is in the frequency band much lower than the band α.

Numerical results as shown in FIGS. 6 and 7 are obtained with respect to the following three types of LNG storage tanks, each having a width of 40 m and a height of 40 m:

(1) LNG storage tank (Comparative example 1) which does not have a bulkhead and an anti-sloshing device, but does not have a durable material at all.

(2) LNG storage tank (comparative example 2) in which the bulkhead which divides the inner area | region of a tank from side to side is provided in the position (in the center of the width direction) of the sloshing prevention apparatus 20.

(3) Tank 10 (Example) of this invention equipped with the sloshing prevention apparatus 20

In LNG storage tanks (Comparative Example 1) without any durable material, the maximum wavelength width (μ) increases rapidly at an excitation frequency in the range of 0.13 Hz to 0.14 Hz (FIG. 6) or at an excitation frequency of about 0.12 Hz (FIG. 7). do. This frequency belongs to the band α. Therefore, in the band α, the tank of Comparative Example 1 has a synchronization point synchronized with ocean waves. Therefore, there is a possibility that synchronization of the liquid and ocean waves in the tank causes sloshing.

In an LNG tank (Comparative Example 2) having a bulkhead that divides the inner region of the tank into two regions, the maximum wavelength width (μ) is the excitation frequency of 0.20 Hz to 0.21 Hz (Figure 6) or about 0.20 Hz (Figure 7) Increases sharply in the excitation frequency. This frequency belongs to a much higher frequency band than the band α. In other words, if the inner region of the tank is divided by the bulkhead, the sync point moves remarkably in the high frequency band, so that the occurrence of synchronization of liquid and ocean waves in the tank can be prevented, and the occurrence of sloshing can be prevented. However, considering the structural stability, strength and yield strength of the bulkhead, which is self-supporting or upright, the joint structure of the bulkhead and the inner surface of the tank, the support structure for supporting the bulkhead, and the like, Splitting involves economic or practical difficulties that are included in the shortcomings of construction costs, complex hull structures, and difficulties in the design and construction of the hull.

On the other hand, in the tank 10 of the present embodiment provided with the anti-sloshing device 20, similarly to Comparative Example 2, the maximum wavelength width (μ) is the excitation frequency in the range of 0.20 Hz to 0.21 Hz (Fig. 6) or about It increases rapidly at an excitation frequency of 0.20 Hz (Figure 7). This frequency belongs to a much higher frequency band than the band α. That is, even if the floating body 24 only divides the liquid level LL and its vicinity, the synchronizing point moves remarkably in the high frequency band, similarly to the tank of Comparative Example 2 with the bulkhead, and thus the liquid and the liquid in the tank. Synchronization of the ocean wave is prevented, so that sloshing can be prevented.

6 (bottom graph) shows the relationship between the excitation frequency and the vertical movement of the float 24 relative to the liquid level LL. The vertical motion of the float 24 is only a relatively small behavior occurring in the frequency band in the range of 0.20 Hz to 0.21 Hz. 6 also shows the occurrence of vertical movement of the floater 24 in the frequency band in the range of 0.14 Hz to 0.15 Hz. This is only due to the natural frequency of the float 24 itself in this frequency band, and this vertical motion is only a relatively small behavior.

FIG. 8 is a graph showing the numerical calculation result of the relationship between the primary natural frequency f ₁ of the liquid motion in the LNG tank and the liquid height h in the tank. In FIG. 8, with respect to an LNG storage tank having a height of H = 40 m without any durable material, the primary natural frequency f ₁ obtained by changing the width W of the tank to 40 m, 20 m, and 15 m. ) And the liquid height h is shown. In addition, FIG. 8 shows the frequency band of the ocean blue, which is likely to occur in the North Atlantic in winter, that is, the frequency band α. In the case where the liquid height h and the width W belong to the zone α in FIG. 8, this situation is considered to be a condition in which a sloshing phenomenon is very likely to occur. Regarding the primary natural frequency f ₁ , its value is obtained by the estimation formula of the natural frequency of sloshing as shown in FIG. 8.

As shown in FIG. 8, if the liquid height h exceeds about 8 m, the primary natural frequency f ₁ of the LNG tank having the width W = 40 m belongs to the band α. That is, in order to surely prevent the sloshing phenomenon, the liquid height h needs to be limited to 8 m or less. On the other hand, if the liquid height h is in the range of about 2 m to 3 m, the primary natural frequency f ₁ of the LNG tank having the width W = 20 m belongs to the band α. However, if the liquid height h exceeds about 4 m, the primary natural frequency f ₁ belongs to the high frequency band rather than the band α. In addition, when the liquid height is about 2 m, the primary natural frequency f ₁ of the LNG tank having the width W = 15 m belongs to the band α. However, if the liquid height exceeds about 3 m, the primary natural frequency f ₁ belongs to the high frequency band rather than the band α. That is, the liquid in the LNG tank having the width W = 20 m or W = 15 m is difficult to synchronize with the ocean blue, so sloshing phenomenon is unlikely to occur. This means that by dividing an LNG tank having a width of W = 40m into a plurality of sections having a width of W = 20m or less, synchronization of the ocean blue with the liquid in the tank is prevented, and as a result, sloshing occurrence can be prevented. Regarding the tank of width W = 20 m, the liquid height h of about 2 m to 3 m belongs to the zone α. In relation to a tank of width W = 15 m, the liquid height of about 2 m belongs to the zone α. However, such liquid heights are only at or below the traditionally permitted liquid level loading state, i.e., the height H x 0.1 (range k1 as shown in FIG. 2b) in which no excessive liquid pressure is generated in the tank causing damage to the tank. It is only a loading state.

That is, as can be understood from FIG. 8, the sloshing can be effectively prevented by dividing the LNG accommodating area 15 into compartments having a width of 20 m or less by the bulkhead. As shown in FIGS. 6 and 7, an anti-sloshing action similar to the effect of completely dividing the area 15 with the use of a bulkhead is obtained by the floating body 24 dividing only the liquid surface LL and its vicinity. Can lose. Thus, similar to the division of the area 15 by the bulkhead, the tank of the present embodiment is divided by the floating body 24 into sections with only the liquid level LL and its vicinity each having a width of 20 m or less. According to 10), sloshing can be effectively prevented. In addition, the gaps 25, 26 (FIGS. 3 and 4) formed between adjacent floats 24 act to hinder the flow of liquid through them to dampen liquid vibrations. Therefore, the generation of sloshing can be more effectively prevented by the formation of the gaps 25 and 26 according to the anti-sloshing device 20 of the present invention. In addition, the configuration of the present invention, in which the liquid level LL and its vicinity is divided by the floating body 24, does not provide a structural disadvantage of installing the bulkhead.

9 is a cross-sectional view of the tank 10 showing the manner of dividing the liquid surface LL by the floating body 24. 10 is a graph showing a change in natural frequency related to the number N of free liquid levels. In Fig. 9, the liquid level rise η is the amount of rise at the edge portion of the liquid level during vibration to the horizontal liquid level at rest. In FIG. 10, the maximum wavelength width η _max is the maximum value of the liquid level rise η obtained when the rolling angle of the hull is set to 1 degree.

In FIG. 9 a tank 10 of width W = 58m and height H = 40m is shown. 9A shows the state (N = 2) of the liquid level LL evenly divided into a single series of floats 24 arranged in one row. FIG. 9B shows the state (N = 3) of the liquid level LL evenly divided by two series of floats 24 arranged in two rows. FIG. 9C shows the state (N = 4) of the liquid level LL evenly divided by three series of floats 24 arranged in three rows. The floats 24 are aligned in the direction of the center line of the tank 10. Each float 24 is a hollow panel having an internal cavity, configured in the shape of a box or housing with a rectangular cross section and having a draft depth D = 14.2m and a width B = 2m. In addition, the liquid height is set to the height h = 25.2m, the position of the center C of the rolling motion is set to the widthwise center (Xc = 20m), the height Zc = 10m of the cross section of the tank.

As shown in FIG. 10, as the free liquid surface number N increases, the sync point moves to a high frequency band. Therefore, according to the hull structure, the tank shape, the liquid level, and the like, the arrangement of the floating body 24, the number of columns of the floating body 24, and the like are appropriately set, so that the synchronization point can be moved to the high frequency band as desired.

FIG. 11 is a graph for explaining the anti-sloshing effect of the floating body provided in the tank having the width W = 58 m and the height H = 40 m in a condition of draft depth D = 5 m and width B = 2 m. FIG. 11A shows the vertical positional change of the liquid level at an instant caused at the end (edge of the liquid level) of the tank 10 without the float 24. FIG. 11B shows the vertical positional change of the liquid level at every moment caused at the end of the tank 10 with the float 24. The vertical position of the liquid level as shown in each figure is the result of numerical analysis of the liquid level, obtained in the state where an irregular wave having an effective wave height of 5.95 m and an average period of 10.1 seconds acts on the hull.

As shown in Fig. 11A, liquid vibration is generated by sloshing in the tank 10 without the float 24, in which the vertical position η of the liquid level is 15 m or more, and as a result, the liquid is tanked. The phenomenon that collides with the ceiling surface 14 of the tank 10 occurs within the tank 10. On the other hand, such excessive vibration does not occur in the tank 10 with the float 24 as shown in FIG. 11B because the float 24 effectively prevents sloshing.

12 is a graph showing the relationship between the change in the height Zc of the center C of vibration and the maximum wavelength width η _max described above.

When the height Zc of the center C is changed from the width direction center position (Xc = W / 2) to Zc = 20m, 10m, and 5m, the center of vibration C is changed to a low position, so that the vicinity of 0.20 Hz The maximum wavelength width η _max increases significantly. Also, as the center C of the vibration is changed to a frequency near 0.10 Hz, the maximum wavelength width η _max near 0.10 Hz increases. This phenomenon is thought to be due to the U-tube mode vibration caused by the deformation of the space inside the tank into the U-tube configuration. This U-tube vibration is a phenomenon caused by continuing the excitation of the same or equivalent conditions over a relatively long time, and thus the possibility of U-tube vibration occurring is relatively low. Even if the U-tube mode vibration occurs, the vibration occurring in the frequency region around 0.10 Hz is relatively small, and therefore, it is thought that the possibility of harmful U-tube vibration occurring by providing the floating body 24 is low.

Although the present invention has been described with respect to the preferred embodiments, the present invention is not limited to the above embodiments but various modifications or changes are possible within the scope of the invention disclosed in the claims.

For example, in the above embodiment, the tank 10 has three floats 24 arranged in a linear large size, but two floats 24, or four or more floats 24 are tanks. Can be arranged in any formation in (10).

Further, in the above embodiment, a single series of floats in one row is arranged on the central axis XX of the tank 10 so that the liquid level LL is divided evenly, but in two or more series of two or more rows. The floating body may be disposed in the tank 10 or the liquid level LL may be divided unevenly.

Also in this embodiment, each float 24 is held by a pair of vertical poles 23, but each float 24 may be held by three or more vertical poles 23. .

In addition, the floats 24 do not necessarily have to be strictly aligned linearly, and the floats 24 may be arranged, for example, in a slightly off position, such as zigzag formations.

Industrial availability

The present invention is preferably applied to a liquid storage tank of a membrane type of a liquid cargo ship or a floating marine installation. The anti-sloshing technique according to the present invention can be preferably used in large LNG carriers or FLNG facilities which have been conventionally recognized as difficult to store or transport liquid cargo in the partially loaded state. The present invention allows such large LNG carriers or FLNG plants to store or transport liquid cargo in partially loaded conditions, so the actual effect of the present invention is significant. In addition, the anti-sloshing device of the present invention can be applied to a tank of a ship carrying any liquid cargo.

1: LNG carrier (liquefied natural gas transport ship) 10: LNG storage tank
20: sloshing prevention device 21, 22: base part
23: vertical pole 24: float
D: draft depth LL: liquid level (free liquid level)

Claims (21)

A sloshing prevention device installed in a liquid storage tank of a membrane type of a liquid cargo ship or a floating marine facility and preventing sloshing occurring in the tank,
A plurality of floats arranged in series in the longitudinal or transverse direction of the transport ship or facility; And
A plurality of vertical poles that support the float and guide the float in a vertical direction, resisting a horizontal external force acting on the float;
Each float has a weight to float on the free liquid surface in the tank while maintaining a predetermined draft depth, and the float floats on the free liquid surface to divide the liquid near the free liquid surface and the free liquid surface. And allow liquids on each side of the float to be contiguous with each other through a lower region of the float. The method of claim 1,
The floats are arranged spaced apart from each other, a gap for the flow of the liquid is formed between the adjacent floats, and another gap for the flow of the liquid between the float and the inner wall surface of the tank is formed anti-sloshing device . 3. The method according to claim 1 or 2,
The draft depth of the float is set to H × 0.1 or more, or the distance between the bottom surface of the tank and the lower end of the float is set to h × 0.80 or less when h is H × 0.5, where “H” is An anti-sloshing device, in which the total height of the tank and "h" represent the liquid height. 4. The method according to any one of claims 1 to 3,
The vertical pole extends through the floating body, and the upper base part and the lower base part supporting the upper end part and the lower end part of the pawl are fixed to the ceiling surface and the bottom surface of the tank, and the base parts are the floating part of the tank. And prevent the collision between the ceiling and the bottom surface of the base portion, the base portion reduces the distance between the support points of the pawl less than the total height of the inner region of the tank. 5. The method according to any one of claims 1 to 4,
The free liquid surface is equally divided by the floats arranged in a row oriented in the longitudinal direction of the hull, each float being supported and guided by a plurality of poles spaced apart from each other in the longitudinal direction of the hull. . 5. The method according to any one of claims 1 to 4,
And said free liquid surface is divided by said floats arranged in substantially parallel rows, each float being vertically movably supported by a plurality of poles spaced in the longitudinal direction of the hull. 7. The method according to any one of claims 1 to 6,
The float is an anti-sloshing device comprising a hollow polyhedron consisting of a vertical plane and a horizontal plane having an internal cavity for securing buoyancy. 8. The method according to any one of claims 1 to 7,
And the float has a divider portion extending vertically to divide the liquid in the free liquid surface or near the free liquid surface, and a side protrusion extending laterally from the divider portion to dampen liquid vibration. The method according to any one of claims 1 to 8,
And the float has buoyancy adjusting means for adjusting the draft depth of the float. 10. The method of claim 9,
And the buoyancy adjusting means has buoyancy reducing means for introducing the liquid in the tank into the internal cavity of the floating body, or a weight adjusting means for adjusting the weight of the floating body. 11. The method according to any one of claims 1 to 10,
And a cross section of the tank cut by the vertical cut surface is rectangular or square. A sloshing prevention method for preventing sloshing occurring in a liquid storage tank of a membrane type in a liquid cargo ship or a floating marine facility.
Arranging a plurality of floats in series in the longitudinal or transverse direction of the transport ship or facility, the floats being supported against horizontal external forces acting on the floats and being vertical in response to changes in liquid level. Is moved to;
The liquid on each side of the float is divided so that the liquid on each side of the float is contiguous with each other through the lower region of the float, so that each float floats on the float level of the float while the liquid remains on the free liquid level. And floating, thereby moving the natural frequency of the liquid vibration in the tank to a high frequency band to prevent occurrence of a sloshing phenomenon. The method of claim 12,
A plurality of vertical poles supported by the ceiling and bottom surfaces of the tank are arranged in the tank, the vertical poles supporting the float and resisting the horizontal external force acting on the float. Guide to preventing sloshing. The method according to claim 12 or 13,
The floats are arranged spaced apart from each other, a gap for the flow of the liquid is formed between adjacent floats, and another gap for the flow of the liquid is formed between the float and the inner wall surface of the tank. 15. The method according to any one of claims 11 to 14,
The draft depth of the float is set to H × 0.1 or more, or the distance between the bottom surface of the tank and the lower end of the float is set to h × 0.80 or less when h is H × 0.5, where “H” is A method for preventing sloshing, wherein the tank represents the total height of the tank and " h " represents the liquid height. 16. The method according to any one of claims 12 to 15,
Base portions supporting the pawls are fixed to the ceiling and bottom surfaces of the tank, and the top and bottom portions of the pawls are fixed to the base portions, so that the base portions prevent the float from colliding with the ceiling and the bottom surface of the tank. And the base portions reduce the distance between the support points of the pawl to less than the overall height of the inner region of the tank. 17. The method of claim 16,
The distance between the lower end surface of the upper base part and the ceiling surface that suppresses the rise of the float is set to H × 0.3 or less, where H represents the overall height of the tank. 18. The method according to any one of claims 12 to 17,
And the free liquid surface is equally divided in the transverse direction of the hull by floating bodies arranged in the front and rear directions of the hull. 19. The method according to any one of claims 12 to 18,
And the free liquid surface is divided by floats arranged in substantially parallel rows. 20. The method according to any one of claims 12 to 19,
And the liquid near the free liquid surface and the free liquid surface is divided by a divider portion extending vertically of the float, and the vibration of the liquid is attenuated by a side protrusion extending laterally from the divider portion. 21. The method according to any one of claims 12 to 20,
A method of preventing sloshing, wherein liquid in the tank is introduced into an internal cavity of the float, or the weight of the float is adjusted to adjust the draft depth of the float.

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