KR970005078B1 - Method for manufacturing a membrane for a gas tank - Google Patents

Abstract

The membrane structure is a ring knot membrane with a ring knot combined with a rumple in the part of a board member. The membrane structure includes four rumples(10) forming a cross shape and being inclined in a straight line towards an anchor bolt fixing hole(9) from an outer edge(1) of the board member(a), and the ring knot(2) connected to end portions(11) of the four rumples(10) and having a ring-shaped outer surface(5) and a ring-shaped inner surface(4) symmetrical to the ring-shaped outer surface(5).

Description

Membrane Structure for LNG Tank and Manufacturing Method

1 to 5 show a conventional membrane structure,

1 is a perspective view showing a membrane structure of a single wrinkled honeycomb structured type.

2 is a perspective view showing a membrane structure of a combination of triangular and square wrinkles.

3 is a perspective view showing a single triangular pleated membrane structure.

4 is a perspective view showing a single triangular pleated membrane structure.

Figure 5 is a perspective view of a bi-parallel pleated membrane structure.

6 is a perspective view showing a membrane structure of the present invention.

7 is a plan view of FIG.

8 is a side view of FIG.

9 is a finite element network of technics membranes.

10 is a finite element network of Kawasaki membrane.

11 is a finite element network of a membrane according to the invention.

12 is a boundary condition diagram of a membrane according to the present invention.

Figure 13 shows the equivalent von mouse stress of the Technics membrane.

Figure 14 shows the equivalent von Mice stress of the Kawasaki membrane.

Figure 15 is an equivalent von mouse stress of the membrane according to the present invention.

* Explanation of symbols for main parts of the drawings

A: Plate 1: Sheet edge

2: ring node 3: ring node top

4: inside ring node 5: outside ring node

6: ring node end 9: anchor bolt fixing hole

10: wrinkles 11: wrinkle ends

12: left side 13: right side

The present invention relates to an improved structure and manufacturing method of a membrane for a liquefied natural gas storage tank, and more particularly, to a ring knot membrane in which ring knots and corrugations are combined with a part of a sheet.

In general, liquefied natural gas is generally stored in a large tank as a cryogenic liquid having a boiling point of -162 ° C under atmospheric pressure, and the storage tank is configured as a vertical cylindrical double structure. The inner tank membrane is made of material resistant to low temperature brittleness, and the outer tank side plate is made of PC (Prestressed concrete) or carbon steel, and insulation material is interposed between them to evaporate due to heat intrusion from outside the tank. Manufacture so that gas (Boil-off gas) is minimized.

Membrane in cryogenic storage tank refers to a special plate designed and manufactured so that the material can expand and contract freely with changes in temperature and load.It is installed on the side and bottom of the tank to prevent leakage of cryogenic liquid. It is primarily responsible for the sealing function, and absorbs the load generated by the liquid pressure or the self-weight while transferring the fatigue cyclic load due to the thermal load and delivers it to the single panel (Insulation panel).

As a conventional membrane, the corrugated plate known from Japanese Patent Application Publication No. 50-21008 shown in FIG. 1 has a continuous hexagonal (honeycomb) corrugation structure that must absorb expansion contraction due to temperature change. It is a corrugated plate which forms a Y-shaped intersection with each corrugation having an angle of 120 °. The single corrugated honeycomb structure of this structure has the disadvantage that stress analysis is difficult, fabrication by press working is difficult, and residual stress exists.

In addition, the non-curvature low temperature tank known from Japanese Patent Application Publication No. 60-14959 shown in FIG. 2 has a triangular wrinkle having a triangular cross section, a trapezoidal wrinkle orthogonal to the triangular wrinkle, and an orthogonal portion. The three sides of the trapezoidal wrinkles are placed on the top surface of the trapezoidal wrinkles, and the three sides of each of the three sides that face each other and the tops of the trapezoidal wrinkles are parallel to each other, and the three sides of the triangles are bent and formed without oversized area. The quadrilateral tetrahedral shaped plate body is provided, but the combination of the triangular and square wrinkles has a difficult structure in stress analysis, residual stress, expensive manufacturing cost, and difficulty in forming.

In addition, the expansion and contraction structure of the membrane known from Japanese Patent Application Publication No. 60-32079 shown in FIG. 3 has branched arrangements in which at least one gathering portion has wrinkles protruding from the membrane surface of the low temperature liquefied gas tank. As the stretched structure, the gathering portion is bent in a shape in which the upper portion is raised with respect to the flat plate portion, and the upper portion and the corrugation are joined to connect the bent surfaces of both sides and are welded, but the stress analysis is difficult. Stress concentration occurs, residual stresses exist, and there is difficulty in manufacturing.

In addition, the single triangular wrinkle membrane known from Japanese Patent Application Publication No. 62-12439 shown in FIG. 4 is a type of expansion joint in which the expansion joint crosses the same height at the intersection of the expansion joint. And the corners are divided by S-shaped bends, Although there are expansion joint methods such as mold spiral winding and mold with concave, stress analysis is difficult, stress concentration occurs, residual stress exists, and there is difficulty in manufacturing.

In addition, the membrane structure known from Japanese Patent Application Publication No. 62-46759 shown in Fig. 5 is a shape of a bending wave with gentle cross-sections on one side and the other side of the unit corrugation, and gentle bending on both ends of the longitudinal section. It is formed to form a surface connected to the flat plate portion, and has a surface shape protruding smoothly three-dimensionally with respect to the flat plate, and the unit membrane mutually welded, but there is a stress concentration phenomenon and residual stress , Rotational heat shrinkage occurred.

As described above, many kinds of conventional membranes are known, but the problems are classified in detail by type.

The first corrugated membrane (FIG. 2) having a structure in which a triangular fold and a trapezoidal corrugation cross each other has a problem that many stress concentrations occur at corners where the bottom and the wall meet and sharp curves of the corrugation.

In addition, the second double-row wrinkle membrane (figure 5) having a structure in which two parallel corrugations cross each other at right angles with a stainless steel plate has been subjected to rotational contraction due to the absorption action of the heat deflection of the parallel parallel wrinkles. Behavior occurs, and since the rotational shrinkage deformation acts symmetrically in opposite directions, wrinkle deformation is not a problem because it is designed to balance each other, but if thermal deformation is unbalanced by external conditions, Locally intense thermal stress concentrations can occur, presenting significant problems. However, welding, molding, and stress analysis are easier than the first corrugated membrane. Since the radius of curvature at the corners of the membrane is relatively large and gentle, there is less risk of local stress concentrations in this region than in the first corrugated membrane.

In addition, the third corrugated membrane welded on the top of the corrugation is formed by forming the end of the plate into a flanged bent shape, and then contacting the top portions in a butt-shaped form, that is, a membrane formed by welding and joining two sheets of the welded joint. It is easy to manufacture, but the stability of the quality can be secured only when the molding and welding technology is very excellent.

In addition, the fourth membrane (Non-corrugation membrane) without wrinkles in the membrane plate (non-corrugation membrane) is a membrane using an Invar (Invar) material having a very low coefficient of thermal expansion is mainly used in LNG carriers. Invar materials have very high heat shrinkage at cryogenic temperatures, but are limited in terms of high price and difficulty in purchasing them.

In order to improve the above-mentioned conventional problems of the present invention, the membrane structure according to an embodiment of the present invention has four pleats that crosswise toward the anchor bolt fixing hole at the outer edge of the plate, and the ends of the four pleats Ring member consisting of an annular outer surface and an annular inner surface symmetrical with the annular outer surface is connected to.

In the above example, the heights raised from the four corrugations and the plate of the ring joint may be the same. Also in the above example, the cross section of the four pleats and ring nodes may be approximately semi-circular, or elliptical, polygonal in which the transverse diameter is narrower than the longitudinal diameter.

Furthermore, the unit board thickness may be thicker than the thickness in the panel board.

The present invention also proposes a method for manufacturing a membrane structure, according to one example, a ring node composed of four corrugations and a ring inner and annular outer surface directed in a straight line from the outer side of the unit plate toward the anchor bolt fixing hole in a cross shape Press-processing, punching holes for inserting the unit board into the panel board, bending the same wrinkles as the four corrugations of the unit board in the horizontal and vertical directions of the panel board, and And inserting the unit board into the punched holes of the panel board to fit the ends of the corrugations and welding the edges of the unit board.

Here, the welding step is preferably an overlap welding method.

An object of the present invention is to provide a convenient and simple structure in manufacturing as a symmetrical form by combining the ring node and the pleats.

In addition, it is an object of the present invention to provide a very practical and stable membrane by reducing the crease curvature is slightly increased when the pressure and temperature load acts on the membrane.

It is also an object of the present invention to maintain a uniform stress distribution and low stress levels when pressure is applied to the membrane.

Embodiments of the present invention are shown in FIGS. 6 and 8. First, the unit membrane plate material A is provided in a square shape. This square shape is a shape for convenience of description, and is usually circular. Since the membrane is in cryogenic conditions and subjected to pulsating fluid pressure loads, stress deformation behavior analysis is very important in terms of material selection and design. As the membrane material, stainless steel, aluminum alloy, Invar, 9% Ni steel, etc. are used, and in the case of SUS material, austenitic stainless steel having a thickness of 1.2 to 2.0 mm is most used. Four corrugations 10 are formed in a straight line from the outer edge 1 of the plate material A of a predetermined size to the anchor bolt fixing hole 9 and form a cross. Each of the four corrugations has the same cross section and has the same curvature. The curvature of the left side 12 and the right side 13 of each corrugation is the same. Thus, four corrugations 10 having a predetermined curvature are formed in the plate A so that the cross section neutral axis of the plate A is stretched and stressed. Do not cause this. The four corrugations 10 are formed to cross each other, but the corrugations do not directly cross from the anchor bolt fixing hole 9 to a predetermined diameter. From the anchor bolt fixing hole 9, a predetermined ring diameter 2 having an inner diameter smaller than the outer diameter is formed from the anchor bolt fixing hole 9 so as to be positioned around the anchor bolt fixing hole 9. . The annular ring segment 2 serves as an intersection where the four pleats 10 intersect. The ring node consists of an annular inner surface 4 and an outer surface 5 with the curvature of the inner surface 4 and the outer surface 5 being the same. The heights raised from the four corrugations and the plate of the ring node, that is, the height from the flat plate A to the top of the corrugation and the top of the ring node are preferably equal. The cross section of the four corrugations and ring nodes may be approximately semi-circular, or elliptical, or polygonal, with a transverse diameter narrower than the longitudinal diameter. Thus, the unit board according to the present invention is easy to manufacture by taking the symmetrical form as a whole.

The panel board, not shown, is made up of the unit boards. This panel board material is attached to the side wall and bottom wall of a tank. The panel board member is punched with holes for accommodating the unit board member, and is bent to form corrugations in the horizontal and vertical directions based on the center points of the holes. On the other hand, the above-mentioned unit board is preferably formed by press working in order to manufacture wrinkles and ring joints integrally. When the unit board is completed in this way, it is put in the hole of the panel board material and welded to the edge of the unit board material to fix the unit board and the panel board material. In addition, in order to reduce the cost of the plate, the plate thickness of the unit plate is preferably thicker than the plate thickness of the panel plate. For example, 2 mm stainless steel plate may be used for the unit plate material for press working, and 1.2 mm stainless steel plate may be used for the panel plate material for bending.

Therefore, the method of manufacturing the membrane according to the present invention includes four corrugations 10 that crosswise from the outer edge of the unit plate A toward the anchor bolt fixing hole 9, the annular inner surface 4 and the annular outer surface. Press-processing the ring node 2 composed of (5), and punching holes at regular intervals in the panel plate formed according to the size and shape of the tank at regular intervals; Bending and forming the panel board in the horizontal and vertical directions about the center point of the punched hole to form a wrinkle of the same shape as the unit board, and inserting the unit board into the hole of the panel board After aligning the ends of the unit plate is composed of the step of welding the edge portion to the panel plate. The welding step is preferably an overlap welding method.

Comparative example

The stress behavior analysis of the membrane structure has relied on experimental analysis since it is very difficult to theoretically analyze because of the complex three-dimensional shape of the membrane knot.

The present invention analyzes the stress deformation behavior of the membrane, which is the most important structure in cryogenic tanks, by using the finite element method, and compares and reviews the existing model to overcome the existing problems and develop a new type of membrane model and optimize the design of the model. To provide. Considering the geometric symmetry of the membrane plate, the model was divided into quarters and three-dimensional behavior analysis was performed. Membrane plates were analyzed by finite enzyme analysis considering the effects of the uniform distribution load acting perpendicular to the corrugation by the pressure of the storage liquid and the effect of thermal contraction and thermal expansion due to cryogenic liquid at -162 ℃.

9, 10 and 11 show a finite element network for the finite element analysis of membranes, consisting of 1100 to 1300 axisymmetric quadrangular elements and 120 to 1400 element nodes. Because the structure is very complex, the quadrangular mesh configuration is reconstructed to match the shape characteristics of the structure. Particularly, the portions where the wrinkle ends, ring nodes, wrinkles and ring nodes meet each other are divided into finer elements as shown in FIG. For the finite element analysis program, MARC (MARC User's Manual, 1993, Version K.5, MARC Analysis Research Co.) was used, and the boundary conditions closest to the load conditions actually applied to the membrane plates were substituted.

The simulation data values for the material properties and boundary conditions used in the numerical analysis are given in Table 1, and the boundary conditions for performing the analytical model of the membrane of the present invention are shown in FIG.

In FIG. 12, boundary 1 excludes the displacement in the Y-axis and rotational displacement in the X and Y-axis directions, and boundary 2 defines the displacement in the X-axis direction and the rotational displacement in the Y- and Z-axis directions. Assumed no. In addition, Boundary 3 excluding ring nodes and wrinkles were analyzed in the absence of displacement in the Z-axis direction. The boundary conditions for the models of other comparative examples (FIGS. 9 and 10) were also applied in the same manner as the boundary conditions of the membrane of the present invention.

Models that performed computer simulations are the French Technics membranes (Fig. 2; corresponding to the first corrugated membrane) and Kawasaki membranes (Fig. 5; the second double row wrinkle membrane), which are representative examples of existing membranes. And ring-shaped membranes (RM) according to the invention. In order to analyze the membrane more precisely, the thickness of the plate was divided into three parts: the upper surface (Layer1), the middle surface (Layer 2), and the lower surface (Layer 3) in contact with the liquid.

13, 14 and 15 degrees show the equivalent von Mises stress distribution on the membrane plate. In the three model analyzes, equivalent ponse stress generated on the upper surface of the membrane was interpreted as 105Kg for the Technics membrane, 78Kg for the Kawasaki membrane, and 32Kg for the membrane of the present invention, as shown in FIGS. 13a, 14a, and 15a. In addition, the equivalent von Maie stress appearing in the middle surface of the membrane was 61Kg, the largest in the Technics membrane, 35Kg in Kawasaki membrane, 21Kg in the membrane of the present invention.

According to these calculations, the Technics membrane has a locally high stress concentration but a relatively uniform stress distribution, while the Kawasaki membrane has a relatively low stress concentration but is slightly unevenly distributed, showing instability of rotational deformation behavior. Membrane of the present invention exhibits excellent performance compared to the two conventional membrane models by symmetrically uniformly distributing the stress and maintaining a concentration of about 65.6% compared to technics membrane and 40% compared to Kawasaki membrane even in stress concentration. . In addition, the cross section and corrugation area, which are pointed to the most vulnerable part of the membrane, were compared with each other for the three models, and the ring node type membrane model of the present invention was interpreted as being most stable with 21 to 32 kg.

FIG. 13 (a) shows the distribution of tensile or compressive stresses applied to the top surface of the technics membrane. In the corrugation where the flat plate and the corrugation of the plate cross, the maximum compressive stress of about 84Kg is applied at the node, but FIG. 13 (b) In the middle folds of the figure, the tensile stress and the compressive stress occurring at the upper and lower sides cancel each other and thus no stress appears. The liquid load acting on the technics membrane plate acts as a pressure load uniformly on the corrugated plate, and the wrinkle top portion has a relatively large effect of the pressure load applied to the corrugated side. Therefore, the shape of the corrugation causes a small displacement of the top of the corrugated plate, so compression deformation occurs in the corrugated plate, and the temperature load causes the membrane in the tank to contract. The membrane plate of the unit membrane is similar to the anchor bolt. Since it is in a welded state, a slip phenomenon occurs due to temperature change, which causes the strain to be bent.

In the upper surface of Kawasaki membrane of FIG. 14 (a), the maximum tensile stress of about 78Kg acts near the end of the wrinkles, and in the middle of FIG. 14 (b), the tensile and compressive stress of about 35Kg is unevenly near the end of the wrinkles. It shows what happens. In the case of Kawasaki membranes, the behavior fluctuations are different from those of technics membranes. In particular, under uniform pressure loads, the behavior of the corrugation part is deformed in the form of lowering the height of the top of the corrugation. Large stresses were found to concentrate. This shows that the strain at the crease increases but the stress value decreases compared to the technics membrane.

In the ring knot type membrane of the present invention shown in Fig. 15 (a), a tensile stress of about 32 Kg occurs near the pleats crossing the flat plate portion, and a compressive stress of about 26 Kg is acting on the top of the ring node. In the middle plane of FIG. 15 (b), the pressure distribution is generated completely symmetrically and uniformly, and a compressive stress of about 21 Kg occurs at the site where the top of the ring node meets the wrinkles.

According to the numerical analysis, the ring-barrier membrane of the present invention is characterized in that the stress is completely symmetrically distributed and the stress difference between the ring-barrier and the wrinkles is small compared with the above two models. In the membrane of the present invention, the behavior of the ring joint portion deforms downward when a uniform pressure load is applied. In other words, the strain is increased at the node, but the stress concentration is greatly reduced, which can greatly alleviate the cause of fatigue failure in the membrane plate. In the case of the membrane of the present invention that the stress is generated in the node can be a very important advantage in terms of stability of the membrane structure. In addition, the shrinkage deformation according to the temperature change occurs in the same way as the Technics membrane, and the overall behavior analysis may be performed by combining the pressure and temperature changes.

As a result of the numerical analysis using the finite element program, the membrane of the present invention was most excellent in terms of stress concentration and stress distribution. Technics membrane or Kawasaki membrane were interpreted as having high stress, but there was no problem in strength.

The three strains of membrane models widely used in cryogenic storage tanks have been analyzed using the finite element analysis.

The behavior analysis of the Technics membrane, the Kawasaki membrane, and the ring-node membrane of the present invention showed that all three membranes exhibited a phenomenon in which a large stress was concentrated in the corner portion and the wrinkle node where the plate and the wrinkle contact each other. The stress concentration in the middle of the membrane model was 61Kg for the Technics membrane, 35Kg for the Kawasaki membrane, and 21Kg for the ring joint type membrane. The node membrane was interpreted as 32 kg.

The ring-membrane membrane of the present invention was interpreted as the most stable and symmetrical and uniformly distributed in comparison with other membrane models in stress concentration or stress distribution. As a result, in the membrane for ship storage tank, it is very important in that the fatigue life, which is particularly problematic by the fluid pressure fluctuation load, can be greatly extended.

Next, the welding of the membrane will be described.

The membrane used to store the cryogenic liquid is installed on the tank wall and the bottom of the storage tank so that the welding area can be divided as follows.

① Overlapped area of membrane plate

② Membrane Plate and Angle Corner

③ membrane plate and angle piece

④ membrane plate and anchor point

In addition, the membrane plate washed for membrane welding is welded to the edge portion and the edge portion to inspect that the membrane plate is partially assembled into a set of membrane panel is sent to the storage tank construction site to perform the installation work.

There are two types of welding used when installing the membrane plate in the storage tank, circumferential welding of the anchor point using an automatic welding machine, and overlapping and welding the membrane plate and the plate. The joint of the single corrugated ring knot membrane steel sheet according to the present invention is joined by adopting the conventional Lap welding and Plug welding method, and the unit board and the panel board Are joined by overlap welding. As such, when the membrane structure is manufactured by overlap welding of the unit plate and the panel plate, the unit plate can be formed by pressing and the panel plate can be formed by bending. The characteristics of the membrane manufactured by the bonding method utilizing the advantages of the two processing methods are as follows.

① Bending of panel plate material can greatly alleviate residual stress and stress concentration phenomenon. When press working, residual stress occurs due to non-uniform changes in the processing thickness, especially in the processing part, so processing such as annealing heat treatment should be performed, but cutting does not require such additional processing.

② The mold to be pressed may be simpler and smaller. When press working to the panel board, the entire membrane must be mold-processed, but here, only the unit board is press-processed, which makes the process relatively easy.

③ 2mm stainless steel plate is used for the unit plate that needs to be press processed, and 1.2mm stainless steel plate is used for the panel plate that is bent, which opens up the possibility of saving significant material.

The present invention has the simplest structure and is excellent in that the stress is uniformly distributed in comparison with other conventional membranes, and exhibits a stress level of only 23 to 29% or less. In particular, when the pressure and temperature loads are applied, the wrinkle curvature is somewhat It is very practical and stable because the size of the swells and wrinkles decreases.

Claims (7)

Four corrugations 10 forming a cross-shaped line from the outer edge portion 1 of the plate A toward the anchor bolt fixing hole 9, and an annular outer surface connected to the end portions 11 of the four corrugations. (5) and an annular inner surface (4) symmetrical with the annular outer surface, characterized in that it comprises a ring node (2) having the same curvature radius and the same cross-sectional shape with the four corrugations (10) Membrane structure for LNG storage tank. 2. The membrane structure according to claim 1, wherein the cross-sections of the four pleats and ring nodes are approximately semicircular. The membrane structure according to claim 1, wherein the cross section of the four corrugations and the ring nodes is in the shape of an ellipse in which the transverse diameter is narrower than the longitudinal diameter. The membrane structure according to claim 1, wherein the cross section of the four corrugations and the ring nodes is polygonal. The membrane structure according to claim 1, wherein the sheet thickness of the unit sheet is thicker than the sheet thickness of the panel sheet. Four corrugations (10) forming a cross-shaped straight line from the outer edge portion (1) of the unit plate (A) toward the anchor bolt fixing hole (9), and the annular inner surface (4) and the annular outer surface (5) Press-processing the ring node 2, punching holes for inserting the unit plate into the panel plate, four corrugations of the unit plate in the transverse and longitudinal directions relative to the center point of the hole of the panel plate; Bending the same wrinkles as the step, and inserting the unit plate into the punched hole of the panel plate material to align the ends of each of the pleat and then weld the edges of the unit plate to the panel plate material Method for manufacturing membrane structure for natural gas storage tank. 7. The method of claim 6, wherein the welding step is an overlap welding method.