KR20090032087A - An arrangement for a cylindical tank for transportation of liquefied gases at low temperature in a ship - Google Patents

Abstract

A horizontal generally cylindrical tank (2) for transportation of liquefied gases at low temperature in a ship is supported in two saddle supports (12) in the ship (1). At each support, the tank has internal reinforcements comprising two adjacent perforated bulkheads (3) and a framework of crossing girders/stiffeners (4-7) welded between the bulkheads (3), thus providing a tank (2) with sufficient strength for a capacity in the range of at least 40.000-60.000 m3. A method for securing accurate roundness of the tank in the support area is also described.

Description

The structure of a cylindrical tank of a ship for transporting liquefied gas at low temperature {AN ARRANGEMENT FOR A CYLINDICAL TANK FOR TRANSPORTATION OF LIQUEFIED GASES AT LOW TEMPERATURE IN A SHIP}

The present invention relates to the design, construction and support for mounting a large, independent, horizontal, generally cylindrical tank on board a vessel for carrying liquefied gas at low temperatures. The present invention is also applicable to a so-called twin tank consisting essentially of two cylindrical tanks dried together in one common tank.

Horizontal independent cylindrical tanks are widely used on vessels with relatively small total cargo loads to carry liquefied gas at low temperatures, and the largest known vessels built with such cargo tanks are approximately 30,000 m 3. Has a total cargo load.

However, over the last 20 to 30 years, larger vessels for the transport of liquefied gas have been built to defined sizes with total cargo loads ranging from 120,000 m 3 to 160,000 m 3. Recently, ships with a total cargo load of more than 200,000 m 3 have been contracted and built.

To date, these large vessels have been largely built according to two different design concepts, namely the membrane cargo tank and the independent spherical cargo tank.

To date, no development has been made to apply cylindrical tanks to such large vessels for liquefied gas.

Although such cylindrical tanks are preferred in comparison to, for example, spherical tanks with regard to design, assembly and in-vessel installation, stand-alone cylindrical tanks have never been applied to large vessels for the transport of liquefied gas as described above.

Spherical tanks have only one degree of freedom (diameter), while cylindrical tanks have two degrees of freedom (diameter and length), which are advantageous for arrangement and installation in the surrounding hull.

In addition, the construction and drying of the cylindrical tank is much simpler compared to the spherical tank.

However, over the last five to ten years, so-called membrane type ships have been the dominant form, replacing large ships for the transport of liquefied natural gas (LNG). However, these vessels have limited performance. In particular, there are limitations with regard to the acceptance durability of the cargo containment system to withstand liquid motion (sloshing) when the vessel is sailing in rough seas. Because of the risk of damage due to sloshing, these types of vessels do not allow partial filling of cargo tanks between about 20% and about 80% of full tanks while at sea.

In addition to these filling constraints, sloshing often caused damage to the insulation boxes and membranes of the cargo containment system. The number of cargo tanks is an important parameter in the cost of building LNG vessels. For very large ships under contract and construction, it is necessary to increase the number of cargo tanks from four to five (compared to some smaller membrane vessels), in which case relatively high costs for the construction of such ships It is worth considering that this is required.

Common weaknesses / disadvantages of LNG vessels in membrane and spherical tanks are in the construction for piping, wiring and internal access between the top and bottom of the cargo tank. The distance between the top and the bottom can range from 40 meters to 45 meters and a self-contained tower must be provided inside each cargo tank to support and secure the piping and wiring as well as the ladder for access to the tank bottom.

In addition, since these towers must have sufficient strength to withstand slushing in the ocean, the towers become quite complex and expensive structures.

A conceptual alternative for applying a standalone cylindrical tank to large and extra large vessels for the delivery of liquefied gas may be to upscale the existing configuration of a cylindrical tank applied to a small liquefied gas carrier.

In this compact liquefied gas carrier, a freestanding cylindrical tank is supported in two saddle structures, which are integrated into the surrounding hull structure. Between the cargo tank (made of aluminum, stainless steel or low temperature steel) and the saddle structure made of steel, an insulating material of sufficient strength to support the weight of the full cargo tank is provided.

The critical load point for this cylindrical tank is at the support and shell and internal reinforcement near the support. Internal reinforcements for small vessels / tanks are generally

1) single ring stiffener with flange, or

2) consists of a single ring stiffener with a flange associated with a single circular perforated wash bulkhead.

This type of reinforcement is sufficient for small and medium liquefied gas carriers, and in these small cargo tanks no restrictions on filling level are usually required.

Large vessels with horizontal cylindrical tanks and internal reinforcements such as 1) will have restrictions on the filling level of the tank due to sloshing during navigation.

Internal reinforcements such as 2) are not practical for large tanks due to the difficulty to reinforce large diameter single wash bulkheads. In addition, these two types of structures / reinforcements limit the radial stiffness and strength, which becomes increasingly important as the tank grows larger. Thus, the lack of stiffness and strength will result in radial deformation along the periphery of the tank in the support area, and this deformation and the accompanying stresses will be difficult to calculate accurately. In addition, the deformation of the surrounding hull due to different draft and marine conditions will be transmitted to the support system and the cargo tank.

The fact that the surrounding hull with the saddle structure is deformed and that the tank in the support area has a radial deformation will make it difficult to make accurate precalculations of the stress level of the cargo tank element. However, accurate precalculation of such stresses is a mandatory requirement from applicable national / international institutions and classification societies, and the structural / reinforcement types applied to cylindrical tanks of small ships are difficult or even impossible to apply to large ships.

The present invention provides a technical solution to which large independent cylindrical tanks for carrying liquefied gas, in particular liquefied natural gas (LNG), can be applied. In addition, the present invention alleviates the major drawbacks / disadvantages of the other design concepts described above. The structure according to the invention is defined by claim 1 and the method according to the invention is defined by claim 12.

In particular, the present invention provides a good technical solution for the following main items.

Avoid significant restrictions on the filling level of cargo tanks during navigation.

A limited minimum number of cargo tanks can be achieved (two, three or four cargo tanks depending on the cargo load).

Simple internal arrangement of cargo tanks for piping, electrical wiring and access between the upper and lower parts of the cargo tanks.

The present invention also provides a structure / reinforcement in the interior of the tank at the support, which enables accurate stress calculations on the surrounding hull structure and the material of the cargo tank under normal loading conditions.

The present invention provides two circular perforated wash bulkheads with other supports inside the cargo tank at each support. The distance between circular wash partitions is generally in the range of 1 meter to 4 meters. A framework of girder / stiffeners is provided and welded between the circular perforated wash bulkheads, whereby the two circular perforated bulkheads are interconnected through the framework. Subsequently, adjacent sections of the outer shell plate are welded to the perimeter of the circular perforated partitions and to the radial girder between the partitions. Thus, the two circular perforated bulkheads, the intervening framework, and the outer shell plate constitute a rigid wheel-like structure.

In this structure the two circular bulkheads have many openings / perforations for quickly balancing the difference in level inside the tank. The two circular perforated bulkheads together with the intermediate framework and the outer shell form a very strong structure. Since the radial stiffness and global stiffness can be almost infinite, any global or local radial deformation of the tank at near external loads is nearly impossible.

On this basis, the stress calculation of the cargo tank is simple and reliable, and thus the requirement for accurate prestress calculation can be realized. In addition, a double bulkhead with an intermediate framework can withstand the force from sloshing in an efficient manner, so that the local stress at the bulkhead attached to the outer shell will be significantly less than the local stress at the single bulkhead. For example, if a ship with a cylindrical cargo tank with a total cargo load of 145,000 m 3 is provided with three cargo tanks, the vessel will not be constrained by partial filling of the cargo tank by optimizing the perforations in the inner bulkhead. It is expected to be.

This is a significant advantage over ships in membrane and old tanks that require at least four cargo tanks for a total cargo load of 145,000 m 3. In addition, ships of membrane-type cargo tanks as described above have limitations due to partial filling in the ocean.

The space between the two circular perforated bulkheads in each support can be used in an efficient manner for piping, wiring, and access between the top and bottom of the tank. Domes with access hatches for the connection of piping and wiring are arranged just above the double bulkheads.

The cargo tank has an ambient temperature when installed and constructed on the saddle support structure of the hull. However, if the cargo tank is cooled by the loading of the first cargo (eg LNG), the diameter of the tank will shrink about 60 mm in diameter for a 30 meter diameter steel tank. At least in theory, the tank is partially deformed from the initial contact surface to the saddle support, thereby exposing the tank to an unstable risk of transverse direction when the vessel is rolled off the ocean.

This risk is ruled out by securing the dome (two for each tank) for lateral movement, whereby the tank remains constant in the same transverse position.

The dome structure on the double wash bulkhead allows for the arrangement of the outer bracket on the dome on the plane of the wash bulkhead and at the same time the arrangement of the inner rigid plate on the dome. This allows the structure of the dome connected to the circular wash bulkhead to resist all common lateral forces and to transfer the forces to the surrounding deck structure. The transfer of lateral forces from the cargo tank to the surrounding deck structure via the dome is made possible by a system of insulating material specifically arranged between the dome and the deck structure. In addition, the arrangement of materials between them handles any vertical and longitudinal movement with temperature changes in the cargo tank. The rear dome may allow vertical movement of cargo tanks. The stern dome may be capable of vertical and longitudinal movement. If the vessel is sagging and hogging, the rear dome may allow some longitudinal movement.

Another advantage of the design concept of the present invention is that it is possible to fabricate cylindrical sections on supports with built-in bulkheads / framework with accurate roundness. Circular perforated bulkheads can be dried to be fully welded and initially made with excess dimensions. In final welding, the partition wall is measured, marked and cut to the correct diameter so that an accurate roundness can be achieved and guaranteed. In the next step, the adjacent shell plate is welded to the circular perforated bulkhead and the adjacent framework, still maintaining the correct roundness.

The exact roundness of the tank at the support makes it easy to fit the tank to the saddle structure of the hull.

Another novel alternative is to apply a pressure sensor along the perimeter of the saddle structure between the tank and the hull. The pressure load on the saddle may be continuously monitored and compared with the precomputed load along the perimeter of the support system.

BRIEF DESCRIPTION OF THE DRAWINGS For a clearer understanding of the present invention, reference will be made in detail to exemplary embodiments schematically illustrated in the accompanying drawings.

1A and 1B are each a side view and a plan view of an LNG carrier embodying the present invention.

2A and 2B are cross-sectional views taken along the line A-A shown in FIG. 1, showing two other embodiments of the present invention.

3 is a cross-sectional view taken along the line B-B shown in FIGS. 2A and 2B.

4 is an enlarged view of the detail part 3 indicated in FIG.

5A and 5B are enlarged views of detail 1 indicated in FIG.

6A and 6B are enlarged views of detail 2 indicated in FIG.

1A and 1B are plan views showing the overall structure of an LNG carrier 1 having a total cargo capacity of about 145,000 m 3, the LNG carrier having three cylindrical cargo tanks.

2A and 2B are cross sectional views through a ship and a cargo tank (see section AA of FIG. 1A), the cross section showing perforated bulkheads 3 at one of the supports for one cargo tank. It shows between.

The figures show two different structures / solutions for the framework between circular perforated bulkheads.

The intervening framework shown in FIG. 2A consists of a vertical plate girder 4 and a horizontal plate girder 5.

Concentric ring girders 6 are arranged on the outside, and radial girders 7 are arranged between the concentric ring girders 6 and the plate 2 of the tank 2. For optimal force transfer between the shell and the girder, it is important that the force is transmitted vertically towards the shell plate.

The intervening framework shown in FIG. 2B consists of a concentric ring girder 6 and a radial girder 7.

2A and 2 schematically show a ladder 8 between the top and bottom of the tank and the piping / wiring 9.

2A and 2B basically shows the perforation / opening 10 of the partition. The final number / position of perforations / openings of the bulkhead will be calculated and taken into account in order to achieve optimal results for minimizing the load / stress on the bulkhead and the shell from sloshing in the tank during navigation.

2A and 2B show a cargo tank provided with an outer insulation 11, a saddle support 12, and insulation and weight support material 13 between the saddle support and the cargo tank.

3 is a cross-sectional view of the section B-B indicated in FIGS. 2A and 2B, showing two perforated bulkheads 3 positioned at a constant distance from each other. The distance is predefined to be in the range of 1 meter to 4 meters. FIG. 3 is an explanatory view for securing the tank 2 to the saddle support with respect to the longitudinal movement of the tank in the support of either of the supports as a whole. On the other support, the tank 2 slides freely in the longitudinal direction.

3 also shows a vertical girder and a horizontal girder (including concentric girders) with openings 10, 14 allowing free flow of liquid cargo and access to all spaces between the circular bulkheads.

FIG. 4 is a view of detail 3 indicated in FIG. 3, for transmitting forces (primarily due to slushing) from partition 3 and radius plate 7 to outer shell plate 17 in the longitudinal direction. The structure is shown.

The bracket 15 at the transition between the partition 3 and the shell 17 is shown inside the tank and the bracket 16 in the same plane is shown outside the tank. Both brackets snip while approaching zero at the end towards the shell. In addition, the outer bracket 18 is shown in the support zone, with the other brackets 15 and 16 and the inner radius plate 7 in the same radial plane. The structure of the bracket 18 described above is formed by cutting the space for the bracket in the interposition material 13 between the tank and the saddle support. In order to fix the tank in the longitudinal direction, the flat bar 19 is arranged externally along the periphery of the support region of the tank.

The corresponding flat bar 20 is arranged on the saddle support 12 to fix the interposition material 13 with respect to the saddle support 12 and the peripheral hull 1.

5A and 5B show the detail 1 indicated in FIG. 1A, showing the principle for securing the tank 2 in the transverse and longitudinal directions at an aft dome 23. FIG. The concentric ring 21 is fixed to the hull 11, the concentric ring 22 is fixed to the rear dome 23, and the surface between the concentric rings is due to the temperature change of the tank 2. Acts as a sliding surface for the vertical movement of the. The material of these concentric rings may be the same material as the material applied between the cargo tank and the saddle support.

However, the rear dome 23 and the tank 2 are fixed for movement in the transverse and longitudinal directions, and the dynamic force acting on the cargo tank 2 when the ship is sailing is the tank 2. From the rear dome 23 and through the concentric rings 21 and 22 to the hull 1. To withstand and transmit force, the tail dome 23 is internally reinforced by the vertical reinforcement plate 24 and the horizontal reinforcement plate 25.

The vertical reinforcement plate 24 may be arranged in the same plane as the two circular wash partitions 3, and also between the shell plate 17 and the rear dome 23 of the tank to reduce the stress concentration in the transmission of force. The brackets 26 are also arranged in the same plane.

FIG. 5B shows the section C-C indicated in FIG. 5A.

6A and 6B show the detail 2 indicated in FIG. 1A, in which the tank 2 is fixed in the transverse direction, and at the same time the stern dome 29 in the vertical and longitudinal directions according to the temperature change and FIG. The principle at the stern dome 29 to ensure free movement of the tank 2 is shown. The intermediate elements 27, 28 are arranged between the stern dome and the surrounding hull 1. The inner element 27 is fixed to the dome, the outer element 28 (two separate pieces) is fixed to the hull 1, and the engagement surface of the inner element 27 and the outer element 28 is the tank 2. It acts as a sliding surface for vertical and longitudinal movement of the stern dome 29 in accordance with the temperature change of. The material of the intermediate element may be the same material applied between the cargo tank and the saddle support.

However, with the structure shown, the stern dome 29 and the tank 2 are fixed against lateral movement, and the lateral dynamic force acting on the cargo tank 2 is transferred from the tank 2 to the stern dome 29. And to the hull 11 via intermediate elements 27 and 28. To withstand and transmit force, the stern dome 29 is internally reinforced by the vertical reinforcement plate 30 and the horizontal reinforcement plate 31.

The vertical reinforcement plate 30 is arranged in the same plane as the two circular wash partitions 3.

In addition, the bracket 32 between the shell plate 17 and the stern dome 29 of the tank is also arranged in the same plane in order to reduce the stress concentration during the force transmission.

FIG. 6B shows the section D-D indicated in FIG. 6A.

It is to be understood that the present invention is not limited to the above-described exemplary embodiments, but may be modified and changed by those skilled in the art within the scope of the appended claims.

Claims (12)

A structure of a horizontal, generally cylindrical, shipboard tank 2 for transporting liquefied gas at low temperatures, the tank 2 being supported on board by at least two supports 12, the tank being reinforced perforated In a structure having an internal reinforcement including a partition, The internal reinforcement comprises two adjacent circular perforated bulkheads (3), wherein a framework system of intersecting stiffeners (4, 5, 6, 7) is arranged and welded to the bulkhead in the intermediate space. 2. The rear reinforcement (24, 25) and the outer reinforcement according to claim 1, wherein the rear dome (23) is arranged with two adjacent circular perforated bulkheads (3) in the saddle support of the rear cargo tank and aligned with the partitions (3). The bracket 26 allows the rear dome to transmit the transverse and longitudinal forces between the cargo tanks 2 via the intermediate elements 22, 23 to the hull 1 and simultaneously slide the rear dome 23 in the vertical direction. Structure to enable. The internal stiffener (30) according to claim 1 or 2, wherein the stern dome (29) is arranged with two adjacent circular perforated bulkheads (3) in the saddle support of the stern cargo tank and aligned with the partition walls (3). And external brackets 32 allow the stern dome to transmit the transverse forces between the cargo tanks 2 via the intermediate elements 27, 28 to the hull 1 and at the same time in the vertical and longitudinal directions. Structure to enable slide). 2. Structure according to claim 1, wherein the partitions (3) have openings (10) in the area of each partition defined by the nearest girders (4, 5, 6, 7). 5. The structure as claimed in claim 1, wherein the space between the partition walls is used to protect the piping and the wiring and to provide access between the top and bottom of the tank. 6. The structure according to claim 1, wherein the stiffener comprises tangential and radial plates (6, 7). 7. 7. Structure according to claim 6, wherein the stiffener comprises vertical and horizontal plates (4, 5). 8. Structure according to claim 6 or 7, wherein an opening (14) is provided in at least some of the stiffener plates (4, 5, 6, 7). The support system according to claim 1, wherein the support system is a saddle support 12 having a support insulation material 13 of the same length as the distance between the inner partitions 3, the interposition material 13. ) Is preferably fixed against movement by a flat bar flange 19, which is reinforced by sniped brackets 16 and 18 and welded to the shell 17 of the tank 2, and the sniped bracket 15 ) Is also provided between the shell (17) of the tank (2) and the partition (3). 10. The structure of claim 9, wherein a pressure transducer is provided and arranged along the periphery of the support system and the vertical pressure from the tank to the support is continuously monitored and recorded. The tank 2 according to any one of the preceding claims, wherein the tank 2 has a cargo load in the range of 40,000 m 3 to 60,000 m 3, and the distance between adjacent bulkheads 3 in the support is preferably 1 meter to 4. Structure that is metric range. A method of constructing a generally cylindrical tank in a vessel for carrying liquefied gas at low temperatures, the tank 2 having at least two regions for support in the vessel 1, each region having a circular perforated bulkhead. In a tank configuration method having an internal reinforcing material, The reinforcement is made in the form of two adjacent perforated bulkheads 3 and is manufactured to have a large diameter, and intersecting stiffeners 4, 5, 6, 7 are welded between the partitions 3, of the tank 2 Method for tank construction, characterized in that the partition plate (3) and the outer stiffener (7) are cut to the correct diameter and roundness before the shell plate (17) is welded to the partition wall (3) and the outer stiffener (7).

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