NO327766B1 - Cylindrical tank and method of manufacture thereof - 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 m<SUP>3</SUP>. A method for securing accurate roundness of the tank in the support area is also described.

Description

The present invention relates to devices for the construction and storage of large, independent and horizontal cylindrical tanks on board ships for the transport of liquefied gases at low temperature. The patent application also applies to so-called twin tanks, which consist of two cylindrical tanks that are assembled into one tank.

Horizontal and independent cylindrical tanks have largely been used for ships with a relatively small cargo capacity for the transport of liquefied gases at low temperatures, and the largest known ships with such cargo tanks that have been built have a total cargo capacity of approx. 30,000 m3.

However, in the last 20-30 years, many larger ships have been built for the transport of liquefied gases, and the usual size of such larger ships has been with a total cargo capacity in the area of 120,000-160,000 m3, and in the very last time it has become ordered such ships with a total cargo capacity of over 200,000 m3. The largest ships are built for the transport of liquefied natural gas (LNG).

Such large ships have so far been roughly built according to two different design concepts,

namely diaphragm tanks and independent spherical tanks.

Failure to use cylindrical tanks for the largest ships for the transport of liquefied gases can be said to be a "missing link" in the development so far.

As mentioned, independent cylindrical tanks have not found application for such large ships, and this despite the fact that both with regard to the design of the ship, production and installation of the tanks on board, cylindrical tanks are preferable to, for example, spherical tanks. A spherical tank has only one degree of freedom (diameter), while a cylindrical tank has two degrees of freedom (diameter + length), and this means that a cylindrical tank can more easily fit into a surrounding hull structure.

However, in the last 5-10 years, new membrane-type ships have been the dominant alternative for large ships to carry liquefied natural gas (LNG). But these ships also have weaknesses in their construction, and this especially applies to the strength-wise ability to resist fluid movements (sloshing) inside the cargo tanks when the ship is at sea. The ships are therefore subject to restrictions on filling the cargo tanks by authorities such as classification companies, and normally it is not permitted to have filling between approx. 20% and 80% of the volume in the cargo tanks when the ship is at sea. However, even with such restrictions, damage to membrane structures has occasionally occurred for some ships due to sloshing in the cargo tanks. The number of cargo tanks is a significant parameter for determining the construction price of such ships, and it can be mentioned in this connection that for the largest membrane-type ships under construction, due to sloshing it has been necessary to increase the number of cargo tanks from four to five, and the ships have thereby become relatively more expensive to build.

A common weakness for membrane ships and ships with ball tanks is the arrangement for routing pipes and cables, as well as access, between the top and bottom of cargo tanks. The distance between top and bottom can be in the order of 40-45 metres, and independent towers must be arranged for this height inside each cargo tank for guiding/clamping pipes and cables, as well as for access via ladders.

In addition, these towers must have sufficient strength to withstand fluid movements in seagoing conditions (sloshing), and the result is complicated and expensive solutions.

A close alternative for the use of independent cylindrical tanks also for large and the largest ships for the transport of liquefied gases is to scale up existing constructions that have been used for smaller ships with cylindrical cargo tanks. For such existing smaller ships, the independent cylindrical tanks are stored in two saddle structures, and where the saddle structures are integral parts of the surrounding hull structures. Between the steel or aluminum cargo tank and the steel saddle structure, a thermally insulating spacer is placed, and with sufficient strength to support the cargo tanks. The critical load points for such cylindrical cargo tanks will be found in connection with the storages, and at the internal reinforcements of the tanks in the storage zones. For existing smaller ships, the internal structures/reinforcements in the cargo tanks at the storage areas usually consist of either,

1) a simple ring stiffener with a flange,

or

2) a simple ring strut with a flange and a single, circular, perforated and braced bulkhead

(splash bulkhead)

For cylindrical cargo tanks of limited size, and correspondingly for smaller ships, these types of constructions/reinforcements are current, and have proven to be very acceptable solutions, and without requirements for restrictions on the level of filling in the tanks.

Possibly large ships with horizontal cylindrical tanks, and with internal ring stiffeners with flanges at the stowages, will probably also have to accept restrictions with regard to the filling level of the tanks due to liquid movements in seaway. The other alternative, consisting of ring stiffeners with a flange and in connection with a simple circular and perforated bulkhead, will be unrealistic for large diameters of the cargo tanks due to the difficult bracing of such a simple bulkhead. These two types of constructions/reinforcements also have a limited radial stiffness and strength, and this will become more and more pronounced the larger the tanks become, and will cause radial deformations of the tanks along the periphery in the bearing zone, and these deformations (and associated stresses ) will be difficult to predict. On the other hand, deformations in the hull will occur due to varying draft and draft, and these deformations will in turn also be transferred to the storage/saddles and cargo tanks. The fact that the hull/saddles are deformed, and that the tanks radially in the storage zones have their own deformation pattern, means that exact pre-calculation of loads and stresses in the tank material becomes difficult. Such exact pre-calculation is, however, a requirement from the authorities and classification societies, and this means that types of constructions/reinforcements used on cylindrical tanks for smaller ships cannot be used without further ado for large ships.

This invention provides technical solutions that enable the use of large independent cylindrical tanks for the transport of liquefied gases, and in particular liquefied natural gas (LNG), and furthermore, these technical solutions eliminate pointed out weaknesses of alternative design concepts (membrane tanks and spherical tanks).

In particular, this applies to technical solutions for the following conditions:

- Avoid significant restrictions on filling the cargo tanks.

- Limited number of cargo tanks (2.3 or maximum 4 depending on the ship's total cargo capacity). - Simplified routing of pipes and cables, as well as access, between the top and bottom of each cargo tank

Furthermore, the invention provides constructions/reinforcements inside the tanks at the storage facilities which will enable relatively accurate stress calculations in materials for cargo tanks and hulls under prevailing load conditions.

The invention, which is defined in claims 1 and 10, essentially consists of providing two circular flapping bulkheads next to each other inside the cargo tank at each storage. The distance between the bulkheads can normally be of the order of approx. 1-3 meters. A framework of stiffeners will be welded in between the flat bulkheads, so that the two bulkheads are inextricably linked to each other. Corresponding sections of the cargo tank's outer shell plates will then be welded to the periphery of the two perforated bulkheads. The two perforated circular bulkheads, intermediate stiffeners and outer shell plates will thus appear as a very rigid and uniform construction. The two bulkheads will have a number of openings/perforations for quick compensation of differences in level that may occur during seagoing. The two bulkheads together with the intervening welded-in framework will have an almost infinite radial stiffness, and the tank will globally become almost radially undeformable at the bearings under all prevailing loads. This, in turn, will simplify the calculation work for mapping voltages in this area, and the requirement for accurate calculations can be met. Furthermore, a double bulkhead with an intermediate framework will be able to resist forces from liquid movements in an effective way, and local stresses in the tank shell where the bulkheads are welded to the shell will, relatively speaking, be significantly less than for a tank with a single brace bulkhead. As an example, if such a ship with a total cargo capacity of approx. 145,000 m3 is equipped with three cargo tanks, so there is reason to believe that this ship will not have any restrictions on partial filling of the tanks at all by optimizing openings in the bulkheads. This will be a competitive advantage compared to membrane type and ball tank type ships, as both of these types must have a minimum of 4 cargo tanks for this cargo capacity, and in addition, a ship with this invention will likely not have filling restrictions for the cargo tanks.

Furthermore, the space between the bulkheads can be used in an elegant and efficient way for routing pipes and cables, as well as for access, between the top and bottom of all tanks. Dom for connecting pipes and cables, as well as with a hatch for access, is arranged directly above the double bulkhead at the aft storage for the tank.

An additional advantage of this design concept is the ability to produce the cylindrical sections with built-in bulkheads/framework at the supports with exact roundness.

The perforated bulkheads with intermediate frameworks can be produced and fully welded first, and the bulkhead plates can be produced with excess dimensions. The bulkheads can then be measured, marked and burned to an exact diameter, and exact circular roundness of the section is achieved. Then the associated shell plates for the support section can be welded on, and exact roundness is maintained.

Another finesse which is thought to be used in connection with this invention is the installation of pressure transmitters along the periphery of the storages. In this way, pressure loads in the saddle bearings can be monitored at all times, and also compared against pre-calculated pressure loads along the bearing periphery.

A closer visualization of constructions and inventions is shown in the following figures: Fig.l shows a general arrangement for an LNG ship 1 with approx. 145,000 m3 cargo capacity, and with three 3 cylindrical cargo tanks 2. Fig.2A and Fig.2B show a cross-ship section through ship and cargo tank, (see Fig.l section A-A) and the section is shown between the two perforated bulkheads 3 at a storage for a cargo tank.

The figures show two alternative solutions for bracing between the perforated bulkheads:

Total

For this option (see Fig.2A), the stiffeners between the bulkheads consist of a framework of vertical 4 and horizontal 5 plate stiffeners. At the outer end, a concentric circular ring stiffener 6 is arranged, and between this and the shell plates, radial stiffening plates 7 are arranged. For optimal power transfer between the bulkheads and shell plates, it is important that the forces are transferred radially towards the outer shell plates.

Everything. 2.

For this option (see fig.2B), the stiffeners between the bulkheads consist of concentric ring stiffeners 6 and radial stiffening plates 7 between the ring stiffeners.

For both figures (and alternatives), conductors 8 and pipe and cable guides 9 between top and bottom of tanks are schematically shown.

Both of these figures show in principle perforations/openings 10 in a bulkhead, but the final number and location of openings in the bulkhead will be the subject of further assessments and calculations, and in view of optimal results with regard to stresses from sloshing in the tanks.

Both figures also show that the tanks are provided with external thermal insulation 11, saddle storage 12 and an insulating and pressure-resistant intermediate layer between saddle storage and cargo tank 13.

Fig. 3.

This figure shows section B-B which is indicated in Fig. 2A, and shows the two perforated bulkheads at a certain distance from each other. This distance has previously been indicated to be in the order of 1-4 metres. The figure also shows the principle of saddle storage in a warehouse where the cargo tank is locked against movement in the longitudinal direction of the tank.

This figure also shows that vertical and horizontal bracing plates (as well as concentric bracing) are provided with openings 14 for free flow of liquid, and for access to all places in the space between the two circular bulkheads.

Fig A

This figure shows Detail A as referred to in fig. 3, and relate to important details for the transfer of forces (mainly due to sloshing) in the longitudinal direction from the bulkheads 3 and radial plates 7 to the outer tank shell 17. Inside is shown knee plate 15 at the transition between bulkhead 3 and tank shell, and externally in the same plan shows knee plate 16. Both dissection plates are nicked and ground to zero at the end against the tank shell. Furthermore, external knee plates 18 are shown in the bearing zone, and in the same radial plane as other knee plates 15 and 16 and internal radial plate 7. The arrangement of the latter knee plate 18 is characterized by the fact that grooves must be milled in the intermediate material between the tank and the saddle bearing. In order to lock the tank against the longship's forward displacement at the one bearing, flat iron 19 is arranged externally along the periphery in the bearing zone of the tank.

Claims (10)

1. Device for a horizontal, largely cylindrical tank (2) for the transport of liquefied gases at low temperature in a ship, which tank (2) is supported in the ship (1) in at least two support rings (12) where the tank has an internal reinforcement which comprises a perforated and braced bulkhead, characterized in that the reinforcement comprises two perforated bulkheads (3) arranged next to each other, where a framework of crossing stiffeners (4-7) is welded between the bulkheads. 2. Device according to claim 1, where the bulkheads (3) have openings (10) whose size depends on the area of the respective bulkhead which is bounded by the nearest stiffeners (4-7). 3. Device according to claim 1 or 2, where there is room between the bulkheads (3) for routing pipes (9) and cables, as well as access (8) from top to bottom of the tank. 4. Device according to claim 1,2 or 3, where the stiffeners comprise tangentially extending and radially extending plates (6,7). 5. Device according to claim 4, where the stiffeners also comprise vertical and horizontal plates (4,5). 6. Device according to claim 4 or 5, where at least some of the stiffening plates (4-7) are provided with openings (14). 7. Device according to one of the preceding claims, where the bearings are saddle bearings (12) with load-bearing insulation material (13) in the same width as the distance between the bulkheads (3), the insulation material (13) being held laterally by flanges (19) welded to the tank's ( 2) shell (17) and reinforced with knee plates (16,18) which are preferably notched, as notched knee plates (15) are also arranged between the tank shell (17) and bulkheads (3). 8. Device according to claim 7, where pressure sensors are arranged along the periphery of the storage, which pressure sensors are connected to a monitoring system. 9. Device according to one of the preceding claims, where the tank (2) has a volume of the order of magnitude 50,000 m , and where the distance between the bulkheads (3) is preferably of the order of magnitude 1-4 m. 10. Procedure for manufacturing a largely cylindrical tank for the transport of liquefied gases at low temperature in ships, which tank (2) is supplied in at least two areas where the tank is to be supported in the ship (1), which reinforcement comprises a perforated and braced bulkhead, characterized in that the reinforcement is produced in the form of two oversized perforated bulkheads (3) arranged next to each other, where a framework of crossing stiffeners (4-7) is welded in between the bulkheads, after which the bulkheads (3) are cut to exact diameter and roundness before the associated shell surfaces (17) of the tank (2) in the support area are welded to the bulkheads (3).