WO2009067017A1

WO2009067017A1 - Cylindrical tank for transport and storage of chilled, liquified gas on a floating unit, with provisions for reducing liquid movements and absorbing deformations due to variations of the internal load

Info

Publication number: WO2009067017A1
Application number: PCT/NO2008/000397
Authority: WO
Inventors: Kåre SYVERTSEN; Arne Smedal; Jan Vidar Aarsnes
Original assignee: Sevan Marine Asa
Priority date: 2007-11-21
Filing date: 2008-11-07
Publication date: 2009-05-28
Also published as: NO20075996L; NO329009B1

Abstract

A tank structure for the transport and storing of refrigerated liquefied gas, where the tank (1) is adapted for a position within the hull of a floating unit (2), said tank (1) comprising inner sheathing / skin plates (3) arranged as a cylinder having a vertical axis, and sheathing / skin plates (4, 5) in the top and bottom of the tank, respectively. A set comprising horizontally and vertically extending frames (11, 12) and horizontally extending struts (20) are welded on the sheathing plates (3, 4, 5) within the tank. The purpose is to contribute to the absorption of interior loads and reduce liquid movements in the tank (1) during the movements of the floating unit (2).

Description

Cylindrical tank for transport and storage of chilled, liquified gas on a floating unit, with provisions for reducing liquid movements and absorbing deformations due to variations of the internal load.

Cylinder tank for transport and storage of chilled, liquefied gas on a floating unit

The present invention is regarding a tank structure for the transport and storage of refrigerated or chilled liquefied gas, where the tank is adapted for a position within the hull of a floating unit, said tank comprising internal sheathing plates or skin plates arranged to form a cylinder having a vertical axis, and sheathing plates in the top and bottom of the tank. A floating unit in the form of a cylindrical platform of the present type is inter alia described in the patent document NO 319 971. The technology of today for the transport and storage of refrigerated liquefied gas is mainly based upon three different structure embodiments. With reference to tanks arranged within a ship, these three structure types can briefly be described as follows:

- Spherical tanks made of aluminium. The tanks are self-supporting and being carried in the ship hull on a "magnetic bearing belt".

Diaphragm tanks consisting of thin plates of stainless steel, bearing against an insulation. The steel plates of this structure type are not contributing as any reinforcement element, just as a gas sealing. The thin steel plates are provided with a certain corrugation for absorbing deformations being caused by temperature variations. In diaphragm tanks there are usually two diaphragm layers, as there is a certain risk for puncturing of the thin diaphragm plates.

- Self-supporting "conventional" tanks. These are tanks that can be built of stainless steel or aluminium. The concept is based upon the construction of free standing self-supporting tanks to be installed in holds/vessels in the ship hull.

At tank installations on land two main types are used:

Tanks burrowed in the ground and being cylindrical or having a different form. Tanks built like a short vertical cylinder of which the internal pressure is taken up by (converted to) circumferential tensions in said cylinder. The tanks are flat bottomed and are often provided with a curved roof of spherical shape.

In all the actual embodiments the tanks are insulated at the exterior of the plates forming said tanks. The insulation should be effective to maintain the stored gas at temperatures down to -170 ⁰C without any excessive heat transfer. This is important in order to reduce evaporation and prevent that surrounding structures become too refrigerated. The tank should also have an exterior barrier or a coffer arrangement to catch possible leakages.

An object of the invention is to provide a tank structure suitable for the construction of large tanks to be positioned within the hull of a cylindrical floating unit, for example having a diameter around 80 m and a height of 30-40 m (ca. 200 000 m³).

Another object of the invention is to provide such a tank structure being provided with an interior stiffening suitable for absorbing forces from an internal pressure, absorbing lateral forces caused by movements of the floating unit, and reducing liquid movements in the tank during movements of the floating unit.

The above objects are achieved by a tank structure of the introductory disclosed type, said tank structure - in conformity with the invention - being remarkable in that a set of frames and struts are welded on the sheathing plates in the interior of the tank so as to contribute to the absorption of internal loads and reduce liquid movements in the tank during movements of the floating unit.

An advantageous embodiment of the tank structure according to the invention is particular in that the frames form a grid pattern in the cylindrical part of the tank since they are arranged both in the vertical and horizontal direction.

Further, it is preferable that the struts are arranged as horizontally extending ring struts being supported by the vertical frames.

The horizontal frames and the ring struts contribute to bear circumferential tensions in the circular part of the tank, together with the sheathing plates. This makes it possible to build huge tanks without the need for thick plates. The forces to be absorbed are distributed in the struts, frames and plates. The frames in the upper part of the tank can be given a sufficient strength to carry the roof structure of the tank.

One of the main problems of the ship carried LNG (Liquefied Natural Gas) tanks of today is the problem connected to the liquid movements and sloshing in said tanks. This in particular is a problem by tanks being only partly filled, a common situation over long periods during the LNG production. In an open tank sloshing will generate great local compressive forces, easily resulting in damages of the tank structure and the underlying insulation material. In the present structure the arrangement of struts and frames will prevent the establishing of liquid movements leading to sloshing at high pressures. Even effects like swirling are greatly reduced due to the vertical frames. The cylinder tank should be insulated along the sides and in the upper part by using a suitable material. In these regions the insulation material is not supposed to transfer loads from the tank to the main hull. Beneath the tank bottom the insulation material must have a sufficient strength to transfer the compression forces from the bottom plates to the main hull bottom structure. The interior pressure of the liquid in the tank is transferred directly to the support, making it unnecessary to arrange stiffening reinforcements at the bottom plates to increase their strength. Frames and struts are, however provided at the bottom to reduce the liquid movements in the tanks during low filling levels. The cylindrical side plates are subject to circumferential tensions due to the liquid pressure. By large diameters (of the tank) this would usually require thick cylinder plates. In order to reduce the plate thickness of the cylinder the arrangement of horizontal ring struts to contribute to the absorption of said circumferential tensions is already mentioned. Further is provided a frame set both in the vertical (axial) and the horizontal ring direction (tangentially). The purpose of the frames is partly to form a support for the horizontal struts carried by the vertical frames, partly to provide a support for transferring horizontal forces from the cylinder tank to the outer hull, and partly to provide an effective damping of wave movements that may be generated in a tank during movements. The stiffening struts are also effectively reducing the effects caused by tank sloshing. By unbraced plane surfaces sloshing can generate rather severe local loads.

When the floating unit is exposed for waves that may generate lateral movements or healing (rolling or pitching), lateral forces are generated in the tank. Lateral forces are also the result of healing resulting from inter alia damages of ballast tanks etc. By the construction of the cylindrical tanks both these load situations should be considered. In order to absorb the lateral forces bearing points (areas) are arranged between the inner cylinder tank and the outer main hull of the floating unit. These bearing points are preferably arranged at the crossings between the vertical and the horizontal frames. The upper part of the tank (the top or roof) is exposed to a lower liquid pressure but may receive additional loads due to waves in the tank and therefore preferably is constructed as a self-supporting roof structure having inner frames and sheathing plates, for obtaining a sufficient structure strength. Alternatively, the upper part of the tank can be hinged in the main hull deck structure. The invention is described further below by examples of embodiments being illustrated in the drawings, where:

Figure 1 is an axial cross section of a tank according to the invention and positioned in a cylindrical hull of a floating unit,

Figure 2 is a horizontal cross section following the line H-II of Figure 1, Figure 3 illustrates details of the inner stiffening of the tank of Figure 1,

Figure 4 is a section of the tank of Figure 1 in a perspective view,

Figure 5 shows the detail A of Figure 3 in an enlarged scale,

Figure 6 shows the detail B of Figure 3 in an enlarged scale, Figure 7 is an enlarged cross section through the tank wall and shows the external insulation and a support block arranged between said tank wall and the outer hull of the floating unit,

Figure 8 is an enlarged cross section through a secondary barrier of the insulation beneath the bottom of the tank,

Figure 9 is a cross section through a structure for transfer of horizontal forces from the tank to a double bottom in the floating unit of which the tank is positioned,

Figure 10 is an axial cross section of a tank according to the invention, where bottom tanks for LPG etc. are provided beneath the tank, Figure 11 is a cross section taken along the line XI-XI of Figure 10,

Figure 12 is axial cross section of a tank according to the invention and provided with lateral tanks for LPG etc., and

Figure 13 is a cross section along the line XIII-XIII of Figure 12.

In Figures 1-4 is shown a circular cylindrical tank 1 according to the invention and being positioned within the hull belonging to a floating unit. In the shown embodiment this floating unit is in the form of a cylindrical platform 2. Although the invention will be described in connection with a cylindrical platform it is obvious that the tank also may be used onboard a ship, such as a LNG tanker.

The tank 1 comprises interior or inner sheathing plates or skin plates 3 forming a cylinder having a vertical axis A-A and sheathing plates 4 and 5 in the top (roof) and bottom of the tank, respectively. The sheathing plates 3 and 4 of the cylindrical tank wall and in the tank top are bearing against an outer (exterior) insulation 6 and 7, respectively. The tank 1 rests on an insulated fundament 8, in its turn resting on a double bottom 9 of the platform. According to the invention the tank 1 is provided with an interior reinforcement by a set horizontally extending ring struts 10 and a set comprising horizontally and vertically extending frames 11 and 12, respectively, said sets being welded to the sheathing plates. As mentioned above the purpose of the horizontal ring struts 10 is to contribute to the absorbtion of circumferential tensions in the tank wall, while the purpose of the frames is partly to form a support for the ring struts 10 carried in the vertical frames 12 and partly provide a support for the transfer of horizontal forces from the cylinder tank to the external platform hull. The ring struts also effectively reduce and partly eliminate local pressures due to sloshing. The horizontal frames in the ring (circumferential) direction effectively reduce the generation of waves in the tank, and the vertical frames reduce the risk for getting rotating waves in the tank (swirling).

In order to absorb generated lateral forces in the tank is arranged bearing points (regions) between the cylinder tank 1 and the exterior (outer) main hull of the floating unit 2. These bearing points are in the drawings shown as support blocks 13 positioned in the crossings between the horizontal and vertical frames 11 and 12.

The details A and B shown in Figure 3 are given in greater scale in Figures 5 and 6. In Figure 5 is seen that the horizontal ring struts 10 being fixed to the sheathing plates 3 by welding, carried in slots in the vertical frames 12. As indicated in the drawing these frames are provided with an inner edge flange 14 having a sufficient width to withstand the expected forces. In the drawing is further illustrated a structure 15 extending through the insulation 6 between the horizontal frames 11 and the support blocks 13, for the transfer of loads in radial direction to said support blocks that in turn are bearing against the hull of the platform 2.

Figure 7 is a schematic perspective cross section showing a part of the tank wall and mainly corresponding to Figure 5.

Figure 8 illustrates an enlarged partial cross section of a lower corner region of the tank and the fundament 8 below. The insulation material of the fundament comprises two layers between which is arranged a plate structure 16 providing a secondary barrier to catch LNG from possible leakages in the main tank. This plate should have an extension to secure that any leakage from the main tank will be taken care of. Figure 9 illustrates a structure for the transfer of horizontal forces from the

LNG tank to the double bottom 9 of the platform 2. The structure comprises a reinforcement 17 welded in the tank bottom, a reinforcement 18 for absorbing horizontal forces and being welded in the double bottom, and further one or more transfer elements 19 forming a bearing surface between said reinforcements and made by an insulating material, for example wood.

The structure is an alternative to transferring generated lateral forces via the above mentioned supporting blocks 13 that bear against the platform side. These forces are generated due to horizontal accelerations or platform healing.

During the production of gas several different products often are produced. A common incidence is that also condensate and LPG (Liquefied Petroleum Gas) are produced, in addition to the production of LNG. LPG is in the form of heavier gas fractions that can be stored at higher temperatures than LNG, such as propane and butane.

Figures W- 13 illustrate two different embodiments of the tank structure where tanks for condensate and LPG are shown as a part of the arrangement within the greater inner cylindrical volume of the platform 2. Thereby Figures 10 and 11 show an embodiment having bottom tanks 20 for LPG/condensate and positioned beneath the main tank 1 for LNG, above the supporting fundament 8. Further, Figures 12 and 13 illustrate an embodiment where a number of lateral tanks 21 for LPG/condensate are arranged within the main tank 1, around its circumference. Condensate tanks have no use for refrigeration and therefore may be positioned in the outer part of the platform hull.

Claims

C l a i m s

1. A tank structure for the transport and storage of refrigerated liquefied gas, said structure comprising a tank (1) adapted for a position within the hull of a floating unit (2), said tank (1) comprising internal sheathing plates (3) arranged to form a cylinder having a vertical axis, and sheathing plates (4, 5) in the top and bottom of the tank, respectively, characterized in that a set of frames (11, 12) and struts (10) are welded on the sheathing plates (3, 4, 5) in the interior of the tank so as to contribute to the absorption of internal loads and reduce liquid movements in the tank (1) during movements of the floating unit (2).

2. A tank structure according to claim 1, characterized in that the frames (11, 12) form a grid pattern in the cylindrical part of the tank (1), as said frames are arranged both in the vertical and horizontal direction.

3. A tank structure according to claim 1 or 2, characterized in that the struts (10) are arranged as horizontally extending ring struts being supported by the vertical frames (12).

4. A tank structure according to claim 3, characterized in that the struts (10) are carried in horizontal slots in the vertical frames (12).

5. A tank structure according to any of the claims 2- 4, characterized in that the frames (11, 12) are provided with a reinforcing inner edge flange (14).

6. A tank structure according to any of the preceding claims, characterized in that a number of supporting blocks (13) for absorbing lateral forces in the tank are provided between the outer wall of the cylinder tank (1) and the outer main hull of the floating unit (2).

7. A tank structure according to claim 6, characterized in that the supporting blocks (13) are provided in crossings between the vertical (12) frames and the horizontal (11) frames.

8. A tank structure according to any of the preceding claims, characterized in that a reinforcement structure (17) for the transfer of horizontal forces from the tank (1) to a structure (18) in the centre of the hull of the floating unit (2) via at least one transfer element (19) of an insulating material, is provided in the central portion of the tank (1) bottom (5).

9. A tank structure according to any of the preceding claims, characterized in that tanks (20, 21) for storing LPG/condensate are arranged beneath the main tank (1) and/or along the inside of said main tank.