NO329009B1 - Cylinder tank for transport and storage of cooled liquid gas on a liquid unit - Google Patents

Abstract

A tank structure for transporting and storing cooled liquid gas, wherein the tank (1) is adapted for placement within the hull of a liquid unit (2), the tank (1) comprising inner skin plates (3) arranged as a cylinder with vertical axis, and skin plates (4, 5) at the top and bottom of the tank. On the skin plates (3, 4, 5), a set of horizontally and vertically extending frames (11, 12) and horizontally extending struts (10) are welded to the interior of the tank in order to help absorb internal loads and reduce fluid movement. in the tank (1) as the float unit (2) moves.

Description

The invention relates to a tank construction for the transport and storage of chilled, liquefied gas, where the tank is adapted for placement inside the hull of a floating unit, the tank comprising inner skin plates which are arranged as a cylinder with a vertical axis, and skin plates in the top and bottom of the tank. A floating unit in the form of a cylinder-shaped platform of the type in question is described, for example, in Norwegian patent document no. 319 971. Another relevant example of a tank construction for the transport and storage of cooled, liquefied gas, and where the tank is arranged as a cylinder, is made in US 3863408 Al

Today's technology for the transport and storage of chilled, liquefied gas is mainly based on three different construction inventions. Based on tanks arranged in a ship, these three types of construction can be briefly described as follows: • Ball tanks built from aluminium. The tanks are self-supporting and are stored in the ship's hull on a "magnetic belt". • Membrane tanks that consist of thin plates of stainless steel that rest against an insulation. The steel plates in this construction do not contribute as a strength element, but only as a seal against gas. The thin steel plates are arranged with a certain corrugation to absorb deformations caused by temperature variations. In membrane tanks, there will usually be two membrane layers as there is a certain risk of puncturing the thin membrane plates. • Self-supporting "conventional" tanks. These are tanks that can be built from stainless steel or aluminium. The concept is based on building independent, self-supporting tanks that are placed in rooms/tanks in the ship's hull.

For tank facilities on land, there are two main types used:

• Tanks that lie downhill in the ground, cylindrical or of a different shape.

• Tanks built as a short vertical cylinder where internal pressure is carried by ring stresses in the cylinder. The tanks have a flat bottom and often a curved roof with a spherical shape.

For all the current designs, the tanks are insulated on the outside of the plates that form the tank itself. The insulation must be sufficient to keep gas stored at temperatures down to -170 °C without excessive heat transfer. This is important to reduce evaporation and to prevent surrounding structures from getting too low temperatures. The tank must also have an external barrier or a collection arrangement to capture any leaks.

It is an object of the invention to provide a tank structure which is suitable for the construction of large tanks which can be placed inside the hull of a cylindrical floating unit, e.g. with a diameter of around 80 m and a height of 30-40 m (approx. 200,000 m<3>).

Another object of the invention is to provide such a tank construction which is provided with an internal bracing which is suitable to absorb forces from internal pressure, to absorb lateral forces due to movements of the floating unit, and to reduce liquid movements in the tank when the floating unit moves.

The above purpose is achieved with a tank construction of the type indicated at the outset which, according to the invention, is characterized by the fact that a set of frames and struts are welded onto the skin plates inside the tank with the intention of helping to absorb internal loads and to reduce fluid movements in the tank when the floating unit moves.

An advantageous embodiment of the tank construction according to the invention is characterized by the fact that the frames form a grid pattern in the cylindrical part of the tank, as they are arranged in both vertical and horizontal directions.

It is further advantageous that the struts are arranged as horizontally extending ring struts which are supported by the vertical frames.

In the circular part of the tank, the horizontal frames and ring stiffeners will help to carry ring stresses together with the skin plates. This makes it possible to build large tanks with plate thicknesses that are not too large. The forces to be taken up are distributed between the struts, frames and plates. The frames in the upper part of the tank can be designed with sufficient strength to support the roof structure in the tank.

One of the main problems with today's LNG tanks in ships is problems related to liquid movement and liquid impact (sloshing) in the LNG tanks. This applies in particular to tanks that are only partially filled, which will be the situation for longer periods when producing LNG. Liquid impact in an open tank will produce large local pressure forces. This can easily cause damage to both the tank structure and the underlying insulation material. In the proposed construction, the arrangement of struts and frames will prevent fluid movements that result in high-pressure fluid impacts. Effects such as rotating liquid movement (swirling) will also be significantly reduced by the vertical frames.

The cylinder tank will be insulated along the sides and in the upper part with a suitable material. In these areas, there is no need for insulation material to transfer loads from the tank into the main hull. Underneath the bottom of the tank, it will be insulated with an insulating material that has sufficient strength to transfer the pressure forces from the bottom plates into the bottom structure in the main hull.

Internal pressure from liquid in the tank will be transferred directly to the substrate in the bottom plates. There will therefore be no need to make stiffeners on the bottom plates to increase the strength of the plates. However, frames and struts at the bottom will be arranged to reduce liquid movement when there is little filling in the tanks.

The cylindrical side plates will experience ring stresses due to fluid pressure. In the case of large diameters, this will place great demands on the thickness of the plates in the cylinder. In order to reduce the plate thickness in the cylinder, as mentioned, horizontal ring stiffeners are arranged to help support the ring stresses. In addition, a set of frames is arranged both in the vertical (axial) and horizontal ring direction (tangential). The purpose of the frames is partly to form support for the horizontal struts which are stored in the vertical frames, partly to provide a support to be able to transfer horizontal forces from the cylinder tank into the outer hull, and partly to provide an effective damping of wave motions that can occur in a moving tank. The struts will also effectively reduce the effect of liquid impact (sloshing) in the tanks. In the case of unbraced flat surfaces, liquid impact can cause very high local loads.

When the floating unit is exposed to waves that can cause lateral movements or heeling (rolling or pounding), lateral forces will arise in the tank. Lateral forces will also occur as a result of heeling caused, for example, by damage to ballast tanks etc. Both of these load situations must be taken into account when designing the cylindrical tanks. In order to take up the lateral forces, attachment points have been arranged between the inner cylinder tank and the outer main hull in the floating unit. The installation points will be placed at the crossing points between vertical and horizontal frames.

The upper part of the tank (the roof) is exposed to a lower liquid pressure, but can receive additional loads due to waves in the tank. This upper part of the tank will preferably be built as a self-supporting roof construction where internal frames together with the skin plates form a construction with sufficient strength. Alternatively, it can be suspended in the deck structure in the main hull.

The invention shall be described in more detail in the following in connection with embodiment examples with reference to the drawings, there

fig. 1 shows an axial sectional view of a tank according to the invention which is placed in a cylindrical hull of a floating unit,

fig. 2 shows a horizontal section along the line II-II in fig. 1,

fig. 3 shows details of the internal bracing in the tank of fig. 1,

fig. 4 shows a section of the tank in fig. 1 in perspective,

fig. 5 shows detail A in fig. 3 on an enlarged scale,

fig. 6 shows detail B in fig. 3 on an enlarged scale,

fig. 7 shows an enlarged section through the tank wall and shows external insulation and a support block placed between the tank wall and the outer hull of the floating unit,

fig. 8 shows an enlarged partial section through a secondary barrier in connection with insulation under the bottom of the tank,

fig. 9 shows a section through a structure for transferring horizontal forces from the tank to a double bottom of the floating unit in which the tank is placed,

fig. 10 shows an axial sectional view of a tank according to the invention where bottom tanks for LPG etc. are arranged below the tank,

fig. 11 shows a section along the line Xl-XI in fig. 10,

fig. 12 shows an axial sectional view of a tank according to the invention which is provided with side tanks for LPG etc, and

fig. 13 shows a section along the line XIII-XIII in fig. 12.

In fig. 1-4 shows a circular cylindrical tank 1 according to the invention which is placed inside the hull of a floating unit which in the shown embodiment is in the form of a cylindrical platform 2. Although the invention will be described in connection with a cylindrical platform, it will be clear that the tank can also be used on board a ship, e.g. an LNG tanker.

The tank 1 comprises inner skin plates 3 which form a cylinder with vertical axis A-A, and skin plates 4 and 5 at the top and bottom of the tank respectively. The skin plates 3 and 4 in the cylindrical tank wall and in the top of the tank rest against an outer insulation 6 respectively. 7. The tank 1 stands on an insulated foundation 8 which rests on a double bottom 9 in the platform.

In accordance with the invention, the tank 1 is provided with an internal reinforcement by welding on the skin plates a set of horizontally extending ring stiffeners 10 and a set of horizontally and vertically extending frames 11 respectively. 12. As mentioned above, the purpose of the horizontal ring stiffeners 10 is to help carry ring stresses in the tank wall, while the purpose of the frames is partly to form support for the ring stiffeners 10 which are stored in the vertical frames 12, and partly to provide support for to be able to transfer horizontal forces from the cylinder tank into the outer platform hull. The ring stiffeners will also provide an effective reduction and partial elimination of local pressures caused by liquid impact (sloshing). The horizontal frames in the ring direction will effectively reduce the formation of waves in the tank, and the vertical frames will reduce the risk of getting rotating waves in the tank (swirling).

In order to absorb lateral forces occurring in the tank, anchor points are arranged between the cylinder tank 1 and the outer main hull of the floating unit 2. These anchor points are shown in the drawings as support blocks 13 which are placed at crossing points between horizontal and vertical frames 11 or 12.

Details A and B in fig. 3 is shown on a larger scale in fig. 5 and 6.

As can be seen from fig. 5, the horizontal ring stiffeners 10 which are welded to the skin plates 3 are stored in slots in the vertical frames 12. As indicated in the figure, the frames are provided with an inner edge flange 14 which has sufficient width to withstand expected forces. The figure also shows a structure 15 which extends through the insulation 6 between the horizontal frames 11 and the support blocks 13, for transferring loads in the radial direction to the support blocks which abut the platform 2 hull.

Fig. 7 is a schematic perspective view showing a section of the tank wall, which largely corresponds to fig. 5. Fig. 8 shows an enlarged partial section of a lower corner area of the tank and the underlying foundation 8. The insulation material in the foundation comprises two layers between which a plate structure 16 is placed which forms a secondary barrier that can capture LNG from a possible leak in the main idea. This plate must have an extent that ensures that any leakage from the main tank will be caught. Fig. 9 shows a structure for transferring horizontal forces from the LNG tank to the double bottom 9 in platform 2. The structure comprises a reinforcement 17 which is welded into the bottom of the tank and a reinforcement 18 for absorbing horizontal forces which is welded into the double bottom, and furthermore one or more transfer elements 19 which form a contact surface between the aforementioned reinforcements and are formed of an insulating material, e.g. three.

The structure is an alternative to transferring occurring lateral forces via the above-mentioned support blocks 13 which lie against the side of the platform. These are forces that arise as a result of horizontal accelerations or from tilting of the platform.

When producing gas, there will often be several different products that are produced. The usual thing is that in addition to the production of LNG, condensate and LPG (Liquefied Petroleum Gas) are also produced. LPG are heavier gas fractions that can be stored at a higher temperature than LNG, e.g. propane and butane.

In fig. 10-13 two versions of the tank construction are shown where tanks for condensate and LPG are drawn as part of the arrangement in the large inner cylinder volume in platform 2. Thus fig. 10 and 11 an embodiment where bottom tanks 20 for LPG/condensate are placed below the main tank 1 for LNG, above the supporting foundation 8. Furthermore, fig. 12 and 13 an embodiment where a number of side tanks 21 for LPG/condensate are arranged inside the main tank 1, around its circumference. Condensate tanks do not need cooling, and they can therefore be advantageously placed in the outer part of the platform hull.

Claims (8)

1. Tank construction for the transport and storage of cooled, liquefied gas, where the tank (1) is adapted for placement inside the hull of a floating unit (2), the tank (1) comprising inner skin plates (3) which are arranged as a cylinder with a vertical axis, and skin plates (4, 5) at the top and bottom of the tank, where a set of frames (11, 12) and struts (10) are welded to the skin plates (3, 4, 5) inside the tank, which occupy internal loads and reduces liquid movements in the tank (1) when the floating unit (2) moves, characterized in that a reinforcement structure (17) is arranged in the central area of the bottom of the tank (1) for the transfer of horizontal forces from the tank (1) to a structure (18) in the center of the floating unit's (2) hull via one or more transfer elements (1) of an insulating material. 2. Tank construction according to claim 1, characterized in that the frames (11, 12) form a grid pattern in the cylindrical part of the tank (1), as they are arranged in both vertical and horizontal directions. 3. Tank construction according to claim 1 or 2, characterized in that the struts (10) are arranged as horizontally extending ring struts which are supported by the vertical frames (12). 4. Tank construction according to claim 3, characterized in that the struts (10) are stored in horizontal slots in the vertical frames (12). 5. Tank construction according to one of claims 2-4, characterized in that the frames (11, 12) are provided with a reinforcing inner edge flange (14). 6. Tank construction according to one of the preceding claims, characterized in that a number of support blocks (13) are arranged between the outer wall of the cylinder tank (1) and the outer main hull of the floating unit (2) to absorb lateral forces in the tank. 7. Tank construction according to claim 6, characterized in that the support blocks (13) are placed at crossing points between vertical (12) and horizontal (11) frames. 8. Tank construction according to one of the preceding claims, characterized in that tanks (20, 21) for receiving LPG/condensate are placed under the main tank (1) and/or along the inside of the main tank.