Field of the invention The invention relates to a tank for liquid or liquefied gas as defined in the preamble of claim 1.

Background of the invention/ State of the art When storing and transporting liquid or liquefied gas it is crucial to ensure stability and durability of the filled storing tank in order to avoid hazards caused by sloshing and leakage of the flammable liquid, as the stored liquid such as petrol or liquefied natural gas is highly flammable. Requirements are high especially when gas or liquid is stored and transported at a pressure higher than ambient pressure. In order to improve security, tanks used for liquid or liquefied gas are equipped with at least one inner pressure vessel and one secondary bunding shell.

Liquefied natural gas is stored at a temperature below -152°, and liquefied oxygen and nitrogen even at lower temperature. It is crucial to keep the liquefied gas at its low temperature in order to reduce vaporization of the liquid and avoid pressure increase inside the tank. Therefore an important part of a liquefied gas tank is insulation provided between a first shell serving as a pressure tank and a consecutive outer shell or outer shells.

It is important to construct a liquid or liquefied gas tank and its components such as the inner shell, outer shell(s) and the base of the tank and the attachment of the tank to a foundation in a way that it can withstand stresses caused by temperature changes and external forces acting on the tank.

Deformation of the inner shell caused by temperature changes is a problem especially in liquefied gas tanks. The outermost tank shell again is subjected to weather and temperature conditions of the environment.

Deformations of the material due to temperature changes put significant stress especially on non-uniform parts of the tank. Therefore spherically and cylindrically shaped tanks have been widely used, as the pressure of the content distributes more evenly within the tank. A spherical tank also thermally expands evenly, which puts less stress on welding seams of the tank.

Cylindrical tanks and especially spherical tanks have the disadvantage of having a relatively high center of gravity in relation to their volume. This can be a problem when tanks are subjected to movement of the foundations, for example when they are used in areas with seismic activity or when tanks are installed on transporting or fuel means, for example on liquid carriers or liquefied gas carriers, such as LNG carriers or LPG carriers or fuel tanks for ships and other vehicles.

Movement of the foundation causes movement of the tank with its shells and content and possibly sloshing of the content. The higher the center of gravity the larger the momentum caused by the movement acting on the tank base and attachment points.

Especially when liquid or liquefied gas tank are used in areas with limited space, for example on liquefied gas carrier and ships in general, it is also important to offer a tank that utilizes the available space as efficiently as possible, and with lowest weight of the installation.

Objective of the invention

It is objective of the invention to eliminate the drawbacks of the prior art that haven been outlined above. In particular it is objective of the invention to provide an improved liquid or liquefied gas tank that is well suited for the use of all kinds of transporting, storing and fuelling applications on vehicles or vessels as well as onshore.

Summary of the invention/ Brief description of the invention

A tank for liquid or liquefied gas according to the invention has an inner shell serving as a pressure tank and at least one outer shell spaced apart from the inner shell. Said inner shell and said at least one outer shell are composed of shaped sheet metal segments joined together and the shape of said at least one outer shell is substantially similar to the shape of the inner shell. According to the invention each tank shell comprises a bottom part and an upper part, each composed of at least one shaped sheet metal segment, joined together along a substantially horizontal line. Furthermore along at least one vertical central plane of the shell the upper part has a partial circle cross-section with radius rl, and the bottom part has a partial circle cross-section with radius r2.

In one embodiment radius r2 is 1.1-3 times, preferably 1.5-2.5 times larger than radius rl. According to another embodiment the shells are of symmetric shape around a vertical central axis.

In one embodiment the bottom part is a partial cylinder and the upper part is a partial cylinder, and together they form an elongated tank middle part. A sheet metal head with protruding shape has been joined to each end of the tank middle part in order to form a closed shell.

One option is that the bottom part of the inner shell has thicker walls than upper part of the inner shell.

According to one embodiment at least three evenly distributed base legs are joined to the tank, wherein each base leg comprises a support ring being attached from its top to the inner shell and from its bottom to a part connecting the support ring to a mounting part provided with means for attaching the tank to a foundation structure, and in its middle area the support ring is equipped with a plurality of evenly distributed relief openings.

It is one option that the support ring inside each base leg is equipped at along its edges with a plurality of evenly distributed recesses.

According to another embodiment a sealing disc is attached to the inner wall of the support ring below the recesses of the upper edge in order to maintain a vacuum between the inner shell and a first outer shell and the support ring is joined hermetically sealed from its outer sides to the first outer shell. This sealing disc can for example be fabricated of the same material as the tank shells. It is another option in case of storing liquefied gases that the base leg is provided at its inside with a mass insulation.

A tank according to the invention has a center of gravity that is 30-50 % lower than the center of gravity of a cylindrical or spherical tank of similar volume. This lowered center of gravity is especially advantageous in rough environment, such as in areas with seismic activity and when tanks are installed on transporting or fuelling means, such as road, rail or sea and river transportation and vehicles, and means the tank is more stable than regular cylindrical or spherical shaped tanks when used in rough environment.

Deformation of the tank due to temperature changes has been taken into account in the design phase. The improved base of the tank accommodates deformations of shell or shells caused for example by temperature changes and the weight of the liquid content. This for example prevents ruptures of the welding seams between the tank and the base and improves durability and longevity of the tank.

The new shape of the tank improves rigidity of the tank in a horizontal direction and reduces remarkably stresses on its structure caused by the dynamic load condition, and thus less support is needed.

List of figures

Figure 1 depicts a transport tank according to the invention installed in a liquid or liquefied gas carrier.

Figure 2 schematically depicts a cross-section of a storage or fuel tank

according to the invention.

Figure 3 depicts another transport, storage or fuel tank embodiment according to the invention.

Figure 4 depict the structure of an integrated base leg used with a tank

according to the invention. Figure 5 depicts a support ring used in an integrated base leg used with a tank according to the invention.

Figure 6 depicts a storage or fuel tank according to the invention installed

onshore or on a deck of a ship.

Detailed description of the invention

A tank according to invention is composed of an inner shell forming a pressure tank and at least one outer shell. The at least one outer shell has usually a similar shape as the inner shell and is arranged spaced apart from the inner shell. Especially in tanks for liquefied gas, heat insulation is usually arranged between the inner shell and the outer shell(s). Heat insulation can be formed by a vacuum insulation or by insulating material. It is also possible to arrange several outer shells with several insulating layers between the shells.

In figure 1 is depicted one embodiment of the invention. Figure shows a tank 1 for liquid or liquefied gas mounted into the hull of a cargo vessel 101. In this case the tank 1 is equipped with base legs 11 that are attached to each side of the bottom part of vessel structure. Here the tank is secured to the hull 101 through flanges on the bottom of each base leg 11. The tank uses the space available inside the hull 101 in an optimal way. Thanks to the compact leg structure and the shape of the tank, the installation also has a lower center of gravity CGI as compared to for example spherical tanks of similar volume.

In figure 2 a schematic cross-section of a tank according to the invention is depicted. Here only one base leg 11 is depicted, but it is obvious that a tank usually needs at least three base legs. The depicted tank has double vacuum insulation. The tank consists of an inner shell 2 serving as the pressure tank encompassing volume 16. The inner shell 2 is composed of an upper part 2a and a bottom 2b that have been welded together. The thickness of the pressure tank wall parts (2a, 2b) is chosen in a way that the largest deformation caused for example by thermal expansion occurs in the upper part of the shell 2a and tank wall deformation in the bottom part 2b at the attachment points of the base legs 11 is minimized. Due to the distribution of stress acting on the pressure tank shell, as the stress is highest in the bottom part of the tank, the bottom part 2b is preferably composed of thicker sheet metal than the upper part 2a. The wall thickness of the bottom part is preferably 1.5 to 2 times the wall thickness of the upper part depending mainly on the radius of the lower part, the pressure and the wall thickness of the upper part. Reinforcing the bottom part of the tank improves stability and strength and reduces stress acting on the base legs 11, thus facilitating a simpler base leg structure. The bottom part of the tank is reinforced preferably in all applications of a tank according to the invention. Similar to the inner shell 2, also the outer shell or shell(s) are composed of an upper part and a bottom part. The outer shell 6 depicted in figure 2 has a double structure, that means it consists of two shells, with vacuum insulation 15 arranged between the shells. The upper and bottom parts of the outer shells can have a similar wall thickness that means reinforcement of the bottom part of the outer shells is not needed.

A vacuum insulation 3 is arranged between the inner shell and the outer shell. The tank is additionally equipped with preferable closable pipes 17 for filling the tank and to remove air during filling.

Upper and bottom parts of the shells are joined together for example by welding and it is an aim to create a smooth transition between the parts. This smooth transition is especially important for the inner shell forming the pressure tank in order to ensure a relatively even distribution of pressure.

Figure 3 depicts another embodiment of a tank 1 according to the invention. The tank 1 in question is an elongated tank which body is composed of two partly cylindrical parts joined together at its wall along a substantially horizontal line. Also in this embodiment of a tank according to the invention, the bottom cylindrical part has of the inner shell has a radius that is preferably 1.5-2 times larger than the radius of the upper part. To each end of the tank body, a so called tank head is attached, preferably by welding it along the circumferential profile of the tank body walls. The tank head is a protruding sheet metal part often composed of several sheet metal segments joined together by welding and is the type as commonly used in pressure tanks. In this case the tank has no integrated base legs, and it is supported by commonly used foundation base legs 103. However, the integrated base legs can be applied for this kind of tank of the combined cylindrical shape as well. Figure 4 depicts an integrated base leg structure 11 to be used with a tank 1 according to the invention. The depicted embodiment is a base leg structure 11 provided with double insulation 8, 9, which is especially beneficial when the tank is used for the storage of liquefied gas. In this case, the tank has two outer shells (4, 6). An essential part of the base leg 11 is a support ring 10 that has a structure that is able accommodate to deformations of the pressure tank. The top of the support ring 10 is attached to the pressure tank 2, for example by welding, while its bottom part is attached to a flange ended ring part, connecting the support ring 10 to a mounting part 7 for the installation of the tank to a foundation, that can for example be the hull of a vessel, the frame of a transporter or foundation base legs when the tank is installed onshore or on deck. Here the mounting part 7 is implemented as a flange with bolt holes.

The base leg structure 11 is implemented with an integrated seal disk 13 that is joined to the inner wall of the support ring 10 in a hermetically sealed way in order to create a first vacuum insulation layer (3, 9). The second insulation layer inside the base leg is filled with insulating material 8 that can be the same material as used between the tank shells.

The integrated base leg structure can be designed according to the tank application. For example, if the tank is designed for a liquid that doesn't need a thermal insulation there only a plane ring, shorter than the presented one in Figure 4, and a mounting part are needed for the leg structure.

Figure 5 depicts an embodiment of a support ring 10 provided with reliefs 12 that are preferably evenly distributed along the circumference of the ring and that serve the purpose making the support structure more flexible so that it can accommodate deformations of especially the inner tank shell that occur for example during temperature changes. Preferably they are round shaped openings, but they can also be of different shape. To improve the accommodation of deformations, the ring is additionally equipped with recesses distributed along the edges of the ring. Preferably all recesses have the same rectangular shape and size and they are distributed along the upper and lower edge of the ring. Preferably the recesses of the upper and lower edge and thus also the remaining protruding portions are distributed similarly at intervals of 60° along the circumference of the ring, but also more or less recesses can be arranged along the circumference of the ring.

Preferably the support ring is attached to the pressure tank by welding it from at least three protruding portions to the tank.

Figure 6 depicts a storage or fuel tank 1 according to the invention installed onshore or on a deck of a ship on top of foundation base legs 102. The integrated base legs 11 of the tank 1 are joined to the foundation base legs 102 via flanges. The tank can also be attached to a foundation base that includes vibration damping. The tank depicted in figure 6 is one embodiment of the invention, a largely spherical tank. In figure 6 it can be seen that the also the bottom part 6b of the outer shell 6 has a larger radius than the upper part 6a.