Method Of Constructing A Low Temperature Liquefied Gas Tank Of A Membrane Type

Abstract

A method of constructing a low temperature liquefied gas tank of the membrane type equipped with a doublelayered inner membranous vessel composed of outside and inside membranous vessels, said double-layered inner membranous vessel being pre-constructed separately from the outer rigid vessel which is adapted to receive said inner vessel, said double-layered inner membranous vessel being thereafter mounted into said outer vessel, characterized in that a vacuum is applied to the space formed between said outside and inside membranous vessels in the process of mounting said inner vessel into said outer vessel.

Description

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a method of constructing a low temperature liquefied gas tank, and more particularly a method of constructing a low temperature liquefied gas tank of the membrane type for containing low temperature liquefied gases such as petroleum gases which are in a gaseous state at room temperature and can be liquefied by being cooled down under atmospheric pressure.

2. Description of the Prior Art

A tank of this kind is generally composed of a rigid outer vessel, a compression resistant heat insulating layer provided at the inside of said outer vessel, and an inner membranous vessel provided further at the inside of said heat insulating layer. The inner membranous vessel is made of a thin plate of low temperature resisting materials and adapted to be flexibly deformed to come in close contact with the inner surface of the heat insulating layer when the tank is loaded with liquefied gases in order to transmit the internal pressure exerted by the liquefied gases to the outer vessel by way of the compression resistant heat insulating layer so that the load of the tank is finally supported by the outer vessel.

A tank of this kind requires an additional secondary barrier wall for temporarily checking a leakage of the liquefied gases in case the leakage has occurred in the inner membranous vessel. Since such a secondary barrier wall is required to have the properties of resisting hydraulic load, resisting low temperature and of liquid tightness, it requires highly expensive materials and complicated structures.

To meet with the conditions required of the secondary barrier wall, it has been already proposed to provide a secondary membranous vessel at the outside of the membranous inner vessel to form as a whole a double-layered inner membranous vessel.

In the case of a low temperature liquefied gas tank equipped with the double-layered inner membranous vessel, if the tank is constructed in a manner of first constructing the outer vessel and the heat insulating layer, and thereafter constructing the double-layered membranous vessel within the tank, the latter requiring a relatively long period of construction, the overall period of construction is very much extended, thereby increasing the overall cost of the tank. In view of such a disadvantage, it is also proposed to construct the membranous vessel separately at the outside of the rigid outer vessel and to mount the pre-constructed inner membranous vessel into a hold space prepared in the outer vessel and adapted to receive the same. However, when the double-layered inner membranous vessel is to be pre-constructed and mounted into the hold space formed in the outer vessel, the outside and inside membranous vessels must be temporariy reinforced by a proper reinforcing frame. Such a reinforcing frame requires a complicated structure and increases the weight of the assembly including the outside and inside membranous vessels, requiring a heavy crane for suspending and mounting the same and thus increasing the overall cost of the construction.

SUMMARY OF THE INVENTION

Therefore, it is the object of this invention to conquer the abovementioned difficulties in the conventional methods of constructing a low temperature liquefied gas tank equipped with a double-layered inner membranous vessel and to provide a method to efficiently mount the pre-constructed double-layered inner membranous vessel into the hold space formed in the outer vessel by utilizing a particular structure of a pre-constructed double-layered inner membranous vessel formed of two gas-tight membranes one enclosing the other in substantially close contact therewith.

The abovementioned object is accomplished, according to this invention, by applying a vacuum to a space left between the outside and inside membranous vessels of the pre-constructed double-layered inner membranous vessel in the process of mounting the same into the hold space formed in the outer vessel.

By applying a vacuum to the space left between the outside and inside membranous vessels, the two membranous vessels come in close and firm contact with each other and thus act as a single membranous vessel having so much increased rigidity that the pre-assembled double-layered inner membranous vessel can be readily suspended and mounted into the hold space formed in the outer vessel without the requirement of utilizing complicated reinforcing frames.

BRIEF DESCRIPTION OF THE DRAWING

The accompanying drawing illustrates the manner of performing the method of this invention applied to the construction of a low temperature liquefied gas tank of a membrane type having a double-layered inner membranous vessel incorporated in a tanker ship.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the following, this invention will be described in more detail with respect to the preferred embodiment and with reference to the accompanying drawing.

The drawing shows a construction process of mounting an inner vessel of the double-layered membranous structure into a hold space of a tanker ship, wherein a dual-walled hull 1 including an inner wall 1a is lined with a compression resistant heat insulating layer 2 over the inside surface thereof to provide a hold space 3 opened at the upper portion thereof. Such a ship body generally designated by reference numeral 4 is constructed separately from an inner vessel assembly generally designated by reference numeral 5.

The heat insulating layer 2 may be made of a material having by itself compression resisting characteristics such as hard polyurethane foam as an organic material or foamed concrete as an inorganic material. The inner surface of the insulating layer is preferably covered with a protecting plate 23 made of plywood, etc. Alternatively, the heat insulating layer 2 may be made as a composite structure including a framework made of wood, etc., the space left therein being filled with pealite powder, etc. to provide as a whole a compression resistant heat insulating layer.

The inner vessel assembly 5 comprising a double-layered membrane structure composed of an outside inner membranous vessel 6 and an inside inner membranous vessel 7 is constructed separately from and preferably in parallel with the construction of the ship body 4. The outside and inside inner vessels 6 and 7 are made of thin plates of low temperature resisting materials such as nickel steel, stainless steel, aluminum, etc., and the two inner vessels may have a same thickness or may have difference thicknesses so that one is thinner than the other.

Above the inner vessel assembly 5, there is provided a carrying structure generally designated by reference numeral 11 including a tank cover 8 of a rigid structure, a heat insulating layer 9 provided at the underside of the cover 8 and a protecting plate 10 covering the side and lower surfaces of the heat insulating layer 9. The tank cover 8 is provided with a rigid dome 12 at a central portion thereof. Top panel portions 6a and 7a respectively of the outside and inside inner vessels 6 and 7 are positioned to follow the underside surface of the protecting plate 10 and are fluid-tightly fixed to a flange 13 mounted at a lower portion of the dome 12. From the lowermost end portion of the rigid dome 12 are radially extended cantilevers 14 adapted to hold the top panel portions of the inner vessels from falling down.

The inner vessel assembly 5 under construction is held in a properly self-standing condition by the carrying structure 11 being supported by proper supporting means (not shown), so as not to cause any buckling of the inner vessels due to their own gravity. The inner vessel assembly 5 thus constructed is attached with a conduit 16 adapted to open to the space formed between the outside and inside inner vessels via an opening 15 formed in either of the outside or inside inner vessel according to the convenience of mounting the conduit 16.

The conduit 16 is connected to a vacuum device 18 such as a vacuum pump by way of a pipe 17 made of a flexible material such as rubber, and the air filling a cavity 19 formed between the outside and inside inner vessels 6 and 7 is exhausted. As the air is exhausted, the two inner vessels 6 and 7 come in close and firm contact with each other. Then the inner vessel assembly 5 is suspended by applying wire ropes 20 to the carrying structure 11 and by suspending the wire ropes by a crane 21. Then the inner vessel assembly is brought to a position just above the hold space 3 and is lowered and mounted into the hold space. When the inner vessel assembly 5 is regularly positioned in the hold space, the tank cover 8 is fixed to the hull 1 by welding or other proper fastening means.

When the inner vessel assembly 5 is suspended by holding the carrying structure 11, the cantilevers 14 support the top panel portions of the inner vessels, whereby stress concentration occurring at the flange 13 where the weight of the inner vessel assembly would be concentrated is avoided. The cantilevers 14 may also support a bottom portion 7b of the inside inner vessel 7 by means of suspension means 22. In this case, since the outside and inside inner vessels are firmly stuck to each other due to the vacuum action, the two inner vessels are supported as a unitary body by suspending only the bottom portion 7b of the inside inner vessel 7. The conduit 16 may be provided with a valve so that it is preserved after the completion of the mounting process to be used as a means for detecting leakage of the inside inner vessel.

The effects and advantages of the method of constructing a low temperature liquefied gas tank of a membranous type according to this invention are summarized as follows:

1. Since the inner vessel assembly and an outer vessel assembly such as the ship body can be constructed separately in parallel so that the two construction processes do not interfere with each other, the overall construction period of the tank is very much shortened.

2. By applying vacuum to the space formed between the outer and inner vessels prior to the mounting process of the inner vessel assembly, the inner vessel assembly can be temporarily provided with an increased rigidity to be able to hold its space under suspension and thus can be easily mounted into the outer vessel assembly.

3. In the process of mounting the inner vessel assembly into the outer vessel assembly, no reinforcing means such as a supporting frame is required thereby shortening, the construction period and lowering the construction cost.

4. After the inner vessel assembly has been mounted in the outer vessel assembly, test gases are introduced inside the inner vessel and the gases filling the space formed between the outer and inner vessels are extracted to test the fluid-tightness of the inner vessel. In this case, the conduit means provided for exhausting gases from the space is used since it has been preserved for this purpose.

Claims

I claim:

1. In a method of constructing a low temperature liquefied gas tank of the membrane type, said tank comprising a rigid outer vessel, a compression-resistant heat insulating layer provided at the inside of said outer vessel, and a double-layered inner membranous vessel including outside and inside membranous vessels disposed at the inside of said heat insulating layer, said method comrising the steps of constructing said outer vessel and said heat insulating layer so as to provide a hold space having an opened upper portion for receiving said double-layered inner membranous vessel, said double-layered inner membranous vessel being constructed separately from said outer vessel and heat-insulating layer, and thus outside of said hold space, and thereafter mounting said double-layered inner membranous vessel into said hold space, the improvement which comprises applying a vacuum to the space formed between said outside and inside membranous vessels in the process of mounting said double-layered inner membranous vessel into said hold space wherein said outside and inside membranous vessels are caused to contact each other substantially over the entire surface thereof, thereby increasing the rigidity of the double-layered inner membranous vessel.

2. The method according to claim 1, wherein said double-layered inner membranous vessel is pre-constructed as an inner vessel assembly including a rigid carrying structure provided above said double-layered inner membranous vessel.

3. The method according to claim 2, wherein top panel portions of said outside and inside membranous vessels are supported by cantilevers firmly mounted to said carrying structure when said inner vessel assembly is suspended.

4. The method according to claim 2, wherein bottom portions of said outside and inside membranous vessels are suspended by suspension means extending from said rigid carrying structure to the bottom portion of the inside membranous vessel.