Liquefied gas storage barge with concrete floating structure

Details Liquefied gas storage barge with concrete floating structure Liquefied gas storage barge with concrete floating structure

2002-04-23

2004-09-07

Swinehart, Ed (Department: 3617)

Ships

Building

Freighters

Reexamination Certificate

active

06786166

ABSTRACT:

BACKGROUND OF THE INVENTION
(1) Field of the Invention
The present invention relates generally to storing liquids in a concrete structure that can be moved while floating, and more specifically, to a barge for storing liquefied gas at sea.
(2) Description of the Related Art
Liquefied gases, such as methane, are stored either in freestanding tanks that are cylindrical, spherical, or prismatic, being made out of sheets of special steel or of thick aluminum, or else in tanks constituted by a thin membrane that provides leakproof confinement associated with a thermal insulation system constituted by blocks of foam, with the insulation system resting continuously on a support structure.
In the second configuration, the membrane is thus dimensioned solely to provide leakproof confinement of the liquid, and the mechanical strength of the assembly is provided by the external support structure which is not subjected to cryogenic cold.
French patent No. 2 271 497 discloses in particular such confinement and insulation systems suitable for liquefied natural gas (LNG) tankers and for storage on land, as developed amongst others by Société Gaz Transport & Technigaz (Trappes, France).
Tankers fitted to receive cryogenic storage tanks are normally made of steel by specialist ship yards. For liquid methane, given that the specific gravity of the substance is low (d≈0.47), the configuration of such tankers is most unusual, and because of the high cost of making thermally insulated tanks, LNG tankers are extremely expensive. They also require many precautions to be taken in operation since in the event of liquefied gas leaking onto structural elements of the steel hull of the tanker, the steel becomes brittle and no longer withstands the stresses from the surroundings, leading to the vessel being lost.
Floating structures that are similar but made of concrete have been envisaged because concrete behaves well when put into contact with liquefied gas at very low temperature, however such structures have been designed for sailing purposes and are much bulkier and more massive than vessels made of steel, so the resulting vessels are not economically competitive with equivalent vessels made of steel. Furthermore, their draft requires them to be built in dry docks that are deep, so as to make it possible for them to be moved into deeper water after the dry dock has been flooded.
DE 2 644 856 and FR 2 366 984 disclose a concrete vessel transporting tanks located in concrete compartments. In order to minimize the wetted surface area of the vessel, it has a flat bottom on which the bottom walls of the compartments rest. The side walls of the compartments are supported by cradle type support structures.
A concrete barge is also known that was built for the Ardjuna field (Indonesia) to store liquefied petroleum gas. Gas is stored therein at a temperature of −45° C. in freestanding cylindrical tanks that are thermally insulated, of circular section, and made of steel of medium thickness. The gas comprises butane and propane only. The tanks are stored on two levels: one series of six tanks is stored on deck and a second series of six tanks is stored inside the hull. Each of the tanks inside the hull rests on two cradles that form part of the concrete structure of the hull of the barge. The function of the cradles is to provide supports that come close to an isostatic system, thus minimizing the stresses generated by differential deformation between the tank and the structure of the barge and enabling the load corresponding to the weight of the tank plus its content, i.e. about 3000 (metric) tonnes, to be transferred under good conditions to the hull of the barge, which hull is subjected to buoyancy thrust over its entire wetted surface.
In that configuration, the load as distributed along the tank is concentrated via the cradles and then transferred through the cradles to the hull of the barge, thus giving rise to large concentrated forces, and then finally the load is distributed over the entire active zone of the hull that is subjected to buoyancy thrust. The barge measures about 140 meters (m) in length, 40 m in width, and 16 m in depth, and it is capable of storing about 60,000 cubic meters (m
3
) of gas distributed between twelve identical lagged tanks.
In the storage zone for the lagged tanks, the concrete walls corresponding to the bottom, to the sides, and to the bulkheads are thus provided with reinforcing structures including thick beams associated with concrete shells or webs that must be over-dimensioned, at least in the transition zone, so that force distribution to the remainder of the structure of the hull can take place in satisfactory manner.
Concrete barges have not yet been proposed for storing quantities of gas in excess of 60,000 m
3
, nor for storing liquefied gas at temperatures lower than −50° C., i.e. gas other than liquefied butane or propane, and in particular for liquid methane.
With the techniques commonly used for making barges of concrete structure, giant barge building cannot be extrapolated from the technology used for the Ardjuna barge since that would require either the number of conventional tanks to be multiplied or else tanks to be made that are small in number but gigantic in size, based on free-standing technology, but in that case there would be very great difficulties of implementation, or even technical impossibilities, because of the considerable loads to be transferred via the cradles (isostatic support).
Such giant tanks, for the cryogenic temperatures of liquefied methane (−165° C.) present significant shortening of the inside wall of the tank when it is cooled down, thus creating differential displacements at the supports between the tank and the structure of the barge, since the structure remains at ambient temperature. The supports become very difficult to design since they must be capable of accommodating these movements without giving rise to significant levels of stress which could create fatigue phenomena in said supports or in the tank, thus making such a barge dangerous to operate. These shrinkage phenomena exist with small tanks storing propane cooled to −50° C., but they can be overcome using supports of appropriate design. Extrapolating such principles to giant tanks working at −165° C. or at even lower temperatures would lead to support systems that are extremely complex, requiring major reinforcement of the concrete hull and thus requiring very large quantities of prestressed concrete to be used.
Furthermore, in spite of the good mechanical behavior of concrete, particularly when in contact with liquefied gas, the risk of micro-cracking appearing in zones of maximum stress (support cradles) can lead to water infiltrating through the solid concrete structure, running the risk of corroding the metal reinforcement inside the concrete and of degrading the performance of the insulation system, and this has dissuaded the person skilled in the art from using such concrete barges for storing liquefied methane at sea.
Such barges are subjected to large mechanical stresses from environmental conditions acting on the hull (swell, wind, currents), and also to forces that are large and very localized as created by the anchor system, which is generally situated at the four corners of the barge.
Furthermore, unlike vessels of the LNG tanker type which are generally not authorized to sail when half-loaded, and must often be either practically empty (less than 10%) or practically full (more than 85%), a floating storage facility can be filled to a level that lies anywhere in the range 0% to 100%, and it must provide very high levels of security regardless of the level to which it is filled.
U.S. Pat. No. 4,275,679 discloses concrete barges for storing liquefied gas, having concrete tanks in the form of hemispherical caps or of three-fourths spherical caps, possibly surmounted by circular cylinders extending vertically. Tanks of that shape having curvature in two directions simultaneously are difficult to make and they also require

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