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SUMMARY OF THE INVENTION
The methods and apparatus of the present invention for transporting compressed gas includes a gas storage system optimized for storing and transporting a compressible gas. The gas storage system includes a plurality of pipes in parallel relationship and a plurality of support members extending between adjacent tiers of pipe. The support members have opposing arcuate recesses for receiving and housing individual pipes. Manifolds and valves connect with the ends of the pipe for loading and off-loading the gas. The pipes and support members form a pipe bundle which is enclosed in insulation and preferably in a nitrogen and enriched environment.
The gas storage system is optimized for storing a compressible gas, such as natural gas, in the dense phase under pressure. The pipes are made of material which will withstand a predetermined range of temperatures and meet required design factors for the pipe material, such as steel pipe. A chilling member cools the gas to a temperature within the temperature range and a pressurizing member pressurizes the gas within a predetermined range of pressures at a lower temperature of the temperature range where the compressibility factor of the gas is at a minimum. The preferred temperature and pressure of the gas maximizes the compression ratio of gas volume within the pipes to gas volume at standard conditions. The compression ratio of the gas is defined as the ratio between the volume of a given mass of gas at standard conditions to the volume of the same mass of gas at storage conditions.
As for example, one preferred embodiment of the gas storage system includes pipes made of X-60 or X-80 premium high strength steel with the gas having a temperature range of between -20°F. and 0°F. The lower temperature in the range is -20°F. For X-100 premium high strength steel, the lower temperature may be negative 40°F. For a gas with a specific gravity of about 0.6, the pressure range is between 1,800 and 1,900 psi and for a gas with a specific gravity of about 0.7, the pressure range is between 1,300 and 1,400 psi. The range of pressures at the lower temperature is the pressure range where the compressibility factor varies no more than two percent of the minimum compressibility factor for a gas with a particular specific gravity.
Once the strength of the steel and the pipe diameter are selected, for a given design factor, the pipe wall thickness is determined by maximizing the ratio of the mass of the stored gas to the mass of the steel pipe. By way of further example, for a gas with a specific gravity of substantially 0.6 and where the design factor is one-half the yield strength of the steel pipe having a yield strength of 100,000 psi and a pipe diameter of 36 inches, the pipe wall thickness will be between 0.66 and 0.67 inches. For a gas with a specific gravity of substantially 0.7 in the above example, the pipe wall thickness will be between 0.48 and 0.50 inches.
The wall thickness of the pipe may be increased by adding an additional thickness of material for a corrosion or erosion lo allowance. This thickness is above the thickness required to maintain the resultant yield stress. This allowance may be as much as 0.063 inches or greater depending on the application. The large diameter pipe used in the current invention allows this allowance to be incorporated without unacceptable degradation of the system efficiency. Although the preferred embodiment of the present invention uses high strength carbon steel pipe, other materials may find application in this system. Materials such as stainless steels, nickel alloys, carbon-fiber reinforced composites, as well as other materials may provide an alternative to high strength carbon steel.
The present invention is particularly directed to methods and apparatus for transporting compressed gases on a marine vessel. Preferably the gas storage system on the marine vessel is designed for transporting a gas with a particular gas composition. Where the gas to be transported varies from the design gas composition for the gas storage system, a gas of a second gas composition may be added or removed from the gas to be transported until the resultant gas has the same gas composition as the particular gas composition for which the gas storage system is designed.
The gas storage system may be an integral part of the marine vessel. The marine vessel may include a hull having a support structure with the pipes of the gas storage system forming a portion of the support structure. The hull may be divided into compartments each having a nitrogen atmosphere with a chemical monitoring system to monitor for gas leaks. A flare system may also be included to bleed off any leaking gas. The hull is insulated preventing the temperature of the gas from raising more than 1/2° per 1,000 miles of travel of the marine vessel. As an alternative, the marine vessel may include a hull constructed from concrete with gas storage pipes built into the hull section. A bow section is, connected to one end of the hull section and a stern section is connected to the other end of the hull section.
The gas storage system may be built as a modular unit with the modular unit either being supported by the deck of the marine vessel or being installed within the hull of the marine vessel. The pipes in the modular unit may extend either vertically or horizontally with respect to the deck.
The stored gas is preferably unloaded by pumping a displacement fluid into one end of the gas storage system and opening the other end of the gas storage system to enable removal of the gas. A displacement fluid is selected which has a minimal absorption by the gas. A separator may be disposed in the gas storage system to separate the displacement fluid from the gas to further prevent absorption. Preferably, the gas is off-loaded one tier of pipes at a time. The gas storage system may also be tilted at an angle to assist in the off-loading operation.
The method of transporting the gas includes optimizing the gas storage system on the marine vessel for a particular gas composition for a gas being produced at a specific geographic location. The system includes a loading station at the source of the natural gas and a receiving station for unloading the gas at its destination. The gas storage system is optimized at a pressure and temperature that minimizes the compressibility factor of the gas and maximizes the compression ratio of the gas.
Although the present invention is particularly directed to methods and apparatus for transporting compressed gas, it should be appreciated that the embodiments of the present invention are also applicable to transporting liquids such as liquid propane.
The embodiments of the present invention provide many unique features including but not limited to:
a) Structural integration of a gas storage system with a marine vessel to structurally stiffen the marine vessel, with the storage system including supports serving as bulkheads, the storage system components serving as bulkheads, the gas storage system serving as buoyancy, and the storage system providing storage of all gases and liquids;
b) Construction of a gas storage system as a containerized system allowing the transport of the system on the deck, or in the hull, of a marine vessel wherein the gas storage system is essentially independent of the structure of the marine vessel;
c) Staged off-loading using low freezing point liquid stored either on-shore or on the marine vessel;
d) Off-loading using liquid driven pigs to separate the gas from the liquid;
e) Matching of gas storage pipe dimensions, such as diameter and wall thickness, to the optimized compressibility factor for the composition of a defined gas supply so as to minimize the weight of the steel per unit weight of stored gas on the vessel;
f) Use of premium pipe, manufactured to accepted standards, such as API, ASME, or class society rules, as storage on a marine vessel with a design factor higher than that for individually built pressure vessels, i.e., the design factor being higher than 0.25 or similar standard;
g) Insulation lining of entire hull or the assembly of containers, reducing temperature rise to an acceptable rate for the desired service, such as less than one degree per 100 hours of travel;
h) Trimming of a marine vessel, or tilting of a gas storage system, in order to decrease surface contact area between gas cargo and displacement liquid and maximize the evacuation of displacement liquid from the gas storage system;
i) Taking pressure drop across control valve during the off-loading phase either on-shore or on the vessel but outside of the primary gas containers;
j) Use of manifolding to isolate the specific pipes of a gas storage system most prone to damage, such as the sides and bottom of the vessel, from external causes;
k) Hydrostatic testing during liquid displacement; and
l) Method of construction of a marine vessel.
An advantage of the present invention is that the high capital costs and cryogenic procedures normally associated o with transporting natural gas across water may be significantly reduced making the profitability of the present invention greater than previously used methods and apparatus.
The present invention includes improvement of CNG storage and transportation methods and apparatus, by optimizing the CNG storage conditions, thereby overcoming the deficiencies of the prior methods of natural gas storage and transportation.
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