In the description which follows. like parts are marked throughout the specification and drawings with the same reference numerals, respectively. The drawing figures are not necessarily to scale. Certain features of the preferred embodiments may be shown in exaggerated scale or in somewhat schematic form and some details of conventional elements may not be shown in the interest of clarity and conciseness. It is understood that the systems disclosed in this application are intended to be designed in accordance with applicable design standards for the uses intended, as published by recognized regulatory agencies, such as the U.S. Coast Guard, American Bureau of Shipping (ABS), American Petroleum Institute (API), American Society of Mechanical Engineering (ASME).

 The present invention is directed to several areas including but not limited to methods and apparatus for gas storage and transportation aboard a marine vessel; methods of construction and apparatus for the marine vessel; methods and apparatus for on-loading and off-loading gas to and from a gas storage system aboard a marine vessel; and methods for port-to-port transportation of gas. The present invention is susceptible to embodiments of different forms. There are shown in the drawings, and herein will be described in detail, specific embodiments of the present invention with the understanding that the present disclosure is to be considered an exemplification of the principles of the invention, and is not intended to limit the invention to that illustrated and described herein.

 In particular, various embodiments of the present invention provide a number of different constructions and methods of operation of the apparatus of the present invention. The embodiments of the present invention provide a plurality of methods for using the apparatus of the present invention. It is to be fully recognized that the different teachings of the embodiments discussed below may be employed separately or in any suitable combination to produce desired results. Reference to up or down will be made for purposes of description with up meaning away from the ocean's surface and down meaning toward the ocean's floor.

 It should be appreciated that the present invention may by used with any gas and is not limited to natural gas. The description of the preferred embodiments for the storage and transportation of natural gas is by way of example and is not to be limiting of the present invention.

　

 The preferred embodiment of the gas storage system is designed for gas temperatures and pressures where the gas is maintained in a dense single-fluid ("supercritical") state, also known as the dense phase. This phase occurs at high pressures where separate liquid and gas phase cannot exist. For example, separate phases for compressed natural gas, or CNG, do occur once the gas drops to around 1000 psia. As long as the natural gas, which is primarily methane, is maintained in the dense phase, the heavier hydrocarbons, such as ethane, propane and butane, that contribute to a low compressibility value, do not drop out when the gas is chilled to the gas storage temperature at the gas storage pressure. Thus, in the preferred embodiment, the natural gas is compressed or pressurized to higher pressures and chilled to lower than ambient temperatures, but without reaching the liquid phase, and stored in the gas storage system. Maintaining the gas as CNG rather than LNG, avoids the requirement of cryogenic processes and facilities with a large initial cost at both the loading and unloading ports.

 The methods and apparatus of the present invention optimize the compression of the gas to be transported. The optimization of the CNG storage increases payload while reducing the amount of material needed for the storage components, thereby increasing the efficiency of transport and reducing capital costs. To calculate the optimized compression of the gas to be transported, the compressibility factor is minimized and the mass of stored gas to mass of container ratio is maximized at a given pressure as compared to standard conditions for a particular gas. In the preferred embodiment described, the gas to be transported is natural gas. However, the present invention is not limited to natural gas and may be applied to any gas. Additionally, the means of maximizing the amount of stored gas per unit of material may be used for stationary storage as well, such as onshore, at-shore, or offshore platforms.

 With any gas, the compressibility factor varies with the composition of the gas, if it is a mixture, as well as with the pressure and temperature conditions imposed on the gas. According to the present invention, the optimum conditions are found by lowering the temperature and maintaining the pressure at a point that minimizes the compressibility factor. For natural gas, the compression ratio for this mode of transportation typically varies from 250 to 400, depending on the composition of the gas. Once the optimum pressure temperature condition is determined for the particular gas to be transported, the required dimensions for the storage containment system may be determined.

 Calculating the compression for the gas determines the conditions where the gas will occupy the smallest possible volume. The gas equation of state determines the volume, V, for a given mass of gas m, namely:

 　

　

 where Z is the compressibility factor, T is temperature, R is the specific gas constant and P is pressure. For a given gas composition, Z is a function of both temperature and pressure and is usually obtained experimentally or from computer models. As can be seen from the equation, as Z decreases so does V for the same mass of gas, thus the lowest value of Z for a given operating temperature is desired.

 Since storage volume also decreases with T, the desired operating temperature is also considered as an important factor. According to the present invention, cryogenics are to be avoided but moderately low temperatures are desirable. As temperatures decrease, metals become brittle and metal toughness decreases. Many regulatory codes limit the use of certain groups of metals to finite ranges of temperatures in order to ensure safe operation. Regular carbon steel is widely accepted for use at temperatures down to -20°F. High strength steel such as X-100 (100,000 psi yield strength) is widely accepted for use at temperatures down to about -60°F. Other high strength steels include X-80 and X-60. The selection of the steel for the storage containment system is dependant upon several design factors including but not limited to Charpy strength, toughness, and ultimate yield strength at the design temperatures and pressures for the gas. It of course is necessary that the storage containment system meet code requirements for these factors as applied to the particular application. By way of example the maximum stress level for the storage containment system is the lower of 1/3 the ultimate tensile strength or 1/2 the yield strength of the material. Since 1/2 the yield strength of X-80 and X-60 steel is less than l/3 their yield strength, these high strength steels may be preferred over X-100 steel.

 By way of example, assuming an X-80 or X-60 high strength steel for the storage containment system, the preferred storage containment system may have a lower temperature limit of -20°F. to provide an appropriate margin of safety for the preferred embodiment of the gas storage containment system, although lower temperatures may be possible depending upon the desired margin of safety and type of material used. For example, a lower temperature limit of -40°F. may be possible using a premium high strength steel such as X-100 and a smaller margin of safety.

 The following is a description of one preferred embodiment of the present invention for a gas having a particular composition including a specific gravity of 0.6. An X-100 high strength steel is used for the storage containment system with the preferred storage containment system having a lower temperature limit of -20°F. to provide a predetermined margin of safety for the system. FIG. 1 is a graph of the compressibility factor Z versus gas pressure for a gas with a specific gravity of 0.6. The 0.6 specific gravity is representative of that obtained from a dry gas reservoir having a composition comprising primarily methane and low in other hydrocarbons. The values of Z have been obtained from the American Gas Association (AGA) computer program developed for this purpose. The AGA methodology as applied at a temperature of -20°F., as the design temperature for the storage components, is presented in FIG. 3. Referring to FIG. 3, it is clear that the lowest value of Z, for a specific gravity of 0.6, occurs at about 1840 psia at -20°F. Based on equation (1), the minimum volume to store this gas is obtained by designing the storage components to withstand at least 1840 psia plus appropriate safety margins. These conditions give a compression ratio of approximately 265 of gas volume at standard conditions to gas volume at storage conditions.

 Another example gas composition is illustrated in FIG. 2 showing a graph of the compressibility factor Z versus gas pressure for a gas with a specific gravity of 0.7. The values for Z were obtained in the same manner as FIG. 1. The temperatures of the gas displayed in FIGS. 1 and 2 go no lower than 0°F. FIG. 3 illustrates the compressibility factor for gasses of 0.6 and 0.7 specific gravity as the temperature decreases below 0°F. Now referring to FIG. 3, looking at Z versus P for a 0.7 specific gravity gas, the minimum value of Z is 0.403 and is found in the neighborhood of 1350 psia at -20°F. Thus, for the 0.7 specific gravity gas, the storage components are designed for at least 1350 psia, plus any applicable safety margin. These conditions produce a compression ratio of approximately 268. FIG. 3 also illustrates how compressibility increases as the gas temperature is reduced to even colder temperatures. For a 0.7 specific gravity gas at -30°F. a minimum value of Z is 0.36 at about 1250 psia. For the same gas at a temperature of -40°F., the value of Z decreases to 0.33 at 1250 psia. At pressures below 1250 psia liquids will begin to dropout of the 0.7 specific gravity gas at -40°F. and it wm no longer be a dense phase gas.

 A key objective, and benefit, of the present invention is to increase the efficiency of gas storage systems. Specifically to maximize the ratio of the mass of the gas stored to the mass of the storage system. FIG. 3A, shows the relationship between the pressure at which the gas is stored and the efficiency of the system for various temperatures. It can be seen in FIG. 3A that, at a given pressure, as the temperature of the gas decreases, the efficiency of the storage system increases. While it is preferred that the system of the present invention be operated at the point 31 that will maximize efficiency, it is understood that this may not be practical in all instances. Therefore, it is also preferred to operate the system of the present invention within a range of efficiencies, such as that illustrated on FIG. 3A, and delineated by line 32 and line 34. It is also preferred that the present invention operate with efficiencies exceeding 0.3.

 Still referring to FIG. 3A, the preferred operating parameters for one embodiment of the present invention is represented by curve 36. This curve is representative of a gas, having a specific composition, being stored at -20℃. It is understood that as the composition of the gas varies the curve will also differ. Although it is possible, and advantageous over the prior art, that the gas may be stored at any pressure falling within the range represented, it is preferred that the gas be stored at a pressure in the range defined by curves 32 and 34. Therefore, a storage system constructed in accordance with this embodiment of the present invention should be capable of storing gas at any pressure defined by this range, nominally between 1100 and 2300 psi, and at -20℃.

 A method for optimizing a gas payload includes: 1) selecting the lowest temperature for the storage system considering an appropriate margin of safety, 2) determining the optimum conditions for the compression of the particular composition gas in question at that temperature, and 3) designing appropriate gas containers, such as pipe, to the selected temperature and pressure, e.g. select pipe strength and wall thickness.

 It would be preferred that the system of the present invention be utilized to store and transport a gas of known, constant composition. This allows the system to be perfectly optimized for use with the particular gas and allows the system to always operate at peak efficiency. It is understood o that the composition of a gas can vary slightly over time for a particular producing gas reservoir. Similarly, the gas storage and transportation system of the present invention may be utilized to service a number of reservoirs producing gases of varying composition with a range of specific gravities.

 The present invention can accommodate these variances. FIG. 3 is a view of the -20°F. curves for 0.6 and 0.7 specific gravity gases. The value of Z for the 0.7 specific gravity gas has a variance of Z of less than 2% over a pressure range of o about 1200 to 1500 psia at -20°F. The 0.7 specific gravity gas maintains a 2% variance from about 1150 to 1350 psia at -30°F., and the variance from 1250 to 1350 psia at -40°F. Thus, depending on the temperature of the system, the design of the storage components may be considered optimum over a range of pressures for which the compressibility factor is minimized or within this 2% variance. It is preferred to operate within this variance range but it is understood that other storage conditions may find utility in certain situations.

 Although reference will be made to the use of the system of the present invention with a gas of a particular composition, it is understood that this particular composition may not be the composition actually produced from the reservoir and a system designed for use with gas of a particular composition is not limited to use solely with a gas of that particular composition. For example, decreasing the temperature slightly will allow commercial quantities of leaner gas to be stored in a containment system optimized for a rich gas.

 For the gas storage containers, the preferred embodiment will use a high strength steel of at least 60,000 psi yield strength, i.e., X-60 steel. The storage component is preferably steel pipe, although other materials, including, but not limited to, nickel-alloys and composites, particularly carbon-fiber reinforced composites, may be used. Any pipe diameter can be used, but a larger diameter is preferred because a larger diameter decreases the number gas containers required in a system of a given capacity, as well as decreasing the amount of valving and manifolding needed. Large diameter pipe also allows repairs to be carried out by methods using means of internal access, such as securing an internal sleeve across a damaged area. Large diameter pipe also allows the inclusion of a corrosion, or erosion, allowance to improve the useful life of the storage container with only a minimal affect on storage efficiency. Very large pipe diameters, on the other hand, increase the wall thickness required and are more subject to collapse and damage during construction. Therefore, a pipe diameter is preferably chosen to balance the above described concerns, as well as availability and cost of procurement. According to one embodiment of the present invention, a pipe diameter of 36 inches is used.

 The preferred pipe is mass produced pipe and is quality controlled in accordance with applicable standards as published by the appropriate regulatory agencies. Initial discussions with certain regulatory agencies indicate that, although no applicable code of standards or regulations exist with respect to the use of such pipe as a gas container in a marine transportation application, the use of a maximum design stress of 0.5 of yield strength, or 0.33 of ultimate tensile strength, whichever is lower, is appropriate. This is a significant improvement over the prior art in that the normal special built storage tank construction used in some prior art methods requires a maximum design stress of 0.25 of yield strength. A design factor of 0.5 means that the structure must be designed twice a strong as required and a 0.25 factor means that the structure must be 4 times as strong. Thus the present invention can meet regulatory and safety requirements while using less steel, and thereby significantly reducing capital costs. Another advantage of the present invention is the margins of safety and levels of quality control that are inherent to mass produced, premium grade pipe.

 The preferred embodiment is designed for a gas temperature of -20°F. as the temperature where the gas can be maintained in the dense phase at the storage pressure targeted. As previously discussed, standard carbon steel is widely accepted for use at temperatures as low as -20°F., while the high strength steel used in premium pipe is accepted for use at temperatures as low as -60°F. This gives a wide margin of safety in the operating temperature of the gas storage system as well as providing some flexibility in its use at temperatures below the design temperature. A further consideration is that the heavier hydrocarbons that contribute to a low Z value do not drop out when the gas is chilled to -20°F. because the gas is in the "supercritical" state, i.e,, dense phase. Separate phases for natural gas do occur once the gas drops to around 1000 psia. This can be allowed to happen, outside of the primary gas containment system, when the gas is off-loaded, if it is desired to collect the heavier hydrocarbons such as ethane, propane and butane, which can have higher economic value, but is not preferred during storage and transportation.

 As discussed above, the preferred embodiment uses a high strength steel for the pipe, i.e., at least 60,000 psi yield strength, and the calculations below assume that the design factor of 0.5 of the yield stress controls. The following is a calculation of the preferred wall thickness for the pipe.

 Initially the mass of gas carried per mass of the gas containing pipe is maximized without regard to the other components such as the support structure, insulation, refrigeration, propulsion, etc. The mass of gas, mg that is contained in the pipe per unit length can be written as

　

　

 where pg is the gas pressure, Vg is the volume of the container. Z is the compressibility factor, R is the gas constant and Tg is the temperature. This mass of gas is contained in one foot length of pipe with a diameter of Di is given by

　

　

 In order to maximize the efficiency of the storage system. as defined by the ratio of the mass of the gas to the mass of the storage container (mg/ms.), the pipe should be as light weight as possible. The hoop stress P of a thin walled cylinder is defined as

　

　

 where S is the yield stress of the pipe material, F is the design factor from Table 841.114A of the ASME B31.8 Code (assumed to be 0.5 for this case), and Do. is the outer diameter of the pipe. Therefore, substituting in equation 4 and using an F of 0.5, the mass of the pipe (ms) can be calculated by

　

　

 where ps is the density of the pipe material. Combining equations 2 and 5 the ratio ψof the mass of gas mg to mass of storage system ms. is can be represented by