A system constructed in accordance with the present invention can be constructed in a variety of methods, several of which are presented here to illustrate the preferred methods of constructing pipe storage systems. A new marine vessel can be specially constructed to carry a storage system for CNG. In this embodiment the CNG system is integral to the structure and stability of the marine vessel. Alternatively, a CNG system can be constructed as a modular system functioning independently of the marine vessel on which it is carried. In yet another alternative an old marine vessel can be converted for use in transporting CNG where the structure of the CNG storage system may or may not be an integral component of the marine vessel's structure.

 Referring now to FIGS. 5-7, in constructing a new marine vessel 10, the hull 16 is laid in dry dock and a base structure 60 is installed on the bottom hull 16 with a base plate 62 for each bulkhead 40, such as bulkhead 40b shown in FIG. 7. Then the remainder of the bulkhead 40b is constructed on top of the base plate 62. A bottom beam 18a, such as shown in FIG. 8, or strap 210, such as shown in FIG. 11, is then laid and affixed onto each of the base plates 62 of each of the bulkheads 40, all of the bulkheads 40 being constructed simultaneously. Once the initial set of bottom cross beams 18a or straps 210 are in place on top of the base bulkhead structure 60, then individual completed lengths of pipe 12 are lowered by cranes and laid in the upwardly facing saddles 50 formed in beams 18 or straps 210. Once the entire initial row 20 of pipes 12 have been laid on the initial set of bottom cross beams 18a or straps 210, then a set of cross beams 18, such as shown in FIG. 9, or straps 210 are laid and installed on top of the initial row 20 of pipes 12 with the downwardly facing saddles 52 receiving the individual pipes 12 in row 20 thereby capturing each of the individual lengths of previously laid pipe 12 between the two cross beams 18, 18a or straps 210. The adjacent cross beams 18, 18a or straps 210 are then either welded or bolted together.

 It is preferred that the pipe 12 be installed in the bulkhead 40 while the pipe 12 is at a temperature of 30°F., assuming that the cargo temperature will be -20°F. and the expected ambient outside temperature will be 80°F. Unless the marine vessel 10 is being built at a location where temperatures are already 30°F. and cooling the pipe is unnecessary, the pipe 12 is cooled by passing coolant through each piece of pipe 12 as it sits in the cross beam 18 or strap 210 but before it is fixed in place in the marine vessel 10. Nitrogen may be used as the coolant to cool the pipe to approximately 30°F. This causes the temperature of the pipe 12, when it is installed within the bulkheads 40 to be at a temperature of 30°F, so that expansion or contraction of the pipe 12 is limited to 1 inch as the temperature in the marine vessel 10 ranges from -20°F. to possibly as much as 80°F.

 The cross beams 18 or straps 210 and rows 20 of pipe 12 are continually laid into the hull 16 of the marine vessel 10 until all pieces of pipe 12 are laid horizontally into the marine vessel 10 and the bulkheads 40 are all formed. The individual lengths of pipe 12 are affixed to the cross beams 18 or straps 210 after the pipe 12 has been laid inside the marine vessel 10. For the nominal design it is anticipated that there are approximately 500 lengths of pipe 12 laid in the marine vessel 10, each being approximately 500 feet long.

 The 500 foot lengths of pipe 12 are preferably welded at a pipe manufacturing plant using plant machines to weld the pipe into 500 foot lengths. This is preferred because the quality of the welds are better in the plant as compared to field welding. The pipe 12 is also tested at the manufacturing plant before it is moved to the site of the construction of the marine vessel 10. The pipe 12 is transported on trolleys and individual pieces of pipe 12 are then set into the saddles 50 in the cross beams 18 or straps 210 mounted in the hull 16 of the marine vessel 10. Each of the rows 20 are individually filled with pipe 12 and the cross beams 18 or straps 210 are laid until the marine vessel 10 is completely filled with approximately 30 miles of 36" diameter pipe. After the pipe has been installed, the remaining hull and the deck 28 are then constructed over the pipe bundle 14 to enclose the compartment(s) 42.

 Referring now to FIGS. 13 and 14, another embodiment of the present invention includes a gas storage system constructed as a self-contained modular unit 230 rather than as a part of the hull structure 16 of the marine vessel 10. The preferred modular unit 230 includes a plurality of pipes 232, forming a pipe bundle 231, with pipes 232 being substantially parallel to each other and stacked in tiers. The pipes 232 are held in place by a pipe support system, such as straps 210 having ends connected to a frame 238 forming a box-1ike enclosure around pipe bundle 231, and having a manifold 233, similar to the manifold system shown in FIG. 12, connected to each end of pipes 232. It should be appreciated that the cross beams 18 of FIGS. 8 and 9 may also be used as the pipe support system. The enclosure 238 isolates the pipe bundle 231 from the environment and provides structural support for the piping and pipe support system. The enclosure 238 is lined with insulation 234 thereby completely surrounding pipe bundle 231 and is filled with a nitrogen atmosphere 236. The nitrogen may be circulated and cooled for maintaining the proper temperature of the pipes 232 and stored gas. If stored on deck, the enclosure may be encapsulated by a flexible, insulating skin of panels or semi-rigid, multi-layered membrane that can be inflated by nitrogen and serve as insulation and protection from the elements.

 The size and design of the modular unit 230 is primarily determined by the vehicle that will be used to transport the modular unit. In a preferred embodiment of the present invention, the modular unit 230 is transported on the deck of a cargo vessel. The modular unit 230 used in this application is comprised of 36" diameter pipe arranged thirty-six pipes across and stacked ten pipes high. Each pipe would be 500' long providing a total of thirty-four miles of pipe.

 In an alternative embodiment, the modular units 230 described above could be constructed with the pipes oriented vertically.

 FIG. 15 illustrates the use of the modular unit 230 in a vertical orientation. The height of the unit 230 would be limited because of increased stability problems as the height of the structure increased. A height of 250' may be considered feasible. The vertical modular units 230 may also be constructed so as to be independent of each other and of the marine vessel in order to allow the loading and unloading of the unit 230 as a whole. FIG. 16 illustrates the modular unit 230 in a tilted orientation to assist in off-loading the gas as hereinafter described. It should be appreciated that modular unit 230 may be disposed in the hull of the marine vessel and/or on the deck of the marine vessel in a preferred orientation such as horizontal or vertical. It is preferable to construct as long a length of pipe as possible in the controlled conditions of a steel mill or other non-shipyard environment in order to maintain quality and reduce costs.

 Although the gas storage system of the present invention is preferably part of a new marine vessel, it should be appreciated that the gas storage system may be used with a used marine vessel. There is a requirement now for ships to have a double hull to protect against oil and chemical spillage. Many current ships now have a single hull. It is contemplated that double hull marine vessels are going to replace single hull marine vessels in the near future with the single hull tankers being forced out due to this requirement of a double bull. The preferred embodiment of the present invention does not require a marine vessel with a double hull because the storage pipe for the gas is considered a protective second hull to the single hull of the marine vessel. Each of the pipes is considered another hull or bulkhead to the stored gas. Thus, a double hull on the marine vessel is not required. Therefore, older single-hull marine vessels can be modified for use with the preferred embodiment of the present invention to meet the double-hull requirements. The reuse of older marine vessels is described in U.S. patent application Ser. No. 09/801,146, entitled "Re-Use of Marine vessels for Supporting Above Deck Payloads" and hereby incorporated herein by reference.

 One concern with utilizing older marine vessels in transporting CNG is that the gas storage system of the present invention is very light, even when fully loaded with gas. In fact, the fully loaded pipes of the preferred embodiment of the present invention will float in water. The weight of the storage system may not be sufficient to achieve the required draft of the marine vessel. Sufficient draft is required for stability of the marine vessel and to make sure the propellers are at the proper depth in the water.

 One way to increase the draft of a marine vessel is by adding ballast. FIGS. 17, 20 shows a cross-section of a marine vessel 240 with a gas storage unit 241 disposed in the hull. Additional ballast 242 is placed around the gas storage unit 241. Less ballast is required as the weight of the cargo increases. In reference to FIGS. 19, 20, an additional modular storage unit 243 may be disposed on the deck of the marine vessel 240 to decrease the amount of ballast required. As shown in FIG. 20a, the modular unit 243 is at an incline for convenience in off-loading.

 Referring now to FIGS. 21, 20 and 23, there is shown another embodiment of a marine vessel that utilizes existing ship components with a hull section constructed from concrete. Referring now to FIGS. 21, 20, the cargo section of the hull 244 is constructed from reinforced concrete and joined to a bow section 245 and a stem 246 section constructed of steel. The CNG carrying pipes may be built into the concrete cargo section. The concrete hull 244 reduces the amount of ballast required, is corrosion resistant, and inexpensive to fabricate. FIG. 23 illustrates another hull 245 having a circular cross section.

 Either of the hull shapes of FIG. 21 or 23 could be made using slipforming concrete construction techniques. In slip-form concrete construction, only a small section of the hull is constructed at a time. After a section is finished the concrete forms are moved up and another small section is built on top of the existing section. This type of construction normally takes place in a calm water location, such as a fjord, and the concrete structure is extruded down into the water as it is built.

 The concrete section of the marine vessel is preferably to be built with sections 249 , 251 to allow ballast to be pumped into the ship to control the trim and draft of the marine vessel. The CNG pipes 247 within the concrete section may also serve as post-tensioned reinforcement to the structure since they will expand when pressurized. The concrete hulled CNG transport marine vessel could also be fitted with a deck cargo module 248 for transporting other cargo such as a modular gas storage unit.

 In reference to FIGS. 20 and 24, alternative embodiments of the present invention includes a barge 250 fitted with a modular gas storage system 253 either within the barge as shown in FIGS. 24,20 or on the deck of the barge as shown in FIG. 23 with the hull252 of the barge being used for oil, or other product, storage.

　

 After construction of the marine vessel, all of the air surrounding the pipe bundle is displaced with a nitrogen atmosphere. Each of the compartments or enclosures are bathed in nitrogen. One of the primary reasons for maintaining a nitrogen atmosphere is that it protects against corrosion of the pipes 12.

 Further, the nitrogen provides a stable atmosphere in each bulkhead compartment 42 or enclosure 238 which can then be monitored to determine if there is any leaking of gas from the pipes 12. In the preferred embodiment, a chemical monitor is used to monitor each compartment 42 or enclosure 238 to detect the presence of any leaking hydrocarbons. The chemical monitoring system is continually operating for leak detection and monitoring of system temperature.

 Referring again to FIG. 5, a flare system 100 communicates with each bulkhead compartment 42 between the bulkheads 40. If a leak is detected then the flare system 100 is activated and bleeds off the gas in the compartment to safely burn off the leaking gas or alternatively, vent the gas to atmosphere. The flare system 100 includes a particular flare stack 102 for burning off any leaking gas. Flaring using the bulkhead flares stacks 102 also allow the nitrogen in the compartment 42 to escape and that compartment has to again be bathed in nitrogen.

 It is anticipated that the possibility of a collision of sufficient magnitude to rupture the side of the marine vessel 10 and produce an escape route for leaking storage containers is very low. As a part of the design of the marine vessel 10, the storage compartment 42 will be encased in a wall of some insulating foam 24. In the preferred embodiment, a polyurethane foam 24 will be used having a thickness of about 12-24 inches, depending on application. This not only serves to keep the compartment 42 sufficiently insulated, but creates an added protective barrier around the storage pipes 12. A collision would have to not only rupture the hull 16 of the marine vessel 10 but also the thick polyurethane barrier 24.

 Another safety advantage of the marine vessel design and gas storage design is that since the density of the gases in the pipes 12 are much less than that of water, the filled pipes 12 create buoyancy for the marine vessel. Even if most of the bulkheads compartments 42 were flooded, the marine vessel 10 would still float. This kind of structure can be viewed as a secondary bulkhead system. Thus, the primary bulkhead system is actually redundant and although required by regulations, may not be needed.

 An additional and separate flare system 104 is also made a part of the marine vessel 10 and communicates directly with the manifolds 76, 78 or directly with the pipes 12 as necessary. For example, if it is necessary to bleed some of the natural gas off, such as because the marine vessel 10 has been stranded at sea and the temperature of the gas can not be maintained in the pipes 12, the natural gas is bled off through the separate flare system 104, without disturbing the nitrogen in the compartments 42.

　

 Based on the ABS, once every five years, 10% of the pipe must be tested or inspected for pressure integrity. One method is to send smart pigs through a sampling of the pipes. These smart pigs examine the pipe from the inside. Another method is to pressurize the pipes when they are full of the displacing liquid during an off-loading procedure. The pressure can be monitored to test the integrity of the pipe on the marine vessel. It is preferred that after the pipe has been tested, underwater hull inspection will also be performed.

　

 Separate manifold systems are used for both on-loading and off-loading the gas, When the marine vessel is loaded with gas for the very first time, natural gas is pumped through the pipe and back through a chiller to slowly cool the pipe to a -20°F. The structure may also be cooled by cooling the nitrogen blanket surrounding the structure. Once the pipe is chilled down, the inlet valves 91, 93 are closed and the natural gas is compressed within the tiers of pipe. Both sets of manifolds 90, 92 could be used.

 If, nevertheless, it is desired to avoid the drop in temperature of the gas in the pipe initially, the natural gas can be pumped into the pipe at a low pressure. The low pressure natural gas expands but will not chill the pipe enough to cause thermal shock or to over pressure the pipe at these low pressures. As the marine vessel continues to be loaded with natural gas, the injection pressure of the natural gas is raised to the optimum pressure of 1,800 psi, while cooling to below -20°F. Ultimately the compressed gas is at a temperature of -20°F. and a pressure of 1,800 psi.

　

 Referring now to FIGS. 12 and 29, the manifold system is used for off-loading by pumping a displacement fluid through the master manifold 90 and into the tier manifolds 76 and column manifolds 76. The valves 145 and 121 are open to pump the displacement fluid through the conduits 72 and into one end 64 of a pipe 12. Simultaneously, the valves 91 and 122 at the other end 66 are opened to allow the gas to pass through conduit 74 and into column manifold 78 and tier manifold 88. The displacement fluid enters the bottom of the end cap 68 and the conduit 72 and the offloading gas exits at the top of end cap 70 and conduit 74 at the other end 66 of the pipe 12. The displacement fluid enters the low side and the gas exits the top side of the pipe 12. Thus during off loading, displacement fluids are injected through one tier manifold 86 forcing the compressed natural gas out through the other tier manifold 88. As the displacing liquid flows into one end of the pipe, it forces the natural gas out the other end of the pipe.

 One preferred displacement fluid is methanol. By tilting the ship, or inclining the gas containers, the interface between the methanol and the natural gas is minimized thereby minimizing the absorption of the natural gas by the methanol. Methanol hardly absorbs natural gas under standard conditions. However, because of the high pressures, there may be some absorption of natural gas by the methanol. It is desirable to keep the absorption to a minimum. Whenever natural gas does get absorbed by the methanol, it is removed in the storage tank by compressing it from the gas cap at the top of the tank. Tilting the marine vessel for off-loading would not be used if the displacing fluid was completely unable to absorb the gas. An alternative displacement fluid is ethanol. The preferred displacement fluid has a freezing point significantly below -20°F., a low corrosion effect on steel, low solubility with natural gas, satisfies environmental and safety considerations, and has a low cost

 One preferred method includes tilting the marine vessel lengthwise at the dock or off-loading station. This is done to minimize surface contact between the displacement fluid and the natural gas. By tilting the marine vessel, the contact area between the displacement fluid and the gas are slightly larger than the cross section of the pipe. The bow would probably be raised because the weight of the engine would be in the stern, although in shallow water lowering the stern may not be possible. The marine vessel would be tilted approximately between 1°− 3°. This tilting could be accomplished by submerging a barge under the marine vessel and then making the barge buoyant. Another way to tilt the marine vessel is to shift the ballast within the marine vessel to create the desired amount of tilt.

 Alternatively, the storage structure may be inclined at an angle while the marine vessel is maintained level. Another preferred method would be to construct the storage system so that the pipes are always at an angle to the horizontal. Vertical storage units such as in FIG. 15 also have the advantage of decreasing the absorption of the gas into the transfer liquid because the contact area between the transfer liquid and the stored gas is minimized. It is preferable to incline the pipes at enough of an angle to overcome any natural sag in the pipe between the supports in order to ensure that any liquid caught in the sagging pipe will be removed.

 In reference to FIG. 27, the modular storage pack is shown with an inlet 237 and outlet 235 on each end of the storage pipe. The outlet 235 on one end is at the top of the pipe bundle while the inlet 237 on the opposite end is at the lower end of the pipe bundle. The lower inlet 237 is used to pump transfer liquid into the pipe bundle while the upper outlet 235 is used for the movement of gas products. This placement of the inlet and outlet helps minimize the inter-face between the transfer liquid and the product gas.

 The feature can be further enhanced by inclining the storage pipes so that the gas outlet 235 is at the high point and the liquid inlet 237 is at the low point. Referring to FIGS. 16 and 19, this inclination can be achieved by inclining the module unit or by installing the individual pipes at an angle during construction. This angle could be any angle between horizontal and vertical with an larger angle maximizing the separation between the transfer liquid and the product.

 The marine vessel will preferably dock at an off-loading station which has been built in accordance with the present invention. Thus the docking station may include means for tilting the marine vessel. The means for tilting the marine vessel may include an underwater hoist for lifting one end of the marine vessel or a crane or a fixed arm that swings over one end of the marine vessel. The fixed arm would have a hoist for the marine vessel. Preferably, the bow is raised causing the liquid to minimize contact with the natural gas. The displacement fluid and gas would form an interface which pushes the gas to the bow manifold for off-loading.

 It is possible that in the transport and storage of certain gases and liquids, the natural separation between the product and the displacing liquid, i.e . density, miscibility, surface tension, etc., is not sufficient to prevent undesired mixing of the two components. In such cases, offloading the gas using a displacement liquid may cause some concern in that the displacing liquid may mix with the gas. In order to prevent this from happening, a pig may be placed in the pipe to separate the displacement liquid from the gas.

 Now referring to FIGS. 30 and 31, pigs 220, such as simple spheres or wiping pigs, can be installed within each pipe 222. Pigs 220 of this type are commonly used in pipelines to separate different products. The pig 220 is located at one end of the pipe 222 with the major end of the pipe 220 being filled with gas 224. The displacement liquid 226 is then introduced in the end of the pipe 222 with the pig 220. As the displacement liquid enters the pipe 222, the pig 220 is forced down the length of the pipe 222 pushing the gas 224 ahead of it until the pig 220 reaches the other end of the pipe 222 and the gas is offloaded from the pipe 222.

 When the storage pipe is essentially evacuated, the liquid pumping stops and valving switches over to a low pressure header allowing the available pressure to push the pig back to the first end of the pipe 222 pushing out all of the displacement liquid 226. One disadvantage is that there may be additional horsepower requirements for the pump to push the displacement liquid 224 against the pig 220 to move it at an adequate velocity to maintain efficient sweeping. The pipes will also have to be fitted with access for the maintaining and replacing of pigs 220.

 The docking station includes a tank full of liquid to be used to displace the natural gas. Even though the marine vessel or pipe bundle is tilted, some of the natural gas will be absorbed by the displacement liquid. When the displacement liquid returns to the storage tank, the natural gas which has been absorbed by the displacement liquid will be scavenged off.

 Alternatively the marine vessel includes a tank of displacing liquid. The tank would be carried by the marine vessel so that the marine vessel can serve as a self-contained unloading station.

 The manifold system accommodates a staged on-loading and off-loading of the gas using the individual tiers of connected pipes. If all the pipes were unloaded at one time, the off loading would require a large volume of displacement fluid and an uneconomic amount of horsepower to move the displacement fluid. The displacement of the fluid requires at least the same pressure as that of the compressed natural gas. Thus, if the gas is all off loaded at one time, all of the displacement fluid must be pressurized to the same pressure as the gas. Therefore, it is preferred that the off-loading of the gas using the displacement liquid be done in stages. In a staged off-loading, one tier of pipes is off-loaded at a time and then a another tier of pipes is off-loaded to reduce the amount of horsepower required at any one time. During off-loading, once the first tier is off-loaded, then as the displacement fluid completely fills the first tier of pipes which previously had compressed natural gas, that displacement fluid may be directed to the next tier of pipes to be off-loaded and is used again.

 After the gas is removed from a tier, the displacement fluid is pumped back out to the storage tank with other displacement fluid in the storage tank being pumped into the next tier to empty the next tier of pipe containing compressed natural gas.

 The natural gas is offloaded in stages to save horsepower and also reduce the total amount of displacement fluid. The displacement fluid is ultimately recirculated back to the onshore or marine vessel storage where any natural gas that has been absorbed by the displacing liquid is scavenged. The onshore or marine vessel storage is kept chilled.

 In transporting heavier composition gases, it may be desirable to remove some or most of the higher molecular weight components before providing the gas to the user. Some users, such as a dedicated power plant, may want the added heating value and not want the heavier hydrocarbons removed. In this scenario, the marine vessel has, for example, 0.7 specific gravity gas which is about 83 mole percent methane but includes other components, such as ethane, and still heavier gas components, such as propane and butane, and is stored at a temperature of -20°F. and at a pressure of about 1,350 psi. The gas will pass through an expansion valve at the dock and is allowed to expand as it is offloaded. As the gas cools down and the pressure drops, the liquids will drop out, or gas leaves the critical phase, and becomes liquid. The liquid hydrocarbons will start to form once the pressure drops to about 1000 psia and will be completely removed from the gas as the pressure approaches 400 psia. As the liquids fall out, they are collected and removed.

 This process will be accelerated by the temperature drop associated with the expansion of the gas, therefore no supplementary cooling is required. The prior art processes require a chiller to chill the gas to remove the liquids. The amount of expansion and resultant chilling is dependent on the gas composition and the desired final product. It is doubtful that the gas will have to be recompressed for the receiving pipeline because of the reduced temperature of the gas. However, if the gas pressure must be reduced to a pressure below that required for the pipeline, the gas would be recompressed.

 Referring again to FIG. 28, the pipe on the marine vessel may be divided into four horizontal tiers 200, 210. 220, and 230. Each tier 200, 210, 220, and 230 represents a bundle of pipes 202, 212, 222, and 232. The bundles may be divided evenly across the cross section or they may be divided as regions, such as the group of pipes around the perimeter as one tier and an even division of the remaining pipes as the other tiers. Each tier 200, 210, 220, and 230 has an entry tier manifold 76, 214, 224, and 234 and an exit tier manifold 91, 216, 226, and 236 at each end of pipes 202, 212, 222, and 232 extending to master manifolds 90 and 88 which extend to connections at the dock where further manifolding takes place .

 Displacement liquid held in storage tank 300 is introduced into tier 200 through manifold 90 where valve 145 is open and valves 272, 274, 276, and 121 are closed. The displacement liquid is pumped under pressure through valve 145 into manifold 90 and into pipes 202. As the displacement liquid enters pipes 202, gas is forced out into manifold 206, through valve 91 and manifold 88 towards the dock. Assuming a 0.28 BCF marine vessel, displacement liquid is pumped into tier 200 at a rate of

　

Q=1.068E6 ft³/10 hrs=13315 gpm　(9)

　

 Where a total offload time of 12 hours has been assumed, with the last two hours reserved for liquid removal from the last tier, tier 232, 10 hours of displacement time results.

 When tier 200 is fully displaced, the displacement liquid is removed back through manifold 76 and out through valve 121 and manifold 260, with valve 145 now closed. The displacement liquid is fed back to the storage tank 300 where displacement liquid is simultaneously being pumped to tier 210. Tier 210 is filled with displacement liquid from storage tank 300 through manifold 90, valve 272 and manifold 214, with valves 145, 274, and 276 closed. Tier 210 gas is forced out in the same fashion as tier 200 with gas evacuating through manifold 216, valve 246 and manifold 88 towards the dock. In effect the displacement liquid used in tier 200 becomes part of the reservoir used to displace the gas in tier 210. Thus, there is less need to store enough displacement liquid to fill the entire set of pipes aboard a ship. This process is repeated with each successive tier 220 and 230 until the gas containment system has been evacuated or as much gas remains in the system as is desired for the return voyage. The electric horsepower for this operation, assuming a pressure rise of 1500 psi from tank to marine vessel, is

　

Hp=1500×l44×1.3315/0.8×2.468E5=14567　(10)

　

 where an overall pump efficiency of 0.8 has been assumed. The gas has been allowed to expand from 1840 to 1500 psi in initial offloading. Converting the horsepower to kw-hrs over the 10 hour period and using the 0.28 BCF (less fuel gas for a 2000 mile round trip) gives a cost per MCF of $0.0157, for a kw-hr cost of $0.04

 The tiered off-load system has other advantages in that the liquid storage tank, which is required, is much smaller, say about 50,000 bbls vs 200,000 bbls for full storage. Also, since the amount of liquid stored on the marine vessel during off-load is about a third of what it would be without tiering, the pipe support structure need not be as strong, i.e. the structure required to support liquid filled pipe can be stronger than that required to support gas filled pipe.

 The displacing liquid is at the same temperatures as the gas and therefore it produces not thermal shock on the pipe. After the natural gas has been off-loaded and the marine vessel is returning for another load of gas, the pipes will still contain a small amount of natural gas reserved to fuel the return trip. This remaining gas on the return voyage is below -20°F. because it has expanded. The temperature will drop even more as the gas is used for fuel. Thus, the pipes may be a little cooler when they return, depending on the effectiveness of the insulation.

 After the pipes are refilled with compressed natural gas, the temperature is returned to -20°F. Preferably the marine vessel is constantly on-loading and off-loading and transporting natural gas such that the temperature of the pipes is maintained within a small range of temperatures. The pipe will hold approximately 50% of the load at ambient temperature. Therefore, if the gas temperature rises to an unacceptable level, the most that needs to be flared is 1/2 of the natural gas. The remaining load and pipes will then be at ambient temperature. Thus, when the marine vessel reaches its destination, the compressed natural gas is off-loaded, and then when the marine vessel is reloaded with natural gas, it is necessary to cool down the pipes using a method similar to that used when the first load of compressed natural gas is loaded onto the marine vessel.

 The displacement fluid is preferably off-loaded to an onshore insulated tank. There are pumps on the marine vessel for pumping the displacement fluid to the onshore tanks. The tank is maintained at low temperatures using a chiller so that when the displacement fluid is circulated onto the marine vessel, low temperature control is not lost. This prevents thermally shocking the pipe. The displacement fluid has a freezing point well below the operating temperature of the gas storage system.

 There must be enough fluid to displace at least one tier of the pipe plus enough to fill the tier manifolding and the pump sump in the onshore tank. However, because there are a plurality of tiers of pipes on the marine vessel, it is unnecessary to have sufficient methanol to completely displace the entire 30 miles of pipe on the marine vessel in one pass. Probably, about 250,000 cubic feet of fluid will be required. This is about 50,000 barrels of fluid which is not a large storage tank.

 One of the reasons to use a displacement fluid is to prevent expanding the natural gas on the marine vessel during off-load. If the natural gas expanded on the marine vessel, there would be a drop in temperature. Therefore, during off-loading, the valves 91, 122 are opened on the marine vessel allowing the natural gas to completely fill the manifold system. The master manifolds 88 extend to closed valve 146 at the on-shore manifolds such that the natural gas completely fills the manifold system to the closed valve 146 on-shore. Thus the pressure drop occurs across the valve 146 which off-loads the gas. The gas will expand some as it fills the manifold system. However this is an insignificant amount as compared to the whole load of natural gas on the marine vessel. There is only a few hundred feet of manifold pipe to the closed valve as compared to 30 miles of 36 inch diameter pipe on the marine vessel.

 When the manifold system extending to the closed valve reaches marine vessel pressure, the closed valve is opened and all expansion takes place across the valve. This keeps the pressure drop from occurring on the marine vessel. At the valve, the temperature is going to drop a lot and that provides an opportunity to remove the heavier hydrocarbons from the natural gas. The gas is then normally warmed, although it need not be warmed if it were being passed directly to a power plant.

 In this example, it takes 12 hours to offload the natural gas. The time to on-load or off-load is a function of the equipment.

 Alternatively, the offloading of natural gas could be achieved by simply allowing the gas to warm and expand. The storage system could be warmed in ambient conditions or heat could be applied to the system by an electrical tracing system or by heating the nitrogen surrounding the system. It may also be necessary to scavenge gas remaining in the storage system through the use of a low suction pressure compressor. This method is applicable to mainly slow withdrawal where the marine vessel remains at the offload station for an extended period of time.