Method for offloading and storage of liquefied compressed natural gas

A method for offloading and storage of liquefied compressed natural gas into a salt dome by using inserting displacement gas with a pressure greater than a pressure of the liquefied compressed natural gas and a temperature of from about 80 degrees Fahrenheit to about 120 degrees Fahrenheit into a created cavern in the salt dome. The liquefied compressed natural gas is offloaded from the vessel to the storage cavern, wherein the liquefied compressed natural gas is at a pressure of from about 750 psi to about 1100 psi and a temperature of from about −80 degrees Fahrenheit to about −110 degrees Fahrenheit. The liquefied compressed natural gas is mixed with gas vapor in the storage cavern, wherein the gas vapor in the storage cavern is at a geostatic temperature and at a pressure lower than a pressure of the liquefied compressed natural gas.

FIELD

The present embodiments relate to a method for storing high pressure, compressed natural gas in a salt dome.

BACKGROUND

The current art teaches that liquefied natural gas (LNG) can be stored in a variety of vessels and tanks. Compressed natural gas (CNG) can be stored in higher pressure rated tanks. Problems exist in current storage processes for small vessels that have to travel long distances. A need exists for storage sites underground that provide access to the CNG and also protect the CNG itself.

Compressed natural gas can be transported by way of a barge or above deck on a ship. CNG is typically cooled to a temperature around −75 degrees Fahrenheit at a pressure of around 1150 psi. The CNG is placed into strong, pressure vessels contained within an insulated cargo hold of a ship. Cargo refrigeration facilities are not usually provided aboard the ship even though the cargo is cool. A disadvantage of these ships is that they only travel short distances. If the distance to be traveled is long, the ship must not be delayed in unloading, or else the CNG bleeds off and the shipment is wasted. Current CNG storage systems have the problem of dealing with the inevitable expansion of gas in a safe manner as the gas warms during transport.

A need exists, therefore, for compressed natural gas storage systems that can contain large quantities at intermediate points on an itinerary, or at a remote location that contains refrigeration or sophisticated CNG containment systems.

SUMMARY

The methods are for offloading and storage of liquefied compressed natural gas. The methods include identifying a salt dome, installing a platform over the salt dome, constructing a storage cavern in the salt dome, and inserting piping into the storage cavern. The method further involves connecting piping to a platform offloading manifold disposed on the platform, using a flexible offloading conduit with a platform end connected to the platform offloading manifold and a vessel end connected to a vessel offloading manifold located on a vessel. The method continues by connecting the vessel offloading manifold to a transport container of liquefied compressed natural gas disposed on the vessel.

The method further includes using a flexible displacement gas conduit with a displacement platform end connected to a displacement gas platform manifold located on the platform and a displacement vessel end connected to a displacement gas vessel manifold located on the vessel. The displacement gas has a pressure greater than a pressure of the liquefied compressed natural gas and a temperature of from about 80 degrees Fahrenheit to about 120 degrees Fahrenheit. Next, the method involves connecting the displacement gas vessel manifold to the transport containers and offloading the displacement gas from the transport containers.

The method ends by flowing the displacement gas from a source into the transport container to initiate offloading of the liquefied compressed natural gas from the vessel to the storage cavern. The liquefied compressed natural gas is at a pressure of from about 750 psi to about 1100 psi and a temperature of from about −80 degrees Fahrenheit to about −110 degrees Fahrenheit. The liquefied compressed natural gas is mixed with the gas vapor in the storage cavern. The gas vapor in the storage cavern is at a geostatic temperature and at a pressure lower than a pressure of the liquefied compressed natural gas.

The present method is detailed below with reference to the listed Figures.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Before explaining the present method in detail, it is to be understood that the method is not limited to the particular embodiments herein and it can be practiced or carried out in various ways.

Methods for offloading, storage and loading of liquefied compressed natural gas are embodied herein.

FIG. 1depicts the equipment used in the method. As a first step of the method, a salt dome100is identified below a sea bed101. A platform102is installed over the salt dome100extending above the surface of the water103. A storage cavern104is formed in the salt dome100using conventional equipment and then piping106is installed into the storage cavern104.

In one embodiment, the platform102includes a jacket and deck, but the platform102can be a jack up rig, a fixed leg platform, a tension leg platform, a Spar™, a floating platform, a floating vessel or a drill ship or another variation of this type of support structure.

Additional equipment for use in the method includes a platform offloading manifold110disposed on the platform102for connecting piping106to the platform102. A flexible offloading conduit112with a platform end connected to the platform offloading manifold and a vessel end connected to a vessel offloading manifold114. The vessel offloading manifold114is located on a vessel116. The vessel offloading manifold114is in fluid communication with a transport container118holding two phase liquefied compressed natural gas. The transport container118is also referred to as the storage element in this method. The transport container is disposed on the vessel116. A plurality of storage elements or transport containers can be used on any floating transport vessel.

The flexible displacement gas conduit120has a displacement platform end connected to a displacement gas platform manifold122located on the platform102. The flexible displacement gas conduit also has a displacement vessel end that is connected to a displacement gas vessel manifold124located on the vessel116. Displacement gas is kept at a pressure greater than the pressure of the liquefied compressed natural gas in the transport container118. The displacement gas is maintained at a temperature ranging from about 80 degrees Fahrenheit to about 120 degrees Fahrenheit.

In one embodiment, the transport container118has a top end and a bottom end with a displacement gas vessel manifold124connected to the top end and a vessel offloading manifold114connected to the bottom end, as shown inFIG. 1andFIG. 1a.

The displacement gas flows from a source126. The source126can be a pipeline or another storage cavity in the salt dome. The displacement gas flows into the transport container118to initiate offloading of the liquefied compressed natural gas from the vessel116to the storage cavern104. The liquefied compressed natural gas is kept at a pressure of from about 750 psi to about 1100 psi and at a temperature of from about −80 degrees Fahrenheit to about −110 degrees Fahrenheit.

The method involves mixing the cold liquefied compressed natural gas with warm gas vapor in the storage cavern104. The gas vapor in the storage cavern104is contemplated to be at a geostatic temperature and at a pressure lower than the pressure of the liquefied compressed natural gas. The cold liquefied compressed natural gas is introduced via the piping106into the top of the storage cavern104. Since the cold liquefied compressed natural gas is denser than the vapor in the storage cavern, the cold liquefied compressed natural gas cascades, rains, or precipitates down through the cavern towards the bottom of the storage cavern104. As the cold liquefied compressed natural gas descends, the gas mixes with the warm gas vapor. The cavern is sized to provide an inventory of warm gas vapor such that the temperature after the intimate mixing will be within the thermo elastic limits of the storage cavern104.

In one embodiment, the storage cavern includes using an atomizer500(see,FIG. 5andFIG. 5a) within the storage cavern104to facilitate intimate mixing of the liquefied compressed natural gas with the gas vapor in the storage cavern104. As shown in bothFIG. 5andFIG. 5a, the atomizer500is connected to piping106and is surrounded by casing550. The atomizer500is held in place using a packer552as shown inFIG. 5.

The atomizer500includes a plurality of orifices502A,502B,502C,502D,502E,502F,502G,502H,5021,502J,502K, and502L formed in a conduit504, as shown inFIG. 5a. The plurality of orifices is configured to disperse the liquefied compressed natural gas. The plurality of orifices502can have the same diameter or varying diameters depending on the offloading rate of the gas and well bore configuration. Further, the plurality of orifices502can be formed in a random pattern in the conduit or in a predetermined arrangement. In one embodiment, one end506of the conduit504is closed so that the liquefied natural gas does not flow out of the end and therefore, is forced to be dispersed through the orifices.

Following intimate mixing of the cold gas with the warm vapor, the now cool two part mix increases as heat is absorbed from the cavern walls. The expansion and pressure increases in the salt dome as the temperature increases. The stress is relieved by venting gas from the storage cavern through line127into gas pipeline126. The method further includes flowing the compressed natural gas from the storage cavern104to a natural gas pipeline126. The natural gas pipeline126can also act as the source of the displacement gas via another line127from the manifold110, as shown inFIG. 1a.

In another embodiment, the method further includes pumping the compressed natural gas from the vessel116to the salt dome100through the piping106using a pump199, as shown inFIG. 1.

In still another embodiment, the compressed liquefied natural gas is kept at a pressure of from about 900 psi to about 1000 psi prior to being inserted into the piping106. In another embodiment, the displacement gas is a natural gas from a pipeline network or a natural gas from another storage cavern.

The method shown with the assembly ofFIG. 1orFIG. 1acontemplates offloading the displacement gas from the transport containers118. For example, the offloading of the displacing gas can include shutting off the displacement gas at source126or connecting the platform offloading manifold110to a low pressure sink111b. The low pressure sink in the embodiment ofFIG. 1is contemplated to be a part of the salt dome100.

The next step involves flowing displacement gas from the transport container118through the flexible offloading conduit112to the low pressure sink until the pressure in the transport container118approaches a residual pressure. The low pressure sink is then shut off to terminate offloading of the displacement gas. In another embodiment shown inFIG. 1a, the low pressure sink is a compressor suction111a.

In one embodiment, the residual pressure of the compressed natural gas provides sufficient inventory of residual natural gas to power the vessel116.

FIG. 2illustrates an embodiment where a plurality of transport containers118A and118B (or storage elements) is further grouped into modules200and each module includes a first structural frame including a first stanchion202and a second stanchion204. The modules200further include a second structural frame (not shown) including a third stanchion and a fourth stanchion. The modules200also include a skid shoe (not shown) disposed on each stanchion, a first rack (not shown) connecting the first stanchion202and the second stanchion204and at least a second rack (not shown) connecting the third stanchion and the fourth stanchion. The transport containers are disposed in the first rack202and the second rack204.

In the preferred embodiment, the transport containers are double walled, having a load bearing inner wall, a protective outer wall and insulation disposed between the wall.

As shown inFIG. 3andFIG. 3a, the storage module is made of a first structural frame210with two stanchions212and214and a second structural frame220with two stanchions222and224. Each stanchion has a skid shoe216,218,226, and228. The skid shoe mountings allow the module to be transported from land to a floating vessel10easily. A first rack215connects the first and second stanchions210and211. A second rack225connects the third and fourth stanchions212and213.

Each storage module holds one or more storage elements100. The storage elements have a first end135and a second end140. An individual storage element100is shown inFIG. 4. The storage element100has an inner wall105forming a cavity110, an outer wall115, and an insulation layer120located between the inner wall105and outer wall115. The cavity110is designed to hold compressed cooled natural gas, natural gas liquid, and condensate.

Returning toFIG. 3andFIG. 3a, the first end135of the storage element is supported in the first rack215and the second end140is supported in the second rack225.

The storage module supports between three and fifteen storage elements. The weight of the storage module when loaded with at least one empty storage element ranges from about 5000 short tons to about 8000 short tons.

The structural frame can support up to five racks between the first and second stanchions and up to five racks between the third and fourth stanchions.

The first and second racks can support up to five transport containers. The rack can further include a plate supported by a plurality of ridges for removably holding the transport containers. The rack can be structurally anchored. The second end, or unanchored end, is allowed to travel or move to accommodate thermal strain.

The transport container's empty weight ranges from about 350 short tons to about 700 short tons when loaded. Each transport container can have a length up to about 350 feet.

Returning toFIG. 4, the storage elements have the outer wall115thinner than the inner wall105, since the outer wall115is not designed to be load bearing. The outer wall115can be steel, stainless steel, aluminum, thermoplastic, fiberglass, or combinations thereof. Stainless steel is preferred since stainless steel reduces radiant heat transfer and is fire-resistant and corrosion-resistant.

The construction material for the inner wall105is a high-strength steel alloy, such as a nickel-steel alloy. The construction material for the inner wall could be a basalt-based fiber pipe.

The shape of the storage element can either be cylindrical or spherical. The cylindrical shape, as shown inFIG. 4, is a preferred embodiment. The inner wall105has a diameter ranging from about 8 feet to about 15 feet with a preferred range from about 10 feet to about 12 feet. The outer wall115has a diameter that is up to four feet larger in diameter than the inner wall.FIG. 4adepicts the spherical embodiment of the storage element.

For the spherical shape, the inner wall has a diameter ranging from about 30 feet to about 40 feet. The outer wall has diameter that is up to three feet larger in diameter than the inner wall.

The insulating layer can be perlite.

The method includes identifying a salt dome, installing a platform over the salt dome, constructing a storage cavern in a salt dome and inserting piping into the storage cavern.

The method further includes connecting piping to a platform offloading manifold disposed on the platform, using a flexible offloading conduit with a platform end connected to the platform offloading manifold and a vessel end connected to a vessel offloading manifold located on a vessel, and connecting the vessel offloading manifold to a transport container of liquefied compressed natural gas disposed on the vessel.

The method further includes using a flexible displacement gas conduit including a displacement platform end connected to a displacement gas platform manifold located on the platform, and a displacement vessel end connected to a displacement gas vessel manifold located on the vessel and wherein the displacement gas has a pressure greater than the liquefied compressed natural gas and a temperature of from about 80 degrees Fahrenheit to about 120 degrees Fahrenheit.

The method further includes connecting the displacement gas vessel manifold to the transport container and offloading the displacement gas from the transport containers.

The method includes flowing the displacement gas from a source into the transport container to initiate offloading of the liquefied compressed natural gas from the vessel to the storage cavern, wherein the liquefied compressed natural gas is at a pressure of from about 750 psi to about 1100 psi and a temperature of from about −80 degrees Fahrenheit to about −110 degrees Fahrenheit and mixing the liquefied compressed natural gas with gas vapor in the storage cavern, wherein the gas vapor in the storage cavern is at a geostatic temperature and at a pressure lower than a pressure of the liquefied compressed natural gas.

While these embodiments have been described with emphasis on the preferred embodiments, it should be understood that within the scope of the appended claims these embodiments might be practiced other than as specifically described herein.