Refrigeration – Storage of solidified or liquified gas – Liquified gas transferred as liquid
Reexamination Certificate
2000-02-25
2001-06-12
Capossela, Ronald (Department: 3744)
Refrigeration
Storage of solidified or liquified gas
Liquified gas transferred as liquid
C062S050700
Reexamination Certificate
active
06244053
ABSTRACT:
DESCRIPTION
1. Technical Field
The present invention relates to a system and method for transferring cryogenic liquids and in one aspect relates to a system and method for transferring cryogenic liquids such as liquefied natural gas (LNG) between an offshore receiving/loading station and an onshore import/export facility wherein the system includes a means for maintaining the temperature within the transfer line of the system low enough to prevent cryogenic liquid from gasifying and forming a two-phase fluid within the transfer line during idle periods between two consecutive unloading/loadings.
2. Background
Large volumes of natural gas (comprised mostly of methane) are produced in many remote areas of the world. This gas has significant value if it can be economically transported to market. Where the production area is in reasonable proximity to the market and the terrain permits, the gas can be transported through submerged and/or land-based pipelines. However, where the gas is produced in locations where laying a pipeline is infeasible or is economically prohibitive, other techniques must be used to get this gas to market.
Probably the most commonly used of these techniques involves liquefying the gas on site and then transporting the liquefied natural gas or “LNG” to market in specially-designed, storage tanks aboard sea-going vessels. To form LNG, natural gas is compressed and cooled to cryogenic temperatures (e.g. −160° C.) to convert it to its liquid phase, thereby substantially increasing the amount of gas which can be carried in the storage tanks. Once the vessel reaches its destination, the LNG is off-loaded through a transfer line into onshore storage tanks from which the LNG can then be revaporized as needed and transported on to end users through pipelines or the like.
At a typical LNG terminal, the storage tanks may be located from 100 to 500 meters from the moored vessel. Thus, transfer lines having lengths of one-half kilometer or more are not uncommon and at one known terminal, a transfer line of about 3.5 kilometers in length has actually been used to load LNG onto transport vessels.
In both loading onto and off-loading LNG from a vessel, it is vitally important that the transfer line is one which is capable of being pre-cooled to cryogenic temperatures before a loading/off-loading operation is commenced so that the stresses and strains of the cool-down operation can be avoided during an actual LNG transfer operation and so that excessive amounts of the LNG will not vaporize within the transfer line and overwhelm the boil-off gas handling system during the early stages of loading/off-loading. That is, before commencing a loading/off-loading operation, the transfer line must be cooled from ambient temperature to a cryogenic temperature of about 110° K. to prevent the formation of excessive amounts of gas in the transfer line.
Due to technical reasons, it is now common practice to cool the transfer line to the necessary cryogenic temperature before its initial use and then maintain it at that temperature at all times thereafter without ever allowing the temperatures in the line to rise above a certain cold temperature. That is, for LNG transfer lines, not only is the transfer line maintained at a certain cryogenic temperature, e.g. approximately 110° K., before and during the transfer operation but also during the idle intervals between transfer operations; i.e. those time intervals which exist between the completion of one loading/off-loading operation and the commencement of another.
Depending on demand, these idle intervals may be relatively long in length. For example, at some terminals only one or two LNG transport vessel may arrive each week. Since the loading/unloading operation is normally completed within about twelve hours, a particular transfer line may only be in active use from about twelve to about twenty-four hours during any one week. Thus, a transfer line may have to be maintained at a cryogenic temperature for a whole week even though the line will only be used sporadically for a short time and will remain idle the rest of the time.
As will be understood by those skilled in this art, it is necessary to avoid repeated warming of the transfer line during these idle intervals since the line would have to be “re-cooled” before each transfer operation. This would be very time consuming which would result in substantial delays in loading/off-loading a transport vessel which, in turn, would significantly increase the costs in transporting the LNG. Further, any repeated warming and cooling of the line induces stresses in the line which are likely to cause early failure of the transfer system.
In known prior art LNG transfer systems of this type, the transfer line is initially cooled and maintained at cryogenic temperatures by installing two parallel lines which extend between a storage tank on shore and an offshore facility for mooring a LNG transport vessel. During a transfer operation (e.g. off-loading), the two parallel lines operate in unison, both delivering LNG from the transport vessel to the storage tank onshore. Upon conclusion of the off-loading operation, the two lines are fluidly coupled together at the offshore mooring facility to form a continuous line having both its inlet and its outlet in the onshore storage tank. Circulation pumps, normally installed inside the onshore storage tank, pick up LNG from within the tank, pressurize it, and pump it through the inlet of the continuous line. The LNG travels from the storage tank to the mooring facility through one of the parallel lines and returns to the tank through the other.
Heat leaking into the lines and energy input from the circulating pumps will cause the temperature in the parallel lines to rise thereby warming the LNG in the lines. This, in turn, results in the partial gasification of the LNG thereby creating undesirable, two-phase flow in at least portions of the lines which, in turn, puts severe limitations on the design and operation of the transfer lines.
To alleviate this problem, typically, both of the parallel lines are insulated to minimize heat leak into the lines. While heavily insulated lines work relatively well where relative short transfer distances are involved, they experience severe drawbacks when used to transfer LNG over longer distances. For example, in the terminal where the transfer line was approximately 3.5 kilometers long, the flow rates required to maintain the desired cryogenic temperature were approximately three times as much as required in other typical LNG terminals having shorter transfer lines (e.g. 100 to 500 meters). Such high flow rates are uneconomical, making cooling of the transfer line during idle intervals impractical for these relatively long length of line.
Recently, transfer systems have been proposed for use in LNG terminals where the transport vessel will be moored offshore at significantly greater distances (e.g. up to 6 kilometers) than are now common. For example, in U.S. Pat. No. 6,012,292, issued Jan. 11. 2000, a transfer system is disclosed wherein the transfer line is constructed by placing the return line inside the main transfer line, thereby greatly improving the insulative properties of the lines which, in turn, substantially reduces the amount of two-phase flow in the longer pipeline. However, there still exists a need to reduce even further the degree of vaporization of LNG in the transfer line, especially as the lengths of these lines continue to increase.
SUMMARY OF THE INVENTION
The present invention provides a system and a method for transferring cryogenic fluids (LNG) between a first point (a first LNG storage tank aboard a sea-going vessel) and a second point (a second LNG storage tank located on shore) wherein the transfer system includes a means for cooling the transfer lines when the system is not in use and no cryogenic fluids are being transferred between the tanks. Basically, the system comprises two transfer lines which extend between the first tank and second tank.
In a normal loading/off-loading opera
Buckles John J.
Gulati Kailash Chandler
Capossela Ronald
Mobil Oil Corporation
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