Enhanced LNG tanker offloading in shallow waters

Abstract

A system for offloading LNG (liquified natural gas) from a tanker (26) in shallow waters, for regasing, or heating the offloaded LNG to produce gaseous hydrocarbons, or gas, for pressurizing the gas, and for flowing the gas to an onshore station (56), includes a structure that is fixed to the sea floor and projects above the sea surface and aids in mooring the tanker. In one system, the structure that is fixed to the sea floor is a largely cylindrical tower (12) with a mooring yoke (20) rotatably mounted on its upper end. A floating structure (14) such as a barge that weathervanes, has a bow end pivotally connected to a distal end of the yoke, so the barge is held close to the tower but can drift around the tower with changing winds, waves and currents. The tanker is moored to the tower so the barge and tanker form a combination that weathervanes as a combination. Regas and pressurizing equipment (32, 34) for heating and pressuring the LNG, and any crew quarters (36), are all located on the barge, so a low cost tower can be used. In another system, the structure is a breakwater (180).

Description

CROSS-REFERENCE [0001] Applicant claims priority from U.S. provisional applications Ser. No. 60 / 515,767 filed Oct. 30, 2003, Ser. No. 60 / 550,133 filed Mar. 4, 2004, and Ser. No. 60 / 559,989 filed Apr. 5, 2004.BACKGROUND OF THE INVENTION [0002] Hydrocarbons that are gaseous at room temperature such as 20° C., are often transported by tanker as LNG (liquified natural gas) at −160° C. and atmospheric pressure. Other cold forms during transport are hydrates (gas entrapped in ice) and cooled CNG (compressed natural gas that has been cooled well below 0° C. to reduce the pressure required to keep it liquid). At the tanker's destination, the LNG (or other cold gas) may be offloaded, heated and pressurized, and carried by pipeline to an onshore station for distribution (or possibly for use as by a power plant at the onshore station). [0003] Proposed prior art offloading and regas / injection systems (for heating and pressuring LNG) include a fixed platform extending up from the sea floor to a ...

AI Technical Summary

Benefits of technology

The invention provides a system for mooring a tanker carrying cold hydrocarbons in shallow depths and regasing them for use in an onshore installation. The system includes a floating structure such as a barge and a simple tower, or a fixed structure in the form of a breakwater. The tanker is attached to the barge so they weathervane together. The system also includes a regas unit, a pressurizing unit, and crew quarters, all located on the barge. The breakwater has a length at least 60% of the tanker length and a width no more than one-fourth of its length. The regas and pressurizing units can be electrically energized, and electric power is carried between an onshore electric power station and the structure on which the regas and pressurizing units lie. The invention provides a cost-effective solution for mooring and regasing tankers in shallow depths.

Problems solved by technology

A platform that is large enough to carry such gas heating and pumping systems can be expensive even in shallow waters.

However, in shallow depths (e.g. less than about 70 meters), drifting of the vessel tends to lift the entire length of chain off the sea floor.

This can result in a sudden increase in chain tension rather that a gradual increase that is required.

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