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Gas on the Water PUBLIC ACCESS

LNG Makes its Move.

[+] Author Notes

Associate Editor

Mechanical Engineering 127(05), 36-38 (May 01, 2005) (3 pages) doi:10.1115/1.2005-MAY-3

Abstract

This article highlights the race to bring liquefied natural gas (LNG) onto America’s continental shelf. The new LNG tanker Excelsior moored to the underwater buoy of Excelerate Energy’s Energy Bridge and, after receiving a Coast Guard go-ahead, began re-gasifying its contents and injecting them into the nation's pipeline grid. Energy Bridge tops a list of nearly a dozen offshore terminals in various stages of planning or construction. It joined an even longer list of LNG terminals in various phases of planning or construction trying to do the same thing on land. Offshore terminals will play a role in this total, as Energy Bridge is demonstrating. Placing terminals at sea eliminates some of the concern about transporting LNG near or through populated coastal regions.

Article

The race to bring liquefied natural gas onto America's continental shelf and from there to an energy-hungry nation met its first finisher this March. The new LNG tanker Excelsior moored to the underwater buoy of Excel er ate Energy's Energy Bridge and, after receiving a Coast Guard go-ahead, began re gasifying its contents and injecting them into the nation's pipeline grid. The United States now has five LNG receiving terminals, one more than it had two decades ago.

Energy Bridge tops a list of nearly a dozen offshore terminals in various stages of planning or construction. It joined an even longer list of LNG terminals in various phases of planning or construction trying to do the same thing on land.

In February, the Houston Chronicle reported that Federal Energy Regulatory Commission chairman Pat Wood told a group of energy experts he expected as many as eight LNG receiving terminals would be built by 2010 and that, by 2020, the U.S. could be bringing in as much as a quarter of its gas—about 15 billion cubic feet a day—via LNG.

Offshore terminals will play a role in this total, as Energy Bridge is demonstrating. Placing terminals at sea eliminates some of the concern about transporting LNG near or through populated coastal regions.

Deepwater sites also offer inexhaustible sources of heat for warming LNG back to a gaseous state. There's some concern, however, that using seawater in this manner could jeopardize the lives of the minute sea creatures drifting along at the bottom of the food chain.

Open-rack vaporizers—essentially curtains of heat exchanger tubes—rely on seawater sprayed or flushed over the surfaces of the tubes to flash the cold liquid within them back to gas.

Closed-loop vaporizers, which use another regasifying process known as submerged combustion vaporizing, burn a small amount of natural gas to bring the liquid through a phase change. It's more expensive than the open-rack vaporizer, and mandating its use could mar the economics of LNG enough to make importing it impractical again. At least one company has said that eliminating OR V would spell the end of its plans to build a terminal.

As far as the design of the offshore terminals themselves is concerned, three styles prevail. Energy Bridge deploys a submerged turret buoy arrangement very similar to what the oil industry uses. Special tankers are needed to moor on one. The vessel Excelsior is the first of three such tankers now under way or under construction capable of using the system. Similar turret buoy arrangements have been proposed for sites off coastal New England.

In the turret buoy arrangement, the ships themselves provide LNG storage until all of a vessel's contents have been turned back to gas and sent into the pipeline. Closed-loop vaporization is the common regasifying technique for them.

More traditional proposals built around gravity-based structures are dotting the map of the Gulf of Mexico. These systems would be capable of accepting LNG shipments from any tanker in the fleet since they adapt the same off-loading technology used in terminals shoreside to an open-water environment. These terminals have typically proposed using open-rack vaporizers, and have thus come under the attack of marine biologists as being unfriendly to wildlife. Extensive use of concrete in these terminals could better withstand the chilling effects of an LNG spill than a steel structure.

California, desperate as it is for natural gas, has twO terminals of its own on the drawing boards. Both are of the closed-loop vaporizer style, and make nods to the state's air quality standards by requiring the ports, the LNG vessels themselves, and any boats serving them to burn natural gas instead of other fuels while in the vicinity of a plant. One proposal calls for building a brand-new floating regasification terminal, while an-other proposes to build on an existi'ng, though currently unused, platform.

According to the Coast Guard, whose jurisdiction en compasses LNG terminals in U.S. waters, three terminals have been approved for licensure, seven are under review, and a handful of others remain in the application stage.

A turret loading buoy of the Energy Bridge system as it appears before being submerged in the Gulf of Mexico.

Grahic Jump LocationA turret loading buoy of the Energy Bridge system as it appears before being submerged in the Gulf of Mexico.

Copyright © 2005 by ASME
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