FEATURE FOCUS: Offshore Developments

two if by sea

The latest thinking on LNG says to locate new terminals far beyond the backyards.

Ever since the Methane Pioneer sailed a cargo of liquefied natural gas from the United States to England in 1959, LNG trading in the U.S. has been a business of fits and starts. A chart plotting the growth of seaborne LNG depicts a flat first decade in the 1960s, 10 years' rise in the '70s during the first oil crunch and a flurry of Japanese activity, then a plateau again in the '80s on the tail of natural gas price deregulation and a bump up in domestic natural gas production.

Two of the four LNG receiving terminals built in the United States in the 1970s sat idle before the end of the next decade. They fell victim to inflexible contracts and drooping demand for the ultrachilled liquid. Those two terminals reopened recently—a testament to where on the natural-gas demand curve we sit today.

Shell's Gulf Landing, to be located 38 miles off the Louisiana coast, could be operational by 2008.

LNG sweats again these days under the bright light of public inquiry. Natural gas is now a primary fuel for power generators that have installed combined-cycle gas turbine plants and watched as a shortage of North American gas supplies upped prices. What's new this time is that the tankers that import LNG and its hope for slaking America's recent natural gas craving also happen to be prime targets for terrorism.

Attacks on the USS Cole and the oil tanker Limburg demonstrate the vulnerability of ships to terrorism, said MIT mechanical engineering professor James A. Fay, an oft-cited expert on the safety of LNG and oil tankers, and the author of a study on LNG ship movements through Boston harbor.

The Coast Guard closed that harbor to LNG traffic in the weeks following Sept. 11, 2001, because of fears that an LNG vessel there could be a next Al Qaeda target. The port reopened late that October after a private study, reportedly commissioned by the Department of Energy, estimated a smaller potential spill and fire than earlier models had predicted.

Nearby Everett is home to Distrigas Corp.'s LNG receiving terminal, which began accepting ships from Algeria in 1971. It was the first of four U.S. LNG terminals. Terminals at Cove Point, Md., and Elba Island, Ga., recently reopened, joining the Everett facility and another in Lake Charles, La., in the LNG import business.

The LNG risk has been variously labeled "overblown" and "potent" in recent newspaper accounts. A fatal explosion at an Algerian LNG facility in January may have stymied the industry's chances for building new regasification plants at a number of sites along U.S. shores. Already, several terminals proposed for Maine and California have been shelved.

According to an April 2004 chart published by the Federal Energy Regulatory Commission, 38 terminals or expansion projects have been either approved or planned for the lower 48 states, Canada, and Mexico.

The main concerns over an LNG tanker spill are the way in which the supercooled liquid would quickly vaporize upon contacting the warmer water and the way in which the vapor, if ignited, would rapidly burn in a huge pool fire.

Another offshore LNG approach (above) uses salt domes for storage. Either way, U.S. will see more LNG carriers (below).

Yet, no one knows exactly how a 6.5-million-gallon LNG pool fire would behave. Studies in the 1970s and '80s looked at spills of about 10,000 gallons, according to Jerry Havens, who directs the Chemical Hazards Research Center at the University of Arkansas. Havens studied LNG shipping hazards for the Coast Guard in the 1970s. Those studies produced fires that measured about 50 feet across and 250 feet high, Havens recalled.

Scaling up to a multimillion-gallon spill scenario from that paltry burn requires a long cognitive leap. There's some debate, of course, but most peer-reviewed predictions agree that such a spill could produce a fire of about a quarter-mile radius. Anyone within a half-mile of the fire's edge would sustain second-degree burns on skin exposed for half a minute. A terrorist attack by an explosives-laden speedboat would surely provide an ignition source to the LNG as it spilled out the sides of a tanker.

Japan has long relied on natural gas imports to generate the nation's electricity. The 135,000 cubic meter LNG carrier Al Wajbah, built in 1997, is one of 10 vessels serving the Qatar-Japan LNG project.

What's more, 6.5 million gallons represents the capacity of just one of an LNG carrier's typical five holds. Havens calculated that the size of the pool fire in a spill from a single hold could be great enough to envelop a tanker, which usually stretches over 900 feet long. What would happen after that could be anybody's guess.

A loaded LNG tanker threatens Boston by the proximity with which it passes the city's shoreline on its inbound leg to Everett. According to maps that Fay appended to his 2003 report on the matter, the zone of thermal radiation resulting from an LNG pool fire would extend well onto the shores of Charlestown, East Boston, and Boston proper. For this reason and similar concerns elsewhere, the LNG industry is directing much of its new terminal efforts out beyond the coast, Fay said.

SAFELY AFLOAT

Mooring oil tankers offshore to unload them is nothing new, said Max Krekel, a senior naval architect with Bluewater Offshore Production Systems USA Inc. of Houston.

Bluewater, together with Conversion Gas Imports LP and Paragon Engineering Services Inc., also of Houston, is cooperating in a DOE-National Energy Technology Laboratory commissioned project that's investigating the offshore discharge of LNG carriers into salt caverns. Salt domes, some of which already house the nation's strategic oil reserves, could make very good gas receivers, especially in light of a process developed at Conversion Gas Imports called the Bishop process.

Briefly, the Bishop process eliminates the need for storing LNG in cryogenic tanks at the terminals. Instead, the LNG moves from the tanker to salt cavern storage in a single step as a dense-phase gas at approximately 40°F.

Bluewater's concepts for offshore LNG terminals are designed to let a tanker weathervane, Krekel explained. Moored at a single point, a weathervaning vessel swings freely to present the lowest area it can to wind and sea. The practice is common with offshore oil handling to keep ship movement from straining the floating hose that carries crude off the ship. It's been the practice for 40 years, Krekel said.

What's different about handling LNG is that no floating cryogenic hoses yet exist, Krekel said, although the problem is being worked on. For now, Bluewater proposes a submerged arm that swivels at one end around a seabed anchor. At the other end of the arm floats a buoyant column that supports the LNG transfer hoses on a hinged structure.

In operation, the LNG tanker would moor to the structure from a single mooring line in accordance with conventional practice. The self-propelled swing arm would then sidle up to the tanker amidships so that the vessel and arm could weathervane together. The hinged arm at the top of the column, supporting two 20-inch-diameter offloading hoses and a vapor return hose, would allow the ship to move up and down in the waves independently of the submerged arm.

In one offshore mooring scheme (above and below), the tanker weathervanes while LNG connections match ship's vertical and horizontal motions.

According to Krekel, the scheme uses proven technology in a new configuration. Only the handling of cryogenic fluids offshore is new, he added.

At least five proposals are further along than the Bluewater project, having been approved by various regulatory bodies in the last year. FERC issued a final environmental impact statement last August for Sempra Energy's Cameron LNG receiving terminal in Hackberry, La., for instance. The company is billing it as the first such new terminal to be constructed in the United States in two decades.

Meanwhile, Excelerate Energy LLC plans to build what it calls the world's first offshore LNG regasification terminal about 100 miles out in the Gulf of Mexico. The Energy Bridge Deepwater Port will use a 186-ton single point mooring designed to fit a specially configured LNG tanker that is now under construction at the Daewoo shipyard in South Korea.

Anchored at a depth of 90 feet, the mooring, known as a submerged turret loading buoy, will mate to the LNG tanker through a special hull fitting. Regasifiers on board the ship will vaporize the LNG and feed the resulting natural gas into the pipeline grid. A tanker would stay on the mooring for the four to six days it takes to discharge its regasified contents to the natural gas pipeline.

According to William Sember, vice president of energy development with the Houston-based marine classification society, the American Bureau of Shipping, the turret loading buoy borrows its design from North Sea buoys used in unloading oil tankers. The Energy Bridge design, because it doesn't need to wait for seafloor structures or storage tanks to be built, is expected to go online late next year, Sember reported.

Calling it the world's first LNG deepwater port, ChevronTexaco will site Port Pelican about 40 miles off the Louisiana coast when construction begins this year. The port will comprise mechanical regasification equipment along with LNG storage and a connection to existing offshore pipeline. The port will be constructed on a gravity-based structure, a free-standing concrete or steel platform heavy enough to rest on the seafloor without anchors.

Two projects awaiting Bahamian approvals call for LNG terminals to be built on those islands. Each will feed natural gas pipelines that will run along the seafloor before coming onshore in Florida. The United States has already approved the pipeline portions of the projects.

This handful of projects marks only the beginning of LNG's resurgence in North America. The Federal Energy Regulatory Commission list of potential LNG terminals stretches from Baja California to Nova Scotia.

DON'T LOSE YOUR COOL

Meanwhile, LNG carriers, the vital links between producers and consumers in the so-called LNG value chain, are destined to grow in response to the expanding demand for natural gas. Unlike most ships, which rate their capacity in dead-weight tons, LNG carriers are rated by volume because the load itself has such a low specific gravity, according to senior staff consultant Jim Gaughan of ABS. LNG tankers under construction today have capacities up to 153,500 cubic meters compared to those built in the 1970s, which held 125,000 cubic meters of LNG.

The international classification society is reviewing several technical areas in antici- pation of the construction of Very Large LNG tankers, or VLLNGs, with 250,000 cubic meters of capacity.

LNG carriers represent the last bastion of steam propulsion in an industry where it once dominated. Diesel rules the waves now because of its greater efficiency, Gaughan said. But boilers offer convenient disposal of LNG boil-off, which can amount to 0.15 percent of the cargo each day or about 3 percent after a 20-day voyage, he said. That's why the International Maritime Organization—the shipping arm of the United Nations—requires LNG tankers to have two means of disposing of boil-off. Steam plants, with their two boilers, provide a ready means to safely dump this energy, even if dumping it or using it for propulsion or ship's service generation makes less economic sense as natural gas prices rise.

Membrane tanks can be built with simpler methods than spherical tanks, but they can be damaged by sloshing product.

Moored at a terminal, an LNG carrier must be ready to sail anytime—a precaution in case a terminal, or the ship, is burning. Because diesels need more maintenance between mandatory vessel overhauls than steam plants, performing even brief procedures on an engine while a ship is discharging at the wharf—the usual place for such operations—would compromise a vessel's operational readiness. For this reason, new vessel designers are considering many diesel options, such as twin-engine, twin-screw platforms that will have reliquefaction plants right on board, Gaughan said.

Today's LNG fleet of approximately 160 vessels divides its members about evenly between two principal designs. The Moss sphere, probably the more familiar of the two, holds cargo in multiple 40-meter-diameter insulated aluminum spheres supported by equatorial rings that also act as thermal breaks, Gaughan said.

Fabricating the spheres is tricky and costly, he explained, which makes the simpler membrane design more attractive for many shipbuilders. Unlike spheres, the membrane design doesn't support the weight of the liquid, but instead transfers it through layers of insulation directly to the hull. Membranes are actually thin sheets of nickel steel Invar or 304 L stainless steel that contain the LNG in much the same way a vinyl pool liner keeps water in but doesn't support it. The insulation is polyurethane foam or pearlite inside plywood boxes.

About half of the world's fleet of LNG tankers uses the spherical, or Moss, design. The sphere is less susceptible to damage from sloshing action, but it is harder to build. A thermal break reduces heat loss.

Membrane tanks lack the structural strength of Moss spheres, and are therefore more susceptible to damage as the LNG sloshes around inside them while the ship moves through the sea. The phenomenon worsens as some of the cargo evaporates. Some of the earliest vessels of this variety sustained tank damage by this very mechanism, Gaughan said.

The American Bureau of Shipping has developed a CFD-based program through which it can approve a membrane-style carrier for a 40-year fatigue life. The software also evaluates LNG hulls that are partially loaded. It compares bending stress of the inner hull against the worst conditions the ship might encounter at sea.

LNG tanker capacities have increased only gradually over the years because the industry tends to play conservatively, Gaughan said. The industry's safety record reflects this conservatism.

Even so, insiders saw the coming demand for natural gas spelled out 10 or 15 years ago by the direction that power generators were headed. But ships stayed small because shallow-water terminals constricted the immediate step up to bigger, deeper draft vessels and their efficiency of scale. Even then, insiders considered the move offshore inevitable, if for no other reason than to avoid maneuvering LNG carriers through congested, populous harbors.

In our age of terror, that reasoning makes more sense than ever.

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© 2004 by The American Society of Mechanical Engineers