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# Double WrappedPUBLIC ACCESS

Cleaner, Safer, Greener, Twin-hulled Tankers Keep Oil Spills Off the High Seas.

[+] Author Notes

Associate Editor

Mechanical Engineering 121(01), 52-56 (Jan 01, 1999) (5 pages) doi:10.1115/1.1999-JAN-2

## Abstract

This article discusses that a probabilistic study showed that the use of double-hulled shuttle tankers could reduce the probability of spillage resulting from collisions, contact with non-ship objects, and groundings by 75 percent. American petroleum companies are already using double-hulled tankers. The efficacy of double-hulled vessels in preventing environmental threats was borne out in October 1997 by Conoco Inc.’s tanker Guardian. Conoco, based in Houston, has operated a 100 percent double-hulled U.S. tanker fleet since August 1998. Indeed, the company decided to build only double-hulled tankers months before Congress passed OPA 1990. Two new craft will join Conoco’s flotilla of four twin-hulled vessels in 1999. Conoco engineers met a number of challenges when they embarked on building a double-hulled fleet. Double-hulled vessels are inherently more expensive than single-hulled vessels. Maritrans Operating Partners LP, a wholly owned subsidiary of Maritrans Inc. in Philadelphia, operator and owner of one of the nation’s largest fleets of oil tankers, tugboats, and oceangoing petroleum tank barges, found a way to reduce those costs, which may serve as a model for other shipping firms. The 65-year-old company converted a two-decade-old single-hulled tank barge into a double-hulled vessel.

## Article

The wreck of the Exxon Valdez oil tanker on March 24, 1989, spilled 11.2 million gallons of crude oil into the sea, and galvanized environmentalists and lawmakers. This event spurred Congress to pass the Oil Pollution Act of 1990. Among the law's provisions is the mandate that all petroleum-carrying tankers operating in U.S. ports by the year 2015 possess double hulls to prevent—or at least minimize—spills from punctures. An act of Congress has opened a new niche in shipbuilding for the American petroleum trade.

"A probabilistic study showed that the use of doublehulled shuttle tankers could reduce the probability of spillage resulting from collisions, contact with nonship objects, and groundings by 75 percent," according to Donald Liu, a doctor of mechanical engineering and senior vice president of the American Bureau of Shipping in New York. The ABS is a classification society that services the marine, offshore, and related industries as a self-regulatory organization.

Simply put, a double-hulled tanker is a ship equipped with a hull within a hull; that is, a double skin of steel separated by a distance of 2 to 3.5 meters, depending on a ship's size. "The two hulls are connected by a grillage network of transverse frames and longitudinal girders," said Liu.

American petroleum companies are already using double-hulled tankers. The efficacy of double-hulled vessels in preventing environmental threats was borne out in October 1997 by Conoco Inc.'s tanker Guardian. A barge rammed the tanker when it was entering the Port of Lake Charles, La., while carrying a cargo of crude oil. The collision tore a 400-square-foot gash in the side of the Guardian, but the second, undamaged inner hull safely contained the oil.

Conoco, based in Houston, has operated a 100 percent double-hulled U.S. tanker fleet since August 1998. Indeed, the company decided to build only double-hulled tankers months before Congress passed OPA '90. Two new craft will join Conoco's flotilla of four twin-hulled vessels in 1999.

Conoco engineers met a number of challenges when they embarked on building a double-hulled fleet. For example, determining the proper coating of the ballast compartment between the two hulls was a demanding task because space was at a premium. In addition, they had to design a ventilation system to prevent gas from accumulating between the two hulls in the event that one hull cracked.

Conoco's two new tankers will be equipped with safety and environmental features beyond their double hulls. They are designed to exchange ballast water at sea to eliminate the transport of foreign microorganisms between ports. The tankers will have tin- free anti-fouling paints, low-emission engines, and an on-board data recorder that will serve much like a black box on commercial airliners.

Like the rest of Conoco's U S. double-hulled fleet, the new tankers are being built by Samsung Shipbuilding and Heavy Industries, based in Koje, South Korea. The Korean company is a maj or player in the double-hulled tanker market and has built a number of these vessels for American oil companies.

For example, Samsung constructed two very large crude oil carriers, or VLCCs, at its Koje Island shipyard for Chevron in San Francisco in late 1998 and early 1999. Each of the 308,000-ton deadweight VLCCs will carry up to two million barrels of crude oil from Middle Eastern countries to Chevron's refineries in the U.S. The new vessels will give Chevron a total of 14 double-hulled tankers, 40 percent of its entire petroleum fleet.

American shipbuilders are also building double-hulled tankers. One is Newport News Shipbuilding in Newport News, Va., the shipwright of the U.S. Navy. Newport News Shipbuilding engineers used their expertise in building American aircraft carriers, cruisers, and nuclearpowered submarines to construct double-hulled tankers for the commercial sector under the company's Double Eagle Tanker program. The American Progress is the first of the Double Eagle Tankers built. Mobil christened the tanker on September 10. The 46,000-ton dead weight vessel transports gasoline and distillates. American Progress is the third double-hull to enter Mobil service, but the first built in an American shipyard.

The double-hulled tankers being built for ARCO at Avondale's New Orleans shipyard are meant to withstand 30 years of service. ARCO has two decades of experience in the Alaskan oil trade.

Crane operators lift the bow of the double-hulled HMI Cape Lookout Shoals tanker into place at the Newport News, Va., shipyard. This was a relatively simple task compared to arranging the vessel's bulkheads.

More recently, Newport News Shipbuilding built three double-hulled tankers for Hvide Marine Inc. of Fort Lauderdale, Fla. These vessels will be used to transport crude oil and petroleum products, such as gasoline, jet fuel, kerosene, and light lubricating oils. Hvide, a leading provider of marine support and transportation services to the energy and chemical industries, operates a fleet of281 vessels.

All three of Hvide's double-hulled tankers were built under the auspices of the U.S. Maritime Administration's Title XI Program. This is a loan guarantee to encourage shipbuilding in American shipyards. The three tankers were christened the HMI Cape Lookout Shoals, the HMI Nantucket Shoals, and the HMI Diamond Shoals, on October 3. The trio were named after lightship s, the stationary vessels opera ted by the Coast Guard that served as beacons in lieu of lighthouses to warn mariners of underwater hazards, because the tankers are designed to avoid the ecological dangers posed by petroleum spills. Two additional lightship Double Eagles, the HMI Brenton Reef and the HMI Ambrose Channel, are scheduled for delivery in 1999.

The new Hvide double-hulled tankers are equipped with a Kawasaki Man B&W 6L60MC Mark V main engine that will enable the 700-foot-long and 105-footwide vessels to make 14.5 knots. Each tanker has three 6DKB-20s and one Daihatsu 6DK-20. Two Hyundai 600-kilowatt generators and one Hyundai 850-kilowatt generator provide electrical power.

Seven independent cargo systems in the Hvide tankers can carry 342,040 barrels of product. The cargo systems are interconnected by a series of valves and line blinds. Cargo valves are controlled either manually or remotely. In fact, cargo handling can be performed remotely from the cargo control room, which carries a computer that measures and displays ullage, cargo temperature, cargo loading rates, vessel stability, and hull stresses. Stainless steel heating coils are fitted in each cargo tank to heat products for pumping and because some products-for example, paraxylene used in textile manufacture-must be maintained at specified temperatures. All the cargo tanks and any lines 6 inches and larger are epoxy-coated to protect them from corrosive cargoes.

The double hulls beneath the ships' bottom tanks are 7 feet apart, while the hulls outside the wing tanks are spaced 6 1/ 2 feet apart. "Probably the biggest challenge to building the lightships was arranging the centerline an d longitudinal bulkheads," said Tom Denning, a marine engineer and vice president of engineering at Hvide.

Denning explained that cargo tanks are designed to keep them free of internal structures that take up space or might interfere with the pumping of cargo. "We use corrugated bulkheads to accomplish this. The resulting intersection of the transverse and longitudinal bulkheads is w h ere the geometry of the construction becomes unique," he said.

Newport News Shipbuilding engineers used a proprietary VIVID computer-aided-design system (which had been used in constructing aircraft carriers), as well as the American Bureau of Shipping's Safe Hull Guidelines, to solve the complex bulkhead arrangement. "In addition, they used American Bureau of Shipping's Dynamic Load Analysis to study how the tanker's structure will act under the dynamic loads-that is, sloshing liquids-over the tanker's lifetime," explained Denning.

Hvide personnel learned lessons from the construction of the lightships that went against some conventional wisdom regarding double-hulled vessels. For example, some tankers are built without centerline bulkheads, to sidestep the difficulty of arranging them with the longitudinal bulkheads. However, this causes free surface effect problems; that is, there is too much space in which liquid cargo can slosh, according to Denning. When cargo tanks are filled to different levels, the liquid can slosh to the point that it makes the ship unsteady. The centerline bulkhead has been arranged to drastically reduce sloshing.

"There has also been a move in recent years to build the hulls out of high-tensile steel to reduce 'the vessel's weight, and increase cargo capacity," Denning said. "However, problems with cracking that could penetrate the hull have been experienced. We used a mild grade ABS A36 steel on the lightships for this reason."

The lOO-by-4-foot gash in the side of Conoco's Guardian gives stark testimony to the efficiency of double- hulled tankers. The second, undamaged inner hull prevented a spill.

## Partners in Design

ARCO Marine Ine. of Long Beach, Calif., a wholly owned subsidiary of Atlantic Richfield Co., intends to place two double-hulled tankers into service in the year 2000, and a third in 2001. Each of these vessels will carry one million barrels of oil from Prudhoe Bay and other Alaskan North Slope fields to Puget Sound in Washington State. These vessels are dubbed the "Millennium. Class," and are being constructed by Avondale Industries Inc. in New Orleans. According to Ken Thompson, executive vice president of ARCO, the company's decision to buy the new tankers partly reflects new optimism about oil production from the North Slope.

The Millennium Class vessels represent a departure from ARCO's traditional tanker acquisition.

"Long ago we purchased whatever vessels the shipyards were building that suited our needs," said Robert Levine, a marine engineer and manager of design and new construction at ARCO Marine. "However, the passage of OPA '90 caused us to take an active role in double-hulled tanker design.

"For example, rather than awarding equipment contracts on the basis of the lowest bid, we created a manufacturers' list for equipment that would give us the best service life, facilitate operator usage, reduce maintenance downtime, and basically give us the most reliable ship possible," he explained.

ARCO Marine drew on its own experience operating a tanker fleet in Alaskan waters since that state's pipeline opened in 1978 to develop double-hulled tankers that could withstand 30 years of service. "We learned a great deal about factors affecting the Alaskan oil trade, such as the effects of hot and cold weather, that we were able to apply to the Millennium design," said Levine.

Just as important, ARCO Marine assembled a team of world-class naval architecture and marine engineering firms for the project, each given tasks according to its expertise. SSPA Maritime Consulting of Gothenburg, Sweden, undertook hull design and model testing. Seattlebased Glosten Associates performed hydrodynamics and maneuvering analysis, while San Francisco's Herbert Engineering designed the cargo tank configuration and performed the necessary trim and stability calculations.

John J. McMullen & Associates in New York worked up the specifications, made the contract drawings, and did the drawing review. MCA Engineers, headquartered in Costa Mesa, Calif., performed finite element analysis, dynamic load analysis, and fatigue analysis.

"The ARCO team served as project managers to coordinate the efforts of the different engineering teams," said Levine.

After the initial design work was completed, Avondale engineers did all the detailed design work. Yard workers then cut and welded the steel plate and structural members for the tankers. These components were joined to create the 338 modular units, with double hulls spaced 10 feet apart, which will form a single Millennium tanker. The modules were assembled along the Mississippi River levee at Avondale's building ways.

As with any new class of ship, Avondale engineers engaged in a learning experience that included building the Millenniums to comply with the dynamic load analysis requirements of the American Bureau of Shipping. This analysis was performed by an independent consultant to ensure that the vessels could withstand 30 years of fatigue life. The analysis considers the effect of cyclic stresses due to weather, cargo loading, and other factors.

"Because we were building the modules while the analysis was being performed, we had to anticipate results as we built the ships," said John Falanga, a mechanical engineer and program manager at Avondale. For example, the Avondale engineers had to alter the detailed structural design of the Millennium tankers to meet the close structural tolerances demanded by a 30- year performance life.

Another design first on the Millennium project was building the vessels to meet the R2S+ Redundancy Requirements of the American Bureau of Shipping. This entailed building two complete propulsion systems on each ship, including engine rooms, engines, propellers, and rudders.

Each Millennium engine room had to house a MAN B&W 7S50MC-C 15,015-horsepower engine and 8,600- kilowatt generators driven off the ship's shafts. "It took a lot of painstaking drawing and engineering to fit all that equipment into the engine rooms, where space was at a premium, because of the fine lines that define the aft shape of the ship," explained Falanga. "It helped that we teamed with ARCO engineers on-site here at Avondale."

Other safety features include a joystick to replace the traditional wheel, and an automated navigation system that will warn the crew if the ship deviates even slightly from its course.

The Millennium Class ships will also be the first tankers taking part in the Alternate Compliance Program of the American Bureau of Shipping. This means that the bureau, rather than the U.S. Coast Guard, will perform inspections. In addition to the three Millennium vessels already under contract, AR e o may exercise its option for Avondale to construct two more double-hulled tankers

Workers stand between the hulls of the retrofitted Maritrans 192 tank barge. They pulled out the inner structure and welded 20 modules inside to create the interior hull. Twelve more conversions are planned.

Mobil Oil's 46,OOO-ton American Progress is the third double-hulled tanker in the petroleum giant's fleet, but the first built in a U.S. shipyard.

## Conversion Trims Costs

Double-hulled vessels are inherently more expensive than single-hulled vessels. "The cost of a double-hulled tanker can be from 5 to 10 percent greater than a single-hulled tanker," said Liu of the American Bureau of Shipping.

Maritrans Operating Partners L.P., a wholly owned subsidiary of Maritrans Inc. in Philadelphia, operator and owner of one of the nation's largest fleets of oil tankers, tugboats, and oceangoing petroleum tank barges, found a way to reduce those costs, which may serve as a model for other shipping firms. The 65-year-old company converted a two-decade-old single-hulled tank barge into a double-hulled vessel.

The 10,549-ton barge, which was originally named the Ocean 192 at its launching in 1979, was rechr istened the Maritrans 192 on Nov. 9, 1998.

"It would have cost us $35 million to build a new tug and double-hulled barge, but less than$10 Inillion to retrofit an existing craft," said Tom Hagner, a naval architect and vice president of engineering and maintenance at Maritrans. Hagner said that the barge entered Tampa Bay Shipyard in April. Workers pulled out the existing structure within the barge and welded 20 modules to the original hull to form a second, inner hull. In addition, the deck was raised 11 feet to maintain the capacity of the barge.

As with most retrofits, the Maritrans designers had to work within the limits of the existing structure. "For example, frame spacing on the original barge may vary by an inch without affecting the performance, but it makes it difficult to install new equipment afterwards," explained Hagner. Maritrans engineers n1.et these challenges and completed the conversion in six months. "We intend 12 more conversions of single-hull barges to double-hulls, to meet OPA '90's deadlines for barges," said Hagner.

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