AET Innovator

Lightering in the Gulf gets new breed of vessel

It's a safe bet that most Americans don't realize how much of the crude oil they import is lightered offshore from supertankers to smaller vessels that can meet the draft restrictions in ports such as Houston, Texas, or Morgan City, La.

But every day, huge quantities of oil snakes its way through heavy-duty hoses as two tankers perform an intricate side-by-side dance, mostly in one of several designated lightering zones in the Gulf of Mexico.
 

None of this takes place without a supporting cast of lightering service vessels, which string giant inflatable fenders between the tankers and help with the transfer. Typically, these boats are offshore supply vessels reaching the end of their working lives.

Not anymore. AET Lightering Services of Galveston, Texas, is taking delivery this fall of AET Innovator, a 185-foot steel vessel that Bill Merritt, the company's offshore services general manager, describes as the first of its kind in the world built specifically to support lightering operations.

"The oil companies are looking at safety," said Merritt. "We think it's important as a company to make safety a priority too."

AET started kicking around ideas with Seattle-based Elliott Bay Design Group back in 2006, and the upshot was the design for Innovator and three sister vessels currently under construction at Leevac Industries in Jennings, La.

Apart from safety, Merritt said, the primary considerations were efficiency, maneuverability and crew comfort. "We feel like we got all of those together," he said.
 

Sam Charters, the project manager for the vessels at Elliott Bay's New Orleans office (since acquired by Resolve Marine Group), said that designing a lightering support vessel from scratch breaks the offshore supply mold in several ways.

"A supply boat carries a mixture of deck cargo and liquid cargo below deck, whereas a lightering support vessel primarily carries Yokohama fenders and hoses, which is entirely a deck cargo and very, very light," he said. "A typical full deck load weighs less than 100 tons."

By contrast, a supply boat might be designed to carry 1,000 tons of cargo. "So the load profile is very different," Charters said.

"The other difference is that a supply boat's offloading is up to a rig, whereas the fenders are deployed off the stern, just floated off. So that means that you want the ability … to keep the stern submerged to about the same depth all the time."

Visually, that results in one of the most distinctive features of Innovator: a huge stern roller over a relatively fine stern that stays just a couple of feet above water, whatever the loading condition. Charters describes the hull shape as significantly different from that of a supply vessel of similar size — it has a lower block coefficient overall — and the difference was apparent on the second vessel before it was launched. The bottom was nowhere near flat; as the hull came astern, it swept up very quickly.
 

"The hull design is a lot different than our typical workboats," said Joe Bailey, Leevac's project manager for the vessels. "They took out a lot of hull from midbody all the way to the stern."

One consequence for the shipyard: the vessels are essentially stick-built. "Normally on our workboats our stern and bow would be built upside down, but on this boat everything had to be built upside down," said Bailey.

To keep the stern down in the water, freshwater ballast is transferred from forward tanks to tanks set around the steering compartment, giving what Charters describes as "a large trimming moment for a fairly small amount of weight."

The design is a little more complex than it might seem because AET needed to be able to carry up to 30 days' supply of water and fuel (lightering support vessels sometimes handle multiple operations in a single tour of duty), so the vessel might be substantially lighter at the end of a trip than at the beginning. The vessel has accommodations for 17, which includes seven crewmembers and up to two assistants and two mooring masters with an occasional company representative or supervisor.

Crew safety was also a concern, both on the cargo deck — the fenders are deployed and retrieved by means of a tugger and winch, and a sturdy rail provides safe passage for personnel from the stern to the forecastle — and in the superstructure design. The forecastle has tumblehome, and deckhouse and pilothouse are narrow, as on a docking tug, and on the starboard forecastle deck there is a clear area — no deck obstructions, no mooring or operating equipment, no tank vents — to allow the tanker's crane to hoist the cargo master aboard in a Billy Pugh basket.
 

The propulsion system is conventional: two CAT 3512 diesels drive fixed-pitch propellers through Twin Disc gearboxes. The vessel has a Schottel tunnel thruster, set very deep, and Becker rudders, to increase maneuverability and reduce impacts with tankers during lightering. Charters said the rudders provide higher lift in two ways: the angle of the main blade is significantly increased, and a trailing-edge flap at a 90-degree angle greatly increases the turning force.

AET Lightering Services has a main operation in Galveston, Texas, and a satellite dock at Port Fourchon, La. A typical offload is half a million barrels, which in the case of a very large tanker may just be part of its cargo. "Our expertise is in the Aframax tankers," said AET's Merritt.

According to Merritt, the lightering support vessel will carry a mooring master and an assistant mooring master. The mooring master has an unlimited license and brings the ships together; the assistant supervises the hose hookups and adjusts the fender and mooring lines.

Merritt says a standard operation takes four fenders with 150 to 300 feet of cable between them. "A Panamax will need six and reverse lightering (from a smaller vessel to a larger) needs six fenders to compensate for the draft of the vessels," he said. The new support vessels will carry eight.

Merritt said AET currently uses seven vessels for lightering, four owned and three chartered. The company declined to disclose the price for the new LSVs.

This summer, AET Offshore moved into a new 8,000-square-foot facility in Galveston, and Merritt sees new opportunities throughout the Western Hemisphere.

"We are looking to expand our transfers outside the Gulf — the East Coast, the West Coast, the Caribbean. And we've recently expanded to a new location in Uruguay. We're interested in expanding our operations with this type of vessel," he said.

As for Leevac, it's happy for the work AET has provided and hopes to build future LSVs for them. In August, Tom Church, Leevac's vice president for sales and marketing, put the company's work force at 291, but the Gulfwide lack of interest in new platform supply vessels was cutting into the heart of the yard's traditional business (the company has diversified by opening a repair yard in Lake Charles, La.).

Meanwhile, the lightering industry receives the benefit of a new type of vessel that will set the pace for lightering operations worldwide. "It's gonna be a real eye-opener," said Merritt.

Categories: American Ship Review, Maritime News