New double-hull tanker sets sail

Newport News, VA--Thanks in large part to a certain Hollywood movie, large ships have been stealing the headlines these days. That was certainly the case last fall when Mobil Shipping and Transportation Co. christened the tanker American Progress. Built by Newport News Shipbuilding, it is the first double-hull vessel built in this country to meet the new standards required by the U.S. Oil Pollution Act of 1990.

Although tanker accidents account for only a small amount of the total oil spilled each year, big spills can be catastrophic, as was the case with the Exxon Valdez in 1989.

Part of the problem is that the single hull design of the standard oil tanker does nothing to stop the flow of oil in the event the hull is pierced. Hence the move to the double hull design, which is believed to considerably lessen the amount of oil spilled.

The American Progress is one of a class of petroleum tankers, called the Double Eagle, having a cargo body with two hulls separated by a distance of two meters. Seven pairs of cargo tanks plus two slop tanks are contained within the inner hull. Instead of a common pump room, each tank has its own hydraulically-driven pumping system.

It might seem like an easier (and cheaper) approach to beef up just a single hull, which is made from steel about 1 inch thick. But even that might not guarantee protection in the event of a collision, and the engineers who design these ships must follow the familiar laws of buoyancy. "The thicker you make the steel, the more deadweight you have to displace, so we have to be smart with respect to how much steel we use," explains Wayne McPhail, design manager for Newport News' Double Eagle Program.

In all, the double hull has a fairly complicated structural configuration. Stiffening of the side shell, for example, is required to withstand the hydrostatic head (draft) from the seawater. Rather than having stiffeners inside the tank, as is the case with single hull ships, the stiffeners (called web frames) on the Double Eagle are located between the two hulls. In this way, explains McPhail, engineers were able to stiffen both the inside of the outer hull and outside of the inner hull.

Instead of using the more common angle iron for the web frames, engineers chose a product called bulb flat. While these contoured vertical members are heavier than angle iron, from a coating perspective they are cheaper to paint because they have a rounded surface with fewer free edges. Because they carry no load, the bulkheads, which form the ballast part of the vessel, are made of lightweight corrugated steel.

To ensure that the Double Eagle design could withstand severe loading, high waves, and other extreme conditions, engineers performed a dynamic load analysis on the hull's structural configuration using software developed by the American Bureau of Shipping (ABS).

"Based on the analysis, we made a few minor changes, like adding brackets inboard of the inner side bulkhead to lower stresses," says McPhail. "But all in all, this vessel is tough enough for a typical winter in the North Atlantic."

American Progress tanker specs

- Flag: U.S. Port of Registry: Norfolk, VA

- Design: Double-hull

- Length: 183 meters (600 ft, 5 inches)

- Width: 32.2 meters (105 ft, 8 inches)

- Deadweight (long tons): 46,000

- Cargo tank capacity: 55,000 cubic meters

- Propulsion: Single screw (slow speed) diesel engine

- Generators: 1x850 kW, 2x600 kW

- Trial speed: 14.5 knots