U.S. shipbuilding sails toward modern fabrication, assembly
September 12, 2002
Bath Iron Works is a shipyard in Bath, Maine, that has large-hull production and launch capacity. It's also an example of how shipbuilding has advanced in the use of materials, CAD programs, and more modern fabrication methods.
Even though boat and ship construction dates back to ancient times, some production technologies have been slow to change, illustrated by an only recent shift in technology at a major U.S. shipyard.
The building of large ships is one of the most obvious applications of heavy plate fabrication. Iron vessels first were built on a regular basis in England in the 1830s, and the first such naval fighting vessel was used in England's First China War of 1841-43. Steel began to replace iron construction in the 1870s as steel became more affordable.
Modern commercial ship hulls continue to be built with 14- to 19-millimeter-thick (0.5- to 0.75-inch) plate. Carbon steel is low-cost and easy to repair. These materials normally are specified American Bureau of Shipping grade A, although sometimes grades B and H are used.
Early hulls were riveted, but this approach evolved to 100 percent welded seams by World War II. The submerged arc welding (SAW) process makes up the majority of welding today, using ceramic backup strips where possible to maximize one-side welding.
Double-hull construction is a fairly recent and major design change that affected fabrication and assembly. This was dictated by the Oil Pollution Act of 1990, with the goal of reducing the risk of major environmental disasters caused by fuel and leaking oil and petroleum cargoes. Tanker hulls must be made with double construction, while other transport vessels, such as those for containers and bulk dry cargo, must have double-hull construction only in their fuel tank areas. While the outer hull is 14 to 19 mm thick, the inner hull may be 12 to 14 mm thick.
Only the outer hull details are shaped to contour; inner hull details are designed to allow fabrication from flat plate. Power rolling shapes the outer hull components that require simple curvature, with contour checked against CAD-generated templates. Parts that need compound curvature are formed by selective heating. The latter method requires the skill and experience of craftsmen who now can refer to a CAD-generated graphic matrix, which predicts specific locations and amounts of heat to be applied.
The newest 3-D CAD software packages also can enhance productivity in other important ways. In the past, plate details were cut oversize and then hand-fitted and trimmed. Today CAD technology facilitates accurate design of net-shape details, which are cut with NC equipment driven directly from the CAD data, which also eliminates separate programming, tracer templates, or hand layout time and effort.
Another advantage of CAD technology is that shipyards that use a standardized hull design can reduce or eliminate design effort and lead-time, stretching or otherwise modifying the design to meet customer requirements.
For today's large Navy combat vessels, aluminum is used for lighter-weight topside structure, and composites that resist corrosion are used for secondary items such as gratings and decking. However, steel continues to be the material of choice for hull structure.
In general, Navy shipbuilders now use fewer low-carbon steel hull structures such as those that were used through World War II and more high-strength, low-alloy (HSLA) steels in critical, vulnerable areas of the hull.
HSLA steel use has evolved over the last 20 to 30 years. Its advantages include increased strength and reduced thickness, which provides a weight savings that, in turn, reduces fuel consumption.
The DDG-51 Arleigh Burke-class destroyers are an example of this evolution. Next-generation combat ship classes, such as the DDG (X) project recently initiated for development, probably will use more composite materials because of their radar avoidance characteristics and resulting stealth.
As with other industries, U.S. shipyards' steel fabrication technology continues to change in the face of global competitiveness. Such competition is not new. The U.S. shipyards converted slowly from wooden hulls in the 1800s because of international economics. At a time when up to 50 percent of a ship's cost was labor, U.S. skilled ironworkers were making $14 per week--44 percent more than their counterparts in Scotland--so even then the industry was pursuing labor-saving technologies.
Manufacturing and assembly technologies for heavy plate used for ship hulls have evolved significantly, if slowly, over the past 120 years or so. New materials have dictated some process changes, such as the long-ago transition from basic iron to steel plate for hulls, the use of welded rather than riveted plate joints, and use of aluminum for some topside structures.
To a great extent, the basic approach to assembly depends on how the hull will be launched, and a change in the basic approach to launch constitutes a "sea change" in the most simple shipyard facilities. This is true especially for yards that build large ships, such as commercial cargo vessels and naval combat ships.
Competition, both domestic and foreign, is driving these changes. The U.S. has more than 280 privately owned shipyards employing nearly 100,000 workers. However, only 43 yards can handle hulls larger than 122 meters (400 feet), and only six are qualified to build combat ships for the U.S. Navy.
A prime example is Bath Iron Works (BIW) in Bath, Maine, a General Dynamics facility and one of 15 U.S. shipyards rated as having large-hull production and launch capacity. BIW launched its first ship in 1892 and has since built more than 400. Among these are 93 destroyers produced during World War II (see Figure 1).
With the Navy continuing to be BIW's prime customer, its products have evolved through the Oliver Hazard Perry-class guided-missile frigates of the 1970s to today's Aegis cruisers and the 47 DDG-51 Arleigh Burke-class guided-missile destroyers planned for production. The next major Navy production contract could be for a batch of Zumwalt-class destroyers with an $18 billion business potential.
BIW is the largest single employer in the state of Maine, so it's important to keep it afloat and competitive into the future. Major contracts are fewer and farther between because the Defense Department has reduced budgets and resources in the past decade and the Navy fleet has been reduced from a Reagan-era high of nearly 600 ships to 260 ships worldwide today.
Although BIW's facility is only 56 acres, it must be able to compete with facilities like the Litton Industries' 600-acre Ingalls Shipyard in Pascagoula, Miss.
With current responsibility for both design and production of a batch of Arleigh Burke-class guided-missile destroyers, BIW uses modern CAD/CAM capabilities that integrate the operations. For example, design data directly drives laser cutting of plate.
However, while advances in part fabrication processes are important, a more fundamental change is necessary to reduce overall costs—the launch process and the related buildup of the hull.
The ancient Phoenicians built their ships on a slideway that used gravity to launch the completed hull by sliding it backward into the water. Although some ships have been launched sideways rather than lengthwise, they still were built up on a cradle and released down an inclined slideway. This build-launch approach has several disadvantages.
While the Phoenicians started by laying the keel and working their way upward, most large modern ships are built up in segments or major subassemblies that are then joined to assemble the ship in lengthwise fashion. With the old-fashioned slideway approach, the ship must be assembled from the stern forward rather than starting with the largest center section and working outward to the ends.
Inclined slideway launching also involves risks, such as the ship gets stuck partway down, develops too much momentum, or even falls off the cradle. One of the biggest cargo ships built at BIW was damaged when part of the cradle collapsed during launch.
The last ship to be launched in this way at BIW was the 6,500-ton Navy destroyer Mason, which slid down the inclined slideway in June 2001. After a $547 million modernization investment, the company will assemble and launch its next ship, the USS Chafee, in the more modern approach that most other shipyards have already adopted.
The company will now build up major subsections of up to 600 tons each from smaller subassemblies and large details and then move them by crane into position onto a hydraulic train rail system, where they will be joined on a level, rather than inclined, surface. These old and new technologies overlapped: A few weeks before the Mason launch, the keel was laid for the Chafee at the dedication of the new $240 million Land Level Transfer Facility (LLTF) (see Figure 2).
When the Chafee hull is complete, launching will be less spectacular but safer and faster. A huge floating dry dock will be positioned inline with the LLTF, and the new ship will be transferred into it. The floating dry dock is 180 by 750 ft. long, itself an example of massive fabrication. It was built by Jiangdu Yuehai Shipbuilding Co., Jiangsu Province, China. Only one North American firm bid on the dry dock, and its bid was more than double that of the Chinese company's.
The new dry dock will be winched out on submerged rails into the Kennebec River, where the dry dock will be flooded to allow the hull to float out. This process will take up to three hours, in contrast to the typical 30 seconds for an inclined slideway launch, but significant assembly productivity and cost benefits will be gained.
Using the old method, ships usually were about only 60 percent complete and had been in production up to 110 weeks at the time of launch. The new LLTF approach will allow ships to be 85 percent completed and launched in 72 weeks.
Increased efficiency comes from the fact that larger--and thus fewer--segments are involved in final assembly, and each segment is more fully pre-equipped with pipes and electrical wiring. BIW will enjoy expanded opportunity because it no longer will be restricted to a 510-ft. hull length, and three ships may be built simultaneously, side by side. Shipway 1, for example, is 1,000 ft. long, capable of building a ship 825 ft. long.