Usually, when it comes to modeling and simulation, the sequence is simulate first, then build. But that is not always possible, especially in the defense sector where major programs can span decades.

The Bath Iron Works shipyard (BIW, Bath, ME) faced this situation with regard to its ongoing contract to build DDG-51 Arleigh Burke-class guided-missile destroyers. The ships' primary armament is the Aegis missile. The Navy regards the DDG-51 as the most technologically advanced warship in the world. BIW had already been building the ships for years when it decided to start using computer simulations to help streamline the construction processes.

Not surprisingly, Navy shipbuilding is highly specialized. But the same general simulation methods apply as with the construction of aircraft, large industrial equipment and specialized trucks, railroad locomotives and construction yards in urban areas. At BIW, the DDG-51 simulations focus on staging and service support (S3). This includes the assignment of workers by skill classifications as well as the positioning of tools and raw materials like steel plate, piping and welding supplies. It also encompasses the arranging of the scaffolding that envelops the ships and makes it possible for workers to reach every part of the massive hull sections.

Much of the simulation work is done with Envision and Envision/Assembly programs from Delmia Corp. (Auburn Hills, MI), a unit of Dassault Systemes, a Paris-based developer of computer-aided design and manufacturing software.

All modern yards build ships modularly, with sections that are assembled like toy building blocks. At BIW, these chunks are typically 60 feet wide, 40 feet long and up to 20 feet deep. Within each module, components are carefully grouped and sequenced for assembly. In addition, BIW is continuously receiving requests from the Navy for new equipment, new bays, new computers, new radars, new missiles and new installation methods.

"This is the heart of S3 work and where most of the Delmia efforts are focused," says Bill Farabee, manager of modeling and simulation for BIW. "Thanks to S3 simulations and refining our construction processes, BIW can move much bigger modules around the yard. That has a big impact on shipyard productivity."

Farabee notes that the yard is always congested with eight to 10 ships in various stages of construction. Today, each major section of the yard, each building with all its facilities and cranes, every station where modules are built up, has been modeled in Envision.

"With Delmia, we are working toward just-in-time work flows," Farabee says. "Among other benefits, this means a much better use of our limited space. The main S3 productivity issue is how efficiently we can use the facilities, everything from prebuild operations to sandblasting and painting."

Simulations also enable the company to make better use of workers' time. This is not trivial. Dozens of people can be working on a given vessel at any given time. Hundreds of thousands of man-hours are needed to build a ship, which has tens of thousands of discrete parts.

Especially big gains are being made in simulating scaffolding arrangements. "You always need scaffolding designed to fit around each ship module," Farabee says. "But you do not want to waste time redesigning scaffolding for each module. Most modules are similar in size and shape, so we copy and modify for more efficient planning."

According to Farabee, planning the building of a typical ship module takes an average of three days, about the same as before BIW began using its current simulation system. The difference is that with its current technology, his office is creating much better plans that can be easily communicated to the yard, and reused with each new ship. That drives down planning costs in the long run.

"If we did this planning without simulation, it would actually take about the same amount of time," he notes. "But the Delmia end product is so much better-more detailed, more accurate and therefore more useful."

According to Farabee, using simulation and a lean manufacturing approach, BIW expects to take out 25 percent of the time and cost of shipbuilding by eliminating wasted effort.

"Simulation is a great tool for capturing and sharing that knowledge," Farabee says. "You can always find a better way to build something, no matter how long you have been doing it. No matter how good they are, human engineers cannot evaluate and compare anywhere near as many alternatives as simulations can."

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