Original Date: 07/21/2003
Revision Date: 01/18/2007
Information : Improved Quality and Flatness of Deck Assemblies
Electric Boat Corporation, Quonset Point Facility implemented standardized procedures and sequencing for the fabrication of deck assemblies for nuclear submarines. As a result of these process improvements, deck flatness improved and length/width distortions have been minimized. The company's new approach to deck structures minimizes the use of secondary processes, reduces overall costs, and improves the quality of the deck structure.
Electric Boat Corporation, Quonset Point Facility (EBQP) utilized multiple shop groups led by different foreman to construct decks and platforms for nuclear submarines. This process entailed many steps: first, the structure was assembled on a platen to design dimensions; strong backs were installed and cross braces were added; the first side was welded, the strong backs were released, the deck was flipped, and the strong backs were re-secured; next, the second side was welded and the plating was attached. While some decks are readily assembled, others (i.e., upper level engine room decks) require complex planning and sequencing. Each group set its own procedures and did its own planning. As a result, EBQP encountered problems due to variations in the final products. Of specific concern were variations in distortion, flatness, and shrinkage. The distortion tolerance for length and width of a deck platform approximately 35’ x 24’ is ± 0.300”, and flatness is held to ± 0.500”. Although strong backs were used, the flatness of a deck fabricated by multiple teams could vary three to four times from tolerance. Consequently, secondary processes such as flame straightening or hot pressing were routinely used. These secondary processes increased production cost and often induced unintended bowing and compounded shrinkage problems. Additionally, decks were constructed to the exact, final dimensions called for in the design. Unfortunately, extensive welding operations tended to lead to deck shrinkage beyond acceptable boundaries. If the deck was shrunk too much, it needed to be cut in half, spread, and re-welded.
In preparation for construction of the Virginia Class, EBQP implemented a Design/Build process improvement, recognizing that dimensional control was more critical for the Virginia Class because all decks float on 50,000 lb. mounts and do not touch the pressure hull. As a result, length/width discrepancies cannot be corrected by cutting and welding to the hull. A team of engineers, technical experts in structural design, and the shop foreman were assembled to determine how best to optimize and standardize the assembly sequence for deck structure. The team resolved that only one foreman supervise all work for each major deck assembly, which immediately eliminated variation due to different procedures and planning. Weld shrinkage was built into the initial fit by using design tools to determine an appropriate amount of spread in the assembly to achieve final dimensions equivalent to design. The foreman also agreed to spread welders across the deck to dissipate heat and further reduce possible shrinkage. Fabrication teams can build the deck in prefabricated assemblies that are then welded together. A new standardized fabrication sequence was established with the fabrication team using photogrammetry to ensure that platens are flat and/or shimmed to create surface flatness; the prefabricated deck assemblies and I-beams (fabricated in-house by EBQP) are fit-up on the platen; the first side of the structure is welded, a one-inch reverse bow strong-back is attached to the structure from forward to aft down the center line, and the assembly is flipped 180° and cranked down onto the platen to incorporate a camber (essentially breaking the back of the structure); then the second side is completely welded, the deck plates are added and welded, and finally the structure is checked for flatness and dimensional tolerance.
EBQP found that these new procedures improved deck flatness, controlled dimensional tolerance, and reduced fabrication time. First-time quality is achievable, as secondary processes such as pressing and flame spraying have been eliminated on large deck structures. EBQP compared engine room deck construction time from SSN774 to SSN776 and found that cycle time had been reduced by three months or 23%.
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