by John D. Holmander and Thomas Plante
Going from Ten Modules to Four
During the six years it took to build New Hampshire, the U.S. Navy, General Dynamics Electric Boat, Northrop Grumman Shipbuilding Newport News, and the many vendors who support the two shipbuilders all focused on continuous improvement on a daily basis. This resulted in hundreds of significant process and facility improvements. Arguably the most noteworthy of these improvements was the four-module build plan. This innovative build plan called for the module manufacturing facilities of both shipbuilders to deliver four “super modules” to the final assembly yards rather than the 10 smaller modules delivered for the final assembly of previous Virginia-class submarines.
The “1-3-8” Rule of Thumb
To appreciate the importance of the four-module build plan, it is necessary to understand the advantages of accomplishing as much as possible early in the construction process. A useful shipbuilder’s rule of thumb called the “1-3-8 rule” compares the amount of labor hours required to accomplish the same work in succeeding states of submarine construction.
The earliest part of the construction process takes place on the shop floor. This is generally the most efficient work environment because plans, tools and jigs to facilitate assembly are readily available, and utilities are easy to access. It is also the easiest place for supervisors to provide guidance when needed and to observe and correct deficiencies early enough to avoid rework. Finally, the shop is a controlled environment, so work there can continue in all weather.
An open shop floor is by far the most efficient work environment. Here, EB employee Shawn Faria checks welds on the firstVirginiapayload tube.
Assembled in an efficient open work environment, a “raft” of equipment for the auxiliary machinery room is now ready to slide into the tight space of a hull section at EB’s Quonset Point Facility.
Photos courtesy of General Dynamics Electric Boat.
As work packages are assembled into larger units called modules, the work moves to outfitting buildings that are specially designed to handle the heavier units and align the various modules for insertion into hull sections. (Note that the “modules” discussed here are not the same as the large hull-section modules discussed earlier; these modules are packages of equipment that will eventually be inserted into the large hull sections.)
An outfitting building is a less efficient workspace than the shop floor because adjacent systems increase congestion, and equipment already installed in modules limits access. A task that takes one hour on the shop floor requires roughly three hours in an outfitting building.
The least efficient work environment is inside a hull. Once hull sections are assembled into a complete hull, personnel working within are much more remote from their bench tools and other resources, and access, congestion and safety become much greater concerns. Thus, a job that could be completed in one hour on the shop floor or three hours in a modular outfitting building can take up to eight hours inside a hull.
Following the 1-3-8 rule of thumb, the best strategy is to maximize work in the shop environment and minimize work within the hull. Among other things, this means increasing the size, weight, and state of completion of modules prior to inserting them into hull sections. Some “rafts” (the term for large assembly packages inserted whole into hull sections) for the Virginia class weigh more than a million pounds. For a comparable portion of the ship, one Virginia raft load typically replaces 12 raft loads for the previous Seawolf (SSN-21) class and as many as 18 for the earlier Los Angeles (SSN-688) class.
Applying the 1-3-8 Rule
to the Virginia class
From the start, the Virginia-class design was tailored to optimize the construction process, facilitate the integration of supplier equipment, and incorporate Navy operational and maintenance experience. The design exploited modularity to maximize the construction that takes place in an open shop environment and minimize what has to be done within the confines of a hull.
The four-module build plan takes that successful approach to the next level. By moving critical-path work earlier in the schedule, it shortens construction cycle times, improves learning, and facilitates timely identification and reduction of risk. Not surprisingly, the first application of this plan, in New Hampshire, reduced costs and accelerated the schedule compared to previous ships. Additional cost and schedule improvements are already being realized in follow-on ships.
In their ongoing search for process improvements that will lower construction costs, General Dynamics and Newport News continually evaluate performance in each phase of Virginia-class construction to discover and implement process and productivity improvements that will reduce labor, service and support costs on subsequent ships.
In parallel, the shipbuilders are pursuing an ambitious effort to optimize the entire construction process and significantly shorten construction time. By further increasing the work completed prior to finally assembly, they plan not only to reduce the cost of that specific work, but also to enable their final delivery yards to focus more exclusively on their core competencies—final construction, test, sea trials, and delivery.
Historic Progress in
The first submarine of the Los Angeles class was authorized in 1970. It was built using the traditional technique of joining empty hull sections and then loading individual systems and pieces of equipment through patches cut in the top of the hull. As Los Angeles–class construction progressed to later ships, pre-assembled packages of systems and equipment were top-loaded through the hull patches. However, construction remained inefficient because most work was still performed within the constrained environment of the hull.
Modern modular submarine construction techniques were pioneered in the building of the Ohio (SSBN-726) class, which got under way in the mid-1970s. The large-diameter Ohio pressure hull and a design that explicitly increased access made it easier to develop rafts that could be inserted in larger pieces, as did the new land-level assembly area that replaced the old inclined building ways. New techniques were only applied to a limited extent in early ships of the class, but later ships transitioned to more fully outfitted hull sections as shipbuilding facilities were upgraded to handle larger modules and heavier hull sections.
As the Ohio class progressed, modules grew increasingly complex, and their weight increased from 600 tons to more than 1,400 tons. The Seawolf class, which began in 1989, included major advances in modular construction even between ships. For example, in USS Seawolf, section four was 28 percent modular and weighed 769 tons, whereas in USS Connecticut (SSN-22), ordered just two years later, section four was 78 percent modular and weighed 1,150 tons. Hull sections for the current Virginia class are more than 95 percent modular, with modules approaching 2,000 tons.
The Four-Module Build Plan Next Page>>