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Another “aircraft carrier” concept that continues to receive support from some officials of the Department of Defense is the Mobile Offshore Base (MOB). A MOB is a large, mobile sea base, made up of modular components that are towed to the crisis area and assembled at sea.

These platforms referred to in the Bottom-Up Review as “floating islands”, would be capable of operating from 150 to almost 300 aircraft, depending on the type, including C-130 and even C-17 transports as well as large amounts of dry and liquid cargo. Although not directly comparable to aircraft carriers, MOBs could reduce the requirement for carriers in some areas, where ample time, resources, and security are available to deploy and assemble the platforms.

These platforms would be non-self-propelled. One CAN study addressed the MOB concept comprising six modules assembled to form a platform 3,000ft (914.6m) and 300ft (91.46m) wide. Another concept being developed by McDermott International and Babcock & Wilcox provides for a platform 4,925ft (1,502m) long and 500ft (152m) wide. This design has five separate modules to be towed and assembled at a remote location. The assembled displacement at operating draft would be 1,700,000 tons. Massive amounts of cargo could be transported and stored in the individual sections.
The MOB concept would be the largest floating structure ever built. However, with the available offshore drilling platform and related technology, and the use of subcomponents, there is considered to be little risk in the construction of the platform.

A MOB also could be used to rearm surface ships and submarines and refuel surface ships.

In 2001, the Institute for Defense Analysis, a DoD sponsored think tank, thinks that a MOB would be less cost-effective than nuclear-propelled carriers or high-speed cargo ships for projecting U.S. military power into distant regions. By one estimate, one MOB module would cost about $1.5 billion, meaning a set of modules 5,000ft long would cost $8 billion.

Critics also have cited the loss to explosion of a huge floating oil platform off Brazil in 2001 to warn that such massive structures filled with ammunition and fuel, are too vulnerable to accidents in sea and enemy attacks.

Supporters point that MOB-type platforms would complement, not replace, aircraft carriers. The Department of Defense is sponsoring ongoing studies of the MOB concept , and the Navy’s 30-year shipbuilding plan submitted to Congress in 2003 has $100 million in FY 2008 and $900 million in FY 2009 for construction of a MOB. However, these funds may have been inserted as a “place holder” to ensure Navy participation in the project should the Department of Defense continue to show interest.

From 2003 onward, the MOB concept has received less support because of the efforts of Chief of Naval Operations Admiral Vern Clark to develop the “Sea Base” concept.