Among the topics being debated at the Defense Department today
is the use of so-called mobile offshore bases (or MOBs).
An MOB is a self-propelled, modular, floating platform that can
be assembled into lengths of up to one mile, as required, to support
fixed-wing, conventional aircraft. The bases would provide logistics
support for U.S. military operations.
MOBs often have been cited as possible replacements for aircraft
carriers or large cargo ships, or have been suggested as permanent
offshore logistics bases. The reality, however, is that MOBs are
a bad idea.
Two independent reports confirm the impracticality of mobile offshore
bases. The first was completed in December 1999 by the Office of
Naval Research (ONR) in response to a congressional mandate. The
second was finished in January 2001 by the Institute for Defense
Analyses (IDA). The IDA study noted that, “the alternatives
to the MOB [such as an aircraft carrier] are generally more effective
and less costly than the MOB itself.”
MOBs are expensive. They do not cost less than aircraft carriers,
as some have suggested.
The ONR report examined four of the leading MOB concepts. Their
costs ranged between $5 billion and $10 billion for a basic 5,000-foot
MOB, built to a commercial level of construction, with appropriate
machinery and outfits for the caretaker crew. It did not include
any self-defense systems.
The research and development costs for MOBs were estimated by IDA
to be about $10 billion. An aircraft carrier—specifically
CVN-77—costs about $5 billion.
MOBs would be an extraordinarily large investment that would be
justifiable only if they produced an equally large increase in capability.
They do not.
MOBs are slow, capable of only 4 or 5 knots when fully assembled.
Yet, transit speeds in excess of 12 knots are required to meet most
mission needs for both inter- and intra-theater operations. Though
some proposed MOBs are capable of achieving speeds up to 12 knots,
they do so only when in a disassembled state. It can take days to
assemble an MOB after it arrives in the theater. Thus, speed alone
limits their ability to respond to crises in a timely manner. Unless,
according to IDA, “it is fortuitously positioned in the right
place at the right time,” it might take several weeks for
an MOB to get to the scene of a crisis—far slower than pre-positioned
conventional sealift ships could have brought equipment or aircraft
carriers could have delivered tactical air power.
An MOB would have to be located close to the battlefield for it
to have any operational or tactical utility. Yet, such proximity
to the battlefield also would place the slow, fairly non-maneuverable
platform well within range of land-based threats, making them tempting,
accessible targets to potential adversaries. MOBs would require
far more defensive assets than a carrier because of their lack of
These characteristics even make them potentially vulnerable to
ballistic missile attack. Perhaps the greatest uncertainty regarding
MOBs is their ability to withstand damage. While the Navy has experience
at designing large ships for survivability, the unprecedented size
of an MOB makes it an unusual case and many of the common analytical
and design practices for survivability used for aircraft carriers
and other large combatants are not applicable to MOBs.
For example, the technology and construction techniques used by
offshore oilrigs are said to be applicable to potential MOB designs.
Yet, the inherent vulnerability of such platforms was graphically
demonstrated when the world’s largest offshore oil platform
sank off the coast of Brazil in March. A single explosion—blamed
on a gas leak—reportedly knocked the platform off one of its
air-filled supporting pillars.
The real question is not whether an MOB could be built, but whether
it could satisfy U.S. military requirements for presence, crisis
response, transition to war and actual combat. Applications for
MOBs range from tactical airfield to logistics pre-positioning,
so their contribution to any particular mission depends on circumstances
and on the concept of operations—where it is deployed, how
it is employed and which missions have higher priority.
A key issue, largely overlooked by MOB proponents, is the concept
of operations. Attempting to use a single MOB to conduct multiple
simultaneous or sequential missions would require performance-limiting
trade-offs. For example, if an MOB had a heavy Army brigade of equipment
aboard, it is likely that little space would be available for other
missions, such as storing parts and equipment to support tactical
According to the IDA report, in a dedicated logistics role, an
MOB would not be capable of effectively replacing conventional sealift.
MOBs are significantly slower than the ships currently used or planned
for the pre-positioning of equipment and munitions. Even if an MOB
happened to be in the region at the start of a crisis or conflict,
it would be too large to enter a port for cargo delivery.
MOBs would require significant numbers of barges, lighters, landing
craft, helicopters or fixed-wing aircraft to move cargo to the shore.
The time required to discharge cargo depends on a number of factors,
such as distance to the shore, weather and sea state and an adversary’s
attempt to disrupt that flow of cargo to the shore. In addition,
conventional sealift ships will be able to use the Joint Logistics
Over the Shore (JLOTS) system of crane ships and causeways to offload
conventional sealift ships directly over the beach. While an MOB
and JLOTS both require a secure area ashore, IDA’s analysis
indicates that an MOB provides an inferior delivery capability to
An MOB would be the largest floating offshore structure ever conceived
by maritime engineers. Because of its novel configuration and unprecedented
size, there are potential modes of damage and failure that have
never been considered before for a marine structure.
IDA’s conclusion about MOBs sums it up. The alternatives
to the MOB, such as carriers or large sealift ships, are more effective
and less costly.
Cmdr. Paul Nagy is a reserve surface warfare officer currently
assigned to the Navy’s OPNAV staff.