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Setting the Record Straight On Mobile Offshore Bases.

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.

High Cost

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 rake 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 maneuverability.

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 nor 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 prepositioning, 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 air operations.

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 JLOTS.

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.
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Title Annotation:independent reports on the feasibility of mobile offshore bases
Author:Nagy, Paul
Publication:National Defense
Geographic Code:1USA
Date:Aug 1, 2001
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