Mobile Offshore Base

In concept, a Mobile Offshore Base (MOB) is a modular floating base that can be deployed to an area of national defense interest to provide flight, maintenance, supply and other forward logistics support operations for U.S. and Allied forces. MOB modules will most likely be semisubmersibles which have significantly smaller wave-induced motions compared to conventional hulls. This modularity supports the widest possible range of air support, ranging from vertical/short takeoff and landing (VSTOL) aircraft using a single module to conventional takeoff and landing (CTOL) aircraft using several serially aligned modules approaching 6,000 feet in length. In addition, a MOB accepts ship-borne cargo, provides nominally 3 million square feet for equipment storage and maintenance, stores 10 million gallons of fuel, houses up to 3,000 troops (an Army heavy brigade), and discharges resources to the shore via a variety of landing craft.

The basic strategy is to deploy semisubmersible "building block" modules which could be deployed in a number of different modes of operation. A typical module is shown in Figure 1. Each module consists of a box-type deck supported by multiple columns on two parallel pontoons. When transiting between operational sites, the module is deballasted and travels with the pontoons on the surface much like a catamaran. When on site, the module is ballasted down so that the pontoons are submerged below the surface wave zone, thereby minimizing the wave-induced dynamic motions. The decks, which store rolling stock and dry cargo, are all located above the wave crests. The columns provide structural support and hydrostatic stability against overturning.

A MOB platform could range anywhere in length from a single, 300 meter-long, module to multiple modules serially aligned to form a runway up to 2 kilometers long. All platforms would provide personnel housing, equipment maintenance functions, vessel and lighterage cargo transfer, and logistic support for rotary wing and short take-off aircraft. The longest platform (nominally 2 kilometers in length) would also accommodate conventional take-off and landing (CTOL) aircraft, including the Boeing C-17 cargo transporter. Upon first inspection, the notion of a 2-kilometer long floating platform seems so far beyond the state-of-practice that it would not be worthy of serious discussion. There are, however, a variety of conceptual approaches that offer promise towards accomplishing that goal.

The Office of Naval Research (ONR) investigated whether a MOB represents credible technical capability for Naval and Marine Forces. In FY96, the Office of Naval Research (ONR) assumed leadership to conduct a Science and Technology (S&T) Program to advance critical design technologies for Mobile Offshore Bases (MOB). There are no historical precedents for designing and building floating platforms as large or as multifunctional as MOB. This introduced a high degree of uncertainty into their design, and justified this government-sponsored S&T program focusing on the feasibility of long, interconnected, open-ocean floating platforms. This S&T program critically examined and advanced existing commercial design standards to provide the offshore classification societies and industry with the capability to confidently design and build MOB platforms consisting of interconnected modules with an acceptable level of risk.

The first task for assessing constructability was to identify a range of probable dimensions for MOB semisubmersible modules; this was accomplished in the four previously described preliminary system designs. The modules proposed in those studies range from 220m to 500m, and are all longer than the 200m length of the longest existing semisubmersible. Equally important is the fact that the nominal 120m to 170m beam of these proposed modules is much larger than the capacities of existing shipyards. Using this information, an assessment study was conducted which concluded that U.S. industry has the capacity to competitively deliver a full (2 kilometer) MOB.

This Program identified and managed an extensive series of advancements using the best of academia, industry, and government experts. An independent group of marine engineering experts from industry, the American Bureau of Shipping, and academia was tasked to review the Program and its products and render an opinion on MOB feasibility and cost. The resulting assessment report was provided to Congress in April 2000. A key conclusion was that all of the key technology issues identified at the inception of the ONR S&T program that put MOB beyond the state-of-practice were either resolved or evaluated sufficiently to conclude there were no inherent showstoppers. It was concluded that the use of Mobile Offshore Bases, ranging from one 300-meter long module to a 2-kilometer long platform consisting of serially-aligned multiple semisubmersibles, in the open ocean as a forward base appears technically feasible.

Estimating the construction cost for a basic MOB (includes hull and basic machinery but excludes military enhancements) was one of the two objectives of this S&T Program. Accurate cost estimates are difficult to project at this time for three reasons: (1) the operational requirements have not been refined, meaning that the platform requirements are unknown (specifically, platform length and beam); (2) the trade-off between acquisition versus life cycle costs have likewise not been decided; and (3) the number of units to be built is not known. Therefore, only approximate information is available regarding cost at this time. Indications are that a single module would cost on the order of $1.5B, with a full MOB platform (2-kilometer length) costing between $5B and $8B.

In early 2001 a study by the Institute for Defense Analyses concluded that the Mobile Offshore Base concept was less cost effective than alternatives such as nuclear-powered aircraft carriers, joint logistics capabilities and Large Medium Speed Roll-on/Roll-off (LMSR) sealift ships.