Structuring the Active and Reserve Army for the 21st Century Section 7 of 7
December 1997


Details About CBO's Mobility Analysis

To evaluate how long it would take to deploy Army equipment overseas for a major regional conflict, the Congressional Budget Office (CBO) developed a model of the U.S. military's intercontinental (or strategic) lift capabilities. CBO used the model to evaluate the feasibility of the Army's schedule for delivering forces to such a conflict. CBO also used the model to determine the impact of its various alternatives on the pace of Army deliveries. The model purposely assumes that deliveries take place under favorable conditions, for two reasons: to see whether the Army could meet its schedule even under the best of circumstances and to provide a conservative assessment of the impact of CBO's alternatives on the Army's ability to deploy forces overseas.

Two major components--airlift and sealift--make up strategic lift. CBO did not model airlift in detail. Rather, it relied on an estimate by the Department of Defense (DoD) to establish the total amount of airlift that will be available shortly after 2000 to move the U.S. military. CBO then assumed that the Army would have access to roughly the same fraction of total airlift that it did during the Persian Gulf War. To determine sealift capability, CBO used its model, which tracks the movement of all ships planned to be in the fleet of the U.S. Transportation Command in 2001.

Assumptions of the Sealift Model

The sealift model was designed to provide general approximations, not precise estimates, of the time needed to deliver Army forces overseas. For that reason the model operates on single-day increments, rounding all activities to the nearest day. For example, if an SL-7 Fast Sealift Ship would take 17 days and five hours to sail from Savannah, Georgia, to Pusan, South Korea, the model treats the operation as requiring 17 days. Additionally, instead of assigning ships to particular ports (such as Long Beach or Oakland), the model assigns all ships based in the continental United States to either the East Coast or the West Coast and assumes that their destinations are somewhere in the Middle East or South Korea. In reality, travel times differ for ships going to or from different ports in the same general area, but those differences are relatively small and would not substantially affect the output of the model.

CBO's model contains three key assumptions that could have a considerable impact on the results. The first assumption is that port constraints are nonexistent. In other words, no matter how many ships (of any size) arrive at a given port on a given day, it is assumed that the harbor is sufficiently deep to accommodate them, that there are enough berths for all of them to dock, and that the port has enough workers and equipment to simultaneously load or unload every ship. That may be true of the U.S. ports that the military uses and large ports in South Korea and the Middle East. It is not the case for all foreign ports, however, some of which may not be deep enough or big enough to handle several large ships at once.

The second assumption is that all ships in the simulated fleet are fully operational over the entire course of the sealift operation. In reality, at any time some ships are in dry dock or otherwise unavailable because of routine maintenance or unanticipated problems. Moreover, it is the nature of all mechanical systems to break down over time, so it seems inevitable that a certain percentage of ships would encounter problems during their sealift operations that would either slow them down considerably or render them temporarily inoperable.

Third, the model does not include any other unforeseen impediments that could adversely affect the transit times of ships. An example of such an impediment would be a critical area (the Panama Canal, Suez Canal, or Straits of Hormuz, for example) that was treacherous or impossible to navigate, possibly because of mines, enemy forces, or physical damage to the waterway. Another impediment might be severe weather conditions. Situations like those would require some ships to alter (and thus lengthen) their course, increasing the amount of time it would take them to reach their destination.

All three of those assumptions represent best-case scenarios for delivering U.S. forces overseas. Thus, it is entirely plausible that in reality, sealift operations under different--and perhaps more likely--conditions would take longer than this model predicts.

The Database and Variables in the Model

The fleet of ships whose movement the model simulates is the fleet that DoD plans to have available for sealift operations by 2001. That fleet includes eight SL-7s; 52 roll-on/roll-off ships (ROROs); 19 large, medium-speed roll-on/roll-off ships (LMSRs); and 14 breakbulks (see Table A-1). Additionally, CBO assumed that the military would supplement its sealift fleet by hiring foreign charters with a combined carrying capacity roughly equivalent to 50 U.S. breakbulks.

Table A-1.
Number of Sealift Ships Under the Control of the U.S. Transportation Command and Their Readiness Status
Type of Ship Number of Ships in Readiness Status
(Days until ready)
of Ships
2 4 5 10 20 35

SL-7 Fast Sealift Ship 0 8 0 0 0 0 8
Roll-on/Roll-off Ship 16a 21 6 9 0 0 52
LMSR 8b 11 0 0 0 0 19
Breakbulk 0 0 10 0 4 0 14
Foreign Charter 0 0 0 20 20 10 50

SOURCE: Congressional Budget Office based on data from the Department of Defense.
NOTE: LMSR = large, medium-speed roll-on/roll-off ship.
a. These ships hold prepositioned Marine Corps equipment in peacetime. CBO assumed that after they delivered that equipment to a conflict, half of the ships would be available immediately to transport Army equipment and the other half would be available 45 days later.
b. These ships hold prepositioned Army equipment in peacetime. After delivering that equipment to a conflict, the ships would be available to transport other Army equipment to the conflict.

Each of those ships has a total deck capacity measured in square feet. However, when equipment is loaded onto the ships, it must be tied down and secured. Furthermore, aisles must be left between rows of equipment for safety reasons. Consequently, only a certain percentage of each ship's total capacity can actually be used to carry equipment, and the model uses a variable (STO, for stowage factor) to capture that relationship. A typical value for STO is 0.75, meaning that only 75 percent of the ship's total capacity is actually used to carry equipment (see Table A-2).

Table A-2.
Characteristics of Various Types of Sealift Ships Under the Control of the U.S. Transportation Command
Type of Ship Days for PORT Procedurea Typical Capacity (Square feet) Stowage Factor (Percent)b Maximum Sustainable Speed(Knots)

SL-7 Fast Sealift Ship 3 213,000 75 27
Roll-on/Roll-off Ship 3-4 150,000 75-80 17-19c
LMSR 3 375,000 75 24
Breakbulk 4 72,000 75 19-20

SOURCE: Congressional Budget Office based on Department of Defense, Military Traffic Management Command, Logistics Handbook for Strategic Mobility Planning (April 1994), and other data from the Department of Defense.
NOTE: LMSR = large, medium-speed roll-on/roll-off ship.
a. The amount of time necessary for a ship to enter a port, dock, load or unload, and then leave the port area.
b. The percentage of a ship's capacity actually used for carrying equipment.
c. One roll-on/roll-off ship in the fleet has a maximum sustainable speed of 25 knots.

The remaining variables in CBO's model determine the amount of time each ship requires to perform an operation. The first of those variables is the readiness status (RS), which indicates the number of days a ship needs to be activated and begin steaming. Most ships in the sealift fleet have an RS of two to five days, but a few have an RS as high as 20 days (see Table A-1). Another time variable (PORT) measures the amount of time a ship needs to maneuver into a port, dock, load or unload, and then head back out to sea. The typical value for the PORT variable is three or four days.

The final variable in CBO's model is the transit time between ports (TRANS). During the model's hypothetical sealift operation, the majority of ships sailed directly between ports in the United States (East Coast or West Coast) and ports in South Korea or the Middle East. Exceptions were ships prepositioned in Diego Garcia, Guam, or Saipan and a few ships that had to load equipment belonging to troops in either Hawaii or Germany. In all cases, the value for TRANS was determined using the maximum sustainable speed of a given ship and the distance between its ports of embarkation and debarkation.

The Sealift Process in the Model

CBO's model begins on day zero, which is defined as the day orders are issued to activate the fleet for sealift operations. (That is assumed to be the day the first major regional conflict begins.) Each ship then becomes available for service at the end of its RS period, at which point it begins the following sealift process: it loads equipment (a delay equal to the PORT variable) and sails to its destination (TRANS). It then unloads the equipment at the theater of conflict (another PORT delay). When the PORT procedure is over, the equipment is defined as having arrived in theater. The ship then returns home (TRANS) to begin the cycle again.

The amount of equipment that each ship delivers is its capacity times its STO. Each time a ship loads equipment, the model takes note of what type of equipment is loaded. Every combat unit sent to a conflict has unit equipment, such as tanks, helicopters, and the like. The movement of combat units (typically divisions) is modeled explicitly. In addition, large numbers of combat-support and combat-service-support (CS/CSS) units and associated equipment accompany each combat unit. Their equipment includes trucks, field kitchens, and myriad other items. The model keeps track of the unit equipment assigned to specific Army divisions and other combat units, but only the overall amount of CS/CSS equipment.(1)

In general, CBO did not determine in what order units would be sent to a conflict or which specific units would be loaded onto each ship. Instead, the model used the following guidelines to determine when different types of equipment would be shipped to the theater.

The equipment sent aboard prepositioned ships is, for obvious reasons, predetermined. Additionally, a number of ships are designated to carry equipment for the Navy and Marine Corps on their first sailing and only become available for Army use once that initial delivery is completed.

For all other ships, the decision to load combat unit equipment or CS/CSS equipment was made according to the following criteria. Certain combat unit equipment takes first priority because it belongs to the halting force (the units that are intended to prevent the enemy from making further advances until the rest of the Army's forces arrive and a counteroffensive can be mounted). CS/CSS equipment is also sent during the initial shipments to provide support for the combat forces. In general, throughout the modeling of sealift, support forces and their CS/CSS equipment are shipped along with combat forces so that the ratio of support equipment to combat equipment does not fall below a minimum of roughly 1 to 1, based on square footage. Once all combat unit equipment has been sent, the model continues sending CS/CSS equipment until the full complement has arrived in theater. (In determining how much of the required equipment has arrived in theater, the model includes not only equipment that has been sent by sealift but also equipment that is in place or airlifted. "In place" refers to equipment that is assigned to forces stationed in that theater as well as equipment prepositioned on land near the theater.)

1. CBO modeled the movement of unit equipment only. It did not estimate the time and ships necessary for deliveries of ammunition or other supplies needed to sustain a major regional conflict.

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