Home | Business News | Browse by Publication | I | Interfaces

Steaming on convex hulls.

Article Excerpt
This is a sea story about using a simple classroom example to save a great deal of money, as well as to convince beginning Postgraduate Naval School operations research students--experienced, skeptical military officers--that mathematical analysis can yield immediate results. The application is planning a ship's transit from one point to another in a fixed amount of time, given that the ship can operate with one or more of its propulsion plants idled to save fuel. Simple analysis yields nonintuitive results that US Navy shipboard energy-conservation guides overlook. One of the authors (Kline) solved this homework problem as a student and subsequently applied this example when he took command of USS AQUILA, a patrol hydrofoil missile ship. AQUILA achieved results so striking in comparison to her sister ships that the squadron material officer inspected her engineering plant to ensure that no safety settings were being overridden to achieve this record. Kline's spreadsheet decision-support tool was provided to other hydrofoil commanders. A more general version has been conveyed to the US Navy. Considering that our navy spends about a billion dollars per year on fuel for surface-combatant ships alone, this development promises substantial, long-term returns.

"But thou, contracted to thine own bright eyes, Feed'st thy light'st flame with self-substantial fuel."

Shakespeare, Sonnet I

Key words: naval operations; optimization; linear programming; energy conservation; fuel conservation; sea-going vessels.

History: This paper was refereed.

**********

Naval surface-combatant ships consume large quantities of fuel. For example, four LM2500 gas-turbine engines power a DDG 51 Arleigh Burke-class, guided-missile destroyer (Figure 1). The LM2500 is a derivative of the engines in the Boeing 747 and other commercial aircraft. Depending on propulsion-plant configuration and speed, these four turbines can collectively consume from 600 to 7,000 gallons per hour (GPH) of distillate marine fuel (DFM) (United States Navy 2005a). That means that the destroyer's fuel consumption while at sea is between 100,000 and 1,000,000 gallons per week. For surface combatants alone, marine fuel costs our navy about one billion dollars per year (United States Navy 2005b).

[FIGURE 1 OMITTED]

The US Navy is keenly aware of its fuel consumption, seeks innovations to conserve fuel, and prescribes how its ships should monitor, control, and minimize fuel use. For instance, since 2000, its Naval Sea Systems Command has administered a program to provide energy-conservation incentives and has saved 10 percent of the total fuel allotment for participating ships. For each ship class, the program conducts sea trials to determine efficient fuel-consumption standards for every propulsion-plant configuration and speed, and then publishes these engineering standards in software that can be used to plan transits and ship plant operating modes (United States Navy 2005b).

First principles of naval architecture (Manning 1956) characterize propulsion energy requirements as a cubic function of speed; therefore, speed is the key determinant of fuel consumption. Ronen (1982) suggests an optimization of civilian cargo-ship employment that trades off fuel savings that result from slow steaming with the resulting decrease in revenue to offset fixed operating costs. This provides insight; however, the mission of navy ships often prohibits moving slowly.

Our analysis pertains to a transit along a great circle between points A and B, with no winds, currents, or obstructions in between. The basic idea is to minimize the amount of fuel required to get from A to B in a specified time. We discuss generalizations in the Other Route-Planning Considerations section and in the appendix.

Fuel Use as a Function of Speed and Mode

Some ships can operate in any of a number of propulsion-plant operating modes. The number of engines that are powering each shaft often distinguishes the mode. When a ship's captain orders maximum speed, the chief engineer brings all plants online to power all shafts. This is the least fuel-efficient plant-operating mode in terms of gallons per nautical mile. Routine operations call for lower speeds that allow the ship to align its plants in different modes to use less fuel.

The twin-screwed DDG 51-class destroyer in Figure 1 has two LM2500 engines for each shaft and screw. Other than cold-plant mode in which the ship is anchored...

View this article FREE - Now for a Limited Time, try Goliath Business News
Free for 3 Days!