Ship Economics, Fuel Efficiency and Trans-Oceanic Tug Barges

While the maritime industry offers the lowest transportation cost per container over extended distances that road or railway transport, the cost of fuel per voyage is usually the largest expense item on the accounting sheets. The ship industry has been open to suggestions as to how to reduce the fuel expense, with some ship owners installing air-foil sails on to the decks of some ships, or attaching large kites to the bow that trade winds will pull at altitudes of up to 1,000-metres above sea level. During the early 20^th century, some ship owners converted steam ships to schooners to increase earnings.

The conversion involved removing the steam engines and coalbunkers to increase revenue payload capacity. During a later period, some lake-ship owners increased payload capacity by removing diesel engines and fuel tanks from ships that they converted to tug-barges, including modifying the stern so as to provide a secure coupling for a push tug. The fuel consumption of the tug barge is only marginally higher than former ship, except the increase in payload capacity greatly increased earnings for ship owners and operators. There has been advanced development of ocean-capable tug barges that carry bulk cargo along the American coast.

The successful precedent of an ocean-capable tug-barge that combines a 600-ft barge with a 200-ft tug suggests that it may be possible to develop barges of 1,000 to 1,500-ft length and pushed by tugs of up to 500-ft length. Future research would need to develop barges that when pushed by a tug, will sail at the identical speeds as self-powered ships of equivalent length, draft and beam. The development of a successful prototype would offer the maritime industry a vessel technology capable of carrying greater payload and earning higher revenue despite marginally higher fuel costs.

The tug may carry the identical engine, as would a ship of equivalent dimension as the concept oceanic barge. Extended length of tug may be the basis by which a tug-barge may sail at the speed of a ship, while carrying greater payload. The ability of an extended length of tug to propel a mega-size of barge across the ocean provides opportunity to explore possible methods by which to improve overall fuel efficiency. Some fuel-saving technologies occupy considerable volume, space that would reduce payload capacity aboard a self-powered ship. Except that space would be available aboard a large tug.

Bottom-Cycle Engine:

If the barge carries large volume like containers, a remote control cable would carry navigation signals the navigation crew located in a forward bridge located above the bow of the barge to the push-tug, the location of the engine room crew. The volume aboard an extended length tug could greatly exceed the available volumetric space of the engine room and fuel supply inside a ship. That volume could allow for the installation of a bottom-cycle engine or low-temperature Rankin-cycle engine that would generate power from the exhaust heat of the main diesel engine.

If an engine of 20,000-Horsepower (15,000kW) operates at 50% thermal efficiency, it will reject the equivalent of 20,000-Hp through the combination of the engine exhaust (6,000-Hp) and liquid-based cooling system (7,000-Hp). Engine room ventilation would remove another 7,000-Hp. A heat exchanger could extract heat from engine room ventilation to provide heating to both engine room crew and via a heater pipe, the navigation crew. A battery of bottom-cycle engines could provide some 1,000-Hp from the combination of engine coolant heat and engine exhaust heat, that may be applied to propulsion and reduce overall fuel consumption by 5%.

Regenerative Brayton-Cycle Engine:

A gas turbine is an example of a Brayton-cycle engine, where combustion or heating of the gas occurs between an air compressor (or air storage reservoir) and the engine. Some Brayton-cycle engines comprise both low-pressure and high-pressure compressors and turbines, where the low-pressure turbine and compressor acting as a turbo-charger to the high-pressure turbine and compressor. In some Brayton-cycle designs, it is possible for a regenerator to transfer engine exhaust heat to preheat the incoming stream of air upstream of the combustion chamber(s), downstream of the air compressor or air reservoir. Such operation boosts engine efficiency.

Except that a regenerative heat exchanger of extreme volume is required to transfer engine exhaust heat to the incoming air stream. While it may be possible to combine a piston air compressor with variable air flow rate capability with a piston engine, the combination of separate compressor and engine plus an large air storage reservoir would also occupy increased volume and would only be possible aboard a large push tug. Researchers associated with Quasiturbine Engine Company of Montreal, Canada evaluated combining a (variable flow rate) positive-displacement air compressor and positive-displacement rotary engine, to prove that such operation is possible.

Fuel Flexibility:

While internal combustion piston engines generally have limited flexibility as to the choice of fuel, a Brayton-cycle engine can offer much greater flexibility in the choice of fuel. In some geographic locations, the ability to change fuel with zero or minimal modifications to the engine increases the competitiveness of maritime transportation. It may also be possible to replace the combustion chamber with an externally heated, high-temperature heat exchanger. External heating would allow for combustion of volatile industrial chemicals and/or solvents or the choice of solid fuel combustion such as wood, biomass or small-scale clean coal technology.

A positive-displacement Brayton cycle engine system may be possible using modified existing marine piston engines that would be converted to combustion or heating of the working gas occurring outside of the cylinders. Such development would assure parts compatibility with existing maritime fleets and the ability of engine room crews to perform maintenance and repair while the ship or tug is at sail on the ocean. Engine room crews may be able to undertake minor adjustments to adapt the modified engines to different fuels. Modified existing engines may be suited to operation on large oceanic tug barges.

Conclusions:

The combination of a mega-size of tug pushing and navigating a mega-size of barge may be able to sail at the same speed as existing large ships

The tug-barge combination would be able to carry greater payload than a self-powered ship of equivalent size as the barge

Remote control cables may link forward control bridge located above the barge bow to the push-tug

Large size of tug allows for engine/power options that would reduce payload capacity aboard self-powered ships.

Variety of engine options for mega-size push tugs offer potential for higher energy efficiency

Combination of higher efficiency engines in mega-tug pushing a mega-barge offers potential gains in productivity and increased competitiveness

---

Harry Valentine can be reached at [email protected].