The Versatility of Tug-Barge Technology

The development of a tug pushing and navigating a barge or a train (tow) of coupled barges has steadily gained greater acceptance within the maritime community. While such operation began along inland waterways, Crowley Maritime and Northern Transportation Company sail tug-barge combinations on the open ocean. Tugs used in such service now exceeds 16,000-Hp (12,000kW) and sail at some 16-knots. Tug-barge conversions developed in the northeastern USA have sailed through waves of 12 to 20-feet or 3.6 to 6-metres while Crowley tug-barges regularly sail the energetic waters of the Gulf of Mexico and Caribbean Sea.

Conversions from ship-to-barge have proven feasible for ship owners who removed engine(s) and fuel tank(s) to increase payload capacity. About a century ago, some Great Lakes ship owners removed coalbunkers, boilers, water tanks and steam engines to increase payload capacity by converting some steam ships into wind-driven schooners. While modern tug-barge operations incur marginally higher operating costs than self-powered ships, the gain in earnings from carrying additional payload aboard the ship-to-barge conversion has exceeded that higher operating cost.

Engines off the Ship:

Tug-barge builders and operators have established technical precedents upon which to develop larger tugs and barges to operate extended oceanic voyages. To propel the greater payload capacity aboard an externally powered vessel such as a barge, a tug may produce mechanical propulsive power to activate tug-mounted propellers while a towed power-generating vessel may produce electrical power to activate azipod-mounted propellers under the barge. The latter concept may be built as a twin-hull catamaran, with the propeller backwash passing between the twin hulls while being able to sail at speeds in excess of 20-knots.

Voith of Germany builds extreme-size universal joint couplings and extreme diameter drive shafts. That technology may allow a diesel engine mounted aboard a twill-hull towed unit to drive into a mechanical power input installed at the stern of a barge, then into a gearless “Swiss-mechanism” transfer drive and to a propeller mounted under the stern of a barge. While the all-mechanical power system could offer the combination of high efficiency and faster sailing speed, there will be need for further development to prolong the service life of the extendible, double-jointed drive shaft.

Cleaner Exhaust Emissions:

Tug-barge development occurs at a time when governments seek to require reductions in ship exhaust emissions, while ship operators seek to reduce fuel consumption per ton or per container. While the additional space aboard the barge can earn additional revenue, the space aboard a large tug or towed power generation vessel that supplies power to ship-mounted azipods, can accommodate the added dimensions of alternative power systems and volume requirements of alternative low-density fuel that would otherwise reduce payload capacity aboard ship. The choice of alternative power plants may include internal combustion and externally heated engines.

Tugs and towed power generating units may carry alternative energy systems that include biomass-steam power, grid scale electrical battery storage, micro-nuclear power, radiation-free fusion energy conversion, fireless steam technology and perhaps tanks of compressed air. Other power options would include conventional diesel powered or natural gas powered engines. The use of compressed or liquefied natural gas would require special high-pressure tanks that may need to be cooled using the ocean as the heat sink. 

Short-distance Ferry Service:

Tug-barges may operate on rapid-rechargeable energy storage technology and be well suited to short-distance ferry services. At locations where a central terminal serves multiple ferry service to multiple nearby destinations (up to 5-miles), a single boiler at the dock may sustain a fleet of thermally rechargeable steam tugboats. During layovers, the boiler may ‘pump’ steam into banks of heavily insulated, high-pressure accumulators partially filled with saturated water, aboard each tug. The technology is well proven in railway shunting yards and is known for incurring very low operating costs while offering greatly extended service longevity.

Longer-distance Re-chargeable Ferry:

Banks of grid-scale, chemical batteries that use lithium technology or metallic-oxide flow battery technology, may be carried aboard a tugboat or aboard a twin-hull towed power supply unit. In both cases, electric motors will drive the propellers(s) installed under the tug or as part of azipod technology installed under the barge. Batteries may recharge from the grid during ferryboat layovers, with chemical battery technology requiring replacement every few years. The ferry service could operate voyages of 100 to 150-miles between recharges.

Wave Power:

One recent advance in wave-driven maritime propulsion involves a short and buoyant tug pulling a barge of several times its length. The up and down bobbing motion of a tug in choppy water drives a submerged ‘wing’ that pushed water to the rear as it moves up and down. A scale model involving a ‘wave-powered tug’ pulling a boat of 10-ft (3-meters) sailed at a measured speed of 6-knots. Longer versions of the technology involving twin-hull catamaran barges are expected to sail at higher speed and be suitable for several short-distance coastal applications, including tourist excursions.

Other Alternative Power Systems:

Where tugs provide propulsion, tug-barges may predominantly operate short oceanic voyages at slower speeds than ships. In regions where diesel fuel or bunker fuel is high, they may operate on lower-density fuels such as biomass. Where a recharge of electrical or thermal energy would sustain the operation of the tug, a tug may undergo an extended recharge while another unit navigates the barge. Towed generating vessels could generate electric power from micro-nuclear technology or in the future, from some form of radiation-free nuclear fusion technology and be well suited to extended distance voyages at elevated speeds.

In Port:

Tug-barges regularly operate many inland waterways as well as ocean coastal services in North America and the Caribbean. Ports along inland waterways regularly accommodate the arrival of coupled barges, often uncoupling the barges as they arrive at dockside. Recent research has focused on a tug pushing and navigating a coupled pair of ship-to-barge conversions, technology intended to sail the Great Lakes with possible calls at several North American east coast ports.

Some seaports may be able to accommodate the extended length of coupled ship-to-barge conversions while other seaports may need to borrow operational precedents from ports on inland waterways. The arrival of an oceanic barge towing a power-generating companion vessel may require that the towed unit be uncoupled due to restricted space at dockside. Port tugs then would maneuver the barge to and from dockside.

Conclusions:

A variety of factors favor further development of tug-barge technology:

-The technology is well proven along inland waterways and on the ocean

-Present ongoing development suggests future scope to develop larger tugs that will push and navigate larger barges

-The demand for cost-competitive transportation service is driving down the price for transporting containers

-Vessels that provide the greatest carrying capacity at the lowest cost are progressively proving more attractive to the market, opening the door for the development of towed power-generating companion vessels

-The increased requirement for lower emissions from ship exhausts allows an engine-less barge to remain in service while tug engines are modified for cleaner emissions

-There may be scope to develop a mechanical power transfer technology between a towed unit and a barge with a propeller, to raise efficiency and sailing speed.

Harry Valentine is a frequent contributor to the MarEx newsletter and can be reached at [email protected] for comments/questions.