The Evolution of the Oceanic Tug-Barge

Tug barges are widely used in the North American Atlantic and Pacific Coastal service, carrying bulk freight. While present generation tug-barges involve a tug coupled to the stern of the barge pushing and navigating the assembly, one company with interest in LNG transportation seems poised for a possible breakthrough in tug-barge propulsion. 

Introduction

While railway lines that run parallel to the North American coast line connect many coastal cities, a segment of the bulk freight transportation market prefers to move the cargo by coastal maritime transportation instead of railway transportation. For the railways, moving heavy bulk pushes axle loadings to the maximum and imposes faster wear and tear on the railway infrastructure and on railway rolling stock. A barge built to a beam of 105 feet and 650 feet length can carry the same volume/payload of bulk freight as a fleet of trains, while incurring the operating cost of a single train.

The development of couple barge trains or barge tows along the American inland waterway systems reduces the cost per ton-mile of moving bulk freight over extended distances, over railway transportation. At the present time, the variation of the same design of tug is that used in both ocean coastal and inland waterway service, involves a diesel engine driving any of Z-drive or vertical-axis thrusters via a mechanical transmission. Steam powered tugs were originally designed to assist in the maneuvering of large ships in the restricted areas of dockyards and to move single barges along rivers.

Tug Advantage

During an earlier era when river transportation and coastal transportation developed in North America, vessels were either driven by wind or by onboard steam engines. The need to carry coal and potable water involved allocating space for fuel and water and restricted the operating range of coastal steam boats. Steam powered river vessels made frequent stops at towns along the route, where coal/wood fuel could be taken aboard for the next leg of the voyage. Adapting a tug to push and navigate a river vessel provided additional capacity aboard the vessel to carry greater payload.

In terms of business economics, it was feasible to utilize a tug to push and navigate a single barge. At a later time as the power output and pulling capability of tugs increased, it became very feasible for a pair of tugs to propel and navigate a coupled assembly of barges that formed a river train, with stern and bow tugs. When a powered vessel moored at port to load/unload cargo, the crew could be out of service and unproductive for several days. With tug operation, barges were dropped off while the tug and crew remained productive. 

Coastal Service

The business economics of tug barge operation was considered to be as valid for coastal service as it was along inland waterways, with tugs initially moving and navigating comparatively barges for short distances between nearby coastal cities. Over time, ocean going barges became wider, deeper and longer, making them more capable to sail oceanic service and carrying bulk freight over greater distances in coastal service and also on the North American Upper Great Lakes. The technology has been developed to sail through waves of 20 feet.

The bulk freight market seems poised to require larger tug barges that sail at greater speed of perhaps as high as 15 knots, in turn requiring larger tugs of greater engine output and much greater thrust capability. It is possible that the requirement could push existing tug barge design, with tug propulsion, to its limit. However, ongoing developments in maritime propulsion that date back over a period of decades, might actually offer an method using much proven technology, by which to build bigger barges of much higher tonnage and tugs of greatly increased engine output. 

The Proven Technology

The propulsion systems of some of largest trans-Atlantic passenger ships of a bygone, the largest cruise ships of the present day and the largest oil tanker ships ever built share a common feature. They were floating power stations with engines driving large electrical generators to provide power for electric motors that in turn drove the propellers. While earlier ships had non-steerable propellers, some modern ships include electrically driven propellers that are steerable, courtesy of Azipod technology. It appears technically feasible to combine a remote engine aboard a companion vessel to generate the propulsive electric power.

The companion vessel may resemble a tug and include some self-propulsive capability while the barge to which it is attached includes Azipod units for propulsion and steering. Power cables would connect between the power generation companion vessel and the Azipod equipped barge, with crew control-bridge being located on either. While the companion vessel could be coupled to the stern of the barge using existing coupling technology, there is also the option of a more flexible coupling system to link between the power generation unit and the propulsive barge that would tow the companion vessel.

Automated Navigation Control

Several years ago, the Northern Transportation Company that operated tug barges along Canada’s Mackenzie River participated in a demonstration of computer controlled automated navigation. The demonstration involved a combination of tugs and several barges that while connected by cable, could be steered independently of each other to negotiate through tight meanders along the river. There was also scope to link the barges using a power cable to activate stern and bow thrusters to assure precise maneuvering through areas of river restricted by river depth or river width. It may be possible to adapt the technology to ocean service.

The particular need to move massive bulk freight could require the capacity of a pair of ocean capable barges, each equipped with steerable electrically driven propellers and with the possible additional need for bow thrusters. While the leading barge would tow the large floating power station, the combination of a towing cable, power cable and navigation control cable would connect to a trailing barge, with towing cable tension load providing less than 10 percent of the propulsion requirement. During the ocean voyage, computer controlled navigation directed from the control bridge would guide the assembly of vessels.

Alternative Application

While the vessel concept is intended for bulk freight transportation, including transportation of LNG along the North American Pacific Coast, the floating power station could also find application in the cruise ship industry. Several years ago, a total engine failure occurred on a large cruise sailing offshore from the southeastern U.S., and initial attempts to use tugs to tow the stricken vessel resulted in the destruction of towing cables. A tug boat that is also a power station capable of delivering massive amounts of electric power would be a compatible rescue technology for stricken cruise ships.

Such a power station vessel could couple to the stern of a cruise ship and provide it with propulsive power, assuming that at least one of the Azipod units were operations. Some of the electrical power could also be directed to sustain cruise ship onboard hotel power, for the benefit of several thousand customers of the cruise ship company. Since the bulk carrier vessels are intended to operate along North America’s Pacific Coast, cruise vessels that operate in that region could occasionally have a rescue technology available to them.

Conclusions

The energy company that seeks to develop super-sized tug-barges to move their product along the North American Pacific coast could make a significant contribution in the evolution and development of ocean going tug barge technology.