AT/B’s are as different from ITB’s, as AT/B’s are different from ships. Some history is in order.
For definition purposes the AT/B must be differentiated from the original ITB (Integrated Tug/Barge) where the tug and barge were locked together in a rigid configuration and became for all practical purposes a single unit. My firm, coined and uses the acronym “AT/B” as a trademark, with a deliberate separation being made with a “/” between T (tug) and B (barge), to show that the two units of the articulated tug-barge, are indeed, separate, independent units joined by choice, not necessity.
The common characteristic of the most widely applied rigid ITB systems is the fairing and coordination of the hull shapes of both the tug and the barge, especially in the area where the “tug” mates to the “barge”. They are actually, in essence, “disconnectable” engine rooms. ITB’s date back many years, and the first practical application of the technology came in the 1950’s, when the ITB “CARPORT” was built. This vessel, which essentially was a tug locked onto a stern “ramp” of a barge, traded successfully on the New York canal system and the Great Lakes for many years, hauling grain products. However, for various reasons, she was not repeated. The trade in which she engaged was also populated by conventional tugs working with “notched” barges, and some towed barges. Such units were simpler, less costly to build and more practical. The added speed CARPORT was able to attain, was of little advantage in the New York Barge Canal, where locks and narrow channels greatly restricted unit speed.
In heavy seas with the ITB, the tug functions in the same way as the stern of a ship. It pitches with the barge and heaves with it. However, because the ITB tug is built to a depth to match the companion barge hull, the motions are no worse, or no better, than a ship of the same size. The exception to this of course would be a case where the ITB tug was built with LESS depth. However, even the earliest ITB’s (for which patents exist even in the 1800’s) such as the “CARPORT”, shared the design feature of a nearly equal depth barge and tug.
ITB’s are also designed to remain coupled in all sea states. To the author’s knowledge, no ITB currently in use, has disconnected and successfully towed the companion barge. The few times such separate operations have been attempted, there were casualties or near-casualties. One such loss was “Cat-Tug” lost off the Azores when it was somehow disconnected from its’ barge, OXY 4102. The “tug” was a total loss, as the unseaworthy hull design of the “tug” foundered after damage in heavy seas. During the attempted delivery of a lone “Cat-Tug”, the vessel so badly pitched and poled offshore, that the tug was forced to turn back and the barge was brought to it from another shipyard. In short, ITB’s were never designed to operate as separate units and were truly conceived as “rule beaters” Stories abound, of ITB’s unable to be disconnected before drydocking, as long-unused and seized connecting devices made separation difficult.
So over time, the ITB fell into disfavor as the cost to build these units spiralled to numbers which were in excess of an equivalent ship. The issue of NAVIC-2-81, by the U.S. Coast Guard, also closed many loopholes in regulations which the ITB was supposed to take advantage of. Thus, no ITB has been built since the early 1980’s. Because the ITB was falling from favor, the continuing need for economical maritime transport meant that another solution had to be found.