The early “notched” barge, then ITB, and now AT/B - all grew out of the demand for low cost, safe, reliable, and more rapid marine transportation.
While transportation using the conventional towed barges was less expensive than a ship, they were extremely weather dependent making them unreliable in some conditions and they were also much slower than the ships they often replaced. Towed petroleum and petrochemical barges have historically suffered horrendously as far as weather-induced delays. In some operations in the Gulf, annualized weather delays for long-term operations of some tug/barge fleets averaged 30% or more. In the Northeast, it ran as high as 40 to 50%, especially in the winter.
The primary reason transportation using conventional, hawser towed tug and barges was less expensive than a ship, was related to things like crew and construction costs (especially in the United States Jones Act Trade). As an example a typical U.S. flag, Jones Act Tanker without a large amount of automation to reduce the overall manning requirements would have a crew of about 22 to 32 people. Whereas a typical U.S. flag, Jones Act conventional tug and barge or for that matter an AT/B with a tank barge, with the same cargo carrying capability can operate with as few as 7 people, but seldom more than 10. The difference in crew cost alone is quite large.
Now the task was to develop a design where you could both reduce the crewing (and costs) and get more speed and reliability, the ultimate goal being something akin to ship-like reliability. The initial attempt to solve this problem was the development of the ITB. When that concept fell on hard times, in both the technical and regulatory environs, the response was the continued development of the AT/B.
What the AT/B did, was to solve most of the technical impediments to being ship-competitive, while maintaining the crew and capital cost advantage of the tug and barge. What you have, is weather reliability, in a REAL tug and barge. An AT/B is not a rule beater. So for many types of services, the AT/B shines, as compared to a ship.
- Individual units for insurance purposes - loss of one does not mean a unit CTL.
- As compared to a ship, a wider availability of shipyard sites for drydocking the powerplant.
- Ability of both tug and barge to function as fully independent units when one or the other requires shipyarding.
- Smaller crew and different, more efficient crew culture.
- Ability to build both vessels in specialized shipyards, lowering costs.
- Ship-reliable ETA’s at greatly reduced operating costs.