Seagoing tugboats are in three basic categories:

3. The "integral unit," "integrated tug and barge," or "ITB,"comprises specially designed vessels that lock together in such a rigid and strong method as to be certified as such by authorities (classification societies) such as the American Bureau of Shipping, Lloyd's Register of Shipping, Indian Register of Shipping, Det Norske Veritas or several others. These units stay combined under virtually any sea conditions and the "tugs" usually have poor sea keeping designs for navigation without their "barges" attached. Vessels in this category are legally considered to be ships rather than tugboats and barges must be staffed accordingly. Such vessels must show navigation lights compliant with those required of ships rather than those required of tugboats and vessels under tow. Articulated tug and barge units also utilize mechanical means to connect to their barges. ATB's generally utilize Intercon and Bludworth connection systems. Other available systems include Articouple, Hydraconn and Beacon Jak. ATB's are generally staffed as a large tugboat, with between seven to nine crew members. The typical American ATB operating on the east coast, per custom, displays navigational lights of a towing vessel pushing ahead, as described in the '72 COLREGS."
http://en.wikipedia.org/wiki/Articulated_tug_and_barge

"Système barge-remorqueur

Dans ce système, le remorqueur pousse une barge, leurs structures s'imbriquant l'une dans l'autre, pour un rendement maximum. Deux systèmes existent, les remorqueurs-barges articulés et intégrés, suivant la mobilité des deux navires une fois reliés.


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Note added at 25 mins (2011-03-02 14:04:45 GMT)
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http://fr.wikipedia.org/wiki/Remorqueur#Syst.C3.A8me_barge-r...