Some of the older composite (or integrated) units we used to see on the West Coast were built by Hess and later owned/operated by US Shipping.* ITB BALTIMORE, GROTON, NEW YORK, JACKSONVILLE, MOBILE, PHILADELPHIA.* I believe these are all retired from the petroleum transporation business now.
The tug part of the unit could only be separated at a shipyard under tightly controlled stability conditions.* Couldn't even safely operate by themselves, had to have tugs push them around.* The ATB's, on the other hand, are able to put the barge on a towline, if needed, and safely tow, although there are typically a number of limitations when compared with conventional towing vessels (some of the ATB's don't have tow winches and the ability to put the barge on a long tow line).
The primary purpose behind the ITB's (and to a lesser extent, the ATB's) was to drastically reduce manning and construction costs when compared with tankers of the same size.* Only recently have ATB's been constructed to the size of*small tankers.
Check out the following link for Crowley's ATB information: