Having sailed on real ships ranging from small product carriers to VLCCs and in between, along with a medium-sized ATB (150,000 bbl), my experiences might be a bit different as most seem to stick to one end of the spectrum or the other. That said, I enjoyed the ATB only to the extent that the crew was great to work with. The work, not so much. The shorter rotations are needed to prevent burnout. I chose not to return there as my pay was nearly the same compared to a new chemical tanker, and a real ship as well. Believe me, companies that run ATBs know every loophole that justifies their existence, but also expects the crew to treat the vessel as a bona fide ship. Their answer for questions about how to accomplish this with half the crew (10 I believe) was that "it's half the size of a ship. There. Problem solved". The oil majors seemed to be a bit hesitant to charter ATBs but this company tried to put their best foot forward and convince them they were one and the same. Do ATBs serve a purpose? Sure. Are they equal to a real ship? No. I have sailed through the middle of a hurricane on a ship, been through the Bering Sea on a ship, and also the North Atlantic in February. We rocked and rolled, came home with a few stories, but in no way did I feel worried or concerned for my safety or for the vessel. I can not say the same for an ATB. They exist only because the USCG allows them (who's pockets were/are lined nobody will ever know). Europe hosts thousands of small coastal ships, feeding cargo from one port to another, or sometimes from one dock to another in the same port. Yet they choose to use ships and not ATBs or even old-school wired units.
When a 350,000 bbl product tanker needs a 9,000 kW engine to make 14.5 knots, using 32 MT of 380/day, and a 150,000 bbl ATB product tank barge needs 14,000 kW (two engines) to make 11 knots, using 44 MT of 380/day, it's clear to see the cost advantage of a tankship based on fuel savings and maintenance reductions alone. Assuming the owner of an ATB has lower P&I premiums due to the unmanned barge status, and also half, or slightly less greater than half, of the crew over a tankship (also lowering P&I premiums) I still feel the cost/benefit ratio works in favor of a tankship, and likely in not as great of a timespan as you would think. Then again, when is the last time you worked for a tanker company that had forward-thinking beyond the end of the business day? No companies seem to look years into the future, but only immediately, and what provides an immediate return for the shareholders.