Most of the old FPSO's are just converted tankers there is also a couple of old submarine repair ships that have been converted. But the oil and gas companies are now building new FPSO's as it costs just as much to convert as to build new, they don't really need propulsion as they are anchored in place to the sea bed so they save alit of cash and room. But they are only going to get bigger as the companies want to increase there production I have seen plans for a BP and Chevron vessels that will be coming.