As I recall, Shell Tankers used to tank clean with an "Over-Rich" atmosphere in the cargo tanks; petroleum vapour will explode with a concentration of between 5% and 15% in air, so if you could keep the atmosphere in the tank either below (lean) or above (rich) that band you are safe; but it is almost impossible to keep it like that, given you have to access the tank to put the tank cleaning machine in. Am I also correct that the jet of water emerging from the nozzles of the machine created static electricity, and it was that combination that caused the explosion. There was a third VLCC, the Kong Haakon VII, which also suffered an explosion at around the same time, and it was these three disasters that set off the research that discovered about the static electricity.
As Steve W has pointed out, inert gas systems had been around for years, since before WW2, if I recall. BP was the pioneer in installing it in the late 60s, partly as a result of the British Crown explosion mentioned earlier. Could Bon Accord confirm that the trials ship was ss British Skill, her sister ship and that the first ships fitted from new were the "Titty" class of product carriers from 1968, and the the first generation VLCCs from 1970 on?
Another advantage of I.G. is supposedly it reduces corrosion, due to the oxygen-poor atmosphere in tanks, I think
Lastly, I remember there was a serious explosion on a VLCC fitted with I.G. in the mid-1970s; the seal water pump was shut down, which allowed vapour to leak back into the funnel uptakes, where it went bang.