Surprise inspections of deepwater drilling rigs in the Gulf of Mexico dwindled to about three a year over the past decade, even as exploratory drilling far from shore increased, according to federal data analyzed by The Wall Street Journal.

And since 2004 federal authorities haven't made a single surprise inspection on any of the 50 or so deepwater natural gas and oil production platforms in the Gulf, despite a law requiring periodic unannounced inspections. Like rigs, these semi-permanent structures also handle enormous amounts...