A ship built by the CIA for a secret Cold War mission in 1974 to raise a sunken Soviet sub is heading to the scrap yard, a victim of the slide in oil prices.

Christened the Hughes Glomar Explorer, after billionaire Howard Hughes was brought in on the CIA's deception, the 619-foot vessel eventually became part of the fleet of ships used by Swiss company Transocean to drill for oil.

But the oil price rout means the former spy ship now called GSF Explorer is just one of 40 such offshore drilling rigs that have been consigned to scrap since last year.