In the mid-1970s, Gulf Canada Resources began developing an Arctic drilling system consisting of two mobile drilling units: a Mobile Arctic Caisson (MAC) that could be submerged and filled with gravel to form an artificial drilling island in waters up to in depth and a floating Conical Drilling Unit (CDU) designed for drilling in water depths between while afloat. The intention of this development was to overcome the relatively short operating window of drillships during the ice-free season (100 to 110 days a year) and the water depth limitations of artificial dredged islands in the Canadian part of the Beaufort Sea. The drilling units, each capable of completing one exploration well per year, would be supported by four Arctic Class 4 vessels: two large icebreakers providing 24-hour ice management and standby services on the drilling site and two smaller icebreaking vessels responsible for anchor handling and supply runs between the drilling rigs and coastal bases. By 1982, both drilling units and all four icebreaking vessels were under construction in Canada and Japan for BeauDril, Gulf Canada's drilling subsidiary, and the company had committed itself to a billion-dollar exploration program between 1983 and 1988.