Royal Dutch Shell PLC has concluded drilling for the season in the Arctic Ocean off Alaska's northwest and north coast.
Shell Alaska spokesman Curtis Smith says the company completed one top hole in both the Chukchi and Beaufort seas.
Top holes are the foundation of an exploratory well. They consist of 20-foot in diameter mud-line cellar dug 40 feet into the sea floor to provide protection from ice for a blowout preventer, plus a hole drilled to about 1,500 feet.
Federal regulators permitted Shell to drill top holes but banned drilling into petroleum reservoirs until the company's spill response barge is ready and on site.
Smith says the company considers the season a success and will incorporate lessons learned this year in the 2013 drilling season.