The Clair Ridge platform sited off the coast of Scotland in the North Sea.

BP has started extracting oil from one of the U.K.'s biggest offshore investments in decades.

The energy giant said in a press release Friday that oil is now being pumped from its Clair Ridge project, a section of a hydrocarbon field in the North Sea first discovered in 1977.

BP said its intention was to extract 640 million barrels of oil, with production peaking at 120,000 barrels a day. The project is designed to last for 40 years.

The second phase development of the oilfield, which lies around 50 miles west of the Shetland Islands, is a joint venture with Shell, Chevron and ConocoPhillips.

When discovered in 1977, the field was estimated to contain 7 billion barrels of hydrocarbons, but complex rock formations had prevented extraction.

BP bosses said the first oil from Clair Ridge was the culmination of "decades of persistence" that required capital investment exceeding £4.5 billion ($5.77 billion).

The company claimed the project is the largest commissioning in the North Sea in 20 years and has employed at least 6,000 people to date. BP said construction involved a "flotel," a floating accommodation vessel that housed 500 workers.

It is intended that BP will hold a 45.1 percent interest in the field, Shell will own 28 percent, Chevron North Sea is to take 19.4 percent and ConocoPhillips 7.5 percent.