Ensco plc is an offshore drilling contractor headquartered in London, United Kingdom.

Ensco is the world's second-largest offshore drilling and well drilling company, and owns 41 offshore jack-ups, 8 drillships, and 12 semi-submersible drilling rigs.

In 2016, 13% of its revenues came from BP and 12% of its revenues came from Total S.A..

The company provides quarterly updates on the status of each rig in its fleet on its website.

ENSCO's predecessor company, Blocker Energy Corporation, was incorporated in 1975 by longtime oilman John R. Blocker.

After graduating from Texas A&M in 1948, Blocker worked on a Gulf of Mexico oil rig for several years. In 1954, he established a South Texas drilling company with his father. When an oversupply of oil on the market crippled the contract drilling business, the company was dissolved. In 1958 Blocker went to work for Dresser Industries as operations manager for the oil equipment division in Argentina and Venezuela, a natural fit because he had grown up in South America, learning Spanish before English. In 1965, he moved to Dresser's Houston office and ultimately rose to the level of a senior vice-president.

In 1975, after the 1973 oil crisis, John Blocker bought Choya Energy, a six-rig contract drilling company based in Alice, Texas, and renamed the company Blocker Energy.

Blocker took advantage of his South American experience to position the company in the international market, believing it was less risky than the United States market, which had 800 to 900 competitors. Blocker Energy took on $44 million in debt to expand rapidly. To fund expansion and pay down debt, the company became a public company via an initial public offering.

By 1980, the company had 1,700 employees worldwide, including 211 at its Texas headquarters. In the early 1980s, Blocker, along with many other oil-related companies, thought the price of oil could only go higher. In 1981 and early 1982, the company borrowed heavily to expand its rig fleet to 54 rigs. However, in late 1982, the price of oil plunged and, to avoid bankruptcy, Blocker Energy restructured, giving 64% of the company to its banks in exchange for $240 million in debt forgiveness. By 1983, the company was only operating 6 rigs, although that number increased to 24 in 1984. By 1985, the worldwide employee count of the company was down to 500.

Blocker Energy lost nearly $3 million in 1985 and needed capital. Richard Rainwater's BEC Ventures made an investment in the company in 1986 and chose Carl F. Thorne to run the company, which he did until his retirement 20 years later.

In 1988, the company acquired Golden Gulf Offshore Inc. and its ten boats that supplied offshore oil rigs and another four vessels that moved the rigs' massive anchors for $64 million in cash and stock.

In 1990, the company bought Penrod Holding Corporation out of bankruptcy and adding 19 rigs to its fleet. Penrod was owned by the Hunt family. The deal was orchestrated by Richard Rainwater who owned 21% of Ensco at that time.

In 2011, Ensco acquired Pride International for $7.3 billion. The acquisition provided Ensco access to the Brazil and West African markets, and diversified its asset base from being largely jack-up rigs to include drillship and semi-submersible rigs. As a result of the acquisition, Ensco closed its Dallas office and consolidated into its Houston office.

In 2010, Ensco moved its headquarters to London and become a UK-registered company.

In 2013, the company was added to the S&P 500 index but it was removed from the index in 2016.

In 2015, Petrobras alleged bribery in the negotiations of the DS-5 rig contract with Pride International in 2008 before its acquisition by Ensco. In 2016, Petrobras terminated the contract for the rig.

In 2017, Ensco acquired Atwood Oceanics.

In 2009, a Spanish government authority initiated proceedings seeking $3 million in damages for an alleged environmental spill originating from the ENSCO 5006 rig. Ensco expects to be indemnified from any damages by the customer of the rig.