Transocean Ltd. is an American company. It is the world's largest offshore drilling contractor based on revenue and is based in Vernier, Switzerland. The company has offices in 20 countries, including Canada, the United States, Norway, United Kingdom, India, Brazil, Singapore, Indonesia, and Malaysia. In 2010, Transocean was found partially responsible (30% of total liability) for the Deepwater Horizon oil spill resulting from the explosion of one of its oil rigs in the Gulf of Mexico. In 2019, Royal Dutch Shell accounted for 26% of the company's revenues, while Equinor accounted for 21% of the company's revenues, and Chevron accounted for 17% of the company's revenues. History Transocean was formed as a result of the merger of Southern Natural Gas Company, later Sonat, with many smaller drilling companies. In 1953, the Birmingham, Alabama-based Southern Natural Gas Company created The Offshore Company after acquiring the joint drilling operation DeLong-McDermott from DeLong Engineering and J. Ray McDermott. In 1954, the company launched Rig 51, the first mobile jackup rig, in the Gulf of Mexico. In 1967, the Offshore Company went public. In 1978, SNG turned it into a wholly owned subsidiary. In 1982, it was changed to Sonat Offshore Drilling Inc., reflecting a change in its parent's name. William C. O'Malley, an executive at Sonat's headquarters in Birmingham, was named the company's first Chief Executive Officer in 1992. In 1993, Sonat spun off the majority of its ownership in the company. Sonat sold its remaining 40% stake in the company during a secondary public offering in late 1995. In 1996, the company acquired Norwegian group Transocean ASA for US$1.5 billion. Transocean started in the 1970s as a whaling company and expanded through a series of mergers. The new company was called Transocean Offshore. The new company began building massive drilling operations with drills capable of going to 10,000 feet (as opposed to 3,000 feet at the time) and operating two drill operations on the same ship. Its first ship, Discoverer Enterprise, cost nearly US$430 million and was . The Enterprise class drillship is the largest of the drilling ships. In 1999, Transocean merged with Sedco Forex, the offshore drilling subsidiary of Schlumberger in a $3.2 billion stock transaction in which Schlumberger shareholders received shares of Transocean. Sedco Forex had been formed from a merger of two drilling companies, the Southeastern Drilling Company (Sedco), founded in 1947 by Bill Clements and acquired by