The Sea Quest was a semi-submersible

Semi-submersible

A semi-submersible is a specialised marine vessel with good stability and seakeeping characteristics. The semi-submersible vessel design is commonly used in a number of specific offshore roles such as for offshore drilling rigs, safety vessels, oil production platforms and heavy lift cranes.The...

 drilling rig

Oil platform

An oil platform, also referred to as an offshore platform or, somewhat incorrectly, oil rig, is a lаrge structure with facilities to drill wells, to extract and process oil and natural gas, and to temporarily store product until it can be brought to shore for refining and marketing...

. She discovered the UK's first North Sea oil

North Sea oil

North Sea oil is a mixture of hydrocarbons, comprising liquid oil and natural gas, produced from oil reservoirs beneath the North Sea.In the oil industry, the term "North Sea" often includes areas such as the Norwegian Sea and the area known as "West of Shetland", "the Atlantic Frontier" or "the...

 on 14 September 1969 in the Arbroath Field. She also discovered the first giant oil field named Forties on 7 October 1970.

The Sea Quest was built by Belfast

Belfast

Belfast is the capital of and largest city in Northern Ireland. By population, it is the 14th biggest city in the United Kingdom and second biggest on the island of Ireland . It is the seat of the devolved government and legislative Northern Ireland Assembly...

 shipbuilders Harland and Wolff

Harland and Wolff

Harland and Wolff Heavy Industries is a Northern Irish heavy industrial company, specialising in shipbuilding and offshore construction, located in Belfast, Northern Ireland....

 for BP

BP

BP p.l.c. is a global oil and gas company headquartered in London, United Kingdom. It is the third-largest energy company and fourth-largest company in the world measured by revenues and one of the six oil and gas "supermajors"...

 at a cost of £3.5 million and launched on 8 January 1966.
The entire structure was 320 feet (97.5 m) high and weighed 150,000 tons, including three legs each 35 feet (10.7 m) in diameter and 160 feet (48.8 m) feet long that could be partially filled with water to control the height of the platform above the sea.

In 1977, Sea Quest was sold to Sedco (now part of Transocean

Transocean

Transocean Ltd. is one of the world's largest offshore drilling contractors. The company rents floating mobile drill rigs, along with the equipment and personnel for operations, to oil and gas companies at an average daily rate of US$282,700...

) and renamed Sedco 135C. She was towed to the west coast of Africa. In 17 January 1980, while drilling in the Warri area, Nigeria

Nigeria

Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

, a blowout

Blowout (well drilling)

A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....

occurred and the rig get extensive fire damage. The rig was then deliberately sunk in deep water.