Sea Quest
A British-built semi-submersible drilling rig associated with two formative North Sea oil discoveries.
Last updated August 24, 2026
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Overview
Sea Quest was a semi-submersible offshore drilling rig built in Belfast by Harland & Wolff for BP. Launched on 8 January 1966, the rig represented the large, mobile offshore drilling units being deployed as exploration moved into the difficult waters of the North Sea. It was approximately 320 feet (98 metres) high and weighed about 150,000 tons. Its structure incorporated three large legs, each approximately 35 feet (11 metres) in diameter and 160 feet (49 metres) long. The legs could be partially flooded with water, allowing the rig to control its height above the sea and operate as a semi-submersible platform. Sea Quest is chiefly remembered for its exploration role in the early development of the United Kingdom's North Sea petroleum industry. On 14 September 1969, it discovered what is described as the United Kingdom's first North Sea oil in the Arbroath Field. On 7 October 1970, it made the discovery of the Forties field, the first giant oil field identified in the North Sea. The Forties discovery was particularly significant because it helped establish the scale and commercial importance of the United Kingdom's offshore petroleum province. The rig was built at a reported cost of £3.5 million. After its BP operating period, Sea Quest was sold in 1977 to Sedco, an offshore drilling contractor that later became part of Transocean, and was renamed Sedco 135C. The renamed rig was moved to the west coast of Africa and continued drilling in that region. Its later career ended in Nigeria. While drilling in the Warri area on 17 January 1980, Sedco 135C suffered a blowout that caused extensive fire damage. The damaged rig was subsequently deliberately sunk in deep water. As a result, Sea Quest's historical importance rests both on its role in major North Sea discoveries and on the hazards associated with early deepwater and offshore exploration. It was an industrial asset rather than a consumer-facing brand, and no continuing commercial identity, product portfolio, or public-facing headquarters is documented in the supplied reference material.
History
Sea Quest was constructed during the expansion of offshore petroleum exploration in the 1960s. Harland & Wolff, the Belfast shipbuilder, built the semi-submersible drilling rig for BP. It was launched on 8 January 1966 at a reported cost of £3.5 million. The vessel's design used three exceptionally large legs, each approximately 35 feet in diameter and 160 feet long. By partially filling the legs with water, operators could adjust the rig's buoyancy and working height, enabling a stable drilling position while retaining the mobility expected of a floating offshore unit. The rig became part of BP's exploration effort in the United Kingdom sector of the North Sea. Offshore drilling in the region involved difficult weather, significant water depths, and limited prior knowledge of the underlying geology. Sea Quest's drilling work contributed to two important discoveries. On 14 September 1969, it found oil in the Arbroath Field, an event described in the reference material as the discovery of the United Kingdom's first North Sea oil. On 7 October 1970, it discovered the Forties field. Forties was identified as the first giant oil field in the North Sea and became a defining discovery in the emergence of the United Kingdom as a substantial offshore oil producer. In 1977, Sea Quest changed ownership. Sedco acquired the rig and renamed it Sedco 135C. Sedco was an offshore drilling company that later became part of Transocean. The renamed rig was towed to the west coast of Africa, where it continued to be used for petroleum drilling. This transfer reflected the international movement of specialized drilling units between exploration regions as operators and contractors pursued new offshore prospects. The rig's operating life ended after a serious accident in Nigeria. On 17 January 1980, while drilling in the Warri area, a blowout occurred. The resulting fire caused extensive damage to the unit. Rather than being repaired and returned to service, the damaged rig was deliberately sunk in deep water. Sea Quest therefore passed from an influential exploration asset in the North Sea to a casualty of offshore drilling operations. Its documented legacy is concentrated in the technical and industrial history of offshore oil exploration, especially the Arbroath and Forties discoveries, rather than in an ongoing corporate or consumer brand.
- 1980
Blowout, fire, and sinking
[1]A blowout during drilling in the Warri area of Nigeria caused extensive fire damage. The rig was later deliberately sunk in deep water.
- 1977
Sale to Sedco and renaming
[1]Sea Quest was sold to Sedco and renamed Sedco 135C before being moved to the west coast of Africa.
- 1970
Forties field discovery
[1]On 7 October, the rig discovered the Forties field, described as the first giant oil field in the North Sea.
- 1969
Arbroath Field discovery
[1]On 14 September, Sea Quest discovered oil in the Arbroath Field, described as the United Kingdom's first North Sea oil discovery.
- 1966
Sea Quest is launched
[1]Harland & Wolff launched the semi-submersible drilling rig in Belfast on 8 January 1966 after building it for BP.
Products and positioning
A large mobile offshore exploration rig designed for petroleum drilling in demanding marine environments.
Semi-submersible drilling rig · 1966
Sea Quest
A large semi-submersible offshore drilling unit built by Harland & Wolff for BP. Its three substantial legs could be partially flooded to regulate buoyancy and the platform's elevation above the sea. The rig supported exploratory petroleum drilling in the United Kingdom North Sea and later in West Africa. It is associated with the Arbroath and Forties discoveries and was later renamed Sedco 135C after its sale to Sedco.
Brand decisions
1980
Decision to sink the damaged rigOtherBackground
A blowout while drilling in the Warri area caused extensive fire damage to Sedco 135C.
Action taken
The damaged rig was deliberately sunk in deep water.
Aftermath
The sinking ended the rig's documented operating life.
1977
Sale and transfer to SedcoM&ABackground
After its period of service for BP, the rig was sold as offshore drilling activity expanded across additional producing regions.
Action taken
Sedco acquired Sea Quest, renamed it Sedco 135C, and towed it to the west coast of Africa.
Aftermath
The rig continued drilling in West Africa and later suffered a fatal operational failure in Nigeria.
Recent events
- 1980
Sedco 135C suffers a blowout and fire in Nigeria[1]
Other
After being renamed Sedco 135C and moved to West Africa, the rig experienced a blowout while drilling near Warri, Nigeria, on 17 January 1980. The incident caused extensive fire damage and ultimately led to the rig being sunk.
- 1970
Sea Quest discovers the Forties field[1]
Other
On 7 October 1970, Sea Quest discovered the Forties field, described as the first giant oil field found in the North Sea.
- 1969
Sea Quest discovers oil in the Arbroath Field[1]
Other
The BP-operated semi-submersible drilling rig discovered what is described as the United Kingdom's first North Sea oil on 14 September 1969 in the Arbroath Field.
Sources1 sources
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