|Port of registry:|
|Builder:||Mitsubishi Heavy Industries|
|Laid down:||13 August 1978|
|Acquired:||01 September 1979|
|Class and type:||American Bureau of Shipping: A1 column stabilized MODU; AMS; ACCU; DP S2|
|Tonnage:||20,461 GT; 6,138 NT; 10,100 DWT|
|Length:||91.44 m (300.0 ft)|
|Beam:||75.8952 m (249.000 ft)|
|Depth:||34.7472 m (114.000 ft)|
|Deck clearance:||12,352 mm (40.525 ft)|
|Propulsion:||6 x Electro-Motive Diesel MD-20 3,600hp|
Transocean Marianas is an Earl & Wright Sedco 700 design semi-submersible drilling unit capable of operating in harsh environments and water depths up to 7,000 feet (2,100 m) using an 18.75 in (47.6 cm), 15,000 psi blowout preventer (BOP), and a 21 in (53 cm) outside diameter (OD) marine riser.
Transocean Marianas currently operates offshore drilling activities in Namibia under contract from HRT Participações em Petróleo S.A. Throughout her career, she has operated under several names: Tharos from 1979 to 1994, Polyportia from 1994 to 1996, and P. Portia from 1996 to 1998.
On October 7, 2009, it began drilling on the Macondo well in the Gulf of Mexico. On November 9, 2009. it was damaged by Hurricane Ida. It was replaced by the Deepwater Horizon, which was responsible for the Deepwater Horizon oil spill when that rig exploded on April 20, 2010.
Piper Alpha incident
In 1988, as Tharos, the vessel operated as a large firefighting, construction, diving support and accommodation vessel equipped with a helicopter and a hospital ship with 22 beds, that rescued survivors from the Piper Alpha oil platform disaster. The vessel carried out firefighting on the night of the disaster and supported the fire fighting and well kill operations that led to the extinguishing of the blaze. However, flaws in the design of the vessel's extensible rescue arm prevented it from rescuing crew from the platform, while errors in the operation of the water cannon delayed their use. Divers from Tharos recovered many bodies from the seabed and from the galley area of Piper Alpha. Tharos's role in the early hours of the disaster was shown in the 1990 Scottish Television series Rescue.
2011 Ghana incident
On July 6, 2011, the Transocean Marianas was evacuated due to it taking on water off the coast of Ghana. 
- "ABS Record: Transocean Marianas". American Bureau of Shipping. 19 October 2009. Retrieved 4 June 2010.
- "Vessel Info: Transocean Marianas". DNV Exchange. Det Norske Veritas. 2010. Retrieved 4 June 2010.
- "Transocean Marianas". Transocean. 2010. Retrieved 7 June 2010.
- "Letter from Henry Waxman to Tony Hayward - June 14, 2010" (press release). nergycommerce.house.gov. 2010-06-14. Retrieved 2010-06-14.
- Lancaster, John Frederick (2000). Engineering Catastrophes: Causes and Effects of Major Accidents. Cambridge: Woodhead Publishing. pp. 121–122. ISBN 1-85573-505-9. Retrieved 11 June 2010.
- Dezember, Ryan. "Transocean Ghana rig stable after taking on water".
- Hamish Gray, Secretary of State for Energy (10 December 1981). "Emergency Support Vessels (North Sea)". Parliamentary Debates (Hansard). House of Commons. col. 468W–469W.
- Peter Morrison, Secretary of State for Energy (16 January 1990). "Oil and Gas (Safety)". Parliamentary Debates (Hansard). House of Commons. col. 188W.
- Alick Buchanan-Smith, Secretary of State for Energy (11 November 1985). "Semi-submersible Tharos (Incident)". Parliamentary Debates (Hansard). House of Commons. col. 53W–55W.