A semi-submersible is a specialised marine vessel used in a number of specific offshore roles such as offshore drilling rigs, safety vessels, oil production platforms, and heavy lift cranes. They are designed with good stability and seakeeping characteristics. Other terms include semisubmersible, semi-sub, or simply semi.
Offshore drilling in water depth greater than around 520 meters requires that operations be carried out from a floating vessel, as fixed structures are not practical. Initially in the early 1950s monohull ships were used like CUSS I, but these were found to have significant heave, pitch and yaw motions in large waves, and the industry needed more stable drilling platforms.
A semi-submersible obtains its buoyancy from ballasted, watertight pontoons located below the ocean surface and wave action. The operating deck can be located high above the sea level due to the good stability of the design, and therefore the operating deck is kept well away from the waves. Structural columns connect the pontoons and operating deck.
With its hull structure submerged at a deep draft, the semi-submersible is less affected by wave loadings than a normal ship. With a small water-plane area, however, the semi-submersible is sensitive to load changes, and therefore must be carefully trimmed to maintain stability. Unlike a submarine or submersible, during normal operations, a semi-submersible vessel is never entirely underwater.
A semi-submersible vessel is able to transform from a deep to a shallow draft by deballasting (removing ballast water from the hull), and thereby become a surface vessel. The heavy lift vessels use this capability to submerge the majority of their structure, locate beneath another floating vessel, and then deballast to pick up the other vessel as a cargo.
Early history 
The semi-submersible design was first developed for offshore drilling activities. Bruce Collipp of Shell is regarded as the inventor. But Edward Robert Armstrong may have paved the way with his idea of "seadrome" landing strips for airplanes in the late 1920s, since his idea involved the same use of columns on ballast tanks below the surface and anchored to the ocean floor by steel cables.
When oil drilling moved into offshore waters, fixed platform rigs and submersible rigs were built, but were limited to shallow waters. When demands for drilling equipment was needed in water depths greater than 100 feet (30 m) in the Gulf of Mexico, the first jackup rigs were built.
The first semisubmersible arrived by accident in 1961. Blue Water Drilling Company owned and operated the four column submersible drilling rig Blue Water Rig No.1 in the Gulf of Mexico for Shell Oil Company. As the pontoons were not sufficiently buoyant to support the weight of the rig and its consumables, it was towed between locations at a draught mid way between the top of the pontoons and the underside of the deck. It was observed that the motions at this draught were very small, and Blue Water Drilling and Shell jointly decided that the rig could be operated in the floating mode.
The first purpose built drilling semi-submersible Ocean Driller was launched in 1963. Since then, many semi-submersibles have been purpose-designed for the drilling industry mobile offshore fleet.
The industry quickly accepted the semi-submersible concept and the fleet increased rapidly to 30 units by 1972.
Drilling rig construction has historically occurred in boom periods and therefore "batches" of drilling rigs have been built. Offshore drilling rigs have been classified in nominal "generations" depending upon the year built and water depth capability as follows:
|First||about 600 ft||200 m||Early 1960s|
|Second||about 1000 ft||300 m||1969–1974|
|Third||about 1500 ft||500 m||Early 1980s|
|Fourth||about 3000 ft||1000 m||1990s|
|Fifth||about 7500 ft||2500 m||1998–2004|
|Sixth||about 10000 ft||3000 m||2005–2010|
Mobile offshore drilling units (MODU) 
Semi-submersible rigs make stable platforms for drilling for offshore oil and gas. They can be towed into position by a tugboat and anchored, or moved by and kept in position by their own azimuth thrusters with dynamic positioning.
Semi-submersible crane vessels (SSCV) 
The advantages of the semi-submersible vessel stability were soon recognized for offshore construction when in 1978 Heerema Marine Contractors constructed the two sister crane vessels called Balder and Hermod. These semi-submersible crane vessels (SSCV) consist of two lower hulls (pontoons), three columns on each pontoon and an upper hull. Shortly after J. Ray McDermott and Saipem also introduced SSCV's, resulting in two new enormous vessels DB-102 (now Thialf) and Saipem 7000, capable of lifting respectively 14,200 and 14,000 tons.
During transit an SSCV will be de-ballasted to a draught where only part of the lower hull is submerged. During lifting operations, the vessel will be ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swell. High stability is obtained by placing the columns far apart. The high stability allows them to lift extreme high loads.
Offshore support vessels (OSV) 
Semi-submersibles are particularly suited to a number of offshore support vessel roles because of their good stability, large deck areas, and variable deck load (VDL). Some of the most prominent vessels are;
- Uncle John – Diving / Construction support vessel, built for Houlder Offshore in 1977
- Seaway Swan – Diving Support Vessel built in 1977
- Tharos – Offshore Safety support vessel, built in 1979 and since converted into a drilling vessel, and rechristened Transocean Marianas.
- Stadive – Diving Support Vessel (DSV) built for Shell in 1982
- Iolair – Offshore safety support vessel, built for BP in 1982
- Safe Karinia – Offshore operations vessel, built in 1982
- Polyconfidence – Offshore accommodation platform, built in 1988.
- Q4000 – Offshore Multiservice Vessel, built for Caldive in 2002
- Ocean Odyssey – Converted semi-submersible drilling rig used as a rocket launch pad.
Offshore production platforms 
When oil fields were first developed in offshore locations, drilling semi-submersibles were converted for use as combined drilling and production platforms. These vessels offered very stable and cost effective platforms. The first semi-submersible floating production platform was the Argyll FPF converted from the Transworld 58 drilling semi-submersible in 1975 for the Hamilton Brothers North Sea Argyll oil field.
As the oil industry has progressed into deeper water and harsh environments, purpose-built production semi-submersible platforms were designed. The first purpose-built semi-submersible production platform was for the Balmoral field, UK North Sea in 1986.
|Vessel||Field||Region||Depth (m)||Displacement (Te)||Operator||Startup||Hull designer||Hull builder||Notes|
|Argyll FPU||Argyll Oil Field||UK North Sea||150||34,000||Agip||1975||Converted from Transocean 58 MODU. First semi-submersible production platform.|
|Buchan A||Buchan oil field||UK North Sea||160||18,995||Talisman||1981||CFEM||CFEM Le Havre - converted Aker Offshore Stornoway||Converted from Drillmaster MODU for BP|
|P-09||Corvina Oil Field||Brazil||230||22,896||Petrobras||1983||Aker||Mitsui||Conversion Aker H-3e design drilling rig|
|P-15||Pirauna||Brazil||243||21,616||Petrobras||1983||Mitsubishi Heavy Industries||Mitsubishi Heavy Industries||Conversion Mitsubishi MD-503 design|
|P-12||Linguado / Badejo Oil Field||Brazil||100||22,896||Petrobras||1984||Aker||Mitsui, Japan||Aker H-3e design|
|P-21||Badejo / Salema Oil Fields||Brazil||112||10,765||Petrobras||1984||Earl & Wright||Montreal Engineering, Rio de Janeiro||Conversion of Sedco Staflo drilling rig|
|Deepsea Pioneer FPU||Argyll & Duncan Oil Fields||UK North Sea||150||34,000||Agip||1984||Converted from Deepsea Saga drilling rig|
|P-22||Morela||Brazil||114||17,440||Petrobras||1986||Frede & Goldman||Montreal Engineering||Conversion of drilling rig Sedco 135F|
|Balmoral FPV||Balmoral (+4 satellites) Oil Field||UK North Sea||150||30,983||Premier Oil||1986||GVA||Götaverken, Sweden||First purpose built production semi-submersible.|
|P-07||Bicudo Oil Field||Brazil||207||20,493||Petrobras||1988||Aker||Raumer-Repola or Ishibras Shipyard, Rio ?||Converted from Aker H-3 drilling rig Bendoran|
|Veslefrikk B||Veslefrikk Oil Field||Norwegian Sea||175||43,305||Statoil||1989||Aker||Daewoo, Korea||Converted from West Vision drilling semi. First floating production facility in Norway.|
|AH001||Ivan Hoe Rob Roy Oil Field||UK North Sea||140||26,639||Amerada Hess||1989||Brown & Root||Highland Fabricators, Nigg||Converted from Philips SS Sedco700 drilling vessel|
|P-20||Marlim||Brazil||625||25,983||Petrobras||1992||GVA||Astileros, Spain||GVA4000 design. Converted from Russian built Illiad drilling semi.|
|P-08||Marimba Oil Field||Brazil||423||20,990||Petrobras||1993||Petrobras||Tangenge, Niteroi, Brazil||Converted drilling rig Songa Star / Belford Dolphin|
|P-13||Bijupira / Salema Oil Field||Brazil||625||22,243||Queriz Galvao Perfuracoes||1993||CFEM||UIE, Clydesbank ?||Conversion|
|P-14||Coral / Esrela / Caravela Oil Fields||Brazil||195||22,243||Petrobras||1993||CFEM||CFEM, Brazil ?||Conversion|
|P-18||Marlim||Brazil||910||36,100||Petrobras||1994||GVA||Tenege / FELS||GVA4500 design|
|Troll B FPU||Troll gas field||Norwegian Sea||339||188,968||Statoil||1995||Kvaerner/Doris||Kvaerner Rosenberg||Concrete construction.|
|Nan Hai Tiao Zhan||Luihua||South China Sea||300||0,000||CNOOC||1995||Reading & Bates||Keppel FELS||Converted Sedco700 drilling rig West Stadrill for Amoco|
|P-25||Albacora II Oil Field||Brazil||252||25,983||Petrobras||1996||CENPES||Ultratec||Conversion of Zapat-4000 drilling rig|
|P-27||Voador||Brazil||533||41,659||Petrobras||1996||FELS / Obdebrecht||Levingston||Conversion of Penrod drilling rig Pardill 71|
|Innovator||Marlim||Gulf of Mexico||914||0,000||ATP||1996||GVA|
|Tahara||PY-3||Indian Ocean||339||55,000||Hardy oil and gas||1997||Earl & Wright||Hup Seng Engineering||Conversion Sedco-135 design|
|Njord A||Njord Oil Field||Norwegian Sea||330||45,077||Statoil||1997||Aker||Aker Verdal||Originally built for Norsk Hydro Design based on P-45 ?|
|P-19||Marlim||Brazil||770||33,400||Petrobras||1997||IVI/Sadevegesa consortium||Hitachi Zosen||Converted from Enhanced pacesetter drilling rig.|
|Janice A||Janice Oil Field||UK North Sea||80||0,000||Anadarko||1999||Aker||McNulty||Converted from Aker H3.2 drilling vessel|
|Visund||Visund||Norwegian Sea||335||52,600||Statoil||1999||GVA||Umoe Mandal||GVA8000 design|
|Troll C FPU||Troll gas field||Norwegian Sea||339||54,377||Statoil||1999||GVA||HHI, S.Korea||GVA 8000 design|
|Åsgard B||Åsgard||Norwegian Sea||320||84,848||Statoil||2000||GVA||Daewoo, S. Korea|
|P-36||Roncador||Brazil Campos Basin||1,360||0,000||Petrobras||2000||SBM Atlantia||Davie Shipbuilding, Canada||Converted from drilling rig Spirit of Columbus and sank in 2001|
|Snorre B FDPU||Snorre Oil Field||Norwegian Sea||350||56,600||Statoil||2001||Aker||Dragados||Originally built for Saga|
|P-51||Marlim Sul Oil Field||Brazil Campos Basin||1,255||80,114||Petrobras||2001||Aker||Keppel FELS||Aker DDS design|
|Nakika||Kepler, Ariel, Fourier, Herschell & E. Anstey||Gulf of Mexico||1936||64,000||BP||2003||ABB Lumus||HHI, S.Korea||Constructed for Shell, but operated by BP|
|P-40||Marlim Sul||Brazil||1,080||0,000||Petrobras||2004||PROJEMAR||Jurong||Conversion of DB-100 design drilling rig|
|Kristin FPU||Kristin||Norwegian Sea||320||56,600||Statoil||2005||GVA||Samsung, S. Korea|
|Atlantis PQ||Atlantis Oil Field||Gulf of Mexico||2,156||89,000||BP||2006||GVA||DSME, S.Korea|
|ATP Innovator||Gomez Oil Field||Gulf of Mexico||914||46,160||ATP||2006||Levingston||Levingston||Converted from Rowan Midland semi-sub|
|Independence Hub||10 fields||Gulf of Mexico||2,015||46,160||Anadarko||2007||SBM Atlantia||Jurong Shipyard|
|P-52||Roncador||Brazil||1,795||80,201||Petrobras||2007||Aker||Keppel FELS||Aker DDS design|
|Thunder Horse PDQ||Thunder Horse Oil Field||Gulf of Mexico||1,849||130,000||BP||2008||GVA||DSME, S.Korea||GVA40000 design, largest semi-submersible platform|
|Blind Faith||Blind Faith||Gulf of Mexico||1,980||40,000||ChevronTexaco||2008||Aker||Aker Verdal||Aker DDS design|
|Northern Producer FPF||was at Galley Oil Field now at Don Oil Field||UK North Sea||350||0,000||Petrofac||2009||Granhearne||McNulty, Newcastle||Re-use of Emerald Producer FPU, originally converted from Aker H-3 vessel Alibaba in 1989.|
|Thunder Hawk||Thunder Hawk||Gulf of Mexico||1740||42,000||Murphy||2009||SBM Atlantia||Dyna-Mac Engineering Services Pte Ltd|
|Gjøa||Gjøa Oil Field||Norwegian Sea||360||58,400||Statoil||2010||Aker||Samsung, S. Korea||To be operated by Gas de France|
|P-56||Marlim Sul||Brazil||1,700||50,000||Petrobras||2010||Aker||Keppel FELS||Copy of P-51 platform. Aker DDS design built in Brazil.|
|Gumusut-Kakap||Pisigan, Malilai, Ubah||Malaysia||1,220||40,000||Sabah Shell Petroleum Company||2012||MMHE, Malaysia||MMHE, Malaysia and MMHE-ATB JointVenture||First production semi-submersible in Malaysia|
|P-55||Roncador||Brazil||1,707||105,000||Petrobras||2012||Petrobras||Atlantico Consortium||Built in Brazil.|
See also 
|Wikimedia Commons has media related to: Semi-submersibles|
- Drydock – facility used to repair and maintain ships; some are floating and can be submerged to load vessels.
- Heavy lift ship – semi-submerging vessel for transportation of very large loads.
- Narco submarine – low profile vessels used to smuggle cocaine from South America to the United States.
- RP FLIP (FLoating Instrument Platform) – a tilting craft designed to form a stable platform for scientific data collection.
- Sea Shadow (IX-529) – a craft designed and built by Lockheed Corporation with the superstructure joined to the hulls with continuous members.
- Small waterplane area twin hull – a "SWATH" vessel consists of two submarine-like structures supported on streamlined pylons.
- Submersible or Midget submarine – for small submarine craft.
- Submersible drilling rig – for submersible drilling vessels.
- USS Monitor – the U.S. Civil War warship considered to be the earliest semi-submerged ship.
- USS Spuyten Duyvil – another U.S. Civil War semi-submerged ship, capable of flooding down to a lower profile.
- Welfreighter – a submersible designed by Britain during the second World War to carry combat swimmers.
- 2000Technology Pioneers. Oceanstaroec.com. Retrieved on 2010-12-30.
- 1998 Industry Pioneers. Oceanstaroec.com. Retrieved on 2010-12-30.
- America-Europe via North Atlantic airways over the Armstrong seadrome system of commercial ocean transit by airplane, Edward Robert Armstrong (1927)
- 60 Years in the Gulf of Mexico, E&P Magazine, 2007
- [dead link]
- Summary report on an investigation into the correlation between full-scale measured and predicted motions of the SSSV 'Uncle John'
- [dead link]
- [dead link]
- *GVAC – GVA 5000. Gvac.se. Retrieved on 2010-12-30.
- 2008 Worldwide Survey of Semi-FPSs and FPUs, September 2008, Offshore Magazine / Mustang Engineering
- Industry Projects. Offshore Technology. Retrieved on 2010-12-30.
- *GVAC – Floating production. Gvac.se. Retrieved on 2010-12-30.
- [dead link]
- A Petrobras – RH. Petrobras. Retrieved on 2010-12-30.
- Title Page for UK Petroleum Data CD. Databydesign.co.uk. Retrieved on 2010-12-30.
- The dominance of the FPSO. Offshore Technology (2008-08-29). Retrieved on 2010-12-30.
- Conversion Of The "Deepsea Pioneer", R.A. Bryans, Hamilton Brothers Oil & Gas Ltd., Offshore Technology Conference, 2–5 May 1988, Houston, Texas (5692-MS)
- GVAC.se – Consultants designers
- Dockwise website – Info on Dockwise transport ships
- Naybook website – Photos of Mighty Servant 2 hauling USS Samuel B. Roberts (FFG 58)
- IMO Website – International Maritime Organisation