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 terms semisubmersible, semi-sub or just semi are also generally used for this vessel design.
Offshore drilling in water depth greater than around 120 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 concept, 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.
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.
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.
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;
Generation Water Depth Dates 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 1990's Fifth about 7500 ft 2500 m 1998–2004 Sixth about 10000 ft 3000 m 2005–2010
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 Enchanced 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 P-26 Marlim Brazil 515 27,656 Petrobras 2000 Astilleros Astilleros, Spain Conversion 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 SS-11 Coral Brazil 145 0,000 Petrobras 2003 Breit Bethlehem Steel Conversion Nakika Kepler, Ariel, Fourier, Herschell & E. Anstey Gulf of Mexico 936 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.
- 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.
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- 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
Modern merchant ships Dry cargo Tankers Passenger Support Other
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