Semi-submersible

For semi-submersible boats used for drug smuggling, see Narco submarine.

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Deepsea Delta semi-submersible drilling rig in the North Sea.

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Comparison of deepwater semi-submersible and drillship.

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.

Characteristics

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

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Blue Water Rig No. 1

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.

Classification

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

Applications

Mobile offshore drilling units (MODU)

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Semi-submersible drilling rig on MS3 Heavy-lift Ship

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Saipem Scarabeo 7 semi-submersible drilling rig docked in Cape Town

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.

The IMO MODU Code is an accredited design and operational guideline for Mobile Offshore Drilling Units of the semi-submersible type.

Semi-submersible crane vessels (SSCV)

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Thialf in Norwegian fjord with Fulmar SALM (Single Anchor Leg Mooring) buoy.

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Iolair on Elbe river, 1990

The Brazilian Petrobras P-51 semi-submersible oil platform

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.

A summary of offshore semi-submersible oil production platforms is given in the following table derived from industry data.

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
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	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

• 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.

External links

• 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

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