View source for Semi-submersible platform

{{Short description|Marine vessel used in offshore roles with good stability and seakeeping}}
{{Use dmy dates|date=August 2021}}
[[File:Oil platform in the North Sea.jpg|thumb|300px|''Deepsea Delta'' semi-submersible drilling rig in the [[North Sea]]]]
[[File:Deepwater drilling systems 2.png|thumb|300px|Comparison of deepwater semi-submersible (left) and [[drillship]] (right)]]

A '''semi-submersible platform''' is a specialised [[marine vessel]] used in offshore roles including as [[offshore drilling]] rigs, safety vessels, oil production platforms, and heavy lift cranes. They have good [[ship stability]] and [[seakeeping]], better than [[drillship]]s.<ref>{{Cite web | url=https://www.rigzone.com/training/insight.asp?insight_id=338&c_id=24 |title=How Do Semisubmersibles Work?|work=RigZone}}</ref>

==Characteristics==
Offshore drilling in water depth greater than around {{convert|520|m}} requires that operations be carried out from a floating vessel, since fixed structures are not practical. Initially in the early 1950s monohull ships such as [[CUSS I]] were used, 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 most of its buoyancy from ballasted, watertight [[Floats (nautical)|pontoons]] located below the ocean surface and wave action. Structural columns connect the pontoons and operating deck.<ref name="oceanstaroec2000">{{cite web | url=http://www.oceanstaroec.com/fame/2000/semisubmersible.htm | title=2000 Technology Pioneers | work=Oceanstaroec.com}}</ref> The operating deck can be located high above the sea level owing to the good [[ship stability]] of the design, and therefore is kept well away from the waves.

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 have their onboard mass trimmed to maintain stability. Unlike a [[Submersible drilling rig|submersible]], a semi-submersible vessel is not supported by resting on the [[seabed]].

Semi-submersible vessels are able to transform from a deep to a shallow [[draft (hull)|draft]] by deballasting (removing ballast water from the hull), thereby becoming surface vessels. Usually they are moved from location to location in this configuration. The [[Heavy-lift ship|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==
[[File:Blue Water Rig No. 1.JPG|thumb|300px|Blue Water Rig No. 1]]
The semi-submersible design was first developed for [[oil platform]] activities in the early 1960s. Bruce Collipp of [[Royal Dutch Shell|Shell]] is regarded as the inventor.<ref>{{cite book | url=https://books.google.com/books?id=Jr8VfSONB-IC&pg=PA16 | title=Deepwater Petroleum Exploration & Production: A Nontechnical Guide | first1=William L. | last1=Leffler | first2=Richard | last2=Pattarozzi | first3=Gordon | last3=Sterling | publisher=[[PennWell]] | year=2011| isbn=9781593702533 }}</ref>

However, [[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.<ref>{{cite web | url=https://airandspace.si.edu/collection-objects/seadrome-ocean-airways-proposal-1930 | title=Seadrome Ocean Airways Proposal, 1930 | publisher=[[National Air and Space Museum]]}}</ref>

The first [[jackup rig]]s, for shallow waters, was built in 1954.<ref>{{cite web | url=https://www.offshore-mag.com/home/article/16756956/special-anniversary-the-history-of-offshore-developing-the-ep-infrastructure | title=Special Anniversary – The history of offshore: developing the E&P infrastructure | work=Offshore Magazine | date=1 January 2004}}</ref>

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 [[Royal Dutch Shell|Shell Oil Company]]. As the [[Pontoon (boat)|pontoons]] were not sufficiently [[buoyant]] to support the weight of the rig and its consumables, it was towed between locations at a draught midway 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.<ref name="oceanstaroec2000"/>

The first purpose built drilling semi-submersible ''Ocean Driller'' was launched in 1963.<ref>{{cite web | url=https://www.offshore-mag.com/drilling-completion/article/16760605/how-offshore-drilling-units-evolved | title=How offshore drilling units evolved | work=Offshore Magazine | date=1 May 1997}}</ref>

By 1972, there were 30 semi-submersible units.{{citation needed|date=November 2018}}

==Classification==
Drilling rig construction has historically occurred in boom periods and therefore "batches" of drilling rigs have been built. Offshore drilling rigs have been loosely classified in nominal "generations" depending upon the year built and water depth capability as follows:{{citation needed|date=July 2014}}{{original research inline|date=July 2014}}

{| class="wikitable"
|-
!Generation !! colspan="2"| Water depth !! Dates
|-
||First||about 600&nbsp;ft||200&nbsp;m ||Early 1960s
|-
||Second||about 1000&nbsp;ft||300&nbsp;m ||1969–1974
|-
||Third||about 1500&nbsp;ft||500&nbsp;m ||Early 1980s
|-
||Fourth||about 3000&nbsp;ft||1000&nbsp;m ||1990s
|-
||Fifth||about 7500&nbsp;ft||2500&nbsp;m ||1998–2004
|-
||Sixth||about 10000&nbsp;ft||3000&nbsp;m ||2005–2010
|}

==Applications==

==={{vanchor|Mobile offshore drilling unit}}s ({{vanchor|MODU}})===
[[File:20061204 MightyServant3 JUSTbefore.JPG|thumb|300px|Semi-submersible drilling rig on [[Mighty Servant 3|MS3 semi-submersible heavy-lift ship]]]]
[[File:Eni saipem scarabeo 7.jpg|thumb|220px|left|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 thruster]]s with [[dynamic positioning]].

The [[International Maritime Organization]] MODU Code is an accredited design and operational guideline for mobile offshore drilling units of the semi-submersible type.<ref>{{cite web | url=https://www.eagle.org/eagleExternalPortalWEB/ShowProperty/BEA%20Repository/References/Capability%20Brochures/MODUCapabilities | title=Mobile Offshore Drilling Units Classification, Certification & Related Services}}</ref>

==={{vanchor|Semi-submersible crane vessel}}s ({{vanchor|SSCV}})===<!-- This section is linked from [[Ship prefix]] -->
[[File:SSCVThialf.jpg|thumb|300px|Thialf in Norwegian fjord with [[Fulmar Oil Field|Fulmar]] single anchor leg mooring (SALM) buoy.]]
[[File:Iolair.jpg|thumb|300px|Iolair on [[Elbe river]], 1990]]
[[File:Oil platform P-51 (Brazil).jpg|thumb|300px|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 vessel]]s called [[DCV Balder|''Balder'']] and [[Hermod (ship)|''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 SSCVs, resulting in two new enormous vessels ''DB-102'' (now [[SSCV Thialf|''Thialf'']]) and ''[[Saipem 7000]]'', capable of lifting respectively 14,200 and 14,000 tons.

During transit an SSCV is de-ballasted to a draught where only part of the lower hull is submerged. During lifting operations, the vessel is ballasted down. This way, the lower hull is well submerged. This reduces the effect of waves and swell. High [[Instantaneous stability|stability]] is obtained by placing the columns far apart. The high stability allows them to lift extremely high loads safely.

==={{vanchor|Offshore support vessel}}s ({{vanchor|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).

Notable vessels are as follows:
<!--- Must have a wikipedia article--->
*''[[Transocean Marianas]]'' – Offshore safety support vessel, built in 1979 and since converted into a drilling vessel, and rechristened ''[[Transocean Marianas]]''
*''[[Iolair]]'' – Offshore safety support vessel, built for [[BP]] in 1982, Sedco/Phillips SS was the first built to Red Adair's recommendations. ''Iolair'' followed.
*''[[Q4000 (vessel)|Q4000]]'' – Offshore multiservice vessel, built for Caldive in 2002

==={{vanchor|Offshore production platform}}s===
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 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 oil field|Balmoral field]] in the UK [[North Sea]] in 1986.<ref>{{cite news | url=https://www.ogj.com/general-interest/companies/article/17224010/schematics-illustrate-complex-north-sea-production-facilities | title=SCHEMATICS ILLUSTRATE COMPLEX NORTH SEA PRODUCTION FACILITIES | work=[[Oil & Gas Journal]] | date=4 October 1993 | url-access=subscription}}</ref>

Notable platforms are as follows:
<!--- Must have a wikipedia article--->
* [[Petrobras 36]] – Converted from drilling rig Spirit of Columbus and sank in 2001
* [[Atlantis PQ]] –  BP and BHP joint venture
* [[Thunder Horse PDQ]] – GVA40000 design, largest semi-submersible platform

=== Offshore rocket launch and landing platform ===
''[[Ocean Odyssey]]'' was converted from a mobile drilling rig to a launch platform in 1997. In total, 36 rockets were launched from the platform. The last launch happened in 2014, shortly before its owner [[Sea Launch]] significantly reduced all operations.

The potential application of converted semi-submersible oil drilling rigs is being explored by Space Exploration Technologies ([[SpaceX]]) for launching and landing of [[SpaceX Starship|Starship]]. SpaceX has acquired two former offshore oil drilling rigs in 2021, similar to the ENSCO/Valaris 8506 offshore model,<ref>{{Cite web|last=Burghardt|first=Thomas|date=2021-01-19|title=SpaceX acquires former oil rigs to serve as floating Starship spaceports|url=https://www.nasaspaceflight.com/2021/01/spacex-rigs-starship-spaceports/|access-date=2021-08-04|website=NASASpaceFlight.com}}</ref> but later sold them in 2023 after they were considered unsuitable.<ref>{{Cite tweet |first=Gav |last=Cornwell |user=SpaceOffshore |number=1625290945523511296 |title=The SpaceX Starship rigs Phobos and Deimos are due to depart | date=2023-02-14 |language=en |retweet=jeff_foust}}</ref>

==See also==
*[[Project 1231]] 
*[[SeaOrbiter]]
*[[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.
*''[[Ocean Ranger]]'' – semi-submersible drill rig that collapsed in a storm off the east coast of Canada in 1982 with the loss of all 84 crew members. 
*{{ship|RP|FLIP||6}} (FLoating Instrument Platform) – a tilting craft designed to form a stable platform for scientific data collection.
*[[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|1864|6}} – another U.S. Civil War semi-submerged ship, capable of flooding down to a lower profile.
*[[Semi-submersible naval vessel]]

==References==
{{Reflist}}

==External links==
{{Commons category|Semi-submersibles}}
*[https://web.archive.org/web/20130813094418/http://www.gvac.se/floating-production/ GVAC.se] – Consultants designers
*[https://www.kbr.com/en KBR] – Consultants designers
*[https://boskalis.com/ Boskalis website] – Info on Boskalis transport ships
*[http://www.navybook.com/no-higher-honor/timeline/aboard-mighty-servant-2/ Naybook website] – Photos of Mighty Servant 2 hauling USS Samuel B. Roberts (FFG 58)
*[http://www.imo.org/en/Pages/Default.aspx IMO Website] – International Maritime Organisation

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{{DEFAULTSORT:Semi-Submersible Platform}}
[[Category:Oil platforms]]
[[Category:Ship types]]
[[Category:Petroleum production]]
[[Category:Semi-submersibles|*]]
[[Category:Drilling technology]]