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Offshore drilling typically refers to the discovery and development of oil and gas resources which lie underwater. Most commonly, the term is used to describe oil extraction off the coasts of continents, though the term can also apply to drilling in lakes and inland seas.
There are many different types of platforms for offshore drilling activities, from shallow-water steel jackets and jackup barges, to floating Semi-submersibles and drillships able to operate in very deep waters.
Around 1891, the first submerged oil wells were drilled from platforms built on piles in the fresh waters of the Grand Lake St. Marys (a.k.a. Mercer County Reservoir) in Ohio. The wells were developed by small local companies such as Bryson, Riley Oil, German-American, and Banker's Oil.
Around 1896, the first submerged oil wells in salt water were drilled in the portion of the Summerland field extending under the Santa Barbara Channel in California. The wells were drilled from piers extending from land out into the channel.
Other notable early submerged drilling activities occurred on the Canadian side of Lake Erie in the 1900s and Caddo Lake in Louisiana in the 1910s. Shortly thereafter wells were drilled in tidal zones along the Texas and Louisiana gulf coast. The Goose Creek Oil Field near Baytown, Texas is one such example. In the 1920’s drilling activities occurred from concrete platforms in Venezuela’s Lake Maracaibo.
One of the oldest subsea wells is the Bibi Eibat well, which came on stream in 1923 in Azerbaijan.[dubious ] The well was located on an artificial island in a shallow portion of the Caspian Sea. In the early 1930s, the Texas Co., later Texaco (now Chevron) developed the first mobile steel barges for drilling in the brackish coastal areas of the Gulf of Mexico.
In early 1947, Superior Oil erected a drilling and production platform in 20 feet of water some 18 miles off Vermilion Parish, La. But it was Kerr-McGee Oil Industries (now Anadarko Petroleum), as operator for partners Phillips Petroleum (ConocoPhillips) and Stanolind Oil & Gas (BP) that completed its historic Ship Shoal Block 32 well in October 1947, months before Superior actually drilled a discovery from their Vermilion platform farther offshore. In any case, that made Kerr-McGee’s well the first oil discovery drilled out of sight of land.
When offshore drilling moved into deeper waters of up to 100 feet, fixed platform rigs were built, until demands for drilling equipment was needed in the 100- to 400-foot depth of the Gulf of Mexico, the first jack-up rigs began appearing from specialized offshore drilling contractors such as ENSCO International.
The first semi-submersible appeared due to an accident in 1961. Blue Water Drilling Company owned and operated the four column submersible 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.
As of June, 2010, there were over 620 mobile offshore drilling rigs (Jackups, semisubs, drillships, barges) available for service in the competitive rig fleet. 
Main offshore fields
Notable offshore fields today are found in the North Sea, the Gulf of Mexico, the Campos and Santos Basins off the coasts of Brazil, Newfoundland and Nova Scotia, several fields off West Africa most notably west of Nigeria and Angola, as well as offshore fields in South East Asia and Sakhalin, Russia.
Offshore oil and gas production is more challenging than land-based installations due to the remote and harsher environment. Much of the innovation in the offshore petroleum sector concerns overcoming these challenges, including the need to provide very large production facilities. Production and drilling facilities may be very large and a large investment, such as the Troll A platform standing on a depth of 300 meters.
Another type of offshore platform may float with a mooring system to maintain it on location. While a floating system may be lower cost in deeper waters than a fixed platform, the dynamic nature of the platforms introduces many challenges for the drilling and production facilities.
In both cases, the ocean adds several hundred meters to the fluid column in the drill string. The addition increases bottom hole pressure as well as the energy needed to lift sand and cuttings for oil-sand separation on the platform.
The trend today is to conduct more of the production subsea, by separating sand from oil and re-injecting sand before it is pumped up to the platform, or even pumping it onshore, with no installations visible above the sea. Subsea installations help to exploit resources at progressively deeper waters, locations which have been inaccessible, and overcome challenges posed by sea ice, such as in the Barents Sea.
Offshore manned facilities also present logistics and human resources challenges. An offshore oil platform is a small community in itself with cafeteria, sleeping quarters, management, and other support functions. In the North Sea, staff members are transported by helicopter for a two-week shift. They usually receive higher salary than other industry workers do. Supplies and waste are transported by ship, and the supply needs to be well planned because floor area on the platform is limited. Today, much effort goes into moving as much of the personnel as possible onshore, where management and technical experts are in touch with the platform by video conferencing. An onshore job is also more attractive for the aging workforce in the petroleum industry, at least in the western world. These efforts among others are contained in the established term integrated operations. The increased use of subsea facilities helps achieve the objective of moving more workers onshore. Subsea facilities are also easier to expand, with new separators or separate modules for different oil types, and are not limited by the fixed floor space of an offshore rig.
Effects on the environment
Offshore oil production involves environmental risks, most notably oil spills from oil tankers or pipelines transporting oil from the platform to onshore facilities, and from leaks and accidents on the platform. Produced water is also generated, which is excess water from well drilling or production and includes varying amounts of oil, drilling fluid or other chemicals used in, or resulting from, oil production. According to the organization Culture Change, a Gulf of Mexico rig dumps about 90,000 tons of drilling fluid and metal cuttings over its lifetime, with its wells also contributing with heavy metals. See ecological effects of oil platforms.
- Deep sea mining
- Jackup barge
- Offshore oil and gas in the United States
- Oil drilling
- Oil platform
- History of the Offshore Oil and Gas Development in Louisiana at the Mineral Management Services, Dept of the Interior
- National Ocean Industries Association
- AzerMAR History Page 
- History of Offshore Oil Wells (1840-2010)
- Worldwide Offshore Rig Utilization
- Shell starts production at Perdido
- Debate Over Offshore Drilling. [internet video]. CBS News. 2008. http://www.webcastr.com/videos/news/debate-over-offshore-drilling.html. Retrieved 2008-09-27.
- "Committee Against Oil Exploration". www.culturechange.org. 2007. http://www.culturechange.org/caoe.html. Retrieved 2007-03-10.
- Center for Biological Diversity v Dept of the Interior 17Apr2009 DC Appellate Decision stopping offshore Alaska Oil Leases.
- Shell starts production at Perdido at Shell.com
- IODP-USIO: Publications: Proceedings of the Integrated Ocean Drilling Program