Offshore Oil Field

a complex of structures for the economical extraction and transportation of petroleum and gas from deposits located beneath the floor of bodies of water. The development of petroleum and gas deposits in offshore areas involves geological prospecting and hydraulic-engineering work, topographical surveying and geological engineering study of the floor, geophysical exploration of the territory, deep prospecting and exploratory drilling to discover and evaluate petroleum and gas deposits, drilling of operational wells, and construction of underwater and surface structures for extraction, collection, and transportation of the petroleum and gas to consumers.

One of the first offshore oil fields was established on the American coast (state of Virginia) in the late 19th century. In the USSR, offshore oil extraction began in the Caspian Sea (Il’ich Bay) during the first years of Soviet power. Wells were drilled on a filled-in area, and production organized. In the 1930’s, petroleum deposits in the Caspian Sea (Artem Island) were developed by directional wells drilled from shore and by the construction of offshore islands. The first pile-supported metal base was built in 1935 according to a plan drawn up by N. S. Timofeev and B. A. Raginskii. In 1949, L. A. Mezhlumov, S. A. Orudzhev, and lu. A. Safarov proposed the MOS large-module platform, which accelerated oil field work and made possible the introduction of industrial construction methods. The building of individual offshore bases made possible drilling in water up to 40 m deep. An important stage in the development of offshore oil fields was the construction of offshore trellises (oil-field complexes with interconnected drilling platforms). Shallow areas (5–8 m) are filled and framed with dikes, and the water is pumped out of the enclosed area. Directional slanted wells are drilled in the exploitation of water areas adjacent to land. The well face may be up to 3 km from the vertical.

For water up to 30 m deep, reinforced-concrete and metal platforms are built to accommodate drilling, operating, and other equipment and ensure comprehensive development of the deposit. In the Caspian Sea, 880 steel platforms and more than 300 km of trellises have been erected (as of 1972). They make possible all-weather servicing of the sites. If the field is close to land, the platform is connected to on-shore bases. Large-module bases of the MOS and GIPROMOS types are used for depths down to 40 m; floating drilling platforms with supports on the sea floor, from which individual wells or clusters of directional wells are drilled, are used for depths to 60–80 m. In this case the wellhead operating equipment is placed on the operating base, a conductor module, or the bottom of the sea.

In the development of marine petroleum and gas deposits at depths down to 120 m, separate stationary islands of various designs are built, and directional wells are drilled from them. At depths of more than 150 m, drilling ships or semisubmerged platforms with moored or dynamic stabilization are used. Exploratory and operational wells, and sometimes a cluster of directional wells, are drilled from the individual offshore bases.

The total area of offshore regions in the USSR that are promising for petroleum and gas and are not deeper than 200 m is 2.5 million sq km; more than 75 percent of this area is above the 100-m isobathic line.

The largest offshore oil fields in other countries are in the Atlantic Ocean (the Gulf of Mexico and the Caribbean Sea basin), the eastern part of the Pacific Ocean (Cook Inlet and the California coast), along the Australian coast in the Bass Strait, in the Indian Ocean (the Persian Gulf and the Gulf of Suez), and in the North Sea. The exploitation of these petroleum deposits is based primarily on the island method, in which man-made earth islands are built with rock framing and gravel and stone fill. Metal islands are built from large modules. In addition to the stationary bases, more than 250 mobile drilling rigs are being used in the seas and oceans (1972).

Bases for drilling wells in ice-covered waters are unique in design. For example, in Alaska the heads of dozens of wells are set in massive shells 10 m in diameter to protect them against the ice. When mobile self-hoisting drilling platforms and semisub-merged platforms are used, the wellhead equipment can be placed on the floor of the sea after drilling. Numerous companies in the USA, Great Britain, and France have developed sets of remote-control equipment for such cases. Such units are being successfully operated on Lake Erie and the Gulf of Mexico. Deep-water diving techniques have been developed in the United States and France for periodic inspection and servicing of underwater equipment (wellheads, separator units, pipelines, gates, and so on). Twenty-four countries are extracting petroleum and gas from underwater basins, 31 countries are conducting exploratory drilling, and 82 countries are doing geological and geophysical work (1971). Total world extraction of petroleum from offshore oil fields is about 300 million tons (1971).