Offshore concrete structure
Concrete structures used in offshore marine environments / From Wikipedia, the free encyclopedia
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Offshore concrete structures, or concrete offshore structures, are structures built from reinforced concrete for use in the offshore marine environment. They serve the same purpose as their steel counterparts in oil and gas production and storage. The first concrete oil platform was installed in the North Sea in the Ekofisk oil field in 1973 by Phillips Petroleum, and they have become a significant part of the marine construction industry. Since then at least 47 major concrete offshore structures have been built.
Concrete offshore structures are mostly used in the petroleum industry as drilling, extraction or storage units for crude oil or natural gas. These large structures house machinery and equipment used to drill for, or extract, oil and gas.[1] Concrete offshore structures are not limited to applications within the oil and gas industry, several conceptual studies have shown that concrete support structures for offshore wind turbines can be competitive compared to the more common steel structures, especially for greater water depths.
Depending on the circumstances, platforms may be attached to the ocean floor, consist of an artificial island, or be floating. Generally, offshore concrete structures are classified into fixed and floating structures. Fixed structures are mostly built as concrete gravity based structures (CGS, also termed as caisson type), where the loads bear down directly on the uppermost layers as soil pressure. The caisson provides buoyancy during construction and towing and acts also as a foundation structure in the operation phase. Furthermore, the caisson could be used as storage volume for oil or other liquids.[1] Floating units may be held in position by anchored wires or chains in a spread mooring pattern. Because of the low stiffness in those systems, the natural frequency is low and the structure can move with all six degrees of freedom. Floating units serve as production units, storage and offloading units (FSO) or for crude oil or as terminals for liquefied natural gas (LNG). A more recent development is concrete sub-sea structures.[1]
Concrete offshore structures are highly durable, constructed of low-maintenance material, suitable for harsh and/or arctic environment (like ice and seismic regions),[1] can carry heavy topsides, may be designed to provide storage capacity, can be suitable for soft ground and are economical for water depths larger than 150 m. Most gravity-type platforms need no additional fixing because of their large foundation dimensions and extremely high weight.[1]