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*Production Platforms

F85: Some offshore hazards

Most oil and gas production platforms in offshore Britain rest on steel supports known as 'jackets', a term derived from the Gulf of Mexico. A small number of platforms are fabricated from concrete. The steel jacket, fabricated from welded pipe, is pinned to the sea floor with steel piles. Above it are prefabricated units or modules providing accommodation and housing various facilities including gas turbine generating sets. Towering above the modules are the drilling rig derrick (two on some platforms), the flare stack in some designs (also frequently cantilevered outwards) and service cranes. Horizontal surfaces are taken up by store areas, drilling pipe deck and the vital helicopter pad.

Concrete gravity platforms are so-called because their great weight holds them firmly on the seabed. They were first developed to provide storage capacity in oilfields where tankers were used to transport oil, and to eliminate the need for piling in hard seabeds. The Brent D platform (F87), which weighs more than 200 000 tonnes, was designed to store over a million barrels of oil. But steel platforms, in which there have been design advances, are now favoured over concrete ones.

F86: Production installations in the central and Northern North sea

Several platforms may have to be installed to exploit the larger fields, but where the capacity of an existing platform permits, subsea collecting systems linked to it by pipelines have been developed using the most modern technology. They will be increasingly used as smaller fields are developed. For very deep waters, one solution was the Hutton Tension Leg Platform: the buoyant platform, resembling a huge drilling rig, is tethered to the sea-bed by jointed legs kept in tension by computer-controlled ballast adjustments.

Alternatively, a subsea collection system may be linked via a production riser to a Floating, Production, Storage and Offloading (FSPO) vessel (F88); either a purpose built ship or a converted tanker or semi-submersible rig. The oil is offloaded by a shuttle tanker.

F87: The Brent D concrete gravity platform    F88: The 'Anasuria' FPSO vessal

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