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19 OFFSHORE PLATFORM TYPES

Larger lake and sea-based oil platforms and oil rigs are some
of the largest moveable man-made structures in the world. There
are several distinct types of platforms and rigs.
Fixed Platforms are built on
concrete and/or steel legs anchored
directly onto the seabed, supporting a
deck with space for drilling rigs,
production facilities and crew quarters.
Such platforms are, by virtue of their
immobility, designed for very long term
use (for instance the Hibernia platform).
Various types of structure are used, steel
jacket, concrete caisson, floating steel
and even floating concrete. Steel jackets
are vertical sections made of tubular
steel members, and are usually piled into the seabed. Concrete
caisson structures, pioneered by the Condeep concept, often have
in-built oil storage in tanks below the sea surface and these tanks
were often used as a flotation capability, allowing them to be built
close to shore (Norwegian fjords and Scottish firths are popular
because they are sheltered and deep enough) and then floated to
their final position where they are sunk to the seabed. Fixed
platforms are economically feasible for installation in water depths
up to about 1,700 feet.
Compliant Towers, consist of narrow, flexible towers and a
piled foundation supporting a conventional deck for drilling and
production operations. Compliant towers are designed to sustain
significant lateral deflections and forces, and are typically used in
water depths ranging from 1,500 and 3,000 feet.
Semi-submersible Platforms having legs of sufficient
buoyancy to cause the structure to float, but of weight sufficient to
keep the structure upright. Semi-submersible rigs can be moved

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