Concrete offshore structures are mostly used in the petroleum industry as drilling, extraction or storage units for crude oil or natural gas.
[1] Floating units may be held in position by anchored wires or chains in a spread mooring pattern.
[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.
Most of the designs have in common a base caisson (normally for storage of oil) and shafts penetrating the water surface to carry the topside.
The most common concrete designs are:[citation needed] Condeep refers to a make of gravity base structure for oil platforms invented by Olav Mo[2] and fabricated by Norwegian Contractors in Norway.
Concrete Gravity Base Structures (CGBS) is a further development of the first-generation Condeep drilling/production platforms installed in the North Sea between the late 1970s and mid '90s.
The CGBS have no oil storage facilities and the topside installations will be carried out in the field by a float-over mating method.
The structure has a shape not unlike a marine sea island and is surrounded by a perforated breakwater wall (Jarlan patent).
The original proposal of the French group C G DORIS (Compagnie General pour les Developments Operationelles des Richesses Sous-Marines) for a prestressed post-tensioned concrete "island" structure was adopted on cost and operational grounds.
The single main leg is surrounded by an outer breaker wall perforated with so called Jarlan holes.
[citation needed] To achieve its goal and extract oil within five years after discovering the Brent reservoir Shell divided up the construction of four offshore platforms.
In addition to FPSO’s, there have been a number of ship-shaped Floating Storage and Offloading (FSO) systems (vessels with no production processing equipment) used in these same areas to support oil and gas developments.
Semi-submersible platforms have the principal characteristic of remaining in a substantially stable position, presenting small movements when they experience environmental forces such as the wind, waves and currents.
FPSO or FSO systems are typically barge/ship-shaped and store crude oil in tanks located in the hull of the vessel.
Their turret structures are designed to anchor the vessel, allow “weathervaning” of the units to accommodate environmental conditions, permit the constant flow of oil and production fluids from vessel to undersea field, all while being a structure capable of quick disconnect in the event of emergency.
This barge is built of reinforced and prestressed concrete containing cylindrical tanks each having a cross-section perpendicular to its longitudinal axes that comprises a preferably circular curved portion corresponding to the bottom.