Page 312 - Hydrocarbon Exploration and Production Second Edition

Surface Facilities 299

steel
cables

concrete gravity
platform

tension leg steel jacket
platform platform

Figure 11.32 Fixed production platforms.

150 m and may support production facilities a further 50 m above mean sea level
(MSL). In deepwater, all the process and support facilities are normally supported
on a single jacket, but in shallow seas it may be cheaper and safer to support drilling,
production and accommodation modules on different jackets. In some areas, single
well jackets are common, connected by subsea pipelines to a central processing
platform (Figure 11.32).
Steel jackets are constructed from welded steel pipe. The jacket is fabricated
onshore and then floated out horizontally on a barge and set upright on
location. Once in position a jacket is pinned to the seafloor with steel piles.
Prefabricated units or modules containing processing equipment, drilling and
other equipment (see Figure 11.33) are installed by lift barges on to the top of the
jacket, and the whole assembly is connected and tested by commissioning teams.
Steel jackets can weigh 20,000 tons or more and support a similar weight of
equipment.
Concrete or steel gravity-based structures can be deployed in similar water depths to
steel jacket platforms. Gravity-based platforms rely on weight to secure them to the
seabed, which eliminates the need for piling in hard seabeds. Concrete gravity-
based structures (which are by far the most common) are built with huge ballast
tanks surrounding hollow concrete legs. They can be floated into position without
a barge and are sunk once on site by flooding the ballast tanks. For example,
the Mobil Hibernia Platform (offshore Canada) weighs around 450,000 tons and is
designed and constructed to resist iceberg impact!
The legs of the platform can be used as settling tanks or temporary storage
facilities for crude oil where oil is exported via tankers, or to allow production to
continue in the event of a pipeline shutdown. The Brent D platform in the North
Sea weighs more than 200,000 tons and can store over a million barrels of oil.