|BRITAIN'S OFFSHORE OIL & GAS
Getting Oil and Gas Ashore
Most offshore oil and all offshore gas are brought to shore by pipelines which operate in all weathers. Pipeline routes are planned to be as short as possible. Slopes that could put stress on unsupported pipe are avoided and seabed sediments are mapped to identify unstable areas and to see if it will be possible to bury the pipe. Pipeline construction begins onshore, as lengths of pipe are waterproofed with bitumen and coated with steel-reinforced concrete. This coating weighs down the submarine pipeline even when it is filled with gas. The prepared pipe-lengths are welded together offshore on a laybarge (F101). As the barge winches forward on its anchor lines, the pipeline drops gently to the seabed, guided by a 'stinger'. The inside of pipelines need to be cleaned regularly to remove wax deposits and water: to do this a collecting device known as a pig is forced through the pipe.
Where tankers transport oil from small or isolated fields, various oil storage systems may be used. These may range from cylindrical cells contained in some of the massive concrete structures, to seabed storage units such as that employed at the Kittiwake field, or integral storage such as that contained in the various Floating, Production, Storage and Offloading vessels. In essence these FPSOs are floating storage tankers, as well as production and processing installations. FPSOs provide an important option for developing fields which may be remote from existing infrastructure or where the field recoverable reserves are uncertain, for example because of difficult geological conditions. Examples of FPSOs are Foinaven and Schiehallion in the Atlantic Margin, and the Anasuria (F88, page 49) in the Central North Sea.
In onshore terminals, carefully landscaped to minimise their environmental impact, crude oil and gas undergo further processing. Any remaining water and gas are removed from oil which is stored at the terminal before transport to refineries. Gas is dried and then given its characteristic smell before entering the national grid. During transportation, great care is taken to avoid or deal effectively with spillage.