Most production platforms are placed directly above the wells from which they are extracting oil or gas. The visible topsides are supported by steel structures called jackets which are secured into the seabed. Many platforms also have satellite wells which are a greater distance from the parent structure and linked back by a network of pipelines. The safety zone around a surface object is there to protect all of the surrounding subsea infrastucture.
Floating Production, Storage and Offloading Facility (FPSO)
An FPSO is an offshore production facility that is typically ship-shaped and stores crude oil in tanks located in the hull of the vessel. The oil is periodically offloaded to shuttle tankers for transport to shore. FPSO's are used as production facilities to develop marginal oil fields or fields in deepwater areas such as the Foinavon and Schiehallion fields on the Atlantic Margin. A typical FPSO collects oil from clusters of subsea wells connected to manifolds and rigid risers which in turn are connected to flexible risers from the FPSO. Mooring chains extend outward from these vessels to the anchors which can be a considerable distance from the FPSO and extend well beyond the 500 metre safety zone around the vessel. Surface buoys indicating the anchor positions may also be present making fishing within these areas extremely dangerous.
Drilling Rigs - Jack-Up Rigs, Semi-Submersibles
Drilling rigs are mobile structures which can either be towed into position with tugs or have their own power to manouevre and are used to drill exploration wells. Jack-Up rigs are used in relatively shallow water and extend legs down onto the seabed to fix position to begin drilling. When the work is completed the legs are rewound back up and the rig becomes a floating vessel which can be towed to a new location. Semi submersible drilling rigs are self propelled structures used for drilling in deeper water.
Where offshore oil or gas fields are being constructed the charts for the area concerned often shows an area marked out as Development Area. Surface and subsea working vessels may be engaged in ongoing construction and development of further installations within these areas and other vessels are strongly advised to remain outside the charted limits of these areas.