23-Jun-2002 -- Shortly before arrival at Pascagoula MS we had the chance to visit a confluence, of which we first thought it is not worth to visit, but then it turned out to offer quite something.
is in a oil-exploration area.
At the sight of this oil equipment Captain Peter remembered the time when he was still young and powerful, and when he worked for a certain period on a supply ship for drilling rigs and platforms. Now he is just good enough to carry some chicken around, but he tells us from his reminscences:
Oil and gasfields are now exploited in many parts of the oceans between the shores and the edges of the continental shelf. The first stage of exploration in areas likely to contain hydrocarbon deposits is usually a magnetic, gravimetric and seismic survey, and bottom cores are obtained.
Mobile rigs are then used for drilling wells to explore and develop a field.
There are three principal types of drilling rigs:
1) Jack-up rigs
which are towed into the drilling position where their steel legs are lowered to the seabed and the drilling platform is then jacked-up clear of the water. They are used in depths down to about 120 metres (400ft).
2) Semi-submergible rigs consist of a platform on columns which rise from a caisson submerged deep enough to avoid much of the effects of sea and swell. Some large semi-submergible rigs are self-propelled. They may have displacements of up to 25,000 tons, and are used for drilling in depths to bout 1,700 m (5,600 ft) in the anchored mode, or in the case of dynamically positioned, even in excess of it.
3) Drillships - a typical drillship has a displacement of 14,000 tons. a length
of 135 metres (440 ft). They carry a tall drilling rig amidships, and usually have a helicopter deck on aft. For drilling in depths of less than about 200m (600 ft), the ship is held by an 8-point anchor system, in greater depths a dynamic positioning system is required. They can then drill in depths to about 2,000 metres (6,600 ft) and to a depth of 6,000 metres (20,000 ft)!!! below the seabed.
On a typical field, oil and gas is obtained from wells drilled from fixed platforms, fitted out like a drilling rig, and usually standing on the seabed. From each wellhead, the oil or gas is carried in pipes, known as flowlines, to a production platform where primary processing, compression and pumpingis crried out. The oil or gas is then transported through pipelines to a nearby storage tank, tanker loading buoy or floating terminal, or direct to a tank farm ashore. One production platform may collect the oil or gas from several drilling platforms and may supply a number of tanker loading
buoys or storage units. Such production platforms are called "field terminal platforms".
The development of an offshore field involves the frequent moving of large structures and buoys and the laying of many miles of pipeline. In the course of developing numerous wells are drilled. Those which will not be required again are sealed with cement below the seabed and abandoned. These are known as plugged and abandoned wells (P & A wells). Other wells which may be required at a later date are known as Suspended Wells. They have their wellheads capped and left with a pipe and other equipment. Wells which are in use for producing oil or gas are termed Production Wells. Their wellheads are surmounted by a complex of valves and pipes.
Several different kinds of platform
are used for development, but they are normally piped steel or concrete structures. They serve a number of purposes, they carry drilling equipment, oil and gas separation and treatment plants, pumpline stations and electricity generators. They are fitted with cranes and a helicoter landing deck and accomodation for the necessary complement.
A number of wells may be drilled from one drilling rig, by using a structure termed a "template", placed on the seabed below the rig to guide the drill. A template may stand as much as 15 m (50 ft) above the seabed.
Platforms may stand single or in groups, connected with pipelines to each other.
On some fields, Sub-sea production systems are used. They consist of one and more wells, which have as much of the production equipment as possible on the seabed instead of on a drilling platform. The output from a number of these wells may be collected in an underwater manifold centre, a large steel structure up to 20 m (60 ft) in height on the seabed, for delivery to a production platform.
Submarine pipelines are layed on the seabed for the conveyance of water, oil or gas and may extend many miles into the open sea.
The pipelines have spiral lines on their inner side, similar to the barrel of a gun. These provide the oil a spin. Further water is added to the oil. Due to the spin and the centrifugal force the water then tends towards the pipe walls and acts as a lubrificant between the oil and the pipe. This reduces friction and pumping labor considerably. The oil becomes more easily transportable.
Pipelines contain flammable oil and gas under high pressure. A ship dropping erroneously its anchor on such a pipeline could face an immediate hazard by loss of buoyancy by gas aereated water or fire/explosion, and result in an environmental catastrophe. Every care must therefore be taken to avoid anchoring close to submarine pipelines.
Explanation of the drawing
Left column - mobile rigs
top: Jack-up rig
centre: semi-submersible rig
Right column - platforms
top: Steel production platform
centre: Concrete production platform
bottom: Tension leg platform
Some people may think that this offshore oil extraction business is a dirty and muddy one and a hazard to the environment. If it's well and professionally done - and one can be sure it is in the United States - it is not so dangerous and absolutely clean. That proves the fact that fish heavens are maintained in the closest vicinity and shrimpboats
are fishing their wonderful seafood there, - and everybody loves and praises the famous Gulf-Coast seafood.
Land could be seen as well, - it's Dauphin Island
Drawings of mobile rigs and platforms obtained from "The Mariner's Handbook", NP100, 7th Edition 1999, British Admiralty, Ministry of Defense, Taunton, England