Geology And Geophysics

How Oil Rigs Work



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Once geologists discovered multiple large oil deposits around the world oil platforms began to be utilised to increase the production rate and profitability of oil for a growing global population. An oil rig is a platform positioned over deeply concealed fossil fuel reserves below the surface of the planet. Oil wells have been around for more than one hundred and twenty years. In fact around 1891 the world’s first sumerged oil wells were drilled from large platforms at the Mercer County Reservoir in Ohio United States. 

There are many different types of oil rig platforms. Some of these are:

a) Fixed Platforms

Are held in place by large steele and concrete support pillars, that are anchored into the sea/ocean floor. The platform itself supports drilling rigs, generators, oil pruction centres and large crew quarters. These strutures are designed to be self sustaining for long periods of time. Some concrete caisson structural versions even have inbuilt oil storage tanks.

b) Jacked Platforms

Are platforms mounted on legs that can be lowered or raised, depending on the depth of the water; and are usualy used in shallow waters where tides are pronounced. The existence of gears on the bottom of the legs allows for the platform to be mobile.

c) Tension Leg Platforms

Are large structures, usualy supported on four large legs that are inbedded into the seaflor in such a manner that minimises vertical and sideways movement, thus reducing stress on the rigging systems. In particular, such structures are designed for deep water oil extraction and can siphen oil from waters as deep as 2000 metres.

d) Spar Platforms

Are seabed platforms that are moored by lines, is made up of a cylindrical hull, truss elements that connects to the boyant hull and the bottom structure containing a ballast, allows the entire structure to minimise any unconventional, distruptive movement thanks to its large counterweight, which holds it upright. Movement is achieved via chains that wind up the tension lines, thus pulling itself towards an oil well.

e) Semi-Submersible Platforms

Are nothing more than buoyant hulls. They are ballasted by the altersation of the amount of liquid in the ballast tanks. This systems are usualy anchored by varius chains, ropes and cables and GPS.

f) Compliant Towers

Are designed to be deployed in medium depth water where a lot of lateral stress is experienced. The structures consist of a platform, that houses flexible towers as well as drilling and rigging systems.

g) Unmanned Platforms

Are much smaller autonomous oil producing platforms that are operate via onboard computers that require minimal human interaction.
Most of these oil rigs comprise of a power generators that delivers electricity to the machinery and the crew. Hoisting sysetms, drilling apparatus, drill bits, concrete casings that line the drilling hole, thus preventing the bore to collapse in on itself; and a circulation system that pumps out drill mud. Also the support structure, Derric, that holds together the drilling structure, thus allowing to add extensions as the drill deepens during the drilling process; and a blow out preventer, which controls pressure and doesn’t allow oil to blow out through the drill due to immense pressure which it is subjected to.

After oil is discovered and verified the crew pre drill a shallow hole on the ocean floor. Then the completed drill is lowered in the hole and the drilling commences. As drilling continues, drill mud and rock are pumped out to clear the hole. Eventualy new sections of the drill are added as the hole is deepened. Once the predetermined depth is reached, the drill is removed and the concrete casing with spacers is lowered into the bored hole to prevent collapse of the walls. Concrete is poured into the spacings and together with the pressure of the water and the drill mud it takes on the form of the spacers and hardens thus creating a secure concrete pipe through which oil or gas can be pumped. The casings continue to be put in place by sections.  Once this stage is complete oil is tested for and then extracted.

Electrical equipment and pressure guages are lowered into the hole to analyse the chemical make up of the environment and samples of rock and mud are brought up to the surface for testing. Once the nature of the hole has been finalised, a perforating gun is lowered to the bottom and holes are blown out in the concrete casings to allow oil to seep through and rush up a conduit pipe, to the surface under immense pressure. A multivalved device is attached to the top of the tubing and then cemented to the top part of the casings, thus allowing the control of oil flow. Sometimes acid is pumped down into the  well to dissolve the rock and other deformities to allow the oil to seep into the well from the surrounding environment. Once oil is flowing the rigging is removed and pumps are set up to extract the oil.

The extracted oil is then passed into storage tanks on the platform or pumped directly into waiting tankers that then deliver the oil to refineries on land. Some larger oil rigs have their own processing centres that refine the oil into different grades of petroliums and gases.



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