Geology And Geophysics

Blow out Preventer

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The awful offshore oil spill on records to date is related to a blowout preventer that even though was operational, fails to totally shut off the well.

The term ‘blowout preventer’ which was up till now known to few or connoisseur only, is now in the limelight through the media since the explosion that took the lives of eleven workers and left a massive span of petroleum which is spreading across the Gulf of Mexico.

So, eventually by now everyone has an elementary idea of the blowout preventer being cut off valves that have failed to stop the oil disaster due to their ill functioning.

In fact, from the oil and gas glossary definition, a blowout preventer (BOP) is a large valve placed at the top of the well that can be closed, whenever the drilling operators lose control of any formation fluids like oil during their drilling operations. So by closing the BOP, the operators take back control of the well. Once the pressure is in control and maintained by increasing the mud density, the BOP is again opened.

Hence, for the safety of the staff, the platform and the well, blowout preventers are vital. For this purpose, they are regularly tested, inspected and fixed up. Inspection and tests are performed according to law requirements, taking into account the region, the installation of the platform and the type of well. For instance, in some dangerous zones, BOP tests are carried out daily while in less risky area, the tests on BOP are done fortnightly or monthly.

There are two types of BOPs, ram and annular. In 1922, James Smither Abercrombie and Harry S Cameron invented the ram BOP which uses two opposed hydraulic rams that will either close around or shear through the drill string, resulting in sealing or closing off the wellbore in a quick horizontal action.  On the other hand, invented by Granville Sloan Knox in 1946, the annular BOP, also known as a spherical BOP will use a hemispherical piece of rubber and steel that will close around the drill string in a soft and instant upward and inward motion.

BOPs normally vary in sizes, styles and pressure ratings. For instance, blind rams are intended to close an open well bore, whereas, pipe rams seal around tubular parts like tubing, casing or drill pipe. On the other hand, when all barriers fail, the shear-seal BOPs, fitted with steel shearing planes can cut through drill pipe. Both annular and ram types of BOPs are normally used together in oil well to form a BOP stack.

The BOP can normally be activated automatically, manually, acoustically or by remote control. Hence, the blowout preventer is a supposed fail-safe device that is mainly intended to squash off a well automatically even if everything else on the platform goes wrong.

Unfortunately in the case of the Deepwater Horizon, all went wrong including the BOP where investigators found that the BOP had a dead battery in the control pod, leakage of hydraulic fluid and a weak cutting tool that was unable to cut through the joints in the pipe. So, the faulty BOP was incapable of assuring safety in this state and therefore the blowout was unavoidable.

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