A blowout preventer (BOP) is a large valve that can seal off at the surface wellhead a well being drilled or worked over. During drilling or well interventions, the valve may be closed if pressure from an underground formation causes fluids such as oil or natural gas to enter the wellbore and threaten the rig. By closing this valve (usually operated remotely via hydraulic actuators), the drilling crew can prevent undesired fluid flow, thus helping to regain control of the wellbore. Once this is accomplished, often the drilling mud density within the hole can be increased until adequate fluid pressure is placed on the influx zone, and the BOP can be opened for operations to resume. The invention and use of BOPs were instrumental in the ending of oil gushers, which were dangerous and costly.
Use
BOPs come in a variety of styles, sizes and pressure ratings, and usually several individual units comprise a BOP stack. Blind rams are designed to close an open wellbore. Pipe rams seal around tubular components in the well (drill pipe, casing, tubing, or coiled tubing). Shear-seal BOPs are fitted with hardened steel shearing surfaces that can actually cut through drill pipe and tool strings, if all other barriers fail. Since BOPs are important for the safety of the crew, as well as the drilling rig and the wellbore itself, BOPs are regularly inspected, tested and refurbished. Tests vary from daily test of functions of critical wells to monthly or less frequent testing of wells with low likelihood of control problems.[1] Any of these BOPs may be installed underwater, normally with two hydraulic actuators.
Deepwater Horizon blowout
After the Deepwater Horizon drilling rig explosion on April 20, 2010, the blowout preventer should have activated itself automatically to avoid an oil spill in the Gulf of Mexico. Underwater robots were sent to manually activate the mechanism's switch, to no avail. As of May 2010[update] it is unknown why it failed.[2] BP representatives suggested that the preventer could have suffered an hydraulic leak.[3] Gamma-ray imaging of the preventer conducted on May 12 and May 13, 2010 showed that the preventer's internal valves were partially closed and were restricting the flow of oil. Whether the valves closed automatically during the explosion or were shut manually by ROV work is unknown.[3]
Types
BOPs come in two types, ram and annular. Both types are usually used together in drilling rig BOP stacks.
Ram BOPs utilize several sets of horizontally opposed hydraulic rams that can be fitted out to 1) close around the drill string, 2) shear through the drill string and then seal, or 3) close off a wellbore when no drill pipe or tubing is in it. The ram BOP was invented by James Smither Abercrombie and Harry S. Cameron in 1922, and was brought to market in 1924.[4]
An annular BOP, also known as a spherical BOP, utilizes a hemispherical piece of rubber reinforced with steel. Unlike a ram BOP, which closes with a sharp horizontal motion, an annular BOP closes around the drill string in a smooth simultaneous upward and inward motion. The geometry of this movement reduces internal stresses and friction between the BOP body and the sealing element, resulting in a longer field life with less maintenance. The annular design also allows a much lower operating pressure, reducing the number of hydraulic accumulators necessary and thereby reducing cost and complexity of the operation. It was invented by Granville Sloan Knox in 1946; a U.S. patent for it was awarded in 1952.[5]
Actuation methods
When rigs are drilling on land or in very shallow water where the wellhead is above the water line, BOPs are activated by hydraulic pressure from a remote accumulator. Several control stations will be mounted around the rig.
In deeper offshore operations with the wellhead on the sea floor, there are five primary means that a BOP may be activated. The means are
- automatic, based on excessive pressure;
- automatic, based on excessive flow;
- remotely by an electrical command from the surface through a cable;
- manually, based on external movement of a valve lever done underwater, or
- acoustically, based on a modulated/encoded pulse of sound transmitted by an underwater transducer.
Of these methods, general requirements in the United States were drawn to require only the first four. Specific requirements for permitting individual wells along the U.S. coastline may also call for acoustic actuation. General requirements of other nations, including Brazil, were drawn to require this method. BOPs featuring this method may cost as much as US$500,000 more than those that omit the feature.
The 2009 Minerals Management Service permitting of Deepwater Horizon in the Macondo Prospect did not require acoustic actuation.
Source
Wikipedia
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