Blowout and Blowout Preventers
When the offshore oil rig called the “Deepwater Horizon” began leaking oil into the Gulf of Mexico, blowout preventers and their function became hot topics. Blowout is not just a fear under water where pollution is the major concern. It is also important when drilling on land, where the potential for fire and explosion is quite high, especially in wells with large amounts of natural gas.
Blowout is nothing more than the uncontrolled expulsion of petroleum from a well. In the early stages of oil extraction, it is quite common for petroleum to be under pressure. This results from the fact that a fraction of nearly all petroleum reserves is gas. Because gases expand, a great deal of pressure can be created in petroleum reserves because they exist in confined and inflexible spaces. It is no somewhat like the pressure created by blowing air into a balloon. The more gas there is (air in the case of a balloon), the greater the pressure.
When a reserve is finally breached during drilling and there is an escape route for the pressure, hydrocarbon is carried up the bore hole and ejected onto the surface. Because the holes that are drilled to access a petroleum reserves are relatively narrow and because there can be a great deal of pressure in a well, hydrocarbon can be ejected up to 60 meters into the air. The more flammable hydrocarbons, like methane and propane, can easily be ignited as a result of friction during blowout, leading to fire and explosion. Underwater, the pressure can be enough to force oil out of a well even at depths of over a kilometer. The Deepwater Horizon blowout, which occurred in the Macondo Prospect oil field in the Gulf of Mexico, was the largest underwater blowout in history. Eleven crew were killed in the explosion. The well spilled an estimated 35,000 to 60,000 barrels of oil into the Gulf each day and was not repaired for over 3 months. Since that incident, blowout preventers have been under intense scrutiny. However, no better device has been proposed and standard blowout preventers are still being used. Many underwater drilling operations use more than one blowout preventer after the Deepwater Horizon accident.
While not full proof, blowout preventers can help to stop blowout from occurring and have made drilling for petroleum safer and more environmentally friendly. Blowout preventers are often abbreviated as BOPs (pronounced B-O-P and not as 'bops' in the industry).
BOPs are mechanical devices and come in two basic types: ram and annular. Modern BOPs often use both mechanisms in tandem to help ensure the reliability of the system. In fact, most BOPs are constructed of at least one annular blowout mechanism atop several ram style mechanisms.
Ram Style Blowout Preventers
Ram blowout preventers were the first to be invented in 1922. Original ram BOPs were not intended to completely seal a well, but rather were intended to reduce the flow of oil to a manageable rate and allow time for capping. The ram BOP is based on the principle of a gate valve and uses two pairs of opposing steal plungers called rams, to restrict flow. The mechanism is simple, when pressure in the well moves from downward (into the well) to upward (out of the well) the rams slam shut like gates and slow or stop the flow of oil. Original rams, called pipe rams, simply sealed off the flow of petroleum around the drill pipe, but did nothing to prevent the flow through the pipe.
Modern ram BOPs use shear rams, which can cut through the drill pipe to completely stop the flow of oil. These ram BOPs rely upon hydraulic systems to provide the force needed to cut through strong metal drill pipe and drill string. To allow for recovery of expensive drill bits, most modern ram BOPs consist of both a shear ram and a pipe ram. The shear ram sits on top of the pipe ram and cuts the drill string to completely seal the well. Below that, the pipe ram is deployed at the same time to capture the drill string and prevent it from falling into the bore hole. This makes eventual recovery of the valuable bit feasible.
Annular Blowout Preventers
Annular BOPs were invented in 1952 and are often referred to as “Hydrils” after the company that produced them. Annular BOPs use rubber seals to close around the drill string and seal the well without cutting. This allows the drill string and bit to be removed from the well while pressure is maintained and blowout prevented. These BOPs work by forcing flexible rubber rings into tight wedges around the drill pipe and string.Annular BOPs are not as effective as ram BOPs in completely sealing a well. As such, annular BOPs are often stacked on top of ram BOPs to provide several levels of blowout prevention. Annular BOPs necessarily rely upon hydraulic pressure to create and maintain their seals. In some cases, drilling can continue even when an annular BOP has been deployed.