Blowout preventer
Encyclopedia
A blowout preventer is a large, specialized valve
used to seal, control and monitor oil and gas wells
. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow (formation kick) emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic event known as a blowout
. In addition to controlling the downhole (occurring in the drilled hole) pressure and the flow of oil and gas, blowout preventers are intended to prevent tubing (e.g. drill pipe
and well casing), tools and drilling fluid
from being blown out of the wellbore
(also known as bore hole, the hole leading to the reservoir) when a blowout threatens. Blowout preventers are critical to the safety of crew, rig
(the equipment system used to drill a wellbore) and environment, and to the monitoring and maintenance of well integrity
; thus blowout preventers are intended to be fail-safe
devices.
The term BOP (an initialism rather than a spoken acronym, i.e., pronounced B-O-P, not "bop") is used in oilfield vernacular to refer to blowout preventers.
The abbreviated term preventer, usually prefaced by a type (e.g. ram preventer), is used to refer to a single blowout preventer unit. A blowout preventer may also simply be referred to by its type (e.g. ram).
The terms blowout preventer, blowout preventer stack and blowout preventer system are commonly used interchangeably and in a general manner to describe an assembly of several stacked blowout preventers of varying type and function, as well as auxiliary components. A typical subsea
deepwater blowout preventer system includes components such as electrical and hydraulic
lines, control pods, hydraulic accumulator
s, test valve, kill and choke lines and valves, riser
joint, hydraulic connectors, and a support frame.
Two categories of blowout preventer are most prevalent: ram and annular. BOP stacks frequently utilize both types, typically with at least one annular BOP stacked above several ram BOPs.
(A related valve, called an inside blowout preventer, internal blowout preventer, or IBOP, is positioned within, and restricts flow up, the drillpipe. This article does not address inside blowout preventer use.)
Blowout preventers are used at land and offshore rigs
, and subsea. Land and subsea BOPs are secured to the top of the wellbore, known as the wellhead
. BOPs on offshore rigs are mounted below the rig deck. Subsea BOPs are connected to the offshore rig above by a drilling riser
that provides a continuous pathway for the drill string
and fluids emanating from the wellbore. In effect, a riser extends the wellbore to the rig.
, indicating that substantial improvement is needed.
Blowout preventers come in a variety of styles, sizes and pressure ratings. Several individual units serving various functions are combined to compose a blowout preventer stack. Multiple blowout preventers of the same type are frequently provided for redundancy
, an important factor in the effectiveness of fail-safe
devices.
The primary functions of a blowout preventer system are to:
Additionally, and in performing those primary functions, blowout preventer systems are used to:
In drilling a typical high-pressure well, drill strings are routed through a blowout preventer stack toward the reservoir of oil and gas. As the well is drilled, drilling fluid
, "mud", is fed through the drill string down to the drill bit, "blade", and returns up the wellbore in the ring-shaped void, annulus
, between the outside of the drill pipe and the casing (piping that lines the wellbore). The column of drilling mud exerts downward hydrostatic pressure to counter opposing pressure from the formation being drilled, allowing drilling to proceed.
When a kick (influx of formation fluid
) occurs, rig operators or automatic systems close the blowout preventer units, sealing the annulus to stop the flow of fluids out of the wellbore. Denser mud is then circulated into the wellbore down the drill string, up the annulus and out through the choke line at the base of the BOP stack through chokes (flow restrictors) until downhole pressure is overcome. Once “kill weight” mud extends from the bottom of the well to the top, the well has been “killed”. If the integrity of the well is intact drilling may be resumed. Alternatively, if circulation is not feasible it may be possible to kill the well by "bullheading", forcibly pumping, in the heavier mud from the top through the kill line connection at the base of the stack. This is less desirable because of the higher surface pressures likely needed and the fact that much of the mud originally in the annulus must be forced into receptive formations in the open hole section beneath the deepest casing shoe.
If the blowout preventers and mud do not restrict the upward pressures of a kick, a blowout results, potentially shooting tubing, oil and gas up the wellbore, damaging the rig, and leaving well integrity
in question.
Since BOPs are important for the safety of the crew and natural environment, as well as the drilling rig
and the wellbore itself, authorities recommend, and regulations require, that BOPs be regularly inspected, tested and refurbished. Tests vary from daily test of functions on critical wells to monthly or less frequent testing on wells with low likelihood of control problems.
Exploitable reservoirs of oil and gas are increasingly rare and remote, leading to increased subsea deepwater well exploration and requiring BOPs to remain submerged for as long as a year in extreme conditions. As a result, BOP assemblies have grown larger and heavier (e.g. a single ram-type BOP unit can weigh in excess of 30,000 pounds), while the space allotted for BOP stacks on existing offshore rigs has not grown commensurately. Thus a key focus in the technological development of BOPs over the last two decades has been limiting their footprint and weight while simultaneously increasing safe operating capacity.
BOP stacks, typically with at least one annular BOP capping a stack of several ram BOPs.
.
A ram-type BOP is similar in operation to a gate valve
, but uses a pair of opposing steel plungers, rams. The rams extend toward the center of the wellbore to restrict flow or retract open in order to permit flow. The inner and top faces of the rams are fitted with packers (elastomeric seals) that press against each other, against the wellbore, and around tubing running through the wellbore. Outlets at the sides of the BOP housing (body) are used for connection to choke and kill lines or valves.
Rams, or ram blocks, are of four common types: pipe, blind, shear, and blind shear.
Pipe rams close around a drill pipe, restricting flow in the annulus (ring-shaped space between concentric objects) between the outside of the drill pipe and the wellbore, but do not obstruct flow within the drill pipe. Variable-bore pipe rams can accommodate tubing in a wider range of outside diameters than standard pipe rams, but typically with some loss of pressure capacity and longevity.
Blind rams (also known as sealing rams), which have no openings for tubing, can close off the well when the well does not contain a drill string or other tubing, and seal it.
Shear rams cut through the drill string or casing with hardened steel shears.
Blind shear rams (also known as shear seal rams, or sealing shear rams) are intended to seal a wellbore, even when the bore is occupied by a drill string, by cutting through the drill string as the rams close off the well. The upper portion of the severed drill string is freed from the ram, while the lower portion may be crimped and the “fish tail” captured to hang the drill string off the BOP.
In addition to the standard ram functions, variable-bore pipe rams are frequently used as test rams in a modified blowout preventer device known as a stack test valve. Stack test valves are positioned at the bottom of a BOP stack and resist downward pressure (unlike BOPs, which resist upward pressures). By closing the test ram and a BOP ram about the drillstring and pressurizing the annulus, the BOP is pressure-tested for proper function.
The original ram BOPs of the 1920s were simple and rugged manual devices with minimal parts. The BOP housing (body) had a vertical well bore and horizontal ram cavity (ram guide chamber). Opposing rams (plungers) in the ram cavity translated horizontally, actuated by threaded ram shafts (piston rods) in the manner of a screw jack. Torque from turning the ram shafts by wrench or hand wheel was converted to linear motion and the rams, coupled to the inner ends of the ram shafts, opened and closed the well bore. Such screw jack type operation provided enough mechanical advantage for rams to overcome downhole pressures and seal the wellbore annulus.
Hydraulic rams BOPs were in use by the 1940s. Hydraulically actuated blowout preventers had many potential advantages. The pressure could be equalized in the opposing hydraulic cylinders causing the rams to operate in unison. Relatively rapid actuation and remote control were facilitated, and hydraulic rams were well-suited to high pressure wells.
Because BOPs are fail-safe devices, efforts to minimize the complexity of the devices are still employed to ensure ram BOP reliability and longevity. As a result, despite the ever-increasing demands placed on them, state of the art ram BOPs are conceptually the same as the first effective models, and resemble those units in many ways.
Ram BOPs for use in deepwater applications universally employ hydraulic actuation. Threaded shafts are often still incorporated into hydraulic ram BOPs as lock rods that hold the ram in position after hydraulic actuation. By using a mechanical ram locking mechanism, constant hydraulic pressure need not be maintained. Lock rods may be coupled to ram shafts or not, depending on manufacturer. Other types of ram locks, such as wedge locks, are also used.
Typical ram actuator assemblies (operator systems) are secured to the BOP housing by removable bonnets. Unbolting the bonnets from the housing allows BOP maintenance and facilitates the substitution of rams. In that way, for example, a pipe ram BOP can be converted to a blind shear ram BOP.
Shear-type ram BOPs require the greatest closing force in order to cut through tubing occupying the wellbore. Boosters (auxiliary hydraulic actuators) are frequently mounted to the outer ends of a BOP’s hydraulic actuators to provide additional shearing force for shear rams.
Ram BOPs are typically designed so that well pressure will help maintain the rams in their closed, sealing position. That is achieved by allowing fluid to pass to pass through a channel in the ram and exert pressure at the ram’s rear and toward the center of the wellbore. Providing a channel in the ram also limits the thrust required to overcome well bore pressure.
Single ram and double ram BOPs are commonly available. The names refer to the quantity of ram cavities (equivalent to the effective quantity of valves) contained in the unit. A double ram BOP is more compact and lighter than a stack of two single ram BOPs while providing the same functionality, and is thus desirable in many applications. Triple ram BOPs are also manufactured, but not as common.
Technological development of ram BOPs has been directed towards deeper and higher pressure wells, greater reliability, reduced maintenance, facilitated replacement of components, facilitated ROV intervention, reduced hydraulic fluid consumption, and improved connectors, packers, seals, locks and rams. In addition, limiting BOP weight and footprint are significant concerns to account for the limitations of existing rigs.
The highest-capacity large-bore ram blowout preventer on the market, as of July 2010, Cameron’s EVO 20K BOP, has a hold-pressure rating of 20,000 psi, ram force in excess of 1,000,000 pounds, and a well bore diameter of 18.75 inches.
An annular-type blowout preventer can close around the drill string, casing or a non-cylindrical object, such as the kelly
. Drill pipe including the larger-diameter tool joints (threaded connectors) can be "stripped" (i.e., moved vertically while pressure is contained below) through an annular preventer by careful control of the hydraulic closing pressure. Annular blowout preventers are also effective at maintaining a seal around the drillpipe even as it rotates during drilling. Regulations typically require that an annular preventer be able to completely close a wellbore, but annular preventers are generally not as effective as ram preventers in maintaining a seal on an open hole. Annular BOPs are typically located at the top of a BOP stack, with one or two annular preventers positioned above a series of several ram preventers.
An annular blowout preventer uses the principle of a wedge to shut in the wellbore. It has a donut-like rubber seal, known as an elastomer
ic packing unit, reinforced with steel ribs. The packing unit is situated in the BOP housing between the head and hydraulic piston. When the piston is actuated, its upward thrust forces the packing unit to constrict, like a sphincter
, sealing the annulus or openhole. Annular preventers have only two moving parts, piston and packing unit, making them simple and easy to maintain relative to ram preventers.
The original type of annular blowout preventer uses a “wedge-faced” (conical-faced) piston. As the piston rises, vertical movement of the packing unit is restricted by the head and the sloped face of the piston squeezes the packing unit inward, toward the center of the wellbore.
In 1972, Ado N. Vujasinovic was awarded a patent for a variation on the annular preventer known as a spherical blowout preventer, so-named because of its spherical-faced head. As the piston rises the packing unit is thust upward against the curved head, which constricts the packing unit inward. Both types of annular preventer are in common use.
In deeper offshore operations with the wellhead just above the mudline on the sea floor, there are four primary ways by which a BOP can be controlled. The possible means are:
Two control pods are provided on the BOP for redundancy. Electrical signal control of the pods is primary. Acoustical, ROV intervention and dead-man controls are secondary.
An emergency disconnect system, or EDS, disconnects the rig from the well in case of an emergency. The EDS is also intended to automatically trigger the deadman switch, which closes the BOP, kill and choke valves. The EDS may be a subsystem of the BOP stack’s control pods or separate.
Pumps on the rig normally deliver pressure to the blowout preventer stack through hydraulic lines. Hydraulic accumulators are on the BOP stack enable closure of blowout preventers even if the BOP stack is disconnected from the rig. It is also possible to trigger the closing of BOPs automatically based on too high pressure or excessive flow.
Individual wells along the U.S. coastline may also be required to have BOPs with backup acoustic control. 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.
incident on April 20, 2010, the blowout preventer should have been activated automatically, cutting the drillstring and sealing the well to preclude a blowout and subsequent oil spill in the Gulf of Mexico, but it failed to fully engage. Underwater robots (ROVs) later were used to manually trigger the blind shear ram preventer, to no avail.
it is unknown why the blowout preventer failed. Chief surveyor John David Forsyth of the American Bureau of Shipping
testified in hearings before the Joint Investigation of the Minerals Management Service
and the U.S. Coast Guard investigating the causes of the explosion that his agency last inspected the rig's blowout preventer in 2005. BP representatives suggested that the preventer could have suffered a hydraulic leak. 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 remotely operated vehicle work is unknown. A statement released by Congressman Bart Stupak revealed that, among other issues, the emergency disconnect system (EDS) did not function as intended and may have malfunctioned due to the explosion on the Deepwater Horizon.
The permit for the Macondo Prospect
by the Minerals Management Service
in 2009 did not require redundant acoustic control means. Inasmuch as the BOPs could not be closed successfully by underwater manipulation (ROV Intervention
), pending results of a complete investigation, it is uncertain whether this omission was a factor in the blowout.
Documents discussed during congressional hearings June 17, 2010, suggested that a battery in the device's control pod was flat and that the rig's owner, Transocean
, may have "modified" Cameron
's equipment for the Macondo site (including incorrectly routing hydraulic pressure to a stack test valve instead of a pipe ram BOP) which increased the risk of BOP failure, in spite of warnings from their contractor to that effect. Another hypothesis is that a junction in the drilling pipe may have been positioned in the BOP stack in such way that its shear rams had an insurmountable thickness of material to cut through.
It was later discovered that a second piece of tubing got into the BOP stack at some point during the Macondo incident, potentially explaining the failure of the BOP shearing mechanism. As of July 2010 it is unknown whether the tubing might be casing that shot up through the well or perhaps broken drill pipe that dropped into the well.
On July 10, 2010 BP began operations to install a sealing cap, also known as a capping stack, atop the failed blowout preventer stack. Based on BP's video feeds of the operation the sealing cap assembly, called Top Hat 10, includes a stack of three blind shear ram BOPs manufactured by Hydril (a GE Oil & Gas company), one of Cameron's chief competitors. By July 15 the 3 ram capping stack had sealed the Macondo well, if only temporarily, for the first time in 87 days.
The U.S. government wants the failed blowout preventer to be replaced in case of any pressure that occurs when the relief well intersects with the well. On September 3 at 1:20 p.m. CDT the 300 ton
failed blowout preventer was removed from the well and began being slowly lifted to the surface. Later that day a replacement blowout preventer was placed on the well. On September 4 at 6:54 p.m. CDT the failed blowout preventer reached the surface of the water and at 9:16 p.m. CDT it was placed in a special container on board the vessel Helix Q4000. The failed blowout preventer was taken to a NASA facility in Louisiana for examination by Det Norske Veritas
(DNV).
On 20 March 2011, DNV presented their report to the US Department of Energy. Their primary conclusion was that the rams failed to shear through the oil pipe and seal it because it has buckled out of the line of action of the rams. They did not suggest any failure of actuation as would be caused by faulty batteries.
Valve
A valve is a device that regulates, directs or controls the flow of a fluid by opening, closing, or partially obstructing various passageways. Valves are technically pipe fittings, but are usually discussed as a separate category...
used to seal, control and monitor oil and gas wells
Oil well
An oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
. Blowout preventers were developed to cope with extreme erratic pressures and uncontrolled flow (formation kick) emanating from a well reservoir during drilling. Kicks can lead to a potentially catastrophic event known as a blowout
Blowout (well drilling)
A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....
. In addition to controlling the downhole (occurring in the drilled hole) pressure and the flow of oil and gas, blowout preventers are intended to prevent tubing (e.g. drill pipe
Drill pipe
Drill pipe, is hollow, thick-walled, steel piping that is used on drilling rigs to facilitate the drilling of a wellbore and comes in a variety of sizes, strengths, and weights but are typically 30 to 33 feet in length...
and well casing), tools and drilling fluid
Drilling fluid
In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...
from being blown out of the wellbore
Wellbore
A wellbore is any hole drilled for the purpose of exploration or extraction of natural resources such as water, gas or oil where a well may be produced and a resource is extracted for a protracted period of time....
(also known as bore hole, the hole leading to the reservoir) when a blowout threatens. Blowout preventers are critical to the safety of crew, rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...
(the equipment system used to drill a wellbore) and environment, and to the monitoring and maintenance of well integrity
Well integrity
Well integrity, in regards to oil wells, is defined by as the "“Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well”...
; thus blowout preventers are intended to be fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
devices.
The term BOP (an initialism rather than a spoken acronym, i.e., pronounced B-O-P, not "bop") is used in oilfield vernacular to refer to blowout preventers.
The abbreviated term preventer, usually prefaced by a type (e.g. ram preventer), is used to refer to a single blowout preventer unit. A blowout preventer may also simply be referred to by its type (e.g. ram).
The terms blowout preventer, blowout preventer stack and blowout preventer system are commonly used interchangeably and in a general manner to describe an assembly of several stacked blowout preventers of varying type and function, as well as auxiliary components. A typical subsea
Subsea
Subsea is a general term frequently used to refer to equipment, technology, and methods employed in marine biology, undersea geology, offshore oil and gas developments, underwater mining, and offshore wind power industries.- Oil and gas :...
deepwater blowout preventer system includes components such as electrical and hydraulic
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
lines, control pods, hydraulic accumulator
Hydraulic accumulator
A 'hydraulic accumulator' is an energy storage device. It is a pressure storage reservoir in which a non-compressible hydraulic fluid is held under pressure by an external source. That external source can be a spring, a raised weight, or a compressed gas...
s, test valve, kill and choke lines and valves, riser
Drilling riser
A drilling riser is a conduit that provides a temporary extension of a subsea oil well to a surface drilling facility. Drilling risers are categorised into two types: marine drilling risers used with subsea blowout preventer and generally used by floating drilling vessels; and tie-back drilling...
joint, hydraulic connectors, and a support frame.
Two categories of blowout preventer are most prevalent: ram and annular. BOP stacks frequently utilize both types, typically with at least one annular BOP stacked above several ram BOPs.
(A related valve, called an inside blowout preventer, internal blowout preventer, or IBOP, is positioned within, and restricts flow up, the drillpipe. This article does not address inside blowout preventer use.)
Blowout preventers are used at land and offshore rigs
Oil platform
An oil platform, also referred to as an offshore platform or, somewhat incorrectly, oil rig, is a lаrge structure with facilities to drill wells, to extract and process oil and natural gas, and to temporarily store product until it can be brought to shore for refining and marketing...
, and subsea. Land and subsea BOPs are secured to the top of the wellbore, known as the wellhead
Wellhead
A wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....
. BOPs on offshore rigs are mounted below the rig deck. Subsea BOPs are connected to the offshore rig above by a drilling riser
Drilling riser
A drilling riser is a conduit that provides a temporary extension of a subsea oil well to a surface drilling facility. Drilling risers are categorised into two types: marine drilling risers used with subsea blowout preventer and generally used by floating drilling vessels; and tie-back drilling...
that provides a continuous pathway for the drill string
Drill string
A drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...
and fluids emanating from the wellbore. In effect, a riser extends the wellbore to the rig.
Use
The invention of blowout preventers was instrumental in reducing the incidence of oil gushers, blowoutsBlowout (well drilling)
A blowout is the uncontrolled release of crude oil and/or natural gas from an oil well or gas well after pressure control systems have failed....
, indicating that substantial improvement is needed.
Blowout preventers come in a variety of styles, sizes and pressure ratings. Several individual units serving various functions are combined to compose a blowout preventer stack. Multiple blowout preventers of the same type are frequently provided for redundancy
Redundancy (engineering)
In engineering, redundancy is the duplication of critical components or functions of a system with the intention of increasing reliability of the system, usually in the case of a backup or fail-safe....
, an important factor in the effectiveness of fail-safe
Fail-safe
A fail-safe or fail-secure device is one that, in the event of failure, responds in a way that will cause no harm, or at least a minimum of harm, to other devices or danger to personnel....
devices.
The primary functions of a blowout preventer system are to:
- Confine well fluid to the wellboreWellboreA wellbore is any hole drilled for the purpose of exploration or extraction of natural resources such as water, gas or oil where a well may be produced and a resource is extracted for a protracted period of time....
; - Provide means to add fluid to the wellbore;
- Allow controlled volumes of fluid to be withdrawn from the wellbore.
Additionally, and in performing those primary functions, blowout preventer systems are used to:
- Regulate and monitor wellbore pressure;
- Center and hang off the drill stringDrill stringA drill string on a drilling rig is a column, or string, of drill pipe that transmits drilling fluid and torque to the drill bit. The term is loosely applied as the assembled collection of the drill pipe, drill collars, tools and drill bit...
in the wellbore; - Shut in the well (e.g. seal the void, annulusAnnulus (oil well)The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
, between drillpipe and casing); - “Kill” the wellWell killA well kill is the operation of placing a column of heavy fluid into a well bore in order to prevent the flow of reservoir fluids without the need for pressure control equipment at the surface. It works on the principle that the weight of the "kill fluid" or "kill mud" will be enough to suppress...
(prevent the flow of formation fluidFormation fluidFormation fluid refers to the naturally occurring liquids and gases contained in geologic formations. Fluids introduced during the drilling process are called drilling fluids. Fluids in an oil or gas reservoir are called reservoir fluids. The fluids flowing from the wellhead of an oil or gas well...
, influx, from the reservoir into the wellbore) ; - Seal the wellheadWellheadA wellhead is a general term used to describe the component at the surface of an oil or gas well that provides the structural and pressure-containing interface for the drilling and production equipment....
(close off the wellbore); - Sever the casing or drill pipeDrill pipeDrill pipe, is hollow, thick-walled, steel piping that is used on drilling rigs to facilitate the drilling of a wellbore and comes in a variety of sizes, strengths, and weights but are typically 30 to 33 feet in length...
(in case of emergencies).
In drilling a typical high-pressure well, drill strings are routed through a blowout preventer stack toward the reservoir of oil and gas. As the well is drilled, drilling fluid
Drilling fluid
In geotechnical engineering, drilling fluid is a fluid used to aid the drilling of boreholes into the earth. Often used while drilling oil and natural gas wells and on exploration drilling rigs, drilling fluids are also used for much simpler boreholes, such as water wells. Liquid drilling fluid...
, "mud", is fed through the drill string down to the drill bit, "blade", and returns up the wellbore in the ring-shaped void, annulus
Annulus (oil well)
The annulus of an oil well refers to any void between any piping, tubing or casing and the piping, tubing, or casing immediately surrounding it. It is named after the corresponding geometric concept...
, between the outside of the drill pipe and the casing (piping that lines the wellbore). The column of drilling mud exerts downward hydrostatic pressure to counter opposing pressure from the formation being drilled, allowing drilling to proceed.
When a kick (influx of formation fluid
Formation fluid
Formation fluid refers to the naturally occurring liquids and gases contained in geologic formations. Fluids introduced during the drilling process are called drilling fluids. Fluids in an oil or gas reservoir are called reservoir fluids. The fluids flowing from the wellhead of an oil or gas well...
) occurs, rig operators or automatic systems close the blowout preventer units, sealing the annulus to stop the flow of fluids out of the wellbore. Denser mud is then circulated into the wellbore down the drill string, up the annulus and out through the choke line at the base of the BOP stack through chokes (flow restrictors) until downhole pressure is overcome. Once “kill weight” mud extends from the bottom of the well to the top, the well has been “killed”. If the integrity of the well is intact drilling may be resumed. Alternatively, if circulation is not feasible it may be possible to kill the well by "bullheading", forcibly pumping, in the heavier mud from the top through the kill line connection at the base of the stack. This is less desirable because of the higher surface pressures likely needed and the fact that much of the mud originally in the annulus must be forced into receptive formations in the open hole section beneath the deepest casing shoe.
If the blowout preventers and mud do not restrict the upward pressures of a kick, a blowout results, potentially shooting tubing, oil and gas up the wellbore, damaging the rig, and leaving well integrity
Well integrity
Well integrity, in regards to oil wells, is defined by as the "“Application of technical, operational and organizational solutions to reduce risk of uncontrolled release of formation fluids throughout the life cycle of a well”...
in question.
Since BOPs are important for the safety of the crew and natural environment, as well as the drilling rig
Drilling rig
A drilling rig is a machine which creates holes or shafts in the ground. Drilling rigs can be massive structures housing equipment used to drill water wells, oil wells, or natural gas extraction wells, or they can be small enough to be moved manually by one person...
and the wellbore itself, authorities recommend, and regulations require, that BOPs be regularly inspected, tested and refurbished. Tests vary from daily test of functions on critical wells to monthly or less frequent testing on wells with low likelihood of control problems.
Exploitable reservoirs of oil and gas are increasingly rare and remote, leading to increased subsea deepwater well exploration and requiring BOPs to remain submerged for as long as a year in extreme conditions. As a result, BOP assemblies have grown larger and heavier (e.g. a single ram-type BOP unit can weigh in excess of 30,000 pounds), while the space allotted for BOP stacks on existing offshore rigs has not grown commensurately. Thus a key focus in the technological development of BOPs over the last two decades has been limiting their footprint and weight while simultaneously increasing safe operating capacity.
Types
BOPs come in two basic types, ram and annular. Both are often used together in drilling rigDrilling rig (petroleum)
This article lists the main components of a petroleum onshore drilling rig.Offshore drilling rigs have similar elements, but are configured with a number of different drilling systems to suit drilling in the marine environment....
BOP stacks, typically with at least one annular BOP capping a stack of several ram BOPs.
Ram blowout preventer
The ram BOP was invented by James Smither Abercrombie and Harry S. Cameron in 1922, and was brought to market in 1924 by Cameron Iron WorksCameron International Corporation
Cameron International Corporation , , is a Fortune 500 company and a global provider of pressure control, processing, flow control and compression systems as well as project management and aftermarket services for the oil and gas and process industries...
.
A ram-type BOP is similar in operation to a gate valve
Gate valve
The gate valve, also known as a sluice valve, is a valve that opens by lifting a round or rectangular gate/wedge out of the path of the fluid. The distinct feature of a gate valve is the sealing surfaces between the gate and seats are planar, so gate valves are often used when a straight-line flow...
, but uses a pair of opposing steel plungers, rams. The rams extend toward the center of the wellbore to restrict flow or retract open in order to permit flow. The inner and top faces of the rams are fitted with packers (elastomeric seals) that press against each other, against the wellbore, and around tubing running through the wellbore. Outlets at the sides of the BOP housing (body) are used for connection to choke and kill lines or valves.
Rams, or ram blocks, are of four common types: pipe, blind, shear, and blind shear.
Pipe rams close around a drill pipe, restricting flow in the annulus (ring-shaped space between concentric objects) between the outside of the drill pipe and the wellbore, but do not obstruct flow within the drill pipe. Variable-bore pipe rams can accommodate tubing in a wider range of outside diameters than standard pipe rams, but typically with some loss of pressure capacity and longevity.
Blind rams (also known as sealing rams), which have no openings for tubing, can close off the well when the well does not contain a drill string or other tubing, and seal it.
Shear rams cut through the drill string or casing with hardened steel shears.
Blind shear rams (also known as shear seal rams, or sealing shear rams) are intended to seal a wellbore, even when the bore is occupied by a drill string, by cutting through the drill string as the rams close off the well. The upper portion of the severed drill string is freed from the ram, while the lower portion may be crimped and the “fish tail” captured to hang the drill string off the BOP.
In addition to the standard ram functions, variable-bore pipe rams are frequently used as test rams in a modified blowout preventer device known as a stack test valve. Stack test valves are positioned at the bottom of a BOP stack and resist downward pressure (unlike BOPs, which resist upward pressures). By closing the test ram and a BOP ram about the drillstring and pressurizing the annulus, the BOP is pressure-tested for proper function.
The original ram BOPs of the 1920s were simple and rugged manual devices with minimal parts. The BOP housing (body) had a vertical well bore and horizontal ram cavity (ram guide chamber). Opposing rams (plungers) in the ram cavity translated horizontally, actuated by threaded ram shafts (piston rods) in the manner of a screw jack. Torque from turning the ram shafts by wrench or hand wheel was converted to linear motion and the rams, coupled to the inner ends of the ram shafts, opened and closed the well bore. Such screw jack type operation provided enough mechanical advantage for rams to overcome downhole pressures and seal the wellbore annulus.
Hydraulic rams BOPs were in use by the 1940s. Hydraulically actuated blowout preventers had many potential advantages. The pressure could be equalized in the opposing hydraulic cylinders causing the rams to operate in unison. Relatively rapid actuation and remote control were facilitated, and hydraulic rams were well-suited to high pressure wells.
Because BOPs are fail-safe devices, efforts to minimize the complexity of the devices are still employed to ensure ram BOP reliability and longevity. As a result, despite the ever-increasing demands placed on them, state of the art ram BOPs are conceptually the same as the first effective models, and resemble those units in many ways.
Ram BOPs for use in deepwater applications universally employ hydraulic actuation. Threaded shafts are often still incorporated into hydraulic ram BOPs as lock rods that hold the ram in position after hydraulic actuation. By using a mechanical ram locking mechanism, constant hydraulic pressure need not be maintained. Lock rods may be coupled to ram shafts or not, depending on manufacturer. Other types of ram locks, such as wedge locks, are also used.
Typical ram actuator assemblies (operator systems) are secured to the BOP housing by removable bonnets. Unbolting the bonnets from the housing allows BOP maintenance and facilitates the substitution of rams. In that way, for example, a pipe ram BOP can be converted to a blind shear ram BOP.
Shear-type ram BOPs require the greatest closing force in order to cut through tubing occupying the wellbore. Boosters (auxiliary hydraulic actuators) are frequently mounted to the outer ends of a BOP’s hydraulic actuators to provide additional shearing force for shear rams.
Ram BOPs are typically designed so that well pressure will help maintain the rams in their closed, sealing position. That is achieved by allowing fluid to pass to pass through a channel in the ram and exert pressure at the ram’s rear and toward the center of the wellbore. Providing a channel in the ram also limits the thrust required to overcome well bore pressure.
Single ram and double ram BOPs are commonly available. The names refer to the quantity of ram cavities (equivalent to the effective quantity of valves) contained in the unit. A double ram BOP is more compact and lighter than a stack of two single ram BOPs while providing the same functionality, and is thus desirable in many applications. Triple ram BOPs are also manufactured, but not as common.
Technological development of ram BOPs has been directed towards deeper and higher pressure wells, greater reliability, reduced maintenance, facilitated replacement of components, facilitated ROV intervention, reduced hydraulic fluid consumption, and improved connectors, packers, seals, locks and rams. In addition, limiting BOP weight and footprint are significant concerns to account for the limitations of existing rigs.
The highest-capacity large-bore ram blowout preventer on the market, as of July 2010, Cameron’s EVO 20K BOP, has a hold-pressure rating of 20,000 psi, ram force in excess of 1,000,000 pounds, and a well bore diameter of 18.75 inches.
Annular blowout preventer
The annular blowout preventer was invented by Granville Sloan Knox in 1946; a U.S. patent for it was awarded in 1952. Often around the rig it is called the "Hydril", after the name of one of the manufacturers of such devices.An annular-type blowout preventer can close around the drill string, casing or a non-cylindrical object, such as the kelly
Kelly drive
A kelly drive refers to a type of well drilling device on an oil drilling rig that employs a section of pipe with a polygonal or splined outer surface, which passes through the matching polygonal or splined kelly bushing and rotary table...
. Drill pipe including the larger-diameter tool joints (threaded connectors) can be "stripped" (i.e., moved vertically while pressure is contained below) through an annular preventer by careful control of the hydraulic closing pressure. Annular blowout preventers are also effective at maintaining a seal around the drillpipe even as it rotates during drilling. Regulations typically require that an annular preventer be able to completely close a wellbore, but annular preventers are generally not as effective as ram preventers in maintaining a seal on an open hole. Annular BOPs are typically located at the top of a BOP stack, with one or two annular preventers positioned above a series of several ram preventers.
An annular blowout preventer uses the principle of a wedge to shut in the wellbore. It has a donut-like rubber seal, known as an elastomer
Elastomer
An elastomer is a polymer with the property of viscoelasticity , generally having notably low Young's modulus and high yield strain compared with other materials. The term, which is derived from elastic polymer, is often used interchangeably with the term rubber, although the latter is preferred...
ic packing unit, reinforced with steel ribs. The packing unit is situated in the BOP housing between the head and hydraulic piston. When the piston is actuated, its upward thrust forces the packing unit to constrict, like a sphincter
Sphincter
A sphincter is an anatomical structure, or a circular muscle, that normally maintains constriction of a natural body passage or orifice and which relaxes as required by normal physiological functioning...
, sealing the annulus or openhole. Annular preventers have only two moving parts, piston and packing unit, making them simple and easy to maintain relative to ram preventers.
The original type of annular blowout preventer uses a “wedge-faced” (conical-faced) piston. As the piston rises, vertical movement of the packing unit is restricted by the head and the sloped face of the piston squeezes the packing unit inward, toward the center of the wellbore.
In 1972, Ado N. Vujasinovic was awarded a patent for a variation on the annular preventer known as a spherical blowout preventer, so-named because of its spherical-faced head. As the piston rises the packing unit is thust upward against the curved head, which constricts the packing unit inward. Both types of annular preventer are in common use.
Control methods
When rigs are drilled 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. They also can be closed manually by turning large wheel-like handles.In deeper offshore operations with the wellhead just above the mudline on the sea floor, there are four primary ways by which a BOP can be controlled. The possible means are:
- Electrical Control Signal: sent from the surface through a control cable;
- Acoustical Control Signal: sent from the surface based on a modulated/encoded pulse of sound transmitted by an underwater transducerTransducerA transducer is a device that converts one type of energy to another. Energy types include electrical, mechanical, electromagnetic , chemical, acoustic or thermal energy. While the term transducer commonly implies the use of a sensor/detector, any device which converts energy can be considered a...
; - ROV InterventionWell interventionA well intervention, or 'well work', is any operation carried out on an oil or gas well during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well....
: remotely operated vehicles (ROVs) mechanically control valves and provide hydraulic pressure to the stack (via “hot stab” panels); - Deadman SwitchDead man's switchA dead man's switch is a switch that is automatically operated in case the human operator becomes incapacitated, such as through death or loss of consciousness....
/ Auto Shear: fail-safe activation of selected BOPs during an emergency, and if the control, power and hydraulic lines have been severed.
Two control pods are provided on the BOP for redundancy. Electrical signal control of the pods is primary. Acoustical, ROV intervention and dead-man controls are secondary.
An emergency disconnect system, or EDS, disconnects the rig from the well in case of an emergency. The EDS is also intended to automatically trigger the deadman switch, which closes the BOP, kill and choke valves. The EDS may be a subsystem of the BOP stack’s control pods or separate.
Pumps on the rig normally deliver pressure to the blowout preventer stack through hydraulic lines. Hydraulic accumulators are on the BOP stack enable closure of blowout preventers even if the BOP stack is disconnected from the rig. It is also possible to trigger the closing of BOPs automatically based on too high pressure or excessive flow.
Individual wells along the U.S. coastline may also be required to have BOPs with backup acoustic control. General requirements of other nations, including Brazil, were drawn to require this method. BOPs featuring this method may cost as much as US$
United States dollar
The United States dollar , also referred to as the American dollar, is the official currency of the United States of America. It is divided into 100 smaller units called cents or pennies....
500,000 more than those that omit the feature.
Deepwater Horizon blowout
During the Deepwater Horizon drilling rig explosionDeepwater Horizon explosion
The Deepwater Horizon drilling rig explosion refers to the April 20, 2010 explosion and subsequent fire on the Deepwater Horizon semi-submersible Mobile Offshore Drilling Unit , which was owned and operated by Transocean and drilling for BP in the Macondo Prospect oil field about southeast of the...
incident on April 20, 2010, the blowout preventer should have been activated automatically, cutting the drillstring and sealing the well to preclude a blowout and subsequent oil spill in the Gulf of Mexico, but it failed to fully engage. Underwater robots (ROVs) later were used to manually trigger the blind shear ram preventer, to no avail.
it is unknown why the blowout preventer failed. Chief surveyor John David Forsyth of the American Bureau of Shipping
American Bureau of Shipping
The American Bureau of Shipping is a classification society, with a mission to promote the security of life, property and the natural environment, primarily through the development and verification of standards for the design, construction and operational maintenance of marine-related facilities...
testified in hearings before the Joint Investigation of the Minerals Management Service
Minerals Management Service
The Bureau of Ocean Energy Management, Regulation and Enforcement , formerly known as the Minerals Management Service , was an agency of the United States Department of the Interior that managed the nation's natural gas, oil and other mineral resources on the outer continental shelf...
and the U.S. Coast Guard investigating the causes of the explosion that his agency last inspected the rig's blowout preventer in 2005. BP representatives suggested that the preventer could have suffered a hydraulic leak. 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 remotely operated vehicle work is unknown. A statement released by Congressman Bart Stupak revealed that, among other issues, the emergency disconnect system (EDS) did not function as intended and may have malfunctioned due to the explosion on the Deepwater Horizon.
The permit for the Macondo Prospect
Macondo Prospect
The Macondo Prospect is an oil and gas prospect in the United States Exclusive Economic Zone of the Gulf of Mexico, off the coast of Louisiana...
by the Minerals Management Service
Minerals Management Service
The Bureau of Ocean Energy Management, Regulation and Enforcement , formerly known as the Minerals Management Service , was an agency of the United States Department of the Interior that managed the nation's natural gas, oil and other mineral resources on the outer continental shelf...
in 2009 did not require redundant acoustic control means. Inasmuch as the BOPs could not be closed successfully by underwater manipulation (ROV Intervention
Well intervention
A well intervention, or 'well work', is any operation carried out on an oil or gas well during, or at the end of its productive life, that alters the state of the well and or well geometry, provides well diagnostics or manages the production of the well....
), pending results of a complete investigation, it is uncertain whether this omission was a factor in the blowout.
Documents discussed during congressional hearings June 17, 2010, suggested that a battery in the device's control pod was flat and that the rig's owner, Transocean
Transocean
Transocean Ltd. is one of the world's largest offshore drilling contractors. The company rents floating mobile drill rigs, along with the equipment and personnel for operations, to oil and gas companies at an average daily rate of US$282,700...
, may have "modified" Cameron
Cameron International Corporation
Cameron International Corporation , , is a Fortune 500 company and a global provider of pressure control, processing, flow control and compression systems as well as project management and aftermarket services for the oil and gas and process industries...
's equipment for the Macondo site (including incorrectly routing hydraulic pressure to a stack test valve instead of a pipe ram BOP) which increased the risk of BOP failure, in spite of warnings from their contractor to that effect. Another hypothesis is that a junction in the drilling pipe may have been positioned in the BOP stack in such way that its shear rams had an insurmountable thickness of material to cut through.
It was later discovered that a second piece of tubing got into the BOP stack at some point during the Macondo incident, potentially explaining the failure of the BOP shearing mechanism. As of July 2010 it is unknown whether the tubing might be casing that shot up through the well or perhaps broken drill pipe that dropped into the well.
On July 10, 2010 BP began operations to install a sealing cap, also known as a capping stack, atop the failed blowout preventer stack. Based on BP's video feeds of the operation the sealing cap assembly, called Top Hat 10, includes a stack of three blind shear ram BOPs manufactured by Hydril (a GE Oil & Gas company), one of Cameron's chief competitors. By July 15 the 3 ram capping stack had sealed the Macondo well, if only temporarily, for the first time in 87 days.
The U.S. government wants the failed blowout preventer to be replaced in case of any pressure that occurs when the relief well intersects with the well. On September 3 at 1:20 p.m. CDT the 300 ton
Ton
The ton is a unit of measure. It has a long history and has acquired a number of meanings and uses over the years. It is used principally as a unit of weight, and as a unit of volume. It can also be used as a measure of energy, for truck classification, or as a colloquial term.It is derived from...
failed blowout preventer was removed from the well and began being slowly lifted to the surface. Later that day a replacement blowout preventer was placed on the well. On September 4 at 6:54 p.m. CDT the failed blowout preventer reached the surface of the water and at 9:16 p.m. CDT it was placed in a special container on board the vessel Helix Q4000. The failed blowout preventer was taken to a NASA facility in Louisiana for examination by Det Norske Veritas
Det Norske Veritas
Stiftelsen Det Norske Veritas is a classification society organized as a foundation, with the objective of "Safeguarding life, property, and the environment". The organization's history goes back to 1864, when the foundation was established in Norway to inspect and evaluate the technical condition...
(DNV).
On 20 March 2011, DNV presented their report to the US Department of Energy. Their primary conclusion was that the rams failed to shear through the oil pipe and seal it because it has buckled out of the line of action of the rams. They did not suggest any failure of actuation as would be caused by faulty batteries.
See also
- Subsea technology
- Christmas tree (oil well)Christmas tree (oil well)In petroleum and natural gas extraction, a Christmas tree, or "tree", is an assembly of valves, spools, and fittings used for an oil well, gas well, water injection well, water disposal well, gas injection well, condensate well and other types of wells...
- Oil wellOil wellAn oil well is a general term for any boring through the earth's surface that is designed to find and acquire petroleum oil hydrocarbons. Usually some natural gas is produced along with the oil. A well that is designed to produce mainly or only gas may be termed a gas well.-History:The earliest...
- Offshore oil spill prevention and responseOffshore oil spill prevention and responseOffshore oil spill prevention and response is the study and practice of reducing the number of offshore incidents that release oil or hazardous substances into the environment and limiting the amount released during those incidents....
External links
- Blowout preventer: Definition from the Schlumberger glossary May 2010
- Blowout preventer: Definition from the US department of Labor, Occupational Safety & Health Administration (OSHA) May 2010
- http://www.osha.gov/SLTC/etools/oilandgas/images/bop_stack.jpg
- http://www.osha.gov/SLTC/etools/oilandgas/drilling/wellcontrol.html
- http://www.asmenews.org/archives/backissues/july03/features/703oilwell.html
- Photograph of subsea BOP stack linked from Oil states Offshore Products
