Field Instrumentation

What is a blowout preventer?

A blowout preventer is a safety device that can be used to prevent the uncontrolled flow of liquid and gas during well-drilling operations. The blowout preventers are large high-pressure valves that can be controlled remotely. So these large valves can prevent the blowout in the oil and gas wells. These valves would seal off the surface of the well which is being worked upon or drilled. The blow-out preventer valve would close during drilling if there is an overpressure created as a result of an underground zone resulting in the formation of fluids such as natural gas or oil to enter the wellbore and it could affect the functioning of the rig. The blow-out preventer valve is mostly closed by the hydraulic actuator and it will be controlled remotely.

A BOP will control the well at any time and it is mostly installed in stacks, the annular BOP can close the wells at different devices such as the drill string. In order to control a blowout, the BOP stack is fitted with hydraulic lines and this would allow the drillers to pump a heavier drilling fluid in the well, and also to evacuate the lighter fluid from the well. The BOP pressure will be selected in a way that it can support the maximum pressure encountered during the drilling operation.

What is a blowout in an oil well?

A blowout is an unwanted or uncontrolled flow of fluid due to the surface formation, and this would be released to the surface and it could lead to many safety-related problems. So the BOP system is inevitable in an oil well, the blowout potential like the amount of blowout of the oil or gas could be according to the scenario at which the blowout happens. The blowout rate will be low in case of a lost pipe, junk or complex escape paths for the fluid could lower the blowout rates in an oil well.

How to install a BOP?

If the BOP’s are installed offshore then, the drilling platform is in contact with the sea bed, so we can install the BOP’s at the surface on the top of the wellhead system. In deep water, the drilling devices will be floating and the floating rig will be disconnected from the well and the BOP will be connected to the well to ensure the safety of the well.

How does a BOP work?

The major function of a BOP control system is to make sure that the proper hydraulic pressure is conveyed to the proper place at the proper pressure and this must be done very quickly. Hydraulic power is mostly used for the BOP operation, so the hydraulic pressure must reach the right place. The response time of the BOP can be improved in deep water with longer hoses and also a reservoir of hydraulic fluid can be stored in the subsea. The signal for the operation of the control valve is initially transmitted hydraulically. By using an electrical signal BOP can achieve a quick response time. Most of the modern BOP’s uses multiplex control systems. So the BOP can control, seal, and control the oil and gas wells to prevent blowouts. The blowout preventers are mostly used in, land wells, subsea wells, and offshore rigs. The land and the subsea BOP’s are secured to the top of the wellbore and it is called a wellhead. The BOP’s on the offshore rigs are mounted below the rig deck.

The BOP will act as a check valve, it would allow the fluid to be pumped down to the drill string and it blocks the back flow.

What are the types of blow out preventer?

There are two types of BOP and they are RAM type and annular type. Both types of BOP’s are used in the stack, it will be placed in a way that one annular BOP stacked above several RAM BOP’s

RAM BOP

The operation of the RAM BOP is similar to a gate valve, basically, it is a valve that uses a pair of opposing pistons and a steel RAM block. The RAM blocks extend through a guide chamber of the BOP housing and it will extend towards the center of the BOP wellbore to halt the returning flow, it will open to permit the flow. So in case if there is any change in the fluid pressure the steel rams would shut with the help of hydraulic pressure with great force.

Types of RAM

Pipe ram – This will close around the drill pipe and restricts the flow in the annulus between the outside of the drill pipe and wellbore and it won’t obstruct the flow in the drill pipe.

Blind ram – This type has no opening for tubing and it closes the well, if the well doesn’t have a drill string and it would seal it

Shear ram – This can shear the pipe in the well and can seal the wellbore. This type has a steel blade to shear the pipe and also has seals to seal the annulus

Blind shear ram – this type will seal a wellbore even if the bore is occupied by the drill string, by cutting through the drill string as the ram close the well.

Annular BOP It is a large circular valve that can control the wellbore fluid, it consists of a rounded elastomer with reinforced steel segments. The rubber will deform radially inwards and seals. It can close around different shapes such as square, hexagonal, etc. The major part of the annular BOP is the sealing element. So if the fluid pressure changes the closing mechanism would be actuated, the piston will receive the hydraulic pressure which would cause it to move upwards and thus forcing the sealing element and it will be extended to the wellbore around the drill string. The steel segment will close over the rubber so that it can prevent the excessive partial close over the rubber in order to prevent the excessive extrusion when sealing under high pressure.

What does a BOP control panel do?

The BOP control panels would actuate all the safety devices in a BOP by a pneumatic hydraulic system. The hydraulic system consists of actuators and accumulators and it will operate without using the power in the rig. Mostly there would be two control panels one will be at the rig floor and the other one is located remotely away from the risky areas. The actuator is composed of a pressure sensor, electronic unit, power supply, electric motor, etc.

When and where should we install a blow-out preventer?

  • It must be attached to the wellhead
  • It must be located below the rig floor in the cellar during the onshore conditions
  • The BOP must be located below the rig floor on the texas deck
  • In the offshore (floating drilling unit) the BOP will be on the seabed
  • The BOP will be attached to the wellhead by using flanged or collet connectors

What is the need for a blowout preventer?

A BOP is a large valve that is placed at the top of the well and this will be closed if the control over the fluid formation is lost. So if this valve is closed then the reservoir control will be regained so it will be opened only after the pressure control. The BOP can determine certain things such as it can determine if the flow from the well could occur or not. The BOP can stop the flow from the annulus with or without the drill string in the hole. It would allow the pipe movement under pressure and also allows the circulation of the fluid. The major purpose of a BOP is to control the well-pressure.

What are the requirements for a subsea BOP system?

  • It should have more than two BOP’s
  • In order to ensure the proper and independent operation of the BOP system, there should be an operable dual pod control system
  • In order to prevent the fast closure of BOP components, an accumulator system should be used and this will also operate in case of power loss to the fluid connection
  • The subsea BOP should have a remotely operated vehicle and it should have intervention capability
  • The ROV should be able to do the shutting down of the well during the emergency situation
  • There should be an operational or physical barrier in a BOP control panel in order to prevent the accidental disconnect function.
  • All the control panels for the BOP systems must be labeled

What are the different types of control method in a BOP?

Electrical control signal

In this type of control method the signal is sent from the surface through a control cable

Acoustical control signal

This signal is sent from a surface based on a modulated/encoded pulse of sound transmitted by an underwater transducer

Remotely operated vehicles

The ROV would provide the hydraulic pressure in case of emergency situations

Dead-man switch

This is used in an emergency situation as a fail-safe activation and also if the control, power, and hydraulic lines have been served.

How to select a BOP?

  • It must be selected according to the maximum surface pressure
  • Size of casing and casing hanger must be considered
  • It must be selected according to the available space between the top of the cellar pit and the bottom of the rotary table
  • It must be selected according to the flanging size and pressure rating of the wellhead flange
  • Service conditions must be considered
  • Rated working pressure
  • It must be selected according to the operation time

What are the limitations of the BOP?

  • The maximum BOP pressure rating struggles to keep pace with the drilling technology
  • Full testing of all the emergency functions is not possible
  • The components in the BOP could easily fail
  • Certain emergency situations are not detected by BOP’s
  • The sheer RAM is not capable to cut through jointed drill pipe connections or other kinds of obstructions in the BOP bore.

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