Blowout Preventers in Upstream Oil & Gas

What are Blowout Preventers?

Blowout preventers (BOPs) are critical pieces of safety equipment used in oil and gas drilling operations. Sitting atop the wellhead, these large, specialized valves are designed to seal, control, and monitor the well to prevent the uncontrolled release of crude oil or natural gas from the well. BOPs are the last line of defense for preventing a catastrophic blowout and keeping rig personnel, equipment, and the environment safe.

How Blowout Preventers Work

The basic function of a BOP is to quickly close in the well to prevent the uncontrolled flow of fluids. During normal drilling operations, drilling mud is circulated through the drill pipe and up the annulus to control formation pressure. If the hydrostatic pressure of the mud column drops below the formation pressure, an influx of gas, oil or water can enter the wellbore, potentially leading to a blowout if not controlled.BOPs contain a series of rams, seals and valves that can rapidly close off the annular space around the drill pipe or casing, or seal an open hole. When a “kick” is detected, the rig crew can remotely activate the BOP hydraulic controls to close the appropriate rams. With the well sealed off, heavier mud can be circulated to regain primary well control before the BOP is opened.Diagram of a typical BOP stack
Diagram of a typical subsea BOP stack showing the main components

Main BOP Components

A typical BOP stack contains the following main components215:

• Annular preventer: Donut-shaped rubber seal that can close around drill pipe, casing or an open hole. Usually located at the top of the BOP stack. Allows pipe to be stripped in or out of the hole with the annular closed.
• Pipe rams: Set of opposing steel rams with circular openings to seal around a specific size drill pipe. Variable bore rams can accommodate a range of pipe sizes.
• Blind rams: Solid, flat rams with no openings that seal against each other to close off an open hole when no pipe is present.
• Shear rams: Hardened steel blades designed to cut through drill pipe in an emergency to seal the well. Blind shear rams seal the well after shearing.
• Kill and choke lines: High-pressure lines that allow fluids to be pumped into or out of the well with the BOP closed. Used for circulating kill-weight mud and bleeding off the pressure.

Additional components include a hydraulic control system with accumulator capacity, connectors, valves, and a support frame. Subsea BOP stacks also have a lower marine riser package (LMRP) that connects the riser to the BOP.

Types of Blowout Preventers

There are two main categories of blowout preventers: ram and annular315.

Ram Blowout Preventers

Ram BOPs use pairs of opposing steel rams that extend toward the center of the wellbore to restrict flow or retract open to allow flow. The ends of the rams that seal against each other are fitted with elastomer seals. The rams move via hydraulic pistons integrated into the BOP body. Multiple sets of rams are typically used for redundancy. The types of ram BOPs include316:

• Pipe rams: Close around a drill pipe, sealing the annulus but allowing flow through the pipe
• Blind rams: Seal against each other to close off an open hole with no pipe
• Shear rams: Cut through drill pipe to seal an open hole in an emergency
• Blind shear rams: Cut pipe and seal the well bore

Annular Blowout Preventers

Annular BOPs use a donut-shaped elastomer packing unit that is mechanically squeezed inward to seal around pipe or seal an open hole. A hydraulic piston compresses the packing unit, which expands inward to seal the annular space or close off the hole. The flexibility of the rubber allows the packing unit to maintain a seal even as pipe is moved through it.Annular BOPs are typically positioned at the top of a BOP stack, with one or more ram preventers below. They allow pipe to be “stripped” in or out of the hole under pressure. Compared to ram BOPs, annular preventers are limited in their pressure ratings and ability to seal an open hole.

BOP Pressure Ratings and Sizes

BOPs are available in a range of sizes and pressure ratings to suit different well conditions. Typical BOP bore sizes range from 7-1/16″ to 21-1/4″ for ram preventers and up to 30″ for annular preventers4714.Working pressures range from 2,000 psi to 20,000 psi for ram BOPs and 1,000 psi to 10,000 psi for annular BOPs4714. The pressure rating must exceed the maximum anticipated surface pressure (MASP) the well may encounter. Higher pressure-rated BOPs are needed as wells are drilled deeper and encounter higher formation pressures.Some common BOP pressure ratings and sizes include4714:

• 11″ 5,000 psi ram BOP
• 13-5/8″ 5,000 – 10,000 psi ram BOP
• 18-3/4″ 15,000 psi ram BOP
• 21-1/4″ 2,000 – 5,000 psi annular BOP
• 13-5/8″ 5,000 – 10,000 psi annular BOP

Table of annular BOP sizes and pressure ratings
Table showing common annular BOP sizes and pressure ratings4

BOP Construction Materials

BOPs are constructed of high-strength, ductile materials to withstand the immense pressures and loads encountered during well control operations. The main BOP body is typically made of forged steel such as AISI 4130 or 8630 alloy steel414. These low-alloy steels have excellent strength and toughness properties.Precision machining is required to achieve the tight tolerances needed for the sealing surfaces and hydraulic components. The rams are also made of forged alloy steel, with the sealing ends often clad with stainless steel for wear and corrosion resistance. Shear rams have inserts made of hardened tool steel to enable pipe shearing.The elastomer seals and packing units are critical components. The annular packing unit consists of a steel-reinforced elastomer, usually nitrile rubber (NBR) or hydrogenated nitrile butadiene rubber (HNBR)414. These materials provide high tensile strength, abrasion resistance and sour gas (H2S) resistance.For the ram BOP seals, HNBR or fluoroelastomer (FKM) is commonly used for their chemical resistance properties14. Explosive decompression resistance is also important to prevent seal failure if gas becomes trapped in the elastomer.

BOP Testing and Maintenance

Stringent testing, inspection and maintenance is required to ensure BOPs will function reliably in the event of a well control emergency.

BOP Testing

BOPs are function and pressure tested at regular intervals, from daily to bi-weekly, as well as whenever they are made up into the stack3911. A typical surface BOP test consists of:

1. Low-pressure test to 250-300 psi to check for leaks
2. High-pressure test to rated working pressure for 5-15 minutes
3. Ram sequencing test to verify each component operates properly

Subsea BOPs undergo similar testing, with additional testing of the control pods, autoshear systems, acoustic emergency functions, and ROV intervention capabilities. Testing frequency is determined by local regulations and industry standards such as API Standard 539.

BOP Maintenance and Inspection

Detailed maintenance and inspection is performed whenever a BOP stack is pulled out of service, such as between wells or during shipyard maintenance. Some key maintenance activities include:

• Complete disassembly, cleaning and visual inspection of components
• Non-destructive examination (NDE) such as magnetic particle or ultrasonic inspection
• Pressure testing of seals, valves and hydraulic circuits
• Repair and refurbishment of rams, seals, hydraulics and structural components
• Verification of shear ram sealing capabilities
• Recertification and documentation of inspection and testing

BOPs are also monitored for wear, leaks and other issues during operations. Seal leakage, hydraulic fluid cleanliness, and ram wear are checked regularly. Subsea BOPs are remotely monitored for pressure and fluid levels in the control pods and functions are routinely operated from the surface controls.Technician performing maintenance on a BOP ram
Technician performing inspection and maintenance on a BOP ram17

BOP Control Systems

The BOP stack is operated by a control system that provides pressurized hydraulic fluid to actuate the functions of the preventers, valves and connectors. The control system allows the well to be rapidly shut in from a safe location, even if power or communication to the BOP is lost.A typical surface BOP control system consists of12:

• Electric, air-operated or mixed power hydraulic pumps
• Fluid reservoir and manifold with regulators and valves
• Accumulator bottles that store pressurized fluid for emergency operation
• Remote control panels at the driller’s station and other locations
• Pilot-operated control valves that direct fluid to operate functions

Subsea BOP control systems are more complex due to the distance between the surface and seafloor. They use multiplex (MUX) electro-hydraulic controls consisting of112:

• Redundant subsea control pods with electronics and hydraulic components
• MUX cables that transmit electrical signals between the pods and surface
• Surface hydraulic power unit and accumulator banks
• Operator stations that communicate with subsea pods via the MUX cables
• Subsea functions such as ram locks, ROV intervention panels and autoshear

The subsea control pods perform the logic to operate the BOP functions via solenoid valves that send hydraulic pressure to the selected component. The pods are the “brain” of the subsea BOP controls and are designed for high reliability and redundancy.Diagram of a MUX BOP control system
Diagram showing the main components and operation of a subsea MUX BOP control system1

Regulations and Standards

The design, manufacture, operation and maintenance of BOPs is governed by various regulations and industry standards to ensure safety and reliability. In the U.S., the Bureau of Safety and Environmental Enforcement (BSEE) regulates offshore drilling operations and has established detailed requirements for BOP systems in the Code of Federal Regulations (CFR).Some key U.S. regulatory requirements include39:

• 30 CFR 250.730: BOP system requirements
• 30 CFR 250.737: BOP testing requirements
• 30 CFR 250.739: BOP maintenance and inspection requirements

The American Petroleum Institute (API) also publishes several standards related to BOPs that are widely followed by the industry:

• API Standard 53: Blowout Prevention Equipment Systems for Drilling Wells9
• API Spec 16A: Specification for Drill-through Equipment414
• API Spec 16D: Specification for Control Systems for Drilling Well Control Equipment12
• API Standard 16AR: Standard for Repair and Remanufacture of Drill-through Equipment17

Other key standards include:

• NORSOK D-001: Drilling Facilities
• NORSOK D-010: Well Integrity in Drilling and Well Operations
• ISO 13533: Petroleum and Natural Gas Industries – Drilling and Production Equipment

These regulations and standards specify requirements for BOP design, manufacture, testing, operation and maintenance to ensure the equipment is fit for purpose and that personnel are properly trained. Compliance with these standards is critical for safe and reliable drilling operations.

The Future of Blowout Preventers

As drilling moves into deeper waters and more challenging environments, the demands on BOP systems will continue to increase. Some key trends and developments in BOP technology include:

• Increased redundancy: Additional shear rams, improved emergency systems, and more rigorous testing and maintenance to reduce failure risk.
• All-electric designs: Replacing hydraulic components with electric actuators and controls to improve reliability, response time and control10.
• Real-time condition monitoring: Sensors and software to provide early warning of degradation and improve maintenance planning5.
• Improved shearing capability: New shear ram designs and materials to reliably cut larger and stronger pipe under HPHT conditions.
• Compact designs: Smaller, lighter BOP stacks to reduce riser loads and adapt to slimmer well designs.

Despite these advancements, the basic function of a BOP remains the same – to serve as the last line of defense against a blowout. Robust design, rigorous testing and maintenance, and highly trained personnel will always be essential to ensure these critical safety devices can rapidly and reliably seal a well in the event of an emergency.

Conclusion

Blowout preventers are the unsung heroes of the oil and gas industry, standing guard day and night to protect rig crews, equipment and the environment. While we hope a BOP is never needed, their presence provides reassurance that a well can be quickly and safely shut in if primary well control is lost.By understanding the different types, functions and components of a BOP stack, as well as the key regulatory and maintenance requirements, the industry can ensure these vital safety systems are always ready to act as the last line of defense against a blowout.

References

https://en.wikipedia.org/wiki/Blowout_preventer[1] https://www.nov.com/-/media/nov/images/products/rig/bop_control_systems_operation.ashx?mw=900[2] https://www.arescotx.com/photos-videos/what-is-a-blowout-preventer/
3 https://en.wikipedia.org/wiki/Blowout_preventer[4] https://www.blowout-preventers.com/annular-blowout-preventers.php
5 https://dspace.lib.cranfield.ac.uk/bitstream/handle/1826/15997/Subsea_blowout_preventer_%28BOP%29-2020.pdf[7] https://www.keystoneenergytools.com/the