The annular preventer, also known as an annular BOP, is a large, donut-shaped device installed at the top of the BOP stack. It uses a rubber sealing element reinforced with steel, known as a packing unit, which is hydraulically compressed to form a tight seal around the drill pipe or completely across the open wellbore. This action prevents the uncontrolled escape of formation fluids—such as oil, gas, or water—that could lead to a blowout.
Unlike ram-type BOPs, which require specific-sized rams to seal around different diameters of pipe, the annular preventer can accommodate a wide range of pipe sizes and shapes. This adaptability is one of its most significant advantages.
Functions of Annular Preventers are as follow;
Sealing the Annular Space
One of the primary functions of the annular preventer is to seal the annular space—the area between the outside of the drill string and the inside of the wellbore or casing. By forming a pressure-tight barrier in this space, the annular BOP prevents formation fluids from migrating to the surface. This is critical during drilling operations when maintaining control over subsurface pressures is essential.
Versatile Sealing Capability
A standout feature of the annular preventer is its ability to seal around various sizes and shapes of tubulars. While ram BOPs are limited by the fixed size of the ram blocks installed, annular preventers can seal around irregular shapes such as tool joints, the Kelly, and even square or non-cylindrical objects. This versatility is crucial during complex drilling operations where the drill string configuration frequently changes.
This flexibility means that the annular preventer can be engaged at nearly any point during drilling without needing to swap out components to fit a specific diameter. As a result, it offers a faster response in emergency scenarios and greater operational efficiency overall.
Full-Bore Closure
Another significant advantage of the annular BOP is its ability to completely seal the wellbore when no pipe is present. This function, often referred to as “full-bore closure,” allows operators to shut in the well during tripping operations—when pipe is being run into or pulled out of the hole—or during emergencies. This capability is essential for maintaining well control even when the drill string is not in place.
Stripping Operations
Stripping is a procedure in which the drill string is slowly moved in or out of the wellbore while maintaining a seal to contain well pressure. With careful application of hydraulic pressure, the annular preventer allows for this controlled movement while still keeping the well sealed. This function is particularly important when the well is under pressure and normal pipe movement could compromise well control.
Through this method, operators can continue necessary operations such as removing or installing the drill string without opening the wellbore to potentially hazardous formation fluids. This ability to “strip” pipe through the annular preventer under pressure highlights its importance during complex well interventions.
Enhancing Overall Safety
The annular preventer acts as a vital safety device in every drilling operation. Often serving as the first line of defense, it can be engaged quickly and effectively in various scenarios, from routine drilling to emergency situations. It works in conjunction with ram-type BOPs to create a comprehensive well control system capable of sealing off the well under almost any condition.
By offering rapid sealing capabilities, adaptability to different pipe sizes and shapes, and the ability to close off an open hole, the annular preventer significantly reduces the risk of blowouts. Its role is not only technical but also strategic—it enhances operational readiness and safety compliance on drilling rigs worldwide.
