What are the types of annular BOP?

The commonly used types of annular BOP in the field can be classified into conical, spherical, and combination types based on the shape of their sealing element.
(1) Conical Element Annular Blowout Preventer
The main components of a conical element annular BOP include a piston, a body, a top cap, and a support sleeve. The top cap is threaded to the body, and the support sleeve is bolted to the lower step of the body. The elastomeric element sits on the support sleeve. The upper inner cavity of the piston is in an inverted conical shape, matching the conical element. A dust seal is installed between the body and the top cap, forming the upper hydraulic chamber with the piston shoulder. The lower hydraulic chamber is formed between the piston shoulder and the body shoulder. Both chambers are connected to the hydraulic control system via oil pipe fittings.
The elastomeric element is conical in shape with a vertical bore slightly larger than its nominal diameter to facilitate drill string passage. It's reinforced with evenly distributed radial support ribs vulcanized into the rubber, forming a rigid skeleton to bear radial and axial loads and control uniform deformation of the element.
During well closure, hydraulic pressure drives the piston upward, compressing the element. The element contracts radially, with the support ribs converging and the rubber between them being forced towards the wellbore, creating an initial seal. Well pressure acting on the piston's lower surface further drives it upward, enhancing the sealing performance.
The conical element has a large rubber volume, allowing all rubber between the ribs to be squeezed towards the wellbore for sealing. This excess volume enables it to seal around various shapes and sizes of drilling tools or fully close an empty wellbore.
As the element wears, the piston stroke must increase to extrude more reserve rubber. When the piston reaches its maximum stroke or the support ribs' upper and lower ends meet, the element's reserve rubber is depleted, and reliable sealing becomes impossible even with increased hydraulic pressure. Thus, element lifespan can be monitored by measuring piston stroke.
(2) Spherical Element Annular Blowout Preventer
Spherical element annular BOPs are designed for offshore drilling and can be installed in subsea equipment or on drilling vessels. When sealing around the drill string, the spherical element can slide up and down with wave motion, and its ball-joint design allows the drill string to tilt in any direction with minimal impact on sealing performance. This design offers easier control and extended element lifespan.
The main components of a spherical element annular BOP include a top cap, spherical element, dust seal, piston, and body. The top cap, with a hemispherical inner surface, is bolted to the body flange (connection methods include flange bolting or wedge-block locking). The element sits on a "Z"-shaped piston. The body has two oil pipe fittings connected to the hydraulic control system. The upper hydraulic chamber is formed by the dust seal, body, and piston shoulder, while the lower chamber is between the piston shoulder and body shoulder. The top connection of spherical element annular BOPs is typically threaded, while the bottom connection can be flanged, threaded, or clamp-type.
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