Ram Blowout Preventer: Definition, Types, and Well Control Applications

What Is a Ram Blowout Preventer?

A ram blowout preventer (BOP) is a hydraulically actuated pressure-control device mounted on the wellhead that closes across the wellbore to contain formation fluids during a well control event. Opposing ram blocks driven by large hydraulic cylinders travel inward to seal against the pipe or open hole, preventing an uncontrolled blowout.

How a Ram BOP Works

Each ram BOP consists of a heavy-steel body (the ram housing or ram block cavity), two opposing ram blocks that sit in horizontal bores on either side of the wellbore centerline, hydraulic actuating cylinders attached to the outer face of each ram block, and a bonnet assembly that closes the cylinder bore and carries the locking mechanism. At rest, the rams are fully retracted and the wellbore is open. When the driller or well control personnel actuate the BOP, hydraulic fluid from the accumulator unit pressurizes the closing side of each cylinder. The rams travel inward along precision-machined guides until the packing elements on the face of each ram compress against the pipe or against the opposing ram face, forming a pressure-tight seal. Sealing forces for a 10,000 psi (690 bar) working pressure BOP can exceed 500,000 lbf (222 kN) per ram block.

Locking mechanisms prevent the hydraulic pressure from backing off once the rams are closed, ensuring the well remains sealed even if the hydraulic supply line is lost. Most modern ram BOPs use a combination of a mechanical hand wheel lock (threaded rod that engages into the back of the ram block) and an automatic lock that engages as the ram travels to the closed position. Opening the rams requires first releasing the lock and then applying hydraulic pressure to the open side of the cylinder. Per API Specification 16A (fourth edition, 2017), a fully rated ram BOP must open and close within 30 seconds under full working pressure, and the hydraulic accumulator system designed to API Specification 16D must be able to complete all required closing functions with no recharge from the rig power supply.

The wellbore pressure acts on the face of the closed ram blocks and is transmitted to the ram housing through the bonnet bolts, which are heavy-hex or studded fasteners sized for the working pressure rating. Elastomeric packing elements in the ram blocks provide the primary seal; these elements are manufactured from nitrile (NBR), hydrogenated nitrile (HNBR), or fluoroelastomer compounds selected for compatibility with the wellbore fluid, temperature, and H2S partial pressure. Under ISO 13533, ram BOPs are classified by bore size (typically 7-1/16 in. to 21-1/4 in. / 179 mm to 540 mm), working pressure, and temperature rating (low, standard, or high), with material class designations (AA, BB, CC, DD, EE, FF) specifying the corrosion-resistant alloy content required for sour service.

Ram Types and Their Applications

Blind Rams

Blind rams carry flat or slightly concave rubber packing on their face and are designed to seal the wellbore when no tubular is in the hole. They do not cut; if drillpipe is present and blind rams are closed, the pipe will prevent the faces from meeting and the well will not be sealed. Blind rams are installed as the uppermost ram set in many stack configurations and serve as a secondary barrier when the drill string has been pulled clear of the BOP. They are also used as test rams to pressure-test the BOP stack from below.

Pipe Rams

Pipe rams carry a semi-circular cutout in the packing face, sized to a specific pipe outer diameter. Common pipe ram sizes in the industry include 3-1/2 in. (88.9 mm), 4-1/2 in. (114.3 mm), 5 in. (127 mm), 5-1/2 in. (139.7 mm), and 6-5/8 in. (168.3 mm) among others, always referencing API tubular OD. When closed, the rubber packing wraps around the pipe circumference and the rams bear against each other at the top and bottom of the cutout, creating a seal that holds wellbore pressure while the pipe string remains in tension. Pipe rams are the workhorse of well control operations: they are closed first when a kick is detected with pipe in the hole, and the well is then circulated out under controlled conditions with the rams holding pressure.

Variable Bore Rams

Variable bore rams (VBRs), sometimes called multi-bore rams, use a thick, deformable elastomeric packing element that can conform to a range of pipe ODs, typically spanning about 3-1/2 in. to 7 in. (88.9 mm to 177.8 mm) in a single set of rams. This flexibility is valuable on wells with mixed tubular programs or when the pipe size in the hole is uncertain. VBRs come with a trade-off: because the packing must deform substantially to accommodate small pipe, the sealing force per unit area decreases at the smallest pipe size end of the range, and VBRs are generally rated at slightly lower pressure-holding capacity than fixed-bore pipe rams of equivalent class. On wells where multiple casing strings, production tubing, and drillpipe may need to be sealed against successively, VBRs reduce the number of ram changes required.

Blind Shear Rams (BSRs)

Blind shear rams, also called shear blind rams, integrate cutting blades on the ram face that sever the drillstring when the rams close, then continue inward to seal the open wellbore. They represent the last-resort closure option: used when all other well control methods have been exhausted and the well must be closed regardless of what is in the hole. Shearing a standard 4-1/2 in. (114.3 mm) S-135 grade drillpipe requires approximately 1,200,000 to 1,500,000 lbf (5,340 to 6,672 kN) of cutting force, which must be delivered by the hydraulic actuator cylinders within the BSR. This requirement drives the BSR to be physically larger and to require greater hydraulic volume from the accumulator than any other component in the stack. BSRs cannot reliably cut tool joints (the thicker threaded connectors between drillpipe joints), heavyweight drillpipe, or drill collars; stack design must position the BSR such that a tool joint is unlikely to be at the BOP when emergency closure is needed. Post-Macondo requirements under the U.S. Bureau of Safety and Environmental Enforcement (BSEE) 30 CFR Part 250 mandate that BSRs be function-tested under actual wellbore conditions and that operators demonstrate the rams can shear the specific tubulars in use on a given well.

Casing Shear Rams

Casing shear rams are heavy-duty shearing rams designed to cut casing strings and large-diameter tubulars rather than drillpipe. They are primarily used in well abandonment operations and in emergency situations where casing must be severed at the wellhead. Casing shear rams are not typically part of a standard drilling BOP stack but may be used in completion and workover BOP configurations. Because casing walls are thicker and harder than drillpipe, casing shear rams require higher hydraulic actuation forces and carry blade geometries optimized for larger cross-sections.

BOP Stack Configuration

A complete blowout preventer stack for an onshore or shallow-water well is assembled from the bottom up in a sequence that provides multiple independent barriers and allows the maximum flexibility during well control operations. A typical configuration on a high-pressure Montney or Duvernay HPHT well in western Canada begins at the casing head with a casing ram sized for the intermediate casing string below, followed by a blind/shear ram set, then two independent pipe ram sets (often a fixed-bore pipe ram below and a VBR above), and finally an annular BOP at the top. This layout ensures that after a kick is detected with the drillstring in the hole, the driller can close the pipe rams with the string in place, observe the shut-in drill pipe pressure and casing pressure to diagnose the kick, and then circulate the influx out using the kill line and choke manifold without opening the BOP.

On subsea deepwater wells, the BOP stack is mounted on the subsea wellhead at the mudline and connected to the drillship or semi-submersible via the drilling fluid riser. Subsea stacks are substantially more complex: they typically carry six or more rams (two sets of pipe/VBR rams, two BSR sets, and casing shear rams), operate at working pressures of 15,000 psi (1,034 bar) for most modern deepwater applications, and must be operable by remotely operated vehicle (ROV) in the event of a loss of hydraulic or electronic control from the surface. Emergency acoustic shut-in systems allow the stack to be closed from the surface or from a rescue vessel even if the umbilical is severed. The deadman system, introduced after Macondo, automatically closes the BSRs if the stack detects a loss of all communication from the surface for a defined period. Subsea BOP stacks for ultra-deepwater wells in the U.S. Gulf of Mexico (GoM), the Norwegian North Sea, Australian offshore basins, and Brazilian pre-salt fields all operate under jurisdiction-specific regulations that overlay the baseline API/ISO requirements.