8.1 Ram-Type Blowout Preventers
Key Takeaways
Ram blowout preventers employ hydraulically driven, horizontally opposed forged steel ram blocks equipped with elastomeric packers to seal the wellbore against pressures up to 15,000–20,000 psi.
Rams seal pressure from below only; fixed pipe rams seal one pipe size and can hang off the string, variable bore rams (VBRs) seal a range of sizes; blind rams seal open hole without pipe; blind shear rams (BSR) sever drill pipe and seal the wellbore.
The open atmospheric telltale weep hole between the inner mud seal and outer hydraulic cylinder seal diagnoses leakage: drilling fluid indicates an inner mud seal failure, whereas hydraulic fluid indicates an operating cylinder seal failure.
Ram BOPs feature wellbore pressure assist, where upward wellbore pressure acts across the rear vertical area and beneath the ram blocks, exerting an inward and upward force that supplements the hydraulic closing force.
Ram locks (manual handwheels or automatic locking systems) are fitted so a closed ram stays closed if hydraulic closing pressure is lost, for example during a long shut-in, hang-off or subsea disconnect.
Ram-type blowout preventers (BOPs) serve as high-pressure mechanical barriers that seal the wellbore when primary well control—the drilling fluid hydrostatic column—fails. While annular preventers provide flexible initial closure on various tubulars, ram preventers deliver definitive metal-backed sealing and mechanical hang-off support capable of holding differential pressures up to 15,000–20,000 psi.
Ram BOP Construction and Operating Principles
A ram BOP consists of an API Specification 16A forged alloy steel body with a vertical bore and two horizontally opposed ram cavities. Each cavity houses a forged steel ram block, elastomeric packing elements, and a double-acting hydraulic operating cylinder. Bonnets housing the cylinders are secured via hinge pins or bolted flanges, swinging open for rapid inspection and ram replacement.
Hydraulic Actuation & Pressures
Hydraulic fluid from the accumulator operates the rams at 1,500 psi regulated manifold pressure:
- Closing Cycle: Fluid enters behind the operating piston, driving the piston rod and ram block horizontally inward until face seals meet at the wellbore center.
- Opening Cycle: Fluid is ported to the rod side of the piston, retracting ram blocks into bonnet cavities and clearing the wellbore drift.
- High-Pressure Shearing: Accumulator systems provide up to 3,000 psi via bypass valves or booster circuits to shear high-strength tubulars.
Sealing Direction
Ram preventers are designed to hold pressure from below (the wellbore side) only. Wellbore pressure acting under and behind the ram helps energise the seal; pressure applied from above does not, and may leak past. This is why rams must be installed the right way up, why BOP pressure tests are applied from below (through a test plug or cup tester), and why closed rams cannot be relied on to hold pressure pumped down on top of them.
Wellbore Pressure Assist
Modern ram BOPs feature hydrodynamic wellbore pressure assist. When closed, wellbore fluid enters the guideways behind and underneath the ram blocks. This fluid pressure acts across the rear vertical face of the ram blocks, exerting an inward force that supplements hydraulic closing force, while also driving the ram block upward against the upper cavity seal face. Consequently, as wellbore pressure rises during a kick, sealing integrity increases mechanically.
Classification of Ram Types
Ram preventers use specialized ram blocks matched to specific well control functions:
1. Fixed Pipe Rams
Machined with semi-circular, steel-reinforced elastomeric packers to pack off around one specific outer diameter (OD) of drill pipe, casing, or tubing (e.g., 5-inch drill pipe). Fixed pipe rams can support the weight of the string when a tool joint is landed on top of the closed rams (hang-off), up to the manufacturer's rated hang-off load, but they cannot seal on tool joints or different pipe sizes.
2. Variable Bore Rams (VBRs)
VBRs incorporate segmented steel reinforcement fingers molded within an elastomeric packer. As rams close, steel segments pivot inward like an iris diaphragm, supporting the elastomer under high differential pressure. A single set packs off across a range of tubular ODs (e.g., to ) and, in some designs, a hexagonal kelly. VBRs provide versatility for tapered strings, but exhibit lower hang-off ratings and shorter packing lifespans than fixed pipe rams.
3. Blind Rams
Blind rams (master rams) feature flat elastomeric face seals without a pipe recess, engineered solely to seal open hole when no tubulars are across the stack. Closing blind rams on drill pipe crushes the tubular without sealing, severely damaging ram blocks and leaving the well uncontrolled.
4. Blind Shear Rams (BSR)
Equipped with high-strength alloy shearing blades and integrated elastomeric seals. In a single stroke, blades sever drill pipe or tubing spanning the wellbore; the upper blade folds the lower pipe section downward, and an integrated seal on the upper blade engages the lower blade face to establish a pressure-tight wellbore seal. Standard BSRs cannot cut drill collars, heavy-weight drill pipe (HWDP) tool joints, or large casing.
When are they used? Blind/shear rams are closed when the well must be sealed and there is pipe across the stack that cannot be dealt with any other way, for example when other preventers have failed, or before an emergency disconnect on a floating rig. They cut the pipe inside the BOP and then close and seal the well. With no pipe in the hole they act as ordinary blind rams.
5. Casing Shear Rams (CSR)
Oversized shearing rams designed to cut large casing strings (e.g., or ) and heavy tubulars in subsea stacks. Certain CSR designs shear without sealing, requiring an adjacent blind ram for well shut-in.
| Ram Type | Sealing Target | Shearing Capability | Hang-Off Capacity | Typical Stack Position |
|---|---|---|---|---|
| Fixed Pipe Ram | Specific pipe OD (e.g., 5" DP) | None | High (manufacturer rating) | Varies with stack design |
| Variable Bore Ram (VBR) | Range of tubular ODs (3.5"–5") | None | Moderate | Varies with stack design |
| Blind Ram | Open hole (no pipe) | None | Zero | Varies with stack design |
| Blind Shear Ram (BSR) | Cut pipe & seal open hole | Drill pipe & tubing | Moderate | Varies with stack design |
| Casing Shear Ram | Casing & heavy tubulars | Large casing / tubulars | Low / None | Subsea Stacks |
Bonnet Assemblies and the Diagnostic "Weep Hole"
The operating piston rod passes between the wellbore cavity and hydraulic cylinder through the bonnet assembly, isolated by two dynamic seal sets:
- Inner Mud Packings: Prevent abrasive, corrosive wellbore fluids (mud, gas, ) from leaking along the rod.
- Outer Hydraulic Packings: Prevent hydraulic fluid (1,500–3,000 psi) from escaping the operating cylinder.
Diagnostic Weep Hole (Vent Port)
An open atmospheric vent port—the weep hole—is drilled vertically through the bonnet between these two seal sets:
- Drilling Mud or Gas Leaking: Indicates failure of the inner mud packing seal. Wellbore fluid has escaped containment, but the hydraulic system remains uncompromised.
- Hydraulic Oil Leaking: Indicates failure of the outer hydraulic rod seal.
Warning
Never Bull-Plug a Weep Hole: A weeping hole must never be plugged or capped. Bull-plugging a leaking mud seal forces high-pressure wellbore fluid past the hydraulic rod packing into the operating system, which can cause hydraulic lock, burst hydraulic lines and loss of the preventer's function.
Ram Locking Mechanisms
Ram locks are fitted so that a closed ram stays closed even if hydraulic closing pressure is lost or bled off, for example during a long shut-in, while the string is hung off, or on a subsea stack after a disconnect.
1. Manual Handwheel Locks
Threaded mechanical lock screws extend horizontally from bonnet heads. Crew members rotate handwheels clockwise until locking shafts shoulder against the rear of operating pistons. Rig procedures normally require manual locks to be fully unlocked during normal operations, with the number of turns to lock and unlock known and posted, so the crew can lock rams quickly and confirm full travel.
2. Automatic Locking Devices
Many ram BOPs, and all subsea stacks, use locks that engage automatically when the ram closes and release only when opening pressure is applied. Designs differ by manufacturer (for example wedge-type locks behind the piston, or multi-position locks that can hold a variable bore ram at whatever position it closed), but the purpose is the same: the ram cannot drift open if hydraulic pressure is lost.
What does drilling fluid discharging from a ram BOP bonnet weep hole indicate, and what is the required operational action?
The hydraulic operating cylinder has ruptured; pump 3,000 psi hydraulic closing fluid immediately.
The inner mud packing seal has failed; the weep hole must remain open to prevent wellbore pressure from pressuring up the hydraulic system, and the seal must be scheduled for replacement.
The weep hole should be immediately bull-plugged with a steel high-pressure pipe plug to contain wellbore pressure inside the stack.
The ram blocks are not fully closed against the drill pipe; increase manifold regulator pressure to 2,000 psi.
Which type of ram-type preventer is engineered with internal segmented steel reinforcement fingers that allow it to pack off across a range of tubular outer diameters?
Fixed pipe rams
Blind shear rams
Variable bore rams (VBRs)
Casing shear rams
What is the primary operational hazard of closing standard blind rams when a drill string is present across the blowout preventer stack?
The drill pipe will be crushed or flattened without sealing the wellbore, severely damaging the ram blocks and compromising well integrity.
The closing chamber hydraulic pressure will instantaneously exceed 5,000 psi and rupture the accumulator bottles.
The blind rams will automatically cut the pipe cleanly, but the lower fish will drop to the bottom of the well.
The mud return flowline will freeze due to sudden Joule-Thomson expansion.
How does wellbore pressure assist contribute to the mechanical sealing efficiency of a closed ram blowout preventer?
It forces hydraulic fluid out of the opening chamber back into the accumulator manifold.
It automatically engages the mechanical manual handwheels from outside the BOP bonnet.
It cools the elastomeric packer elements, preventing thermal breakdown at high bottom-hole temperatures.
It acts across the rear vertical faces and underneath the ram blocks, exerting an inward and upward force that supplements the hydraulic piston closing force.
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