Blowout Preventer Stack Product

Overview

A blowout preventer stack is the last mechanical barrier between a drilling rig and an uncontrolled release of formation fluid. It is a vertical column of large forged-steel valves bolted to the wellhead, each able to close the wellbore in a different way: the Annular Preventer squeezes shut around whatever is in the hole, the Pipe Ram Preventer preventers seal around a known pipe diameter, and the Blind Shear Ram cuts the pipe entirely and seals the open bore. A surface stack for a land rig may weigh 30 t; a deepwater subsea stack with its riser package reaches 400 t and stands 16 m tall.

During normal drilling the stack is wide open and drilling fluid circulates through it unimpeded. When the well "kicks" — formation pressure pushes gas or oil into the wellbore faster than the mud column can hold it back — the driller closes a preventer within seconds. The well is then shut in, and the influx is circulated out under controlled pressure through the Choke and Kill Outlets outlets while heavy mud is pumped down the Kill Line.

How it works

Every preventer in the stack is hydraulically operated. The Annular Preventer uses a conical Annular Piston that rises inside the Annular Body and wedges the Packing Element — a steel-reinforced rubber donut — radially inward. Because the rubber conforms to any shape, the annular can seal on drill pipe, tool joints, casing, wireline, or the open hole, and it can even allow pipe to be stripped through it under pressure.

Ram preventers are more positive. Two opposed Ram Block assemblies slide horizontally on Ram Operating Piston rods until their elastomer Ram Packer faces meet around the pipe. Pipe rams are bored for one diameter; variable-bore rams cover a range. Wellbore pressure acting on the back of the rams adds to the sealing force, so a closed ram holds better as pressure rises. The hinged Ram Bonnet doors open for ram changes between well sections.

The Blind Shear Ram is the emergency option. Its hardened Upper Shear Blade and Lower Shear Blade overlap as they close, flattening and cutting the drill pipe body; tandem Shear Booster Piston cylinders raise the closing force to the 1.5–3 MN needed to shear modern S-135 pipe. After the cut, the Shear Seal Packer closes the bore completely, abandoning the lower pipe string in the hole.

Control system

Speed matters in well control, so hydraulic energy is stored rather than pumped on demand. The Hydraulic Accumulator System bank holds water-glycol fluid pressed to 3,000–5,000 psi by nitrogen precharge behind each Accumulator Bladder. API Standard 53 requires enough usable volume to close, open, and close every preventer with pumps dead, and to close the annular within 45 seconds. Precharge in each Accumulator Bottle is checked through its Nitrogen Charging Valve and set near 1,000 psi.

Subsea stacks carry two fully redundant Control Pod packages, conventionally painted blue and yellow. An electrical command from the rig travels down a multiplexed umbilical to a Pilot Solenoid Valve, which ports pilot pressure to the corresponding SPM Valve; that valve in turn directs main hydraulic power to the chosen function. Shuttle Valve elements at every function accept fluid from whichever pod fires first, so a single pod failure never disables the stack. Deadman and autoshear circuits close the shear rams automatically if both communication and hydraulics from the surface are lost.

Outlets, connectors, and testing

Below each ram cavity, flanged side outlets carry Failsafe Gate Valve pairs — gate valves held open by hydraulic pressure and spring-closed on failure — feeding the Choke Line and Kill Line. All flanged joints use API 6A metal ring gaskets on Outlet Flange faces; elastomer seals appear only where movement requires them.

The Wellhead Connector latches the stack onto the subsea wellhead with a hydraulically driven collet that preloads the hub to several meganewtons, and the Riser Connector lets the upper riser package pull away in an emergency disconnect while the lower stack stays sealed on the well. The whole column hangs in the Stack Frame, guided onto the hub by the Guide Funnel during landing.

Regulation keeps the stack honest: every function is exercised weekly, and the full stack is pressure-tested against a test plug at least every 21 days, first at low pressure (250–350 psi) to prove the seals seat, then at rated or maximum anticipated pressure. Packing elements, ram rubbers, and bonnet seals are routine consumables replaced between wells.