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BOP Stacks
A BOP stack is the complete assembly of annular and ram preventers, choke and kill valves, spools and connectors on one frame. A subsea stack splits into the lower stack and the lower marine riser package (LMRP), and the two usually change hands together. This Category covers surface and subsea BOP stacks, with or without their control equipment, in common bores from 13-5/8 to 21-1/4 in.
How a stack is built
A surface stack bolts to the wellhead under the rotary table of a jackup or platform. Annulars sit on top, ram preventers below, and the choke and kill outlets come off drilling spools or ram body side outlets. A subsea stack lands on the seabed wellhead and is built in two parts. The lower stack carries the wellhead connector, the ram preventers and the failsafe choke and kill valves. The LMRP carries one or two annulars, the control pods, the riser adapter and the LMRP connector. The LMRP can unlatch and leave the lower stack shut in on the well during an emergency disconnect.
Configurations
- Surface stacks, usually one annular over three or four rams, worked from an accumulator unit on deck
- Subsea stacks on discrete hydraulic controls, common in shallower water and older floaters
- Subsea stacks on MUX controls, standard on deepwater floaters
- Stacks with five, six or seven ram cavities, which leave room for dual shear rams plus test or casing rams
In US federal waters, 30 CFR 250.734 requires a subsea stack to carry at least one annular, two pipe rams and two shear rams. At least one shear ram must seal after shearing, and any non-sealing shear ram must sit below a sealing one. The stack also needs ROV intervention, autoshear and deadman functions, and an emergency disconnect sequence on a DP rig. An older four-ram stack with one shear ram does not meet this without another cavity.
Key specifications
| Parameter | Why it matters | Typical classes or units |
|---|---|---|
| Bore | Must clear the casing program and match the wellhead | 13-5/8, 18-3/4 or 21-1/4 in. (346, 476 or 540 mm) |
| Rated working pressure | Must exceed the maximum anticipated surface pressure | 5,000, 10,000, 15,000 or 20,000 psi (34.5 to 138 MPa) |
| Ram and annular count | Sets which rules and well plans it satisfies | Cavities above and below the outlets |
| Shear capability | Must cut the heaviest pipe in your string at MASP | Documented shear tests per pipe grade |
| Temperature and service | Elastomers and steels limit HPHT and sour work | API 16A temperature rating, NACE MR0175 trim |
| Height and weight | Must fit the BOP handling system, moonpool and carrier | ft, m, short tons, tonnes |
Standards and certification
API 16A covers the design, materials and testing of each preventer, connector and spool, and assigns a product specification level, with PSL 3G adding a gas test. API Standard 53 covers the assembled system, including test pressures, response times and the five-year major inspection. In US waters, 30 CFR 250.732 requires an independent third party to verify that the shear rams cut and then seal on the pipe you plan to run, with test results and calculations corrected for MASP. Section 250.737 sets pressure tests at least every 14 days, with a 21-day option where BSEE approves a health monitoring plan.
Failure modes and inspection
Ram cavities, seal seats and ring grooves wear and pit, especially after idle periods in seawater. Shear blades dull and lose their margin. Bonnet seals, operator pistons and ram locks leak. Annular packers fatigue with each stripping run. A survey on a used stack should include visual and dimensional checks against OEM limits, magnetic particle testing of high-stress areas, a stump test with low and high pressure holds, and function tests of every circuit with timed responses.
Documentation to ask for
- Data book with traceability from fabrication and certificates of conformity
- Shear test reports and the third-party shear verification
- Last major inspection report with its independent review
- Pressure and function test charts, with closing times
- Elastomer manufacture dates and storage history
- Equipment schematics showing any modifications or added cavities
Integration with the rig
Check the stack against the BOP carrier or skid, the moonpool opening, the gantry crane capacity and the rig's control system. A stack from a different design may need a new control system, new choke and kill hoses or a different LMRP adapter to suit your riser.
Refurbishment and recertification
Recertification strips each preventer, inspects it against OEM criteria, replaces elastomers and seals, and ends in a full pressure and function test. An unused stack from a cancelled newbuild skips service wear but still needs elastomers replaced if they have aged out. Well-known stack makers include Cameron, now part of SLB, NOV under the Shaffer name, and Hydril, now part of HMH. For single preventers, see annular and ram BOPs.
FAQ
Frequently asked questions
What documents should come with a used BOP stack?
A used BOP stack should come with its data book and fabrication traceability, certificates of conformity, shear test and verification reports, the last major inspection report and its repair history. Recent pressure and function test charts and elastomer dates show how much work it needs before it can be stump tested.
How many rams does a subsea BOP stack need?
In US federal waters, a subsea stack needs at least one annular, two pipe rams and two shear rams under 30 CFR 250.734. At least one shear ram must seal the wellbore after cutting. Other regulators set their own minimums, so check the rules where the rig will work.
How often must a BOP stack be pressure tested?
Under 30 CFR 250.737, a BOP must be pressure tested at installation and at least every 14 days, or every 30 days for blind shear rams. BSEE can approve a 21-day interval if you run a BOP health monitoring plan. API 53 sets its own test intervals for wells outside US waters.
Can a surface BOP stack be used subsea?
No, a surface stack lacks the subsea control pods, accumulators, ROV panels, autoshear and deadman systems, and pressure-balanced design that subsea work needs. Converting one is effectively a new build. A subsea stack can sometimes be used on the surface, with its own control system adapted.
What does a shear ram verification cover?
Shear verification shows that the shear rams cut the pipe, tubing and lines you will run and then seal, at the maximum anticipated surface pressure. Under 30 CFR 250.732 an independent third party reviews the test protocol, the results and the calculated shearing and sealing pressures.
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