BOP Accumulator Unit (Koomey): How It Works, Step by Step, with Daily Checks and the Drawdown Test
Key takeaways
- The accumulator unit stores hydraulic fluid under nitrogen pressure so the BOP can be closed in seconds, even if rig power is lost.
- Typical surface system: 3,000 psi in the bottles, 1,000 psi nitrogen precharge, 1,500 psi on the ram manifold and a separately adjusted annular pressure.
- Usable fluid is the volume delivered between operating pressure and 200 psi above precharge. Worked example: a 10 gal bottle at 1,000 / 3,000 / 1,200 psi gives 5 gal.
- The drawdown test (pumps off) must leave at least 200 psi above precharge; the pump test and recharge times are defined in API Standard 53.
- 4-way valves stay in OPEN or CLOSE, never in BLOCK, during operations. Only nitrogen is used for precharge.
Educational content only: procedures and operating values vary between companies and wells. Always follow your company’s approved procedures, your supervisor’s instructions and the permits in force at your site.
What is the accumulator unit and why does it matter?
The BOP stack is closed by hydraulic pressure. The accumulator unit, also called the BOP control unit or closing unit, is the system that stores that pressure and sends it to each function. Many people on the rig call it the "Koomey", after one well-known brand.
The key idea is stored energy. Pumps alone are too slow to close a large preventer in a few seconds. So the unit stores hydraulic fluid under pressure in bottles precharged with nitrogen. When the driller pushes CLOSE, this stored fluid moves to the BOP at once. If rig power or air is lost, the bottles can still close the well.
The main components
- Reservoir: a tank of control fluid at atmospheric pressure. The fluid is the one specified by the manufacturer, often a water-based fluid with additives (and glycol in cold weather) or a suitable hydraulic oil. Diesel or motor oil must never be used.
- Electric pump (often triplex): the main pump. A pressure switch starts it when accumulator pressure falls and stops it at operating pressure.
- Air pumps: a back-up power source driven by rig air, with their own pressure switch. Some units also have a nitrogen back-up bank.
- Accumulator bottles: steel pressure vessels, bladder type or float/piston type. The upper part holds nitrogen at the precharge pressure; the pumped fluid compresses the nitrogen and is stored under the same pressure.
- Manifold regulator: reduces accumulator pressure to the ram manifold pressure (commonly 1,500 psi) for rams and hydraulic valves such as the HCR.
- Annular regulator: a separate regulator for the annular preventer. Its setting depends on the preventer model, the pipe size and wellbore pressure, from the manufacturer's chart.
- Bypass valve: sends full accumulator pressure to the ram manifold when needed, for example to shear pipe with the blind/shear rams. It is normally closed.
- 4-way valves: one per function (annular, each ram, HCR). Three positions: OPEN, BLOCK (centre) and CLOSE.
- Gauges: accumulator pressure, manifold pressure and annular pressure, on the unit and repeated on the remote panels.
- Remote panels: the driller's panel on the rig floor and at least one more panel at a safe place, so the BOP can be operated away from the unit.
- Relief valves: protect the system against over-pressure.
How a closing sequence works, step by step
- In standby, the bottles are charged to operating pressure (for example 3,000 psi), the manifold shows 1,500 psi and the annular gauge shows its set pressure.
- The driller presses CLOSE for a function on the remote panel. The panel moves the 4-way valve on the unit to CLOSE, by air or electric signal.
- Regulated fluid flows to the closing side of the preventer, and fluid from the opening side returns to the reservoir.
- The accumulator gauge drops as fluid leaves the bottles, and the manifold gauge dips then recovers.
- When accumulator pressure falls below the pressure-switch setting, the electric pump (and, if needed, the air pumps) start and recharge the bottles.
- The pumps stop when operating pressure is reached. The crew confirms the function on the panel indicator and with the gauges.
Reading the gauges is a skill. If a function is operated and the accumulator pressure does not drop at all, the fluid may not have moved: suspect a blocked line, a valve in BLOCK or a panel fault. If pressure keeps falling and the pumps run continuously, suspect a leak or a function that did not seal.
Typical pressures in a 3,000 psi surface unit
| Item | Typical value | Note |
|---|---|---|
| Accumulator operating pressure | 3,000 psi | Rated working pressure of the unit |
| Nitrogen precharge | 1,000 psi (minimum for 3,000 psi bottles) | Checked before each well, adjusted to within 100 psi |
| Minimum pressure after functions | 1,200 psi | 200 psi above precharge |
| Ram manifold | 1,500 psi | Raised with the bypass only when required |
| Annular | Per manufacturer chart | Depends on preventer, pipe and well pressure |
These are common values described in API Standard 53 and industry training. Your unit's values are on its data plate and in the company procedures.
Usable fluid: Boyle's law with a worked example
Nitrogen in the bottle follows Boyle's law at constant temperature: P1 × V1 = P2 × V2. The usable fluid is the volume the bottle delivers between operating pressure and the minimum allowed pressure (200 psi above precharge).
Usable fluid per bottle = V bottle × (P pre ÷ P min − P pre ÷ P max)
Bottle volume 10 gal, precharge 1,000 psi, operating 3,000 psi, minimum 1,200 psi.
Gas volume at 3,000 psi = 10 × 1,000 ÷ 3,000 = 3.33 gal
Gas volume at 1,200 psi = 10 × 1,000 ÷ 1,200 = 8.33 gal
Usable fluid = 8.33 − 3.33 = 5.0 gal per bottle
If the closing volumes of the functions that must be operated add up to, say, 30 gal, the bank needs at least 30 ÷ 5 = 6 such bottles, before any extra factor required by the standard or the company. The closing volumes come from the manufacturer's data for each preventer.
A low precharge reduces usable fluid. For example, a bottle with no precharge at all stores almost nothing useful. This is why precharge is checked before each well.
Tests the technician must know
1. Accumulator drawdown test
- Record accumulator, manifold and annular pressures, and confirm the bottles are at operating pressure.
- Stop and isolate the pumps (electric and air).
- Operate the functions required by the test procedure, typically close the annular, close a pipe ram and open the HCR, with pipe in the hole as the procedure specifies.
- Record the closing time of each function and the final accumulator pressure.
- Acceptance: final pressure at least 200 psi above precharge (1,200 psi in the example). Closing times within the limits, for a surface stack about 30 s for each ram and for annulars smaller than 18-3/4 in, and 45 s for annulars of 18-3/4 in and larger.
- Restore the pumps and record the time to recharge to operating pressure.
2. Pump capacity test
With the accumulator bottles isolated, the pumps alone must close the annular on pipe, open the HCR and reach 200 psi above precharge within 2 minutes. The pump system must also recharge the bottles from precharge to operating pressure within the time set by API Standard 53 (15 minutes for the main system).
3. Precharge check
Bleed the fluid of the bottle being checked back to the reservoir, then read the nitrogen pressure with a charging and gauging assembly. Add only nitrogen. Follow the manufacturer's procedure: never open a bottle that is still under fluid pressure.
Daily and shift checks
- Reservoir level correct and no water or debris in the fluid.
- Accumulator, manifold and annular gauges at their normal values, and the same readings on the remote panels.
- Electric pump in AUTO and air supply available; pumps start and stop at the correct pressures.
- 4-way valves in OPEN or CLOSE matching the actual BOP position, never in BLOCK.
- Blind/shear ram control protected with a cover or guard against accidental operation.
- No leaks on hoses, fittings or the manifold; hoses in good condition and secured.
- Function test results and times recorded as required by the company.
Common problems
| Symptom | Possible cause | First check |
|---|---|---|
| Pumps start very often | Leak, or a valve passing to the reservoir | Look for leaks; listen at the 4-way valves |
| Slow closing times | Low precharge, restricted line, low regulator setting | Precharge, regulator, hoses |
| Pressure drops but the BOP does not move | Line or connection fault at the stack | Hose connections, closing-side line |
| No pressure drop when operating | Valve in BLOCK, panel fault | Valve positions, panel air/electric supply |
| Unit pressure above normal | Pressure switch fault | Relief valve and switch, stop pump |
HSE: stored energy
- The unit is a pressure vessel system. Before any maintenance, isolate the pumps, bleed the bottles to zero through the correct valves and apply lock-out / tag-out.
- Nitrogen only. Never precharge with air or oxygen: mixed with hydraulic oil under pressure it can explode.
- Injection injuries. A pinhole leak at 3,000 psi can inject fluid through the skin. Never feel for a leak with your hand; use a piece of cardboard and get medical help at once if injured.
- Hoses. Secure high-pressure hoses and keep people out of the line of fire during tests.
- Authority. Do not operate any BOP function without the instruction of the driller or the supervisor, and announce every function test.
Summary for the technician
The accumulator unit stores energy so the BOP can close fast, with or without rig power. Know its three pressures (accumulator, manifold, annular), keep the 4-way valves in OPEN or CLOSE, check precharge with nitrogen only, and understand the drawdown test: after the required functions, at least 200 psi above precharge must remain.
Test your understanding
Glossary terms
Related lessons
- BOP Equipment: Types and Closure Methods
- What Is a BOP? Quick Introduction
- Kill Sheet Step-by-Step: Driller's Method or Wait and Weight?
- Warning Signs: How to Detect a Kick Early
Sources
- API - Standards catalogue (Standard 53: well control equipment systems for drilling wells)
- IADC - API Standard 53 conformance assessment tool (accumulator, pump and closing-time requirements)
- NOV - BOP control unit (product flyer)
- Drilling Formulas - 4-way valve operation in the accumulator (Koomey) unit
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