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OIL & GAS EQUIPMENT

Koomey Unit: The Complete Guide to Accumulator Units in Drilling Operations

Everything you need to know about the Koomey Unit — the heart of BOP control systems and the last line of defense against blowouts.

In the oil and gas drilling industry, safety is everything — and few pieces of equipment embody that principle more than the Koomey Unit. Named after Koomey (Koomey Accumulators Inc.), the manufacturer that pioneered this technology, this hydraulic accumulator unit is the power source that keeps Blowout Preventers (BOPs) ready to seal the well at any moment.

Koomey accumulator unit with red nitrogen bottles on a drilling site

Fig 1: A typical Koomey accumulator skid with nitrogen bottles, control manifold and hydraulic hoses

What Is a Koomey Unit?

A Koomey Unit — also called a BOP accumulator unit or hydraulic control unit — is a skid-mounted system that stores pressurized hydraulic fluid (oil) and delivers it on demand to operate the BOP stack.

Think of it as a hydraulic battery bank: it stores energy in nitrogen-charged accumulator bottles and releases it instantly to close rams, shear pipes, or seal the annular space — even if rig power is completely lost.

🔑 Key Fact: The Koomey Unit must have enough stored energy to close every ram and the annular BOP once, with a minimum pressure reserve remaining — even with all rig pumps shut down.

Main Components of a Koomey Unit

1. Accumulator Bottles

The most recognizable part — the row of red bottles seen in the image. Each bottle contains:

  • A nitrogen pre-charge (typically ~1000 psi)
  • A bladder that separates nitrogen from hydraulic oil
  • Hydraulic oil pressurized up to 3000 psi (or 5000 psi in HP systems)

Standard bottle sizes are 80 US gallons or 120 US gallons of usable oil volume.

2. Triplex Pumps

Used to recharge the accumulators to full pressure:

  • Air-operated pump — runs on rig air, works without electricity (critical during power failure)
  • Electric pump — the primary charging unit during normal operations

3. Reservoir (Storage Tank)

Holds the hydraulic fluid supply, usually sized at least 2× the total accumulator volume.

4. Control Manifold

The brain of the unit. Includes:

  • Four-way valves for open/close control of each BOP function
  • Pressure regulators (manifold pressure & annular pressure)
  • Pressure gauges — accumulator, manifold, and annular readings
  • Relief valves for overpressure protection

5. High-Pressure Hoses

Heavy-duty hydraulic hoses connecting the unit to the BOP stack — clearly visible in the photo, coiled along the front of the skid.

How Does a Koomey Unit Work?

The working principle is based on compressed nitrogen energy storage:

  • Step 1: Pumps push hydraulic oil into the bottles, compressing the nitrogen bladder.
  • Step 2: Oil pressure rises and stabilizes at the working pressure (e.g., 3000 psi).
  • Step 3: When an operator opens a four-way valve, the stored pressure pushes oil instantly to the BOP piston.
  • Step 4: The ram closes and seals the well in seconds.
  • Step 5: Pumps automatically recharge the system back to full pressure.
💡 Why nitrogen? Nitrogen is used because it's inert, non-flammable, doesn't react with hydraulic oil, and compresses predictably — making it ideal for safe high-pressure energy storage.

Why the Koomey Unit Is Critical for Well Safety

The Koomey Unit is the last line of defense against a blowout. Its importance comes from three key roles:

  • Instant Response: Closing a BOP takes seconds — stored hydraulic energy makes this possible without waiting for pumps to build pressure.
  • Redundancy: With air-operated pumps, the unit works even during a total rig power failure.
  • Regulatory Compliance: API 53 and international drilling standards mandate accumulator capacity sufficient to close all BOP functions with pressure to spare.

Simply put: if the Koomey Unit fails, the well is unprotected. That's why pre-charge tests, pressure monitoring, and weekly accumulator function tests are mandatory on every rig.

Technical Specifications

ParameterTypical Value
Working Pressure3000 psi (HP systems: 5000 psi)
Nitrogen Pre-Charge1000 psi (1/3 of working pressure)
Bottle Capacity80 or 120 US gallons each
Bottle Quantity4 – 20+ (depends on BOP stack size)
Manifold Pressure1500 psi (for ram-type BOPs)
Annular Pressure500 – 1500 psi (adjustable)
Pump TypeAir-operated + Electric triplex pumps
Reservoir Size≥ 2× total accumulator volume
StandardsAPI 16A / API 53

Operation & Maintenance Essentials

To keep a Koomey Unit reliable, rig crews follow strict procedures:

  • ✔ Daily: Check accumulator, manifold, and annular pressures.
  • ✔ Weekly: Function test all BOP rams and the annular from the accumulator unit.
  • ✔ Monthly: Verify nitrogen pre-charge pressure in each bottle (drift indicates bladder leakage).
  • ✔ Annually: Inspect bladders, test relief valves, and recalibrate gauges.
  • ✔ Critical Rule: Never open accumulator bottles to atmosphere without bleeding pressure first — explosion risk.

Frequently Asked Questions

Why is it called a "Koomey" unit?
The name comes from Koomey Accumulators Inc., the original manufacturer that popularized this design in the drilling industry. Today, "Koomey" is used generically for any BOP accumulator unit.
How much pressure does a Koomey unit store?
Standard units operate at 3000 psi, while high-pressure systems work at 5000 psi. Bottles are pre-charged with nitrogen at 1000 psi.
Can a Koomey unit work without electricity?
Yes. The stored nitrogen energy closes the BOPs instantly, and the air-operated pump can recharge the system using rig air — making the unit fully functional during a total power failure.
How many accumulator bottles are needed?
It depends on the BOP stack. The rule (API 53) requires enough usable oil to close every ram once, close the annular, plus open one HCR valve — all with at least 200 psi above minimum closing pressure remaining.
What are the red bottles in the photo?
They are accumulator bottles — each one holds a nitrogen-charged bladder separated from hydraulic oil. Their bright red color is a safety standard marking high-pressure equipment.

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