A blowout preventer is only useful if it can be closed fast when a well kicks, and that has to be true even if the rig loses electrical power or the hydraulic pumps fail at the worst moment. The BOP accumulator, widely known by the trade name Koomey unit, is the bank of pressurized bottles that stores the hydraulic energy needed to operate the BOP stack on demand. This guide explains how nitrogen-precharged accumulator bottles store and release that energy, how the usable volume and closing time are sized, and why the closing unit's pressures are treated as safety-critical values to be monitored.
BOP Accumulator (Koomey Unit) in one line: A BOP accumulator, commonly called a Koomey unit or a closing unit, is a system of hydraulic bottles precharged with nitrogen gas that stores pressurized hydraulic fluid ready to operate the blowout preventer. Because the stored energy is in the bottles, the BOP can be closed even if rig power and the charging pumps are unavailable. The system is sized so its usable fluid volume can close and open the required functions within a set closing time, and its precharge and system pressures are monitored continuously as a safety-critical part of well control.
An accumulator bottle stores energy by compressing a gas. Each bottle is charged with nitrogen to a set precharge pressure before hydraulic fluid is added, with the gas and the fluid separated by a piston, bladder, or float depending on the design. When the system pumps push hydraulic fluid into the bottle, they compress the nitrogen further, so the bottle ends up holding fluid at high pressure with the compressed nitrogen acting like a spring behind it. When a BOP function is operated, that compressed nitrogen pushes the stored fluid back out at pressure to close a ram or annular preventer - no pump action required at the instant of use.
The reason this design is used, rather than relying on a pump alone, is response and reliability. Closing a BOP quickly demands a large slug of hydraulic fluid delivered almost instantly, far faster than pumps could supply, so the accumulator acts as the energy reservoir that discharges on demand while the pumps merely recharge it afterward. Just as important, the stored energy is independent of external power: if the rig loses electricity or the charging pumps fail during a well control event, the nitrogen-driven fluid in the bottles can still operate the stack. That fail-safe stored energy is the whole point of a Koomey unit.
Not all the fluid in an accumulator is usable. As fluid is drawn off, the nitrogen expands and the system pressure falls, and the pressure can only fall so far before it is too low to operate the preventers reliably. The usable volume is therefore the fluid that can be delivered between the fully charged pressure and a minimum operating pressure, above the precharge; fluid below that point cannot do useful work. Sizing an accumulator system means providing enough bottles that this usable volume covers the functions that must be operated, with margin, from the stored energy alone. Because nitrogen pressure changes with temperature, the precharge is set for the expected conditions, which matters on subsea stacks where the bottles sit in cold water.
The other half of sizing is speed. Well control standards define how quickly the preventers must close - the ram and annular functions must actuate within specified closing times - and the accumulator plus the closing lines must be able to deliver that. Systems are typically required to close the essential functions and retain adequate pressure using only the accumulators, without the charging pumps running, so the design assumes the worst case of no pump support. The unit also includes the charging pumps, usually both electric and air-driven for redundancy, that recharge the bottles between operations and maintain system pressure, along with regulators that set the operating pressure delivered to the annular and ram preventers, which often differ.
Because the accumulator is the last line of hydraulic energy for well control, its condition is watched closely. The key pressures are the accumulator system pressure, which shows how much stored energy is available, the manifold and regulated operating pressures supplied to the preventers, and the nitrogen precharge on the bottles, which is checked periodically because a bottle that has lost precharge cannot store its share of energy. The charging pumps' cut-in and cut-out behavior is monitored too, since pumps that fail to maintain pressure quietly erode the reserve. Well control regulations require regular testing and function tests of the whole closing unit, and the recorded pressures are the evidence that the system will perform when called upon.
These pressures are increasingly instrumented and trended rather than only read from local gauges. Continuous monitoring of accumulator and manifold pressure can reveal a slow leak, a drifting regulator, or a pump that is cycling too often long before it becomes a well control problem. A cloud SCADA platform such as Merobix, which reads live pressure and status tags from field equipment over protocols like Modbus and MQTT and presents them as trends and alarms in browser dashboards, is the kind of layer that lets a rig supervisor or an engineer in town watch closing-unit pressures continuously and be alerted if system pressure sags or precharge appears low. The accumulator itself provides the fail-safe stored energy at the well; monitoring turns its critical pressures into an early warning rather than a surprise discovered during a function test.
The nitrogen precharge is what stores the energy. When hydraulic fluid is pumped into a bottle it compresses the nitrogen, which then acts like a spring and pushes the fluid back out at pressure when a BOP function is operated. Nitrogen is used because it is inert and stable, and the compressed gas lets the accumulator deliver a large slug of fluid instantly, far faster than a pump could, and without needing external power at the moment of use.
Usable volume is the amount of hydraulic fluid an accumulator can deliver between its fully charged pressure and the minimum pressure at which the preventers still operate reliably. Fluid remaining below that minimum cannot do useful work because the nitrogen pressure is too low. Sizing an accumulator system means providing enough bottles that this usable volume, above the precharge, covers the required functions with margin from stored energy alone.
Koomey unit is a common trade name for the BOP closing unit - the skid of nitrogen-precharged accumulator bottles, charging pumps, regulators, and control manifold that stores and delivers the hydraulic energy to operate a blowout preventer. The name has become a generic term in the field for any BOP accumulator and control system, regardless of manufacturer.
This page references the protocol specifications published by the organizations below. Editions, product capabilities, and documentation change over time - confirm current requirements and specifications directly with the source.
Last reviewed: July 27, 2026. Merobix is not affiliated with, endorsed by, or sponsored by these organizations; their names are used only to identify the standards and products discussed.
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