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ACCUMULATOR
SYSTEMS
ACCUMULATOR
SYSTEMS
P r e s e n t e d b y : B a b a t u n d e M o b i s o l a .
2 0 1 5

Objectives
To discuss:
• What an accumulator is
• Basic operating principle of an accumulator
• Types of accumulators
• Functions of accumulators
• Accumulator Systems
• Components of accumulator systems
• Sizing accumulator systems.

What's An Accumulator?
 Accumulator: a unit used to hydraulically operate Rams BOP, Annular
BOP, HCR and some hydraulic equipment.
 Accumulator (Pressure Control Device): Canisters of hydraulic fluid,
pressurized with a nitrogen gas cap of sufficient pressure and volume
to operate all the rams on a BOP in case of power failure to the BOP.
 Accumulator is a device in which potential energy is stored in the
form of a compressed gas or spring, or by a raised weight to be used
to exert a force against a relatively incompressible fluid.

What's An Accumulator? – Schematic Diagram

What's An Accumulator? – Sample Images

Basic Operating Stages of an Accumulator.

Basic Operating Principle of an Accumulator.
A. The accumulator is empty, and neither gas nor hydraulic sides are pressurized.
B. The accumulator is precharged.
C. The hydraulic system is pressurized. System pressure exceeds precharge pressure,
and fluid flows into the accumulator.
D. System pressure peaks. The accumulator is filled with fluid to its design capacity.
Any further increase in hydraulic pressure would be prevented by a relief valve in
the system. Equilibrium is reached.
E. System pressure falls. Precharge pressure forces fluid from the accumulator into
the system.
F. Minimum system pressure is reached. The accumulator has discharged its design
maximum volume of fluid back into the system.

Working Stages of a Bladder Accumulator
• Precharge pressure - Precharge pressure is a percentage of the
minimum or (Operating pressure) maximum working pressure of the
system and determined by the type of application. The calculation is
part of all sizing formulas.
• Minimum Operating Pressure - Is the minimum pressure at which the
system will still function.
• Operating Pressure – Is the maximum working pressure for the
accumulator system. The pump system on the accumulator unit
supplies this pressure in the hydraulic system, so the final charge
pressure is equal to the pump stopping pressure.

Types of Accumulators
Gas-charged bladder
Diaphragm accumulators
Gas-charged piston
Spring-loaded piston
Weight loaded piston

Gas-charged bladder
• A bladder accumulator consists of seamless high
pressure cylinder with an internal elastomeric
bladder with pressurized nitrogen in it and
hydraulic fluid on the other(external) side.
• The bladder is used to separate gas from the
hydraulic fluid.
• A poppet valve in the discharge port keeps the
bladder from extruding when the pump is off.

Gas-charged bladder
• The original design was the bottom-repair
style which is still offered by most
manufacturers.
• The top-repair style is now available and
makes bladder replacement simple and fast.
• These accumulators are available from 1-gallon
to 15-gallon sizes.
• maximum working pressures of 3,000 and
5,000 psi

Gas-charged bladder
A D V A N T A G E S
 Reduction in working costs
 Reduction in maintenance cost
 Highest efficiency with tests showing 97 percent energy
retainment.
D I S A D V A N T A G E S
 Compressed ratio is limited, approximately 4:1
 Damaged Bladder
 Broken poppet valve
 Nitrogen will permeate the bladder material over time
and need to be periodically recharged.

Diaphragm accumulators
 Diaphragm accumulator may be spherical
or cylindrical.
 An elastic diaphragm is used in place of a
rubber bladder which would typically
reduce the usable volume of the
accumulator, so the diaphragm
accumulator may not have volume
capacity of a bladder accumulator.
 It is low weight, compact design and
good for shock applications (good
response characteristics).

Diaphragm accumulators
• Diaphragm accumulators have a quick
response time than piston accumulators
• The main difference with bladder
accumulator is an increased maximum
compression ratio of approximately 8:1.
• They usually can hold a volume of fluid less
than 1 gallon.
• Maximum operating pressure of 3600 psi
• Diaphragm accumulators behavior will be
similar to bladder accumulator and share the
same advantages and disadvantages.

Gas-charged piston
• The gas-charged piston accumulator has a free-
floating piston with seals to separate the liquid and
gas.
• It operates and performs similarly to the bladder
type, but has some advantages in certain
applications.
• An increase of liquid volume decreases the gas
volume and increases gas pressure, which provides
a work potential when the liquid is allowed to dis-
charged.
• A gas-charged piston accumulator can cost twice as
much as an equal-sized bladder type.

Gas-charged piston
ADVANTAGES
• High compression ratio up to 10:1
• Higher flow rate than bladder type.
• Virtually no nitrogen escapes so they will not have to
be recharged
DISADVANTAGES
• A bit heavier, and less efficient than the bladder
model.
• Lower response time than the bladder and diaphragm.
• Prone to leakage .
• Susceptible to fluid contamination.

Spring-loaded piston
 A spring-loaded piston accumulator is identical to
a gas-charged unit, except that a spring forces
the piston against the liquid.
 The load characteristics of a spring are such that
the energy storage depends on the force
required to compress s spring.
 The free (uncompressed) length of a spring
represents zero energy storage.
 As a spring is compressed to the maximum
installed length, high pressure value of the liquid
in a ram assembly is established.

Spring-loaded piston
ADVANTAGES
• Its main advantage is that there
is no gas to leak.
DISADVANTAGES
• A main disadvantage is that this
design is not good for high
pressure and large volume.
• Spring fatigue.

Weight loaded piston
 A raised weight accumulator consists of a
vertical cylinder containing fluid to the hydraulic
line.
 The cylinder is closed by a piston on which a
series of weights are placed that exert a
downward force on the piston and thereby
energizes the fluid in the cylinder.
 Gravity acts on the weight to pressurize the
hydraulic system fluid, thus storing energy.

Weight loaded piston
ADVANTAGES
• Extremely high capacity at low cost.
DISADVANTAGES
• The major drawback to weight-loaded
accumulators is their physical size.
• Problem in sealing.

Functions of Accumulators
1. Stores Energy: Potential energy is stored in compressed gas to be
released upon demand. This energy can be compared to that of a raised
pile driver ready to transfer its tremendous energy upon the pile.
2. Absorbs Pulsations: An accumulator can be used to cushion the pressure
spike from sudden valve closure, the pulsation from pumps or the load
reaction from sudden movement of parts connected to hydraulic
cylinders.
3. Cushions Operating Shock: In many fluid power applications the driven
member of the hydraulic system stops suddenly, creating a pressure
wave which is sent back through the system. The gas cushion in an
accumulator, properly placed in the system, will minimize this shock.

4. Emergency and safety: An accumulator which is kept constantly
under pressure is valuable in the event of an electrical power failure
as it can provide flow and pressure to perform an additional
function or complete a machine cycle.
5. Leakage compensation: An accumulator can be used to maintain
pressure and make-up for lost fluid due to internal leakage of
system components including cylinders and valves.
6. Thermal expansion: An accumulator can absorb the pressure
differences caused by temperature variations in a closed hydraulic
system.

7. Supplements Pump Delivery: An accumulator, capable of storing
power, can supplement the fluid pump in delivering power to the
system.
8. Maintains Pressure: Pressure changes occur in a hydraulic system
when the liquid is subjected to rising or falling temperatures. An
accumulator compensates for such pressure changes by delivering
or receiving a small amount of hydraulic liquid.
9. Dispenses: An accumulator may be used to dispense fluids under
pressure, such as lubricating greases and oils.

Accumulator Systems
Subsea Accumulator Unit Surface Accumulator Unit

Accumulator Systems
• Accumulator system simply accumulates (or stores) hydraulic fluids under pressure
for use in functioning BOPs in matter of seconds.
• The hydraulic fluid is piped to the opening and closing ports of the BOP stack
(including the HCR valve) via steel and coflex hoses.
• The BOP can then be functioned from the accumulator unit or from a remote
control panel. For surface stacks, hydraulic fluid returning from the BOP stack is
sent back to the accumulator reservoir so that the entire system is a “closed loop”.
• According to API RP 53, there must be 2 or 3 independent sources of power that
will be available for each closing unit (accumulator system). Typically, you will use
these following sources:
• Hydraulic with pressure charged in the bottles.
• Pneumatic
• Electric

Accumulator Systems
Main components of an accumulator unit:
• Accumulator Bottles
• Pumping system (electric and pneumatic
pumps)
• Manifold system
• Reservoir tank

Accumulator Systems
Accumulator Remote Control Panels:
Remote control panels can be used to
function the BOP from strategic positions
at the wellsite.
Typical strategic positions are: Driller’s
position, Toolpusher’s office or near and
escape route.

Accumulator Pressure Charging System

Accumulator Pressure Charging System
• In an accumulator unit as shown in the previous slide as an example, there
is one electric pump and two pneumatic pumps, which will automatically
pump hydraulic fluid stored in a reservoir tank when pressure in bottles is
below set pressure in order to maintain bottle pressure at the operating
pressure.
• The electric pump is mainly used to maintain pressure. Pneumatic pumps
will be used when:
1) The electric pump fails and
2) To help the electric pump increase bottle pressure quicker.

SIZING BOP ACCUMULATOR SYSTEMS
• Accumulator sizing calculations are performed to determine if the
accumulator can provide sufficient usable hydraulic fluid to operate the
BOP.
• The requirement for sizing an accumulator system is based on hydraulic
fluid volume and pressure. calculations are based on variations of Boyle's
law.
• Usable hydraulic fluid – the hydraulic fluid recoverable from the
accumulator system between the maximum accumulator operating
pressure and minimum operating pressure.
• Stored hydraulic fluid – the hydraulic fluid recoverable from the
accumulator between the maximum operating pressure and precharge
pressure.

EXAMPLE :
Accumulator Data:
• 10 gallons accumulator with
• 1,000 psi precharge pressure
• 1,200 psi Minimum operating
pressure
• 3,000 psi operating pressure
BOP Data:
• Hydril GK annular BOP (Volume to close
annular = 3.86 gallons)
• Three Shaffer Ram BOPs (3 x 1.19 gallons
each)(Volume to close pipe rams = 3.57
gallons)
• One Mc Envoy “AC” HCR Valve (Volume
to open hydraulic valve = 0.46 gallons)

Solution:
Step 1 Determine hydraulic fluid required to increase pressure from
pre-charge pressure to system operating pressure:
Boyle’s Law for ideal gases:
P1 V1 = P2 V2
1000 x 10 = 3000 x V2
10000 = 3000 x V2
V2 = 10,000/3000
V2 = 3.33 gallons of Nitrogen

Solution:
It means that N2 will be compressed from 10 gallons to 3.33 gallons in order
to reach system operating pressure. Therefore, 6.67 gallons (10 – 3.33 = 6.67
gallons) of hydraulic fluid is used for compressing to system operating
pressure.
Step 2 Determine hydraulic required increasing pressure from pre-charge
to minimum operating pressure:
P1 V1 = P2 V2
1,000 psi x 10 gallons = 1,200 psi x V2
10,000 = 1200 x V2
V2 = 10,000/1,200
V2= 8.33 gallons of Nitrogen

Solution:
It means that N2 will be compressed from 10 gallons to 8.33 gallons.
Therefore, 1.67 gallons (10 – 8.33 = 1.67 gallons) of hydraulic fluid is
used for compressing to minimum operating pressure.
Step 3 Determine usable fluid volume:
Usable volume per bottle = Hydraulic used to compress fluid to
operating pressure – hydraulic used to compress fluid to minimum
pressure
Usable volume = 6.67 – 1.67 = 5 gallons

Solution:
Step 4 Determine fluid volume needed to function preventers:
• Volume to close annular = 3.86 gallons
• Volume to close rams = 3.57 gallons
• Volume to open hydraulic valve = 0.46 gallons
• Total volume = 7.89 gallons
Determine the number of accumulator bottles needed to give the
amount of fluid determined.
7.89 gallons ÷ 5 gallons usable fluid/bottle = 1.578 or 2 Bottles

Conclusion
All BOP components can be activated separately by hydraulic pressure,
maintained in accumulator cylinders. Their operation does not depend
on the rig’s prime mover. Workers can trigger the BOP from any of
several control stations on or near the drilling rig.
The worst case in an emergency well control situation is a complete
blackout in the infrastructure. So the accumulator has to provide the
energy for shutting in the well without any supply from a pump system.

References
• “Chevron Well Control Guide” 2005.
• Chandan Nagaraja: “Hydraulic Accumulator”.
• http://www.tobul.com/
• http://dta.eu/hydraulics/hydraulic-accumulators/
• http://www.drillingformulas.com/accumulator-koomey/
• http://hydraulicspneumatics.com/200/TechZone/Accumulators/Articl
e/False/7241/TechZone-Accumulators
• http://hydraulicspneumatics.com/200/TechZone/Accumulators/Articl
e/False/85523/TechZone-Accumulators

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