Babatunde Mobisola - Accumulator Systems

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 .

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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 b ladder

Diaphragm accumulators

Gas-charged p iston

Spr ing- loaded p iston

Weight loaded p iston

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 bladderA 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 pistonADVANTAGES

• 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 Accumulators1. 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.

Functions of Accumulators

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.

Functions of Accumulators

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.

SIZING BOP ACCUMULATOR SYSTEMSEXAMPLE :

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)

SIZING BOP ACCUMULATOR SYSTEMS

SIZING BOP ACCUMULATOR SYSTEMSSolution:

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

SIZING BOP ACCUMULATOR SYSTEMSSolution:

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

SIZING BOP ACCUMULATOR SYSTEMSSolution:

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

SIZING BOP ACCUMULATOR SYSTEMSSolution:

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/Article/False/7241/TechZone-Accumulators

• http://hydraulicspneumatics.com/200/TechZone/Accumulators/Article/False/85523/TechZone-Accumulators

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