FRAC TANK EXPLOSIONS Side Vents at back of Frac Tank Gas

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FRAC TANK EXPLOSIONS
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Introduction

• R.J. Goodman, EHS Operations Training Manager,
  XTO Energy
• Investigated 4 separate frac tank explosions that
  occurred on two separate jobs.
• 8 years of EP safety experience
• Degree in Fire Science

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References

• API RP 2003 Sixth Edition, September 1998.
• Protection Against Ignitions Arising Out of
  Static, Lightning, and Stray Currents
• API 545, Working Group, Standard for Lightning
  Protection for Hydrocarbon Storage Tanks

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Learning Objectives

• Explore 4 investigations
• Understand how static electricity generation
  relates to frac tank explosions
• Demonstrate the usefulness of flowback gas buster
  systems to prevent frac tank explosions

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XTO Energy Explosions

• Four Explosions Occurred
• November 10, 2005 Major County, OK (2 Events)
• March 31, 2006 Major County, OK
• April 3, 2006 Major County, OK
• All four explosions had the following
  characteristics in common
• An air/foam mixture was pumped down the tubing
  and returned up the tubing/casing annulus.

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XTO Energy Explosions

• 2. Air/foam assisted flowback operations had been
  engaged for 12 15 hours
• Gas, water, oil, sediment and oxygen were piped
  from the wellhead to temporary non-pressure rated
  frac tanks
• Each frac tank was internally lined with an Epoxy
  type liner
• Each tank had a screwed together downcomer
  protruding into the frac tank 2 3

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XTO Energy Explosion Site 1Nov. 10, 2005 Major
County, OK
Location Configuration
Downcomer entered the front of the tank.
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XTO Energy Explosion Site 1Nov. 10, 2005 Major
County, OK
Picture of Downcomer
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Internal Configuration of Frac Tank Site 1
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XTO Energy Explosion Site 2March 31, 2006
Major County, OK
Downcomer Configuration
Static Arc Point
Downcomer is chained to the tank
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XTO Energy Explosion Site 2March 31, 2006
Major County, OK
2 Part Epoxy Lining on Wall
Downcomer
Static Builds and Arcs from Fluid to Downspout
Oil/Water/Natural Gas contents
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XTO Energy Explosion Site 2April 3, 2006 Major
County, OK
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Investigation Conclusions

• Static electricity was generated when oil, water,
  gas and sediment passed through flowline.
• The lined frac tanks reduced static discharge
  through tank walls.
• While the tanks were externally bonded and
  ground, none were internally grounded to the
  charged liquids.
• All explosions occurred while airing back.

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Fire Triangle Application
Static Electricity Spark
Air/Foam Flowback
Natural Gas Oil Vapors
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Fire Triangle Application

• Oil, Water, Gas Sediment flowing
• Velocity of liquids flowing through a 2 pipe
• Insufficient Relaxation time for the charged
  particles
• Insulated Lined Tanks

• Air containing 21 O2 is injected down hole
• Air containing 21 O2 is at the tank hatch

Static Electricity Spark
Air/Foam Flowback
Natural Gas Oil Vapors

• Oil and Natural Gas vapors are forced out of the
  tank and pass through the narrow 1 x 1 tank
  hatch.

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Fire Triangle AIR

• An air/foam mixture is pumped down the tubing and
  returned up the tubing/casing annulus. The
  air/foam mixture contains 21 oxygen prior to the
  mixture being pumped down hole. Therefore,
  oxygenated gas is returned to the surface after a
  few hours of stimulation.
• The process of pumping an air/foam mixture serves
  two functions
• Lightens formation fluids which allows faster
  fluid recovery.
• Pressure forced down the tubing annulus allows
  the formation fluid to flow more easily up the
  casing annulus.

AIR
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Fire Triangle AIR

• Atmospheric air containing 21 oxygen co-mingle
  with oxygen enriched flowback gases at the tank
  hatch.
• The oxygenated atmosphere presents a problem
  because the ignition source (flowback downcomer)
  penetrates the tank where maximum oxygen
  saturation occurs.

AIR
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Fire Triangle FUEL

• The Fuel Sources can be one of the following
• Natural Gas
• Oils that emit vapor
• Fuel vapors collect inside the tank and are
  forced through a 1 x 1 hatch on the tank.

FUEL
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Fire Triangle Static Generation

• Turbulent contact of dissimilar fluids such as
  water or gas flowing through a liquid
  hydrocarbon. API RP 2003, page 3
• Fluid velocity and turbulence are key components
  in static generation.
• Nonconductive Flammable Liquid - Oil is
  considered nonconductive and holds a charge
  better than produced water. Saltier liquids
  allow electric charges to flow easier through
  them.

HEAT
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Fire Triangle Spark GapAPI RP 2003 4.1.3.4

• Loose floating conductive objects or debris
  inside the container.
• Conductive downcomer which does not reach the
  bottom of the tank.
• Gage rods or side wall probes which are not
  connected to the bottom.
• Gage tapes, sample containers or thermometers
  which are lowered into the tank vapor space.
• Ungrounded couplings or hoses in the tank.

HEAT
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Fire Triangle Static Spark

• In order for ignition to take place there must be
  enough heat generated to ignited the air/fuel
  mixture.
• In the case of the frac tank explosions, heat is
  generated when a static generated spark jumps the
  gap created between the downcomer and charged
  liquid below.

HEAT
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Typical Frac Tank Configuration
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Review Why is Static Energy building?

• As oil, water, gas and sediment travel through
  the flowline they become positively charged.
• Charged particles are projected into a tank and
  do not have sufficient relaxation time.
• Charged particles are further charged when they
  fall into the liquid level below.
• The liquid level in the tank contains
  non-conductive flammable liquids.

HEAT
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Review Why is Static Energy building?

• 5. As the Charged liquids continue to build,
  they look for a path to ground.
• Lined tanks further insulate the non-conductive
  flammable liquids and increase static build up.
• Caution Static can build in an unlined tank if
  build up surpasses discharge to ground.
• Eventually, the charges build enough to arc from
  charged liquids inside the tank up to the
  elevated downcomer.
• When the air/fuel mixture reaches the appropriate
  ratio BANG!!!!!

HEAT
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What Are The Mitigation Options?

• Remove the air?
• Cant do that because we want faster stimulation.
• Air will always be at the tank hatch.
• Remove the fuel?
• Not unless we want to be out of a job!
• Remove the ignition source?
• Now were talking!

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Mitigation Option 1Grounding, Bonding Charge
Reduction

• Most people think externally grounding the tank
  is the best answer.
• API RP 2003 4.5.3 Grounding
• Storage tanks on grade-level foundations are
  considered inherently grounded for dissipation of
  electrostatic charges regardless of the type of
  foundation. The addition of grounding rods and
  similar grounding systems will not reduce the
  hazard associated with electrostatic charges in
  fluid.

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Mitigation Option 1Grounding, Bonding Charge
Reduction

• Internal grounding or bonding is the better
  answer.
• API RP 2003 4.5.9.1.b.2
• The tank should have a metal plate with a
  surface area no less than 30 in.² per 100 gallons
  located at the tank bottom, and bonded to an
  external ground. The plate provides an
  electrical path between the liquid contents and
  ground through which the charge can dissipate.

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Mitigation Option 1Grounding, Bonding Charge
Reduction

• API RP 2003 4.1.5.
• To Prevent charge generation
• Avoid Splash and Misting Operations
• Limit initial fill rates and maximum flow rates
• Use sufficient relaxation time downstream of
  pumps and filters
• Ground conductive fluids while filling insulated
  containers
• Remove or ground spark promoters in tanks
• Use sufficient waiting period before sampling

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Mitigation Option 2Gas Buster Systems

• Use the right equipment for the task at hand.
• For years companies have used tanks designed to
  hold non-pressurized liquids as flowback tanks.
  This process will continue to work until the
  forces of nature and the fire triangle align.
  After the forces align, you will experience your
  first fire or explosion.

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Mitigation Option 2Gas Buster Systems

• Gas buster systems eliminate static build up and
  arc gaps.
• This process is accomplished by
• Using an unlined open top tank (grounding
  principle)
• Flowback fluid and gas enter the tank through a
  2 line. After a few feet of travel, the line
  size increases to 4 then to 8 then to 16 and
  finally to 24 in diameter. (charge relaxation)
• The increase in line diameter serves 3 functions
• Decreases fluid velocity
• Increases fluid charge relaxation time
• Increases surface area and allows charges to
  escape

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Progression of Gas Buster Piping
The line splits at a header system. The header
split, along with the increase in pipe diameter,
slows the velocity of fluid!
2 Line
4 Line
8 Line
16 Line
24 Line
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Mitigation Option 2Gas Buster Systems

• Slots approximately one foot in length are cut in
  the bottom of the 24 section of piping. (too
  rich or too lean to ignite)
• The slots allow the liquids to fall out the
  bottom while allowing the gases to escape up the
  sides.
• Large tank hatches located along the sides of the
  tank can be opened to allow for faster dispersion
  of gas vapors.
• This process eliminates the rich gas volume
  between the fluid and the top of the closed tank
  while reducing static build up potential.

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Gas Buster Piping and Vents
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Side Vents at back of Frac Tank
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Gas Buster System in Wyoming
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Gas Buster System in Wyoming
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Mitigation Option 3Slotted Downcomer System

• Use a slotted downcomer that extends from the end
  of the riser at the top of the tanks back hatch
  to approximately 1 from tank bottom. Submerge
  the downcomer in fluid. Submerging the downcomer
  will bond the charged liquid to the downcomer and
  should eliminate spark gap. Cutting 1 vertical
  slots in the downcomer pipe should allow a lot
  of the gas to break out of fluid and escape
  upward out of the hatch (less agitation). This is
  considered a poor-boy gas buster.
• Ensure the downcomer is bonded to the tank
  hatch lip.
• Per API RP 12R1, discharging flowback contents
  into the fluid should reduce the risk of sand or
  solids causing metal contact spark.

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Mitigation Option 3Slotted Downcomer System

• Larger diameter pipe will allow the velocity of
  fluid and debris to decrease and result in lower
  static potential.
• If flowing back through 100 of 2 3/8 tubing
  with steel connections increase the output or
  downspout diameter to 4. Doubling the pipe
  diameter decreases the velocity by approximately
  4 times thus lowering the static potential
  significantly.
• API RP 2003 recommends using uncoated frac tanks.
  This provides protection against ignition
  arising out of static, lightning and stray
  currents while allowing for maximum gas/air
  dilution.
• Allow no plastic or rubber connections in the
  flowback line.

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Summary

• Multiple frac tanks have exploded throughout the
  U.S.
• Static builds up any time fluids, gases and
  solids flow through pipe or hoses at a sufficient
  velocity.
• Static dissipation is hampered when it is
  generated inside lined tanks.
• Static may not dissipate through unlined tank
  walls when nonconductive flammable liquids are
  involved in the flowback process.
• Flowing through an open top working pit with gas
  buster system is the safest option to prevent
  closed top frac tank explosions.

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• Questions?