Presentation on theme: "FRAC TANK EXPLOSIONS. Introduction R.J. Goodman, EHS & Operations Training Manager, XTO Energy Investigated 4 separate frac tank explosions that occurred."— Presentation transcript:
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 E&P safety experience Degree in Fire Science
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
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
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: 1.An air/foam mixture was pumped down the tubing and returned up the tubing/casing annulus.
XTO Energy Explosions 2.Air/foam assisted flowback operations had been engaged for 12 – 15 hours 3.Gas, water, oil, sediment and oxygen were piped from the wellhead to temporary non- pressure rated frac tanks 4.Each frac tank was internally lined with an Epoxy type liner 5.Each tank had a screwed together downcomer protruding into the frac tank 2 – 3’
XTO Energy Explosion Site 1 Nov. 10, 2005 – Major County, OK Location Configuration Downcomer entered the front of the tank.
XTO Energy Explosion Site 1 Nov. 10, 2005 – Major County, OK Picture of Downcomer
XTO Energy Explosion Site 2 March 31, 2006 – Major County, OK Downcomer Configuration Downcomer is chained to the tank Static Arc Point
XTO Energy Explosion Site 2 March 31, 2006 – Major County, OK Oil/Water/Natural Gas contents Downcomer 2 Part Epoxy Lining on Wall Static Builds and Arcs from Fluid to Downspout
XTO Energy Explosion Site 2 April 3, 2006 – Major County, OK
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.
Fire Triangle Application Air/Foam Flowback Static Electricity Spark Natural Gas & Oil Vapors
Fire Triangle Application Air/Foam Flowback Static Electricity Spark Natural Gas & Oil Vapors 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 Oil and Natural Gas vapors are forced out of the tank and pass through the narrow 1’ x 1’ tank hatch.
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: 1.Lightens formation fluids which allows faster fluid recovery. 2.Pressure forced down the tubing annulus allows the formation fluid to flow more easily up the casing annulus. AIR
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
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
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
Fire Triangle: Spark Gap API RP 2003 22.214.171.124 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
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
Review: Why is Static Energy building? 1.As oil, water, gas and sediment travel through the flowline they become positively charged. 2.Charged particles are projected into a tank and do not have sufficient “relaxation” time. 3.Charged particles are further charged when they fall into the liquid level below. 4.The liquid level in the tank contains non-conductive flammable liquids. HEAT
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. 6.Eventually, the charges build enough to arc from charged liquids inside the tank up to the elevated downcomer. 7.When the air/fuel mixture reaches the appropriate ratio…… BANG!!!!! HEAT
What Are The Mitigation Options? Remove the air? –Can’t 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 we’re talking!
Mitigation Option 1 Grounding, 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.”
Mitigation Option 1 Grounding, Bonding & Charge Reduction Internal grounding or “bonding” is the better answer. –API RP 2003 126.96.36.199.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.”
Mitigation Option 1 Grounding, 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
Mitigation Option 2 Gas 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.
Mitigation Option 2 Gas 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
Progression of Gas Buster Piping 2” Line 4” Line8” Line 16” Line24” Line The line splits at a header system. The header split, along with the increase in pipe diameter, slows the velocity of fluid!
Mitigation Option 2 Gas 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.
Mitigation Option 3 Slotted Downcomer System Use a slotted downcomer that extends from the end of the riser at the top of the tank’s 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.
Mitigation Option 3 Slotted 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.
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.