Mariner East 2 Hazards Quantification
The Mariner East 2 pipeline is proposed to carry Highly Volatile Liquids (artificially liquefied Ethane, Propane, Butane gas) through several densely populated areas in Southeastern Pennsylvania. Much of route through Chester and Delaware counties would be considered Class 4 High Consequence Areas by Natural Gas standards. However, these artificially liquefied gases have hazards greater than traditional Natural Gas. “Natural Gas Liquids”, or “NGLs” are maintained in an artificially liquefied state inside the pipeline. This liquid state is a higher energy state containing more combustion energy than the ambient state these gases have in the natural environment. If leaked, highly condensed liquids rapidly convert back to a gas, expanding by hundreds of times in volume into a highly explosive gas cloud. These gases are naturally odorless, colorless, and heavier-than-air. The heavier-than-air characteristic allows these gases to migrate in unpredictable ways that are highly dependent on wind conditions and topography. The gases, being heavier-than-air, remain low to the ground, will settle in low lying areas, can enter sewer systems or porous rock and use them as conduit to migrate indeterminate distances from a leak site. These gases only require a 3% gas-to-air ratio to remain combustible. A model has been developed to quantify the hazards for the proposed Mariner East 2 pipeline carrying artificially liquefied ethane, propane, and butane from Houston, PA to Marcus Hook, PA. The model focuses on the heavily populated segment between the pumping station in Derry Township, Dauphin County, PA and the pipeline terminus in Twin Oaks, Delaware County, PA. Methane Natural Gas hazards model is shown for reference and comparison throughout the modeling process. Data for this assessment utilizes results from an independent Hazards Study performed by Oklahoma-based Quest Consultants. Quest ran several simulations of ME2 pipeline ruptures using liquefied Ethane as the piped material. A summary of their results is shown:
Also of note, and not covered in detail in this assessment, is that independent of how leaked gas ignites, the ensuing jet fire can have a thermal radius over 600 feet. This 25-acre area surrounding the leak would be inaccessible until the jet fire was extinguished. Because these materials are artificially liquefied within the pipe, any leak will continue even after valve stations close and isolate the leak site. Therefore, any jet fire will continue until the gas fully self-ventilates and there is not more fuel to burn. With 800 to 1,200 tons of material condensed within each 6-mile long segment between valve stations, the time to fully extinguish a jet fire an access people within the 25-acre jet fire affected area would likely be measured in hours, likely many hours. Properties of Material Contained within 20in diameter 1,440psi Pipeline
Immediate Ignition Model This model addresses immediate ignition and delayed ignition plausible worst-case rupture scenarios. The math model uses the PHMSA Derivation Potential Impact Radius Formulae for Flammable Gases Other Than Natural Gas as a baseline for Instant Ignition scenarios. Data obtained from an independent hazards study performed by Quest Consultants using ethane at 2 local operating pressures was used to correlate the Instant Ignition model. These local pressures represent the operating pressure of the pipeline at the simulated location of rupture accounting for pressure dropping at a steady rate as the material gets farther from the pump. A pressure drop rate of 8.5 psi per mile downstream from the closest pumping station was determined from the Quest Study and has been used in this assessment.
The resulting model shows significant increase in severity when comparing the liquefied gases (ethane, propane, butane) to traditional Natural Gas (methane). The large variance is apparent even at lower operating pressures, demonstrating that severity is more sensitive to the highly condensed liquid state than to pressure itself. While higher pressure does show increased severity, there is a sharp increase in severity at the operating pressure at which each material converts from gas to liquid. This sharp severity increase correlates well with the density vs. pressure curve for each material. The Instant Ignition model of Potential Impact Radius vs. Local Operating Pressure was then rearranged to show PIR based on distance from the closest pumping station.
The pumping station layout provided in the approved Mariner East 2 applications specifies a pumping station in Derry Township, Dauphin County and another intermediate pumping station in Brecknock Township, Berks County. However, the pipeline operator claims that they are not using the Berks County pumping station. Because of this uncertainty, the model was run both ways. The local operating pressure from Derry Township to Twin Oaks was therefore derived for both pumping station configurations.
The local pressure trends were then input into the Instant Ignition PIR model to obtain the Instant Ignition PIR for Ethane, Propane, and Butane along the entire ME2 segment, with Methane Natural Gas shown for reference.
3 Minute Ignition Model
This scenario exists because of the heavier-than-air nature of the ME2 materials. This characteristic means that these materials, when converted back to a gaseous state outside the pipeline, will remain close to the ground and have the potential to migrate. Data from the Quest hazards study was also used to model a 3-minute delayed ignition leak scenario. A best-fit curve was applied between the 2 simulated data points resulting in a model that shows delayed ignition impact radius for Ethane as a function of local operating pressure.
The Delayed Ignition model of Potential Impact Radius vs. Local Operating Pressure was then rearranged to show PIR based on distance from the closest pumping station.
Once again, local operating pressures along the entire segment from Derry Township to Twin Oaks were derived both ways: assuming the approved Berks County pump station is active and in-active.
The local pressure trends were then input into the 3-Minute Delayed Ignition PIR model to obtain the Delayed Ignition PIR for Ethane along the entire ME2 segment, with Methane Natural Gas Instant Ignition PIR shown for reference. Methane is lighter than air and as a result does not have a delayed ignition scenario for a pipeline.
Case Study Comparable
20” ATEX Ethane Pipeline Rupture in Follansbee, WV, January 26, 2015
The ATEX pipeline, operated by Enterprise is a 20in diameter pipeline designed to carry liquefied ethane. This pipeline is equivalent to Mariner East 2 in many ways and serves as the best real-world correlation.
The rupture site was just a few miles downstream from the closest pumping station.
The resulting impact radius exceeded 2,000 feet, with a home that distance away from the rupture sustaining thermal damage.
Approximately 5 acres of unoccupied woodlands was burned in the fire that took 24 hours to extinguish.
The Follansbee accident has been overlaid onto both the Instant Ignition and 3-minute Delayed Ignition Impact Radius Models and shows good correlation with the delayed ignition scenario.
Now consider that the local operating pressure just prior to rupture was just over 1,000psi according to the released Failure Investigation Report. With this information, we can compare to both the Instant Ignition and 3-minute Delayed Ignition Impact Radius Models:
Affected Populations Assessment This model was then cross-linked with 2010 municipal census data to determine approximate number of people within the derived impact radius.
People with Instant Ignition Impact Radius Along the assessed segment from Derry Township to Twin Oaks, there are approximately 32,000 to 38,000 total people living within the 3-minute delayed instant ignition zone of the Mariner East 2 pipeline.
People with 3-Minute Delayed Ignition Impact Radius Along the assessed segment from Derry Township to Twin Oaks, there are approximately 50,000 to 60,000 total people living within the 3-minute delayed ignition zone of the Mariner East 2 pipeline.
Affected populations for all scenarios overlaid for comparison:
PHMSA Evacuation Recommendation PHMSA recommends an “initial downwind evacuation for at least 800 meters (1/2 mile).
Along the assessed segment from Derry Township to Twin Oaks, there are approximately 90,000
total people living within ½ mile of the Mariner East 2 pipeline.
While a ½ mile evacuation radius is recommended, the unpredictable migration of these gases released in high quantities may require a greatly increased evacuation zone. Another factor that contributes to the unpredictability of these gas migration patterns is the presence of sewers, ventilation, and other systems that can serve as conduit for the gas to spread out. The heavier-than-air nature of these gases means that it will flow downhill but also down into these open-air systems that are below grade (ground level). Karst, a highly porous rock formation, is prevalent in the Uwchlan area providing additional underground passageways that can allow released gas to spread randomly throughout the surrounding areas. "You’ve got to be nuts to put a large diameter HVL in a karst terrain” (Richard Kuprewicz, president of Accufacts) The threat of gas spreading out and traveling through these open-air systems is identified in the PHMSA emergency guidebook for these materials and provides the potential for isolated explosions in isolated locations, all of which can trace back to the initial leak site.
Because of the unpredictable gas migration that these gases can take, there are numerous large-scale releases that have required a 3-mile evacuation radius. This size radius has been used in remote locations where there likely are few underground open-air systems. In a densely populated area with a vast network of sewers (both storm water and septic), an equivalent evacuation would be far more complex. Given the sewer system’s ability to transfer gas far and wide, it is not inconceivable to see combustible concentrations of gas make their way many miles away. To maintain simplicity, the number of people has been determined within a potential 3-mile evacuation radius. Along the assessed segment from Derry Township to Twin Oaks, there are approximately 500,000 total people living within 3 miles of the Mariner East 2 pipeline. If a 3-mile evacuation radius is executed in Delaware County, the number of people involved in a single evacuation could conceivably approach 100,000.
An example is illustrated in the middle of Delaware County. This location is designated on the chart above with the red star. The number of affected people, based on Middletown’s population density, likely grossly underestimates the people in this large evacuation zone since this area includes Media, Aston, and many other much more populous municipalities. Also not included in these numbers are the numerous schools, businesses, and other centers with large populations that are also not included in municipal census data.