Friday, December 18, 2009

Vehicle Barriers and VBIEDs

There is an interesting article over on about choosing the right antiterrorism vehicle barriers. The article includes a link to a .PDF table showing a general comparison of the characteristics of different types of vehicle barriers. Both the article and the table should be valuable references for high-risk chemical facilities considering the use of vehicle barriers for their Site Security Plan.

The article is targeted at audiences looking to use vehicle barriers to prevent terrorist attacks on buildings using vehicle borne improvised explosive devices (VBIED). Since the targets discussed here are public buildings housing offices or apartments, the discussion of the blast analysis of a target structure is not really applicable to attacks on chemical facilities.

VBIED Targets 

While there might be some reason that a terrorist might want to conduct a VBIED attack on the office complex at a chemical facility, that is not the type target that most VBIED attacks will be pointed at in the event of an attack on a high-risk chemical facility. The most likely target of such an attack would be storage containers of toxic, flammable or explosive release chemicals of interest (COI). Since a VBIED attack is a high-cost attack strategy, it is most likely to be pointed at the highest value target, the storage container(s) with the highest off-site consequence.

If the facility had a large acrolein storage tank and a warehouse containing a small number of acrolein drums, the storage tank would certainly be a more likely VBIED target. A facility with similar sized tanks of allyl alcohol and allylamine would probably decide that the allyl alcohol tank would be a more valuable terrorist target because it is a toxic release (vs a flammable release) COI; and it would have more of an immediate off-site consequence if released into the environment.

That value calculus can be deceptive in some cases. If a facility has a high-pressure containment vessel like a chlorine tank, it may be easier for a terrorist to attack a nearby flammable release COI storage tank. The resulting fire, if close enough to the chlorine tank, could cause a catastrophic rupture of that tank through weakening the thick metal walls of the tank and increasing the pressure inside of the tank above the design limits.

Storage Tank Damage 

The article has a pretty good discussion of the graduations of the types of damages that a VBIED can inflict on a target building. Understanding these damage levels is important in crafting an anti-VBIED barrier plan. Ideally a security planner would like to prevent any damage, but a realistic assessment might be made that a few broken windows from the overpressure effect might be an acceptable level of damage.

This would affect the required standoff distance between the barriers and the building. A similar decision tool is required for chemical storage tanks. The damage assessment criteria for storage tanks should be based on the resulting release rate of the chemical the chemical of interest. The higher the release rate the more likely there would be an adverse off-site impact from the release. The comparable structural damage limit states for storage might be categorized (lowest to highest risk) as:
Damage to associated piping; Limited damage to gaskets, seals or flanges on the tank proper; Small punctures (<1 in="" major="" punctures="" tank="" the="" walls="">1”) in the tank walls; or Catastrophic failure of the storage tank.
The overpressure effects required to produce the damage are going to depend in large part on the type of construction of the storage container. High-pressure systems will require higher levels of overpressure to sustain similar levels of damage. I have not seen any specific figures for the overpressure required to sustain these levels of damage, but I suspect that they would be available from the Center for Chemical Process Safety or one of the engineering associations.

Size of VBIED 

One important factor in determining the placement of vehicle barriers that is kind of glossed over in this article is the size of the explosive charge in the VBIED. This is a critical variable in determining the overpressure produced by detonation. A 40,000 lb ammonium nitrate VBIED will produce catastrophic storage tank failure over a much larger area than a 500 lb VBIED. It would not be reasonable to assume that a terrorist team would use a 40,000 lb VBIED to attack a 5,000 gallon flammable release storage tank. The VBIED detonation would be more dangerous than the subsequent release.

Parking such a VBIED outside of the security perimeter would have more of an off-site consequence and would be a less risky operation. The security system design team is going to have to determine what is a reasonable sized VBIED that they could expect to face in a serious attack on the facility. They will have to take into account the fact that a larger VBIED is much more costly to a terrorist organization (from both a materials and operational perspective) than a smaller VBIED.

The design team is going to have to weigh the cost of VBIED against the utility of the facility as a target. A large chlorine storage facility in a major urban area will have to design for a larger VBIED than a similar sized facility in a rural setting. Unless DHS decides to get involved in dictating VBIED sizes for the design basis for facility security systems (and an argument could be made for a Tier ranking based size standard), the VBIED size determination will be a facility security management decision. The basis for the VBIED size used in the security system design should be explicitly spelled out in the design documents.

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