CRITICAL CONSIDERATIONS WHEN SELECTING A BLAST- RESISTANT STRUCTURE

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JANUARY 2017

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Critical Consideration #1: API Recommended Practices Blast resistance of buildings and structures is a critical factor when placing them and their occu-pants on an oil, gas, or chemical processing facility site. When considering the options for selec-tion of structures, the principle guidance comes from API Recommended Practices: 752 for per-manent plant buildings, 753 for portable buildings, and 756 for tents.

API RP 756 outlines recommended practices for managing the risk from explosions, fires, and toxic material releases to personnel located in temporary on-site structures. To gather data, the API performed vapor cloud explosion (VCE) tests to determine the response of tents to the poten-tial explosion hazards that may be present at refineries, petrochemical, and chemical operations. As a result of those tests, API RP 756 includes the following recommendations:[1]

• Manage the occupancy of tents in close proximity to process areas. • Design, construct, install, modify, and maintain tents intended for occupancy to mitigate

hazards that the tent site could present to occupants in the event of explosion, fire, and toxic material release.

• Manage the use of tents intended for occupancy as an integral part of the design, construction, maintenance, and operation of a facility.

Image 1: 1000kg NM, 5.9psi x 36ms incident blast near a DAS tent.

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Critical Consideration #2: Structural Integrity The most important feature of a blast resistant structure is the ability to absorb blast energy with-out causing catastrophic failure to the structure as a whole. Structural failure is the predominant factor in injuries and fatalities associated with plant explosions, so API guidelines for permanent buildings, portable buildings, and tents are offered to reduce risk to personnel.

In theory, any blast resistant structure designed to the relevant overpressure and impulse criteria should protect its occupants from the effects of explosion blast waves, but does this happen in practice?

While that is the implied conclusion of the API guidance documents and the conventional wisdom within the industry, tests and analyses show that when it comes to blast resistant designs, not all structures are equal. While typical rigid structures can be designed to avoid structural failure, thus protecting the occupants from the hazards of collapse, occupants’ experiences during the blast can be dramatically different when compared to their experiences inside a Dynamic Air Shelter.

Consider a vapor cloud explosion within a process plant. As the blast wave propagates outward, the peak side-on overpressure experienced at any point decreases with distance. As the wave passes a point, there is a nearly instantaneous rise in pressure, followed by a more gradual decay. The duration of the blast is typically in the order of 50-200 milliseconds, and the integral of the positive phase pressure over time—the area under the pressure-time curve—is known as the im-pulse. A simplified impulse curve is shown in Figure 1.

When the blast wave reaches a building, several things happen. Assuming it is a rigid structure designed for the blast load, the structure should withstand the blast without failing, although some level of damage is anticipated. Inside the building the experience will depend on a number of factors, principally the type of structure and whether or not there are any openings. The open-ings may be due to open doors or windows, or to failure of doors or windows due to the blast. On the side of the building facing the explosion source, some of the blast wave energy is reflected, some is transmitted through the wall due to flexing, and some is absorbed by both plastic and elastic deformation of structural components. The transmitted blast energy causes a pressure wave

Figure 1: Simplified impulse curve.

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within the building that can in turn reflect off the far wall, resulting in reverberation and possible amplification as reflected pressure waves pile on each other. Pressure wave energy can also enter through openings, resulting in similar effects. While gross structural failure should be avoided by proper design, injuries may still be possible due to internal pressure transients. In general, the more rigid the structure, with steel blast resistant modules being the most rigid, the greater the likelihood of significant internal pressure spikes and associated risk of injury. [2]

When the same blast wave reaches a Dynamic Air Shelter structure, the response is very differ-ent. First, the facing wall flexes, resulting in distortion of the shape of the structure. The bulk of the energy travels through the inflated support columns and down to the ground on the other side. The portion of wave energy transmitted through the wall by deflection results in similar deflection on the far side, resulting in the wave energy almost entirely passing through, rather than being reflected internally. The net result is both a much lower peak internal overpressure and minimal reflected peaks. [3]

Image 2: Conventional rigid-framed tent interior.

Image 3: Conventional rigid-framed tent under blast load-ing. Results in structural failure.

Image 4: Inside of shelter prior to a 5.9psi x 36ms incident blast.

Image 5: Inside of shelter during a 5.9psi x 36ms incident blast. Structure “kneels” downward before it encroaches into primary habitable space (PHS). Minimal effect on the occupants.

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In a series of field tests at the Suffield Laboratory of Defence R&D Canada in 2009, a single-wall DAS shelter was subjected to three successive blasts of increasing magnitude. The first gave an incident pressure at the shelter of 1.8 psi and duration of 24 ms; the second gave 3.0 psi at 400 ms; and the final 5.9 psi at 36 ms. Deflections of the wall within the habitable space were negli-gible in the first case, rising to just 4% in the third and largest trial. The shelter fully rebounded to its original shape after each test, with minor, non-structural seam damage noticed after the third and final test. Inside the shelter, the blast strength was reduced in terms of amplitude and impulse, while the rise time was extended, as shown in Figures 2 and 3. [3] By comparison, an individual inside the rigid framed shelter would be hit harder and repeatedly by overpressure than if they had been outside in the free field wave. [3]

Figure 3: DAS shelter incident and transmitted blast waves. Peak overpressure reduced 50%, impluse re-duced 28%, and rise time extended 4ms.

Incident wave

Figure 2: Conventional shelter with Hesco Barrier. Peak overpressure, impulse, and rise time largely unaffected.

Transmitted wave

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Test Run F: Double/No Gap - Shot 1

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Critical Consideration #3: Personnel Safety With the latest innovative configuration developed by DAS, the internal pressure rise is both mini-mal in magnitude and extended in time (dampened), as illustrated in the test results shown in Figure 4. What this means for the occupant is a much less severe shock load and a significantly less traumatic experience.

All of Dynamic Air Shelters products have built in redundancies to be fail safe in the unlikely event of a failure, with no rigid components to collapse. With the newest innovation from Dynamic, not only is potential structural collapse eliminated but also injury to personnel is almost entirely eliminated. [4] This newest innovation allows for work closer to hazards with significantly reduced risk.

DAS shelters offer improved safety for occupants during a blast, coupled with rapid deployment and relocation. They can also be designed and specified for toxic shelter in place, fire resistance (external thermal radiation), and hurricane-strength winds. They can meet short-term as well as long-term needs for occupied shelters in hazardous locations in accordance with API recommend-ed practices.

Figure 4: Test results, 5.9 psi, 36 ms. The maximum transmitted pressure is 78.5% lower than incident.

Red: Incident overpressure at the building (measured)Blue: Internal overpressure (measured)

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Citations:

[1] API Recommended Practice 756. Management of Hazards Associated with Location of Process Plant Tents. First ed., September 2014. http://www.api.org/~/media/files/publications/whats%20new/756_e1%20pa.pdf

[2] Cormie, D., Mays, G., and Smith, P., 2012, Blast Effects on Buildings, 2nd Ed., Thomas Tel-ford, London.

[3] Ritzel, D., Parks, S., & Crocker, J., 2009, Blast, Ballistic, and Earthquake Response of a Deployable AirBeam Shelter System, Shock & Impact Loads on Structures, p. 545-552, Adelaide: CI-Premier Conference.

[4] Stratton, J.C., 2016, “Blast Tube Tests of Multi-Layer Pneumatic Structures,” Dynamic Air Shelters, Calgary, Alberta.

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For more information:http://www.dynamicairshelters.com/products/blast-resistant/https://www.youtube.com/user/DynamicAirSheltershttp://www.dynamicairshelters.com/

About Dynamic Air Shelters:Dynamic Air Shelters has been providing protection and safety for the at-risk workforce, theirequipment, and their environment for the past 15 years. Dynamic’s blast-resistant shelters meetthe strictest safety standards, providing protective indoor space – and can withstand the harshestconditions in tropical hurricane zones, the remote Arctic Circle, and on plant sites with inherenthazards. Several independent live field tests have confirmed Dynamic’s air-inflated structures’ability to defend against a serious explosive threat and withstand high-proximity impact.Dynamic’s Blast Resistant Shelters are considered to be the best solution to comply with theAmerican Petroleum Institute (API 756) recommended guidelines.

Dynamic Air Shelters is a ISO 9001 2008 Certified manufacturer of flexible industrial solutions –ensuring the highest level of quality management systems are in place to serve its customers andstakeholders. The company serves several specific industries: Oil and Gas, Industrial,Construction, Mining, Military, Warehousing, Educational, Sports Facilities, Trade Shows, AircraftHangars, and Disaster Recovery/First Response with approximately 100 employees, andcurrently fields a significant footprint in Canada, United States, Europe, Australia, Japan, Russia,Azerbaijan, Netherlands, Qatar, Afghanistan, Cypress, South Africa, United Kingdom, andTrinidad.

Dynamic Air Shelters’ manufacturing is conducted in North America with facilities in Canada andthe USA, and offices in Calgary, Alberta; Grand Bank, Newfoundland; and Houston, Texas.

Contact our office:Toll Free 1-877-772-7734

sales@dynamicairshelters.com

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