IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

AMMONIUM NITRATE IN PORTS – STORAGE AND TRANSPORTATION

Robert Hutchison and Philip Skinner

Lloyd’s Register; e-mail: robert.hutchison@lr.org and philip.skinner@lr.org

Large quantities of ammonium nitrate are manufactured and transported by ship around Australia

supplying the main raw material for explosives used by the mining industry. Thus there is the poten-

tial for accidental explosions involving hundreds or thousands of tonnes of ammonium nitrate in

stockpiles near ports awaiting loading to ships or on board a ship.

The ammonium nitrate used as a raw material for explosives has a lower density and a higher

porosity than the ammonium nitrate used in the fertiliser industry, which makes it more sensitive

to explosion propagation.

There have been numerous accidents involving ammonium nitrate around the world over the past

century involving various grades and mixtures of ammonium nitrate. These include the major acci-

dents at Oppau, Texas City and Toulouse.

More recently, there have been terrorist attacks that have used ammonium nitrate based explo-

sives such as the Oklahoma City bombing.

Risk assessments of the transportation of ammonium nitrate must take account of all these

factors, which requires addressing the following questions:

. How should the consequences of an explosion of ammonium nitrate be modelled? What is the

TNT equivalence of pure and contaminated ammonium nitrate? Will the ammonium nitrate

detonate or deflagrate?

. How should the consequences of a fire involving ammonium nitrate be modelled? Can the grade

of ammonium nitrate support combustion? What are the products of combustion?

. How should the likelihood of an accidental explosion involving ammonium nitrate be esti-

mated? Is history a good predictor of future explosions?

. How should the likelihood of terrorist activities be estimated?

. What fraction of a load or stockpile of ammonium nitrate could explode? What fraction of a

load or stockpile of ammonium nitrate is likely to explode?

Risk assessments prepared for Australian ports that handle significant quantities of ammonium

nitrate are used to provide guidance on the above questions. This will assist the preparation of

future risk assessments to accurately assess the risk associated with transportation of large quan-

tities of ammonium nitrate by ship.

KEYWORDS: ammonium nitrate, fire, explosion, transportation, ports, shipping

INTRODUCTIONWithin Australia, there is a large market for explosives tosupport the mining industry. The main explosive used inAustralian mining has ammonium nitrate as a precursor.Ammonium nitrate is both manufactured in Australia andimported from overseas. Due to the size of the marketin ammonium nitrate, large quantities are transported bothinternationally and within Australia.

Ships are used for the international transportation ofammonium nitrate and potentially for movements betweenthe east and west coasts of Australia.

The local manufacturers and the importers ofammonium nitrate want to minimise their costs and solarge shipment sizes have occurred and are proposedfor the future within Australia. Where the load is sufficientlylarge that a ship can be chartered for exclusive use, themanufacturer or importer has increased control over thecondition of the ship and other specific aspects of the trans-portation. This can improve the delivered quality of the

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material through reduced potential for contamination andless damage to the bags. This also reduces the transportcost per tonne.

However, larger shipments have potentially largeraccident consequences. The larger a shipment of ammoniumnitrate, the larger is the maximum possible explosion, thedistance travelled by smoke from a fire and the pollutionpotential.

This paper examines the changes in risk and costs thatoccurs with changes in the shipment size of ammoniumnitrate. This paper focuses on the accidental explosionrisks associated with transportation of ammonium nitrateand does not consider smoke from fires and the risksassociated with terrorism.

Previous Quantitative Risk Assessments (QRAs) thathave been undertaken in Australia were used to provide theexplosion scenarios and likelihoods. Indicative costingestimates have been provided by people involved in thetransportation of ammonium nitrate.

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

This paper focuses on the differences in cost and riskassociated with an annual trade of 100 000 tonnes (te) ofammonium nitrate through an Australian port. This is alarge but credible annual trade volume for an Australianport. The shipment sizes considered are 100 te, 200 te,500 te, 1000 te, 2000 te, 5000 te, 10 000 te and 20 000 te.

SCENARIO IDENTIFICATIONMany of the scenarios that are considered in QRAs are notaffected by the shipment size. For example the risksassociated with explosion of a truck carrying ammoniumnitrate to or from the port are only affected by the truckload size and the annual trade, not the quantity that is onthe ship transporting the ammonium nitrate.

The scenarios that have been considered in this paperare those considered in previous QRAs in Australia:

1. A small explosion of 20 te of ammonium nitrate. This isconsidered a small explosion only by comparison to thequantities that can be carried on a ship. A fire on a shipcould conceivably directly affect 20 te of ammoniumnitrate, causing it to be heat affected or contaminatedby fuel or other organic material. The fire could thencause the 20 te of contaminated heat affected AN toexplode.

2. A partial explosion of 20% of the ammonium nitrate inthe shipment. The scenario that is envisaged here is theexplosion of 20 te of ammonium nitrate boosting afraction of the ammonium nitrate carried on the ship.The historical record suggests that only a fraction ofstockpiles of ammonium nitrate involved in explosionshave contributed to the overpressure wave.

3. A complete explosion of 100% of the ammoniumnitrate in a full shipment. This scenario is consideredthe worst credible accident and could occur due to anuncontrolled fire in a fully laden ship with all theammonium nitrate stored in one hold or in close proxi-mity. The uncontrolled fire could cause a fraction of theammonium nitrate potentially contaminated and heataffected by the fire to detonate and then to propagatethrough the rest of the shipment.

CONSEQUENCES OF SCENARIOSIn assessing the consequences of the explosion scenarios,there are a number of important parameters.

1. TNT equivalence. The TNT equivalence of pureammonium nitrate is considered to be in the range of30% to 55% based on both theoretical and experimentalresults. In this study an equivalence of 34.6% isused based on the ratio of the heat of detonation ofammonium nitrate of 378 kcal/kg to the heat ofexplosion of TNT of 1094 kcal/kg (Lawrence Liver-more National Laboratory 2002).

If the ammonium nitrate is contaminated during theaccident with a fuel, the TNT equivalence is increasedto approximately 1.0. For the large shipments ofammonium nitrate being considered in this analysis, it

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is unlikely for an accident to cause contamination of asignificant fraction� and a TNT equivalence of 34.6has been used.Despite the variation in the values of TNT equivalencefor ammonium nitrate, this is a parameter with lessuncertainty than the others in the analysis and thedifferences in equivalence do not greatly affect the con-sequence distances.

2. Fraction of inventory that contributes to the explosion.The fraction of inventory that contributes to the over-pressure shockwave produced by the explosion is acritical parameter but is very uncertain.In historical accidents, the fraction of the inventory thathas contributed to the overpressure has ranged over twoorders of magnitude from less than 1% at CherokeeNitrogen in 1973 (Freeman 1975), to 10% at Oppauin 1921(Medard 1990), to 10%–60% for Toulouse in2001 (Creemers, et al. 2002) and close to 100% atTexas City in 1947 (Klintz, et al. 1947).The reasons for the differences in the inventory frac-tions that contributed to the explosions include differ-ences in the material or grade of ammonium nitrate,the degree of confinement and the proximity of theinitial explosion to the bulk pile.In QRAs within Australia, the uncertainty surroundingthe fraction of inventory that will contribute to theshock wave has been accommodated by postulatingdifferent likelihoods for various explosion scenarios.In this study, the three different accident scenarios pri-marily differ in the quantity of ammonium nitrate thatexplodes and thus are considered with different likeli-hood estimates.This method implicitly utilises a risk based frameworkwhere both the consequence and likelihood are con-sidered. The regulators in Australia have establishedrisk-based criteria for assessing new developments.

3. Explosion modelling. Most recent QRAs in Australia onammonium nitrate have modelled explosion of anequivalent quantity of TNT to represent the explosionof the ammonium nitrate. The relationship betweenquantity of TNT and overpressure is well known anddocumented (Mannan 2005).The explosion modelling estimates the overpressure asa function of distance. There are also rudimentarymodels that estimate the shrapnel distribution from anexplosion but they are not considered further in thisstudy.The relationship between overpressure and likelihoodof fatality varies whether the affected person is insideor in the open air. In this study, an overpressure of14 kPa was considered to have negligible fatality risk,21 kPa to have a 20% fatality risk, 35 kPa to have a

�One accident that has occurred was where a bunker hatch at the bottom

of a hold was not sealed correctly. After loading with ammonium nitrate

bags, the ship filled its bunker tanks. The fuel flowed into the base of the

hold and over the duration of the voyage soaked into the bags. If this

material was exploded a TNT equivalence of 1 would be appropriate.

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

50% fatality risk and 70 kPa to have a 100% fatality risk(DIPNR, 1990).

4. People in the vicinity of the explosion. The modellingmust consider the people who may be in the dangerzone when an explosion occurs. In addition, the prop-erty that may be damaged also should be considered,particularly to assess the potential for domino orknock-on accidents.In this study, consequences have been restricted to fatal-ities to people. The port considered in this study isfictitious but is based on the numbers of people atdistances from a number of the ports that handleammonium nitrate in Australia.In the immediate vicinity of the ship (,50 m from thecentre of the accidental explosion), there are likely tobe 20 people including ship personnel, stevedores andinspectors. In the area between 50 m and 100 m fromthe centre of the accidental explosion, there is likelyto be 4 people, primarily associated with security, portadministration and truck flow management. Between100 m and 500 m there are likely to be other shipsand local storages and buildings on the port. 50people are assumed to be present in this region. Thearea between 500 m and 1000 m is likely to includeadministration buildings, as well as general port facili-ties, including warehouses, storages, container loadingonto trucks or rail cars. In this large area 1000 peopleare assumed to be present. Beyond 1000 m is likely tobe commercial areas and residential areas. The popu-lation density in this area is assumed to be 20/ha(which is the average residential density in Sydney).Figure 1 illustrates this data and shows the concen-tration of people at the ship during the loading/unload-ing operation. However the density of populationgenerally increases with distance from the ship, asdoes the numbers of people.In most of the explosion scenarios, a fire precedes theexplosion. This period can be used to evacuate peoplefrom the vicinity of the ammonium nitrate, which cansignificantly reduce the numbers of people who maybe killed or injured by an explosion. This factor hasnot been considered in this study.

Local Population

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Figure 1. Populations surrounding shipment of ammonium

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LIKELIHOODThe likelihood of the explosion scenarios is the area ofgreatest uncertainty. Since the three ship explosions in1947 and the one in 1953, I am unaware of any shipexplosions involving ammonium nitrate. Following thoseexplosions (over 50 years ago), significant changes weremade to the composition of ammonium nitrate and the emer-gency response actions that would occur in the event ofa fire.

However, the historical record cannot show that a shipexplosion is impossible. Thus, fault trees have been devel-oped for numerous QRAs, which identify the causalsequences that are required for an explosion to occur andsuggest likelihood or frequency values for the scenarios.

The likelihood estimates of the explosion scenariosconsidered in this study are based on recent QRAs preparedin Australia. These likelihood estimates are:

. 20 te AN explosion on a ship in a port for loading orunloading of ammonium nitrate: 1.1 � 1028 per cargo.

. Partial explosion of shipment (20% of full load):2.5 � 1029 p.a.

. Complete explosion of shipment (100% of full load):1.1 � 1029 p.a.

RISK OF ACCIDENT SCENARIOSThe risk of the explosions is estimated taking into accountthe physical consequences and the likelihood. In Australia,the New South Wales Department of Planning criteria areused by many state governments to assess the risk of pro-posed developments. The criteria are based on locationspecific individual fatality risk contours and the risk ofdefined overpressure and heat radiation levels. The criteriaapply to various land uses e.g. a new development shouldnot expose any residential land to fatality risks above1 � 1026 p.a.

Another criterion that is often considered is societalrisk expressed as an FN curve. There are no official FN cri-teria in Australia but there are various criteria that have beenapplied elsewhere in the world.

However, in a comparative risk assessment, such asthe subject of this study, a single measure of risk wasdesired. The Potential Loss of Life (PLL) is thesummation of the individual fatality risk levels at all thelocations where a person is assumed to be located. This esti-mate of risk does not take account of the potential forpeople to be injured but not killed, neither does it includethe potential for shrapnel to strike people, for people tobe injured by smoke from a fire or the potential for propertyto be damaged.

COST OF SHIPPINGThe cost of shipping ammonium nitrate comprises two com-ponents:

1. A fixed administration cost per shipment which is inde-pendent of the size of the shipment. This is assumed tobe $5000.

Average cost per tonne

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Figure 2. Average cost per tonne of ammonium nitrate

shipments

Fatality Risk

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Small shipexplosion

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Total

Figure 4. Location specific individual fatality risk surrounding

10 000 te shipments

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

2. The cost of the shipping. For loads smaller than a fullship, the cost is a fixed price per tonne (assumed to be$120 per tonne). For loads of 5000 te and above,a full ship can be chartered. The cost for loads of5000 te to 20 000 te is assumed to be $120 � 5000 ¼$600 000.

The costing is shown in Figure 2. It is significantly lesscostly to transport larger shipments, particularly if a shipcan be chartered for a full load.

RESULTSThe overpressure produced by explosions of shipments ofammonium nitrate is shown in Figure 3. The distances tofatal overpressures of 35 kPa range from less than 200 mfor 100 te of AN to approximately 1000 m for 20 000 teof AN. The distances to overpressure that will not causefatality and only has a low likelihood of injury (3.5 kPa)range from 1 km for 100 te AN to 5.6 km for 20 000 te AN.

The fatality risk to people located in the vicinity of aport undertaking 10 000 te shipments of ammonium nitrateis shown in Figure 4. The fatality risk to people located closeto the ship (,150 m) is dominated by the small shipexplosion because the likelihood is higher. The risk topeople located between 150 m and 500 m is dominated bythe partial ship explosion and for distances between 500 mand 1400 m the risk is dominated by the complete shipexplosion. The magnitude of the risks at all locations is

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Figure 3. Overpressure from explosions of various sized

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very low, less than 1.5 � 1027 p.a. This value should be jux-taposed with the NSW Department of Planning individualfatality risk criterion for sensitive locations in the vicinityof a proposed development, which is 5 � 1027 p.a.

The societal risk as a function of shipment size isshown in Figure 5. The larger shipments have generallylower likelihoods of explosions but the consequencesare substantially larger. With the smallest shipment size(100 te), the likelihood of a complete ship explosion is1.1 � 1026 p.a. and this could kill 28 people. All thesepeople are working on the port and have some degree ofvoluntary acceptance of risk. With the largest shipmentsize, the likelihood of a complete ship explosion is muchlower at 5.5 � 1029 p.a. but the number of people whocould be killed is much higher at 3600. Also, the majorityof these people would be members of the public with novoluntary acceptance of the risk. The criteria lines shownare the indicative societal risk criteria suggested in NSWbut are not mandatory. The societal risk associated with1000 te and 2000 te shipments is closer to the lower criterialine. The smaller shipments lie closer to the upper criterialine and the 10 000 te and 20 000 te shipments extendbeyond the upper criteria line because they could cause inexcess of 1000 fatalities, which is the limit tolerable usingthese criteria lines. However, other criteria lines are usedin other jurisdictions and the conclusions of the analysiswould be different.

Societal Risk

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Figure 5. Societal risk as a function of shipment size

Cost vs Benefit

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Figure 6. Costs and benefits as a function of shipment size

IChemE SYMPOSIUM SERIES NO. 153 # 2007 IChemE

Figure 6 shows the comparison between the cost oflarger shipments to society due to the potential explosionrisk and the benefits to the owner of the shipment throughlower costs. The overall fatality risk, measured by PLL,decreases as a function of shipment size until the 2000 teshipment size is reached. The PLL is larger for 5000 te ship-ments, lower for 10 000 te shipments and higher for 20 000te shipments. These changes in fatality risk are due tothe combination of the further extent of larger explosions,the population distributions and the lower likelihoodof larger explosions. The lowest PLL is at the 10 000 teshipment size.

The ratio between the shipment cost and the PLL islowest at the 2000 te shipment size. Either side of the 2000te shipment size, the changes in PLL are greater than thechanges in the shipment cost. However the risks are stillvery low for all the shipment sizes.

CONCLUSIONSThe risks of transporting large shipments of ammoniumnitrate through Australian ports is low due to the very lowlikelihood of large explosions coupled with the significantdistances (.1 km) to large populations. The risk is likelyto meet current individual fatality risk based criteria.

The societal risk associated with shipments ofammonium nitrate varies significantly with larger ship-ments. The distance from the ship to residential or commer-cial populations is an important factor in determining thefatality risk. For the population distribution and shipmentcosts considered in this study, the lowest ratio betweenPLL and shipment cost is for a shipment size of 2000 te.This shipment size also corresponds to the societal risk

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that is closest to the lower societal risk criterion suggestedfor use in NSW.

When considering the tolerability of the risk fromlarger shipment sizes, it is important to consider societalrisk and not just the individual fatality risks as the magni-tude of the consequences can be significant if a largeshipment of ammonium nitrate explodes.

REFERENCESCreemers, A.F.L., Kersten, R.J.A., van der Steen, A.C.,

Opschoor, G. 2002, The ammonium nitrate explosion in Tou-

louse, France – The incident and its consequences for indus-

trial activities, TNO web site.

DIPNR 1990, Risk Criteria for Land Use Planning, NSW

Department of Infrastructure, Planning and Natural

Resources: Hazardous Industry Planning Advisory Paper

No. 4

Freeman, R. 1975, The Cherokee Ammonia Plant Explosion,

Chemical Engineering Progress 71(11), November 1975

Klintz, G.M., Jones, G.W. and Carpenter, C.B. 1947, Report of

Investigations Explosions of Ammonium Nitrate Fertilizer on

Board the S.S. Grandcamp and S.S. High Flyer at Texas City,

Tex., April 16, 17, 1947.

Lawrence Livermore National Laboratory 2002, Cheetah

Model Description, web page current in September 2002,

http://www.llnl.gov/str/Fried.html.

Mannan, S. 2005, Lees’ Loss Prevention in the Process Indus-

tries, 3rd Edn.

Medard, L.A. 1990, Accidental Explosions Volume 2: Types of

Explosive Substances, Translator P. Fawcett, Ellis Horwood

Limited, chapter 23 (Ammonium nitrate and its thermal

decomposition) and chapter 24 (The explosive properties

of ammonium nitrate).