Paul Martino T h e Aerospace Corporation* G e r m a n t o w n , M d . 20767
LNG risk management A broad perspective of the hazards of liquefied natural gas ( L N G ) storage and transportation is presented
*The author was employed b j Ecology and Environment, Inc., Buffalo, N.Y.14225, when this article was written.
Natural gas is desirable as a fuel because it burns with very little pollution and is relatively safe for distribution to a diverse group of consumers. I t serves as a heating fuel in many homes, public buildings, and manufacturing plants. In addition, it is a major raw material feedstock used in the petrochemical industry to produce a number of chemical products-ammonia, methanol, and propylene, to name a few. The United States needs natural gas as an energy source. Domestic production currently satisfies essential requirements, but neither domestic sources nor pipeline imports from Canada or Mexico are likely to meet incremental demand, except at costs eauivalent to or exceeding the cost of importing liquefied natural gas (LNG). The U S . has been importing L N G primarily from Algeria. Currently, the approved level of imported LNG is 800 billion ft3/y. Imports could double by 1990 to meet the nation’s natural gas needs, according to a study conducted by the Office of Technology Assessment ( I ) , increasing by as much as 1.3-1.8 trillion ft3/y. To supplement the declining supplies of natural gas in the U S . , we could import large quantities of L N G from countries that have plenty of gas but no internal market for distribution, such as Saudi Arabia and Iran. However, because of the volatile political situation in Middle East countries adjacent to the Persian Gulf, the reliability of supply, particularly from Iran, is questionable. It is likely that in the not-too-distant future, the major sources of L N G will be Trinidad, Colombia, Chile, Australia, Indonesia, and Nigeria. To accommodate the flow of natural gas into the interior distribution system of the United States, it will be necessary to develop a number of terminals along the coastline. Therefore, the principles of risk management will continue to play an increasingly important role during the planning, design, and operation phases of these L N G terminals. A holistic approach It would be preferable to analyze L N G risk on the basis of past experience with such operations in the U S .
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Environmental Science & Technology
0013-936X/80/0914-1446$01,00/0 @ 1980 American Chemical Society
However, in contrast to the number in operation for many years in other parts of the world, there have been only a few L N G terminals operating in the
us.
For the most part, L N G in the United States has been handled primarily at “peak-shaving plants,” which produce liquefied natural gas and store it for use only during peak demand (normally during the cold winter months). There are approximately 90 peak-shaving plants in the U S . today, and all have operated safely for years. The nation has thus accumulated 860 tank-years of safe operating experience at L N G peak-shaving facilities, liquefaction plants, and import receiving terminals. This excellent safety record is attributed to proper project planning and the thoroughness of industry safety codes and government regulations that control the design, construction, and operation of these facilities. An accident, by definition, is an event that is not foreseen or intended. However, the probability and effects of an accident can be minimized to the extent that its conceivable causes and consequences can be envisioned by an analyst. A systematic analysis of the consequences of an L N G spill provides additional information for safe system design and for evaluation of risk. “Basically, the structure of risk is the probability of an occurrence of a consequence in combination with the manner in which that consequence is valued” (2). Figure 1 is a logic diagram illustrating the functional units of a n analytical process for risk analysis of a proposed L N G terminal. The diagram is a modified version of a more general approach (3). By examining the requirements of each functional unit, the analyst can synthesize a range of situations, each classified according to the appropriate category. Table 1 is a listing of salient parameters and variables for each element of the logic diagram. This article discusses, in general terms, each functional unit of the logic diagram as it applies to risk aspects that should be considered during the planning stages of a typical L N G terminal. T h e goals of an analysis of this nature are: to identify the considerations necessary to prevent or limit the uncontrolled release of L N G to assure that, even in the remote event of an L N G spill, public risk is reasonably small. The first step is the performance of a hazards and system analysis of the proposed L N G terminal site. This is a critical study of the proposed trans-
portation and storage system, the properties of the hazardous material, potential system accidents, natural occurrences, and abnormal events. The study refers to external environmental factors possibly affecting the system or its credible failure events. These factors include traffic levels, demography, land use planning, precipitation, wind speed distribution, and atmospheric stability conditions. The environment in which the proposed system is to operate may, to a large extent, determine the probability of accidents or failure and their consequences. The second step is a probability analysis, in which accident scenarios and safety impacts are considered in a probabilistic sense. Historical accident data, traffic data, and hypothetical accident scenarios are some of the inputs necessary to conduct a proper probability analysis. The third step, analysis of the consequences of credible accidents, is perhaps the most complex aspect of a risk analysis. It is here that the physics of a spill are modeled and the implications to population and property become evident. Once the analyses have been completed, the risk assessment function can be initiated. The assessment of risk is rather straightforward. It compares risk values to other activities and identifies unacceptable risk values. The discovery of unacceptable risks and their sources leads the way to mitigation actions, which comprise the fourth step. The risk mitigation function is shown as part of a feedback loop (a dashed line) in Figure 1. Examples of mitigation measures include redesign of storage tank dikes, deepening the ship channels, or implementing vessel traffic control measures in the ship channels. A repeat of the sequential analytical process, now taking mitigation measures into account, is essentially a sensitivity analysis that indicates the amount of risk reduction that has been achieved. It also determines whether the mitigation measures are reasonable and whether the proposed terminal site will be acceptable through the proper integration of data inputs, thereby providing meaningful results for risk management. Hazards and system analysis Natural gas is composed almost entirely of methane with small amounts of ethane, propane, butane, pentane, and nitrogen. It can be liquefied a t atmospheric pressure by reducing its temperature to approximately -260 O F , in which state it physically occupies a space 1/600 of its
original volume. It is an odorless, nonreactive hydrocarbon which is nontoxic to plants or animals ( 4 ) . Methane, however, is flammable when its concentration in air is 5-15 vol %. The flammable property of natural gas makes it a desirable energy source. However, its hazardous nature under certain conditions necessitates the design of transportation, storage, and handling facilities to minimize the risk to employees and the general public. When conducting a comprehensive analysis of LNG-terminal risks, one considers the effects of natural and abnormal events. Potential hazards from existing and future land use are examined; for example, flight patterns at major airports and their relationship to the proposed site are determined. Other factors include sabotage, meteorite impacts, and seismic activity. Although a reality elsewhere in the world, the threat of terrorists sabotaging L N G terminals in the U S . is considered unlikely. Nevertheless, appropriate security measures to prevent such events are usually considered in the design and operation of a terminal. An uncontrollable but unlikely event is an accident caused by meteorites. It has been estimated that each year approximately 65 meteorites weighing more than one pound strike the surface of the earth ( 5 , 6). L N G terminals in operation today and those planned for the near future can have as many as four storage tanks, each having a capacity of 600 000 bbl, and can have moored a t the dock two 125 OOO-m3 L N G tankers, each 1000 ft long. The total horizontal surface area of the tanks and tankers is relatively small in comparison to the surface area of the U.S. (432 000 ft2 versus 1.05 X 1014 ft2). From these data and other formulas (6, 7), it has been calculated that the annual probability of a meteorite impacting a terminal’s storage tanks and tankers is 6.5 X lo-’, or one chance in 1.5 million per year. Seismic events are also uncontrollable; but through proper project planning, L N G terminals can be sited where seismic disturbances have not been recorded. Seismic risk zone classifications for different geographical areas of the U S . can be obtained from the National Earthquake Information Center (8). The zone classifications are based on the distribution of damaging earthquakes, but do not reflect the probability of earthquake occurrence. Uncontrollable natural events which occur frequently and may have an impact on public risk in the vicinity of the terminal include hurricanes. torVolume 14, Number 12, December 1980
1447
i I! J
+,
#
i
c
f
1
I
Routine weather1 climate Maritime environment Material properties Abnormal events Terminal accidents
Vessel spill probability
, .
Terminal spill probability
i
Pool fire Vapor plume Plume fire
1
Thermal criteria Exposure Risk assumptions Risk computation Risk comparison
1 ~
1
I
i i
Vessel Terminal
nadoes, electrical thunderstorms, and flooding. The frequency of tropical cyclones or hurricanes in a proposed terminal site area is of major concern. Historical information on the occurrence of hurricanes in the U.S., their maximum wind velocities, and the damage they cause is available ( 9 ) . Despite the fact that the preceding events are of low probability, a systematic analysis considers all possible accident scenarios. A scenario, in this context, is simply a guide to the changes that could accompany a specific event. The major accidents that are possible a t or in the vicinity of an L N G terminal are the collapse of an L N G storage tank, and a tanker accident which causes the rupture of a cargo tank. Tank collapse and vapor cloud The collapse of a fully loaded L N G storage tank can release a large amount of methane vapor to the atmosphere. Initially, the L N G vaporization rate is very high, but it gradually diminishes as the ground cools. Assuming nonignition, the downwind vapor flow follows the same pattern-a high initial flow and then a decline. There are a number of influential parameters which determine whether or not a fire could exist be1448
Environmental Science & Technology
yond the diked area: nonignition at the time of tank failure wind speed and direction atmospheric stability the moisture content of the impoundment floor the number of ignition sources in the plume's path. Figure 2 presents estimates of how far a flammable vapor plume can flow downwind from a collapsed storage tank, given various wind speeds and atmospheric stability classes. These distances were calculated by a computer employing well-known equations for L N G vaporization rate and vapor dispersion (10). The equations for vapor dispersion are derived from the statistical fluctuation of concentration about a mean value. This statistical theory leads to prediction of a Gaussian concentration distribution in the downwind and crosswind directions. As shown in Figure 2, only certain conditions cause a flammable plume containing 5% methane vapor to travel any significant distance downwind. For example, the downwind distance can extend as far as 7500 ft when a wind speed of 3.6 knots and atmospheric stability class F (Table 2) occur concurrently. Because of the low wind speed and stable atmospheric
conditions, dispersion of the methane vapor takes a long time and the flammable vapor plume extends a considerable distance. Different wind speeds and stability classes occur with known probabilities; therefore the probabilities of vapor transport distances also may be calculated. Annual probabilities of maximum downwind distances of a flammable vapor plume for a given set of meteorological conditions can be computed by use of S T A R Program Wind Distribution data by Pasquill Stability Classes from the National Climatic Center, Asheville, N.C. Table 2 presents a typical example of the probability distribution of atmospheric stability classes for various wind speed intervals in one U S . coastal region. From these data, it has been calculated that there is a 90% chance that flammable vapor will not travel more than 2350 ft from the spill of a 600 000-bbl storage tank. This distance falls within property boundaries of many L N G terminals in operation today, as well as of those designed for the future. Tank collapse and fire It is likely that a tank collapse will result in immediate fire within the diked area. A fire of this nature forms
TABLE 1
Risk parameters and variables Hazards 8 system analysis
Material properties Physical Chemical
Hazardous Abnormal events
Sabotage Aircraft crashes Meteorite crashes Seismic events Terminal accidents
Storage tank collapse Pipeline rupture Unloading line spill
Probability analysis
Consequence analysis
Terminal spill probability
Historical accident data Credible accidents
Risk assessment
Pool fire Immediate ignition Burning rate
Thermal criteria Thermal exposure time
Thermal radiation flux
Thermal dosage Fatalitv/iniurv I
Vapor plume Vessel spill Drobabllitv
L
Risk mitigation
Terminal Design Operation Vessel Design
,
Pool spreading rate Vaporization rate
Operation Vessel traffic control
Immediate ignition exposure Pool size Delayed ignition Vapor dispersion exposure Flammable vapor zone Number of exposed Vapor exposure time persons
Historical vessel accident data Vessel traffic data Potential dangerous locations Hypothetical Populated area accident scenarios exposure
Fatalities/injuries Risk assumptions
Tanker accidents
Plume fire Delayed cloud ignition
Collisions Groundings Rammings
Burning time Thermal radiation flux
Population density Number of ignition sources/person Ignition probability Nonignition probability Individual exposure probability
Routlne weather/ climate
Wind speedldirection Atmospheric stability Humidity/precipitation Temperature
Risk computation
Flammable zone Probable weather conditions Ignition probability each sector Average persons exposedf year Risk of fatality/ injury Annual individual risk
Maritime environment
Site accessibility Ship channels Tanker behavior Berthing arrangements Vessel traffic Demography Land use
Risk comparison
Everyday risks Acceptable/ unacceptable risks
TABLE 2
Annual stability class probabilities for various wind speeds Stablllty class
A = Extremely unstable B = Unstable C = Slightly unstable D = Neutral E = Slightly stable F = Moderately stable
0-3
0.001575 0.010777 0.005160 0.012421 0.000000
0.069070
4-6
0.002580 0.019317 0.018403 0.036556 0.038245 0.071947
Wind speed class Interval, knots 7-10 11-16 17-21
>21
-
-
-
0.025048 0.306694
0.003927 0.046991
-
-
0.015298 0.062768 0.155493 0.090510
-
Source: Wind Distribution by Pasquill Stability Classes (STAR Program)-Station
Total
0.004156 0.045392 0.000183 0.115490 0.007033 0.565189 0.128756 0.141017 1 .oooooo
No. 12926, 1965-69, National Climatic Center, Asheville, N.C.
Volume 14, Number 12, December 1980
1449
a tall flaming cylinder tilted in the direction of the wind. Thermal radiation fluxes from the fire can be calculated from radiant heating formulas which are applicable specifically for L N G spillage on land (10). Figure 3 illustrates the maximum thermal radiation flux at ground level from a fire in a diked area containing the entire contents of an L N G storage tank. Thermal exclusion area criteria (recommended by the National Fire Protection Association (NFPA-59A) and required by the U S . Department of Transportation) are noted on the figure; these correspond to radiation fluxes that can be absorbed safely by various vulnerable targets. In general, the target criteria are intended to protect off-site persons and structures and to prevent secondary fires. The implications of prolonged human exposure to thermal radiation can be gained from a study of the data relating to physiological effects as a function of thermal radiation and exposure time (11, 12). For example, prolonged exposure of bare skin to 1600 Btu/ft2.h for more than 30 s can cause skin blisters similar to sunburn. Exposure of bare skin to the same thermal radiation flux for about 100 s represents the threshold for significant injury. Once the effects are known, the potential for human injury can be determined by examining the thermal radiation fluxes for various distances. Caution should be exercised when applying these data to an analysis of an L N G fire, since the initiation and slow
FIGURE 2
looked information that is essential for proper and safe project design. For example, the waterway dimensions may not be adequate to accommodate L N G tankers that are large enough for profitable operation of a terminal.
LNG tanker accidents L N G tankers range in size from about 40 000 m3 to 165 000 m3. The standard size is 125 000 m3, Although unique design and materials are required to contain the cryogenic gas, a typical 125 OOO-m3 L N G tanker is similar to modern bulk cargo ships, with the propulsion machinery and deckhouse aft. The cargo tanks have either a spherical or prismatic configuration, and are recessed within the vessel hull to provide protection against tank rupture in the event of awn accident. Normally, there are five cargo tanks, each with a capacity of 25 000 rn3, within the hull of a 125 OOO-m3 vessel. The LNG cargo becomes dangerous to the public only if there is a spill and ignition. There are a number of factors that mitigate the probability of both an accident and a spill. To understand the mechanisms involved, an analysis of the shipping approaches to the terminal should be conducted. It should include an examination of ship channel dimensions, channel 6ottom composition, water depth variations caused by tidal flow, vessel traffic, and navigational systems. An in-depth study identifies critical features that may indicate high risk. The analysis may also reveal over-
Vessel spill probability A reasonable estimate of spill probability is based on an analysis of historical accident data concerning transportation on the waterways approaching the terminal. The U S . Coast Guard, Washington, D.C., compiles and makes available accident data for a number of waterways and ports in the U S . Computer printouts for individual waterways list the type of accident, extent of damage, characteristics of the vessel involved, and conditions existing at the time of the accident. Since it is of interest to assess the public risk from vessel accidents, only those accidents that result in a spill (such as collisions, rammings, and groundings) will be considered here. An accident rate per vessel transit can be estimated by relating the number of accidents to the number of vessel transits made on the terminal’s approach waterways for the same period of time. Annual summaries of vessel transits for ports and waterways in the U S . are available in “Waterborne Commerce in the US.,”compiled by the US.Army Corps of Engineers. The number of inbound and outbound transits is given according to vessel draft and type. T h e Coast Guard’s computer
on land
Vapor plum-pill 10000
buildup to peak thermal radiation flux allows adequate time and stimulus for most individuals to flee and seek shelter.
1
1 Vapor ptume vs. wind speed Storage tank cotlapre, 800 OOO bM, 5% methane vapor conccmtntlon
100
0
I
I
I
I
I
I
I
I
I
I
I
I
I
2
4
6
a
10
12
14
16
18
20
22
24
26
Wind speed (knots)
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EnvironmentalScience & Technology
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printout of vessel accidents classifies vessel size according to gross tons and length. Each sample set for vessel transits and accident data should include the same size range of vessels. Therefore, vessel draft should be related to either vessel gross tonnage or vessel length. These correlations can be developed by the use of empirical relationships available in naval architecture literature. The calculated accident rate estimates the probability of an accident without considering the magnitude of damage. Only a small portion of the accidents are severe enough to cause a cargo tank to rupture and spill LNG. Hence, estimates of the probability of an L N G spill should be based on pollution-causing incident data, available from the U S . Coast Guard.
For forecasting the consequences of an LNG tanker accident in the vicinity of an L N G terminal, the normal procedure is to analyze the instantaneous release of the entire contents of a single cargo tank as a result of a vessel collision. The likely outcome would be a large fire in the vicinity of the tanker. However, it is also conceivable that immediate ignition may not occur and that a flammable vapor cloud could travel downwind before ignition (13). LNG spill on water The behavior of L N G on water is fairly well understood from laboratory experiments and from field tests (14, 15). Variables such as L N G spreading rate, vaporization rate, and pool size have been measured and evaluated by
FIGURE 3
Thermal radiatioMpill on land 100 000
Thermal radiation vs. distance 800 000-bbl LNG fire
50 000
in tank Impoundment area
20 000
10 000
6700 6000
4000
1600
1000
425
100 100
500
1000
2000 3000 4000
Distance from center of fire (ft)
10 000
experimenting with spills under controlled conditions. A 25 OOO-m3 spill on water spreads to a maximum pool diameter of about 1800 ft in 440-720 s, with a mass vaporization rate varying from 93 000 to 105 000 Ib/s, respectively. As the uncontained pool forms, the spilled L N G vaporizes rapidly into a cold, visible, methane cloud at a rate of about 0.04 Ib/ftz of pool area per second, and moves with the wind. The extremity of the downwind plume flow is a function of atmospheric stability and wind speed and is shown for a number of atmospheric stability classes and wind speeds in Figure 4. As can be seen, the results of the L N G tanker spill are similar to those of the storage tank spill. The flammable vapor plume distances were computed from one of several mathematical models, available in the literature, which describe the physical phenomena of L N G spills on water (16). Only atmospheric stability classes A through D were included, since they generally occur during the day in a maritime environment and since it was assumed that L N G tankers operate only by day. As was the case for the storage tank accident scenario, the flammable vapor plume distance is dependent upon probabilistic meteorological conditions. It has been calculated that there is a 90% chance that the vapor plume would not extend beyond 27 000 ft, or about 4.5 nautical miles, A comparison of this distance with the calculated probable distance of 2350 ft for the storage tank collapse indicates a 10-fold difference in magnitude for even a smaller spill volume (a 25 OOO-m3 spill on water versus a 600 000-bbl or 95 400-m3 spill on land). This great difference is attributed to the uncontrollable nature of an L N G spill on water.
LNG fire on water The characteristics of an L N G fire on water differ from those of a storage tank fire on land because of the unconfined nature of the spill and the higher heat transfer from water to the floating L N G pool. The level of thermal radiation from an L N G fire depends on the rate of burning, which is determined by how fast the L N G is vaporized. The vaporization rate, in turn, depends on the rate at which heat is received by the L N G as a result of heat transfer from water and radiation from the flame. In the case of a burning L N G spill, the heat fed back as radiation from the flame results in a vaporization rate of about 0.01 lb/ft2 of L N G surface per second. The vaporization rate on water Volume 14, Number 12, December 1980
1451
FIGURE 4
Vapor plume-spill on water
1
F
100 000
Vapor plume vs. wind speed Cargo tank release, 25 OOOmJ, daytime cases
L
50 000
I
10 000
5000
-
3000
1
1
1
1
,
,
1
1
,
l
I
,
I
,
,
,
,
I
I
,
,
,
I
Wind speed (knots)
FIGURE 5
Thermal radiation-spill on water Distance from fire center 10 000
=
9000 8000 1 7000 6000 5000 4000
=E
3000 2000
-
3
2
= .-5
1000
900
2
800 700 600 500 400
E"z
200
3
5
-
e
300
100 90 80
70 60 50 40 30 20
: --
-1
-
-
Thermal radiation vs. time Cargo tank fire, 25 OOOm3, 10-mph wind speed, 75% relative humitity 100
300
200
400
Time (seconds)
1452 EnvironmentalScience & Technology
500
600
700
in the absence of burning is about 0.04 Ib/ft2-s. The combined vaporization rate for the LNG pool is, therefore, about 0.05 Ib/ft2.s. The flame diameter and flame height of a burning 25 OOO-m3 LNG spill are 1344 ft and 906 ft, respectively. More than 400 s elapse between ignition and the time of maximum thermal radiation. Figure 5 indicates the time dependence of the thermal radiation flux at various distances from the center of the spill. The thermal radiation data illustrated in Figure 5 are useful for evaluating the potential damage to people and property. For example, ignition of a wooden structure in the vicinity of a fire requires at least 5 min of exposure to thermal radiation flux of 6600 Btu/ft*.h. Figure 5 reveals that for the hypothetical fire considered here, a wooden structure within 1500 ft of the flame center, or 828 ft from the flame surface, would burn since thermal radiation levels at that distance are high enough to stimulate ignition. To determine the extent of danger to humans, it is appropriate to apply probability relationships given in the Vulnerability Model (12). In this model, the thermal radiation flux is integrated with time to compute a dosage which is related to a probit (substitute probability) function for an estimate of the fatality probability. Human fatality probabilities calculated from the model are given in Table 3 .
TABLE 3
Thermal radiation fatality probability Fatailty probability (%)
1
50 100
Distance from fire center (11) Upwind Downwind
3650 3050 2400
4200 3600 2900
Since the flaming cylinder of burning methane is tilted in the direction of the wind, the lethal distances vary, depending on whether a person is exposed to thermal radiation from upwind or downwind. The fatality probability values are applicable only to those individuals who are outdoors, immobile, and do not seek shelter. A 100%fatality probability exists within a distance of 2900 ft downwind from the center of a 25 OOO-m3 methane fire. Risk assessment and mitigation The principal concern expressed by the public is the occurrence of a transportation accident releasing L N G that is not immediately ignited, but in which flammable vapor is transported downwind, causing a fire some distance from the spill. Remote fires can be the most hazardous because they involve areas that can be more heavily populated than the spill site. The consequences of an L N G spill from storage tanks are of lesser concern for two reasons: The storage facilities can be designed so that impounding areas and dikes contain the entire contents of a storage tank. The design can control the L N G spill vaporization rate so that flammable vapor will have a low probability of extending beyond the facility’s property line and exposing the public to risk. In a collision involving an LNG tanker, the results of a potential spill are not as readily controlled. In order to assess public risk from a tanker collision, the annual probability of a spill is first estimated. This probability value is multiplied by the population exposure and divided by the entire population within the flammable risk zone. The annual individual risk of injury or fatality then can be determined. A realistic assessment of public risk from an L N G tanker collision includes identification of a hypothetical spill location on an aerial photograph, map, or nautical chart (usually a t the intersection of ship channels or con-
verging shipping lanes). A flammable risk zone is defined by a circle drawn about the spill location, with a radius equal to the probable maximum distance of the resulting flammable vapor plume. The circled area can be divided into sectors to evaluate the population a t risk. From census data or other relevant sources of demographic information (such as city or regional planning reports), the population density within each sector can be estimated. The probability of ignition a t any point depends upon the population within an area, since the number of potential ignition sources is directly related to the population density. In risk analyses, it is usually assumed that on the average there are four persons for every ignition source, and that each ignition source has a 1% probability of igniting the flammable vapor. The probability that a person will be injured because of delayed ignition of the vapor plume is thus the product of the probability that ignition occurs, that the person is outdoors and does not take shelter, and that the wind is blowing in his direction. As previously stated, for a 25 000m3 L N G spill on water, there is a 90% chance that the flammable vapor plume will not extend beyond 27 000 ft. However, on the U S . coast, population within this distance can be dense. Indeed, many environmental impact statements for L N G terminals indicate that populations range from zero to 20 000 within 5 mi. For simplicity, a uniformly distributed population of 10 000 within a 27 000-ft radius is assumed. If the circle is divided into 16 sectors for each compass heading corresponding to wind direction data, one obtains a population density of 625 persons per sector. Since there is one ignition source for every four persons, there are about 156 ignition sources in each of the 16 sectors. Each ignition source has only a 1% probability of igniting the vapor plume; hence, the probability of igniting the vapor plume in each sector is calculated to be 79.2%. Given random wind directions, the probability that the direction will lie in the sector of interest is assumed to be 20%. Similarly, there is a 20% probability that the person is outdoors and does not take shelter. Thus, the average number of persons exposed to injury in each sector is: 0.792 X 0.2 X 0.2 X 0.2 X 625 = 3.96 persons So for all 16 sectors, there are 63.4
persons exposed to injury. Therefore, the probable number of exposed persons in a large populated area can be very small, because of a combination of very small probability values. The exposure values from this simple calculation answer only part of the question since these values are valid only if a spill occurs. The probability of the occurrence of an event is the other important aspect of public risk. Environmental and risk assessment reports submitted to the Federal Energy Regulatory Commission for the approval of L N G terminals contain estimated spill probability values expressed in the number of L N G spills per tanker trips to the terminal. A review of these values (17) reveals that probability values range from a low of about 4 X IO-’ spills per tanker trip to a high of about 3.7 X 1 0-5 spills per tanker trip. Public risk t en can be estimated on an individual basis for a given number of L N G tanker trips per year. Assume that there will be about 100 L N G tanker trips per year to the L N G terminal. The upper limit to the annual probability of a spill is then 3.7 X which is the product of the number of tanker trips per year and the spill probability per tanker trip. Because of frictional heating, there is only a 10% chance that ignition will not occur on collision. In view of this, the probability of a vapor plume without immediate ignition and fire is 3.7 X For the 63.4 exposed persons in the flammable risk zone of 10 000 people, the individual risk of fatality or injury per person per year is 2.35 X This is a very small risk when compared to everyday human activity. According to U.S. National Safety Council statistics, the risk from motor vehicle accidents is 2.5 X fatalities per exposed person per year. Thus, the hypothetical case discussed here presents an acceptable risk; consequently, risk mitigation would not be required. If the calculated risk were too high, action would have to be taken to rectify the situation, either by vessel control or by altering the waterway design, or both. A general methodology The conceptual approach discussed in this paper can be thought of as a general methodology that may be employed to identify major shortcomings during the planning phase of any risky L N G venture. This approach is the result of an evolutionary process based upon years of knowledge and experience gained through the analysis of several proposed L N G terminal sites Volume 14, Number 12, December 1980
1453
along the eastern and southern coasts of the United States. By no means does this article explore all possibilities, but it does provide a basic framework which may be used as a guide by engineering designers, project planners, and general managers who are in the business of constructing and operating safe LNG terminals.
References (1) Oil Gas J., April 21, 1980. (2) Rowe, William D. “An Anatomy of Risk”; John Wiley and Sons: New York, 1977, p. 38. (3) Silvestro, F. B.; Mazurowski, M. J. “Computer Model for LNG/LP Risk Simulation,” presented a t The Transportation Research Board of the National Research Council, Washington, D.C., Jan. 16, 1978. (4) Patty, Frank A,, Ed. “Industrial Hygiene and Toxicology. Vol. 11, Toxicology,” Interscience Publishers, Inc.: New York, 1958. (5) Solmon, D. A,; Erdmann, R. C.; Hicks, T. E.; Okrent, D. “Estimate of the Hazards to a Nuclear Reactor from the Random Impact of Meteorites,” UCLA-ENG-7426; University of California-Los Angeles: March 1974. (6) “LNG Terminal Risk Assessment Study for Point Conception California,” Science Applications, Inc.: La Jolla, Calif., Jan. 23, 1976.
I
i I
II I I I
I I
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(7) Blake, V. E. “A Prediction of the Hazards from the Random Impact of Meteorites on the Earth’s Surface,” SC-RR-68-838; Sandia Laboratory, Aerospace Nuclear Safety: December 1968. ( 8 ) Fed. Regist. 1974,39(168). (9) “Climates of the States, Volumes 1 & 11”; Water Information Center, Inc.: Port Washington, N.Y., 1974. (10) “Project IS-3-1 LNG Safety Program, Phase 11 Report”; American Gas Association. (11) Stoll; Green, “Relationship Between Pain and Tissue Damage Due to Thermal Radiation,” J . Appl. Physiol. 1959, 14(3). (12) “Vulnerability Model: Assimilation System for Assessing Damage Resulting from Marine Spills,” U.S. Coast Guard Report No. CG-D-75; June 1975. (13) “Liquefied Natural Gas, Views and Practices, Policy, and Safety”; Department of Transportation, U S . Coast Guard: Washington, D.C., Feb. 1,1976. (14) Burgess, D. S., et al., “Hazards of LNG Spillage in Marine Transportation,” Final Report No. Z-70099-9-92317; U S . Department of the Interior, Bureau of Mines, Pittsburgh, Pa., prepared for the Commandant of the U.S. Coast Guard, Washington, D.C., February 1970. (15) “Spills of LNG on Water-Vaporization and Downwind Drift of Combustible Mixtures,” Report No. EE61E-72; Esso Research and Engineering Company: American Petroleum Institute, LNG Steering Group, Nov. 4, 1972. (16) Feldbauer, G. F.; Heigl, J. J.;McQueen,
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