thetic gas would be supplying only 7 percent of the projected demand.

Gas from Alaska will total less than a trillion cubic feet by 1980, and somewhat more than 2 trillion cubic feet by 1990. The potential gas supply of Alaska has been estimated at more than 300 trillion cubic feet, about 28 percent of the total estimated gas potential of the U.S. Alaskan gas is found mostly with oil, and its production will therefore require the production of the oil as well.

Potentially significant amounts of gas could also be obtained from liquid fuels, such as naptha, and from the conversion of waste products such as cattle manure, sewage sludge, urban garbage, and so on.

The largest single supplementary source of gas is expected to be direct importation of gas in a liquefied form, via ocean tankers,

from Africa, South America, the North Sea, and Alaska. About 2 trillion cubic feet will be imported annually by 1980 and 4 trillion cubic feet by 1990.

This article reviews the properties of natural gas affecting its transportability, some of the historical developments leading up to the presently planned importation program, and some of the safety hazards posed by the transportation of large amounts of liquid natural gas (LNG) to major U.S. metropolitan ports.

Transporting Natural Gas

Natural gas is composed of methane, although there are usually small amounts of heavier hydrocarbons (ethane, propane) mixed in. Methane is a colorless, odorless gas at room temperature. At its boiling point of -259 degrees F. liquefied methane can be stored at atmospheric pressure.

Methane has a heating value of about 1,000 BTU per cubic foot of gas (measured always at atmospheric pressure, 14.7 pounds per square inch and at 60 degrees F.); that is, the combustion of a cubic foot of methane gives 1,000 BTU of energy. Less than 20 BTU need be removed from a cubic foot of methane to reduce it to a liquid at -259 degrees F. Since one cubic foot of liquid methane expands to a volume of 625 cubic feet of gas (at 60 degrees F.) there is a great advantage in shipping methane in its liquid state. LNG is a light liquid, with a density 0.42 times that of water. The molecular weight of methane is slightly more than half that of air, so that pure methane vapor is lighter than air at the same pressure and temperature. Methane leaking from a pipeline thus rises into the atmosphere. However, methane vapor evaporated from ING at atmospheric pressure is heavier than air because it is so cold (-259 degrees F.). Even when warmed by mixing with air, the resultant vapor-air mixture is still heavier than pure air. Consequently, the methane evaporated from an LNG spill will not rise, but will spread along the ground like gasoline vapor.

The first major shipments of LNG by tanker occurred in 1959 when seven successful tanker shipments were made from Lake Charles, Louisiana, to the United Kingdom. The tanker carried a modest cargo of 5,000 cubic meters of LNG equivalent to 110 million cubic feet of gas. In 1969 the Boston Gas Company imported from Algeria the first shipload of LNG into the U.S. By 1970 the first applications for long-term imports of LNG had been filed with the Federal Power Commission which has jurisdiction over the transportation and pricing of natural gas.

Over the past thirteen years the size and
number of ING tankers has grown dramat
ically from one tanker carrying 5,000 cubic
meters of LNG in 1959 to thirteen 1 NG tank

ers in service in 1971 with an average capa-
(continued on page 27)

(continued from page 22)

city of 40,000 cubic meters. By 1990 it is expected that the US. alone will need 130 large ING tankers for importing gas. Tankers of 125.000 cubic meters (2.7 billion cubic feet of gas) are now on order and can be expected to be the "standard" size for future operations.

A number of specific projects to import LNG into the US. have been proposed, and some have already been approved by the Federal Power Commission (FPC). Distrigas Corporation, a subsidiary of the Cabot Corporation, plans to import about 15.4 billion cubic feet of gas annually over the next twenty years in about fourteen shiploads of LNG per year. LNG for meeting peak demands or "peakshaving," will be delivered to Everett, Massachusetts and to Staten Island, New York. Initially the LNG will be regasified and distributed to seven gas companies serving New York and New England. Distrigas has indicated eventual expansion of its program when its second tanker of 45 billion cubic feet annual capacity is completed.

The FPC recently approved a much larger second project. By 1976 the El Paso Algeria Corporation plans to start importing from Algeria about one billion cubic feet of gas per day for use on the East Coast. Terminal facilities are planned for Elba Island off Savannah and for Cove Point on Chesapeake Bay in Maryland. Environmentalists have already threatened legal action

prevent construction at the Maryland

Lite, preferring a state park at the location.

A summary of planned and proposed LNG projects is presented in Table 1. If all of these projects are implemented as now proposed, the average daily import of LNG by 1980 will be about 250,000 cubic meters. This would require two large tankers of the 125,000 cubic meter size (or a larger number of smaller tankers) making deliveries on the East Coast each day. By 1990 this figure would be expected to double to four large tankers each day.

Question of Safety

Liquid natural gas can be transported by ocean tanker, by river and ocean-going barges, and by trucks. Since the steel used in conventional carriers becomes brittle at extremely low temperatures and because LNG boils easily, LNG carriers must have specially insulated tanks. Currently, two ocean-going tanker designs are employed, both carrying LNG at atmospheric pressure. The first kind uses self-supporting, insulated nks which are independent of the ship's ull. Insulation is attached to either the ship's hull or to the exterior of the 'tanks. Recent designs employ spherical self-supporting tanks. The second form of tanker uses "membrane tanks." In these tankers the hull of the ship serves as the tank wall. Insulation, and a thin (one millimeter or less) metallic cover or "membrane" over

it, keep the LNG from coming in contact with the ship's hull. The membrane tanker makes better overall use of space and the majority of tankers under construction or on order are of this type. Both kinds of tankers use the methane vapor "boil-off" for power during ocean crossings. Metal fatigue is considered to be a potentially serious problem with LNG tankers, and their long-term performance characteristics have not yet been determined.

As with any new rapidly expanding technology, questions concerning safety eventually arise. In the case of LNG, the question of safety can be posed: does the transportation, unloading, or storage of liquid natural gas entail any unique or extraordinary

TABLE 2

hazards that decision makers and, particularly, the public should be aware of? The answer at this time appears to be yes.

The potential hazard of LNG handling arises from the possibility of a major tanker accident during which a substantial amount of LNG could be lost and later catch fire. To the industry's credit, no such accident has occurred to date. Presumably, however, one will eventually occur. Preliminary theoretical calculations of the course of such an accident have been performed at the Massachusetts Institute of Technology and the major conclusions are presented

below.

Liquid natural gas in some ways poses less of a fire threat than conventional fossil

fuel liquids such as gasoline. Gallon for gallon, gasoline and similar oil products have 60 percent more energy than LNG. On a simple energy-per-volume basis, therefore, one would not expect a fire fueled by LNG to pose any more of a hazard than what is already accepted in the use of conventional oil products.

More hazardous would be a spill of LNG on the surface of waters without immediate ignition. Under these circumstances a cloud of combustible natural gas would form by the rapid evaporation of the spill. The cloud could then blow ashore and be ignited in populated areas. Given the many possible sources of ignition on land, such as automobiles and home heating units, it is very likely that such a cloud would eventually ignite and burn as long as some portion of I was of flammable composition. The details of such spills are considered below and estimates of the times needed for evaporation and the size of the resulting clouds are presented.

As stated above, LNG is much lighter than water. Therefore, if it is spilled on water it will float, although at first it will boil furiously. The heat required to evaporate the LNG will be supplied by the cooling and then freezing of the water beneath the LNG layer. As explained previously, the density of the cold methane vapor formed is greater than that of air at standard conditions even though its molecular weight is less than that of air.

It is possible to estimate the length of time necessary to evaporate a given volume of LNG after it has spilled onto water. One finds the approximate result that the evaporation time varies only as the one-third power of the spill volume. This means that large spills will evaporate at rates much greater than small ones. As an example, a large spill, which is eight times larger than a small one, will take only twice as long to evaporate.

Table 2 lists the evaporation times for a small (10 cubic meters), moderate (1,000 cubic meters) and very large (100,000 cubic meters) spill, the latter being equivalent to the entire cargo of an LNG supertanker. It can be seen that these times (ranging from one-half minute to eight minutes) are very short. Also listed is the maximum radius of the spill which is estimated at the time of total evaporation. These range from 53 feet for a small spill to 2,400 feet for a large one. Thus, a large tanker accident could result in a liquid spill of almost a mile in diameter.

The volume of the cold vapor cloud Evolved from the spill would be about 250 times larger than the original volume of LNG. This cloud would he expected to spread horizontally because of its weight. Estimates of the radius of the cloud are also presented in Table 2 for a time of fifteen minutes after the spill, which would be a typical travel time for a vapor cloud to drift a few miles to shore. The radius

of the cloud-if it continued to spread because of its weight-would be considerably larger than that of the liquid spill. We shall see, however, that a cloud would not be likely to get as large as this because it would continue to absorb heat from the ground or water below it.

A major effect not yet accounted for in the motion of the vapor cloud is that of the wind. The cloud would drift downwind, and wind forces would tend to elongate the cloud in the downwind direction. At the same time turbulence may tend to mix the cloud vertically with the air above it. Most important of all, heat would be added to the vapor cloud by virtue of its contact with the water and ground beneath it. When enough heat has been added to raise the vapor density to that of the surrounding atmosphere, the cloud would lose contact with the ground, and further heating from this source would cease. The cloud would then continue to drift downwind as a neutrally-buoyant air mass.

It is possible to estimate the radius of the cloud at the time when it becomes neutrally-buoyant. The radius, and the corresponding cloud height, are presented in Table 2 for various spills. These are substantially smaller than the computed radii for the cloud due to gravitational spreading, but are larger than the maximum radius of the liquid spill. Accordingly, it seems unlikely that a vapor cloud would continue to spread due to gravitational forces for as long as fifteen minutes, but the cloud would reach the width and depth given in Table 2 earlier and then would drift downwind without much further change in shape.

The principal conclusion to be drawn from the foregoing discussion is that an LNG spill on water will very rapidly vaporize and spread to cover a large area at ground level. A 100,000 cubic meter spill, possible from a supertanker, would form a vapor cloud more than a mile wide and twenty feet deep in less than fifteen minutes. Without making a more detailed calculation, it is evident that if a given volume of LNG is leaked over a period of time greater than what is considered here, it will vaporize as fast as it leaks.

It might be argued that a cloud of pure vapor would not burn because it is too rich to be flammable. However, it will certainly be diluted with air at its edges due to mixing, and thus could be ignited. It seems likely that once ignited a cloud would burn to completion as a turbulent diffusion flame, much as does a field of grass, although at a much higher rate. A closer analogy might be the burning of the zeppelin Hindenburg in 1937, in which 200,000 cubic meters of pure hydrogen burned in several minutes. The burning of the full cargo of an LNG supertanker would be equivalent to the burning of 100 Hindenburgs. A conflagration of that size in a major city could be a disaster.

It might seem unlikely that the full cargo

of a supertanker could be spilled at once. But because of the brittle fracture behavior of steel when cooled to LNG temperatures, the puncturing of one tank in a vessel by collision or grounding could lead to the cooling and then fracture of adjacent tanks, propagating the failure to both ends of the vessel.

Some research on the explosive properties and hazards of LNG is being carried out by the gas industry. Shell Pipeline Corporation, along with eleven gas and oil companies, conducted a study which indicated "that 'explosions' of weathered LNG spilled on water have a very low damage potential and that the possibility of an 'explosion occurring under marine transportation conditions is highly remote." These (vapor) "explosions" do not involve combustion, but merely very rapid boiling. They occur for spills of LNG samples which have become rich in the heavier, non-methane compo nents. Such enrichment would occur by methane boiling off during storage over very long periods of time. The large bulk quantities of LNG being shipped by tankers would not exhibit this effect because of the comparatively short times needed for transportation. The Institute of Gas Technology and the National Bureau of Standards are studying the thermodynamic properties of LNG under a contract with the American Gas Association. Thus there is a recognition on the part of the gas industry that safety is a relevant fact in the future use of LNG.

Perhaps the most immediate problem posed by this analysis is the planning and siting of LNG loading and storage facilities in the near future. Very careful consideration should be given to safety questions and the possible consequences of a major accident if large-volume facilities are to be constructed near major population centers. At the same time, further research, both experimental and theoretical, should be pursued and the results published in the open literature. Only in this way can the full impact of the LNG program on both the nation's energy problems and the public safety be assessed and the program's merits judged accordingly.

NOTES

1. "United States Energy, A Summary Review," US Dept. of the Interior, USGPO, Jan. 1972

2. BTU is the British Thermal Unit, a unit of energy which is equal to the heat required to raise the temper ature of one pound of water one degree Fahrenheit 3. "How the Arabs Changed the Dil Business Fortune, Aug. 1971, p. 115.

4. "National Gas Supply and Demand, 1971 1990 Staff Report No. 2, Federal Power Commission, USGPO Feb 1972, p. 3.

5. For further background material suo ibid., chapter 4 6. Fay, James A., Unusual Fire Hazard oil (NG Tanker The Fa Spills to be published Hoult, David P. 1972 Proceedings of Hazard of LNG Spilled on Water the Conference on LNG Importation and Terminal Safety. National Academy of Sciences. Boston Massachusetts. June 13-14, 1972, p. 87 102

7.

"LNG Continues Spectacular Growth ̈ Pipeline and Gas Joumal, June 1972