Analysis / Perspective
Despite Disclaimers & Caveats, Sobering LNG Scenarios In Report To FERCTable 2.6 Effects on People for 1,600 BTU/hr/ft2 (5 kW/m2) Thermal Radiation
(May 17, 2004) -- A 128-page report by a firm under contract to the Federal Energy Regulatory Commission (FERC) announces: "This work considers the flammable vapor and thermal radiation hazards created by unconfined LNG spills on water resulting from an LNG cargo release."
What follows in the report (Consequence Assessment Methods For Incidents Involving Releases From Liquefied Natural Gas Carriers by Houston-based ABSG Consulting) is sobering...arguably more so because of its caveats. Example: "[I]t is important to keep in mind that the recommended methods cannot provide precise estimates of effects because of variability in actual incident circumstances as well as uncertainty inherent in the methods used")...plus the report's admission that scant data exist showing exactly what would happen in an accident or act of sabotage involving an LNG tanker vessel.
Given these admissions, we believe what the report does say is significant.
Among other things, it indicates that breaching an LNG tanker's hull and cargo tanks could produce a pool fire, burning people and property up to 4,600 feet (nearly a mile) away. The inferno could cause "severe pain" in 13 seconds, second-degree burns in 30 seconds, and third-degree burns within 50 seconds of exposure. A flammable vapor cloud could travel several thousand feet before dissipating into a stable condition.
And the report says that until models "are better developed and supported (experimentally and/or theoretically), a more conservative approach is recommended." In other words, since better data don't yet exist, it's prudent to assume the high end of consequential damage.
We poured over the report's data tables and dense verbiage and highlighted items we considered important. We endeavored to maintain the detail that our readers have come to expect along with caveats in the original.
This writer is not an LNG engineer...but neither are the government officials. We believe they have a duty to do what we did: read the material and try to understand it. [If we've misinterpretted any of the data, we'll gladly stand corrected if it's pointed out with specific references to the report text.]
Just in case our interpretation is in err, we have provided a link to the original complete text for our readers' reference.
We have transferred portions of the text we consider significant into viewable html format below. The text is not pleasant reading but we found it understandable with a little effort.
The first section discusses LNG hazards and is self-explanatory.
The second section discusses methods for addressing consequences. We noted the portion discussing the effects of thermal radiation on people (i.e. how much heat it would take to produce 1st, 2d and 3d degree burns). As we understand the text, based on federal agency and Nat'l Fire Protection standards, a heat level of concern is 1,600 BTU/hr/ft2 (5 kW/m2).
Chart 2.6 (which we've reproduced below) indicates that if exposed to that heat level, a person would experience "severe pain" in 13 seconds, first degree burns in 20 seconds, second degree burns in 30-40 seconds, third degree burns (1% fatality) in 50 seconds, an a 72% probability of 1st degree burns in 40 seconds.
The third section indicates times and distances at which LNG clouds will stay flammable and the amount of heat they could produce.
Table 3.1 (which we've reproduced below) posits an LNG release from hole diameters of 3.3 feet and 16 feet and indicates LNG pool fire calculations. For the 16 foot diameter hole, the flame length (height) could reach 1,400 feet. The downwind distance to the heat level of 1,600 BTU/hr/ft2 (the heat level causing the 1st, 2d and 3d degree burns described above) is 2,800 feet for a hole 3.3 feet in diameter...and 4,600 feet -- nearly 9/10 of a mile downwind -- for a hole 16 feet in diameter.
In other words (if we read the data correctly) these scenarios postulate that if a pool fire occurred after a spill from a vessel with a hole 16 feet in diameter, people nearly a mile away could suffer 1st, 2d or 3d degree burns in less than a minute.
Table 3.2 (which we've reproduced below) indicates distances by which an LNG vapor cloud would remain flammable (LFL means lower flammability limit, below which the cloud is too dilute for ignition). It indicates that from a 16 foot hole, the downwind distance (with a 20 mph wind) to the lower level of flammability would be 2.46 miles (13,000 feet); from the 3.3 foot hole, it's 2.08 miles (11,000 feet).
Left unstated is the obvious: the report discusses fires, burns and the like in the abstract -- as if the LNG were in a remote location -- not (as locally proposed) within the LB-L.A. Port complex, which harbors toxic, explosive and other hazardous materials on shore and aboard some vessels.
The report doesn't discuss the real world damage that might result if LNG got loose, ignited...and went on to ignite other things afloat and onshore at the Ports.
With those caveats, the report states in pertinent part:
When spilled onto water, LNG will initially produce a negatively buoyant vapor cloud (i.e., the cold vapors are more dense than air and stay close to the water or ground). As this cloud mixes with air, it will warm up and disperse into the atmosphere. If not ignited, the flammable vapor cloud would drift downwind until the effects of dispersion dilute the vapors below a flammable concentration. At a 5 percent concentration of gas in air, LNG vapors are at their lower flammability limit (LFL). Below this vapor/air ratio, the cloud is too dilute for ignition. At a 15 percent concentration of gas in air, LNG vapors are at their upper flammability limit (UFL). Above this vapor/air ratio, the cloud is too rich in LNG for ignition.
The downwind distance that flammable vapors might reach is a function of the volume of LNG
spilled, the rate of the spill, and the prevailing weather conditions. Also, in order to disperse to significant downwind distances, a vapor cloud must avoid ignition. Evaluation of ignition probability is beyond the scope of this study; however, it is noted that the large releases from an LNG carrier would likely require a significant energy source to initiate (i.e., to puncture the outer hull, inner hull, and cargo tank). An event of sufficient magnitude to rupture an LNG cargo tank may also provide ignition sources. If a flammable cloud is ignited by the initiating event or by other ignition sources (e.g., on the ship, on other nearby vessels, or on shore), the flame will burn back to the vapor source, and the flammable cloud would not travel a significant distance over land.
If a flammable vapor-air mixture from an LNG spill is ignited, it may result in a flash fire, which is a short duration fire burning the vapors already mixed with air in flammable concentrations. The flame front will burn back through the vapor cloud to the spill site, provided the vapor concentration along this path is high enough to continue burning. The rate at which this flame front travels relative to the unburned gas is called the laminar burning velocity. An unconfined methane-air mixture will burn slowly, tending to ignite combustible materials within the vapor cloud, whereas fast flame speeds tend to produce flash burns rather than self-sustaining ignition.
Although LNG vapors can explode (i.e., create large overpressures) if ignited within a confined space, such as a building or structure, there is no evidence suggesting that LNG is explosive when ignited in unconfined open areas. Experiments to determine whether unconfined methane-air mixtures will explode have been conducted and, to date, have been negative.
The principal LNG hazards of interest for this study are those posed by flammable vapor
dispersion and thermal radiation. Secondary hazards, such as cryogenic burns and asphyxiation, are typically localized to LNG transport and storage areas and are outside the scope of this study.
LNG is less hazardous than liquefied petroleum gas (LPG) and liquefied ethylene, which have (1) higher specific gravities, (2) a greater tendency to form explosive vapor clouds, (3) lower minimum ignition energies (MIEs), and (4) higher fundamental burning velocities. LNG is not toxic, and it rapidly evaporates; therefore, long-term environmental impacts from a release are negligible if there is no ignition of natural gas vapors.
1.4.1 Fire Hazards
LNG vaporizes quickly as it absorbs heat from the environment, and the resulting vapor is
flammable when mixed in air at concentrations from 5 to 15% (volume basis). Its fire-related
properties are comparable to other light hydrocarbon fuels (see Table 1.1). The only significant difference is that its molecular weight is considerably less than air, so once it warms above approximately -162 ºF (-108 ºC) it will become less dense than air and tend to rise and disperse more rapidly. However, LNG vapor at its normal boiling point -259 ºF (-162 ºC) is 1.5 times more dense than air at 77 ºF (25 ºC). Typically, LNG released into the atmosphere will remain negatively buoyant until after it disperses below its LFL.
Three types of fires -- pool fires, jet fires, and flash fires -- are postulated for the purposes of this study.
Pool Fire -- When a flammable liquid is released from a storage tank or pipeline, a liquid pool may form. As the pool forms, some of the liquid will evaporate and, if flammable vapor finds an ignition source, the flame can travel back to the spill, resulting in a pool fire, which involves burning of vapor above the liquid pool as it evaporates from the pool and mixes with air.
Jet Fire -- If compressed or liquefied gases are released from storage tanks or pipelines, the materials discharging through the hole will form a gas jet that entrains and mixes with the ambient air. If the material encounters an ignition source while it is in the flammable range, a jet fire may occur. For LNG stored at low pressure as a liquid, as it is in an LNG carrier, this type of fire is unlikely. Jet fires could occur during unloading or transfer operations when pressures are increased by pumping. Such fires could cause severe damage but will generally affect only the local area. This report focuses on large spills on water, and analysis of jet fires is outside the scope.
Flash Fire -- When a volatile, flammable material is released to the atmosphere, a vapor cloud forms and disperses (mixes with air). If the resultant vapor cloud is ignited before the cloud is diluted below its LFL, a flash fire may occur. The combustion normally occurs within only portions of the vapor cloud (where mixed with air in flammable concentrations), rather than the entire cloud. A flash fire may burn back to the release point, resulting in a pool or jet fire but is unlikely to generate damaging overpressures (explode) when unconfined.
2.7 EFFECTS OF THERMAL RADIATION ON PEOPLE
This section provides an overview of data and methods for estimating the effects on people and structures that result from thermal radiation exposure...
...Selecting Levels of Concern
As in all hazard assessment activities, thermal radiation levels of concern must be chosen with the nature of (1) the potentially exposed population and (2) the potential fire events. For example, permissible levels could be defined differently for (1) workers in a process plant who are wearing protective clothing, (2) areas where people are generally not present but could have access, or (3) sensitive populations, such as the elderly.
For purposes of onshore facility siting analysis, 49 CFR 193 and NFPA 59A specify a level of concern of 1,600 BTU/hr/ft2 (5 kW/m2). Based on the information presented in Section 2.7.1 [of the report], 1,600 BTU/hr/ft 2 (5 kW/m2) could be expected to cause the effects summarized in Table 2.6.
|Effect||Exposure Time (seconds)||Data Source [citations to report unless otherwise indicated]|
|Severe pain||13||Table 2.2|
|First-degree burns||20||Table 2.4|
(5 kW/m2for 20 seconds corresponds to a thermal dose of 100 kJ/m2)
(5 kW/m2 for 30 seconds corresponds to a thermal dose of 150 kJ/m2)
|Third-degree burns (1% fatality)||50||Table 2.4|
(5 kW/m2 for 50 seconds corresponds to a thermal dose of 250 kJ/m2)
|72% probability of first-degree burns||40||Table 2.5|
This level of 1,600 BTU/hr/ft 2 (5 kW/m2) is applicable to short duration events, such as fireballs. It is also applicable to the early stages of longer duration events, such as pool fires, provided that the potentially exposed population will have both opportunity and capability to quickly take cover...
CONSEQUENCE ASSESSMENT EXAMPLES
This section presents some example consequence analysis results using the methods
recommended in Section 2 [including portion above]. The scenarios presented here are chosen to illustrate the various analysis methods and are not intended to model any specific facility. For these examples, site-specific parameters (e.g., atmospheric conditions, surface roughness) were chosen either to match values used in other studies (for comparison purposes) or values required by 49 CFR 193 for LNG facility siting.
For actual analysis of a site, these parameters must be selected based on site conditions and the goals of the analysis...
This section presents the results of example pool fire calculations for an LNG spill on water. The scenarios examined are fires following spills from 3.3-ft (1-m) and 16-ft (5-m) holes in an LNG carrier just above the waterline. In both cases, the holes are assumed to be uniform in diameter and to penetrate through the outer and inner hulls and the cargo tank. The 3.3-ft (1-m) diameter hole represents a relatively long duration event, and the 16-ft (5-m) diameter hole represents a more rapid, short duration release. These example calculations are intended only as demonstrations of the modeling methods. The results should not be taken as a consequence assessment for any specific facility. Evaluation of a specific facility requires input parameter values based on site-specific conditions, and analysis of different or additional scenarios may be appropriate.
Figure 3.1 Estimated LNG Release Rates Based on Orifice Model
For these examples, it is assumed that the amount of LNG above the hole is 4.4 × 10 5 ft 3 (12,500m3), and the orifice model is used to estimate outflow, with flow rate dropping as the liquid level above the hole drops. It is assumed that the spill is ignited immediately upon release. The scenarios and input parameter are summarized as follows:
Hole diameters: 3.3 ft (1 m) and 16 ft (5 m)
Initial liquid height above hole: 43 ft (13 m)
Total spill quantity: 4.4 × 105 ft3 (12,500 m3)
Air temperature: 80 °F (27 °C)
Relative humidity: 70%
Wind speed: 20 mph (8.9 m/s)
Flame surface emitted flux: 84,000 BTU/hr/ft2 (265 kW/m2)
Burning rate: 0.058 lb/s/ft2 (0.282 kg/s/m2)
Table 3.1 summarizes the results of the pool fire calculations for these scenarios...Note that the results for the 3.3-ft (1-m) hole show that the total fire duration matches the total spill duration. This occurs because the pool spreads until the burning rate matches the spill rate into the pool. For the 16-ft (5-m) hole, the spill is much more rapid, lasting only 1.3 minutes, and the pool does not have enough time to spread to the point where the burn rate matches the spill rate. However, the large mass in the pool does result in a much larger
pool, and therefore the spilled material burns in a much shorter time.
Table 3.1 Summary of Results for Example Pool Fire Calculations
|Hole diameter||3.3 ft (1 m)|| 16 ft (5 m)|
|Initial spill rate||11,700 lb/s (5,300 kg/s)||290,000 lb/s (130,000 kg/s)|
|Total spill duration||33 min||1.3 min|
|Maximum pool radius||240 ft (74 m)||440 ft (130 m)|
|Total fire duration||33 min||6.9 min|
|Flame length (height)||910 ft (280 m)||1,400 ft (430 m)|
|Flame tilt at maximum radius||35 deg||31 deg|
|Downwind distance to 12,000 BTU/hr/ft2 (38 kW/m2)||
1,200 ft (370 m)||2,000 ft (600 m)|
|Downwind distance to 7,900 BTU/hr/ft2 (25 kW/m2)||
1,500 ft (450 m)||2,400 ft (720 m)|
|Downwind distance to 3,800 BTU/hr/ft2 (12 kW/m2)||
2,000 ft (600 m)||3,200 ft (980 m)|
|Downwind distance to 1,600 BTU/hr/ft2 (5 kW/m2)||
2,800 ft (860 m)||4,600 ft (1,400 m)|
3.3 FLAMMABLE VAPOR DISPERSION
This section presents the results of example dispersion calculations for an LNG spill on water. The scenarios examined are spills that are not immediately ignited and disperse downwind. Spills are from 3.3-ft (1-m) and 16-ft (5-m) holes in an LNG carrier just above the waterline. For these examples, it is assumed that the amount of LNG above the hole is 4.4 × 10 5 ft 3 (12,500 m 3 ) and the orifice model is used to estimate outflow, with flow rate dropping as the liquid level above the hole drops.
These are the same scenarios presented in Section 3.2 for pool fires, except in this case it is assumed that ignition does not occur immediately and ambient conditions are different. Also, as stated above for pool fires, these example calculations are intended only as demonstrations of the modeling methods. The results should not be taken as a consequence assessment for any specific facility. Evaluation of a specific facility requires input parameter values based on site-specific conditions, and analysis of different or additional scenarios may be appropriate...
Table 3.2 Summary of Results for Example Dispersion Calculations
|Hole diameter||3.3 ft (1 m)||16 ft (5 m)|
|Initial spill rate||11,700 lb/s (5,300 kg/s)||290,000 lb/s (130,000 kg/s)|
|Total spill duration||33 min||1.3 min|
|Heat transfer to LNG pool||11,700 BTU/hr/ft 2 (37 kW/m2)||11,700 BTU/hr/ft 2 (37 kW/m2)|
|Maximum pool radius||430 ft (130 m)||550 ft (170 m)|
|Total evaporation duration||34 min||18 min|
|Downwind distance to LFL||11,000 ft (3,300 m)||13,000 ft (3,900 m)|
|Time at which LFL reaches maximum distance||44 min||35 min|
|Time at which entire cloud drops below LFL||48 min||38 min|
|Downwind distance to ½ LFL||16,000 ft (4,800 m)||18,000 ft (5,500 m)|
|Time at which ½ LFL reaches maximum distance||53 min||45 min|
|Time at which entire cloud drops below ½ LFL||58 min||48 min|
The report also states:
It is also important to note that this study addresses the potential consequences of large scale LNG cargo releases without regard to the sequence of events leading to such an incident or their probabilities of occurrence. As such, this report does not and was not intended to provide a measure of risk to the public. A thorough risk assessment would consider both the probabilities and consequences of hazardous events. And finally it should not be assumed that the levels of hazards presented in this study are the assured outcome of an LNG vessel release, given the conservatisms in the models and the level of damage required to yield such large-scale releases...
While studying the results of this report, readers should keep the following key points in
Models have limitations. The recommended models/approaches represent a reasonable set of
tools to aid decision making. However, it should be recognized that models have an inherent
level of uncertainty, as described in this report. In some cases, the scientific community’s
understanding of the fundamental physical phenomena is better than its ability to simulate such phenomena. Also, models cannot make decisions for us. They inform decision makers, who
must integrate information on many different factors in their decision-making process.
Consequence assessment is only one piece of the risk picture. Understanding risk requires an understanding of (1) what can go wrong, (2) what the consequences might be, and (3) how likely the losses are to occur. This report focuses only on consequence modeling of potential release scenarios, not how likely such scenarios are to occur. Decision making related to scenario risks should be considered in the context of both potential consequence and expected likelihood (or frequency). With regard to potential attack scenarios, the expected likelihood is a function of both threat (the likelihood that someone would try to carry out a specific type of attack) and vulnerability (the likelihood that a specific type of attack would be successful and produce the expected consequences of concern). As noted in the report, LNG vessel and associated facility operations are highly regulated and closely monitored/controlled by authorities, so many layers of protection exist against losses. The dependability of these layers of protection was not addressed in this project, but are important considerations in understanding the total risk picture.
Risk perception and risk acceptance are complex issues. How individuals and groups of
individuals perceive or accept risks depends on many factors, which are often subjective with no clear right and wrong answers. Even when very precise/certain risk information is available, different people often react to the information in different ways. This project made no attempt to place value judgments on what risks people should or should not accept.
To view the report to FERC in pdf form, click www.soundenergysolutions.com." target="_blank">"Consequence Assessment For Incidents Involving Releases From Natural Gas Carriers.
The Mitsubishi subsidiary seeking to build and operate the LB LNG facility maintains a web site with information on its proposal, accessible at www.soundenergysolutions.com.
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