Advances in LNG Hazard and Risk Assessment

8/12/2019 Advances in LNG Hazard and Risk Assessment

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INTRODUCTION

In LNG plant, as on other petrochemical sites, it is important to understand thehazards and risks associated with operation of the plant. It is crucial to have accurateassessment of such risks so that they can be minimized by the design and operation of the

plant.

In the Health, Safety and Environment Consultancy Group in Shell Global Solutionswe have developed a methodology, known as Shepherd, that integrates risks from avariety of fire, dispersion and explosion hazards. It presents the risks to people and assets,on- and off-site, in a way that is easy to assimilate. In Shepherd, items such as processunits and buildings are placed on a map and their relevant properties are specified. Outputis principally via a range of graphs and contour plots such as Figure 1. The capability totrace back the contributors to risk at a particular location is also provided.

Figure 1. Frequency of flammable vapour clouds for an exampleof a large petrochemical plant.

The hazards of vapour-cloud explosions were originally assessed in Shepherd byusing a worst-case explosion in each unit. However, even if an explosion occurs, attainingthe worst-case overpressure is unlikely; the gas cloud may only fill part of the plantarea, and its concentration may not be near stoichiometric. For this reason, in most areasof explosion assessment, we have moved to using a probabilistic assessment of the entirerange of possible explosion events. Results are often expressed as an exceedance curve,which is a plot of the frequency with which a given overpressure will be exceededagainst that overpressure. (An example appears later in this paper in Figure 9.) Such an

exceedance approach has been incorporated into Shepherd, and is described here.


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FULL EXCEEDANCE

The most comprehensive method for analyzing the range of explosion hazards that a plant is exposed to, is the full probabilistic approach described by Puttock [1]. This isoften used for offshore facilities, where personnel may be housed relatively close to plant,and a very detailed assessment is needed. We also carried out this full explosionexceedance study for all the processing units at a UK refinery. This involved a completecount of equipment, flanges, fittings and associated pipe-work with the potential to leakflammable fluids. Added to this were data on process streams (normal operating pressure,temperature and fluid composition), layout of the main equipment items (vessels, pumps,compressors, heat exchangers, furnaces, etc.), building construction types and occupancylevels. The methodology then calculated tens of thousands of vapour cloud dispersion andexplosion scenarios to predict the consequences of all foreseeable explosions at occupied

buildings on the site. Associated with each scenario is a frequency of individual riskderived from the frequency of the original leak multiplied by the ignition probability, thedegree of overlap of the flammable cloud with congested areas of plant and vulnerability

to explosion following damage to the building. To repeat this exercise for other siteswould not be cost effective so a generic version has been developed and implementedinto the Shepherd software and is described here. In this generic exceedancemethodology, the input and processing of data is considerably reduced over the fullexceedance and significant effort has been applied to minimise loss of accuracy.

THE RISK ASSESSMENT METHODOLOGY

The Shepherd risk software comprises a family of graphical risk integrators,containing the explosion exceedance methodology as well as other predictions such asrisk of flammable gas, flame impingement and toxic hazards. Shepherd has beendeveloped to carry out fit-for-purpose Quantified Risk Assessment (QRA) for a broadrange of onshore industrial sites such as LNG plants, refineries, gas plants, chemicals

plants, LPG distribution sites, pipeline systems, etc.

For explosion exceedance calculations, in comparison to the full method described inthe previous section, the Shepherd exceedance methodology is simpler, but more

pragmatic. It described in detail by Chamberlain and Puttock [2]. For this approach thegas dispersion scenarios and their associated explosion overpressures and impulses have

been collapsed onto two generic exceedance curves, one for flammable releasesoriginating within the process unit comprising the congestion area and one for releasesoriginating in adjacent areas.

A flow chart outlining the Shepherd explosion methodology is shown in Figure 2.The information in the numbered boxes is further described below:

1. The dispersion distance to the lower flammable limit is calculated, to examinewhether a flammable cloud released in one unit extends to adjacent congestedareas. The release rate is determined by the fluid properties and hole size. Arelease frequency must also be assigned using, for example, EP Forum [3]. Anignition probability must be assigned to each release. The data recommended byCox et al. [4] is one such source. The wind rose (direction, speed and stabilityhistogram) is also defined at this stage.


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Figure 2. The simplified explosion exceedance methodologyimplemented in Shepherd

2. For each fuel type impacting the unit, a worst-case explosion calculation is performed to determine the source size, pulse duration and overpressure. The rulesets used are the Congestion Assessment Method, CAM [5-7], for vapour cloudexplosions (using data from box 2a) and a modified BLEVE (Boiling LiquidExpanding Vapour Explosion) model [8-10] for vessel runaway explosions (usingdata from Box 3a). For enclosed process areas we assume that all flammable leaks

within the enclosures are capable of filling it with a flammable mixture, that leakswithin the enclosures will not migrate to neighbouring congested areas and thatleaks from neighbouring process areas will not migrate into the enclosures.

3. The total frequencies of releases and worst case explosions are mapped ontogeneric exceedance curves for each congested area, one for internal vapour cloudhits and one for vapour cloud hits from external sources. The blast decay as

pressure pulses move away from the source into the surroundings is alsocalculated at this stage.

4. The structural response of the building and building damage is coupled to the blast loading calculated in Box 3 by using the generic data contained in the 1995

Technology Co-Operative report prepared by Barker et al. [11]. Clearly moresophisticated building response calculations could be carried out if necessary. The

The SHEPHERD Methodol ogy for Assessi ng Vulnerabilit y of Buildi ng Occupantsto Process Plant Explosions

DATA COLLECTION SHEPHERD METHODOLOGY DATA COLLECTION

1a. Fluid Properties. START

CompositionPressureTemperatureHole size and frequencyDirection and probabilityLocation

1b. Distance to Lo wer Flammable limit 1. Ignition probability

2. Worst Case Explosion 2a. Area congestion.

3a. Reactive vessel 3. Generic exceedance curverunaway properties for overpressure andand frequency impul se. Generic decay of

blast into di stance.4a. Buil ding d ata.Occupancy 4. Generic buil ding d amageBuilding type and occupant vulnerability

5. Fatality rate in terms of: A. Ind ividual ri sk to the mostexposed person in the buildingB. Potential loss o f lif e (PLL)/yr for building and siteC. Building and s ite F(N)

END


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vulnerability of occupants to building damage has been derived from studies byOswald et al. [12], and by Jeffries et al. [13]. There are three vulnerability models

programmed into Shepherd: the model published in API RP752 [14], a pressure-only method based on assumed pulse durations and a Pressure-Impulse (PI)method. The last two methods are based on the generic building types of Barker et

al. [11].5. The fatality rates are derived from the time each individual spends in that building

and the total amount of time that all occupants spend in the building. All fatalityrates are summed and can be expressed in terms of risk markers such as IndividualRisk, risk contours, Potential Loss of Life per annum, or F(N) (group risk) plots.

Note that Shepherd exceedance ends with a prediction of the overall explosion risk per building or site. Application of risk tolerance and effectiveness of safety measures aredecisions for the site. Shepherd exceedance is not intended to replace the full riskmethodology, but it is a considerably more efficient way of carrying out the calculations.It is recommended that more detailed calculations should be carried out when makingcritical decisions, such as ones that involve severe consequences with borderline risks.

The calculations are designed to satisfy the guidance given in API RP752 [14] andCCPS [15] on explosion risks. The results are expressed as the risk of fatality for theindividual who is most exposed to building damage following an explosion in the plant.Also the group risk is calculated in terms of Potential Loss of Life (PLL)/yr and F[N] forthe site employees who may also be exposed to the building explosion risk. These riskmarkers enable the derivation of guidance to satisfy a sites risk criteria by way of:

1. design overpressures for new buildings or upgrades to existing buildings,

2. the siting of temporary buildings,

3. the siting of new buildings,

4. the siting of new processing units

5. levels of building occupancy, and

6. identity of the main risk contributors

THE LAYOUT TOOL

The Shepherd exceedance method is very useful for quantifying explosion risk onexisting plant. However, such risks are most easily controlled in the early stages of

design, when plant layout is being planned. The layout tool, now incorporated inShepherd, assists in this optimisation.

At such an early stage, the data needed to run the normal explosion exceedancecalculation are not available. For example, it is not possible to identify quantities andtypes of equipment on a unit that has not yet been designed. Thus a major part of thedevelopment of the layout tool was to gather data on typical characteristics of plant unitsof various types. These characteristics include level of congestion (needed for explosionassessment) and equipment counts. They can be scaled to take account of varying unitsize. Leak frequencies used for the layout tool are based on these equipment counts andthe Shepherd generic leak frequencies. This data gathering was performed both for

refineries and LNG units.


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Specifically for the LNG layout tool, it is assumed that an LNG train can contain thefollowing units

Liquefaction

Fractionation

Sulphur removal

Dehydration & Mercury Removal

QUALITY CHECK OF THE LNG LAYOUT METHODOLOGY

As a quality check on the layout tool, the original process data for an LNG plant have been entered into Shepherd using conventional process blocks and generic leakfrequencies for 10, 25, 50 and 100mm leaks. For comparison, the layout tool assessmentwas also performed.. The LNG layout tool was applied to all trains assuming that the onlyinformation available was the train footprints, building locations and building types.

Figure 3 shows the flammable gas frequency from all the trains. Figure 4 shows theindividual risk contours for occupants of B4 type buildings (steel or concrete frame withmasonry fill or cladding). An occupancy level of 40hr/week is assumed. The yellow areafor example indicates that the individual risk would be between 10 -5/year and 10 -4/yearfor someone spending forty hours per week in a B4 building in that location.

Figure 3. Flammable gas frequency


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Figure 4. Individual risk contours for occupants of B4 buildings.

Assumes 40hr/week occupancy.

Figure 5 shows a comparison of predictions of individual risk from the normalShepherd assessment and the layout tool. An occupancy level of 40hr/week has beenassumed. Clearly there is good agreement between the two approaches.


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LNG TrainsShepherd Exceedance v Layout

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Figure 5. A comparison of the individual risk calculations for occupants of B3buildings, as calculated by the original Shepherd method and the layout tool.

A SIMPLE EXAMPLEAs a example of the use of the layout tool, we show (Figure 6) a hypothetical LNG

train with an occupied building of B2 type (steel frame, metal sides) and maximumoccupancy level of 40 hr/week located 87m away. Let us assume that it is desired to keepthe maximum individual risk below 10 -4/year.

The proposal is to locate an identical train as close as possible to the existing one andto predict the increased risk to building occupants. The maximum individual risk (MIR)to building occupants from the existing train is 6.9E-05 per year, which meets the chosencriterion.


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Figure 6. A hypothetical LNG train with a nearby building. Contours of

individual risk for occupants of B2 buildings are plotted.

Figure 7 shows the proposed location for the new train. Risks from both trainsoperational. The MIR to building occupants is now 1.3E-04 per year, which exceeds the

project criterion.

Figure 7. The effect on explosion risk of adding a new train.


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There are then several options to satisfy the chosen risk criterion:

Re-locate the building. The choice of location can be based on the risk contoursand confirmed by dragging the building to the new location and recalculating

Strengthen the building based on the free-field overpressure and impulse

exceedance graphs. An example is shown in Figure 9 and described below. Choose a new location for the new train. This can be assessed by dragging the

process block to various new locations with the mouse and recalculating. Anexample is shown in Figure 8

Figure 8. As Figure 7, with the new train moved.

Graphical results can also be obtained. Figure 9 is an exceedance plot for free-fieldoverpressure at the building, that is showing the frequency with which any specifiedoverpressure will be exceeded. Impulse can be plotted in the same way.


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Figure 9. An exceedance plot of free-field overpressure at the building location.

MORE COMPLEX LAYOUT

A final example (Figure 10) shows the layout tool applied to a plant with a morevaried mix of units and buildings. In this case the contours of the frequency ofoverpressure exceeding 100 mbar are plotted.

Figure 10. The layout tool applied to a plant with a more varied mixof units and buildings.


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11. Barker, D.D., M.J.Lowak, C.J.Oswald, J.P.Peterson, M.W.Stahl, andJ.H.Waclawczyk Final Report: Conventional Building Blast PerformanceCapabilities, 1995 Technology Cooperative, Wilfred Baker Engineering Inc., 1996(Confidential).

12. Oswald, C. J. and Q.A.Baker Vulnerability Model for Occupants of BlastDamaged Buildings, 34 th Annual Loss Prevention Symposium, March 6-8, 2000.

13. Jeffries, R.M., L.Gould, D.Anastasiou and R Pottrill Derivation of fatality probability functions for occupants of buildings subject to blast loads, Phase 4. WSAtkins Science and Technology Contract Research Report 147/1997.

14. API Recommended Practice 752, Management of Hazards Associated withLocations of Process Plant Buildings. May 1995.

15. Center for Chemical Process Safety (CCPS) of the American Institute of ChemicalEngineers Guidelines for Evaluating Process Plant Buildings for ExternalExplosions and Fires., 1996.

Advances in LNG Hazard and Risk Assessment

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