ASSESSING EXPLOSION HAZARDS OF LARGE HYDROCARBON CLOUDS FORMED IN CALM CONDITIONS: ARE WE DOING IT WRONG? Jerry Havens Department of Chemical Engineering University of Arkansas, USA 55 th UKELG Meeting on “Dispersion and Consequences of LNG Releases” April 26, 2016 - HSE Laboratory, Buxton Derbyshire
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ASSESSING EXPLOSION HAZARDSOF LARGE HYDROCARBON CLOUDS
FORMED IN CALM CONDITIONS:ARE WE DOING IT WRONG?
Jerry HavensDepartment of Chemical Engineering
University of Arkansas, USA
55th UKELG Meeting on Dispersion and Consequences of LNG ReleasesApril 26, 2016 - HSE Laboratory, Buxton Derbyshire
LNG IMPORT / EXPORT HISTORY (UNITED STATES) First LNG Cargo (27,400 m3 ), 1964 Lake Charles, Louisiana to Canvey Island. Federal regulation 49 CFR 193 (Department of Transportation) Liquefied Natural Gas
Facilities: Federal Safety Standards, promulgated in 1980, applicable to land portion of terminals. Approving authority for operations beyond the ship-to-shore boundary is Coast Guard.
49 CFR 193 is primarily consequence based, requiring the applicant to ensure public safety by enforcement of exclusion zones that do not extend beyond property lines -for two specific hazard categories: Unignited vapor cloud travel - distance to the LFL concentration level for
prescribed Design spills. Prescribed fire heat flux limits distance from ignited Design spills.
To now, UVCE hazard has been neglected based on assumption of handling pure LNG (treated as methane). U. S. NOW APPROVING LNG EXPORT TERMINALS, WITH HAZARDS OF ACCIDENTAL RELEASE OF HEAVIER-THAN-METHANE HCs. THIS PAPER IDENTIFIES POTENTIAL DEFICIENCIES IN DISPERSION MODELING AND EXPLOSION PRESSURE-LOADING (OVERPRESSURE) METHODS IN CURRENT USE.
IMPROVED REGULATORY PROCEDURES AREREQUIRED TO ADDRESS EXPORT TERMINAL HAZARDS
Large design spills presently being considered for both LNG and heavy hydrocarbons make it difficult for applicants to meet the required vapor exclusion distances (distances to the lflconcentration level cannot extend beyond the property line). The new way-around this problem is confinement of the gas cloud within property boundaries with gas-impermeable vapor fences --- introducing uninvestigated potential for increases in damaging explosion loading (overpressure).
Confinement of vapor clouds by fences, in addition to increasing explosion potential, can severely limit the utility of newly approved CFD dispersion computer methods for estimating cloud concentrations and explosion overpressures. CFD dispersion models recently approved have not been evaluated adequately for
predicting gas-cloud concentration fields confined by fences introducing additional uncertainty in the accurate prediction of concentration profiles and prediction of flame-runup overpressures most importantly with vanishingly low wind, stable, conditions.
Recently approved predictions of overpressures for flammable HC clouds formed from Design Spills in low or no- wind conditions for several export terminal applications in the U.S. are using methodology for estimating the overpressures (explosive loading) that have been shown by industry evaluations to be seriously non-conservative.
JORDAN COVE EXPORT TERMINAL (JCET)Proposed for Coos Bay, Oregon Artists Rendition VERY LARGE, HIGH-COST, PLANT
FULL CONTAINMENT TANK DIAMETER ~ 80 MFOUR LIQUEFACTION TRAINS SHIP OVERALL LENGTH ~ 300 MCONSTRUCTION COST ~ $6+ BILLION
This paper focuses on possible errors and unacceptable uncertaintiesIn the EIS for the JCET. The primary concern is the methodology used in the EIS to evaluate the possible consequences of unconfined vapor cloud explosions (UVCEs).
5
Predicting Explosion Overpressures(from documents supporting the JCET EIS)
The analysis of overpressure hazards from a VCE requires two primary simulation steps. The dispersion of the flammable cloud from a given release; The flame front and overpressure propagation from ignition of the flammable cloud.
Each of the above steps is a complex function of several parameters, including the following: Release characteristics, location and orientation Wind speed and direction; Ignition timing and location.
The process to identify a worst-case vapor cloud explosion is a complex task typically requiring numerous simulations. The following considerations were made (for JCET) to reduce the simulation matrix:
Releases from liquefaction trains 2 and 3 were considered; The same scenarios selected for vapor dispersion were used; Ignition locations selected to maximize flame acceleration to property lines.
In order to determine which scenarios will produce the largest overpressure hazards, a mapping methodology is needed to convert the inhomogeneous vapor clouds into equivalent stoichiometric clouds. The methodology used the Q9 criterion developed by Gexcon.
Focus on Selected Hydrocarbon Design Spills Specified for JCET
Storage Impoundment Design SpillVolume, gallons Volume, gallons Volume, gallons
LNG Tanks (2) 89,662,000 112,338,200 0 (Full Containment*)Propane (1) 15,670 43,935 (shared) 15,670Isopentane (1) 31,030 43,935 (shared) 31,030Amine (1) 17,205 17,245 17,205Ethylene (1) 14,000 43,935 (shared) 14,000LNG line 36 inch** NA 833,400 (shared) 827,740MR*** line 6 inch NA 833,400 (shared) 61,060
*ASSUMES NEGLIGIBLE PROBABILITY OF ANY LIQUID RELEASE**SHIP UNLOADING LINE (LARGEST LINE)
***MIXED REFRIGERANT
900000
800000
700000
600000
500000
400000
300000
200000
100000
0
Vapor Cloud Exclusion Zone IssuesVast Range of Design Spills for Tank and Largest Operated Line specified by Applicants
Ship Unloading Line design spills: range 28,900 gallons to 812,000 gallons; 2004-2009 (Left to Right)Jordan Cove Ship-Unloading Line Spill appears to be the largest to date.
spill
size
, gal
lons
Jordan Cove ExportShip Unloading Line
827,740 gallonsInto ConcreteImpoundment
833,400 gallonswidth length depth (ft)
85 85 ~15
The Large Spills at Jordan Coveare confined to the Property by Vapor Fences
VAPORFENCES
For Scaling
LNG TANKDIAMETER
~ 80 M
SHIPLENGTH~300 M
Overall PlantDimensions (fence location extremes)
900
m
800 m
SHIP UNLOADING LINE DESIGN SPILL827,740 Gallons
Estimated Cloud Area (> lfl)720 m by 400 m = 288,000 m2
Estimated Cloud Volumeassuming 2 5 m depth576,000 1,440,000 m3
Containment by Dikes and Fences?
JETTING AND FLASHING LNGASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,FROM WHICH EVAPORATION
IS PREDICTED
A SHIP AT THE DOCK IS ENGULFED.THE CONCENTRATION DISTRIBUTION
OF THE CLOUD IS NOT REPORTED-No concentration field specified;
preventing independent evaluationof explosion overpressure
method used.
ETHYLENE DESIGN SPILL14,000 Gallons
Estimated Cloud Area (> lfl)320 m by 400 m = 128,000 m2
Estimated Cloud Volumeassuming 2 4 m depth256,000 512,000 m3
JETTING AND FLASHING ETHYLENEASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,FROM WHICH EVAPORATION
IS PREDICTED
PLANT IS LARGELY ENVELOPEDCONCENTRATION DISTRIBUTIONOF THE CLOUD IS NOT REPORTEDNo concentration field specified;
preventing independent evaluationof explosion overpressure
method used.
MIXED REFRIGERANT (MR) DESIGN SPILL61,060 Gallons
Estimated Cloud Area (> lfl)720 m by 400 m = 288,000 m2
Estimated Cloud Volumeassuming 2 4 m depth576,000 1,152,000 m3
JETTING AND FLASHING REFRIGERANTASSUMED PARTLY CAPTURED IN
CONCRETE SPILL IMPOUNDMENT,FROM WHICH EVAPORATION
IS PREDICTED
THE PLANT AND SHIP ARE ALMOST ENGULFED.THE CONCENTRATION DISTRIBUTION
OF THE CLOUD IS NOT REPORTED.No concentration field specified;
preventing independent evaluationof explosion overpressure method used.
THE MR IS PROPRIETARY AND IS NOTIDENTIFIED TO THE PUBLIC THIS CREATES
ADDITIONAL UNCERTAINTY
12
The Dispersion Phase1. The first step in predicting overpressures for a UVCE is to estimate the
concentration field throughout the cloud as a function of time. In cases considered here (U.S.) this is now done almost exclusively with the FLACS model.
2. Since the gas concentration field is not uniform, the overpressures experienced will depend critically on the time and location of ignition; for each vapor cloud predicted to result from a given Design Spill, numerous simulations must be made to determine the importance of ignition time and location. Additional variables that must be considered are wind direction and speed for Jordan Cove wind speeds of 1 m/s and 2 m/s were specified for (stationary) wind directions. For each Design Spill numerous executions of FLACS were necessary.
3. Only after these concentration fields are specified can the calculations for overpressures proceed to attempt identification of the worst cases resulting from the various times and locations of ignition.
13
Assumed 2D Flow Area Parking LotConcentration Decreasing Right to Left
110 m
Focusing onModelingDispersion
in CalmConditions
RevisitBuncefield
ParkingLot
If we can understand and predict this approximately 2-D sections concentration dependence with time, can proceed confi-dently with estimation of explosion overpressures.
Considering Gas Gravity Current Dispersion PredictionsNo-wind, sufficiently dense, gas gravity currents can be turbulence-free. This has been demonstrated in laboratory (wind tunnel) experiments, justifying a realistic prediction of the dispersion in the parking lot at Buncefield.
Laminarization Criteria (from published literature)where
g = gravitational acceleration, m/s2g = reduced gravitational
acceleration, m/s2Qo= volumetric flow rate, m3/sU = fluid velocity, m/sU* = friction velocit
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