Prepared by:

Weiping Dai, Ph.D., P.E.

BREEZE Software

12700 Park Central Drive | Suite 2100 | Dallas, TX 75251

+1 (972) 661-8881 | breeze-software.com

Managing Risks Utilizing Air Dispersion Modeling for Offshore Production Industry

Managing Risks Utilizing Air Dispersion Modeling for

Offshore Production Industry

Weiping Dai, Ph.D., P.E.Ventura, CAMay 14, 2004

trinityconsultants.com

AWMA West Coast Section Annual Meeting

Objectives

Understand potential air emissions from offshore oil & gas exploration and production operations.Study available air dispersion modeling tools to manage risks due to air emissions from offshore operations.Demonstrate the proper application of dispersion modeling tools.

Outline

Air Emission Characteristics from Offshore Operations (Pollutants/Release/Hazards)Potential Risks of Air Emissions (Health and Safety Hazards)Air Dispersion Modeling Tools and Applications

Air Emissions from Offshore OperationsProduct Characterization

Crude Oil – mixture of many different hydrocarbon (HC) compounds including natural gas.Natural Gas – associated gas from oil wells or non-associated gas from gas wells

Mixture of methane (dominant), ethane, propane, butane, etc.

Liquefied Natural Gas (LNG)Natural Gas Liquids – Propane and butane liquefied via cooling and compressionAll these products contain other constituents and impurities that could be emitted to the atmosphere

Air Emissions from Offshore OperationsProcesses with Potential Air Emissions

Drilling/Well DevelopmentFugitive natural gas, VOCs, CO2, CO, H2S, etc.

ProductionCrude oil separation - Separation of gaseous components such as natural gas and H2SNatural gas conditioning - Removal of hydrogen sulfide and/or carbon dioxide; fugitive BTEX

Maintenance (workovers) VOCs, paints, HCl (scale removal)

Spills and BlowoutsAccidental release from leaking tanks, flowlines, valves, joints, and gauges, etc. – VOCs and in-site burning productsBlowouts of associated natural gas and fire

Air Emissions from Offshore Operations

Air Emission SourcesFlaring (CO, NOx, SO2, VOC) and Venting

Combustion products from flaring natural gas or acid gas

Exhaust from diesel engines and turbines (NOx, PM, O3, CO, SO2)Fuel combustions from pumps, heater-treaters, and motors

Potential Risks due to Air EmissionsHealth & Safety Concerns

Hydrocarbons/VOCs (benzene, toluene, naphthalene, etc.) –HAP, toxic/fire hazardsHydrogen Sulfide – toxic (and even fatal at certain concentrations) to humans and corrosive for pipesCarbon Dioxide – Greenhouse gas contributing to global warmingGlycols from natural gas processing – volatile and hazardousChlorofluorohydrocarbons (CFCs) – Good fire fighting agents but causing ozone depletion; gradually phasing-outCriteria pollutants: SO2, CO, NOx, PM10, O3

Flaring of combustible/poisonous gases like methane and hydrogen sulfide reduces health and safety risks in the vicinity of the well.

Potential Risks due to Air Emissions

Personnel on-site or at nearby locations may expose to excessive toxic concentration or heat radiation.Damage to on-site or nearby property due to fires caused by accidental release.Adverse impact on ambient air quality.

Air Regulatory Requirements for Offshore Operations

Clean Air Act – covers coastal areas and the offshore regions of the Pacific, Atlantic, Arctic Oceans, and region of the Gulf of Mexico adjacent to Florida

National Emission Standards for Hazardous Air Pollutants (NESHAP)New Source Performance Standards (NSPS)

Air Regulatory Requirements for Offshore Operations

Minerals Management Service (MMS) Air Quality Standards (30 CFR Part 250) –covers Gulf of Mexico adjacent to Texas, Louisiana, Mississippi, and Alabama.

Limits for VOC, CO, NO2, SO2, and TSPBlowout prevention regulationsVenting and flaring of natural gas

Air Dispersion Modeling

Why, or When, to Model?Modeling typically conducted for one of the following reasons

Regulatory RequirementsAir Quality StandardsToxic air pollutantsDevelopment of mitigation strategies

Engineering AssessmentsSite planningEmissions control

Hazardous ReleasesEmergency response planning and operationsRisk assessment

Air Dispersion Modeling ToolsCriteria to select proper dispersion model:

Continuous Release (> 1 hr) vs. Short-duration Accidental Release (< 1 hr)Neutrally Buoyant vs. Dense GasElevated Source or Ground-level SourceGas/Aerosol (two-phase accidental release)Toxic or Fire Hazards

Major Types of ModelsGaussian Models for continuous releasesShort-term accidental release models

Structure of a Dispersion ModelFor Each Source

Physical HeightPollutant Emission RateCoordinatesStack DiameterStack Gas VelocityStack Gas TemperatureDimensions Used to

Characterize Building Wake Effects

MeteorologyPasquill Stability ClassWind DirectionMixing HeightAmbient TemperatureWind Speed

For Each ReceptorCoordinatesGroundlevel ElevationHeight Above Ground

Simulation of Atmospheric Physics

Estimate of Air PollutantConcentrations at Receptors

Gaussian Dispersion Models

Suitable for steady-state continuous emissions from

FlaringVentingFugitive Emissions

Example Models: ISCST3, SCREEN3, OCD, AERMOD, ISC-PRIME, CALPUFF.

Ambient Concentration Calculation with Gaussian Models

The concentration at the receptor at x, y, z from a source with effective height, H is:

Emissions Q

Downwind factor 1u

Crosswind factor

2

2yy

1 yexp2 σσ2 π

−

Vertical factor 2 2

2 2z zz

1 (H z) (H z)exp exp2 σ 2 σσ2 π

− + − + −

χ( , , : )x y z H =

Ambient Concentration Calculation with Gaussian Models

Ambient concentration is a function of emissions, downwind, lateral, and relative vertical distance from the source, cross-wise distance from the flow direction, wind speed, and PGT stability class

Any effects on plume behavior have to be parameterized in terms of dispersion coefficients and/or source height.

++

−−

2

z

eH

2

1-exp

2

z

eH

2

1-exp

2

y

y

2

1exp

zyu2

Q=C

σσσσσπ

zz

Modeling Flare

Considerations for Flare Modeling

Heat Release and Radiation LossEffective Diameter

Emissions – Combustion Products and Unburned Emitted PollutantsHigh Exhaust TemperatureSignificant Buoyancy Plume Rise

Distance From Flare (m)

Distance From Flare (m)

Alti

tud e

(m)

155

175

195

215

Accidental Release ModelsToxic Models

Neutrally buoyant: INPUFF, AFTOX, etc.Dense Gas: DEGADIS, SLAB, or SOURCE5 for LNG source term calculation, etc.

Fire ModelsLFGRISK for LNG including Jet Fire model and Pool Fire model

Considerations for Accidental ReleaseRelease RateRelease DurationRelease Conditions (temperature, pressure, hole/pipe size)Dense gas vs. Neutrally buoyantIdeal gas vs non-ideal gasLiquid vapor equilibrium

Single compound flashingMulti-compound flashing

Single-phase vs two-phase Choked vs non-choked flow

Dense Gas Modeling

Dense gas models consider the non-Gaussian behavior of concentrated, heavier-than-air, releases to the atmosphere.Heavier-than-air releases tend to display three distinct transport regimes

SlumpingGround-huggingPassive dispersion (neutrally buoyant)

Utilizing Modeling to Determine Distance to Level of Concern

Modeled H2S Concentration at Sea Level

050

100150200

0 50 100 150 200 250 300 350 400 450 500Downwind Distance (m)

Con

c (p

pm)

Utilizing Modeling to Identify Impact Area

Model Accuracy40 CFR 51 Appendix W – EPA Model Guideline

“Models are more reliable for estimating longer time-averaged concentrations than for estimating short-term concentrations at a specific location.The models are reasonably reliable in estimating the magnitude of highest concentrations occurring sometime, somewhere within an area.Errors in highest estimated concentrations of 10 to 40 percent are found to be typical. Estimates of concentrations that occur at a specific time and site are poorly correlated with actually observed concentrations and are much less reliable.Uncertainties do not indicate that an estimated concentration does not occur, only that the precise time and locations are in doubt.”

Advantages of ModelingCan be used to simulate as many actual or potential “what-if” emission scenarios as necessary.Useful to identify or predict areas of concern due to air emissions.Useful to determine the radius of impact to specific level of concerns for released toxic/flammable compounds.Useful to develop prevention/emergency response plan to reduce risk to potential toxic/fire hazards.Cost-effective

Contact InformationWeiping Dai, Ph.D., P.E. Phone: 972-661-8100Fax: 972-385-9203Email: wdai@trinityconsultants.comAddress: Trinity Consultants

12801 N. Central Expressway, Suite 1200Dallas, TX 75243