Presentation on the use of air dispersion modeling in understanding potential risks of offshore oil and gas exploration and production.

1. 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

2. Managing Risks Utilizing Air
Dispersion Modeling for
Offshore Production Industry
Weiping Dai, Ph.D., P.E.
Ventura, CA
May 14, 2004
trinityconsultants.com
AWMA West Coast Section
Annual Meeting

3. 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.

4. 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

5. Air Emissions from Offshore Operations
Product 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 compression
All these products contain other constituents and
impurities that could be emitted to the atmosphere

6. Air Emissions from Offshore Operations
Processes with Potential Air Emissions
Drilling/Well Development
Fugitive natural gas, VOCs, CO2, CO, H2S, etc.
Production
Crude oil separation - Separation of gaseous components
such as natural gas and H2S
Natural gas conditioning - Removal of hydrogen sulfide
and/or carbon dioxide; fugitive BTEX
Maintenance (workovers)
VOCs, paints, HCl (scale removal)
Spills and Blowouts
Accidental release from leaking tanks, flowlines, valves,
joints, and gauges, etc. – VOCs and in-site burning products
Blowouts of associated natural gas and fire

7. Air Emissions from Offshore Operations
Air Emission Sources
Flaring (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

8. Potential Risks due to Air Emissions
Health & Safety Concerns
Hydrocarbons/VOCs (benzene, toluene, naphthalene, etc.) –
HAP, toxic/fire hazards
Hydrogen Sulfide – toxic (and even fatal at certain
concentrations) to humans and corrosive for pipes
Carbon Dioxide – Greenhouse gas contributing to global
warming
Glycols from natural gas processing – volatile and hazardous
Chlorofluorohydrocarbons (CFCs) – Good fire fighting agents
but causing ozone depletion; gradually phasing-out
Criteria 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.

9. 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.

10. 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)

11. 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 TSP
Blowout prevention regulations
Venting and flaring of natural gas

12. Air Dispersion Modeling

13. Why, or When, to Model?
Modeling typically conducted for one of the
following reasons
Regulatory Requirements
Air Quality Standards
Toxic air pollutants
Development of mitigation strategies
Engineering Assessments
Site planning
Emissions control
Hazardous Releases
Emergency response planning and operations
Risk assessment

14. Air Dispersion Modeling Tools
Criteria to select proper dispersion model:
Continuous Release (> 1 hr) vs. Short-duration
Accidental Release (< 1 hr)
Neutrally Buoyant vs. Dense Gas
Elevated Source or Ground-level Source
Gas/Aerosol (two-phase accidental release)
Toxic or Fire Hazards
Major Types of Models
Gaussian Models for continuous releases
Short-term accidental release models

15. Structure of a Dispersion Model
For Each Source
Physical Height
Pollutant Emission Rate
Coordinates
Stack Diameter
Stack Gas Velocity
Stack Gas Temperature
Dimensions Used to
Characterize Building Wake
Effects
Meteorology
Pasquill Stability Class
Wind Direction
Mixing Height
Ambient Temperature
Wind Speed
For Each Receptor
Coordinates
Groundlevel Elevation
Height Above Ground
Simulation of
Atmospheric Physics
Estimate of Air Pollutant
Concentrations at Receptors

16. Gaussian Dispersion Models
Suitable for steady-state continuous
emissions from
Flaring
Venting
Fugitive Emissions
Example Models: ISCST3, SCREEN3,
OCD, AERMOD, ISC-PRIME, CALPUFF.

17. 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
1
u
Crosswind factor
2
2
yy
1 y
exp
2 σσ2 π
 
− 
  
Vertical factor
2 2
2 2
z zz
1 (H z) (H z)
exp exp
2 σ 2 σσ2 π
    − + 
− + −    
     
χ( , , : )x y z H =

18. 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
e
H
2
1
-exp
2
z
e
H
2
1
-exp
2
y
y
2
1
exp
zy
u2
Q
=C
σσσσσπ
zz

19. Modeling Flare

20. Considerations for Flare Modeling
Heat Release and Radiation Loss
Effective Diameter
Emissions – Combustion Products and
Unburned Emitted Pollutants
High Exhaust Temperature
Significant Buoyancy Plume Rise

21. Distance From Flare (m)
Distance From
Flare (m)
Altitude(m)
155
175
195
215

22. Accidental Release Models
Toxic Models
Neutrally buoyant: INPUFF, AFTOX, etc.
Dense Gas: DEGADIS, SLAB, or
SOURCE5 for LNG source term calculation,
etc.
Fire Models
LFGRISK for LNG including Jet Fire model
and Pool Fire model

23. Considerations for Accidental Release
Release Rate
Release Duration
Release Conditions (temperature, pressure, hole/pipe size)
Dense gas vs. Neutrally buoyant
Ideal gas vs non-ideal gas
Liquid vapor equilibrium
Single compound flashing
Multi-compound flashing
Single-phase vs two-phase
Choked vs non-choked flow

24. 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
Slumping
Ground-hugging
Passive dispersion (neutrally buoyant)

25. Utilizing Modeling to Determine
Distance to Level of Concern
Modeled H2S Concentration at Sea Level
0
50
100
150
200
0 50 100 150 200 250 300 350 400 450 500
Downwind Distance (m)
Conc(ppm)

26. Utilizing Modeling to Identify Impact Area

27. Model Accuracy
40 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.”

28. Advantages of Modeling
Can 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

29. Contact Information
Weiping Dai, Ph.D., P.E.
Phone: 972-661-8100
Fax: 972-385-9203
Email: wdai@trinityconsultants.com
Address: Trinity Consultants
12801 N. Central Expressway, Suite 1200
Dallas, TX 75243