The proposed Alaska LNG Gas Treatment Plant project required dispersion modeling to evaluate impacts on ambient air quality standards. Initial modeling found inconsistent results due to complex building arrangements of two nearby facilities. Wind tunnel testing was used to determine equivalent building dimensions to refine building inputs, which significantly reduced predicted concentrations. The environmental regulator approved the wind tunnel analysis approach. The modeling now demonstrates operations will not cause exceedances of air quality standards.

1. Sergio Guerra, PhD | GHD
Ron Petersen, PhD, CCM | CPP
Jim Pfeiffer | BP/Alaska Gasline Development Corporation
June 28, 2018
Use of Wind Tunnel Refinements in the
Dispersion Modeling Analysis of the
Alaska LNG Gas Treatment Plant
A&WMA's 111th Annual Conference & Exhibition,
Hartford, CT

2. Abstract
The proposed Alaska LNG GTP project includes the construction of a natural gas treatment plant on the Alaska North
Slope. The Gas Treatment Plant (GTP) is proposed to be located on the west coast of Prudhoe Bay and would treat
natural gas produced on the North Slope.
Initial dispersion modeling of the Alaska LNG Gas Treatment Plant (GTP) found results inconsistent with local and
regional measurements when evaluating compliance with the 1-hour NO2 National Ambient Air Quality Standard
(NAAQS) due in part to two adjacent nearby sources. These existing sources include the Central Gas Facility (CGF)
and Central Compression Plant (CCP) located immediately east of the GTP. The prevailing winds at the site are east-
northeast and west-southwest which align with the arrangement of the facilities.
The building downwash inputs generated by the Building Profile Input Program for PRIME (BPIPPRM) were evaluated
for the CGF and CCP facilities. This analysis confirmed that the building dimension inputs for numerous wind directions
were outside of the tested theory used to develop the building downwash algorithms in AERMOD. Previous
studies2,8,11,12,13 suggest that AERMOD predictions are biased to overstate downwash effects for certain building
input ratios.
Wind tunnel determined equivalent building dimensions (EBD) were conducted for the most critical stacks and wind
directions to refine AERMOD-derived predicted concentrations. The current paper covers the EBD method used to
refine the building inputs for the CGF and CCP facilities. The regulatory process and benefits from this physical
modeling method is also discussed.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

3. Outline
1. Introduction
2. WT Study
3. Regulatory Review
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

4. Alaska LNG Project
From: https://agdc.us/alaskas-lng-project/project-overview/
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

5. Locations of the existing CCP and CGF facilities
with respect to the proposed GTP facilities
Figure 1-1 from the Alaska LNG 7 Oct 2016 air modeling report.

6. Initial Screening Assessment
The following ratios are analyzed:
1. Hs/Hb ratios below 3.5
2. Width/height ratio (Wb/Hb) or
the length/height ratio (Lb/Hb)
and compare to ratio of 3.5
3. Wb/Hb and Lb/Hb ratios below
3.5 can also produce
overpredictions
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

7. Aerial views of Central Gas Facility (left) and
the Central Compression Plant (right) at
Prudhoe Bay, Alaska
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

8. Justification for Wind Tunnel Refinements
• Used for over two decades in regulatory permit modeling (ISC and
AERMOD).
• Source characterization technique used in lieu of BPIP, not subject to
alternative modeling requirements from Section 3.2.2 of Appendix W.
• The theoretical basis derived from the basic equations of motion in
dimensionless notation.
• Uses established methodology in EPA’s Fluid Modeling Guideline.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

9. Wind Tunnel vs BPIP-Derived Building
Dimensions
• BPIPPRM uses simplifications to
determine bdg dimensions used by
PRIME to calculate downwash
effects.
• Wind tunnel-derived building
dimensions can be used to refine
the traditional way of obtaining
building dimension inputs with
BPIPPRM.
Figure created in BREEZE® Downwash Analyst
BREEZE is a registered trademark of Trinity Consultants, Inc.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

10. Model Design and Construction
Obtain source/site data
Specify test wind speeds and
directions
Compute model operating conditions
Construct scale model

11. CGF facility looking northeast and
corresponding photograph of 3D printed model
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

12. CCP facility looking northeast and
corresponding photograph of 3D printed model
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

13. Identify Wind Tunnel Determined EBD
that Match Dispersion with Site
Structures Present
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

14. Acceptance Criteria
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

15. Regulatory Review
• The ADEC requested the technical assistance of the Environmental Protection
Agency Region 10’s Office of Air and Waste technical staff (EPA Region 10) in
reviewing this project.
• Review from EPA Region 10:
• (1) found that the use of EBDs was justified due to the complex building
arrangements of the nearby sources evaluated (CGF and CCP);
• (2) found that the methodology used in the study was in compliance with current
EPA guidance and recommendations for EBD studies and wind tunnel modeling;
• (3) recommended that ADEC allow the use of the EBDs for the GTP project as
presented in the final EBD report; and
• (4) identified possible issues when the lateral displacement of a stack from the
building center (YBADJ) is significant.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

16. Regulatory Review
• EPA R10 suggested a criterion where the use of the EBD method is most
appropriate when applied to Wb/Hb ratios that are outside the limits specified
in Equation 2 (less than 0.5 and more than 8.0). However, cases that fall
within the range of Equation 2 may still qualify for an EBD analysis, but
must be evaluated on a case-by-case basis.
• EPA R10 was concerned that using a different YBADJ from BPIPPRM could
create a realignment of near source plumes that could misrepresent the
concentration field and may prevent plume overlapping.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

17. Example of Plume Distortion due to YBADJ
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

18. PRIME Cavity and Wake Sources
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

19. Cavity Plume Unrealistic Concentration
Amplification Issue
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

20. Conclusions
• Downwash effects were evaluated for the two adjacent facilities (CGF and
CCP) next to the Alaska LNG GTP.
• This case study is unique because of the highly complex and elongated
arrangement of the two facilities.
• The EBD refinements obtained from the wind tunnel studies were reviewed
and approved by the regulatory agency. The predicted concentrations for
key receptors were significantly reduced when running AERMOD with the
building input refinements obtained from the wind tunnel study.
• As a result, AK LNG GTP is able to demonstrate that operation of these
nearby facilities will not cause an exceedance of the ambient air quality
standards.
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG Gas Treatment Plant

21. Sergio A. Guerra, PhD Ron Petersen, PhD, CCM
sergio.guerra@ghd.com rpetersen@cppwind.com
Mobile: + 612 584 9595 Mobile:+1 970 690 1344
Questions?

22. www.ghd.com
Use of Wind Tunnel Refinements in the Analysis of the Alaska LNG
Gas Treatment Plant