April 24, 2013

Mr. Stan Krivo

USEPA Region 4

Air Permits Section

61 Forsyth Street SW

Atlanta, GA 30303-8960

Re: Statoil Updated Air Quality Analysis – Remaining Issues

Statoil Gulf Services, LLC

Title V and PSD Permit Application

OCS Drilling and Exploration Activities

Dear Mr. Krivo:

Statoil Gulf Services, LLC (Statoil) submitted a Title V and Prevention of Significant 

Deterioration (PSD) permit application for proposed exploratory drilling in the Outer Continental 

Shelf (OCS) in the eastern Gulf of Mexico to EPA on September 5, 2012. On October 10, 2012 

EPA responded with a letter of incompleteness and a request for additional information required 

to continue the application review process. On behalf of Statoil, ENVIRON International 

Corporation (ENVIRON) provided EPA with an initial response letter in December 7, 2012, and 

another response letter with further information in January 28, 2013, which included a revised 

Air Quality Impact Analysis (AQIA).

On March 14, 2013, ENVIRON’s Bart Brashers received an email from Stan Krivo, requesting 

further information/explanations of Statoil’s revised Section 6 of the PSD application (dated 

January 28, 2013). This letter reiterates EPA’s additional questions, and provides the answers 

to each.

Question 1. Stack Exit Parameters – The following comments are associated with the 

modeled stack exit parameters provide in Tables 6.2.3-1, 2, and 3.

- The units for the columns Exit Velocity and Exit Temperature are incorrect (meters) in 

Tables 6.2.3-1 and 2. The correct units should be provided. 

ENVIRON has prepared revised Tables 6.2.3-1, 2, and 3 with corrected units and attached them 

to this letter. 

Question 2. - All stacks with horizontal stack orientation were indicated to be modeled 

with an exit velocity of 0.001 m/s. Horizontal stacks in Table 6.2.3-3 are not adjusted to 

this value and appear to have been used in the impact modeling.
The exit velocities of the Crewboats and Other Support Vessels (OSV), shown in Table 6.2.3-3 

were not directly used in the CALPUFF modeling. They were only used in the SCREEN3 runs 

that were used to determine the initial σz values. 

Question 3. - The basis for the stack heights should be provided (e.g., water surface or 

vessel deck).

All stack heights are relative to the water surface. 

Question 4. Volume Sources – The following comments are associated with Tables 6.2.3-

4 and 5.

- The forklifts are associated with the Discoverer America drill ship and their emissions 

have been included with the Discoverer America in Attachment 2 emission 

calculations. The reason the forklifts are included with the support vessel as a volume 

source should be explained.

The forklifts are mobile sources, working on the deck of the Discoverer Americas. They were 

therefore modeled as a volume source, centered on the location of the drilling vessel, and were 

not one of the string of volume sources representing the OSV/Crewboats. 

The location listed for the source FORK in table 6.2.3-4 of the January 28, 2013 AQIA is 

incorrect, but the location in the CALPUFF input files is correct, at LCC coordinates (941.964, -

1186.269). Revised tables are included as an attachment to this letter.

Question 5. - The vertical dimension of the modeled representative volume sources was 

the average height of the offshore support vessels divided by 2.15 (29.5 m or 96.6 

feet). The heights of the fleet of available support vessels used to develop this height 

should be provided.

The vertical dimension (σz) of the volume sources used to represent the OSV/Crewboats was 

not, in fact, based on the heights of the vessels as the document indicated. The last sentence 

in the paragraph in question (bottom of page 6-11 of the January 2013 Revised AQIA) is 

incorrect. 

Instead, σz was based on two SCREEN3 runs (provided on the modeling DVD). The primary 

emitter (main engines) of each vessel was used as a point source in a SCREEN3 run, using 

stack parameters provided by support vessel operators and listed in Table 6.2.3-3. The 

SCREEN3 output file lists the plume parameters at the point of maximum 1-hour concentration. 

From this line of output, the σz value for that vessel type was taken. The σz from the two 

vessels types were then averaged to find the value used in the CALPUFF modeling.

The σz value used for the crewboats is 3.79 meters, and the value for the OSV is 23.55 meters, 

which average to 13.7 meters, the value for σz found in the CALPUFF modeling.
Question 6. - The volume source heights were indicated to be based on SCREEN3 runs 

using actual stack parameters, building dimensions bases on ship’s dimensions, 

stability of 4, and wind speed of 20 m/s. As noted above for Table 6.2.3-3, the crew boat 

horizontal stacks should have been modeled with 0.001 m/s exit velocity. Review of the 

SCREEN3 modeling files reveals no building dimensions were used for these supply 

boats and the selected plume height for the crew boats was associated with 3 stability 

and 8 m/s. The bases for the heights (19.41 m) of the volume sources should be 

provided and explained.

The SCREEN3 runs did not use a specific wind speed and stability class, as indicated in the 

Revised AQIA near the bottom of page 6-11. Instead, the SCREEN3 runs used the more 

conservative “full meteorology” approach. This approach finds the maximum concentration over

a wide range of wind speeds and stability classes. It is a very conservative approach, 

guaranteeing the maximum model-predicted concentration.

The SCREEN3 output file contains, in the same line as the σz value found at the point of 

maximum 1-hour concentration, the plume height. The center of the plume for the crewboats is 

5.39 meters, and the center of the plume for the OSV is 33.42 meters, which average to 19.41 

meters. This value was used in the CALPUFF modeling.

Had the SCREEN3 run used an exit velocity of 0.001 m/s for the crewboat, the initial plume 

height would have been lower than 5.39 meters. Using this exit velocity yields 0.12 meters, in 

which case the average plume height for the crewboat and OSV would have been 16.77 meters, 

which is close to the value of 19.41 meters. Given the distance from the source to the nearest 

receptors, this error is exceedingly unlikely to change the outcome of the modeling.

The Protocol, the original AQIA, and the revised AQIA all took the position that the details of the 

initial plume rise are not important at the relevant source-receptor distances, and that 

downwash would not be included in the CALPUFF modeling. EPA did not dispute this 

contention. It is therefore consistent that the SCREEN3 modeling did not include building 

downwash.

Question 7. - The horizontal dimension for the representative volume sources are for 

both the longitudinal and lateral dimensions. The horizontal dimension modeled was the 

center to center distance between adjacent volume sources divided by 2.15 (5,121 m for 

long-term and 1,664 m for short-term). This appears not appropriate for the lateral path 

dimension and not following the guidance provided in the AERMOD User’s Guide for 

volume sources. Further explanation and/or demonstration of the appropriateness of the 

support vessels modeling procedure should be provided.

Table 3-1 of the AERMOD User Guide lists the suggested σy for a “Line Source Represented by 

Adjacent Volume Sources (see Figure 1-8(a) in EPA, 1995)” as the length of the side of a box 

divided by 2.15. Figure 1-8(a) from the ISCST3 Users Guide (EPA ,1995) is shown below:
Here, W is the length of a side of a box, which is equal to the center-to-center distance between 

adjacent volume sources. As agreed to in the Protocol, the mobile sources (crewboats and 

OSV) are modeled using ten (10) volume sources placed along their line of travel during the 

time they are within 25 miles of the drilling vessel. The CALPUFF modeling therefore followed 

this EPA guidance (the ISCST3 User Guide, essentially reiterated in the AERMOD User Guide). 

The essence of this guidance was recently re-affirmed by the Haul Roads Working Group in 

their 2012 document. 

Question 8. Recent Court Decision - PM2.5 SMC and SIL - The recent court decision 

concerning use of the PM2.5 significant monitoring concentration (SMC) and significant 

impact levels (SIL) as the basic for exemption from pre-construction air quality 

monitoring and cumulative NAAQS and PSD increment compliance modeling affects this 

application.

- The court has vacated and remained [sic] the PM2.5 SIL. Project impacts less than the 

SIL cannot, by itself, be used as justification to eliminate cumulative NAAQS and PSD 

increment compliance modeling. Additional information should be provided to support a 

conclusion that the project’s impacts would not cause or contribute to a NAAQS or PSD 

increment exceedance.

The requested justification for using the PM2.5 SIL can be found in the attached report, in 

Section 4. The report also provides an analysis of secondary PM2.5 formation in the region, in 

Section 5. According to a recently published peer-reviewed journal article, the region’s PM2.5 is 
dominated not by ammonium nitrate, but instead by ammonium sulfate. Increases in NOx in the 

region are unlikely to bring about corresponding increases in PM2.5, due to competition for 

available ammonia. 

Question 9. - The court also vacated the PM2.5 SMC. Project impacts less than the SMC 

can no longer be used to exempt the project from pre-construction ambient air quality 

monitoring. If existing air quality observations are proposed in lieu of pre-construction 

monitoring, supporting information should be provided to demonstrate that existing 

ambient air quality data would provide representative or conservative ambient 

concentrations for the impact area.

The requested supporting information in lieu of pre-construction monitoring can be found in the 

attached memo, in Section 4.

Question 10. PSD Class I Area Air Quality Related Values (AQRV) Assessment - The 

federal land managers for the Breton Class I area should be provided an opportunity to 

review and comment on the updated PSD Class I area AQRV impact analyses.

ENVIRON will cc: the relevant FLM (Jill Webster) with this correspondence, and verify she has 

received the January 2013 AQIA. 

Please contact me if you have further questions.

Sincerely,

Bart Brashers, PhD

425-412-1812

bbrashers@environcorp.com
Table 6.2.3-1

Developer Point Source Stack Parameters 

Used in CALPUFF Modeling Analysis

Stack Modeling 

ID

X-Coord Y-Coord Height Elev. Dia. Exit 

Vel.

Exit 

Temp. Orientation

(LCC) (LCC) (m) (m) (m) (m/s) (K)

Gen. Engine 1 GEN1 941.93 -1186.23 28.93 0 0.8 0.001 623.15 Horizontal

Gen. Engine 2 GEN2 941.93 -1186.23 28.93 0 0.8 0.001 623.15 Horizontal

Gen. Engine 3 GEN3 941.93 -1186.23 26.82 0 0.8 0.001 623.15 Horizontal

Gen. Engine 4 GEN4 941.93 -1186.23 26.82 0 0.8 0.001 623.15 Horizontal

Gen. Engine 5 GEN5 941.93 -1186.3 28.93 0 0.8 0.001 623.15 Horizontal

Gen. Engine 6 GEN6 941.93 -1186.3 28.93 0 0.8 0.001 623.15 Horizontal

Gen. Engine 7 GEN7 941.93 -1186.3 26.82 0 0.8 0.001 623.15 Horizontal

Gen. Engine 8 GEN8 941.93 -1186.3 26.82 0 0.8 0.001 623.15 Horizontal

E. Gen Engine EGEN1 942.01 -1186.23 9.45 0 0.4 0.001 888.15

Downward with 

45 degree angle 

to horizon

Lifeboat 1 LB1 942.02 -1186.26 19.7 0 0.9 0.001 830.15 Horizontal

Lifeboat 2 LB2 942.02 -1186.27 19.7 0 0.9 0.001 830.15 Horizontal

Lifeboat 3 LB3 941.94 -1186.26 19.7 0 0.9 0.001 830.15 Horizontal

Lifeboat 4 LB4 941.94 -1186.27 19.7 0 0.9 0.001 830.15 Horizontal

MOB Engine 1 MOB1 941.93 -1186.26 19.7 0 1.9 0.001 773.15 Horizontal

Cement Unit 

Engine 1 CMU1 941.98 -1186.3 9.45 0 0.2 0.001 794.15

Downward with 

45 degree angle 

to horizon

Cement Unit 

Engine 1 CMU2 941.98 -1186.3 9.45 0 0.2 0.001 794.15

Downward with 

45 degree angle 

to horizon
Table 6.2.3-2

Discoverer Americas Point Stack Parameters 

Used in CALPUFF Modeling Analysis

Stack Modeling 

ID

X-Coord Y-Coord Height Elev. Dia. Exit 

Vel.

Exit 

Temp. Orientation

(LCC) (LCC) (m) (m) (m) (m/s) (K)

Gen. Engine 1 GEN1 941.853 -1186.259 21.33 0 1 0.001 640 Horizontal

Gen. Engine 2 GEN2 941.852 -1186.261 21.33 0 1 0.001 640 Horizontal

Gen. Engine 3 GEN3 941.853 -1186.262 21.33 0 1 0.001 640 Horizontal

Gen. Engine 4 GEN4 941.853 -1186.268 21.33 0 1 0.001 640 Horizontal

Gen. Engine 5 GEN5 941.852 -1186.269 21.33 0 1 0.001 640 Horizontal

Gen. Engine 6 GEN6 941.853 -1186.271 21.33 0 1 0.001 640 Horizontal

E-Gen. Engine EGEN 942.063 -1186.25 21.33 0 0.6 0.001 640 Horizontal

Lifeboat 1 LB1 942.072 -1186.283 12.74 0 0.1 0.001 830.15 Horizontal

Lifeboat 2 LB2 942.072 -1186.247 12.74 0 0.1 0.001 830.15 Horizontal

Lifeboat 3 LB3 942.061 -1186.283 12.74 0 0.1 0.001 830.15 Horizontal

Lifeboat 4 LB4 942.061 -1186.247 12.74 0 0.1 0.001 830.15 Horizontal

Lifeboat 5 LB5 941.843 -1186.282 12.74 0 0.1 0.001 830.15 Horizontal

Lifeboat 6 LB6 941.843 -1186.248 12.74 0 0.1 0.001 830.15 Horizontal

MOB Engine 1 MOB 942.05 -1186.285 12.74 0 0.1 0.001 830.15 Horizontal

Air Compressor 

1 ACOMP1 941.843 -1186.282 15.85 0 0 0.001 773 Horizontal

Air Compressor 

2 ACOMP2 941.843 -1186.282 1.83 0 0 0.001 773 Horizontal

Table 6.2.3-3

Support Vessel Point Source Stack Parameters

Used in Developing CALPUFF Modeling Analysis

Stack Modeling 

ID

X-Coord 

(LCC)

Y-Coord

(LCC)

Height

(ft)

Elev.

(ft)

Diameter 

(ft)

Exit 

Velocity 

(ft/s)

Exit 

Temp 

(K)

Orientation

OSV (Main #1) OSV1 895.858 -1186.624 57.04 0 1.33 157.32 718 Vertical

OSV (Main #2) OSV2 895.858 -1186.624 57.04 0 1.33 157.32 718 Vertical

OSV (Main #3) OSV3 895.858 -1186.624 57.04 0 1.33 157.32 718 Vertical

OSV (Main #4) OSV4 895.858 -1186.624 57.04 0 1.33 157.32 718 Vertical

Crewboats (Main 

#1) C1 895.858 -1186.624 3.28 0 1.00 188.32 607 Horizontal

Crewboats (Main 

#2) C2 895.858 -1186.624 3.28 0 1.00 188.32 607 Horizontal

Crewboats (Main 

#3) C3 895.858 -1186.624 3.28 0 1.00 188.32 607 Horizontal

Crewboats (Main 

#4) C4 895.858 -1186.624 3.28 0 1.00 188.32 607 Horizontal

Crewboats 

(Generator #1) CG1 895.858 -1186.624 3.28 0 0.33 152.25 610 Horizontal

Crewboats 

(Generator #2) CG2 895.858 -1186.624 3.28 0 0.33 152.25 610 Horizontal
Table 6.2.3-4

Volume Source Stack Parameters for Support Vessels

Used in Visibility and Long-Term CALPUFF Modeling Analysis

ID X-UTM Y- UTM Height (m) Elev (m) Sigma Y (m) Sigma Z (m)

VOL01 895.858 -1186.62 19.41 0 2382 13.7

VOL02 900.98 -1186.58 19.41 0 2382 13.7

VOL03 906.101 -1186.55 19.41 0 2382 13.7

VOL04 911.223 -1186.51 19.41 0 2382 13.7

VOL05 916.345 -1186.47 19.41 0 2382 13.7

VOL06 921.467 -1186.43 19.41 0 2382 13.7

VOL07 926.589 -1186.39 19.41 0 2382 13.7

VOL08 931.71 -1186.35 19.41 0 2382 13.7

VOL09 936.84 -1186.31 19.41 0 2382 13.7

VOL10 941.964 -1186.269 19.41 0 2382 13.7

FORK (a) 941.964 -1186.269 2.74 0 10 1.4

(a) Forklift applicable only to the Discoverer Americas analysis

Table 6.2.3-5

Volume Source Stack Parameters for Support Vessels

Used in Short-Term CALPUFF Modeling Analysis

ID X-UTM Y- UTM Height (m) Elev (m) Sigma Y (m) Sigma Z (m)

VOL01 895.858 -1186.624 19.41 0 774 13.7

VOL02 897.523 -1186.619 19.41 0 774 13.7

VOL03 899.188 -1186.613 19.41 0 774 13.7

VOL04 900.853 -1186.608 19.41 0 774 13.7

VOL05 902.518 -1186.603 19.41 0 774 13.7

VOL06 904.184 -1186.597 19.41 0 774 13.7

VOL07 905.849 -1186.592 19.41 0 774 13.7

VOL08 907.514 -1186.587 19.41 0 774 13.7

VOL09 909.179 -1186.581 19.41 0 774 13.7

VOL10 910.844 -1186.576 19.41 0 774 13.7

FORK (a) 941.964 -1186.269 2.74 0 10 1.4

(a) Forklift applicable only to the Discoverer Americas analysis