;11 bhpbilliton 

BHP Billiton Petroleum (Americas) Inc. 

1360 Post Oak Boulevard Suite 150 

Houston Texas 77056-3020 USA 

Tel 713 9618500 Fax 713 499 5994 

www.bhpbilliton.com 

Via Federal Express Overnight Service 

August 2, 2011 

Mr. Gregg Worley 

Section Chief, Air Permits 

Air, Pesticides and Toxics Division 

U.S. Environmental Protection Agency - Region IV 

Mail Code: 9T25 

61 Forsyth Street, S.W. 

Atlanta, Georgia 30303 

Attn: Andrew Porter 

RE:	 BHP Billiton Petroleum (GOM) Inc. 

Response to EPA Comments on Modeling Analysis 

DeSoto Canyon Area (Sake Prospect) 

Eastern Gulf of Mexico, OCS Waters 

Gentlemen: 

BHP Billiton Petroleum (GOM) Inc., herein referred to as "BHPB", designated Operator of 

Federal leases in the Outer Continental Shelf (OCS), Gulf of Mexico (GOM), Eastern Planning 

Area, submitted an application on October 14, 2010 for a Pre-Construction and Operating 

Permit. 

The purpose of the permit will be to conduct exploratory drilling activities utilizing a mobile 

offshore drilling unit (MODU) with associated support vessels in the DeSoto Canyon Area 

located in the Eastern Gulf of Mexico. The lease blocks are located east of the 87.5 degree 

longitude jurisdictional boundary. 

EPA Region IV personnel, Andrew Porter and Art Hofmeister, contacted our office on July 6, 

2011 to provide comments regarding the modeling analyses that was submitted in support of 

the PSDlTitle V Major Operating Permit application. 

This letter is being submitted in response to those comments. 

EPA Comment 1: Support Vessel Model Methodology 

BHPB Response: The proposed support vessel for the proposed exploratory activity include a 

combination of work and crew boats that will be used to transport personnel and supplies to the 

MODU. 

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For the purposes of the modeling analysis, the worst-case support vessels (work and crew 

boats) potentially available to BHPB for the Sake Project were determined based on comparison 

of total engine ratings and the applicable EPA Marine Engine Standard Tiers (40 CFR Part 94) 

for each work and crew boat. 

Potential hourly and annual emissions were then estimated for those worst-case boats to 

conservatively account for the worst-case MODU support vessels' emissions rates. The support 

vessels' maximum hourly emissions have been considered for modeling analysis while the 

support vessels are (1) at the MODU (2) within 25 miles of the MODU and (3) while enroute to 

or from the MODU. 

The support vessels can take multiple routes within the 25-mile radius enroute to or from the 

MODU. Therefore, BHPB modeled the emissions from the support vessels assuming that all 

emissions would be emitted through a single stack at the center of the MODU. The center of 

the MODU (also the center of the 25 mile radius circle) is considered to be most representative 

of the support vessel location due to the various vessel route options that can be taken to reach 

the MODU. 

Additionally the support vessels' emissions have been modeled through a main engine stack 

along with all proposed emissions from the MODU. As stack parameters vary between 

proposed support vessels, the stack parameters associated with the support vessel emission 

will also change. Although all support vessels' stack parameters cannot be defined as a single 

set, the conservatism built into the support vessel emissions along with modeling all MODU 

emissions as a single plume is expected to provide a representative estimate of the actual Class 

I area impact from the proposed drilling operations. 

EPA Comment 2: Main Engine Stack Height 

BHPB Response: The modeling analyses' single stack parameters were based on the stacks of 

each MODU's main engines. These are the largest source of emissions. The stack height used 

in the model is 35.92 meters which was calculated from the bottom of the hull. The stack height 

calculated from above the waterline is 25.01 meters which is a stack height difference of 10.91 

meters. 

Based on the conservatism added to the model (by including worst-case emission scenario 

through a single stack and generation of single plume including the support vessels), BHPB 

believes that modeled impacts will continue to provide a representative estimate of the actual 

Class I area impacts. 

Additionally, considering that even the shortest distance for the Class I areas from the worst 

case modeled location is a distance of 214 kilometers, BHPB does not expect that the 10.91 

meters difference in stack height will result in a significant change in Class I area modeled 

concentrations. 

EPA Comment 3: PM2.5 Initial Screening Modeling Impacts 

BHPB Response: Modeling of visibility impairment due to PM emissions required PM 

speciation because different sizes and phases of particulate matter affect visibility to varying 

extents. Therefore a PM speciation summary including documentation of PM2.

5 emissions, was 

provided in the modeling report. 
,
 
Page 3 of 3 

Although the proposed OCS source has the potential to emit PM2.5 in excess of the Significant 

Emission Rate (SER), a PSD Increment analysis is not currently required for PM 2.5; as the PM2.5 

Increment standards are not effective until October 20, 2011. 

A Class I PSD Increment analysis was completed for PM1Q emissions from the proposed 

operations and the modeled impacts for the 24-hour and annual averaging periods were 

provided. 

Based on the results of this analysis, the modeled impacts of PM1Q are below the Class 1 PM2.5 

Significant Impact Levels (SILs) of 0.06ug/m3 and 0.07ug/m3 for the annual and 24-hour 

averaging periods, respectively. As such, since the PM10 modeling analysis includes all species 

that contribute to PM2.5 BHPB expects to comply with the PM2.5 Class I SILs via compliance 

with the PM10 Class I SILs. 

Please do not hesitate to contact me at (713) 599-6349 or Susan.Wilson@bhpbiliiton.com if you 

have any questions or require additional information regarding the information contained within. 

Sincerely, 

Susan Wilson 

HSE Analyst Regulatory 

Gulf of Mexico Production Unit 

cc:	 Andrew Porter, U.S. EPA, Region 4, porter.andrew@epa.gov 

William Bruscino, Trinity Consultants 
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