# ;11 bhpbilliton

BHP Billiton Petroleum (Americas) Inc. 1360 PostOak Boulevard Suite 150 HoustonTexas77056-3020 USA Tel 713 9618500 Fax713499 5994 www.bhpbilliton.com Via Federal Express Overnight Service August2, 2011 Mr. GreggWorley 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 EPAComments on Modeling Analysis DeSoto Canyon Area (Sake Prospect) Eastern GulfofMexico, 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 longitudejurisdictional 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 MajorOperating Permitapplication. This letter is being submitted in response to those comments. EPAComment 1: SupportVessel Model Methodology BHPB Response: The proposed support vessel for the proposed exploratory activity include a combination ofwork and crew boats that will be used to transport personnel and supplies to the MODU.Page 2of 3 For the purposes of the modeling analysis, the worst-case support vessels (work and crew boats) potentially available to BHPB forthe Sake Projectwere determined based on comparison oftotal engine ratings and the applicable EPA Marine Engine Standard Tiers (40 CFR Part 94) foreach work and crew boat. Potential hourly and annual emissions were then estimated for those worst-case boats to conservativelyaccountfortheworst-case MODU supportvessels' emissions rates. The support vessels' maximum hourly emissions have been considered for modeling analysis while the supportvessels are (1) at the MODU (2) within 25 miles ofthe MODU and (3) while enroute to orfrom 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 ofthe MODU. The center of the MODU (also the center ofthe 25 mile radius circle) is considered to be most representative ofthe supportvessel 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 asingle plume is expected to provide a representative estimate ofthe actual Class Iarea impactfrom the proposed drilling operations. EPAComment2: 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 ofemissions. The stack height used in the model is 35.92 meters which was calculated from the bottom ofthe 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 Iarea 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. EPAComment3: 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 of3 Although the proposed OCS source has the potential to emit PM2.5 in excess ofthe Significant Emission Rate (SER), a PSD Incrementanalysis is notcurrently required for PM2.5; as the PM2.5 Incrementstandards are not effective until October20, 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 ofthis 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 PM 10 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 ISILs. Please do not hesitate to contact me at (713) 599-6349 orSusan.Wilson@bhpbiliiton.com ifyou have anyquestionsor require additional information regarding the information contained within. Sincerely,

SusanWilson HSEAnalyst Regulatory GulfofMexico Production Unit

cc: Andrew Porter, U.S. EPA, Region 4, porter.andrew@epa.gov William Bruscino, Trinity Consultants\ \