ENVIRON International Corp. 19020 33rd Avenue West, Suite 310

, Lynnwood, WA 98036 V +1 425.412.1800 F +1 425.412.1840 environcorp.com July 11, 2013

Via E

- mail

Jill Webster US Fish and Wildlife Service Jill_Webster@fws.gov

Re: Statoil PSD permit

Dear Ms

. Webster

, The recent PSD permit ENVIRON prepared on beh alf of StatOil Gulf Services, LLC (Statoil) sought the flexibility to use one of two drilling vessels, the Maersk Developer and the Transocean Discoverer Americas . The revised air quality impact analysis (AQIA) we submitted for your review sh owed the l atter (Discoverer) did not “ pass ” the visibility degradation target of 5% change in 98 th percentile predicted extinction. The values for the three years of modeling were 7.19%, 5.06%, and 5.07%. Because the two latter years were jus t barely above the target of 5%, and because the emissions methodology we used was very conservative, we submitted the report for your review.

The purpose of this memo is to further clarify the conservative assumptions associated wit h the emission rates used in the visibility modeling, to highlight why we think the two years that are slightly above the 5% target change are over

- estimated and are likely less than 5%.

Table 6.4.11

- 4 of the r evised AQIA shows th e main components of the predi cted extinction change. It is clear that the secondary aerosols are largely controlled by NOx emissions, because the ammonium nitrate (NO 3 ) column is at least an order of magnitude larger than those of the other components , and two order of magnitude larger than most of the components . Therefore , the visibility impacts will be largely controled by NOx emission s, despite the fact that this is modeled as a

non

- linear process.

The crew boats and o ther support vessels ( OSV), collectively called support vessels herei n , were assumed to be the same for both drilling vess e l s. ENVIRON used the maximum 1

- hour emission rate for the visibility modeling, effectively assuming that the support vessels were operating all 24 hours of every day. In reality, although the frequency of visits to the drilling vessel by the support vessels is difficult to predict, they would almost certainly not be runn ing at 100% load (under full power as they approach o r depart th e drilling vessel) all the time. The OSV stated speed 1 is 12 knots (about 22 km/hr) and the crew boats 2 speed is 21 knots (about 39 km/hr). They would therefore cover the 25 nautical miles (about 46 km) in about two hours or less . During the time they are near the drilling vessel, they use their much smaller bow thruster engines to “ hover ” , so the emissions while near the drilling vessel are relatively small.

T he t otal short

- term NOx emission rate from the supp ort vessels is 257.80 lb/hr, taken from Table 6.2.3

- 14 in the r evised AQIA. The total NOx emissions from the dril ling vessel, is 554.94 lb/hr, taken

1 http://www.ottocandies.com/specs/peyton.pdf
2 http://www.grahamgulf.com/sybil_graham.htmJ ill Webster

- 2

- 7 /11/2013

from T able 6.2.2

- 10 of the revised AQIA. Thus the support vessels account for ab out 32 % of the total NOx emission rate of 812.74 lb/hr.

Without considering if any of the emission units on the drilling vessel would actually be operating all 24 hours of each d ay, can still make a reasonable assumption that the support vessels would not visit the drill ship more than once per day. Further, we can assume they s pend two hours within the 25 nautical mile radius w hile approach ing , and another two hours w hile depart ing , leading to four hours (not 24 hours) of operation per day within the radius

. This l eads to a 24

- hour “ reasonable” PTE of 257.80 × 4/24 + 554.94 = 597.91 lb/hr, or about 74% of the maximum 1

- hour emission rate of 812.74 lb/hr

.

Because NOx conversion to ammonium nitrate is the primary driver of visibility impairment in this situation, we can assume roughly that the 98 th percentile extinction changes for the Dis coverer scenario would be about 5.32%, 3.74%, and 3.75%, respectively for the 2001

- 2003 model years.

Even assuming two vis its per day for both the crew boat and support vessels on every day modeled

, which is still a very unlikely scenario, would yield 98 th percentile extinction changes of 5.68%, 3.99%, and 4.00%.

Please contact me as soon as possible if this clari fication is ins ufficient for you to decide whether or not to object to the permit, as EPA is very close to issuing a draft permit.

Sincerely,

Bart Brashers Atmospheric Scientist