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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 behalf 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 showed the 

latter (Discoverer) did not “pass” the visibility degradation target of 5% change in 98th 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 just 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 with 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 revised AQIA shows the main components of the predicted extinction change. 

It is clear that the secondary aerosols are largely controlled by NOx emissions, because the 

ammonium nitrate (NO3) 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 emissions, despite the fact that this is modeled as a 

non-linear process.

The crew boats and other support vessels (OSV), collectively called support vessels herein, were 

assumed to be the same for both drilling vessels. 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 running at 100% load (under full 

power as they approach or depart the drilling vessel) all the time. The OSV stated speed1 is 12 knots 

(about 22 km/hr) and the crew boats2 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.

The total short-term NOx emission rate from the support vessels is 257.80 lb/hr, taken from Table 

6.2.3-14 in the revised AQIA. The total NOx emissions from the drilling vessel, is 554.94 lb/hr, taken 

 

1 http://www.ottocandies.com/specs/peyton.pdf

2 http://www.grahamgulf.com/sybil_graham.htm
Jill Webster - 2 - 7/11/2013

from Table 6.2.2-10 of the revised AQIA. Thus the support vessels account for about 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 day, 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 spend two hours within the 25 

nautical mile radius while approaching, and another two hours while departing, leading to four hours 

(not 24 hours) of operation per day within the radius. This leads 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 98th percentile extinction changes for the Discoverer 

scenario would be about 5.32%, 3.74%, and 3.75%, respectively for the 2001-2003 model years. 

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

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

and 4.00%.

Please contact me as soon as possible if this clarification is insufficient 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