A Spill Risk Assessment of the Enbridge Northern Gateway Project

of a collision spill exceeding 20,000 m 3 and 50,000 m 3 is 10% and 0.3%, respectively
(Brandsæter and Hoffman 2010 p. 6-­‐81 -­‐ 6-­‐85).
DNV estimates unmitigated spill return periods for each type of tanker incident based
on scaled incident frequencies, conditional probabilities, the length of each segment,
and the number of times the route is travelled per year. DNV presents unmitigated risk
as a return period, which is the number of years between spill events, and is an
alternative to stating the annual probability of a spill. According to DNV, an
unmitigated tanker incident will result in an oil/condensate spill once every 78 years
(see Table 6). DNV also estimates unmitigated return periods for various oil spill sizes
and, based on Method 2 of the consequence assessment, DNV estimates that a spill
exceeding 5,000 m 3 will occur every 200 years while a spill exceeding 40,000 m 3 , will
occur every 12,000 years (see Table 7).
DNV conducts a sensitivity analysis in its marine shipping QRA examining impacts of
several parameters on spill probabilities. The sensitivity analysis tests the impact of
changes in parameters on all three potential routes separately under the assumption
that all 220 tankers forecast to call at Kitimat Terminal use the route being tested
exclusively. Therefore, the spill probabilities do not account for using a combination of
routes. Results from the sensitivity analysis suggest that return periods change
modestly compared to baseline return periods (Table 4).
Table 4: Summary of Sensitivity Analysis on Spill Return Periods (years)
Sensitivity Parameter North Route
South Route
via
Caamano
Sound
South Route
via
Browning
Entrance
Baseline 69 83 84
Sensitivity 1: Increase in Scaling Factors for Grounding 59 71 71
Sensitivity 2: Increase in Traffic Density for Collisions 67 81 82
Sensitivity 3a: Decrease to 190 Tankers per year 80 96 97
Sensitivity 3b: Increase to 250 Tankers per year 61 73 74
Sensitivity 4: 200 nm Extension to Segments 5 and 8* 67 81 82
* DNV does not provide overall return periods for the trip extension sensitivity analysis and thus return periods were calculated
based on Brandsæter and Hoffman (2010).
The next step in the DNV analysis estimates the impact of various mitigation measures
on spill return periods. DNV examines several risk reduction measures qualitatively
including enhanced navigational aids, vessel traffic management system, environmental
limits for safe operation, and the establishment of places of refuge along tanker routes.
The marine shipping QRA only quantitatively evaluates a single mitigation measure: the
tug escort plan. DNV obtains the risk reducing effect of escort tugs from a confidential
study it completed in 2002 and applies the downward adjustments directly to powered
grounding, drift grounding, and collision incidents for the ENGP (Table 5).
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