The number you reported in your Enbridge editorial regarding the risk of a pipeline spill is the probability of their largest category of tanker disaster, not the probability of a major pipeline rupture.
In addition, your rough calculation works well for return periods like the 15000 years (0.2 per cent is pretty close) but it doesn't work as well for smaller return periods.
Using the spill return period for physiographic regions along the pipeline route from Enbridge's 2010 report on environmental performance, the probability of one or more large spills (greater than one million litres) from the pipeline over a 30 year period is about 27 per cent. Note that this is actually lower that what you would get if you use the rough calculation dividing 30 by the combined spill return period of 95 years. If you use the 50 year operational life given immediately below the table, the probability is about 41 per cent.
Shane Rollans
Department of Mathematics and Statistics
Thompson Rivers University
Kamloops
