Estimated occurrence rates for analysis of accidental oil spills on the US outer continental shelf

The Minerals Management Service estimates the likelihood of oil spills of 1000 barrels and greater occurring in association with the production and transportation of offshore oil on the US Outer Continental Shelf (OCS). The estimation process uses a spill rate constant, based on historical accidents, expressed in terms of number of spills per 109 barrels of oil produced or transported. The mean spill occurrence estimate is obtained by multiplying the rate constant by the volume of oil projected to be handled. The probability of one or more spills occurring in a given production period is then estimated by using the mean number of spills in a Poisson process. The calculated occurrence rates of 0·60 spills per 109 barrels produced on US OCS platforms and 0·67 spills per 109 barrels transported in US OCS pipelines represent a decline of 40 and 58%, respectively, since last evaluated in 1983. Spill occurrence rates for worldwide tanker transport remained unchanged, since last evaluated in 1983, at 0·90 for ‘at-sea’ spills and 0·40 for ‘in-port’ spills.