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Branch of
Environmental Sciences
Oil-Spill Modeling Program:
Oil-Spill Occurrence Rates for OSRA
Estimates
of occurrence rates for offshore oil spills are useful for analyzing
potential oil-spill impacts and for oil-spill response contingency
planning. With the implementation of the Oil Pollution Act of 1990 (U.S.
Public Law 101-380, August 18, 1990), estimates of oil-spill occurrence
became even more important to natural resource trustees and to
responsible parties involved in oil and gas activities.
The
Oil-Spill Risk Analysis (OSRA) model, developed in 1975 by the
Department of the Interior (DOI), is a tool that evaluates offshore
oil-spill risks (Smith et al., 1982; LaBelle & Anderson, 1985). This
model is used to develop probabilistic estimates of oil-spill occurrence
and contact. A realistic, objective methodology for estimating oil-spill
occurrence rates is required for the model's application. The MMS
developed and maintains oil-spill databases on
U.S. OCS Spill Incidents: 1996-2008,
U.S. OCS Spill Incidents: 1964-1995, and tanker spills, which
are used to support these estimations (Lanfear & Amstutz, 1983; Anderson
& LaBelle, 1990, 1994).
Oil-spill occurrence rate estimates were revised
(Anderson & LaBelle, 2000) based on U.S. Outer Continental Shelf
(U.S. OCS) platform and pipeline spill data (1964 through 1999),
worldwide tanker spill data (1974 through 1999), and barge spill data
for U.S. waters (1974-1999). These spill rates are expressed and
normalized in terms of number of spills per volume of crude oil handled.
All estimates of spill occurrence rates were restricted to spills
greater than or equal to 1,000 barrels (159 cubic meters, 159
kiloliters, 136 metric tonnes, 42,000 U.S. gallons). This paper presents
a simple approach for estimating oil-spill occurrence, normalized as a
function of the volume of oil handled. For this paper, volume is
reported in barrels (bbl) to assist policy- and decision-makers in
government and industry.
Confidence Intervals
As a
supplement to this paper,
95-Percent Confidence Intervals are presented. Further
statistical information supporting this approach can be found in
documents identified in the Additional Statistical Background discussion
below.
Additional Statistical Background
Anderson
& LaBelle (2000) is the fourth of a series of independently
peer-reviewed papers presented in support of oil-spill rate assumptions
used for the DOI OSRA Model, with two earlier Anderson & LaBelle efforts
(1994 & 1990) and
Lanfear & Amstutz (1983). Lanfear & Amstutz (1983) examines the
cumulative frequency distributions of oil spills, tests pipeline miles
as an alternative exposure variable for pipeline spills, and discusses
the trend analysis of offshore spills performed by
Nakassis (1982). These spill rate papers tier off earlier work
performed by DOI in support of the OSRA Model, and work performed by
other oil-spill researchers, as referenced in the papers.
Smith et al. (1982)
documents the fundamentals of the DOI OSRA Model. It describes the
approach of using lambda, the unknown spill occurrence rate for a fixed
class of spills, as a parameter in a Poisson process, with volume of oil
handled as an exposure variable to predict the probability of spill
occurrence
(pages 18-24). A Bayesian methodology, described in detail in
Appendix A, “Distribution Theory of Spill Incidence,”
provides one way to weight the different possible values of lambda given
the past frequency of spill occurrence for a fixed class of spills.
Smith et al. (1982) selects volume as an exposure variable in that it
is a quantity that would be more practical to estimate future exposure
(a necessity for using it to forecast future spill occurrence) than the
other exposure variables considered.
In
support of using the Poisson process for spill occurrence and
examinations of different exposure variables, Smith et al. (1982)
references the works of Devanney & Stewart (1974), Stewart (1976), and
Stewart & Kennedy (1978). These references, and other pertinent ones,
can be found at
Oil Spill Rates - Additional References.
You can download the linked PDF
documents by right-clicking on the link and then selecting SAVE TARGET
AS, which will allow you to save the PDF file to your computer. |
