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STUDY TITLE: Estimation of Oil Spill Risk From Alaska North Slope, Trans-Alaska Pipeline, and 

Arctic Canada Oil Spill Data Sets 

REPORT TITLE: Estimation of Oil Spill Risk From Alaska North Slope, Trans-Alaska Pipeline, 

and Arctic Canada Oil Spill Data Sets

CONTRACT NUMBER: 01-99-PO-16128 

SPONSORING OCS REGION: Alaska Outer Continental Shelf Region 

APPLICABLE PLANNING AREA(S): Alaska Beaufort Sea 

FISCAL YEAR(S) OF PROJECT FUNDING: FY1999 

COMPLETION DATE OF REPORT: March 2000 

COST(S): FY 1999 - $95,338 

CUMULATIVE PROJECT COST: $95,338 

PROJECT MANAGER: Jeffery L. Mach 

AFFILIATION OF PROJECT MANAGER: Hart Crowser, Inc. 

ADDRESS: 2550 Denali Street, Suite 705, Anchorage, Alaska 99503 

PRINCIPAL INVESTIGATOR(S): Jeffery L. Mach, Robert L. Sandefur, and Jean Z. Lee* 

KEY WORDS: Oil Spill, Risk, Alaska, Canada, Arctic, Beaufort Sea, Trans-Alaska Pipeline 

System, Oil Exploration, Oil Development, Oil Production, Oil Transportation. 

BACKGROUND: In Alaska environmental assessments and environmental impact statements for 

proposed Outer Continental Shelf (OCS) oil exploration and development activities, the Minerals 

Management Service (MMS) estimates the potential risks for the occurrence of large oil spills. 

The MMS uses national OCS statistics to estimate the potential for oil spills of 1,000 barrels 

(42,000 gallons) or more. The historical record for OCS platforms and pipeline spills used to 

calculate the national OCS oil spill rates is based primarily from Gulf of Mexico OCS oil 

exploration and development experience. The MMS proposed to calculate Beaufort Sea oil spill 

frequency based on Alaska North Slope, Trans-Alaska Pipeline System, and Canadian Arctic 

experience, rather than oil spills in the Gulf of Mexico. 

OBJECTIVES: The objectives of this study were to: 

• Obtain and collate all available data on oil industry spills for designated Alaska North 

Slope, Trans-Alaska Pipeline System, and Arctic Canada study areas; 

• Review the reliability and completeness of the oil spill data; 

• Obtain and collate data on crude oil production and crude oil and diesel fuel pipeline 

mileages and throughput by year for the designated Alaska North Slope, Trans-Alaska 

Pipeline System, and Arctic Canada study areas; 

• Evaluate the appropriateness and statistical robustness of the data for estimating 

onshore and offshore oil spill risks from Beaufort Sea oil development; and 

• Estimate draft onshore and offshore spill rate(s) using the optimum data sets. 

DESCRIPTION: The study gathered: 

• Data on oil spills of 100 barrels and greater in volume that occurred in the Alaskan and 

Canadian study areas and which were associated with oil industry; 

• Documentation on oil spills of 500 barrels and greater; 

• Data on crude oil production, pipeline mileage, pipeline throughput and other crude oil 

transportation volumes by year in the study areas; and 

• Information on current and historical government oil spill notification requirements. 

Oil spill, crude oil production, pipeline mileage, and pipeline throughput and other crude oil 

transportation data were collated and entered into a Microsoft Access 97® database. The 
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comprehensiveness and completeness of the oil spill data was evaluated. The oil spill data was 

analyzed statistically and oil spill risk rate estimates were calculated. 

SIGNIFICANT CONCLUSIONS: Data for Alaskan oil spills was sufficiently comprehensive and 

complete for use in statistical analyses and calculation of oil spill risk rates. Canadian oil spill 

data was not sufficiently comprehensive for analysis and use in risk rate estimate calculations. 

An estimated oil spill risk rate of approximately 45 gallons of oil spilled per million barrels of crude 

oil produced was calculated to be the average, if trends that started in 1980 continue. This rate is 

subject to considerable uncertainty in the mean (±50% at the 95% level of confidence) and the 

value derived from logarithmic values is 55 gallons of oil spilled per million barrels of crude oil 

produced, as opposed to 45 gallons of oil spilled per million barrels of crude oil produced. These 

two values agree within the standard deviation of the means. The 95 percent log confidence 

limits on spills for a given year are ±440 percent at the 95 percent level of confidence. Hart 

Crowser is more inclined to believe the log values than the untransformed values, because the 

cumulative frequency of the data is more lognormal than normal. These very wide confidence 

limits and individual yearly values are consistent with the small number of data points available 

for this prediction. 

Oil spill risk estimation rates based on the number of spills of a given size per given volume of 

crude oil produced were calculated to be: 

• 0.0053 spills of 100 barrels or greater per million barrels of crude oil produced (or 1 spill 

per 188.7x106

 barrels of crude oil produced), ±24 percent; 

• 0.00093 spills of 500 barrels and greater per million barrels of crude oil produced (or 1 

spill per 1.1x109

 barrels of crude oil produced), ±58 percent, 

• 0.00039 spills of 1,000 barrels and greater per million barrels of crude oil produced (or 1 

spill per 2.6x109

 barrels of crude oil produced), ±89 percent, and 

• 0.000078 spills of 10,000 barrels and greater per million barrels of crude oil produced (or 

1 spill per 12.8x109

 barrels of crude oil produced), ±200 percent, for spills.

Incorporation of the Canadian oil spill data and re-calculation of the estimated spill risk rate based 

on the number of spills did not significantly change the conclusions. 

STUDY RESULTS: A total of 126 oil spills of 100 barrels and greater were identified within the 

Alaskan and Canadian study areas; 111 spills in Alaska and 15 in Canada. Alaskan oil spill data 

were obtained from nine sources and spanned the years from 1970 to 1999. Canadian oil spill 

data were obtained from two sources and spanned the years 1973 to 1999. The most recent 

spills of 100 barrels or larger associated with the oil industry in the Alaska and Canada study 

areas occurred in 1997. 

Data, by year, on crude oil production, pipeline mileage and throughput for larger pipelines, and 

other crude oil transportation in the study areas were obtained. Limited, single year data on the 

mileages of smaller field gathering pipelines was obtained. 

Descriptions of historical government oil spill notification requirements and a summary of current 

oil spill notification requirements in the study areas were prepared. More than 60 percent of all 

Alaskan oil spills appeared in two or more of the data sets. Spills of 500 barrels and greater and 

1000 barrels and greater appeared in two or more data sets at a higher percentage than spills of 

100 barrels. 

Of 28 spills that were 500 barrels and larger, documentation was obtained for 24 of the spills. For 

both the Alaskan and Canadian oil spill data, the volumes assigned for the spills of 500 barrels 

and larger are reliable, but must be considered as general estimates in most cases. 

Documentation for these spills often does not describe how the spill volume was determined. 

Alaska oil spill data was used to conduct statistical analyses and estimate oil spill risk rates. A 

series of box and cumulative frequency plots of the Alaska spill data were constructed to analyze 

the data and determine trends. Exploratory data analysis on relevant independent variables 

indicated little statistical difference in terms of spills that occurred within the Alaska – Onshore 
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North Slope, East of NPRA (ONS) and in the Trans-Alaska Pipeline System (TAPS) study areas. 

A general check on the fluctuation of the data set indicated spill occurrence to appear random. 

There appeared to be little difference in the size of spills between the types of spills associated 

with the various facilities, with the exception of pipelines, which had larger spills. Analysis of 

variance by oil type showed that, in general crude oil spills tend to be larger than other types of oil 

spills. 

A statistical analysis of individual spill volumes by study area, facility type, oil type, affected media 

and spill cause combined did not indicate any particularly interesting correlation. Annualized 

groupings of spills, where total spill volumes by year were accumulated and plotted on a 

cumulative frequency plot, showed a mixture of several populations. Re-plotted on a log scale, a 

single lognormal population emerged. A count of the number of spills per year in the database 

showed a possible Poisson distribution. 

When spill size was plotted by year to see if regulatory or reporting requirements had a significant 

affect, it appeared that from 1975 to 1979 there were a considerable number of large spills, and 

then the number of spills dropped to a more or less constant rate. The year of 1977 is significant 

because crude oil production on North Slope and operation of TAPS began in the middle of that 

year. However, the years of 1978 and 1979 visually fit with years of 1975 and 1977 better than 

breaking the data at 1977. The 1975 to 1979 period appear to have the most number of spills. 

When Alaskan spill data were plotted on a yearly basis, it appears that prior to 1977, spill rates 

were considerably greater than in the subsequent years. When re-plotted on a log scale, it is 

apparent that prior to 1980 spill rates were considerably greater than after 1980. 

Oil spill risk rates were calculated based on the number of spills and on volume. Hart Crowser 

concluded that spill rate is the best variable to use in predicting the volume of further oil spills. A 

rate based on volume was selected because of the greater visual variability in the data. The 

statistical significance of this visual analysis showed a highly statistically significant correlation 

with spill rate and year if all of the Alaska spill data is included. If data earlier than 1980 is 

excluded, then there is still a correlation between spill rate and year that is significant at the 1 

percent level of confidence. However, if data earlier than 1985 is excluded, then there is a 

correlation between spill rate and year, which is significant at the 17 percent level of confidence. 

Hart Crowser also calculated oil spill risk rates based on the number of spills of a given volume 

per million barrels of crude oil produced, using Alaskan oil spill data from 1978 through 1999. A 

spill risk rate calculation also was made with the Canadian data included. 

STUDY PRODUCTS: 

Mach, J. L.; P. E. Sandefur, and J. H. Lee. 2000. Estimation of Oil Spill Risk from Alaska North 

Slope, Trans-Alaska Pipeline, and Arctic Canada Oil Spill Data Sets. OCS Study MMS 2000-007. 

Final Report Contract No. 01-99-PO-16128. USDOI, MMS Alaska OCS Region: Anchorage, AK. 

Final Oil Spill and Supporting Data Sets 

* PI’s affiliation may be different than that listed for Project Manager(s)