ACCESS NUMBER: 17141

STUDY TITLE: Alternative Oil Spill Occurrence Estimators for the Beaufort/Chukchi Sea OCS – 

Statistical Approach 

REPORT TITLE: Alternative Oil Spill Occurrence Estimators for the Beaufort/Chukchi Sea OCS – 

Statistical Approach 

CONTRACT NUMBER: 1435-01-00-PO-17141 

SPONSORING OCS REGION: Alaska 

APPLICABLE PLANNING AREA: All 

FISCAL YEARS OF PROJECT FUNDING: 2000 

COMPLETION DATE OF REPORT: September 2006 

COST: FY 2004: $100,000 CUMULATIVE PROJECT COST: $100,000 

PROJECT MANAGER: Ted G. Eschenbach 

AFFILIATION: TGE Consulting 

ADDRESS: 4376 Rendezvous Circle, Anchorage, AK 99504 

PRINCIPAL INVESTIGATOR*: Ted G. Eschenbach

KEY WORDS: Oil spill occurrence, Beaufort Sea, Chukchi Sea, statistics, Gulf of Mexico, Outer 

Continental Shelf, spill rate estimation 

BACKGROUND: The MMS Alaska Offshore Continental Shelf (OCS) Region uses oil spill occurrence 

predictions for National Environmental Protection Act assessments for all parts of their area of jurisdiction, 

ranging from the shore through shallow water, to deeper water. In 2000 to 2006, a study was carried out 

by TGE Consulting to develop spill rate estimates with confidence intervals for the Beaufort and Chukchi 

Seas. Statistical analysis supported Poisson models for spill rates for the Gulf of Mexico (GOM) using 

exposure variables of time, volume of production, pipeline-mile-years, and platform-years. Models for the 

last three exposure variables were extended to the Arctic for non-GOM specific spill causes, and models 

for the last two are recommended for use. An innovative approach to modeling pipeline spill risks that 

specifically considers platform-related pipeline spills was introduced. 

OBJECTIVES: The objectives of this study are as follows: 

Assimilate and analyze available oil spill statistics and evaluate their applicability to lease 

tracts which could be offered in the upcoming Beaufort Sea sales. 

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Develop statistically appropriate approaches to estimating oil spill rates for the Gulf of Mexico 

Develop statistically appropriate methods to extend GOM models to the Beaufort Sea and 

Chukchi Sea lease sales from spills of at least 50 – 1,000 bbl or more 

Provide statistical support to MMS in evaluation of statistical issues in estimation of oil spill 

rates. 

DESCRIPTION: This study analyzes GOM statistics for pipeline and platform oil spills, and with

statistically appropriate techniques develops Poisson models for spill rates in the GOM and extended to 

the Arctic. These models are tested through analysis of exponential inter-spill intervals for several 

exposure variables. These exposure variables include time, oil production, pipeline mile-years, and 

platform-years. Weibull and lognormal models for spill volumes are developed and used to support 

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analyses at spill thresholds of 50, 100, 500, and 1000 barrels. In each case confidence limits are 

calculated and reported. The stability of the results has been confirmed for different time periods. 

SIGNIFICANT CONCLUSIONS: 

The Poisson distribution for pipeline and platform spill rates is satisfactory. Other distributions could be 

chosen, but the Poisson 

1. Fits with historical practice 

2. Has a theoretical foundation – it is not just an empirical curve fit 

3. Is “not rejected” at reasonable levels of statistical confidence 

4. Even though the fit of any distribution may be imperfect, the key question when estimating rates, 

is “how much do these imperfections change the estimated rate? Generally, the answer is very 

little.” 

For the GOM, any of the exposure variables can be used to model the rate of oil spills, due to the high 

level of correlation between them. Furthermore, with production as the exposure variable, the results are 

generally consistent with MMS published results. 

The best exposure variables for extension to the Arctic are pipeline mile-years and platform-years. Using 

production volumes as the exposure variable seems to significantly overstate the risk. Time cannot be 

used as the exposure variable. 

The level of uncertainty for Arctic extension is very high due to the many uncertainties. However, it is 

possible to make reasonable, justifiable extensions from the GOM data to the Arctic. 

Preliminary results from a new approach to modeling pipeline spill rates based on “platform-related” spills 

and pipeline mile-years suggest that this is a better approach for extension to new environments, such as

the Arctic or new GOM facilities in very deep waters.

STUDY RESULTS: Significant differences from past MMS publications include Poisson confidence 

intervals, exact binomial confidence intervals, detailed analyses for the exposure variables of pipeline 

mile-years and platform-years, the use of the larger spill data set of spills exceeding 50 barrels to 

estimate rates at higher thresholds, and the inclusion of more recent data (through 2005). A declining 

rate of platform spills is statistically verified, so that platform results are generally based on spills 1990 to 

2005, while pipeline results are based on data from 1972 to 2005. 

Spill causes are analyzed to identify spills with GOM-specific causes, such as hurricanes and fishing trawl 

gear. This is modeled as a binomial proportion for GOM-specific and applicable to the Arctic spills at the 

spill size thresholds. These results are extended to the Arctic with a method that provides a conservative 

confidence interval and a method that provides a minimum width confidence interval – both at each spill 

size threshold. 

For extension from the GOM data to the Arctic using a Beaufort development scenario, spill rates per 

production volume are found to be unreliable as compared with spill rates per pipeline mile-year and per 

platform-year. There was insufficient data on the Beaufort development scenario to estimate the spill 

rates for ice keel gouging, but a rough approximation for strudel scour suggested that this was less 

significant than applicable causes, such as corrosion, human error, and operational impacts. 

The estimated rates of Arctic applicable pipeline spills per pipeline mile-year and of platform spills per 

platform-year at spill thresholds of 50, 100, 500, and 1000 barrels are presented with minimum and 

conservative confidence intervals. Thus, the existing data from the GOM is extended through statistically 

supported techniques. 

Preliminary results from a new approach to modeling pipeline spill rates based on “platform-related” spills 

and pipeline mile-years suggest that this is a better approach for extension to new environments, such as

the Arctic or new GOM facilities in very deep waters.

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STUDY PRODUCTS: 

Eschenbach, T.G., and W. V. Harper. 2006. Alternative Oil Spill Occurrence Estimators for the 

Beaufort/Chukchi Sea OCS – Statistical Approach, Final Report to US Department of the Interior, 

Minerals Management Service, Alaska OCS Region, OCS Study MMS 2006-059. 

 

Eschenbach, T.G. 2001. Initial Work on a Statistical Approach to Oil Spill Occurrence Estimates for the 

Beaufort/Chukchi Sea OCS. MBC Applied Environmental Sciences. Alaska OCS Region Eighth 

Information Transfer Meeting Proceedings. OCS Study MMS 2001-049. Anchorage, USDOI, MMS, 

Alaska OCS Region. 

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