ACCESS NUMBER: 17199

STUDY TITLE: Alternative Oil Spill Occurrence Estimators for the Beaufort and Chukchi Seas – Fault 

Tree Method

REPORT TITLE: Alternative Oil Spill Occurrence Estimators for the Beaufort and Chukchi Seas – Fault 

Tree Method

CONTRACT NUMBER: 01-00-PO-17199 

SPONSORING OCS REGION: Alaska 

APPLICABLE PLANNING AREA: Beaufort and Chukchi Seas 

FISCAL YEARS OF PROJECT FUNDING: 2000, 2002 

COMPLETION DATE OF REPORT: August 2002 

COST: FY 2000: $97,900; FY 2002 $14,000 CUMULATIVE PROJECT COST: $111,900 

PROJECT MANAGER: Frank G. Bercha 

AFFILIATION: Bercha International Inc. 

ADDRESS: 2926 Parkdale Blvd. NW, Calgary, Alberta, Canada, T2N 3S9 

PRINCIPAL INVESTIGATOR*: Frank G. Bercha, Seymour L. Ross 

KEY WORDS: Oil spill occurrence, Beaufort Sea, Chukchi Sea, fault tree analysis 

BACKGROUND: 

The MMS Alaska OCS Region uses oil spill occurrence predictions for National Environmental Protection 

Act assessments for all parts of their area of jurisdiction, ranging from onshore through shallow water, to 

deeper water. Based on a study carried out in 1999-2000 (OCS Study MMS 2000-007), MMS estimated 

pipeline and facility spill rates from Alaskan North Slope and Trans-Alaska Pipeline onshore oil spill 

experience to shallow coastal waters and the near shore Beaufort Sea. However, as water depth 

increases and one moves further from shore, extrapolation of these statistics is not necessarily valid due 

to the change in operational modes and environmental conditions. There are no adequate historical 

statistics to characterize spill rates in deeper waters in the Beaufort and Chukchi Seas, for forthcoming 

lease areas. Accordingly, MMS implemented the present study to develop and apply alternative 

methodologies for the assessment of oil spill rates associated with exploration and production facilities 

and operations in deeper waters in the Chukchi and Beaufort Seas. 

OBJECTIVES: 

The objectives of this study are as follows: 

To assimilate and analyze world-wide and US OCS oil spill statistics and evaluate their applicability to 

deeper lease tracts which could be offered in the upcoming Beaufort Sea sales or in subsequent 

Chukchi Sea sales. 

•

•

•

•

To develop the fault tree method for estimating oil spill occurrences from Beaufort Sea and Chukchi 

Sea developments associated with spills less than and greater than 1,000 bbl. 

To develop alternative oil spill indicators and assess their robustness using the fault tree approach. 

To provide statistical support to MMS in evaluation of statistical issues in estimation of oil spill rates. 

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ACCESS NUMBER: 17199

DESCRIPTION: 

Oil spill occurrences were generated for several expected future oil and gas development scenarios 

(including exploration, production, and abandonment) in the Beaufort and Chukchi Seas Offshore 

Continental Shelf (OCS) lease sale regions (Figure 1). Because sufficient historical data on offshore oil 

spills for these regions do not exist, an oil spill occurrence model based on fault tree methodology was 

developed and applied. Using the fault trees, base data from the Gulf of Mexico were modified and 

augmented to represent expected Arctic offshore oil spillage frequencies. Three principal spill occurrence 

indicators, as follows, were quantified: 

 Annual spill frequency 

 Annual spill frequency per barrel produced 

 Spill index, the product of spill size and spill frequency 

These indicators were quantified for the following spill sizes; small (> 50 < 100 bbl), medium (> 100 < 

1,000 bbl), large (> 1,000 < 10,000 bbl), and huge (> 10,000 bbl). 

Quantification of spill indicators was carried out for each future year for four different Beaufort Sea 

development scenarios, ranging in duration up to 38 years, and for two Chukchi Sea scenarios of 10-year 

duration. In addition, comparative scenarios for non-Arctic locations were formulated and analyzed for oil 

spill occurrence. Generally, it was found that the non-Arctic spill frequency indicators were likely to be 

significantly higher than those for similar scenarios in the Arctic. The computations were carried out using 

a Monte Carlo process to permit the inclusion of estimated uncertainties in the Arctic effects. A wide 

range of details for each scenario was generated, and conclusions and recommendations were 

developed. 

SIGNIFICANT CONCLUSIONS: 

1. All non-Arctic scenario spill indicators are greater than their Arctic counterparts. Non-Arctic spill 

frequencies are approximately 40% greater; spill indices, 8% greater for the non-Arctic scenarios 

(Table 1). 

2. Spill frequencies and the spill index can be seen to follow the facility build-up and phase-out, as they 

are directly proportional to facility quantities (Figure 2). 

3. Spill frequency per barrel produced continues to rise beyond the peak production year due to the rate 

of production decline being disproportionately higher than the rate of abandonment chosen for the 

scenarios (Figure 2). 

4. Spill frequency per year and per barrel produced decreases significantly with increasing spill size for 

all scenarios (Table 1). 

5. Each spill indicator for Beaufort Sea Sale 1, 2, and 3 is similar in value. The indicators are higher for 

the composite “Sale All” scenario (Table 1). 

6. Chukchi Sea spill indicators are all higher than Beaufort Sea indicators (Table 1). 

7. The spill index increases dramatically with spill size for all scenarios (Table 1). 

8. For both the Beaufort and Chukchi scenarios, platforms contribute the most (50% and 61% 

respectively) to the two spill frequency indicators, but the least (5% and 6% respectively) to the spill 

index (Figure 4). 

9. Wells are by far the highest contributors to spill index in the Beaufort and Chukchi Seas, at 85% and 

89% respectively, while platforms and wells are each responsible for 10% or less contribution to the 

spill index (Figure 4). 

10. The Monte Carlo results give higher oil spill occurrence indicators than the expected value results. 

This is due to the skewness of the distributed values, which are inputs to the Monte Carlo 

calculations. 

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ACCESS NUMBER: 17199

11. Uncertainties in the Arctic spill variations resulting from uncertainties expressed as numerical 

distributions of the Arctic effect inputs were quantified using the Monte Carlo method. The variance in 

the spill indicators ranged for upper bounds from 20 to 50% of the mean, and for lower bounds from 

20 to 35% of the mean. 

STUDY RESULTS: 

The following principal groups of results, illustrated by attachments, were generated: 

 Expected time history of spill occurrences over the scenario life (Figure 2). 

 Spill occurrence variations by spill volumes in the main spill size ranges (Figure 3 and Table 1). 

 Spill occurrence variation by spill cause such as boat anchoring or ice gouging. 

 Spill occurrence contribution from each main facility type, including pipelines, platforms, and wells 

(Figure 4). 

 Comparison of spill occurrence predictions between Arctic and non-Arctic scenarios (Figure 2 and 

Table 1). 

 The variability in the results due to uncertainties in the Arctic effects introduced, expressed as 

cumulative distribution functions and statistical measures. 

 In the final report, a detailed description of the methodology, results, and conclusions and 

recommendations is given, as well as a section on limitations of the study. 

STUDY PRODUCTS: 

1. Bercha International Inc., “Alternative Oil Spill Occurrence Estimators for the Beaufort and Chukchi 

Seas – Fault Tree Method”, Volumes I and II, Final Report to US Department of the Interior, Minerals 

Management Service, Alaska OCS Region, OCS Study MMS 2002-047, August 2002. 

2. Bercha, Frank G., and Milan Cerovšek, “Ice Pressure Ridge Impacts on Oil Spills in the Alaskan 

OCS”, paper accepted for presentation at 16th International Symposium on Ice, Dunedin, International 

Association for Hydraulic Engineering and Research, New Zealand, 2-6 December 2002. 

3. Bercha, Frank G., “Alternative Oil Spill Occurrence Estimators for the Beaufort/Chukchi Seas OCS – 

Fault Tree Approach”, Presented at the 8th ITM, Minerals Management Service, Anchorage, Alaska, 

3-5 April 2001. 

* Pertinent figures and tables are included hereafter 

 

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ACCESS NUMBER: 17199

FIGURES AND TABLES: 

Figure 1 

Beaufort and Chukchi Seas Study Areas 

Table 1 

Summary of Spill Indicators for All Scenarios 

Beaufort Sea Chukchi Sea 

Year 

2016 

Year 

2019 

Year 

2024 

Year 

2020 

Year 

2020 

Year 

2010 

Year 

2010 

Year 

2010 SPILL INDICATORS 

Spill Size 

bbl x 1000 

Sale 1 Sale 2 Sale 3 Sale All

Sale All

Non 

Arctic 

Base 

Case 

High 

Case 

High C 

Non 

Arctic 

SM 9.97 10.17 9.84 29.98 43.90 37.66 70.18 95.17 

L 4.53 4.42 4.07 13.02 17.83 15.23 25.34 36.70 

H 2.39 2.34 2.21 6.93 8.31 7.68 14.38 17.85 

Spill Frequency 

per 103 years 

All 16.88 16.93 16.12 49.93 70.04 60.58 109.91 149.72 

SM 0.21 0.24 0.25 0.40 0.59 0.41 0.31 0.42 

L 0.10 0.11 0.11 0.17 0.24 0.17 0.11 0.16 

H 0.05 0.06 0.06 0.09 0.11 0.08 0.06 0.08 

Spill Frequency 

per 109 bbl 

produced 

All 0.36 0.40 0.42 0.67 0.94 0.66 0.48 0.66 

SM 2 2 2 6 9 8 13 19 

L 28 27 26 81 102 92 171 218 

H 170 169 165 505 529 534 1150 1211 

Spill Index [bbl] 

(Product of spill frequency 

and mean spill size) 

All 200 199 193 592 640 633 1335 1448 

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Beaufort Sea Sale All Spill Frequency 

0

5

10

15

20

25

30

35

40

45

50

55

60

65

70

75

20042005200620072008200920102011201220132014201520162017201820192020202120222023202420252026202720282029203020312032203320342035203620372038

Spill Frequency per 1000 years

Non Arctic Spills

Arctic Spills

Beaufort Sea Sale All Spill Frequency per 109 bbl Produced

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

2.0

2.2

2.4

2.6

2.8

3.0

3.2

3.4

3.6

200420062008201020122014201620182020202220242026202820302032203420362038

Spill Frequency per 109 bbl Produced

Non Arctic Spills

Arctic Spills

Beaufort Sea Sale All Spill Index - Non Arctic

0

50

100

150

200

250

300

350

400

450

500

550

600

650

20042005200620072008200920102011201220132014201520162017201820192020202120222023202420252026202720282029203020312032203320342035203620372038

Spill Index [bbl]

Non Arctic Spills

Arctic Spills

Figure 2 

Beaufort Sea Composite Scenario Annual Variation 

in Arctic and Non-Arctic Spill Occurrence Indicators 

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ACCESS NUMBER: 17199

Beaufort Sea-Sale All - 2024

 Spill Frequency per 10^3 years

for Spill size x 1000 bbl

60%

26%

14%

SM L H 49.93

Beaufort Sea-Sale All - 2024

 Spill Frequency per 10^9 bbl produced

for Spill size x 1000 bbl

26%

60% 14%

SM L H 0.67

Beaufort Sea-Sale All - 2024

 Spill Index [bbl]

for Spill size x 1000 bbl

1%

14%

85%

SM L H

592

Figure 3 

Beaufort Sea ‘Sale All’ Spill Indicators – Year 2024 

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ACCESS NUMBER: 17199

Beaufort Sea-Sale All - 2024

 Spill Frequency per 10^9 bbl produced

50%

31%

19%

P/L

Platforms

Wells 0.67

Beaufort Sea-Sale All - 2024

 Spill Frequency per 10^3 years

31%

50%

19%

P/L

Platforms

Wells 49.93

Beaufort Sea-Sale All - 2024

 Spill Index [bbl]

10% 5%

85%

P/L

Platforms

Wells 592

Figure 4 

Beaufort Sea ‘Sale All’ Spill Indicators – Year 2024 

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