Appendix K 

________________________________

Information, Models, and the 

Assumptions Used to Analyze the 

Effects of Oil Spills 

TABLE OF CONTENTS 

K.1. Oil Spill Size Categories .................................................................................................... K-3 

 K.1.2. Probability of a Small Crude Oil Spill - Less Than 500 Barrels.......................... K-9 

K.2. Oil Weathering and Spreading........................................................................................ K-10 

 K.2.1. Modeling Simulations of Oil Weathering............................................................ K-10 

 K.2.2. Observations of Historic North Slope Spill Patterns.......................................... K-13 

K.3. Cumulative Analysis of Oil Spills ................................................................................... K-14 

 K.3.1. Preparing the Cumulative Analysis.................................................................... K-14 

 K.3.2. Estimating Possible Future Spills From All Sources ......................................... K-14 

K.4. Bibliography..................................................................................................................... K-17 

List of Tables 

K-1. Oil Spill Scenario Assumptions for the Alternatives ....................................................... K-4 

K-2. Large Crude Oil Spills Estimated Over the Production Life of the Northeast National 

Petroleum Reserve - Alaska.............................................................................................. K-8 

K-3. Small Crude Oil Spills Estimated Over the Production Life of the Northeast National 

Petroleum Reserve – Alaska ........................................................................................... K-10 

K-4. Small Refined Oil Spills <500 bbl Estimated Over the Production Life of the Northeast 

National Petroleum Reserve – Alaska............................................................................ K-11

K-5. Fate and Behavior of a Hypothetical 500 bbl Oil Spill From Lagoon Pipelines ........... K-12 

K-6. Fate and Behavior of a Hypothetical 900 bbl Oil Spill From a Lagoon Facility........... K-13 

K-7. Oil Spill Rates and Spill-size Categories Used to Estimate Large Crude Oil Spills for the 

Cumulative Analysis ....................................................................................................... K-14 

K-8. Cumulative Oil-Spill-Occurrence Estimates ≥500 bbl and ≥1,000 bbl over Assumed 

15-20 Year Production Life of the Northeast National Petroleum Reserve – Alaska..... K-16 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-2
Northeast NPR-A Supplemental IAP/EIS 

K-3 Appendix K: Oil Spill Scenarios 

APPENDIX K 

INFORMATION, MODELS, AND ASSUMPTIONS USED 

TO ANALYZE THE EFFECTS OF OIL SPILLS

This Supplement to the Amended IAP/EIS analyzes oil spills, and their potential impacts to 

environmental, economic, and sociocultural resources and resource areas, which could result 

from onshore oil exploration and development in the Northeast National Petroleum Reserve – 

Alaska (NPR-A). Predicting an oil spill is an exercise in probability. There is uncertainty 

associated with the location, number, and size of oil spills, the chemistry of the oil, and the 

environmental conditions at the time of a spill. Although some of the uncertainty reflects 

incomplete or imperfect data, there is also a considerable amount of uncertainty involved in 

predicting events 15 to 25 years into the future. However, the chance of an oil spill occurring 

can be estimated using historical data. 

Assumptions about oil spills are used to analyze the effects of oil spills. These assumptions 

pertain to the type of oil, the source of an oil spill, the general location and size of a spill, the 

chemistry of the oil, how the oil will weather, how long the oil will remain, and where the oil 

will go. Project-specific information, statistical analysis, and professional judgment support the 

assumptions. Based on these assumptions, a scenario is created to reflect a spill, and the effects 

of such a spill are analyzed. These steps constitute a “what if a spill occurs” analysis. 

This oil spill analysis considers the entire production life of the planning area, and assumes 

that commercial quantities of hydrocarbons are present in the planning area and that these 

hydrocarbons will be developed and produced at the estimated resource levels presented in the 

Supplemental IAP/EIS. Uncertainties exist, such as 1) the actual resource levels, 2) the actual 

size of a crude or refined oil spill, 3) the approximate location of oil assumed to be produced, and 

4) whether production would occur at all. If no hydrocarbons exist, there is no chance of a crude 

oil spill occurring in the planning area. 

K.1 Oil Spill Size Categories 

This Supplement analyzes what is likely to happen in the future, using assumptions about the 

likely size, duration, and type of a spill to analyze the effects. To estimate these parameters, oil 

spills are divided into two types: crude oil and refined oil spills. Crude oil spills are divided into 

three size categories: small, large, and very large. Within each of these categories, generalized 

and specific assumptions are made. Refined spills fall into the small spill size category. 

Small spills are defined as those less than 500 barrels (bbl; 1 bbl = 42 gallons); large spills are 

greater than or equal to 500 bbl or 1,000 bbl (depending upon the data source); and very large 

spills are greater than or equal to 120,000 bbl. Table K-1 shows the assumed source of a spill(s), 

type of oil, size of spill(s) in bbl, and the receiving environment that is assumed in the analysis 

of the effects of oil spills in this Supplement. The effects of spill(s) are analyzed in Chapter 4 

(Environmental Consequences). The following sections discuss the oil spill analysis, and the 

assumptions used for analysis, for each of these three size categories. 

 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-4

K.1.1 Probability of a Large Crude Oil Spill 

Large spills are defined as greater than or equal to 500 bbl for the Alaska North Slope and 

Trans-Alaska Pipeline System (TAPS), and greater than or equal to 1,000 bbl for the TAPS 

tankers. Historical information about previous large spills on the Alaska North Slope, from 

TAPS, and from TAPS tankers was used to estimate the hypothetical size of large spills and the 

rate at which such large spills would be expected to occur in the future. 

Table K-1. Oil Spill Scenario Assumptions for the Alternatives 

Assumed Number of Spills Under Each 

Source of Spill Alternative Type of 

Oil 

Size of 

Spill 

(bbl) A B C D 

Receiving 

Environment

Small Spills (< 500 bbl) Onshore and Offshore 

Operational Crude 3 561 596 721 659 

spills from all 

sources Refined 0.7 1,276 1,474 1,782 1,628 

Ice, tundra, 

snow, gravel 

pad, and 

water 

Large Spills (≥ 500 bbl) Onshore or Offshore 

Pipeline Crude 4,800 

Platform/grave

l pad Crude 900 

Storage 

tank/gravel pad Diesel 900 

2 3 3 3 

Ice, tundra, 

snow, gravel 

pad, and 

water 

Very Large Spills (≥ 120,000 bbl) 

Well blowout Crude 120,000 Not 

Applicable 

Not 

Applicable 

Not 

Applicable 

Not 

Applicable 

Ice, tundra, 

snow, gravel 

pad, and 

water 

 K.1.1.1 Historical Large Crude Oil Spill Sizes 

Assumptions for large spills from production in the planning area are based on the historic spill 

sizes from onshore Alaska North Slope oil industry spills from 1985 to 2000, TAPS spills from 

1977 to 2001, and TAPS tanker spills from 1977 to 1999. Additional consideration is given to 

the large spill that occurred in March 2006 known as the GC-2 Oil Transit Line Release. 

Historical Crude Oil Spills Greater Than or Equal to 500 Barrels on the North Slope 

The Alaska North Slope oil spill analysis includes onshore oil and gas exploration and 

development spills from the Point Thompson Unit, Badami Unit, Kuparuk River Unit, Milne 

Point Unit, Prudhoe Bay West Operating Area, Prudhoe Bay East Operating Area, and offshore 

Duck Island Unit (Endicott). Alaska North Slope data include spills from onshore pipelines and 

offshore and onshore production and gathering facilities. The following information does not 

include spills on the Alaska North Slope from the TAPS, which were evaluated separately. 

For the Alaska North Slope, all available information on historic spills greater than or equal to 

100 bbl during the period 1968 through 2000 was obtained from industry and regulatory 

agencies and collated (Anderson and LaBelle 2000, Hart Crowser, Inc. 2000). Information on 

the 2006 GC-2 Oil Transit Line Release came from Situation Reports posted on the Alaska 

Department of Environmental Conservation website. The USDOI MMS and Hart Crowser, Inc. 
Northeast NPR-A Supplemental IAP/EIS 

K-5 Appendix K: Oil Spill Scenarios 

collected data for crude oil spills for the U.S. Beaufort Sea, the NPR-A, and Onshore Alaska 

North Slope east of the NPR-A from the following sources: 

• British Petroleum (BP) Exploration (Alaska), Inc., electronic database files of oil spills in 

the Prudhoe Bay Unit Western Operating Area (1989 through 1996), Duck Island 

(Endicott) Unit (1989 through 1996), and Milne Point (1994 through 1996). 

• BP Exploration (Alaska), Inc., electronic spreadsheet containing all industry and 

contractor oil spills from January 1997 to May 2001. 

• Atlantic Richfield Company (ARCO) electronic spreadsheet files of oil spills for the 

Prudhoe Bay Unit Eastern Operating Area (1977 through 1996), Kuparuk River Unit 

(1977 through 1985 and 1986 through 1996), and Kuparuk River Unit exploration (1986 

through 1996). 

• Alyeska printed summary report of oil spills greater than 1,000 bbl along the TAPS from 

1977 through 1989. 

• Joint Pipeline Office electronic database of oil spills along the TAPS (1970 through 

1994). 

• Bureau of Land Management (BLM) printed reports of oil spills along the TAPS during 

1981 and 1982. 

• Alaska Department of Environmental Conservation (ADEC) electronic text and 

spreadsheet files of oil spills from the agency’s current oil and hazardous substances 

spill database (July 1995 through February 1997) and an earlier oil and hazardous 

substances spill database (1971 through July 1995). 

• Alaska Department of Environmental Conservation electronic spreadsheet containing 

all oil spills in their current oil and hazardous substance spill database to December 

2000. 

• An unattributed printed summary of oil spills over 100 gallons on the Alaska North 

Slope and along the TAPS from 1970 through 1981. 

• An electronic spreadsheet summary of Alaskan and Canadian oil spills of 100 bbl or 

greater, from 1978 through 2000, as reported by the Oil Spill Intelligence Report. 

• An MMS report that no oil spills of 100 bbl or larger have occurred in the Alaska Outer 

Continental Shelf Beaufort or Chukchi sea(s) study area. 

• Alyeska electronic spreadsheet file containing all oil spills of 100 bbl or larger from the 

company’s oil-spill database to September 1999. 

A review of the reliability and completeness of the data for spills greater than or equal to 500 

bbl (Hart Crowser, Inc. 2000) indicates that the available information was most reliable for 

1985 through 2000, based on written documentation or lack of documentation and spills before 

that period. The MMS determined that spills greater than or equal to 100 bbl were documented 

and included in the database since 1985. In 1985, the ADEC began tracking spills in an 

electronic format. Although Hart Crowser, Inc. (2000) states that the database is complete for 

the years since production began, the BLM prefers to use 1985 as the starting point of 

reliability. Any uncertainty in documenting spills before that time is a concern because it is 

typical for spills to occur more frequently during field and pipeline startup. 

Six crude oil spills greater than or equal to 500 bbl associated with onshore or offshore Alaska 

North Slope oil production occurred from 1985 to 2000. No spills greater than or equal to 1,000 

bbl were documented during this time period. Of the six spills, one (i.e., a leak in either a 20- or 

24-inch flow line from the wells in Kuparuk to the Central Processing Facility) is classified as a 

pipeline spill. The other five spills are classified as facility spills. The 2006 spill was from a 34 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-6

inch crude oil transit pipeline with an estimated volume of 201,000 gallons, +/- 33%, or 4800 

bbls +/- 33%. 

From 1985 to 2000, the median facility spill greater than or equal to 500 bbl on the Alaska 

North Slope was 663 bbl, and the mean (or average) was 680 bbl. The one pipeline spill had a 

volume of 510 bbl. For spill analysis, the largest recorded facility spill is used. The largest 

facility spill on record is 925 bbl. This oil spill analysis uses a pipeline spill of 4800 bbl. 

Rounded to the nearest 100 (to reflect the uncertainty associated with spill estimates), the 

hypothetical spill sizes become 900 bbl for the facility spill and 4800 bbl for the pipeline spill. 

Historical Crude Oil Spills Greater Than or Equal to 500 Barrels From the TransAlaska Pipeline 

The TAPS oil-spill analysis includes the pipeline and the pump stations, but excludes the 

Valdez marine terminal. Eight crude oil spills greater than or equal to 500 bbl associated with 

TAPS occurred from 1977 through 2001. Most large crude oil spills were associated with the 

start-up of the pipeline. No large spills greater than or equal to 1,000 bbl occurred from 1981 to 

2000. On October 4, 2001, a bullet punctured the 48-inch TAPS mainline; approximately 6,800 

bbl of crude oil were released from this intentional sabotage. Using the highest reported spillquantity values, the mean (average) recorded crude oil spill greater than or equal to 500 bbl 

from 1977 to 2001 is 5,462 bbl, and the median is 4,381 bbl. Using the Alyeska Pipeline Service 

Company reported values, the mean and median spill sizes are 4,089 and 1,650, respectively. 

For spill analysis, the highest reported spill quantity mean is used and rounded to the nearest 

100. Therefore, the mean hypothetical TAPS spill size is 5,500 bbl (median 4,400 bbl) for this 

oil spill analysis. 

Historical Crude Oil Spills Greater Than or Equal to 1,000 Barrels from Tankers 

Eleven crude oil spills greater than or equal to 1,000 bbl associated with the TAPS tankers have 

occurred from 1977 to 1999. The mean size for all TAPS tanker spills is 27,000 bbl and the 

median is 5,000 bbl. For in-port spills, the mean and median are 5,600 bbl and 5,300 bbl, 

respectively. For at-sea spills, the mean and median are 40,600 bbl and 4,900 bbl, respectively. 

The TAPS tanker spills are smaller than worldwide tanker spills and slightly smaller than 

tanker spills in U.S. waters (Anderson and Labelle 2000). 

Historical Crude Oil Spills From Blowouts on the Alaska North Slope 

The record for Alaska North Slope blowouts is not validated, but is presented as the best 

available information. There are two written reports regarding blowouts on the Alaska North 

Slope: Mallory (1998) and Fairweather (2000). Fairweather (2000) found 10 blowouts—six that 

Mallory had identified for the period 1974 to 1998 and four that occurred before 1974. Of the 10 

blowouts, nine were gas and one was oil. The 1950 oil blowout was unspectacular and could not 

have been avoided, as there were no casings or blowout preventors available (Fairweather 

2000). Drilling practices from 1950 would not be relevant today. A third study confirmed that 

no crude oil spills greater than or equal to 100 bbl from blowouts occurred from 1985 through 

1999 (Hart Crowser, Inc. 2000). A recent report titled Blowout Frequency Assessment of 

Northstar (Scandpower AS 2001) uses statistical blowout frequencies modified to reflect specific 

field conditions and operative systems at Northstar. This report concludes that the blowout 

frequency for drilling in the oil-bearing zone at Northstar is 1.5 x 10-5 per well drilled. In 

comparison, the average statistical blowout frequency for a development well in the North Sea 

and U.S. Gulf of Mexico is 7.4 x 10-5 per well. This same report estimates that the statistical 
Northeast NPR-A Supplemental IAP/EIS 

K-7 Appendix K: Oil Spill Scenarios 

frequency of a blowout spill with a size greater than 130,000 bbl is 9.4 x 10-5 per well drilled for 

Northstar. 

However unlikely a blowout may be, it is an important concern to the public; therefore, the 

effects of a 120,000 bbl (15 day) spill are analyzed in section 4.10 (Low Probability, Very Large 

Oil Spill). 

 K.1.1.2 Historical Large Crude Oil Spill Rates 

Oil spill rates are the number of spills that occur over some exposure variable. The exposure 

variable can be bbl of oil produced or pipeline miles per year. Oil spill rates are estimated for 

the Alaska North Slope, the TAPS, and the TAPS tankers using historical spill data. 

Alaska North Slope Spill Rate 1985-2000 Based on Volume 

Only one Alaska North Slope facility or pipeline spill greater than or equal to 1,000 bbl from 

Alaska North Slope production has occurred since 1985. No documentation for crude oil spills 

greater than or equal to 100 bbl occurring prior to 1985 was found, but spill records dated prior 

to 1985 have not been validated as complete because of missing or incomplete documentation. 

As noted above, five facility spills and one pipeline spill are documented from 1985 to 2000 and 

a large pipeline spill occurred in 2006. Total Alaska North Slope production was estimated to 

be 9.36 billion barrels (Bbbl) of crude oil and condensate (Alyeska Pipeline Service Company 

2001, McMaines 2001). Anderson and LaBelle (2000) calculated Alaska North Slope spill rates 

from 1985 to 1998, hence they are slightly different from the spill rates calculated, using the 

1985 to 2000 information, for this Supplement. The spill rate of 0.53 large spills per Bbbl 

handled was calculated for Alaska North Slope facility spills, using the entire record of five 

spills from 1985 to 2000. BLM and MMS use the 1985 to 2000 time period because spills 

greater than 100 bbl have been documented since 1985. In addition, the ADEC began an 

electronic database of oil spills in 1985. BLM and MMS consider the database most reliable 

from 1985 forward. The Alaska North Slope pipeline spill rate of 0.11 large spills per Bbbl 

handled was based on the record of one pipeline spill from 1985 to 2000. Including the spill in 

2006, without considering the increased production that occurred in that additional time period, 

gives a very conservative figure of 0.22 spills per Bbl handled. The combined large crude oil 

spill rate for facilities and pipelines is 0.75 spills per Bbbl handled. 

Trans-Alaska Pipeline Spill Rate 1977-2001 and 1985-2001 Based on Volume and 

Pipeline-Mile-Year 

Flow in the TAPS began on June 20, 1977, with throughput of 112 million barrels (MMbbl) by 

the end of 1977. Throughput increased to almost 400 MMbbl in 1978, peaked at 744 MMbbl in 

1988, and was 370 MMbbl in 2001. The estimated total volume transported through the TAPS 

during the period 1977 through 2001 is 13.62 Bbbl. The TAPS is 800 miles long. 

1977-2001 

There have been 12 crude oil spills greater than or equal to 100 bbl attributed to TAPS 

operation, four of which were less than 500 bbl. Eight spills were greater than or equal to 500 

bbl, of which six were greater than or equal to 1,000 bbl. The last spill greater than or equal to 

1,000 bbl occurred in 2001. The spill rate for spills greater than or equal to 500 bbl of 0.59 spills 

per Bbbl transported for TAPS was calculated based on the record of six accidental and two 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-8

sabotage spills over 13.62 Bbbl of production. The spill rate of 0.000425 large spills per 

pipeline-mile-year for TAPS was calculated based on the record of six accidental and two 

sabotage spills over 18,835 pipeline-mile-years during the period 1977 through 2001. 

1985-2001 

For purposes of this oil spill analysis, approximately the same time period (1985-2001) and the 

same class size (greater than or equal to 500 bbl) as the Alaska North Slope data in section 

K.1.1.1, Historical Large Crude Oil Spills, are used. The spill rate of 0.21 large spills per 

Bbbl transported for TAPS was calculated based on two spills over 9.7 Bbbl of oil transported. 

The TAPS spill rate is 0.00015 large spills per pipeline-mile-year. The rate was also calculated 

based on two spills over 13,605 pipeline-mile-years from 1985 to 2001. 

Trans-Alaska Pipeline Tanker Spill Rate 1977-1999 Based on Volume 

Eleven tanker spills occurred in association with the transportation of Alaska North Slope 

crude: the Exxon Valdez spill and 10 other spills less than or equal to 15,000 bbl (Anderson and 

LaBelle 2000). No large spills have occurred since 1991. The spill rate of 0.87 spills per Bbbl 

transported was calculated based on the record of 11 accidental spills over 12.6 Bbbl of 

production (Anderson and LaBelle 2000). 

 K.1.1.3 Estimated Mean Number and Probability of One or More Large Crude 

 Oil Spills for the Northeast National Petroleum Reserve – Alaska 

The mean number of large crude oil spills, estimated over the production life of the planning 

area for Alternative A, Alternative B, Alternative C, and Alternative D are shown in Table K-2. 

The mean number of spills is derived from the projected resource volumes and the historic spill 

rate. The estimated total spill volume in Table K-2 is the total volume for all of the spills 

estimated for the given alternative. For instance, if two spills of 4800 bbl each were likely to 

occur, then the estimated total spill volume would be 9600 bbl. 

Table K-2. Large Crude Oil Spills Estimated Over the Production Life of the 

Northeast National Petroleum Reserve – Alaska 

Alternative Resources 

(Bbbl) 

Spill Rate 

(Spills/Bbbl) 

Assumed 

Spill Size 

(bbl) 

Estimated 

Mean Number of 

Spills1

Estimated 

Total Spill Volume2

(bbl) 

Crude Oil 

A 2.9 0.75 4,800 2.175 10,440 

B 3.35 0.75 4,800 2.513 12,062 

C 4.05 0.75 4,800 3.038 14,582 

D 3.7 0.75 4,800 2.775 13,320 

1

 The estimated mean number of oil spills is based on the estimated resource volume multiplied by the spill rate. 2

 The estimated total spill volume is the total volume for all of the estimated spills for the given alternative and price of oil. 

The projected mean number of spills (listed in Table K-1) is used to estimate the chance of one 

or more large spills occurring. 

 

 
Northeast NPR-A Supplemental IAP/EIS 

K-9 Appendix K: Oil Spill Scenarios 

K.1.2 Probability of a Small Crude Oil Spill - Less Than 500 Barrels 

A total small spill rate of approximately 618 spills per Bbbl handled, calculated from the Alaska 

North Slope record of small spills, is used here. This spill rate consists of 178 small crude oil 

spills per Bbl and 440 small refined product spills per Bbbl. Since the companies and regulators 

that now operate onshore will likely participate onshore in the Northeast NPR-A, it seems 

reasonable to assume that the spill rate in the Northeast NPR-A will be similar to the rate on 

the Alaska North Slope. 

Historical oil spill information and simple statistical methods are used to derive the following 

information about small crude and refined oil spills that occur on the Alaska North Slope: 

• estimates of how often a spill occurs for every Bbbl of oil produced (oil-spill rates); 

• estimates of the mean number of oil spills; and 

• estimates of the mean and median size of oil spills from facilities, pipelines, and flow 

lines combined. 

This information is used to estimate the number, size, and distribution of operational small 

spills that may occur in the planning area. 

The historical information consists of crude and refined oil spills reported to the ADEC and the 

Joint Pipeline Office by the oil industry. Crude and refined oil spill rates and patterns from 

Alaska North Slope oil and gas exploration and development activities are determined for spills 

greater than or equal to one gallon and less than 500 bbl. Refined oil includes aviation fuel, 

diesel fuel, engine lubricants, fuel oil, gasoline, grease, hydraulic oil, transformer oil, and 

transmission oil. The Alaska North Slope oil spill analysis includes onshore and offshore oil and 

gas exploration and development spills from the Point Thompson Unit, Badami Unit, Kuparuk 

River Unit, Milne Point Unit, Prudhoe Bay West Operating Area, Prudhoe Bay East Operating 

Area, and Duck Island Unit. 

Oil spill information is provided to the ADEC by private industry in accordance with State of 

Alaska Regulations, 18 AAC § 75. The ADEC figures are based on initial spill reports and may 

not contain updated information. Because of increased scrutiny after the Exxon Valdez oil spill, 

information in the ADEC database is most reliable for 1989 and later. Even though the 

integrity of the database cannot be validated thoroughly, the information in the database is still 

valuable because it is the only available data on small spills. For this oil spill analysis, ADEC 

records were spot checked against spill records from ARCO Alaska and BP. All spills greater 

than or equal to one gallon and less than 500 bbl occurring in the 1989 through 2000 time 

period were included in the oil spill analysis. A simple analysis of operational small oil spills 

was performed, and spill rates were estimated without regard to differentiating operation 

processes. The ADEC database structure does not facilitate quantitative analysis of Alaska 

North Slope oil spill rates separately for platforms, pipelines, or flow lines without further 

documentation and validation. 

 K.1.2.1 Historical Small Crude Oil Spill Rates and Patterns on the North Slope 

Because this analysis of crude oil spills was performed collectively for all Alaska North Slope 

facilities, pipelines, and flow lines, the pattern that emerged was one of numerous small spills. 

Of the crude oil spills that occurred between 1989 and 2000, the ADEC database indicates that: 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-10

• 18% were less than or equal to 1 gallon; 

• 54% were less than or equal to 5 gallons; and 

• 99% were less than 25 bbl. 

The small spill sizes in the database range from less than 1 gallon to 425 bbl. The mean crude 

oil spill size on the Alaska North Slope is 2.7 bbl, and the median spill size is 5 gallons. For 

purposes of the oil spill analysis in this Supplement, a mean crude oil-spill size of 3 bbl is 

assumed for small spills. 

The database indicates that the causes of small crude oil spills on the Alaska North Slope, in 

decreasing order of frequency, are: 

• leaks 

• faulty valves/gauges 

• vent discharges 

• faulty connections 

• ruptured lines 

• seal failures 

• human error 

• explosions 

Approximately 30% of the spills in the database do not include information on the causes. 

The estimated small crude oil spill rate for the Alaska North Slope is 178 spills per Bbbl 

produced. The mean number, size, and total volume of small spills for each of the alternatives 

are shown in Table K-3. For this oil spill analysis, the mean number of small spills is used as 

the assumed number of spills. 

Table K-3. Small Crude Oil Spills Estimated Over the Production Life of the 

Northeast National Petroleum Reserve – Alaska 

Alternative Resources 

(Bbbl) 

Spill Rate 

(Spills/Bbbl) 

Assumed 

Spill Size 

(bbl) 

Estimated 

Mean Number of 

Spills1

Estimated 

Total Spill Volume 

(bbl) 

Crude Oil 

A 2.9 178 3 516 1,548 

B 3.35 178 3 596 1,782 

C 4.05 178 3 721 2,163 

D 3.7 178 3 659 1,977 1 The estimated mean number of oil spills is based on the estimated resource volume multiplied by the spill rate and is rounded 

to the nearest whole number. 

 K.1.2.2 Historical Small Refined Oil Spill Rates and Types of Spills on the 

 North Slope 

Typical refined products spilled are aviation fuel, diesel fuel, engine lube, fuel oil, gasoline, 

grease, hydraulic oil, transformer oil, and transmission oil. On the Alaska North Slope, diesel 

spills represent 61% of refined oil spills by frequency and 75% by volume. Engine lube oil spills 

are 10% by frequency and 3% by volume. Hydraulic oil spills are 26% by frequency and 10% by 

volume. All other categories of spills are less than 1% by frequency and volume. Refined oil 
Northeast NPR-A Supplemental IAP/EIS 

K-11 Appendix K: Oil Spill Scenarios 

spills occur in conjunction with oil exploration and production, and correlate to the volume of 

Alaska North Slope crude oil produced. As production of crude oil has declined, so has the 

number of refined oil spills. However, this apparent relationship could be coincidental, as 

emphasis on pollution prevention has also increased in the last several years. From 1989 to 

2000, the spill rate for refined oil was 440 spills per Bbbl produced. 

The mean number of refined oil spills during the lifetime of the alternatives is shown in Table 

K-8. 

Table K-4. Small Refined Oil Spills <500 bbl Estimated Over the Production Life of 

the Northeast National Petroleum Reserve – Alaska 

Alternative Resources 

(Bbbl)

Spill Rate 

(Spills/Bbbl)

Assumed Spill 

Size (bbl)1

Estimated 

Mean Number of Spills2

Estimated Total 

Spill Volume (bbl)

A 2.9 440 0.7 1,276 893 

B 3.35 440 0.7 1,474 1,032 

C 4.05 440 0.7 1,782 1,247 

D 3.7 440 0.7 1,628 1,140 

1

 The mean spill size for refined spills on the Alaska North Slope from 1989 through 2000; equivalent to 29 gallons. 2 The fractional estimated mean spill number and volume are rounded to the nearest whole number. 

 K.1.3 Probability of a Very Large Oil Spill - Greater Than or Equal to 120,000 

 Barrels 

Size assumptions for very large spills for planning area facilities and pipelines are based on 

response planning standards and discharge estimates for the Alpine oil field (ARCO Alaska 

1999, Phillips 2001). Blowouts are unlikely events. While blowouts are often equated with 

catastrophic spills, very few blowout events have resulted in spilled oil, and the volumes that 

are spilled are often small. 

K.2 Oil Weathering and Spreading 

Information about oil weathering and the aerial extent of an oil spill were estimated from oil 

weathering models and historical information. 

 K.2.1 Modeling Simulations of Oil Weathering 

To judge the effect of an oil spill, the following volumes must be estimated: 

• the amount of oil that evaporates; 

• the amount of oil that disperses; and 

• the amount of oil that remains after a certain time period. 

Alpine field crude oil was used as the analog of oil types in the planning area. Weathering 

estimates of Alpine field crude oil and Arctic diesel (over a 30-day period) were derived by the 

SINTEF Oil Weathering Model (OWM), Version 2.0 (Reed et al. 2000). 

Individual weathering results for Alpine field crude oil spills from the SINTEF OWM model are 

shown in Table K-5 and Table K-6. The SINTEF OWM changes both oil properties (density, 

viscosity, pour point, flash point, and water content) and physical properties (spreading, 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-12

evaporation, oil-in-water dispersion, and water uptake) of the oil. The OWM performs a 30-day 

time horizon on the model weathering calculations, but with a warning that the model is not 

verified against experimental field data for more than 4 to 5 days. The SINTEF OWM has been 

tested extensively with results from three full-scale field trials of experimental oil spills (Daling 

and Strom 1999). 

The SINTEF OWM does not incorporate the effects of: 

• currents 

• beaching 

• containment 

• photo-oxidation 

• microbiological degradation 

• adsorption to particles 

• encapsulation by ice 

The spill sizes chosen for oil weathering were 500 and 900 bbl for the Alpine field-type crude oil 

spill, and 900 bbl for a diesel spill. Two general scenarios were simulated—one in which oil 

spills into open water, and another in which oil freezes into the ice and melts into 50% ice cover. 

It was assumed that open water occurs July through September, and that a winter spill melts 

out in July. For open water, the weathering of the 500- and 900-bbl spills was modeled as 

instantaneous spills. For the meltout spill scenario, the entire spill volume was modeled as an 

instantaneous spill. Although different amounts of oil could melt out at different times, the 

MMS assumed a conservative approach−all oil was released at the same time. Results are 

reported for the end of 1, 3, 10, and 30 days. The assumed fate and behavior of Alpine field 

crude oil and diesel oil, information that was used in the analysis of the effects of oil on 

environmental and social resources, are summarized in Table K-5 and Table K-6. 

Table K-5. Fate and Behavior of a Hypothetical 500 bbl Oil Spill From Lagoon 

Pipelines.1

Features Summer Spill2 Meltout Spill3

Time after spill in days 1.0 3.0 10.0 30.0 1.0 3.0 10.0 30.0 

Oil remaining (percent) 75.4 68.0 44.0 38.0 77.0 71.9 64.3 57.6 

Oil dispersed (percent) 0.6 2.0 8.0 22.0 0.0 0.1 0.7 2.4 

Oil evaporated (percent) 24.0 30.0 36.0 40.0 23.0 28.0 35.0 40.0 

Thickness (mm) 3.1 1.9 1.1 1.0 4.6 2.7 1.5 1.0 

Discontinuous area (mi2)

4

 0.6 3.1 15.5 63.9 0.6 4.3 10.5 83.0 

Estimated coastline oiled (mi)5 10.5 9.9 1 Calculated with the SINTEF Oil Weathering Model Version 2.0 (Reed et al. 2000), assuming an Alpine field crude type. 2

 Summer (July through September) and assumes: 12-knot wind speed, 33 degrees Fahrenheit, and 1.3-feet (0.4-meter) wave height. 3 Spill is assumed to occur in May into first-year ice, pools 0.8 inches (2 cm) thick on ice surface for 2 days at 32 degrees Fahrenheit 

before meltout into 50 percent ice cover, 11-knot wind speed, and 0.3 feet (0.1 meter) wave heights. 4 Calculated from Equation 6 of Table 2 in Ford (1985), and is the discontinuous area of a continuing spill or the area swept by an 

instantaneous spill of a given volume. Ice dispersion occurs for about 30 days before meltout. 

5

 Calculated from Equation 17 of Table 4 in Ford (1985), and is the result of stepwise multiple regression for length of historical 

coastline affected.

The structure of the ADEC Alaska North Slope spill database does not facilitate a quantitative 

analysis of pipeline spill rates for small spills. The ADEC database specifically identifies five 

pipeline leaks among 975 spill records. The volumes of these pipeline leaks are 0.7, 5, 18, 125, 
Northeast NPR-A Supplemental IAP/EIS 

K-13 Appendix K: Oil Spill Scenarios 

and 510 bbl. Additionally, any spills occurring or moving off pads would have some potential to 

enter a river or water body. For the purposes of this oil spill analysis, the percent of crude oil 

spills occurring on a pad versus off the pad and onto the surrounding environment was 

estimated. Approximately 65 to 80% of all crude oil spills would occur on a pad and have little 

or no effect on the environment. Approximately 20 to 35% could occur in or reach the 

surrounding environment. 

Table K-6. Fate and Behavior of a Hypothetical 900 bbl Oil Spill From a Lagoon 

Facility.1

Features Summer Spill 2 Meltout Spill 3

Time after spill in days 1 3 10 30 1 3 10 30 

Oil remaining (percent) 75.5 68.4 57.9 40 76.9 71.8 64 56.5 

Oil dispersed (percent) 0.5 1.6 6.1 20 0.1 0.2 1 3.5 

Oil evaporated (percent) 24 30 36 40 23 28 35 40 

Thickness (millimeters) 4.1 2.5 1.5 1 6.1 3.9 1.9 1.2 

Discontinuous area (square miles)4 0.6 4.3 21.1 86.8 1.2 5.6 26.7 112.2 

Estimated coastline oiled (miles)5 13.6 13.0 1 Calculated with the SINTEF Oil Weathering Model Version 2.0 (Reed et al. 2000), assuming an Alpine field crude type. 2

 Summer (July through September) assumes: 12-knot wind speed, 33 degrees Fahrenheit, and 1.3-feet (0.4-meter) wave 

height. 

3 Spill is assumed to occur in May into first-year ice, pools 0.8 inches (2 cm) thick on ice surface for 2 days at 32 degrees 

Fahrenheit before meltout into 50 percent ice cover, 11-knot wind speed, and 0.3 feet (0.1 meter) wave heights. 4 Calculated from Equation 6 of Table 2 in Ford (1985), and is the discontinuous area of a continuing spill or the area swept by 

an instantaneous spill of a given volume. Ice dispersion occurs for about 30 days before meltout. 5

 Calculated from Equation 17 of Table 4 in Ford (1985), and is the result of stepwise multiple regression for length of 

historical coastline affected. 

 K.2.2 Observations of Historic North Slope Spill Patterns 

The development scenarios for alternatives A, B, C, and D include an onshore pipeline. Of 

greatest concern would be the possible contamination of the Colville River, because a pipeline 

could cross or underlie the Colville River and some of its tributaries, and Teshekpuk Lake. 

Those spills reaching the surrounding environment generally remain restricted to a limited area 

of the tundra unless they reach a river, stream, or other water body. The ADEC records are not 

accurate enough to provide statistical spill size areas. The following are comments based on 

information from the ADEC database and Behr-Andres et al. (2001). Off-pad spills that occur in 

or reach the environment generally cover a small area (less than or equal to 500 ft2). Larger 

areas of contamination occur when wind blows a fine oil mist over a large area. The largest 

area ever covered was the result of a pipeline spill on December 30, 1993, at drill site 5, well 23, 

which misted a fine oil spray of 4 bbl over a tundra area of 100 to 145 acres (Mueller 1997). 

Crude oil from a failed flowline spilled onto a gravel pad, reserve pit, and impoundment. High 

winds resulted in the crude oil being misted over the snow-covered tundra in an area 

approximately 330 feet wide and 1,300 feet long (Behr-Andres et al. 2001). Of the off-pad spills 

that occur, many contact snow or ice, which is cleaned up before the oil reaches the tundra. 

Smaller spills are likely to be contained within the snow layer, depending on snow depth and 

density. Larger spills are more likely to reach the ground surface. The ADEC database 

documents that a spill at Point McIntyre covered approximately 23 acres of snow-covered 

tundra with 142 bbl of crude oil. Because this area was snow covered, there was little impact to 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-14

the surrounding environment. If this spill had occurred during the summer, the impacts would 

have been very different. 

K.3 Cumulative Analysis of Oil Spills 

This section discusses how the oil spills for Effects of the Cumulative Case (section 4.7) were 

estimated. 

 K.3.1 Preparing the Cumulative Analysis 

The TAPS pipeline, onshore Alaska North Slope, TAPS tankers, and the Alaska Outer 

Continental Shelf have varying spill rates and spill-size categories. For a summary of the spill 

rates and spill size categories that were assumed for analysis of oil spills in the cumulative case, 

see Table K-7. One noteworthy fact is that most oil originating from either onshore or offshore 

on the North Slope of Alaska flows through the TAPS pipeline and into TAPS tankers. 

Table K-7. Oil Spill Rates and Spill-size Categories Used to Estimate Large Crude Oil 

Spills for the Cumulative Analysis 

Beaufort OCS Alaska North Slope

1985-2001

TAPS Pipeline

1985-2001

TAPS Tanker

1977-1999

Location Spill Rate

(Spills/Bbbl)

Size 

Category

(bbl)

Spill Rate

(Spills/Bbbl)

Size 

Category

(bbl)

Spill Rate

(Spills/Bbbl)

Size 

Category

(bbl)

Spill Rate

(Spills/Bbbl)

Size 

Category

(bbl)

Offshore 0.23 >1,000 - - 0.21 >500 0.88 >1,000 

Onshore - - 0.64 >500 0.21 >500 0.88 >1,000 

Sources: Anderson and LaBelle (2000), Bercha Group, Inc. (2002), and USDOI MMS (2002). 

Estimates of past, present, and reasonably foreseeable production are used for the quantitative 

analysis of oil spills. Past, present, and reasonably foreseeable production contributes 14.4 Bbbl 

in reserves and resources, with the planning area contributing an additional 1.5 Bbbl (the mean 

resource value for the planning area), for a total of 15.9 Bbbl. 

 K.3.2 Estimating Possible Future Spills From All Sources 

The estimated mean number and volume of spills for the cumulative case are shown in Table 

K-8. The likely number of additional oil spills in the Beaufort Sea, onshore, along the TAPS 

pipeline, or tanker route due to projects in the planning area is two. Thus, for purposes of 

analysis of the cumulative case, it is assumed that the planning area would contribute a total of 

two additional oil spills offshore in the Beaufort Sea, onshore, or along the TAPS pipeline or 

tanker route. 

The Beaufort Sea pipeline and platform spill size range used in the analysis is 1,500 to 4,600 

bbl. The onshore spill size range used is 500 to 900 bbl. For the cumulative case, a TAPS 

pipeline spill of 4,400 bbl is estimated. The average spill sizes from TAPS tankers and the 

distribution of the number of spills used for this analysis is as described in the Northwest 

National Petroleum Reserve – Alaska IAP/EIS (USDOI BLM and MMS 2003). 

It is estimated that one spill greater than or equal to 1,000 bbl would occur as a result of 

activities in the Beaufort Sea over the lifetime of planning area projects. This estimate is based 

on production from past, present, and reasonably foreseeable development. Possible offshore 
Northeast NPR-A Supplemental IAP/EIS 

K-15 Appendix K: Oil Spill Scenarios 

sources in these categories (past, present, and reasonably foreseeable development) include 

Endicott, Northstar, Kalubik, Gwydyr Bay, Flaxman Island, Liberty, Kuvlum, and 

Hammerhead. This estimate also includes potential production from undiscovered resources on 

Federal leased tracts in the Beaufort Sea. 

It is estimated that eight spills greater than or equal to 500 bbl would occur onshore before 

entering the TAPS pipeline. One of these spills is likely to be related to planning area projects. 

It is estimated that three spills greater than or equal to 500 bbl would occur along the TAPS 

pipeline, although it is unlikely that the additional throughput given planning area projects 

would increase the number of spills. 

Fourteen spills greater than or equal to 1,000 bbl are expected to occur as a result of projects 

along the TAPS tanker route, one would be expected to be due to the additional volume from 

planning area projects. Of these: 

• nine spills with a mean size of 4,000 bbl—four in port and two at sea—would be expected 

to occur; 

• four spills with a mean size of 13,000 bbl would be expected to occur at sea; and 

• one spill with a size ranging from 200,000 to 260,000 bbl (for purposes of analysis 

250,000 bbl) would be expected to occur at sea. 

Previous studies show that the chance of one or more spills both occurring and contacting land 

along the U.S. coast adjacent to the TAPS tanker route is less than or equal to 3% (LaBelle et 

al. 1996). 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-16

Table K-8. Cumulative Oil-Spill-Occurrence Estimates ≥500 bbl and ≥1,000 bbl over 

Assumed 15-20 Year Production Life of the Northeast National Petroleum Reserve – 

Alaska 

Crude-Oil Spills 

Spill Location and 

Timeframe 

Reserves and 

Resources 

(Bbbl) 

Spill Rate 

(Spills/ Bbbl) Size Category Assumed Size (bbl) 

Most Likely 

Number 

Estimated 

Mean 

Number of 

Spills 

Offshore 

Past, present, and 

reasonably foreseeable 

2.80 0.23 ≥1,000 bbl NA 1 0.64 

Planning Area NA 0.23 ≥1,000 bbl NA NA NA 

Total 2.80 0.23 ≥1,000 bbl NA 1 0.64 

Onshore 

Past, present, and 

reasonably foreseeable 11.6 0.64 ≥500 bbl 500–900 7 7.42 

Planning Area 1.5 0.64 ≥500 bbl 500–900 1 0.96 

Total 13.1 0.64 ≥500 bbl 500–900 8 8.38 

TAPS (Pipeline) 

Past, present, and 

reasonably foreseeable 14.4 0.21 ≥500 bbl 4,400 3 3.02 

Planning Area 1.5 0.21 ≥500 bbl 4,400 0 0.32 

Total 15.9 0.21 ≥500 bbl 4,400 3 3.34 

TAPS (Tanker ) 

Past, present, and 

reasonably foreseeable 14.4 0.88 ≥1,000 bbl varies 13 12.67 

Planning Area 1.5 0.88 ≥1,000 bbl varies 1 1.32 

Total 15.9 0.88 ≥1,000 bbl varies 14 13.99 

Note: The ADEC database has no significant crude oil spills on the North Slope resulting from well blowouts and no facility or onshore pipeline spills 

greater than 1,000 barrels for the years 1985-2000. 

NA = Data not available or not applicable. 

Source: USDOI MMS (2002). 
Northeast NPR-A Supplemental IAP/EIS 

K-17 Appendix K: Oil Spill Scenarios 

K.4 Bibliography for Appendix K 

Alyeska Pipeline Service Company. 2001. Pipeline Facts: 

[http://www.alyeska-pipe.com/Pipelinefacts/PipelineOperations/Throughput.html]. 

Anderson, C.M., and R.P. LaBelle. 2000. Update of Comparative Occurrence Rates for 

Offshore Oil Spills. Spill Science and Technology Bulletin 6(5/6): 303-321. 

ARCO Alaska, Inc. 1999. Oil Discharge Prevention and Contingency Plan, Alpine 

Development Participating Area North Slope Area. ARCO Alaska, Inc., Anchorage, 

Alaska. 

Behr-Andres, C., J. Conn, S. Forester, and J. Wiegers. 2001. Tundra Spill Cleanup and 

Remediation Tactics: A Study of Historic Spills and Literature. AMEC Earth and 

Environmental and Alaska Department of Conservation. 

Bercha Group, Inc. 2002. Alternative Oil Spill Occurrence Estimators for the Beaufort and 

Chukchi Seas – Fault Tree Method. MMS 2002-047. U.S. Department of the Interior, 

Minerals Management Service, Alaska Outer Continental Shelf, Anchorage, Alaska. 

Daling, P.S., and T. Strom. 1999. Weathering of Oil at Sea: Model/Field Data Comparisons. 

Spill Science and Technology 51:63-74. 

Fairweather E&P Services, Inc. (Fairweather). 2000. Historical Blowout Study, North 

Slope of Alaska. BP-Amoco Exploration Alaska. Anchorage, Alaska. 

Ford, R.G. 1985. Oil Slick Sizes and Length of Coastline Affected: A Literature Survey and 

Statistical Analysis. U.S. Department of the Interior, Minerals Management Service, 

Pacific Outer Continental Shelf Region, Los Angeles, California. 

Hart Crowser, Inc. 2000. Estimation of Oil Spill Risk from Alaska North Slope, Trans Alaska 

Pipeline and Arctic Canada Oil Spill Data Sets. Outer Continental Shelf Study MMS 

2000-007. U.S. Department of the Interior, Minerals Management Service, Alaska Outer 

Continental Shelf Region, Anchorage, Alaska. 

LaBelle, R.P., C.M. Marshall, C.M. Anderson, W. Johnson, and E. Lear. 1996. Oil Spill 

Risk Analysis for Alaska North Slope Oil Exports (Domestic Movement). U.S. 

Department of the Interior, Minerals Management Service, Branch of Environmental 

Operations and Analysis, Herndon, Virginia. 

Mallory, C.R. 1998. A Review of Alaska North Slope Blowouts, 1974-1997. Document II-9 In

Preliminary Analysis of Oil Spill Response Capability in Broken Ice to Support Request 

for Additional Information for Northstar Oil Spill Contingency Plan. Volume 2. Prepared 

for BP Exploration (Alaska), Inc., and ARCO Alaska, Anchorage, Alaska. 

McMaines, S. 2001. Electronic Mail from Steve McMaines, State of Alaska, Alaska Oil and Gas 

Conservation Commission, to Caryn Smith, Oceanographer, U.S. Department of the 

Interior, Minerals Management Service, Alaska Outer Continental Shelf Region; 

Subject: North Slope Oil Production by Field to December 2001. 
Northeast NPR-A Supplemental IAP/EIS 

Appendix K: Oil Spill Scenarios K-18

Mueller, (no initial given). 1997. Cited In Northwest National Petroleum Reserve-Alaska, 

Final Integrated Activity Plan/ Environmental Impact Statement. Volumes 1 and 2. U.S. 

Department of the Interior, Bureau of Land Management and Minerals Management 

Service, Anchorage, Alaska. 

Phillips Alaska, Inc. 2001. Colville River Unit Satellite Development Environmental 

Evaluation Document. 

Reed, M., N. Ekrol, P. Daling, O. Johansen, M.K. Ditlevsen, and I. Swahn. 2000. SINTEF 

Oil Weathering Model User’s Manual, Version 1.8. SINTEF Applied Chemistry, 

Trondheim, Norway. 

Scandpower AS. 2001. Blowout Frequency Assessment of Northstar. Report No. 27.83.01/R1. 

British Petroleum Exploration (Alaska), Anchorage, Alaska. 

Smith, R.A., J.R. Slack, T. Wyant, and K.J. Lanfear. 1982. The Oil Spill Risk Analysis 

Model of the U.S. Geological Survey. U.S. Geological Survey Professional Paper 1227, 

U.S. Geological Survey, Reston, Virginia. 

U.S. Department of the Interior, Bureau of Land Management and Minerals 

Management Service (USDOI BLM and MMS). 2003. Northwest National 

Petroleum Reserve – Alaska, Final Integrated Activity Plan/Environmental Impact 

Statement. Volumes 1 and 2. U.S. Department of the Interior, Bureau of Land 

Management and Minerals Management Service, Anchorage, Alaska. 

U.S. Department of the Interior, Minerals Management Service (USDOI MMS). 2002.

Liberty Development and Production Plan, Final Environmental Impact Statement. 

Outer Continental Shelf Environmental Impact Statement/Environmental Assessment 

MMS 2002-019. Three Volumes. U.S. Department of the Interior, Minerals Management 

Service, Alaska Outer Continental Shelf Region, Anchorage, Alaska.