Our Reference: P1206 

OCS Study BOEM 2014-772 

FINAL REPORT 

Loss of Well Control Occurrence and Size Estimators 

for Alaska OCS

BOEM Contract Number M12PC00004 

October 2014

By 

Bercha International Inc. 

Calgary, Alberta, Canada 

 U.S. Department of the Interior

 Alaska Outer Continental Shelf Region 

 Environmental Sciences Management
OCS Study BOEM 2014-772 

FINAL REPORT 

Loss of Well Control Occurrence and Size Estimators 

for Alaska OCS

BOEM Contract Number M12PC00004 

October 2014

Principal Investigator: Dr. Frank G. Bercha, P.Eng. 

Bercha International Inc. 

2926 Parkdale Boulevard N.W. 

Calgary, Alberta, T2N 3S9, Canada 

Email: berchaf@berchagroup.com 

Study concept, oversight, and funding were provided by the U.S. Department of the Interior, 

Bureau of Ocean Energy Management, Environmental Studies Program, Washington, DC 

under Contract Number: M12PC00004. 

DISCLAIMER 

The opinion, findings, conclusions, or recommendations expressed in this report or product are 

those of the authors and do not necessarily reflect the views of the U.S. Department of the 

Interior, nor does mention of products constitute endorsement or recommendations for use by the 

Federal Government. 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

i

EXECUTIVE SUMMARY 

A. Introduction 

Probabilistic estimates of oil spill occurrences are used by the United States Department 

of the Interior, Bureau of Ocean Energy Management (BOEM), to support the 

development of environmental impact assessments for hypothetical and proposed 

developments in the U.S. Chukchi and Beaufort seas. Due to the limited offshore oil 

development in this region, it was not feasible to base these oil spill probability estimates 

on empirical data from that region alone. Rather, statistically significant non-Arctic 

empirical data from the U.S. Outer Continental Shelf (OCS) – including the Gulf of 

Mexico (GOM) and Pacific OCS – and world-wide sources, together with their variance, 

are used as a starting point, to be adjusted using fault and event tree methodologies to 

emulate Arctic conditions. One of the sources of oil spills, and likely the largest spill 

volume potential, is the Loss of Well Control (LOWC) leading to a blowout during 

drilling, production, workover or abandonment. Accordingly, BOEM has retained Bercha 

International Inc. (Bercha) to conduct a study of world-wide data on LOWC incidents, 

and generate statistics and information characterizing regional, incident type, causal, and 

other characteristic variations of LOWC frequencies and associated consequences 

including hydrocarbon spill volumes. LOWC events include both blowouts and well 

releases. 

B. Summary of Report 

The present report deals with the scope of work in successive chapters as follows: 

 Chapter 2 – Databases and Exposure 

 Chapter 3 – U.S. GOM and PAC Statistics 

 Chapter 4 – UK, Norway, and North Sea Statistics 

 Chapter 5 – U.S. GOM and North Sea Statistics 

 Chapter 6 – Australia, Holland, and Canada Statistics 

 Chapter 7 – Temporal and Regional Comparisons 

 Chapter 8 – LOWC Spills

 Chapter 9 – Conclusions and Recommendations

Chapter 8 deals with LOWC hydrocarbon (HC) spills and their characteristics, including 

volume distributions. The value and variability of specific spill volume LOWC 

frequencies for a specified development can be more accurately derived using 

quantitative risk analysis (QRA) approaches including fault trees. Accordingly, an 

algorithm to assess LOWC spill frequencies and characteristics for specified 

developments using the regional frequencies presented in this report as a starting point, 

and incorporating the principal development characteristics and risk factors is described 

and illustrated with an example. 

In its entirety the work provides a perspective on world wide and U.S. OCS LOWC spill 

characteristics, as well as statistics and methodology to facilitate BOEM’s LOWC spill 

potential evaluations in support of its environmental impact assessments of hypothetical 

and proposed specific U.S. OCS projects. 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

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C. Summary of Conclusions

General conclusions of the work can be summarized as follows: 

 Generally adequate data on LOWC occurrences and their characteristics in 

western waters such as the North Sea and the U.S. GOM, are available from the 

SINTEF database for a sufficiently large exposure for the period from 1980 to 

2011. 

 More detailed data, on LOWC occurrences and their characteristics, including 

spill volumes, for the U.S. OCS are available from the BOEM/BSEE database for 

a sufficiently large exposure for the period from 1980 to 2011. 

 The above data are of sufficient quantity and quality to permit the generation of 

statistics, including occurrence rates for different operational phases and 

products spilled, associated confidence intervals, and other statistical measures. 

 Certain data, however, were not available, including spill volumes for locations 

other than the U.S. OCS, well exposure populations by water depth intervals for 

all locations, or detailed characterization of the products spilled from LOWC 

incidents. 

Table 1 summarizes the key high level LOWC parameters for the principal regions 

studied. These are only the high level results; far more details are in the report. 

Table 1: Summary of Principal LOWC Parameters for Key Regions 

EXPOSURE LOWC FREQUENCY LOWC DURATION 

Drilling Production Drilling Production Interventions 50 % 

stopped 

90 % 

stopped REGION 

wells drilled well-years 

per 1000 

wells 

drilled 

per 1000 

well-years 

per 1000 

well-years minutes days 

U.S. GOM 31,574 197,721 3.45 0.106 0.314 200 8 

North Sea 13,727 59,141 2.99 0.051 0.355 3 20 

Holland 1,143 2,948 0 0.339 0.339 n/d* n/d 

Australia 2,559 9,589 1.56 0.104 0 n/d n/d 

Canada East Coast 679 3,955 2.95 0 0 n/d n/d 

* n/d = no data 

It should be noted that the LOWC DURATION values give the % chance that an LOWC 

incident will cease within the time given in each case. 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

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D. Summary of Recommendations

The following recommendations based on the work summarized here: 

 Reconcile any minor differences between the SINTEF and BOEM/BSEE data for the 

U.S. OCS. 

 Explain the main differences among the regional LOWC parameters displayed in 

Table 1, by further review of associated conditions having an effect on LOWC 

frequencies. 

 The depth interval well exposure data are not available for the GOM and other 

regions, resulting in a less meaningful assessment of LOWC occurrence variation 

with depth. Obtain the number of wells by depth interval and evaluate the variation of 

LOWC frequency and its characteristics with depth. 

 LOWC duration data for many incidents indicates a 0 duration. The significance of 

this datum should be investigated and durations adjusted. 

 Full exposure data for Australia, Holland, and Canada were not available, and 

accordingly were estimated for the number of production well-years. Obtain 

exposure data from the stated regional administrations and re-evaluate LOWC 

frequencies. 

 In earlier editions of SINTEF documentation (pre 2000) spill volumes were included, 

but are not included in current data. SINTEF has been advised and are considering 

options. 

 Surface and subsea LOWC data in SINTEF are not adequately documented to assess 

likelihood of releases into the marine environment as opposed to underground 

releases. Determine which of the subsea incidents release into the sea and which stay 

underground. 

 Review of the Chapter 9 LOWC algorithm which gives frequency as a function of 

various regulatory, operational, and reservoir parameters, with experts, and by 

specific application to known regions. As appropriate, modify aspects of the 

algorithm following such review and application. 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

iv

ACKNOWLEDGEMENTS 

Grateful acknowledgement for funding and direction is made to BOEM, Alaska OCS 

Region. In particular, the following BOEM and BSEE personnel are acknowledged 

together with their roles: 

 Dr. Heather Crowley, Contracting Officer’s Representative (COR) 

 Caryn Smith, Advisor 

 Warren Williamson, Petroleum Engineer, IT Advisor 

 Geoffrey Wikel, Advisor 

 Christy Tardiff, Contracting Officer 

This work was carried out by Bercha International Inc. Key Bercha personnel on the 

project team were as follows: 

 Dr Frank G Bercha, Project Manager and Principal Engineer 

 Milan Cerovšek, Reliability Engineering Specialist 

 Wesley Abel, Offshore Operations Advisor 

 Dr. Ove Tobias Gudmestad, Norwegian and SINTEF liaison 

 Dr. Philip Laird, Australian liaison 

 Susan Bercha, Editorial and Word Processing Manager 

 Scientific Advisory Panel: 

 Dr. Richard Prentki, Oceanographer and Alaska OCS expert 

 Wesley Abel, P.Eng. , Retired OIM and VP of multinational oil company 

 Dr. Elliott Taylor, Oil Spill Specialist 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

v

TABLE OF CONTENTS 

CHAPTER PAGE 

Executive Summary.............................................................................................................. i 

Acknowledgements.............................................................................................................. ii 

Table of Contents ............................................................................................................... iii 

List of Tables........................................................................................................................v 

List of Figures................................................................................................................... vii 

Glossary of Terms and Acronyms.................................................................................... viii 

1 Introduction ............................................................................................1.1 

1.1 General Introduction .............................................................................................1.1 

 1.1.1 Background .........................................................................................................1.1 

 1.1.2 Objectives............................................................................................................1.2 

1.2 Scope of Work ......................................................................................................1.2 

 1.2.1 Task 3 Detailed Scope of Work ...........................................................................1.3 

1.3 Organization and Cross-reference of this Report with Task 3..............................1.4 

1.4 Principal Definitions .............................................................................................1.4 

2 Databases and Exposure ..........................................................................2.1 

2.1 Principal Databases Used......................................................................................2.1 

2.2 SINTEF Database .................................................................................................2.1 

 2.2.1 Characteristics of SINTEF Database..................................................................2.1 

 2.2.2 SINTEF Database Exposure ...............................................................................2.5 

 2.2.3 SINTEF Database Confidentiality.......................................................................2.6 

2.3 BOEM and BSEE Data.........................................................................................2.7 

3 U.S. GOM and PAC Statistics .................................................................3.1 

3.1 General Description of U.S. GOM and PAC LOWC Statistics............................3.1 

3.2 U.S. GOM LOWC Exposure ................................................................................3.1 

3.3 U.S. GOM LOWC Frequency Characteristics......................................................3.1 

3.4 U.S. GOM LOWC Duration .................................................................................3.6 

3.5 U.S. GOM LOWC Depth Variation .....................................................................3.7 

 3.5.1 Approach and Water Depth Ranges....................................................................3.7 

 3.5.2 U.S. GOM Shelf Water Depth Range ..................................................................3.7 

 3.5.3 U.S. GOM Offshore Water Depth Range ............................................................3.7 

3.6 U.S. Pacific OCS (PAC) LOWC Characteristics................................................3.10 

4 North Sea Statistics...................................................................................4.1 

4.1 General Description of North Sea LOWC Statistics.............................................4.1 

4.2 North Sea LOWC Exposure..................................................................................4.1 

4.3 North Sea LOWC Frequency Characteristics.......................................................4.1 

4.4 North Sea LOWC Duration ..................................................................................4.6 

4.5 North Sea LOWC Depth Variation.......................................................................4.7 

 4.5.1 Approach and Water Depth Ranges....................................................................4.7 

 4.5.2 North Sea Shelf Water Depth Range ...................................................................4.7 

 4.5.3 North Sea Offshore Water Depth Range .............................................................4.7 

4.6 North Sea UK Sector LOWC Characteristics.....................................................4.10 

4.7 North Sea Norwegian Sector LOWC Characteristics.........................................4.10 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

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5 U.S. GOM and North Sea Statistics ........................................................5.1 

5.1 General Description of U.S. GOM and North Sea LOWC Statistics ...................5.1 

5.2 U.S. GOM and North Sea LOWC Exposure ........................................................5.1 

5.3 U.S. GOM and North Sea LOWC Frequency Characteristics..............................5.1 

5.4 U.S. GOM and North Sea LOWC Duration .........................................................5.6 

5.5 U.S. GOM and North Sea LOWC Depth Variation..............................................5.7 

 5.5.1 Approach and Water Depth Ranges....................................................................5.7 

 5.5.2 U.S. GOM and North Sea Shelf Water Depth Range ..........................................5.7 

 5.5.3 U.S. GOM and North Sea Offshore Water Depth Range ....................................5.7 

6 Australia, Holland, and Canada Statistics .............................................6.1 

6.1 General Description of Australia, Holland, and Canada Statistics .......................6.1 

6.2 Australian LOWC Statistics..................................................................................6.1 

6.3 Dutch LOWC Statistics.........................................................................................6.2 

6.4 Canadian East Coast LOWC Statistics .................................................................6.3 

7 LOWC Frequency Regional and Temporal Comparisons ...................7.1 

7.1 Approach to Comparisons.....................................................................................7.1 

7.2 Regional Exposure................................................................................................7.1 

7.3 LOWC Principal Characteristic Variation for U.S. and North Sea Regions ........7.1 

7.4 Regional and Temporal LOWC Detailed Comparisons .......................................7.5 

7.5 Shallow and Deep Drilling LOWC Characteristic Comparisons .......................7.12 

8 LOWC Hydrocarbon Spills .....................................................................8.1 

8.1 Introduction on LOWC Hydrocarbon Spills.........................................................8.1 

8.2 Spill Data Sources.................................................................................................8.1 

 8.2.1 SINTEF GOM LOWC Release Data ...................................................................8.1 

 8.2.2 BOEM GOM LOWC Spill Data ..........................................................................8.3 

8.3 General U.S. GOM LOWC Incident Characteristics and Exposure.....................8.4 

 8.3.1 SINTEF GOM Spill Frequency Characteristics..................................................8.4 

 8.3.2 BOEM GOM LOWC Spills..................................................................................8.4 

8.4 U.S. OCS LOWC Spill Volume Distributions (1964-2010).................................8.5 

8.5 U.S. GOM Drilling Spill Volume Frequencies.....................................................8.7 

8.6 General Description of Largest Gulf of Mexico LOWC Spills ............................8.9 

8.7 Risk Analytic Approaches to Predicting LOWC Spill Frequencies and Volumes . 8.10 

 8.7.1 Introduction on Risk Analysis Approaches to Specific LOWC Characteristics .....8.10 

 8.7.2 Review of Risk Analysis Approaches to Evaluating Specific LOWC Characteristics ..8.10 

 8.7.3 Fault Tree Analysis Algorithm for Specific Development LOWC Characteristics ...8.11 

9 Conclusions and Recommendations........................................................9.1 

9.1 General Summary .................................................................................................9.1 

9.2 General Conclusions .............................................................................................9.1 

9.3 Specific Conclusions.............................................................................................9.2 

 9.3.1 U.S. OCS .............................................................................................................9.2

 9.3.2 North Sea.............................................................................................................9.3 

 9.3.3 North Sea and GOM............................................................................................9.3 

 9.3.4 Australia, Holland, and Canada .........................................................................9.3 

 9.3.5 Regional and Temporal Comparisons.................................................................9.4 

 9.3.6 LOWC Hydrocarbon Spill Characteristics .........................................................9.5 

 9.3.7 Other Information Generated..............................................................................9.5 

9.4 Recommendations for Further Work ....................................................................9.6 

References........................................................................................................R.1 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

vii

LIST OF TABLES 

TABLE PAGE 

2.1 SINTEF Database Fields.......................................................................................2.2 

2.2 SINTEF Incident Details.........................................................................................2.3 

2.3 SINTEF Regions ....................................................................................................2.5 

2.4 Well Drilling Exposure (1980-2011)....................................................................2.5 

2.5 Production Well-Years Exposure (1980-2011) ....................................................2.6 

2.6 Ratios of Production Well-Years to Development Wells Drilled and 

 Oil to Gas Producers (1980-2011) ........................................................................2.6 

2.7 U.S. OCS LOWC Crude and Condensate Spill Volume Data (1980-2010).........2.8 

3.1 U.S. GOM LOWC Frequency Summary..............................................................3.4 

3.2 U.S. GOM Drilling LOWC Frequency Details ....................................................3.4 

3.3 U.S. GOM LOWC Frequency Variability ............................................................3.5 

3.4 U.S. GOM Shelf Water Depth Range LOWC Incident Distribution....................3.8 

3.5 U.S. GOM Offshore Water Depth Range LOWC Incident Distribution..............3.9 

3.6 OCS 1980-2011 Exposure - Drilling ..................................................................3.10 

3.7 PAC Region LOWC Frequencies Summary.......................................................3.11 

4.1 North Sea LOWC Frequency Summary (1980 – 2011)........................................4.4 

4.2 North Sea LOWC Drilling Frequency Details (1980 – 2011) ..............................4.4 

4.3 North Sea LOWC Frequency Variability .............................................................4.5 

4.4 North Sea Shelf Water Depth Range LOWC Incident Distribution.....................4.8 

4.5 North Sea Offshore Water Depth Range LOWC Incident Distribution ...................4.9 

4.6 UK Sector LOWC Statistics Summary...............................................................4.10 

4.7 Norwegian Sector LOWC Statistics Summary...................................................4.10 

5.1 U.S. GOM and North Sea LOWC Frequency Summary......................................5.4 

5.2 U.S. GOM and North Sea LOWC Drilling Frequency Details.............................5.4 

5.3 U.S. GOM and North Sea Blowout Frequency Variability ..................................5.5 

5.4 U.S. GOM and North Sea Shelf Water Depth Range LOWC Incident Distribution...5.8 

5.5 U.S. GOM and North Sea Offshore Water Depth Range LOWC Incident Distribution...5.9 

6.1 Ratios of Production Well-Years to Development Wells Drilled and 

 Oil to Gas Producers (1980-2011) ........................................................................6.1 

6.2 Australian LOWC General Frequency (1980-2011).............................................6.2 

6.3 Dutch LOWC General Frequency (1980-2011) ...................................................6.2 

6.4 Canadian East Coast LOWC General Frequency (1980-2011) ............................6.3 

7.1 LOWC and Exposure Information (1980-2011)...................................................7.2 

7.2 LOWC Frequencies for U.S. GOM and North Sea Regions (1980-2011) ...........7.3 

7.3 Regional LOWC Frequency (1980-2011) ............................................................7.6 

7.4 Regional LOWC Frequency (2001-2011) ............................................................7.7 

7.5 Regional LOWC Frequency (1980-2000) ............................................................7.8 

7.6 U.S. GOM Shallow Gas and Other Drilling LOWC Frequencies ......................7.13 

7.7 North Sea Shallow Gas and Other Drilling LOWC Frequencies .......................7.13 

7.8 U.S. GOM and North Sea Shallow Gas and Other Drilling LOWC Frequencies...7.13 
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Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

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List of Tables – Continued 

8.1 SINTEF General LOWC HC Spill Data - U.S. GOM (1980-2011) .....................8.2 

8.2 SINTEF LOWC HC Spill Data With Pollution - U.S. GOM (1980-2011) ..........8.3 

8.3 BOEM GOM Spill (>= 50 bbl).............................................................................8.3 

8.4 SINTEF GOM LOWC General Spill Frequency Characteristics.........................8.4 

8.5 SINTEF U.S. GOM OCS 1980-2011 Blowout Spill Frequency Variability........8.4 

8.6 BOEM GOM LOWC General Spill >= 50 bbl Frequency Characteristics ..........8.5 

8.7 BOEM U.S. GOM OCS 1980-2011 LOWC Spill >= 50 bbl Frequency Variability.. 8.5 

8.8 U.S. OCS LOWC Spill Distribution Summary (1964-2010) ...............................8.6 

8.9 U.S. OCS LOWC Spill Distribution Summary Including 106 bbl Spills..............8.7 

8.10 U.S. OCS LOWC Spill Size Frequency Distribution ...........................................8.7 

8.11 Enormous Class B Blowout Frequency and Its Variability..................................8.8 

8.12 Annual Regional Frequency Input Table............................................................8.13 

8.13 LOWC Frequency Factor Input Table ................................................................8.13 

8.14 Regional Frequency Resultants Output ..............................................................8.14 

8.15 Specific Development Annual Regional Frequency Input Table .......................8.14 

8.16 Specific Development LOWC Frequency Input Table.......................................8.15 

8.17 Specific Development Frequency Resultants Output .........................................8.16 

9.1 Summary of Principal LOWC Parameters for Key Regions ................................9.2 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

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LIST OF FIGURES 

FIGURE PAGE 

3.1 U.S. GOM Annual Well Drilling Exposure..........................................................3.2 

3.2 U.S. GOM Annual Producing Well Exposure......................................................3.3 

3.3 Blowout Frequency – U.S. GOM OCS Production Variability (1980-2011).......3.5 

3.4 Blowout Frequency – U.S. GOM OCS Drilling Variability (1980-2011)............3.5 

3.5 U.S. GOM OCS Loss of Well Control – Duration by Minutes ............................3.6 

3.6 U.S. GOM OCS Loss of Well Control – Duration by Hours to 8 Days ...............3.6 

3.7 U.S. GOM Shelf Water Depth Range LOWC Incident Distribution Graphs ...........3.8 

3.8 U.S. GOM Offshore Water Depth Range LOWC Incident Distribution Graphs...........3.9 

4.1 North Sea Annual Well Drilling Exposure ...........................................................4.2 

4.2 North Sea Annual Producing Well Exposure .......................................................4.3 

4.3 Blowout – North Sea Production Variability (1980-2011)...................................4.5 

4.4 Blowout – North Sea Drilling Variability (1980-2011)........................................4.5 

4.5 North Sea Loss of Well Control – Duration by Minutes ......................................4.6 

4.6 North Sea Loss of Well Control – Duration by Hours to 20 Days .......................4.6 

4.7 North Sea Shelf Water Depth Range LOWC Incident Distribution Graphs ........4.8 

4.8 North Sea Offshore Water Depth Range LOWC Incident Distribution Graphs...4.9 

5.1 U.S. GOM and North Sea Annual Well Drilling Exposure..................................5.2 

5.2 U.S. GOM and North Sea Annual Producing Well Exposure ..............................5.3 

5.3 Blowout – U.S. GOM and North Sea Production Variability (1980-2011)..........5.5 

5.4 Blowout – U.S. GOM and North Sea Drilling Variability (1980-2011) ..............5.5 

5.5 U.S. GOM and North Sea Loss of Well Control – Duration by Minutes.............5.6 

5.6 U.S. GOM and North Sea Loss of Well Control – Duration by Hours to 20 Days...5.6 

5.7 U.S. GOM and North Sea Shelf Water Depth Range LOWC Incident Distribution Graphs..5.8 

5.8 U.S. GOM and North Sea Offshore Water Depth Range LOWC Incident Distribution Graphs ..5.9 

7.1 Frequency Graphs: GOM and North Sea (1980-2011).........................................7.4 

7.2 U.S. GOM Temporal LOWC Frequency Comparisons........................................7.9 

7.3 North Sea Temporal LOWC Frequency Comparisons .......................................7.10 

7.4 All Well (North Sea and U.S. GOM) Temporal LOWC Frequency Comparisons...7.11 

7.5 Comparative Regional Shallow Gas and Other Drilling Blowout 

 Frequencies (1980-2011) ....................................................................................7.14 

8.1 Cumulative Frequency and Percentage Spill Size Exceedance ............................8.6 

8.2 U.S. OCS (1980-2011) Spills Blowout Frequency Variability >= 50 bbl............8.7 

8.3 U.S. OCS GOM (1980-2011) Enormous Class B Spill Frequency Variability....8.8 

8.4 Specific Development LOWC Characteristic Evaluation Conceptual Fault Tree...8.12 

8.5 Quantified Specific Development LOWC Characteristic Evaluation Fault Tree ... 8.15 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

x

GLOSSARY OF TERMS AND ACRONYMS 

bbl Barrel. A volumetric unit used in the petroleum industry; equivalent to 

42 U.S. gallons or 158.99 liters.

Blowout A blowout is an incident where formation fluid flows out of the well 

or between formation layers after all the predefined technical well 

barriers or the activation of the same have failed. 

BOEM Bureau of Ocean Energy Management, U.S. Department of the 

Interior

BSEE Bureau of Safety and Environmental Enforcement, U.S. Department 

of the Interior

Consequence The direct effect of an accidental event. 

Delineation well A well drilled specifically to determine the boundary of a discovered 

reservoir. 

Development well A well drilled for the extraction of reservoir hydrocarbons. 

Exploration well A well drilled to test a potential, but unproven hydrocarbon trap. Also 

called a “wildcat” well. 

FT Fault Tree

FTA Fault Tree Analysis 

GOM Gulf of Mexico OCS

GOR Gas to Oil Ratio is the ratio of volumetric flow of produced gas to the 

volumetric flow of crude oil for a specific crude oil and gas mixture 

sample. 

Hazard A condition with a potential to create risks such as accidental leakage 

of hydrocarbons from a pressurized vessel. 

LOWC Loss of Well Control

MMbbl Million Barrels 

MMS Minerals Management Service. On October 1, 2011, the Bureau of Ocean Energy

Management, Regulation and Enforcement (BOEMRE), formerly the Minerals Management Service

(MMS), was replaced by the Bureau of Ocean Energy Management (BOEM) and the Bureau of Safety

and Environmental Enforcement (BSEE) as part of a major reorganization

NPS Nominal Pipe Size or diameter 

OCS Outer Continental Shelf

PAC Pacific OCS 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

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Risk A compound measure of the probability and magnitude of adverse 

effect. 

SINTEF The Foundation for Scientific and Industrial Research at the 

Norwegian Institute of Technology 

Spill frequency The number of spills of a given spill size range per year. Usually 

expressed as spills per 1,000 years (and so indicated). 

Spill frequency 

per barrel produced 

The number of spills of a given spill size range per barrel produced. 

Usually expressed as spills per billion barrels produced (and so 

indicated). 

Spill index The product of spill frequency for a given spill size range and the 

mean spill size for that spill size range. 

Spill occurrence Characterization of an oil spill as an annual frequency and associated 

spill size or spill size range. 

Spill occurrence 

indicator 

Any of the oil spill occurrence characteristics; namely, spill 

frequency, spill frequency per barrel produced, or spill index (defined 

above). 

Spill sizes Small (S): 50 - 99 bbl 

Medium (M): 100 - 999 bbl 

Large (L): 1,000 - 9,999 bbl 

Huge (H): 10,000 - 149,999 bbl 

Enormous: >=150,000 bbl 

 

TIMS Technical Information Management System (of BSEE) 

Well One or more wellbores into the earth for the purpose of either finding 

or producing underground resources or providing services related to 

the production of underground resources, specifically hydrocarbons in 

this document. 

Wellbore Also called “borehole,” means a unique oriented hole from the bottom 

of a drilled interval to the surface. If more than one path exists from a 

surface location to a bottom hole point then more than one wellbore 

exists. 

Well interventions Activities to remediate an existing well, such as workovers. 

Well release The reported incident is a well release if oil or gas flowed from the 

well from some point where flow was not intended and the flow was 

stopped by use of the barrier system that was available on the well at 

the time the incident started. 
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

1.1

CHAPTER 1 

INTRODUCTION 

1.1 General Introduction 

1.1.1 Background 

In order to provide context for the present report, background and details of the project 

are provided. The project is entitled, “Loss of Well Control Occurrence and Size 

Estimators Alaska OCS” and identified as Project Number M12PS00020. It was awarded 

to Bercha International Inc. (Bercha) by the Bureau of Offshore Energy Management 

(BOEM). Its completion is estimated in March of 2016. 

BOEM uses the historical blowout record for the U.S. Outer Continental Shelf (OCS) 

and the North Sea as an input to the fault tree model to develop oil spill occurrence 

rates for oil-and-gas-lease sales and any development projects in the Chukchi and 

Beaufort Sea OCS Planning Areas proposed under BOEM and industry planning [12, 

13, 14]*. In recent years, the Alaska OCS Region has frequently been tasked to provide 

frequency estimates and analysis of potential loss of well control (LOWC) occurrence 

during lease sale, exploration and development in National Environmental Policy Act 

(NEPA) assessments. The largest spill from a single well control incident in the history 

of the U.S. offshore oil industry, the Macondo blowout in the U.S. Gulf of Mexico 

( G O M ) OCS [23, 51], has further focused interest in consideration of very large spills 

from well control incidents in NEPA analyses. 

Under the Bureau of Safety and Environmental Enforcement regulations 30 CFR 

§ 250.188 (3) [52] industry must report all losses of well control. 

"Loss of well control" means: 

(i) Uncontrolled flow of formation or other fluids. The flow may be to an 

exposed formation (an underground blowout) or at the surface (a surface 

blowout); 

(ii) Flow through a diverter; or 

(iii) Uncontrolled flow resulting from a failure of surface equipment or 

procedures. 

In general, well control incidents can be separated into two categories: (i) loss of well 

control without release to the environment, and (ii) loss of well control with release to 

the environment. An additional categorization of the latter is made in this work; 

namely, distinguishing between principally gaseous releases and principally 

hydrocarbon (HC) liquid releases or spills. 

 

*

 Numbers in square brackets refer to publications and documents listed in the “References” section of this 

report. 
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1.2

Although the offshore LOWC has received attention over the last few decades [3, 4, 9, 

11, 32, 33, 40], Offshore Blowouts, Causes and Control by Holand [35], has the most 

comprehensive analysis of worldwide blowout data published to date, but those data are 

in need of updating. The primary world database for this information has been 

compiled by and is held by SINTEF (http://www.sintef.no/Home/Technology-andSociety/Safety-Research!Projects/SINTEF-Offshore-Blowout-Database/) [46], with 

SINTEF and members of SINTEF (including Bercha) with access to the data and various 

statistical analyses based on the database [34, 38, 39, 43, 44]. The current work has 

included continued membership and acquisition, analysis, and reporting on the 

SINTEF database in the context of the present contract by Bercha. 

1.1.2 Objectives 

The general objectives of the project reported herein are to: 

 Update offshore loss of well control frequency information through 2011 for the 

U.S., Canadian and Australian offshore regions, the North Sea, and other areas 

with a comparable regulatory regime. Collate exposure variable information. 

 Apply statistical procedures to develop loss of well control occurrence rates for 

different operational phases and product spilled (e.g., gas, crude and condensate, 

drilling mud). 

 Estimate confidence intervals for occurrence rates. 

 Provide statistical measures such as mean and median spill sizes including 

appropriate methods for statistical outliers such as the Macondo blowout. 

 Provide professional support to BOEM in regard to statistical issues of 

occurrence rates, size estimator(s) and confidence intervals related to this study 

and study results.

 The products should be suitable for use in BOEM NEPA documents and in 

BOEM Fault Tree analyses of Oil Spill Occurrence Estimators.

1.2 Scope of Work 

The technical scope of work for the study may be summarized as follows: 

 Task 1: Post Award Meeting with BOEM and Identify a Scientific Review Panel 

 Task 1a – Post Award Meeting

 Task 1b – Scientific Review Panel candidates -- 3 total 

 Task 2: Update and Collate Offshore Loss of Well Control Records 

 Task 2a – Update and collate offshore loss of well control records for the 

U.S., Canadian, and Australian offshore regions, the North Sea, and other 

areas with a comparable regulatory regime.

 Task 2b – Collate exposure variable information

 Task 2c – Coordinate with the study “Updates to the Fault Tree for Oil Spill 

Occurrence Estimators” 
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 Task 3: Develop Information and Occurrence Rates for Well Control Incidents 

from Compiled Databases 

 Task 3a – Develop general information about well control incidents.

 Task 3b – Develop statistical approach for handling small sample size.

 Task 3c – Provide occurrence estimators for well control incidents by 

operational phase and product spilled based on the historical record.

 Task 3d – Provide occurrence estimators for well control incidents in the 

Gulf of Mexico and Pacific OCS by geographical area based on water depth 

and reservoir properties. 

Additional management and consulting tasks are also included, and will be completed in the 

project timeframe. 

1.2.1 Task 3 Detailed Scope of Work

Task 3: Develop Information and Occurrence Rates for Well Control Incidents from 

Compiled Databases 

 Task 3a – Develop general information about well control incidents.

 The Contractor shall develop general information about loss of well control that 

can be used to estimate small, large and very large spill impact producing factors. 

Some examples include: 

 U.S. OCS versus other regions. 

 Loss of well control by product spilled (gas, crude and condensate, and drilling 

mud). 

 Loss of well control by spill size. 

 Loss of well control by spill duration. 

 Statistical measures of loss of well control such as mean and median spill sizes 

and durations, and confidence intervals. 

 Task 3b – Develop statistical approach for handling small sample size. 

 The Contractor shall develop methodology/statistical approach to deal with the 

inherent challenges associated with a small population size. This may include 

development of a methodology that addresses phenomena such as temporal and 

spatial autocorrelation, ensuring that the sample population consists of 

independent observations, and other such issues. It was found that in fact the 

populations sampled were large, with ample statistical significance. 

 In progress meetings it was agreed that the primary approach will be to identify 

LOWCs with spill occurrences for the GOM and worldwide, and among these to 

isolate the probability of occurrence of spills such as Macondo, and to comment 

on the principal factors leading to such spills possibly using fault trees to illustrate 

the key causal factors. 
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 Task 3c – Provide occurrence estimators for well control incidents by 

operational phase and product spilled based on the historical record. 

 The Contractor shall develop occurrence estimators as the number of well control 

incidents per number of wells drilled for the following categories: all operations 

(production, workover, completion, drilling), drilling operations (exploration and 

development), exploration drilling, and development drilling. 

 Provide additional occurrence estimators within each category based on volume 

and type of product spilled (gas, crude and condensate, and drilling mud). 

 Use statistical measures to identify any spatial and temporal trends in well control 

incident rates, volume spilled, and confidence intervals by region, phase of 

operation, type of product spilled. 

 Task 3d – Provide occurrence estimators for well control incidents in the Gulf of 

Mexico and Pacific OCS by geographical area based on water depth and 

reservoir properties.

 The Contractor shall develop well control incident occurrence estimators for the 

Gulf of Mexico and Pacific OCS for very shallow (0-200 feet), shallow (201-500 

feet), deep (501-5000 feet), and ultra-deep (> 5000 feet) water depth ranges, and 

reservoir properties such as differences with high-temperature, high-pressure 

wells or other such variables. 

1.3 Organization and Cross-reference of this Report with Task 3 

In order to logically address Task 3 and additional work, this report – following this 

introductory chapter – has been organized into 8 chapters, intended to cover the above 

Task 3 Scope of Work, as follows: 

 Chapter 2: Databases and Exposure – Task 3a 

 Chapter 3: U.S. GOM and PAC Statistics – Tasks 3a and 3d 

 Chapter 4: UK, Norway, and North Sea Statistics Task 3a 

 Chapter 5: U.S. GOM and North Sea Statistics – Tasks 3a and 3d 

 Chapter 6: Australia, Holland, and Canada Statistics – Task 3a 

 Chapter 7: Temporal and Regional Comparisons – Tasks 3a and 3d 

 Chapter 8: LOWC Spills – Task 3b 

1.4 Principal Definitions 

In 2006, the Minerals Management Service (MMS) [40] published the following 

definition of a Loss of Well Control (LOWC) event: 

 A blowout is an uncontrolled flow of formation or other fluids either to an 

exposed formation (underground blowout) or to the surface or sea floor (surface 

blowout). 
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1.5

 Flow through a diverter. 

 Uncontrolled flow from the well resulting from a failure of surface equipment or 

procedures. 

In the SINTEF analysis [34], these LOWC events have generally been subdivided into 

two principal categories; namely, Blowouts as described above in the first bullet, and 

Well Releases, as described in the second and third bullets above. The following 

definitions are used for these terms by SINTEF: 

 A blowout is an incident in which formation fluids continue to flow out of the 

well to the surface or sea floor or between formation layers after all existing 

technical well barriers have failed to stop the flow. 

 A well release is an incident in which formation fluids flow out of the well or 

between formation layers but the flow is stopped using existing technical well 

barriers. 

Detailed descriptions of other technical terms and acronyms used are given in the 

Glossary of Terms and Acronyms on page viii at the front of this report. Other related 

terms and acronyms are given in [34, 50]. 
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2.1

CHAPTER 2 

DATABASES AND EXPOSURE 

2.1 Principal Databases Used 

The principal databases reviewed for this worldwide LOWC study were the SINTEF 

worldwide database [46], U.S. GOM and PAC data [22, 24, 41], and several other 

databases and publications relating to offshore LOWC statistics [2, 25, 36, 42]. 

Of these, the principal database is that generated by SINTEF. SINTEF is the Norwegian 

acronym for Stiftelsen for Industriell og Teknisk Forskning, meaning "The Foundation for 

Scientific and Industrial Research". It was established at the Norwegian Institute of 

Technology in Trondheim, Norway, in 1950, and has expanded rapidly to its current 

configuration with over 2,000 employees (most of whom are located in Trondheim, 

Norway). Details of the SINTEF organization are available from their website 

(www.sintef.no) [46]. 

One of the activities in which the SINTEF organization has engaged is in the collection 

of databases to support risk analyses of various industrial activities, including offshore 

drilling. Thus, the SINTEF offshore blowout database is used as the foundation for most 

of the blowout characteristic and statistical results generated under the present project. 

In addition to providing the database (once membership status has been acquired, as has 

been done by Bercha), not only can one access the raw data, but also receive publications 

by one of its affiliated companies, ExproSoft headed by Per Holand [34], as well as other 

publications indicating various analyses of the database [38, 43, 44, 45, 46]. 

Information sources utilized under the present project include the SINTEF [46] database, 

the BOEM/BSEE database [22, 24] and several anecdotal sources of information, 

including the National Energy Board and the Transportation Safety Board of Canada [9], 

the Australian Offshore Regulator, and direct review of reports by agencies such as the 

U.S. Coast Guard (U.S.CG) [51], BOEM [23], and TransOcean [49]. 

2.2 SINTEF Database 

2.2.1 Characteristics of SINTEF Database

An initial description of the SINTEF database is provided below. 

The database fields for LOWCs are summarized in Table 2.1. 
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2.2

Table 2.1: SINTEF Database Fields 

BlowoutID BlowoutDate MainCategory SubCategory CountryName Field 

WaterDepth Operator InstallationName InstallationType WellDepth WellStatusType 

CasingSize CasingDepth MudWeight BH_Pressure MaxMeasuredShutIn

Pressure 

MaxTheoreticShutInWH

_Pressure 

API_grade GasVolume OilVolume WaterVolume GasOilRatio RockType 

FormationAge FormationName LossOfBarrier1 LossOfBarrier1Desc LossOfBarrier2 LossOfBarrier2Desc 

NorthSeaStandards ExternalCause HumanError PhaseType Activity OperationType 

FlowPath FlowPathDesc ReleasePoint ReleasePointDesc FlowMediumType Flowrate 

PollutionType LostProduction Duration Fatalitie IgnitionTime IgnitionType 

ConsequenceClass MaterialLoss ControlMethod ControlMethodDesc RevisionDate 

DataQuality ActivityDesc OperationDesc InstallationTypeDesc ExternalReference Remarks 

In addition to providing tabular results for each of the database fields as available, 

SINTEF also contains detailed incident reports. A typical (but somewhat more detailed 

than most) incident report is shown in Table 2.2, in this case for the Macondo blowout. 

The regions covered by the SINTEF database include the offshore drilling jurisdictions of 

Australia, Canada, Holland, Norway, UK, and of course the U.S.A, and are shown in 

Table 2.3. Norway and the UK comprise the North Sea. 

Unfortunately, other drilling regions such as the Java Sea, the Indian Ocean, the Gulf of 

Bohai, the Sea of Okhotsk, as well as off the coast of Nigeria – which have extensive 

offshore drilling and production activity – are not covered by the SINTEF database. To 

some degree, the drilling standards in these regions are not the same as those of the 

regions covered by SINTEF, so that statistical data would be biased to lower standards if 

drawn from these regions. 
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Table 2.2: SINTEF Incident Details (typical but more detailed than most)

Category and location Field : Mississippi Canyon Block 252, 

Macondo, lease G32306 

X 

Blowout ID : 611 X Water depth : 1521 [m] X

Date : 20.04.2010 X Operator : BP Exploration & Production Inc. X 

Category : Blowout (surface flow) Installation name : Deepwater Horizon X 

Sub category : Totally uncontrolled flow, 

from a deep zone 

Installation type : SEMISUBMERSIBLE 

Country name : U.S./GOM OCS Remark 

Well description API grade : 0 X 

Well depth : 5579 [m] X Gas volume : 0 [1.000 m3/day] X

Well status : KILLED Oil volume : 0 [m3/day] X

Casing size : 9,625 [inch] X Water volume : 0 [m3/day] X

Casing depth : 5579 [m] X Gas/oil ratio : 0 [Sm3/Sm3] X

Mud weight : 0 [kg/m3] X Rock type : A.SANDSTONE 

B.H. Pressure : 817 [bar] X Formation age : B.MIOCENE 

MMSIP : 0 [bar] X Formation name X 

MTSIP : 0 [bar] X

Blowout causes Loss of barrier 2 : B1.FAILED TO CLOSE BOP 

Loss of barrier 1 : C14.CASING PLUG 

FAILURE Remark 

Remark : Cement in casing failed External causes : NO 

 : No, acoustic backup 

BOP control system 

Human error : Failed to observe kick before well was 

flowing 

X 

Present operation Activity : B1.CIRCULATING 

Phase : EXPL.DRLG WILDCAT Remark 

Operation : W9.ABANDON WELL 

Op Remark X 

Blowout characteristics : DRILLFLOOR - THROUGH ROTARY 

: A.THROUGH DRILL 

STRING/TUBING 

Release point 

Flowpath SUBSEA BOP 

B.THROUGH ANNULU.S. Remark 

Remark Flowrate : 8000 [m3/day] X

Flow medium : Oil, Gas (deep) Ignition time : 0 [hrs] X

Pollution : LARGE Ignition type : EXPLOSION 

Lost production X Consequence Class : TOTAL LOSS 

Fatalities : 11 X Material loss : 0 [mil U.S.$] X

Duration : 85 [days] X

Other 

Control method : CAPPED 

Remark 

Revision date : 06.10.2010 X 

Data quality : VERY GOOD 

Reference : www.bp.com X 
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Table 2.2: SINTEF Incident Details ~ Continued ~

: From BP Investigation published September 2010. 

(http://www.bp.com/sectiongenericarticle.do?categoryId=9034902&contentId=7064891) 

A sequence of failures involving a number of different parties led to the explosion and fire which killed 11 people and caused 

widespread pollution in the Gulf of Mexico earlier this year. 

Decisions made by “multiple companies and work teams” contributed to the accident which it says arose from “a complex and 

interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces.” 

The report found that: 

• The cement and shoe track barriers – and in particular the cement slurry that was used – at the bottom of the Macondo well failed 

to contain hydrocarbons within the reservoir, as they were designed to do, and allowed gas and liquids to flow up the production 

casing; 

• The results of the negative pressure test were incorrectly accepted by BP and Transocean, although well integrity had not been 

established; 

• Over a 40-minute period, the Transocean rig crew failed to recognise and act on the influx of hydrocarbons into the well until the 

hydrocarbons were in the riser and rapidly flowing to the surface; 

• After the well-flow reached the rig it was routed to a mud-gas separator, causing gas to be vented directly on to the rig rather than 

being diverted overboard; 

• The flow of gas into the engine rooms through the ventilation system created a potential for ignition which the rig’s fire and gas 

system did not prevent; 

• Even after explosion and fire had disabled its crew-operated controls, the rig’s blow-out preventer on the sea-bed should have 

activated automatically to seal the well. But it failed to operate, probably because critical components were not working. 

“To put it simply, there was a bad cement job and a failure of the shoe track barrier at the bottom of the well, which let 

hydrocarbons from the reservoir into the production casing. The negative pressure test was accepted when it should not have 

been, there were failures in well control procedures and in the blow-out preventer; and the rig’s fire and gas system did not prevent 

ignition. 

“We have said from the beginning that the explosion on the Deepwater Horizon was a shared responsibility among many entities. 

This report makes that conclusion even clearer, presenting a detailed analysis of the facts and recommendations for improvement

both for BP and the other parties involved. We have accepted all the recommendations and are examining how best to implement 

them across our drilling operations worldwide. 

BP said the report was based on information available to the investigating team. It noted that additional relevant information may 

be forthcoming, for example, when Halliburton’s samples of the cement used in the well are released for testing and when the rig’s 

blow-out preventer is fully examined now that it has been recovered from the sea-bed. 

Other recommended reports: 

Chief Counsel’s Report, http://www.oilspillcommission.gov/chief-counsels-report (Published February 17th, 2011) 

Report to the President, http://www.oilspillcommission.gov/final-report (Published January 11 2011) 

DNV report on the BOP, http://www.deepwaterinvestigation.com/go/site/3043/ (Published March 23 2011) 

The DNV report related to failed to close shear rams concluded: 

The primary cause of failure was identified as the BSRs failing to fully close and seal due to a portion of drill pipe trapped between 

the blocks. 

Contributing causes to the primary cause included: 

• The BSRs were not able to move the entire pipe cross section into the shearing surfaces of the blades. 

• Drill pipe in process of shearing was deformed outside the shearing blade surfaces. 

• The drill pipe elastically buckled within the wellbore due to forces induced on the drill pipe during loss of well control. 

• The position of the tool joint at or below the closed Upper Annular prevented upward movement of the drill pipe. 

• The Upper VBRs were closed and sealed on the drill pipe. 

• The flow of well fluids was uncontrolled from downhole of the Upper VBRs 

Remarks 

The blowout was capped. A relief well was drilled to finally control the well. 
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Table 2.3: SINTEF Regions 

Nationality Region Well Type Well Subtype 

Australia All Drilling As available 

Canada Canada Drilling As available 

Holland All Drilling As available 

Norway All Drilling Wildcat, appraisal, development 

 Production Oil, gas, condensate, injection, suspended/abandoned 

UK All Drilling Wildcat, appraisal, development 

 Production Oil, gas, condensate, injection, suspended/abandoned 

U.S. GOM Drilling Wildcat, appraisal, development 

 Production Oil, gas, condensate, injection, suspended/abandoned 

 Pacific Drilling Wildcat, appraisal, development 

 Production Oil, gas, condensate, injection, suspended/abandoned 

2.2.2 SINTEF Database Exposure

Tables 2.4 and 2.5 summarize the regional totals for drilling and production wells, 

respectively. 

As will be discussed later, data on production well-years is not available for certain 

regions. Accordingly, to estimate production well-years and volumes of oil to gas 

producers, ratios from available regional data, as shown in Table 2.6, are used here as a 

rough estimate. 

Table 2.4: Total Wells Drilled Exposure (1980-2011) 

Region Exploration Development Drilling 

Total 

U.S. GOM OCS 12,299 19,275 31,574

U.S. PACIFIC OCS 130 745 875

UK 3,302 5,807 9,109

Norway 1,251 3,367 4,618

Holland 703 440 1,143

Australia 1,628 931 2,559

Denmark 214 455 669

Canada East Coast 295 384 679

TOTAL 19,822 31,404 51,226
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Table 2.5: Production Well-Years Exposure (1980-2011) 

Region Oil 

Producers 

Gas/-

Cond 

Producers 

Total 

Producers Injection 

Wells 

U.S. GOM 101,262 96,459 197,721 9,045 

UK 23,301 15,807 39,108 10,800 

Norway 16,703 3,330 20,033 5,365 

Total: 141,266 115,596 256,862 25,210 

Table 2.6: Ratios of Production well-years to Development Wells Drilled and Oil to 

Gas Producers (1980-2011) 

Region Oil 

Producers 

Gas/-Cond 

Producers 

Total 

Producers 

Development 

Wells 

Drilled 

Ratio 

Producers / 

DevDrill 

Ratio 

Oil/Gas 

Producers 

U.S. GOM 101,262 96,459 197,721 19,275 10.3 1.0 

UK 23,301 15,807 39,108 5,807 6.7 1.5 

Norway 16,703 3,330 20,033 3,367 5.9 5.0 

TOTAL 141,266 115,596 256,862 33,442 7.7 1.2 

2.2.3 SINTEF Database Confidentiality

SINTEF database is proprietary, with access subject to membership and annual update 

participation based on fees. Bercha is both a member and has access for the years 2013 

and 2014. 

In accordance with ExproSoft AS, the agents of SINTEF, on June 21, 2012, Bercha 

received the following advice from them in an email from Dr. Per Holand: 

“Use of data extracted from the Blowout database externally. 

One of the participants requested a clarification related to use of data extracted from the 

database for external clients. This is regulated in § 8.3 in the agreement as follows: 

§8.3 During the confidentiality period specified in Annex C, each Party has the right to 

divulge data from the Project to Affiliates, clients, consultants, contractors, members of 

production groups for which it is operator or technical assistant, relevant regulatory 

authorities and certifying agencies to the extent that their participation in a specific 

project would so necessitate. Any Party making use of this right shall require the 

recipient to sign a confidentiality agreement limiting the use and divulging of Project data 

to such specific and named project only. 
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2.7

It was agreed that our interpretation of this is: 

 The main goal is that the data shall not be released in such a way that they may be 

used to establish a competing database 

 Raw data shall not be presented, i.e. a large number of unedited blowout descriptions 

should not be included, only cases 

 Divulged data, i.e. frequencies and distributions can be presented 

This means that you cannot dump all the information in the database but make reasonable 

extracts.” 

Bercha as the Contractor, interprets this to mean that BOEM as “relevant regulatory 

agency” can view and receive the data we assimilate from the SINTEF database under a 

confidentiality agreement, and that there exist no restrictions on publication of statistics 

and summaries from those data, as is done in the present report. 

2.3 BOEM and BSEE Data 

LOWC data from the U.S. GOM and Pacific (PAC) regions is provided. These data, 

however – just as the SINTEF data – have limited information on oil spills. More recent 

analysis of LOWC spill volumes provided by BOEM [22] provides detailed information 

on those from 1964 to 2010. Data for the period of focus, 1980-2010, from BOEM [22], 

are given in Table 2.7. The data show 9 spills exceeding or equal to 50 bbl. 
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Table 2.7: U.S. OCS LOWC Crude and Condensate Spill Volume Data (1980-2010) [22]

# Blowout 

Date 

Water 

Depth 

Well 

Type* Duration 

(days) Operation** Spillage (bbl) 

Product 

Spilled 

OCS 

Region 

26 18-Sep-1980 105 D 4 PR 1.00 crude oil GOM 

27 12-Jan-1981 36 E 1 DR 0.90 1 condensate GOM 

28 27-Feb-1981 48 D 1 WO 0.90 1 crude oil GOM 

29 26-Jul-1981 48 D 6 CO 0.90 1 crude oil GOM 

30 19-Oct-1981 44 D 1 CO 0.90 1 crude oil GOM 

31 28-Nov-1981 340 D 1 WO 64.00 crude oil GOM 

32 7-Feb-1982 141 D 0.50 WO 0.90 1 crude oil GOM 

33 14-Jul-1982 38 D 57 WO 0.90 1 crude oil GOM 

34 20-Jul-1983 68 E 1 DR 2.00 crude oil GOM 

35 23-Feb-1985 190 D 0.33 WO 50.00 crude oil GOM 

36 20-Mar-1987 126 D 3 CO 60.00 condensate GOM 

37 6-Sep-1987 104 D 1 WO 1.00 Unknown GOM 

38 9-Apr-1988 48 D 0.08 PR 4.50 crude oil GOM 

39 9-Sep-1990 214 D 4 WO 8.00 condensate GOM 

40 9-Oct-1990 186 D 0.04 WO 0.50 crude oil GOM 

41 11-Nov-1991 80 E 1 DR 0.80 condensate GOM 

42 26-Dec-1992 186 E 3 DR 100.00 condensate GOM 

43 22-Feb-1998 87 D PR 1.10 crude oil GOM 

44 8-Jul-1998 51 D 2.2 PA 1.50 condensate GOM 

45 9-Sep-1999 463 D 11 WO 125.00 condensate GOM 

46 12-Jan-2000 309 E 7 DR 0.50 crude oil GOM 

47 28-Feb-2000 2,223 E 0.17 DR 200.00 crude oil GOM 

48 19-Nov-2000 739 D 0.001 WO 0.02 crude oil PAC 

49 6-Jul-2001 169 D WO 1.00 crude oil GOM 

50 12-Jan-2002 153 D 0.0003 WO 0.01 crude oil GOM 

51 3-Oct-2002 50 D PRH 350.00 crude oil GOM 

52 6-Dec-2002 133 D 0.5 PR 0.50 crude oil GOM 

53 8-Mar-2003 30 D 0.17 WO 10.00 condensate GOM 

54 12-Apr-2003 198 D 1 PR 0.02 condensate GOM 

55 9-Feb-2004 23 E 0.25 DR 5.40 condensate GOM 

56 22-Feb-2004 419 D CO 2.50 crude oil GOM 

57 21-Oct-2004 3,855 E 0.21 DR 11.00 crude oil GOM 

58 20-Feb-2006 189 D PA 10.00 condensate GOM 

59 26-Aug-2009 169 D 4 PA 2.44 condensate GOM 

60 29-Dec-2009 6,100 E PA 62.00 crude oil GOM 

61 20-Apr-2010 4,992 E 87 DR-TA 4,900,000.00 2 crude oil GOM 

* Well Type ** Operation

E = Exploration CO = Completion 

D = Development DR = Drilling 

 PA = Permanent Abandonment or Leaking PA 

 PR = Production 

 PRH = Production - Hurricane 

 SIH = Shut-In - Hurricane 

 TA = Temporary Abandonment or Leaking TA 

 WO = Workover 

Notes:

1. A volume of 0.9 bbl signifies that the historical records state “minimal oil pollution”. 

2. Spill #61 spillage is a preliminary volume estimate, not a final volume determined by BOEM. 
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3.1

CHAPTER 3 

U.S. GOM AND PAC STATISTICS 

3.1 General Description of U.S. GOM and PAC LOWC Statistics 

This chapter is intended to provide a comprehensive set of statistics and characteristic 

descriptions for LOWC events in the U.S. GOM and PAC Regions. The results are based 

on the SINTEF data covering the period from 1980 to 2011 [46], except when specified 

otherwise. 

This chapter covers the following principal areas: 

 U.S. GOM exposure 

 U.S. GOM LOWC frequency characteristics 

 U.S. GOM LOWC durations 

 U.S. GOM LOWC water depth distribution 

 Available statistics for U.S. PAC LOWC events 

The SINTEF data [46] are for the OCS only, and exclude state waters. 

3.2 U.S. GOM LOWC Exposure 

The exposure for U.S. GOM and other regions was briefly summarized in Section 2.2.2. 

In the period from 1980 to 2011, a total of 12,299 exploration wells and 19,275 

development wells were drilled in the U.S. GOM, and a total of 197,721 production wellyears occurred [46]. Figure 3.1 graphically illustrates the variation in the number of wells 

of each type drilled annually for the subject period, while Figure 3.2 graphically 

illustrates the number of wells in production annually for the same period. Statistics 

generated in the balance of this chapter are based on the above exposure values, and are 

reproduced in relevant tables. Exposure for production and well interventions has been 

assumed to be the same, as most interventions occur during production. 

3.3 U.S. GOM LOWC Frequency Characteristics 

The characteristics considered in this section include general drilling and production 

LOWC frequencies, details of drilling LOWC frequency characteristics, and the 

statistical variability of the frequency statistics developed; all based on SINTEF data. 

Table 3.1 summarizes the general statistics associated with the general U.S. GOM 

LOWC frequencies based on SINTEF [46] data. As can be seen, the events considered 

are surface blowouts and underground blowouts, and well releases characterized by 

equipment failure release or flow through a diverter – in accordance with the definition 

provided in Section 1.4. Wells are principal exposure variables; one well is counted as 

one exposure unit – whether or not it contains one or more boreholes. These occurrences 

are considered for production wells and well drilling. Statistics for production wells are 

given per well-year. These production well statistics are provided for both oil producers 
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and gas and/or condensate producers. A separate category of LOWCs for event categories 

is given for well intervention. Drilling, again for the same event categories, is given for 

exploration drilling and development drilling as well as their summary. All well drilling 

statistics are based on a frequency per well drilled. 

Figure 3.1: U.S. GOM Annual Well Drilling Exposure 

USGOMOCSDrillingWells

0 100 200 300 400 500 600 700 800 900 1,000

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number ofWells

Explorationwells Developmentwells
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Figure 3.2: U.S. GOM Annual Producing Well Exposure 

USGOMOCSProducingWells

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number ofWells

Total oil producers Gasandcondensateproducers
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Table 3.1: U.S. GOM LOWC Frequency Summary 

Oil 

Production 

Gas/Condensate

 Production 

All 

Production 

Well 

Interventions 

Exploration 

Drilling 

Development 

Drilling 

All 

Drilling 

U.S. GOM OCS Wells 101,262 96,459 197,721 197,721 12,299 19,275 31,574 

1980-2011 

# 

Frequency 

per wellyear # Frequency

per well-year #

Frequency

per wellyear # Frequency

per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 9 8.89E-05 7 7.26E-05 16 8.09E-05 34 1.72E-04 40 3.25E-03 28 1.45E-03 68 2.15E-03 

Blowout (underground flow) 1 9.88E-06 1 5.06E-06 1 5.06E-06 4 3.25E-04 5 2.59E-04 9 2.85E-04 

Blowout Total 10 9.88E-05 7 7.26E-05 17 8.60E-05 35 1.77E-04 44 3.58E-03 33 1.71E-03 77 2.44E-03 

Well Release 1 9.88E-06 3 3.11E-05 4 2.02E-05 26 1.31E-04 1 8.13E-05 3 1.56E-04 4 1.27E-04 

Diverted Well Release 1 5.06E-06 9 7.32E-04 19 9.86E-04 28 8.87E-04 

Well Release Total 1 9.88E-06 3 3.11E-05 4 2.02E-05 27 1.37E-04 10 8.13E-04 22 1.14E-03 32 1.01E-03 

TOTAL 11 1.09E-04 10 1.04E-04 21 1.06E-04 62 3.14E-04 54 4.39E-03 55 2.85E-03 109 3.45E-03 

Table 3.2 gives further details of the well drilling statistics. It is important to separate out 

shallow gas LOWCs from other or deep well drilling LOWCs. The reason for this is that 

by definition, under SINTEF, shallow gas drilling occurs when the surface hole is drilled 

without a blowout preventer (BOP). The exposure for shallow gas drilling is the same as 

that for all drilling, because all offshore wells are initiated by shallow drilling prior to the 

installation of a BOP. One can see that shallow gas drilling frequencies tend to be 

somewhat higher than well drilling with BOP in place. 

Table 3.2: U.S. GOM Drilling LOWC Frequency Details 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow Gas Other All Shallow Gas Other All 

12,299 12,299 12,299 19,275 19,275 19,275 31,574 31,574 31,574 

U.S. GOM OCS 

Wells 

1980-2011 

# Frequency 

per well # Frequency per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequencyper well # Frequencyper well 

Blowout 

(surface flow) 17 1.38E-03 23 1.87E-03 40 3.25E-03 17 8.82E-04 11 5.71E-04 28 1.45E-03 34 1.08E-03 34 1.08E-03 68 2.15E-03 

Blowout 

(underground flow) 4 3.25E-04 4 3.25E-04 1 5.19E-05 4 2.08E-04 5 2.59E-04 1 3.17E-05 8 2.53E-04 9 2.85E-04 

Blowout Total 17 1.38E-03 27 2.20E-03 44 3.58E-03 18 9.34E-04 15 7.78E-04 33 1.71E-03 35 1.11E-03 42 1.33E-03 77 2.44E-03 

Well Release 1 8.13E-05 1 8.13E-05 1 5.19E-05 2 1.04E-04 3 1.56E-04 1 3.17E-05 3 9.50E-05 4 1.27E-04 

Diverted Well 

Release 8 6.50E-04 1 8.13E-05 9 7.32E-04 19 9.86E-04 19 9.86E-04 27 8.55E-04 1 3.17E-05 28 8.87E-04 

Well Release Total 8 6.50E-04 2 1.63E-04 10 8.13E-04 20 1.04E-03 2 1.04E-04 22 1.14E-03 28 8.87E-04 4 1.27E-04 32 1.01E-03 

 

TOTAL 25 2.03E-03 29 2.36E-03 54 4.39E-03 38 1.97E-03 17 8.82E-04 55 2.85E-03 63 2.00E-03 46 1.46E-03 109 3.45E-03 

Finally, Table 3.3 gives the variability for blowout and well release frequencies which 

were given as average values in Table 3.1. The variability calculations are based on those 

presented by Holand [35], indicating the upper and lower 90% confidence intervals for 

the value based on a Chi Square distribution [47]. Thus, for example, for exploration 

drilling, the expected or mean value is 3.58E-03, with an upper 90% confidence interval 

value of 4.51E-03 and a lower 90% confidence interval of 2.74E-03. The same applies to 

the other values indicating the variability of the statistics presented. Figures 3.3 and 3.4 

graphically show this variability for production and drilling blowouts, respectively. These 

figures also present the tabular values for convenience. 
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Table 3.3: U.S. GOM LOWC Frequency Variability 

U.S. GOM OCS Wells Blowout Frequency per Well-Year Well Release Frequency per Well-Year 

Production 

1980-2011 High Low Expected High Low Expected 

Oil Production 1.55E-04 5.36E-05 9.88E-05 2.96E-05 5.07E-07 9.88E-06 

Gas/Condensate Production 1.23E-04 3.41E-05 7.26E-05 6.53E-05 8.48E-06 3.11E-05 

All Production 1.23E-04 5.48E-05 8.60E-05 3.92E-05 6.91E-06 2.02E-05 

Well Interventions 2.29E-04 1.31E-04 1.77E-04 1.71E-04 9.64E-05 1.37E-04 

U.S. GOM OCS Wells Blowout Frequency per Well Well Release Frequency per Well 

Drilling 

1980-2011 High Low Expected High Low Expected 

Exploration Drilling 4.51E-03 2.74E-03 3.58E-03 1.28E-03 4.41E-04 8.13E-04 

Development Drilling 2.23E-03 1.25E-03 1.71E-03 1.57E-03 7.73E-04 1.14E-03 

All Drilling 2.91E-03 2.00E-03 2.44E-03 1.33E-03 7.38E-04 1.01E-03 

Figure 3.3: Blowout Frequency – U.S. GOM OCS Production Variability 

(1980-2011) 

Figure 3.4: Blowout Frequency – U.S. GOM OCS Drilling Variability (1980-2011) 

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

0.0045

Frequencyper

W

High 4.51E‐03 2.23E‐03 2.91E‐03

Low 2.74E‐03 1.25E‐03 2.00E‐03

Expected 3.58E‐03 1.71E‐03 2.44E‐03

ExplorationDrilling Development Drilling All Drilling

Frequency per Well Drilled

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

FrequencyperWell‐Y

High 1.55E‐04 1.23E‐04 1.23E‐04 2.29E‐04

Low 5.36E‐05 3.41E‐05 5.48E‐05 1.31E‐04

Expected 9.88E‐05 7.26E‐05 8.60E‐05 1.77E‐04

Oil Production Gas/Condensate

Production

All Production Well Interventions

Frequency per Well‐Year
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3.4 U.S. GOM LOWC Duration 

Duration of LOWC incidents based on the SINTEF data from 1980 to 2011 can be 

determined from the SINTEF database. The expected duration is displayed both over the 

initial hours (in Figure 3.5 by the minute, up to 5 hours) and in Figure 3.6 by the hour up 

to roughly 8 days. As can be seen, durations up to 200 minutes account for roughly 50% 

of LOWC incidents. The longer view, displayed in Figure 3.6 for up to 8 days, indicates 

that roughly 90% of LOWCs stop within 8 days. Longer duration blowouts, such as 

Macondo, will be discussed in Chapter 8. 

Figure 3.5: U.S. GOM OCS Loss of Well Control – Duration by Minutes 

Figure 3.6: U.S. GOM OCS Loss of Well Control – Duration by Hours to 8 Days 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

xDuration[min]

Probabiltyof

<= Probability of <=x

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 24 48 72 96 120 144 168 192

xDuration[hour]

Probabiltyof

<= Probability of <=x
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3.5 U.S. GOM LOWC Depth Variation 

3.5.1 Approach and Water Depth Ranges

The variation of the number of LOWC incidents with water depth for the U.S. GOM was 

extracted from the SINTEF database. It should be noted that only the number of LOWCs, 

rather than their rate per exposure unit, was available from the data which only gives the 

LOWC incidents by water depth without an exposure variable such as number of wells 

drilled in the subject water depth range; thus, the variable presented in this section is not 

directly comparable to any of the spill occurrence indicators. 

Two water depth ranges were used; namely, the Shelf Range and the Offshore Range. 

The Shelf Range corresponds to water depths used in general data updates [12] specific 

OCS development studies for BOEM [14, 16] and may be summarized as follows: 

 Shallow Shelf: <10 m (<33ft) 

 Inner Shelf: 10 to 29m (33 to 95ft) 

 Middle Shelf: 30 to 60m (95 to 197ft) 

 Outer Shelf and Basin: >60m (>197ft) 

The Offshore Range corresponds to water depths more representative of operations in the 

GOM and North Sea and may be summarized as follows: 

 Very Shallow: <61 m (<200ft) 

 Shallow: 61 to 152m (201 to 500ft) 

 Deep: 153 to 1524m (501 to 5000ft) 

 Outer Shelf and Basin: >1524m (>5000ft) 

Although the individual range intervals are different, the entire range from 0 to an open 

unlimited interval results in the coverage of the same water depth, with just a different 

subdivision of the initial intervals. That is, as will be seen the total number of LOWCs is 

the same for both ranges, but the depth intervals below the maximum open interval are 

different. 

3.5.2 U.S. GOM Shelf Water Depth Range

Table 3.4 gives the distribution of number of LOWC incidents in each of the four Shelf 

water depth ranges. Clearly the maximum number of incidents occurs in the deepest 

range, over 60m. This trend may indicate either that many more wells are drilled in this 

range or that the incident rate is higher in this range or both. Figure 3.7 gives a bar graph 

and a pie chart graphically illustrating these incident number distributions in this range. 

3.5.3 U.S. GOM Offshore Water Depth Range

Table 3.5 gives the distribution of number of LOWC incidents in each of the four 

Offshore water depth ranges. Clearly the maximum number of incidents occurs in the 

Very Shallow range, under 61m. This trend may indicate either that many more wells are 

drilled in this range or that the incident rate is higher in this range or both. Figure 3.8 

gives a bar graph and a pie chart graphically illustrating these incident number 

distributions in this range. 
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Table 3.4: U.S. GOM Shelf Water Depth Range LOWC Incident Distribution 

U.S. GOM OCS 1980-2011 

Shallow Shelf Inner Shelf Middle Shelf Outer Shelf 

and Basin Total 

< 10 m 

(< 33 ft) 

10 to 29 m 

(33 to 95 ft) 

30 to 60 m 

(95 to 197 ft) 

> 60 m 

(>197 ft) All 

7 53 51 82 193 

Figure 3.7: U.S. GOM Shelf Water Depth Range LOWC Incident Distribution Graphs 

U.S.GOMOCS1980‐2011

4%

27%

26%

43%

ShallowShelf <10m

(<33 ft)

Inner Shelf 10 to29m

(33 to95 ft)

MiddleShelf 30 to60m

(95 to197 ft)

Outer Shelf&Basin>60m

(>197 ft)

WaterDepth

0

10

20

30

40

50

60

70

80

90

Numberof LOW

U.S.GOMOCS1980‐2011 7 53 51 82

<10m 10to29m 30to60m >60m

ShallowShelf InnerShelf MiddleShelf OuterShelf&Basin

Number of LOWC 
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Table 3.5: U.S. GOM Offshore Water Depth Range LOWC Incident Distribution 

U.S. GOM OCS 1980-2011 

Very Shallow Shallow Deep Ultra Deep Total 

≤ 61 m 

(≤ 200 ft) 

61 to 152 m 

(201 to 500 ft) 

153 to 1524 m 

(501 to 5,000 ft) 

> 1524 m 

(> 5,000 ft) All 

112 57 23 1 193 

Figure 3.8: U.S. GOM Offshore Water Depth Range LOWC Incident Distribution Graphs 

U.S.GOMOCS1980‐2011

58.0%

29.5%

11.9% 0.5%

Very Shallow ≤  61m

(≤ 200 ft)

Shallow61 to152m

(201 to500 ft)

Deep153 to1524m

(501 to5,000ft)

UltraDeep>1524m

(>5,000 ft)

WaterDepth

0

20

40

60

80

100

120

Number of LOW

U.S.GOMOCS1980‐2011 112 57 23 1

≤  61m 61to152m 153to1524m >1524m

VeryShallow Shallow Deep UltraDeep

Number of LOWC 
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3.6 U.S. Pacific OCS (PAC) LOWC Characteristics 

A paucity of data, both in BOEM and SINTEF for LOWC incidents exists for the U.S. 

Pacific OCS (PAC). Only well drilling information is available from SINTEF. This 

drilling exposure, for exploration wells and development wells is summarized in Table 

3.6. 

Table 3.6: OCS 1980-2011 Exposure - Drilling 

Year Exploration 

Wells 

Development 

Wells 

All Drilling 

Wells 

1980 10 40 50 

1981 14 50 64 

1982 27 58 85 

1983 38 44 82 

1984 19 45 64 

1985 6 39 45 

1986 5 34 39 

1987 4 39 43 

1988 3 29 32 

1989 4 15 19 

1990 17 17 

1991 8 8 

1992 5 5 

1993 21 21 

1994 25 25 

1995 19 19 

1996 31 31 

1997 29 29 

1998 19 19 

1999 11 11 

2000 13 13 

2001 16 16 

2002 21 21 

2003 18 18 

2004 20 20 

2005 23 23 

2006 17 17 

2007 12 12 

2008 5 5 

2009 7 7 

2010 8 8 

2011 7 7 

Total 

1980-2011 130 745 875 

Offshore drilling and hydrocarbon production off the U.S. Pacific coast has occurred 

predominantly off the coast of the County of Santa Barbara. The Santa Barbara oil spill 

created by a blowout on January 28, 1969 on Union Oil Platform A is estimated to have 

released 80,000 to 100,000 bbl of crude oil at a location 10 km from the coast. Although 

there were many regulatory and legal consequences for this and other spills [57], on the 

Pacific OCS, the data in Table 3.7 reflects the moratorium on new leases established in 

1981, showing a rapid decrease in exploration drilling to zero wells in 1990. 
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Table 3.7: PAC Region LOWC Frequencies Summary 

Oil 

Production 

Gas/Condensate

 Production 

All 

Production 

Well 

Interventions

Exploration 

Drilling 

Development 

Drilling All Drilling 

Estimated 

3,837 

Estimated 

3,837 

Estimated 

7,674 

Estimated 

7,674 130 745 875 California OCS 

Wells 

1980-2011 

# 

Frequency 

per wellyear # Frequency 

per well-year #

Frequency

per wellyear #

Frequency

per wellyear # Frequency 

per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 1.34E-04 

Blowout (underground flow) 

Blowout Total 1 1.34E-04 

Well Release 3 4.03E-04 

Diverted Well Release 1 1.34E-03 1 1.14E-03 

Well Release Total 3 4.03E-04 1 1.34E-03 1 1.14E-03 

TOTAL 4 5.37E-04 1 1.34E-03 1 1.14E-03 

SINTEF does not report on the number of wells in production in the PAC region. 

However, the ratios of development drilling to producing well-years evaluated in Table 

2.6 can be used to give an approximate exposure and frequencies are shown in Table 3.7. 

Basically, from Table 2.6, the total number of production well-years is equal to 10.3 

times the number of development wells drilled; and the number of oil versus gas 

producers is 50% of the production well years. As can be seen, the largest number of 

LOWCs occurred during well interventions, but because of the large number of wellyears of exposure, LOWC frequencies result in a relatively low value per year. 
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CHAPTER 4 

NORTH SEA STATISTICS 

4.1 General Description of North Sea LOWC Statistics 

This chapter is intended to provide a comprehensive set of statistics and characteristic 

descriptions for LOWC events in the North Sea, which consists of the United Kingdom 

(UK) and Norwegian sectors. Following production of North Sea results, some details on 

the UK and Norwegian sectors per se are also given. The results are based on the 

SINTEF data [46] covering the period from 1980 to 2011. 

This chapter covers the following principal areas: 

 North Sea exposure 

 North Sea LOWC frequency characteristics 

 North Sea LOWC durations 

 North Sea LOWC water depth distribution 

 Selected statistics for UK and Norwegian sectors 

4.2 North Sea LOWC Exposure 

The exposure for the North Sea and other regions was briefly summarized in Section 

2.2.2. In the period from 1980 to 2011, a total of 4,553 exploration wells and 9,174 

development wells were drilled in the North Sea, with a total of 59,141 production wellyears for the period. Figure 4.1 graphically illustrates the variation in the number of wells 

of each type drilled annually for the subject period, while Figure 4.2 graphically 

illustrates the number of wells in production annually for the subject period. Statistics 

generated in the balance of this chapter are based on the above exposure. 

4.3 North Sea LOWC Frequency Characteristics 

The characteristics considered in this section include general drilling and production 

LOWC frequencies, details of drilling LOWC frequency characteristics, and the 

statistical variability of the frequency statistics developed. 

Table 4.1 summarizes the general statistics associated with the general North Sea LOWC 

frequencies. As can be seen, the events considered are surface blowouts and underground 

blowouts, and other well releases – in accordance with the definition provided in Chapter 

1, Section 1.4. These occurrences are considered for production wells and well drilling. 

Statistics for production wells are given per well-year, for both oil producers and gas 

and/or condensate producers. A separate category of LOWCs is given for well 

interventions. Drilling is given for exploration drilling and development drilling, as well 

as their summary. All well drilling statistics are based on a frequency per well drilled. 
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Figure 4.1: North Sea Annual Well Drilling Exposure 

NorthSeaDrillingWells

0 50 100 150 200 250 300 350 400 450 500

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number ofWells

Explorationwells Developmentwells
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Figure 4.2: North Sea Annual Producing Well Exposure 

NorthSeaProducingWells

0 200 400 600 800 1,000 1,200 1,400 1,600 1,800 2,000

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number ofWells

Oil producers Gasandcondensateproducers
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Table 4.1: North Sea LOWC Frequency Summary (1980 - 2011) 

Oil 

Production 

Gas/Condensate

 Production 

All 

Production 

Well 

Interventions 

Exploration 

Drilling 

Development 

Drilling 

All 

Drilling 

North Sea Wells 40,004 19,137 59,141 59,141 4,553 9,174 13,727 

1980-2011 

# 

Frequency 

per wellyear # Frequency

per well-year #

Frequency

per wellyear # Frequency

per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 2.50E-05 1 5.23E-05 2 3.38E-05 4 6.76E-05 16 3.51E-03 4 4.36E-04 20 1.46E-03 

Blowout (underground flow) 4 8.79E-04 4 2.91E-04 

Blowout Total 1 2.50E-05 1 5.23E-05 2 3.38E-05 4 6.76E-05 20 4.39E-03 4 4.36E-04 24 1.75E-03 

Well Release 1 5.23E-05 1 1.69E-05 17 2.87E-04 8 1.76E-03 5 5.45E-04 13 9.47E-04 

Diverted Well Release 4 8.79E-04 4 2.91E-04 

Well Release Total 1 5.23E-05 1 1.69E-05 17 2.87E-04 12 2.64E-03 5 5.45E-04 17 1.24E-03 

TOTAL 1 2.50E-05 2 1.05E-04 3 5.07E-05 21 3.55E-04 32 7.03E-03 9 9.81E-04 41 2.99E-03 

Table 4.2 gives further details of the well drilling statistics. It is important to separate out 

shallow gas LOWCs from other or deep well drilling LOWCs. The reason for this is that 

by definition, under SINTEF, shallow gas drilling occurs when the surface hole is drilled 

without a BOP. The exposure for shallow gas drilling is the same as that for all drilling, 

because all offshore wells are initiated by shallow drilling prior to the surface completion 

and installation of a BOP. One can see that shallow gas drilling frequencies tend to be 

somewhat higher than other or deeper well drilling with BOP activities. 

Table 4.2: North Sea LOWC Drilling Frequency Details (1980 - 2011) 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow Gas Other All Shallow Gas Other All 

4,553 4,553 4,553 9,174 9,174 9,174 13,727 13,727 13,727 North Sea Wells 

1980-2011 

# Frequency 

per well # Frequency per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequency per well # Frequencyper well 

Blowout 

(surface flow) 15 3.29E-03 1 2.20E-04 16 3.51E-03 4 4.36E-04 4 4.36E-04 19 1.38E-03 1 7.28E-05 20 1.46E-03 

Blowout 

(underground flow) 4 8.79E-04 4 8.79E-04 4 2.91E-04 4 2.91E-04 

Blowout Total 15 3.29E-03 5 1.10E-03 20 4.39E-03 4 4.36E-04 4 4.36E-04 19 1.38E-03 5 3.64E-04 24 1.75E-03 

Well Release 2 4.39E-04 6 1.32E-03 8 1.76E-03 1 1.09E-04 4 4.36E-04 5 5.45E-04 3 2.19E-04 10 7.28E-04 13 9.47E-04 

Diverted Well 

Release 4 8.79E-04 4 8.79E-04 4 2.91E-04 4 2.91E-04 

Well Release Total 6 1.32E-03 6 1.32E-03 12 2.64E-03 1 1.09E-04 4 4.36E-04 5 5.45E-04 7 5.10E-04 10 7.28E-04 17 1.24E-03 

TOTAL 21 4.61E-03 11 2.42E-03 32 7.03E-03 5 5.45E-04 4 4.36E-04 9 9.81E-04 26 1.89E-03 15 1.09E-03 41 2.99E-03 

Finally, Table 4.3 gives the variability for the blowout and well release frequencies which 

were given as average values in Table 4.1. The variability calculations are based on those 

presented by Holand [35], indicating the upper and lower 90% confidence intervals for 

the value based on a Chi Square distribution [47]. Thus, for example, for exploration 

drilling, the expected or mean value is 4.39E-03, with an upper 90% confidence interval 

of 6.12E-03 and a lower 90% confidence interval of 2.91E-03. The same applies to the 

other values indicating the variability of the statistics presented. Figures 4.3 and 4.4 

graphically show this variability for production and drilling blowouts, respectively. These 

figures also present the tabular values for convenience. 
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Table 4.3: North Sea LOWC Frequency Variability 

North Sea Wells Blowout Frequency per Well-Year Well Release Frequency per Well-Year 

1980-2011 High Low Expected High Low Expected 

Oil Production 7.49E-05 1.28E-06 2.50E-05 0 0 0 

Gas/Condensate Production 1.57E-04 2.68E-06 5.23E-05 1.57E-04 2.68E-06 5.23E-05 

All Production 8.02E-05 6.01E-06 3.38E-05 5.07E-05 8.67E-07 1.69E-05 

Well Interventions 1.31E-04 2.31E-05 6.76E-05 4.11E-04 1.83E-04 2.87E-04 

North Sea Wells Blowout Frequency per Well Well Release Frequency per Well 

1980-2011 High Low Expected High Low Expected 

Exploration Drilling 6.12E-03 2.91E-03 4.39E-03 4.00E-03 1.52E-03 2.64E-03 

Development Drilling 8.45E-04 1.49E-04 4.36E-04 9.98E-04 2.15E-04 5.45E-04 

All Drilling 2.37E-03 1.21E-03 1.75E-03 1.77E-03 7.89E-04 1.24E-03 

Note: 90% confidence that Frequency is in the Low-High Interval 

Reference: Offshore Blowouts, Per Holand (page 132) 

Figure 4.3: Blowout – North Sea Production Variability (1980-2011) 

Figure 4.4: Blowout – North Sea Drilling Variability (1980-2011) 

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

0.00030

0.00035

FrequencyperWell‐Y

High 7.49E‐05 1.57E‐04 8.02E‐05 1.31E‐04

Low 1.28E‐06 2.68E‐06 6.01E‐06 2.31E‐05

Expected 2.50E‐05 5.23E‐05 3.38E‐05 6.76E‐05

Oil Production Gas/Condensate

Production All Production Well Interventions

Frequency per Well‐Year

0.0000

0.0010

0.0020

0.0030

0.0040

0.0050

0.0060

Frequencyper

W

High 6.12E‐03 8.45E‐04 2.37E‐03

Low 2.91E‐03 1.49E‐04 1.21E‐03

Expected 4.39E‐03 4.36E‐04 1.75E‐03

ExplorationDrilling Development Drilling All Drilling

Frequency per Well Drilled
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4.4 North Sea LOWC Duration 

Duration of well control incidents based on the SINTEF data from 1980 to 2011 can be 

determined. The expected duration is displayed both over the initial period (by minutes, 

up to 20 minutes) and by the hour up to roughly 20 days. Figure 4.5 shows the initial 

blowout duration for up to 20 minutes. As can be seen, durations up to 3 minutes account 

for roughly 50% of LOWC incidents. The longer view, displayed in Figure 4.6 for up to 

20 days, indicates that roughly 90% of LOWCs stop within 20 days. Longer duration 

blowouts, such as Macondo, will be discussed in Chapter 8. 

Figure 4.5: North Sea Loss of Well Control – Duration by Minutes 

Figure 4.6: North Sea Loss of Well Control – Duration by Hours to 20 Days 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 432 456 480

xDuration[hour]

Probabiltyof

<= Probability of <=x

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0 2 4 6 8 10 12 14 16 18 20

xDuration[min]

Probabiltyof

<= Probability of <=x
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4.5 North Sea LOWC Depth Variation 

4.5.1 Approach and Water Depth Ranges

The variation of the number of LOWC incidents with water depth in the North Sea was 

extracted from the SINTEF database. It should be noted that only the number of LOWCs, 

rather than their rate per exposure unit, was available from the data which only gives the 

LOWC incidents by water depth without an exposure variable such as number of wells 

drilled in the subject water depth range; thus, the variable presented in this section is not 

directly comparable to any of the spill occurrence indicators. 

Two water depth ranges were used as described in Section 3.5; namely, the Shelf Range 

and the Offshore Range. 

4.5.2 North Sea Shelf Water Depth Range

Table 4.4 gives the distribution of number of LOWC incidents in each of the four Shelf 

water depth ranges. Clearly the maximum number of incidents occurs in the deepest 

range, over 60m. This trend may indicate either that many more wells are drilled in this 

range or that the incident rate is higher in this range or both. Figure 4.7 gives a bar graph 

and a pie chart graphically illustrating these incident number distributions in this range. 

4.5.3 North Sea Offshore Water Depth Range

Table 4.5 gives the distribution of number of LOWC incidents in each of the four 

Offshore water depth ranges. Clearly the maximum number of incidents occurs in the 

Very Shallow range, under 61m. This trend may indicate either that many more wells are 

drilled in this range or that the incident rate is higher in this range or both. Figure 4.8 

gives a bar graph and a pie chart graphically illustrating these incident number 

distributions in this range. 
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Table 4.4: North Sea Shelf Water Depth Range LOWC Incident Distribution 

North Sea 1980-2011 

Shallow Shelf Inner Shelf Middle Shelf Outer Shelf 

and Basin Total 

< 10 m 

(< 33 ft) 

10 to 29 m 

(33 to 95 ft) 

30 to 60 m 

(95 to 197 ft) 

> 60 m 

(>197 ft) All 

7 1 4 54 66 

Figure 4.7: North Sea Shelf Water Depth Range LOWC Incident Distribution Graphs 

NorthSea1980‐2011

11%

2%

6%

81%

ShallowShelf <10m

(<33ft)

InnerShelf 10to29m

(33to95ft)

MiddleShelf 30to60m

(95to197ft)

OuterShelf&Basin>60m

(>197ft)

WaterDepth

0

10

20

30

40

50

60

Number of LOW

NorthSea1980‐2011 7 1 4 54

<10m 10to29m 30to60m >60m

ShallowShelf InnerShelf MiddleShelf OuterShelf&Basin

Number of LOWC 
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Table 4.5: North Sea Offshore Water Depth Range LOWC Incident Distribution 

North Sea 1980-2011 

Very Shallow Shallow Deep Ultra Deep Total 

≤ 61 m 

(≤ 200 ft) 

61 to 152 m 

(201 to 500 ft) 

153 to 1524 m 

(501 to 5,000 ft) 

> 1524 m 

(> 5,000 ft) All 

12 41 13 0 66 

Figure 4.8: North Sea Offshore Water Depth Range LOWC Incident Distribution Graphs 

NorthSea1980‐2011

18%

62%

20%

0%

VeryShallow ≤  61m

(≤ 200ft)

Shallow61to152m

(201to500ft)

Deep153to1524m

(501to5,000ft)

UltraDeep>1524m

(>5,000ft)

WaterDepth

0

5

10

15

20

25

30

35

40

45

Number of LOW

NorthSea1980‐2011 12 41 13 0

≤  61m 61to152m 153to1524m >1524m

VeryShallow Shallow Deep UltraDeep

Number of LOWC 
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4.6 North Sea UK Sector LOWC Characteristics 

Table 4.6 summarizes the UK Sector LOWC statistics, and also provides the measures of 

exposure. 

Table 4.6: UK Sector LOWC Statistics Summary 

Oil Production Gas/Condensate

 Production All Production Well Interventions 

Exploration 

Drilling 

Development 

Drilling All Drilling 

23,301 15,807 39,108 39,108 3,302 5,807 9,109 UK 

Wells 

1980-2011 

# Frequency 

per well-year # 

Frequency

per wellyear 

# Frequency

per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 4.29E-05 1 6.33E-05 2 5.11E-05 3 7.67E-05 6 1.82E-03 3 5.17E-04 9 9.88E-04 

Blowout (underground flow) 

Blowout Total 1 4.29E-05 1 6.33E-05 2 5.11E-05 3 7.67E-05 6 1.82E-03 3 5.17E-04 9 9.88E-04 

Well Release 1 6.33E-05 1 2.56E-05 14 3.58E-04 1 3.03E-04 3 5.17E-04 4 4.39E-04 

Diverted Well Release 

Well Release Total 1 6.33E-05 1 2.56E-05 14 3.58E-04 1 3.03E-04 3 5.17E-04 4 4.39E-04 

TOTAL 1 4.29E-05 2 1.27E-04 3 7.67E-05 17 4.35E-04 7 2.12E-03 6 1.03E-03 13 1.43E-03 

4.7 North Sea Norwegian Sector LOWC Characteristics 

Table 4.7 summarizes the Norwegian Sector LOWC statistics, and also provides the 

measures of exposure. 

Table 4.7: Norwegian Sector LOWC Statistics Summary 

Oil Production Gas/Condensate

 Production All Production Well Interventions 

Exploration 

Drilling 

Development 

Drilling All Drilling 

16,703 3,330 20,033 20,033 1,251 3,367 4,618 

Norway 

Wells 

1980-2011 

# Frequency 

per well-year # Frequencyper well-year # Frequencyper well-year # Frequency per well-year # Frequencyper well # Frequency per well # Frequency per well 

Blowout (surface flow) 1 4.99E-05 10 7.99E-03 1 2.97E-04 11 2.38E-03 

Blowout (underground flow) 4 3.20E-03 4 8.66E-04 

Blowout Total 1 4.99E-05 14 1.12E-02 1 2.97E-04 15 3.25E-03 

Well Release 3 1.50E-04 7 5.60E-03 2 5.94E-04 9 1.95E-03 

Diverted Well Release 4 3.20E-03 4 8.66E-04 

Well Release Total 3 1.50E-04 11 8.79E-03 2 5.94E-04 13 2.82E-03 

TOTAL 4 2.00E-04 25 2.00E-02 3 8.91E-04 28 6.06E-03 
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5.1

CHAPTER 5 

U.S. GOM AND NORTH SEA STATISTICS 

5.1 General Description of U.S. GOM and North Sea LOWC Statistics 

This chapter is intended to provide a comprehensive set of statistics and characteristic 

descriptions for LOWC events in the U.S. GOM and North Sea. The results are based 

primarily on the SINTEF data covering the period from 1980 to 2011 [46]. 

This chapter covers the following principal areas: 

 U.S. GOM and North Sea exposure 

 U.S. GOM and North Sea LOWC frequency characteristics 

 U.S. GOM and North Sea LOWC durations 

 U.S. GOM and North Sea LOWC water depth 

5.2 U.S. GOM and North Sea LOWC Exposure 

The exposure for U.S. GOM and North Sea was briefly summarized in Section 2.2.2. In 

the period from 1980 to 2011, a total of 16,852 exploration wells and 28,449 

development wells were drilled, and a total of 256,862 production well-years accrued in 

the period. Figure 5.1 graphically illustrates the variation in the number of wells of each 

type drilled annually for the subject period, while Figure 5.2 graphically illustrates the 

number of wells in production annually for the subject period. Statistics generated in the 

balance of this chapter are based on the above exposure. 

5.3 U.S. GOM and North Sea LOWC Frequency Characteristics 

The characteristics considered in this section include general drilling and production 

LOWC frequencies, details of drilling LOWC frequency characteristics, and the 

statistical variability of the frequency statistics developed. 

Table 5.1 summarizes the general statistics associated with the general U.S. GOM 

LOWC frequencies. As can be seen, the events considered are surface blowouts and 

underground blowouts, and other well releases – in accordance with the definition 

provided in Section 1.4. These occurrences are considered for production wells and well 

drilling. Statistics for production wells are given per well-year, for both oil producers and 

gas and/or condensate producers. A separate category of LOWCs for event categories is 

given for well intervention. Drilling, again for the same event categories, is given for 

exploration drilling and development drilling, as well as their summary. All well drilling 

statistics are based on a frequency per well drilled. 
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Figure 5.1: U.S. GOM and North Sea Annual Well Drilling Exposure 

USGOMOCS+NorthSeaDrillingWells

0 200 400 600 800 1,000 1,200 1,400

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number ofWells

Explorationwells Developmentwells
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Figure 5.2: U.S. GOM and North Sea Annual Producing Well Exposure 

US GOM OCS + North Sea Producing Wells

0 500 1,000 1,500 2,000 2,500 3,000 3,500 4,000 4,500 5,000 5,500

1980

1981

1982

1983

1984

1985

1986

1987

1988

1989

1990

1991

1992

1993

1994

1995

1996

1997

1998

1999

2000

2001

2002

2003

2004

2005

2006

2007

2008

2009

2010

2011

Number of Wells

Total oil producers Gas and condensate producers
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Table 5.1: U.S. GOM and North Sea LOWC Frequency Summary 

Oil 

Production 

Gas/Condensate

 Production 

All 

Production 

Well 

Interventions 

Exploration 

Drilling 

Development 

Drilling 

All 

Drilling 

141,266 115,596 256,862 256,862 16,852 28,449 45,301 

U.S. GOM OCS 

+ North Sea 

Wells 

1980-2011 # 

Frequency 

per wellyear # Frequency

per well-year #

Frequency

per wellyear # Frequency

per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 10 7.08E-05 8 6.92E-05 18 7.01E-05 38 1.48E-04 56 3.32E-03 32 1.12E-03 88 1.94E-03 

Blowout (underground flow) 1 7.08E-06 1 3.89E-06 1 3.89E-06 8 4.75E-04 5 1.76E-04 13 2.87E-04 

Blowout Total 11 7.79E-05 8 6.92E-05 19 7.40E-05 39 1.52E-04 64 3.80E-03 37 1.30E-03 101 2.23E-03 

Well Release 1 7.08E-06 4 3.46E-05 5 1.95E-05 43 1.67E-04 9 5.34E-04 8 2.81E-04 17 3.75E-04 

Diverted Well Release 1 3.89E-06 13 7.71E-04 19 6.68E-04 32 7.06E-04 

Well Release Total 1 7.08E-06 4 3.46E-05 5 1.95E-05 44 1.71E-04 22 1.31E-03 27 9.49E-04 49 1.08E-03 

TOTAL 12 8.49E-05 12 1.04E-04 24 9.34E-05 83 3.23E-04 86 5.10E-03 64 2.25E-03 150 3.31E-03 

Table 5.2 gives further details of the LOWC statistics. It is important to separate out 

shallow gas LOWCs from other or deep well drilling LOWCs. The reason for this is that 

by definition, under SINTEF, shallow gas drilling occurs when the surface hole is drilled 

without a BOP. The exposure for shallow gas drilling is the same as that for all drilling, 

because all offshore wells are initiated by shallow drilling prior to the surface completion 

and installation of a BOP. One can see that shallow gas drilling frequencies tend to be 

somewhat higher than other or deeper well drilling with BOP activities. 

Table 5.2: U.S. GOM and North Sea LOWC Drilling Frequency Details 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow Gas Other All Shallow Gas Other All 

16,852 16,852 16,852 28,449 28,449 28,449 45,301 45,301 45,301 

U.S. GOM OCS 

+ North Sea 

Wells 

1980-2011 # Frequency 

per well # Frequency per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequencyper well # Frequencyper well 

Blowout 

(surface flow) 32 1.90E-03 24 1.42E-03 56 3.32E-03 21 7.38E-04 11 3.87E-04 32 1.12E-03 53 1.17E-03 35 7.73E-04 88 1.94E-03 

Blowout 

(underground flow) 8 4.75E-04 8 4.75E-04 1 3.52E-05 4 1.41E-04 5 1.76E-04 1 2.21E-05 12 2.65E-04 13 2.87E-04 

Blowout Total 32 1.90E-03 32 1.90E-03 64 3.80E-03 22 7.73E-04 15 5.27E-04 37 1.30E-03 54 1.19E-03 47 1.04E-03 101 2.23E-03 

Well Release 2 1.19E-04 7 4.15E-04 9 5.34E-04 2 7.03E-05 6 2.11E-04 8 2.81E-04 4 8.83E-05 13 2.87E-04 17 3.75E-04 

Diverted Well 

Release 12 7.12E-04 1 5.93E-05 13 7.71E-04 19 6.68E-04 19 6.68E-04 31 6.84E-04 1 2.21E-05 32 7.06E-04 

Well Release Total 14 8.31E-04 8 4.75E-04 22 1.31E-03 21 7.38E-04 6 2.11E-04 27 9.49E-04 35 7.73E-04 14 3.09E-04 49 1.08E-03 

TOTAL 46 2.73E-03 40 2.37E-03 86 5.10E-03 43 1.51E-03 21 7.38E-04 64 2.25E-03 89 1.96E-03 61 1.35E-03 150 3.31E-03 

Finally, Table 5.3 gives the variability for the blowout and well release frequencies which 

were given as average values in Table 5.1. The variability calculations are based on those 

presented by Holand [35], indicating the upper and lower 90% confidence intervals for 

the value based on a Chi Square distribution [47]. Thus, for example, for exploration 

drilling, the expected or mean value is 1.31E-03, with an upper 90% confidence interval 

of 1.79E-03 and a lower 90% confidence interval of 8.84E-03. The same applies to the 

other values indicating the variability of the statistics presented. Figures 5.3 and 5.4 

graphically show this variability for production and drilling blowouts, respectively. These 

figures also present the tabular values for convenience. 
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Table 5.3: U.S. GOM and North Sea Blowout Frequency Variability 

U.S. GOM OCS + North Sea Wells Blowout Frequency per Well-Year Well Release Frequency per Well-Year 

1980-2011 High Low Expected High Low Expected 

Oil Production 1.20E-04 4.37E-05 7.79E-05 2.12E-05 3.63E-07 7.08E-06 

Gas/Condensate Production 1.14E-04 3.44E-05 6.92E-05 6.71E-05 1.18E-05 3.46E-05 

All Production 1.04E-04 4.84E-05 7.40E-05 3.56E-05 7.67E-06 1.95E-05 

Well Interventions 1.94E-04 1.14E-04 1.52E-04 2.05E-04 1.31E-04 1.71E-04 

U.S. GOM OCS + North Sea Wells Blowout Frequency per Well Well Release Frequency per Well 

1980-2011 High Low Expected High Low Expected 

Exploration Drilling 4.61E-03 3.05E-03 3.80E-03 1.79E-03 8.84E-04 1.31E-03 

Development Drilling 1.67E-03 9.70E-04 1.30E-03 1.27E-03 6.70E-04 9.49E-04 

All Drilling 2.61E-03 1.88E-03 2.23E-03 1.35E-03 8.41E-04 1.08E-03 

Note: 90% confidence that Frequency is in the Low-High Interval 

Reference: Offshore Blowouts, Per Holand (page 132). 

Figure 5.3: Blowout – U.S. GOM OCS and North Sea Production Variability (1980-2011) 

Figure 5.4: Blowout – U.S. GOM OCS and North Sea Drilling Variability (1980-2011) 

0.00000

0.00005

0.00010

0.00015

0.00020

0.00025

FrequencyperWell‐Y

High 1.20E‐04 1.14E‐04 1.04E‐04 1.94E‐04

Low 4.37E‐05 3.44E‐05 4.84E‐05 1.14E‐04

Expected 7.79E‐05 6.92E‐05 7.40E‐05 1.52E‐04

Oil Production Gas/Condensate

Production All Production Well Interventions

Frequency per Well‐Year

0.0000

0.0005

0.0010

0.0015

0.0020

0.0025

0.0030

0.0035

0.0040

0.0045

Frequencyper

W

High 4.61E‐03 1.67E‐03 2.61E‐03

Low 3.05E‐03 9.70E‐04 1.88E‐03

Expected 3.80E‐03 1.30E‐03 2.23E‐03

ExplorationDrilling DevelopmentDrilling All Drilling

Frequency per Well Drilled
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5.4 U.S. GOM and North Sea LOWC Duration 

Duration of well control incidents based on the SINTEF data from 1980 to 2011 can be 

determined from the SINTEF database. The expected duration is displayed both over the 

initial hours (by the minute, up to 5 hours) and by the hour up to 20 days (480 hours). 

Figure 5.5 shows the initial blowout duration for up to 5 hours. As can be seen, durations 

up to 1.5 hours account for roughly 50% of LOWC incidents. The longer view, displayed 

in Figure 5.6 for up to 20 days, indicates that roughly 90% of LOWCs stop within 9 days. 

Longer duration blowouts, such as Macondo, will be discussed in Chapter 8. 

Figure 5.5: U.S. GOM and North Sea Loss of Well Control – Duration by Minutes 

Figure 5.6: U.S. GOM and North Sea Loss of Well Control – Duration by Hours to 20 Days 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0 50 100 150 200 250 300

x Duration [min]

Probabilty of <= Probability of <= x 

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0 24 48 72 96 120 144 168 192 216 240 264 288 312 336 360 384 408 432 456 480

x Duration [hour]

Probabilty of <= Probability of <= x 
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5.7

5.5 U.S. GOM and North Sea LOWC Depth Variation 

5.5.1 Approach and Water Depth Ranges 

The variation of the number of LOWC incidents with water depth in the U.S. GOM and 

North Sea was extracted from the SINTEF database. It should be noted that only the 

number of LOWCs, rather than their rate per exposure unit, was available from the data 

which only gives the LOWC incidents by water depth without an exposure variable such 

as number of wells drilled in the subject water depth range; thus, the variable presented in 

this section is not directly comparable to any of the spill occurrence indicators. 

Two water depth ranges were used as described in Section 3.5; namely, the Shelf Range 

and the Offshore Range. 

5.5.2 U.S. GOM and North Sea Shelf Water Depth Range 

Table 5.4 gives the distribution of number of LOWC incidents in each of the four Shelf 

water depth ranges. Clearly the maximum number of incidents occurs in the deepest 

range, over 60m. This trend may indicate either that many more wells are drilled in this 

range or that the incident rate is higher in this range or both. Figure 5.7 gives a bar graph 

and a pie chart graphically illustrating these incident number distributions in this range. 

5.5.3 U.S. GOM and North Sea Offshore Water Depth Range 

Table 5.5 gives the distribution of number of LOWC incidents in each of the four 

Offshore water depth ranges. Clearly the maximum number of incidents occurs in the 

Very Shallow range, under 61m. This trend may indicate either that many more wells are 

drilled in this range or that the incident rate is higher in this range or both. Figure 5.8 

gives a bar graph and a pie chart graphically illustrating these incident number 

distributions in this range. 
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Table 5.4: U.S. GOM and North Sea Shelf Water Depth Range LOWC Incident 

Distribution 

U.S. GOM OCS and North Sea 1980-2011 

Shallow Shelf Inner Shelf Middle Shelf Outer Shelf 

and Basin Total 

< 10 m 

(< 33 ft) 

10 to 29 m 

(33 to 95 ft) 

30 to 60 m 

(95 to 197 ft) 

> 60 m 

(>197 ft) All 

14 54 55 136 259 

Figure 5.7: U.S. GOM and North Sea Shelf Water Depth Range LOWC Incident 

Distribution Graphs 

U.S.GOMOCS+NorthSea1980‐2011

5%

21%

21%

53%

ShallowShelf <10m

(<33ft)

InnerShelf 10to29m

(33to95ft)

MiddleShelf 30to60m

(95to197ft)

OuterShelf&Basin>60m

(>197ft)

WaterDepth

0

20

40

60

80

100

120

140

160

Number of LOW

USGOMOCS+NorthSea1980‐ 14 54 55 136

<10m 10to29m 30to60m >60m

ShallowShelf InnerShelf MiddleShelf OuterShelf&

Basin

Number of LOWC 

U.S. GOM OCS + North Sea 1980-2011 
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5.9

Table 5.5: U.S. GOM and North Sea Offshore Water Depth Range LOWC Incident 

Distribution 

U.S. GOM OCS and North Sea 1980-2011 

Very Shallow Shallow Deep Ultra Deep Total 

≤ 61 m 

(≤ 200 ft) 

61 to 152 m 

(201 to 500 ft) 

153 to 1524 m 

(501 to 5,000 ft) 

> 1524 m 

(> 5,000 ft) All 

124 98 36 1 259 

Figure 5.8: U.S. GOM and North Sea Offshore Water Depth Range LOWC Incident 

Distribution Graphs 

WaterDepth

0

20

40

60

80

100

120

140

Numberof LOW

U.S.GOMOCS+NorthSea1980‐2011 124 98 36 1

≤  61m 61to152m 153to1524m >1524m

VeryShallow Shallow Deep UltraDeep

Number of LOWC 

U.S.GOMOCS+NorthSea1980‐2011

47.9%

37.8%

13.9%

0.4%

VeryShallow ≤  61m

(≤ 200ft)

Shallow61to152m

(201to500ft)

Deep153to1524m

(501to5,000ft)

UltraDeep>1524m

(>5,000ft)
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6.1

CHAPTER 6 

AUSTRALIA, HOLLAND AND CANADA STATISTICS 

6.1 General Description of Australia, Holland, and Canada Statistics 

This chapter is intended to provide an outline of statistics and characteristic descriptions 

for LOWC events in the Australian, Dutch, and Canadian offshore production regions. 

The results are based on the SINTEF data covering the period from 1980 to 2011 [46]. 

SINTEF and other sources only provided the exposure as exploratory and development 

wells drilled in each year over the period 1980-2011. No production well-years were 

explicitly given. However, from inspection of other regional data, it was found for this 

period that the total production well-years are 6 to 10 times the total number of 

development wells drilled. Also, oil to gas or condensate producers ratios run between 5 

and 1.0. Table 6.1 gives details of these ratios for regions with available data. 

Table 6.1: Ratios of Production Well-Years to Development Wells Drilled and Oil 

and Gas Producers (1980-2011) 

Region Oil 

Producers 

Gas/-Cond 

Producers 

Total 

Producers 

Development 

Wells Drilled

Ratio 

Producers / 

Dev Drill 

Ratio Oil/Gas 

Producers 

Analogous 

Regions 

U.S. GOM 101,262 96,459 197,721 19,275 10.3 1.0 

U.S. PAC, 

Australia, 

Canada 

UK 23,301 15,807 39,108 5,807 6.7 1.5 Holland, 

Denmark 

Norway 16,703 3,330 20,033 3,367 5.9 5.0 ~ 

TOTAL 141,266 115,596 256,862 33,442 9.0 1.2 ~ 

6.2 Australian LOWC Statistics 

Table 6.2 summarizes the general statistics associated with the Australian LOWC 

frequencies. As can be seen, the events considered are surface blowouts and underground 

blowouts, and well releases and diverted well releases. These occurrences are considered 

for production wells and well drilling. Statistics for production wells are given per wellyear. These production well statistics are provided for both oil producers and gas and/or 

condensate producers. A separate category of LOWCs for event categories is given for 

well intervention. Drilling, again for the same event categories, is given for exploration 

drilling and development drilling, as well of course, as their summary. All well drilling 

statistics are based on a frequency per well drilled. Production and well intervention 

exposures were calculated on the basis of the U.S. GOM data in Table 6.1. 
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6.2

Table 6.2: Australian LOWC General Frequency (1980-2011) 

Oil 

Production* 

Gas / 

Condensate

 Production* 

All 

Production* 

Well 

Interventions*

Exploration

Drilling 

Development 

Drilling 

All 

Drilling 

4,795 4,795 9,589 9,589 1,628 931 2,559 

Australian Wells 

1980-2011 

# Frequency 

per well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequency per well 

Blowout (surface flow) 1 2.09E-04 1 6.14E-04 1 1.07E-03 2 7.82E-04 

Blowout (underground flow) 2 1.23E-03 2 7.82E-04 

Blowout Total 1 2.09E-04 3 1.84E-03 1 1.07E-03 4 1.56E-03 

Well Release 

Diverted Well Release 

Well Release Total

TOTAL 1 2.09E-04 1.04E-04 3 1.84E-03 1 1.07E-03 4 1.56E-03 

* Estimated from U.S. GOM, Table 6.1 

6.3 Dutch LOWC Statistics 

Table 6.3 summarizes the general statistics associated with the Dutch LOWC 

frequencies. As can be seen, the events considered are surface blowouts and underground 

blowouts, and well releases and diverted well releases. These occurrences are considered 

for production wells and well drilling. Statistics for production wells are given per wellyear. These production well statistics are provided for both oil producers and gas and/or 

condensate producers. A separate category of LOWCs for event categories is given for 

well intervention. Drilling, again for the same event categories, is given for exploration 

drilling and development drilling, as well of course, as their summary. All well drilling 

statistics are based on a frequency per well drilled. Production and well intervention 

exposures were calculated based on the UK data in Table 6.1. 

Table 6.3: Dutch LOWC General Frequency (1980-2011) 

Oil 

Production* 

Gas / 

Condensate

 Production* 

All 

Production* 

Well 

Interventions*

Exploration

Drilling 

Development 

Drilling 

All 

Drilling 

1,769 1,179 2,948 2,948 703 440 1,143 

Dutch Wells 

1980-2011 

# Frequency 

per well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequency per well 

Blowout (surface flow) 1 8.48E-04 

Blowout (underground flow) 

Blowout Total 1 8.48E-04 

Well Release 1 3.39E-04 

Diverted Well Release 

Well Release Total 1 3.39E-04 

TOTAL 1 8.48E-04 3.39E-04 1 3.39E-04 

* Estimated from UK, Table 6.1 
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6.3

6.4 Canadian East Coast LOWC Statistics 

Table 6.4 summarizes the general statistics associated with the Canadian LOWC 

frequencies. As can be seen, the events considered are surface blowouts and underground 

blowouts, and well releases and diverted well releases. These occurrences are considered 

for production wells and well drilling. Statistics for production wells are given per wellyear. These production well statistics are provided for both oil producers and gas and/or 

condensate producers. A separate category of LOWCs for event categories is given for 

well intervention. Drilling, again for the same event categories, is given for exploration 

drilling and development drilling, as well of course, as their summary. All well drilling 

statistics are based on a frequency per well drilled. Production and well intervention 

exposures were calculated based on the GOM data in Table 6.1. 

Table 6.4: Canadian East Coast LOWC General Frequency (1980-2011) 

Oil 

Production* 

Gas / 

Condensate

 Production* 

All 

Production* 

Well 

Interventions*

Exploration

Drilling 

Development 

Drilling 

All 

Drilling 

1,978 1,978 3,955 3,955 295 384 679 

Canada East Coast 

Wells 

1980-2011 

# Frequency 

per well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequency per well 

Blowout (surface flow) 1 3.39E-03 1 1.47E-03 

Blowout (underground flow) 1 3.39E-03 1 1.47E-03 

Blowout Total 2 6.78E-03 2 2.95E-03 

Well Release 

Diverted Well Release 

Well Release Total 

TOTAL 2 6.78E-03 2 2.95E-03 

* Estimated from U.S. GOM, Table 6.1 
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7.1

CHAPTER 7 

LOWC FREQUENCY REGIONAL AND TEMPORAL COMPARISONS 

7.1 Approach to Comparisons 

Having presented the LOWC frequency characteristics for several different regions from 

the SINTEF database [46], primarily for the same period from 1980 to 2010 or 2011, the 

logical questions that arise are: How do the LOWC characteristics vary from region to 

region? and How do they vary for different periods or epochs during the data gathering 

period? Accordingly, the following areas are discussed in successive sections in this 

chapter: 

 Regional exposure. 

 LOWC principal characteristic variation for U.S. GOM and North Sea regions. 

 Regional and temporal LOWC detailed comparisons. 

 Shallow and deep drilling LOWC characteristic comparisons. 

7.2 Regional Exposure 

The SINTEF database contains a variety of quantitative characterizations of LOWC 

events for a variety of regions. Table 7.1 summarizes the regions for which LOWC 

characteristics, at different levels of detail, are provided in SINTEF. Note that the 

quantities in bold italics were derived utilizing the ratios of development wells drilled to 

producing well years (described earlier in Table 6.1), together with the corresponding 

ratios of oil to gas producers. Ratios chosen were those from the U.S. GOM for all but the 

Dutch and Danish regions, which were based on the UK. As can be seen, only the first 

three regions are fully characterized by data. Essentially, this comes down to the U.S. 

GOM OCS and the UK and Norway regions, which together constitute the North Sea 

region. Accordingly, in the balance of this chapter, it is the U.S. GOM and the North Sea 

regions that are used in the comparisons provided. 

7.3 LOWC Principal Characteristic Variation for U.S. and North Sea 

Regions 

Table 7.2 gives the principal LOWC frequency comparisons for the principal categories 

of well operations, including oil production, gas or condensate production, and 

exploration and development drilling. It should be noted that well interventions, which 

apply to both drilling and production, have not been included – although well 

interventions are considered in the subsequent chapter, which provides quantitative 

estimates of hydrocarbon spill characteristics. 
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7.2

As can be seen, in Table 7.2 and the accompanying bar chart (Figure 7.1), there is notable 

variation between U.S. GOM LOWC characteristics and North Sea LOWC 

characteristics. Generally speaking, for oil production, the LOWC frequencies for the 

GOM are roughly four times as high as those for the North Sea LOWC. For gas 

production, however, the frequencies are roughly the same. For drilling, a somewhat 

different trend is apparent, the exploratory drilling LOWC frequencies in the GOM are 

somewhat lower than those in the North Sea, while the development drilling LOWC 

frequencies for the GOM are higher than those in the North Sea. The resultant for all 

drilling is that LOWC frequencies for the GOM and North Sea are roughly the same. 

Table 7.1: LOWC and Exposure Information (1980-2011) 

LOWC and Exposure Data available in SINTEF and estimated (1980-2011) 

Region 

Blowouts and 

Well Releases 

(LOWC) 

Oil 

Producing

Wells 

Gas / 

Condensate

Producing

Wells 

Total 

Producing

Wells 

Exploration

Drilling 

Wells 

Development

Drilling 

Wells 

Total 

Drilling 

Wells 

U.S. GOM OCS 193 101,262 96,459 197,721 12,299 19,275 31,574 

Norway – North Sea 32 16,703 3,330 20,033 1,251 3,367 4,618 

UK – North Sea 34 23,301 15,807 39,108 3,302 5,807 9,109 

Australia 5 4,795 4,795 9,589 1,628 931 2,559 

U.S. Pacific OCS 4 3,837 3,837 7,674 130 745 875 

Canada East Coast 2 1,978 1,978 3,955 295 384 679 

Holland 2 1,769 1,179 2,948 703 440 1,143 

Denmark 0 1,829 1,219 3,049 214 455 669 

U.S. GOM not OCS 28 

U.S. Alaska State 4 

U.S. California State 2 

Indonesia 18 

Mexico 11 

Nigeria 4 

India 8 

U.S.SR 6 

Egypt 7 

Trinidad 4 

Brazil 6 

Azerbaijan 5 

Venezuela 4 

Saudi Arabia 4 

Brunei 2 

China 3 

Dubai 1 

Congo 2 

Other in SINTEF 31 

Total SINTEF and 

estimated 422 155,473 128,604 284,077 19,822 31,404 51,226 
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Table 7.2: LOWC Frequencies for U.S. GOM and North Sea Regions (1980-2011) 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

101,262 96,459 197,721 12,299 19,275 31,574 U.S. GOM OCS

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout 10 9.88E-05 7 7.26E-05 17 8.60E-05 44 3.58E-03 33 1.71E-03 77 2.44E-03 

Well Release 1 9.88E-06 3 3.11E-05 4 2.02E-05 10 8.13E-04 22 1.14E-03 32 1.01E-03 

TOTAL 11 1.09E-04 10 1.04E-04 21 1.06E-04 54 4.39E-03 55 2.85E-03 109 3.45E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

23,301 15,807 39,108 3,302 5,807 9,109 UK 

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout 1 4.29E-05 1 6.33E-05 2 5.11E-05 6 1.82E-03 3 5.17E-04 9 9.88E-04 

Well Release 1.10E-06 1 6.33E-05 1 2.56E-05 1 3.03E-04 3 5.17E-04 4 4.39E-04 

TOTAL 1 4.29E-05 2 1.27E-04 3 7.67E-05 7 2.12E-03 6 1.03E-03 13 1.43E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

16,703 3,330 20,033 1,251 3,367 4,618 Norway 

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout 14 1.12E-02 1 2.97E-04 15 3.25E-03 

Well Release 11 8.79E-03 2 5.94E-04 13 2.82E-03 

TOTAL 0.00 0.00 0.00 25 2.00E-02 3 8.91E-04 28 6.06E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

40,004 19,137 59,141 4,553 9,174 13,727 

North Sea 

(Norway, UK) 

 1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout 1 2.50E-05 1 5.23E-05 2 3.38E-05 20 4.39E-03 4 4.36E-04 24 1.75E-03 

Well Release 1 5.23E-05 1 1.69E-05 12 2.64E-03 5 5.45E-04 17 1.24E-03 

TOTAL 1 2.50E-05 2 1.05E-04 3 5.07E-05 32 7.03E-03 9 9.81E-04 41 2.99E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

141,266 115,596 256,862 16,852 28,449 45,301 

All 

U.S. GOM OCS 

+ North Sea 

 1980-2011 # Frequency 

per well-year # Frequency per well-year # Frequencyper well-year # Frequencyper well # Frequency per well # Frequencyper well 

Blowout 11 7.79E-05 8 6.92E-05 19 7.40E-05 64 3.80E-03 37 1.30E-03 101 2.23E-03 

Well Release 1 7.08E-06 4 3.46E-05 5 1.95E-05 22 1.31E-03 27 9.49E-04 49 1.08E-03 

TOTAL 12 8.49E-05 12 1.04E-04 24 9.34E-05 86 5.10E-03 64 2.25E-03 150 3.31E-03 
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7.4

Figure 7.1: Frequency Graphs: GOM and North Sea (1980-2011) 

NS andGOMProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y Blowout 7.79E‐05 6.92E‐05 7.40E‐05

Well Release 7.08E‐06 3.46E‐05 1.95E‐05

TOTAL 8.49E‐05 1.04E‐04 9.34E‐05

Oil Production Gas/Condensate

Production All Production

USGOMProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

FrequencyperWell‐Y Blowout 9.88E‐05 7.26E‐05 8.60E‐05

Well Release 9.88E‐06 3.11E‐05 2.02E‐05

TOTAL 1.09E‐04 1.04E‐04 1.06E‐04

Oil Production Gas/Condensate

Production All Production

NorthSeaProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

FrequencyperWell‐Y Blowout 2.50E‐05 5.23E‐05 3.38E‐05

Well Release 1.00E‐08 5.23E‐05 1.69E‐05

TOTAL 2.50E‐05 1.05E‐04 5.07E‐05

Oil Production Gas/Condensate

Production All Production

Frequency per Well‐Year Frequency per Well‐Year Frequency per Well‐Year

0.00 

USGOMDrillingWells1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W Blowout 3.58E‐03 1.71E‐03 2.44E‐03

Well Release 8.13E‐04 1.14E‐03 1.01E‐03

TOTAL 4.39E‐03 2.85E‐03 3.45E‐03

Exploration

Drilling

Development

Drilling All Drilling

NorthSeaDrillingWells1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W Blowout 4.39E‐03 4.36E‐04 1.75E‐03

Well Release 2.64E‐03 5.45E‐04 1.24E‐03

TOTAL 7.03E‐03 9.81E‐04 2.99E‐03

Exploration

Drilling

Development

Drilling All Drilling

NS andGOMDrillingWells1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W Blowout 3.80E‐03 1.30E‐03 2.23E‐03

Well Release 1.31E‐03 9.49E‐04 1.08E‐03

TOTAL 5.10E‐03 2.25E‐03 3.31E‐03

Exploration

Drilling

Development

Drilling All Drilling

Frequency per Well Drilled Frequency per Well Drilled Frequency per Well Drilled
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7.5

7.4 Regional and Temporal LOWC Detailed Comparisons 

For each of the two principal regions discussed in Section 7.3, the U.S. GOM and the 

North Sea, the LOWC characteristics over three principal periods were considered; 

namely, 1980 to 2011, 2001 to 2011, and 1980 to 2000. These three epochs can be 

considered to roughly represent the entire data collection period (1980-2011), the most 

recent 10 years (2001 to 2011), and the earlier two decades (1980 to 2000). 

Table 7.3 provides LOWC frequency characteristics for the entire data period from 1980 

to 2011, Table 7.4 provides these for the most recent 10 years, from 2001 to 2011, and 

Table 7.5 provides these for the earlier decade from 1980 to 2000. 

What is interesting, rather than the regional comparison, is the temporal variation for 

each of the two principal regions considered, the U.S. GOM and the North Sea. Figure 

7.2 shows the three characteristic periods for the U.S. GOM. It is interesting to note, in 

fact, that in the U.S. GOM, gas/condensate production operations in the most recent 

decade have higher LOWC rates than those in the earlier two decades from 1980 to 2000, 

while oil production LOWCs are lower. In the case of U.S. GOM drilling operations, 

again a similar but much less notable difference is evident between these two epochs. For 

the North Sea, in case of production, the opposite trend is evident, with zero LOWC 

events over the recent decade, and a distribution roughly similar to that of the GOM in 

the earlier two decades. Again, in terms of drilling, the North Sea shows a distinct 

reduction in LOWC frequencies in the most recent decade over that during the earlier two 

decades (Figure 7.3). 

Finally, Figure 7.4 shows the variation in LOWC frequencies for both regions, the U.S. 

GOM and the North Sea. Many of the distinct trends that were associated with each 

region described above, have now been compensated to make the overall regional trend 

temporal variations closer together, but still distinct. In terms of production, one can see 

that the more recent gas production has had a higher rate of LOWCs than that of the 

earlier two decades, while the oil producers show the opposite trend. All production, 

however, tends to be lower in LOWC frequency for the most recent decade. In the case of 

drilling, LOWC frequencies are similar for all three epochs, with a slight decrease in the 

most recent decade. 
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7.6

Table 7.3: Regional LOWC Frequency (1980-2011) 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

101,262 96,459 197,721 12,299 19,275 31,574 U.S. GOM OCS

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 9 8.89E-05 7 7.26E-05 16 8.09E-05 40 3.25E-03 28 1.45E-03 68 2.15E-03 

Blowout (underground flow) 1 9.88E-06 1 5.06E-06 4 3.25E-04 5 2.59E-04 9 2.85E-04 

Well Release 1 9.88E-06 3 3.11E-05 4 2.02E-05 1 8.13E-05 3 1.56E-04 4 1.27E-04 

Diverted Well Release 9 7.32E-04 19 9.86E-04 28 8.87E-04 

TOTAL 11 1.09E-04 10 1.04E-04 21 1.06E-04 54 4.39E-03 55 2.85E-03 109 3.45E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

23,301 15,807 39,108 3,302 5,807 9,109 UK 

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 4.29E-05 1 6.33E-05 2 5.11E-05 6 1.82E-03 3 5.17E-04 9 9.88E-04 

Blowout (underground flow) 

Well Release 1 6.33E-05 1 2.56E-05 1 3.03E-04 3 5.17E-04 4 4.39E-04 

Diverted Well Release 

TOTAL 1 4.29E-05 2 1.27E-04 3 7.67E-05 7 2.12E-03 6 1.03E-03 13 1.43E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

16,703 3,330 20,033 1,251 3,367 4,618 Norway 

1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 10 7.99E-03 1 2.97E-04 11 2.38E-03 

Blowout (underground flow) 4 3.20E-03 4 8.66E-04 

Well Release 7 5.60E-03 2 5.94E-04 9 1.95E-03 

Diverted Well Release 4 3.20E-03 4 8.66E-04 

TOTAL 0.00 0.00 0.00 25 2.00E-02 3 8.91E-04 28 6.06E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

40,004 19,137 59,141 4,553 9,174 13,727 

North Sea 

(Norway, UK) 

 1980-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 2.50E-05 1 5.23E-05 2 3.38E-05 16 3.51E-03 4 4.36E-04 20 1.46E-03 

Blowout (underground flow) 4 8.79E-04 4 2.91E-04 

Well Release 1 5.23E-05 1 1.69E-05 8 1.76E-03 5 5.45E-04 13 9.47E-04 

Diverted Well Release 4 8.79E-04 4 2.91E-04 

TOTAL 1 2.50E-05 2 1.05E-04 3 5.07E-05 32 7.03E-03 9 9.81E-04 41 2.99E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

141,266 115,596 256,862 16,852 28,449 45,301 

All 

U.S. GOM OCS 

+ North Sea 

 1980-2011 # Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 10 7.08E-05 8 6.92E-05 18 7.01E-05 56 3.32E-03 32 1.12E-03 88 1.94E-03 

Blowout (underground flow) 1 7.08E-06 1 3.89E-06 8 4.75E-04 5 1.76E-04 13 2.87E-04 

Well Release 1 7.08E-06 4 3.46E-05 5 1.95E-05 9 5.34E-04 8 2.81E-04 17 3.75E-04 

Diverted Well Release 13 7.71E-04 19 6.68E-04 32 7.06E-04 

TOTAL 12 8.49E-05 12 1.04E-04 24 9.34E-05 86 5.10E-03 64 2.25E-03 150 3.31E-03 
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7.7

Table 7.4: Regional LOWC Frequency (2001-2011) 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

27,863 26,142 54,005 3,080 4,539 7,619 U.S. GOM OCS

2001-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 3.59E-05 4 1.53E-04 5 9.26E-05 9 2.92E-03 6 1.32E-03 15 1.97E-03 

Blowout (underground flow) 1 1.85E-05 3 9.74E-04 4 8.81E-04 7 9.19E-04 

Well Release 1 3.59E-05 3 1.15E-04 2 4.41E-04 2 2.63E-04 

Diverted Well Release 7 2.27E-03 15 3.30E-03 22 2.89E-03 

TOTAL 2 7.18E-05 7 2.68E-04 6 1.11E-04 19 6.17E-03 27 5.95E-03 46 6.04E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

7,559 6,019 13,578 745 2,068 2,813 UK 

2001-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 4.84E-04 1 3.55E-04 

Blowout (underground flow) 

Well Release 2 9.67E-04 2 7.11E-04 

Diverted Well Release 

TOTAL 0.00 0.00 0.00 0.00 3 1.45E-03 3 1.07E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

10,120 1,565 11,685 399 1,674 2,073 Norway

2001-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 2.51E-03 1 4.82E-04 

Blowout (underground flow) 

Well Release 

Diverted Well Release 

TOTAL 0.00 0.00 0.00 1 2.51E-03 1 4.82E-04 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

17,679 7,584 25,263 1,144 3,742 4,886 

North Sea 

(Norway, UK) 

2001-2011 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 8.74E-04 1 2.67E-04 2 4.09E-04 

Blowout (underground flow) 

Well Release 2 5.34E-04 2 4.09E-04 

Diverted Well Release 

TOTAL 0.00 0.00 0.00 1 8.74E-04 3 8.02E-04 4 8.19E-04 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

45,542 33,726 79,268 4,224 8,281 12,505 

All 

U.S. GOM OCS 

+ North Sea 

2001-2011 # Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 2.20E-05 4 1.19E-04 5 6.31E-05 10 2.37E-03 7 8.45E-04 17 1.36E-03 

Blowout (underground flow) 1 1.26E-05 3 7.10E-04 4 4.83E-04 7 5.60E-04 

Well Release 1 2.20E-05 3 8.90E-05 4 4.83E-04 4 3.20E-04 

Diverted Well Release 7 1.66E-03 15 1.81E-03 22 1.76E-03 

TOTAL 2 4.39E-05 7 2.08E-04 6 7.57E-05 20 4.73E-03 30 3.62E-03 50 4.00E-03 
Loss of Well Control Occurrence and Final Report – P1206 

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BOEM October 2014 

7.8

Table 7.5: Regional LOWC Frequency (1980-2000) 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

73,399 70,317 143,716 9,219 14,736 23,955 U.S. GOM OCS 

1980-2000 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 8 1.09E-04 3 4.27E-05 11 7.65E-05 31 3.36E-03 22 1.49E-03 53 2.21E-03 

Blowout (underground flow) 1 1.36E-05 1 6.96E-06 3 3.25E-04 4 2.71E-04 7 2.92E-04 

Well Release 2 1.36E-04 2 8.35E-05 

Diverted Well Release 7 7.59E-04 15 1.02E-03 22 9.18E-04 

TOTAL 9 1.23E-04 3 4.27E-05 12 8.35E-05 41 4.45E-03 43 2.92E-03 84 3.51E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

15,742 9,788 25,530 2,557 3,739 6,296 UK 

1980-2000 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 6.35E-05 1 1.02E-04 2 7.83E-05 6 2.35E-03 2 5.35E-04 8 1.27E-03 

Blowout (underground flow) 

Well Release 2 5.35E-04 2 3.18E-04 

Diverted Well Release 

TOTAL 1 6.35E-05 1 1.02E-04 2 7.83E-05 6 2.35E-03 4 1.07E-03 10 1.59E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

6,583 1,765 8,348 852 1,693 2,545 Norway 

1980-2000 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 9 1.06E-02 1 5.91E-04 10 3.93E-03 

Blowout (underground flow) 4 4.69E-03 4 1.57E-03 

Well Release 7 8.22E-03 2 1.18E-03 9 3.54E-03 

Diverted Well Release 3 3.52E-03 3 1.18E-03 

TOTAL 0.00 0.00 0.00 23 2.70E-02 3 1.77E-03 26 1.02E-02 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

22,325 11,553 33,878 3,409 5,432 8,841 

North Sea 

(Norway, UK) 

 1980-2000 

# Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 1 4.48E-05 1 8.66E-05 2 5.90E-05 15 4.40E-03 3 5.52E-04 18 2.04E-03 

Blowout (underground flow) 4 1.17E-03 4 4.52E-04 

Well Release 7 2.05E-03 4 7.36E-04 11 1.24E-03 

Diverted Well Release 3 8.80E-04 3 3.39E-04 

TOTAL 1 4.48E-05 1 8.66E-05 2 5.90E-05 29 8.51E-03 7 1.29E-03 36 4.07E-03 

Oil Production Gas/Condensate

 Production All Production Exploration Drilling 

Development 

Drilling All Drilling 

95,724 81,870 177,594 12,628 20,168 32,796 

All 

U.S. GOM OCS 

+ North Sea 

 1980-2000 # Frequency 

per well-year # Frequency per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequencyper well 

Blowout (surface flow) 9 9.40E-05 4 4.89E-05 13 7.32E-05 46 3.64E-03 25 1.24E-03 71 2.16E-03 

Blowout (underground flow) 1 1.04E-05 1 5.63E-06 7 5.54E-04 4 1.98E-04 11 3.35E-04 

Well Release 7 5.54E-04 6 2.98E-04 13 3.96E-04 

Diverted Well Release 10 7.92E-04 15 7.44E-04 25 7.62E-04 

TOTAL 10 1.04E-04 4 4.89E-05 14 7.88E-05 70 5.54E-03 50 2.48E-03 120 3.66E-03 
Loss of Well Control Occurrence and Final Report – P1206 

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BOEM October 2014 

7.9

Figure 7.2: U.S. GOM Temporal LOWC Frequency Comparisons 

USGOMOCSProductionWells1980‐2000

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y USGOMOCSWells

1980‐2000

1.23E‐04 4.27E‐05 8.35E‐05

Oil Production Gas/Condensate

Production All Production

USGOMOCSProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y USGOMOCSWells

1980‐2011

1.09E‐04 1.04E‐04 1.06E‐04

Oil Production Gas/Condensate

Production All Production

USGOMOCSProductionWells2001‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y USGOMOCSWells

2001‐2011

7.18E‐05 2.68E‐04 1.11E‐04

Oil Production Gas/Condensate

Production All Production

Frequency per Well‐Year Frequency per Well‐Year Frequency per Well‐Year

USGOMOCSWellDrilling1980‐2000

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W USGOMOCSWells

1980‐2000

4.45E‐03 2.92E‐03 3.51E‐03

Exploration

Drilling

Development

Drilling All Drilling

Frequency per Well Drilled

USGOMOCSWellDrilling1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W USGOMOCSWells

1980‐2011

4.39E‐03 2.85E‐03 3.45E‐03

Exploration

Drilling

Development

Drilling AllDrilling

USGOMOCSWellDrilling2001‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W USGOMOCSWells

2001‐2011

6.17E‐03 5.95E‐03 6.04E‐03

Exploration

Drilling

Development

Drilling AllDrilling

Frequency per Well Drilled Frequency per Well Drilled
Loss of Well Control Occurrence and Final Report – P1206 

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7.10

Figure 7.3: North Sea Temporal LOWC Frequency Comparisons 

NorthSeaWellDrilling  1980‐2000

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W NorthSeaWells

(Norway,UK)

1980‐2000

8.51E‐03 1.29E‐03 4.07E‐03

Exploration

Drilling

Development

Drilling All Drilling

NorthSeaWellDrilling2001‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W NorthSeaWells

(Norway,UK)

2001‐2011

8.74E‐04 8.02E‐04 8.19E‐04

Exploration

Drilling

Development

Drilling All Drilling

NorthSeaWellDrilling1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency per

W NorthSeaWells

(Norway,UK)

1980‐2011

7.03E‐03 9.81E‐04 2.99E‐03

Exploration

Drilling

Development

Drilling All Drilling

Frequency per Well Drilled Frequency per Well Drilled Frequency per Well Drilled

NorthSeaProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y NorthSeaWells

(Norway,UK)

1980‐2011

2.50E‐05 1.05E‐04 5.07E‐05

Oil Production Gas/Condensate

Production All Production

NorthSeaProductionWells2001‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y NorthSeaWells

(Norway,UK)

2001‐2011

1.00E‐08 1.00E‐08 1.00E‐08

Oil Production Gas/Condensate

Production All Production

NorthSeaProductionWells1980‐2000

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y NorthSeaWells

(Norway,UK)

1980‐2000

4.48E‐05 8.66E‐05 5.90E‐05

Oil Production Gas/Condensate

Production All Production

Frequency per Well‐Year Frequency per Well‐Year Frequency per Well‐Year

0.00 0.00 0.00 
Loss of Well Control Occurrence and Final Report – P1206 

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7.11

Figure 7.4: All Well (North Sea and U.S. GOM) Temporal LOWC Frequency Comparisons 

NSandUSGOMWellDrilling  1980‐2000

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

Frequency perWell Dri USGOMOCSWells

+NorthSeaWells

1980‐2011

5.10E‐03 2.25E‐03 3.31E‐03

Exploration

Drilling

Development

Drilling All Drilling

NorthSeaWellDrilling2001‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

FrequencyperWell Dri NorthSeaWells

(Norway,UK)

1980‐2011

7.03E‐03 9.81E‐04 2.99E‐03

Exploration

Drilling

Development

Drilling All Drilling

USGOMWellDrilling  1980‐2011

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

FrequencyperWell Dri USGOMOCSWells

1980‐2011

4.39E‐03 2.85E‐03 3.45E‐03

Exploration

Drilling

Development

Drilling All Drilling

NS andUSGOMProductionWells1980‐2000

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y USGOMOCSWells

+NorthSeaWells

1980‐2011

8.49E‐05 1.04E‐04 9.34E‐05

Oil Production Gas/Condensate

Production All Production

NorthSeaProductionWells2001‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y NorthSeaWells

(Norway,UK)

1980‐2011

2.50E‐05 1.05E‐04 5.07E‐05

Oil Production Gas/Condensate

Production All Production

USGOMProductionWells1980‐2011

1.00E‐06

1.00E‐05

1.00E‐04

1.00E‐03

Frequency perWell‐Y USGOMOCSWells

1980‐2011

1.09E‐04 1.04E‐04 1.06E‐04

Oil Production Gas/Condensate

Production All Production

Frequency per Well Drilled Frequency per Well Drilled Frequency per Well Drilled

Frequency per Well‐Year Frequency per Well‐Year Frequency per Well‐Year
Loss of Well Control Occurrence and Final Report – P1206 

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BOEM October 2014 

7.12

7.5 Shallow and Deep Drilling LOWC Characteristic Comparisons 

For the North Sea, the definition of shallow gas according to Holand [34], is 

unambiguous. Specifically, the definition put forward by Holand and accordingly 

SINTEF [34] is as follows: “Shallow gas release. The release from any gas zone 

penetrated before the BOP has been installed. Any zone penetrated after the BOP is 

installed is not shallow gas.” 

King, in her MSc Thesis of 2009 [36], defines a shallow gas blowout in a similar fashion, 

as follows: “A shallow gas blowout will be defined as a blowout before the blowout 

preventer (BOP) is set.” 

There is some ambiguity, however, for shallow gas LOWC releases in the U.S. GOM. 

According to Holand [34], the following qualifications need to be used in the context of 

GOM incidents: 

 U.S. GOM OCS reservoirs vary greatly in depth. Some reservoirs are as shallow as 

200 metres. In some LOWC incidents, a full BOP stack had been set, but cannot be 

used because it would likely cause a blowout outside the casing and a possible crater. 

 For some incidents, very deep drilling has occurred without running an extra casing 

string and setting a BOP. 

 For some LOWC incidents, a combination of a BOP and diverter is used to control 

flow. 

In the present section, shallow gas LOWC incidents are taken to be those defined by 

SINTEF, which in the North Sea are relatively unambiguous, meaning drilling without a 

BOP. Those for the GOM are interpreted similarly by SINTEF, and so reported in the 

database. 

Tables 7.6, 7.7, and 7.8 report LOWC incidents and associated frequencies for drilling in 

the U.S. GOM, the North Sea, and both the North Sea and GOM, respectively. As can be 

seen from Table 7.6, each of the drilling categories, exploration, development, and oil 

drilling, is subdivided into shallow gas and other, with “Other” generally meaning deeper 

drilling with a BOP. From an inspection of the bottom line, which summarizes blowouts 

and well releases, one can see that the shallow gas incident frequency is generally higher 

than the deep or other frequency for all of the regions, the U.S. GOM, the North Sea, and 

the combined U.S. GOM and North Sea. Finally, Figure 7.5 shows the comparative 

shallow gas and deep drilling blowout (not LOWC) frequencies. While this is true for the 

North Sea, the frequencies for All drilling blowouts are similar but somewhat higher for 

deep drilling in GOM, and similar (but somewhat lower) in the combined region. 
Loss of Well Control Occurrence and Final Report – P1206 

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7.13

Table 7.6: U.S. GOM Shallow Gas and Other Drilling LOWC Frequencies 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow Gas Other All Shallow Gas Other All 

12,299 12,299 12,299 19,275 19,275 19,275 31,574 31,574 31,574 

U.S. GOM OCS 

Wells 

1980-2011 

# Frequency 

per well # Frequency per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequency per well # Frequencyper well # Frequencyper well 

Blowout 

(surface flow) 17 1.38E-03 23 1.87E-03 40 3.25E-03 17 8.82E-04 11 5.71E-04 28 1.45E-03 34 1.08E-03 34 1.08E-03 68 2.15E-03

Blowout 

(underground flow) 4 3.25E-04 4 3.25E-04 1 5.19E-05 4 2.08E-04 5 2.59E-04 1 3.17E-05 8 2.53E-04 9 2.85E-04

Blowout Total 17 1.38E-03 27 2.20E-03 44 3.58E-03 18 9.34E-04 15 7.78E-04 33 1.71E-03 35 1.11E-03 42 1.33E-03 77 2.44E-03

Well Release 1 8.13E-05 1 8.13E-05 1 5.19E-05 2 1.04E-04 3 1.56E-04 1 3.17E-05 3 9.50E-05 4 1.27E-04

Diverted Well 

Release 8 6.50E-04 1 8.13E-05 9 7.32E-04 19 9.86E-04 19 9.86E-04 27 8.55E-04 1 3.17E-05 28 8.87E-04

Well Release Total 8 6.50E-04 2 1.63E-04 10 8.13E-04 20 1.04E-03 2 1.04E-04 22 1.14E-03 28 8.87E-04 4 1.27E-04 32 1.01E-03

LOWC TOTAL 25 2.03E-03 29 2.36E-03 54 4.39E-03 38 1.97E-03 17 8.82E-04 55 2.85E-03 63 2.00E-03 46 1.46E-03 109 3.45E-03

Table 7.7: North Sea Shallow Gas and Other Drilling LOWC Frequencies 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow 

Gas Other All Shallow Gas Other All 

4,553 4,553 4,553 9,174 9,174 9,174 13,727 13,727 13,727 

North Sea Wells 

1980-2011 

# Frequency 

per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequencyper well # Frequencyper well 

Blowout 

(surface flow) 15 3.29E-03 1 2.20E-04 16 3.51E-03 4 4.36E-04 4 4.36E-04 19 1.38E-03 1 7.28E-05 20 1.46E-03

Blowout 

(underground flow) 4 8.79E-04 4 8.79E-04 4 2.91E-04 4 2.91E-04

Blowout Total 15 3.29E-03 5 1.10E-03 20 4.39E-03 4 4.36E-04 4 4.36E-04 19 1.38E-03 5 3.64E-04 24 1.75E-03

Well Release 2 4.39E-04 6 1.32E-03 8 1.76E-03 1 1.09E-04 4 4.36E-04 5 5.45E-04 3 2.19E-04 10 7.28E-04 13 9.47E-04

Diverted Well 

Release 4 8.79E-04 4 8.79E-04 4 2.91E-04 4 2.91E-04

Well Release Total 6 1.32E-03 6 1.32E-03 12 2.64E-03 1 1.09E-04 4 4.36E-04 5 5.45E-04 7 5.10E-04 10 7.28E-04 17 1.24E-03

LOWC TOTAL 21 4.61E-03 11 2.42E-03 32 7.03E-03 5 5.45E-04 4 4.36E-04 9 9.81E-04 26 1.89E-03 15 1.09E-03 41 2.99E-03

Table 7.8: U.S. GOM and North Sea Shallow Gas and Other Drilling LOWC 

Frequencies 

Exploration Drilling Development Drilling All Drilling 

Shallow Gas Other All Shallow Gas Other All Shallow Gas Other All 

16,852 16,852 16,852 28,449 28,449 28,449 45,301 45,301 45,301 

North Sea Wells 

+U.S. GOM OCS 

Wells 

1980-2011 # Frequency 

per well # Frequency per well # Frequencyper well # Frequencyper well # Frequencyper well # Frequency per well # Frequency per well # Frequencyper well # Frequencyper well 

Blowout 

(surface flow) 32 1.90E-03 24 1.42E-03 56 3.32E-03 21 7.38E-04 11 3.87E-04 32 1.12E-03 53 1.17E-03 35 7.73E-04 88 1.94E-03

Blowout 

(underground flow) 8 4.75E-04 8 4.75E-04 1 3.52E-05 4 1.41E-04 5 1.76E-04 1 2.21E-05 12 2.65E-04 13 2.87E-04

Blowout Total 32 1.90E-03 32 1.90E-03 64 3.80E-03 22 7.73E-04 15 5.27E-04 37 1.30E-03 54 1.19E-03 47 1.04E-03 101 2.23E-03

Well Release 2 1.19E-04 7 4.15E-04 9 5.34E-04 2 7.03E-05 6 2.11E-04 8 2.81E-04 4 8.83E-05 13 2.87E-04 17 3.75E-04

Diverted Well 

Release 12 7.12E-04 1 5.93E-05 13 7.71E-04 19 6.68E-04 19 6.68E-04 31 6.84E-04 1 2.21E-05 32 7.06E-04

Well Release Total 14 8.31E-04 8 4.75E-04 22 1.31E-03 21 7.38E-04 6 2.11E-04 27 9.49E-04 35 7.73E-04 14 3.09E-04 49 1.08E-03

LOWC TOTAL 46 2.73E-03 40 2.37E-03 86 5.10E-03 43 1.51E-03 21 7.38E-04 64 2.25E-03 89 1.96E-03 61 1.35E-03 150 3.31E-03
Loss of Well Control Occurrence and Final Report – P1206 

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BOEM October 2014 

7.14

Figure 7.5: Comparative Regional Shallow Gas and Other Drilling Blowout Frequencies 

 (1980-2011) 

ShallowGas(noBOP)

0.00E+00

1.00E‐03

2.00E‐03

3.00E‐03

4.00E‐03

5.00E‐03

Blowout Frequencyper NorthSeaWells

1980‐2011

3.29E‐03 4.36E‐04 1.38E‐03

USGOMOCSWells

1980‐2011

1.38E‐03 9.34E‐04 1.11E‐03

NorthSeaWells

+USGOMOCSWells

1980‐2011

1.90E‐03 7.73E‐04 1.19E‐03

ExplorationDrilling DevelopmentDrilling AllDrilling

Deep

0.00E+00

1.00E‐03

2.00E‐03

3.00E‐03

4.00E‐03

5.00E‐03

Blowout Frequencyper NorthSeaWells

1980‐2011

1.10E‐03 0.00E+00 3.64E‐04

USGOMOCSWells

1980‐2011

2.20E‐03 7.78E‐04 1.33E‐03

NorthSeaWells

+USGOMOCSWells

1980‐2011

1.90E‐03 5.27E‐04 1.04E‐03

ExplorationDrilling DevelopmentDrilling AllDrilling

ShallowGas+Deep

0.00E+00

1.00E‐03

2.00E‐03

3.00E‐03

4.00E‐03

5.00E‐03

Blowout Frequencyper NorthSeaWells

1980‐2011

4.39E‐03 4.36E‐04 1.75E‐03

USGOMOCSWells

1980‐2011

3.58E‐03 1.71E‐03 2.44E‐03

NorthSeaWells

+USGOMOCSWells

1980‐2011

3.80E‐03 1.30E‐03 2.23E‐03

ExplorationDrilling DevelopmentDrilling AllDrilling

Frequency per Well‐Year Frequency per Well‐Year Frequency per Well‐Year 1980‐2011

1980‐2011

1980‐2011
Loss of Well Control Occurrence and Final Report – P1206 

Size Estimators for Alaska OCS BOEM Contract No.: M12PC00004 

BOEM October 2014 

8.1

CHAPTER 8 

LOWC HYDROCARBON SPILLS 

8.1 Introduction on LOWC Hydrocarbon Spills 

A critical area of LOWC information, both qualitative and quantitative, is that of the 

hydrocarbon (HC) spill characteristics associated with LOWC incidents. Clearly, this is 

important because of the concern over the impact of oil and other hydrocarbon fluid 

releases into the marine environment. Unfortunately, SINTEF [46] does not provide any 

quantitative information on the volumes of spills associated with LOWC incidents, 

restricting its characterization to a qualitative one indicating the degree (no, low, 

medium, large, unknown) of “pollution” associated with each specific incident. Pollution 

in SINTEF means a release into the marine environment. A more recent analysis by 

BOEM [22], however, does provide quantitative data and analysis of U.S. OCS LOWC 

spills dating back to 1964. In the balance of this chapter, characterization of spill 

frequencies will be based primarily on the BOEM [22] data and analysis, supplemented 

as appropriate by the SINTEF [46] data. 

The following areas relating to LOWC HC spill volumes and characterization of LOWC 

spill frequencies are discussed herein: 

 Spill Data Sources 

 General Spill Frequency Characteristics and Exposure for U.S. GOM 

 U.S. OCS Spill Size Distributions 

 U.S. GOM Spill Volume Frequencies 

 Risk Analysis Approaches to Predicting LOWC Spill Frequencies and Volumes 

8.2 Spill Data Sources 

8.2.1 SINTEF GOM LOWC Release Data 

As indicated above, SINTEF [46] does not provide quantitative estimates of the volumes 

of HC liquids released, restricting itself only to a qualitative description of the degree of 

pollution as defined above and the severity of consequences. Table 8.1 summarizes the 

U.S. GOM HC spill data as characterized by SINTEF, including the qualitative level of 

pollution and severity of consequences, as well as the phase or operation associated with 

the spills. SINTEF defines HC liquids as a Flow Medium which is oil or condensate, and 

oil is defined as an HC liquid with a Gas to Oil Ratio (GOR) less than 1000 Sm3

/Sm3. 

Thus, the first entry, Oil, represents oil with GOR less than 1000, while the next two 

entries represent HC liquids with GOR higher than 1000. 

Table 8.2 gives data for HC liquid spills which cause pollution or enter into the marine 

environment. It excludes the “unknown” pollution spills. Clearly polluting incidents are 

less in number than those in Table 8.1, as all LOWC HC spills do not enter into the 

ocean. 
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Table 8.1: SINTEF General LOWC HC Spill Data - U.S. GOM (1980-2011) 

U.S. GOM OCS Pollution Type Consequence Class Phase Ty

1980-2011 

SINTEF Data 

Flow Medium 

Type 

Only HC Liquids 

Total No Small Medium Large Unknown No Small Damage Severe Total Loss Unknown Oil Production Gas/Cond Production Production Total Exploratio

Drilling 

Oil 4 4 1 3 4 4 

Oil, Gas (deep) 20 10 7 1 1 1 4 12 1 1 2 5 5 4 

Condensate, 

Gas(deep) 8 1 7 4 3 1 1 1 

Blowout 

(surface flow) 

Total 32 11 14 5 1 1 8 16 2 1 2 3 9 1 10 4 

Oil 

Oil, Gas (deep) 1 1 1 1 1 

Condensate, 

Gas(deep) 

Blowout 

(underground flow) 

Total 1 1 1 1 1 

Oil 3 2 1 3 1 1 

Oil, Gas (deep) 10 6 3 1 1 8 1 1 

Condensate, 

Gas(deep) 2 2 2 2 2 Well Release 

Total 15 6 7 2 6 8 1 1 2 3 1 

Oil 

Oil, Gas (deep) 

Condensate, 

Gas(deep) 

Diverted 

Well Release 

Total 

Oil 7 2 4 1 3 1 3 5 5 

Oil, Gas (deep) 31 17 10 1 1 2 5 20 1 1 2 2 6 6 5 

Condensate, 

Gas(deep) 10 1 9 6 3 1 3 

TOTAL 

TOTAL 48 18 21 5 1 3 14 24 2 1 2 5 11 3 14 5 
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Table 8.2: SINTEF LOWC HC Spill Data With Pollution - U.S. GOM (1980-2011) 

Phase Type U.S. GOM OCS 1980-2011 

SINTEF Data - General 

Spill Type 

HC Liquids 

with Pollution 

Total Production Exploration

Drilling 

Development 

Drilling 

All 

Drilling 

Well 

Interventions

Oil 4 4 

Oil, Gas (deep) 9 1 4 4 4 

Condensate, Gas(deep) 7 2 2 5 Blowout (surface flow) 

Total 20 5 4 2 6 9 

Oil 

Oil, Gas (deep)

Condensate, Gas(deep) Blowout (underground flow) 

Total 

Oil 2 1 2 

Oil, Gas (deep) 3 1 1 2 

Condensate, Gas(deep) 2 2 Well Release 

Total 7 3 1 1 4 

Oil 

Oil, Gas (deep)

Condensate, Gas(deep) Diverted Well Release 

Total 

Oil 6 5 2 

Oil, Gas (deep) 12 1 5 5 6 

Condensate, Gas(deep) 9 2 2 2 5 TOTAL 

TOTAL 27 8 5 2 7 13 

8.2.2 BOEM GOM LOWC Spill Data 

BOEM data provides a level of detail necessary for the quantification of spill volumes. 

For this analysis, spills equal to or greater than 50 barrels (bbl) are considered. These are 

given in Table 8.3, with their subdivision among oil and condensate hydrocarbon liquids. 

The median LOWC spill size in the GOM, when a spill of hydrocarbon liquids does 

occur (1964 to 2010), is 2 bbl [22]. BOEM data also include spills down to 1 bbl, which 

is the reportable quantity. Between the SINTEF and GOM data reported here, the 

analysis includes all spill sizes down to 1 bbl. 

Table 8.3: BOEM GOM Spill (>= 50 bbl) Data 

U.S. GOM OCS Phase Type 

1980-2011 

BOEM Data 

Spill Type Total Production Exploration 

Drilling 

Development 

Drilling All Drilling Well Interventions 

Oil 6 1 3 3 2 

Condensate 3 1 1 2 

Spill 

>= 50 bbl 

LOWC Total 9 1 3 1 4 4 
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8.3 General U.S. GOM LOWC Incident Characteristics and Exposure 

8.3.1 SINTEF GOM Spill Frequency Characteristics 

To be consistent with the previous chapters in this report, exposure from 1980 to 2011, 

for the categories of producing wells, well interventions, and exploration and 

development well drilling, has been based on the SINTEF [46] data. Table 8.4, using the 

incident qualification as hydrocarbon liquid releases of 1 bbl or more with pollution from 

SINTEF, gives the general frequency distribution for the principal well operation 

categories, subdivided into blowouts and well releases. Utilizing the Chi Square 90% 

confidence interval assessment [47], Table 8.5 gives the associated distributions. 

Table 8.4: SINTEF GOM LOWC General Spill Frequency Characteristics 

Production Well 

 Interventions 

Exploration 

Drilling 

Development 

Drilling All Drilling 

197,721 197,721 12,299 19,275 31,574 

U.S. GOM OCS 

HC Liquids 

With Pollution

SINTEF

1980-2011 # Frequency 

per well-year # Frequency per well-year # Frequency per well # Frequency per well # Frequency per well 

Blowout 5 2.53E-05 9 4.55E-05 4 3.25E-04 2 1.04E-04 6 1.90E-04 

Well Release 3 1.52E-05 4 2.02E-05 1 8.13E-05 1 3.17E-05 

Table 8.5: SINTEF U.S. GOM OCS 1980-2011 Blowout Spill Frequency Variability 

U.S. GOM OCS Blowout Frequency per Well-Year 

HC Liquids 

With Pollution

SINTEF

1980-2011 

High Low Expected 

Production 4.63E-05 9.96E-06 2.53E-05 

Blowout Frequency per Well High Low Expected 

Exploration Drilling 6.30E-04 1.11E-04 3.25E-04 

Development Drilling 2.46E-04 1.84E-05 1.04E-04 

All Drilling 3.33E-04 8.28E-05 1.90E-04 

Note: 90% confidence that Frequency is in the Low-High Interval 

REF: Offshore Blowouts -Per Holand /page 132 

8.3.2 BOEM GOM LOWC Spills 

As indicated above, spills have been considered as those with spills of hydrocarbon 

liquids equal to or in excess of 50 bbl. Table 8.6 gives the general frequency 

characteristics associated with these releases based on the BOEM data from 1980 to 2011 

[22]. Although the category of release incidents is described as LOWC, all of these 

incidents were in fact blowouts, rather than diverted well releases or other well control 

incidents. Next, Table 8.7 gives the statistical distribution again based on the Chi Square 

statistical algorithm [47]. Comparing the results in Tables 8.5 and 8.7, although they are 

for distinctly different ranges of spill sizes, with SINTEF covering all spills and BOEM 

data limited to HC spills greater than or equal to 50 bbl, one can see that the drilling 

LOWC frequencies are quite similar, while the production well LOWC frequencies are 

lower for the BOEM data, since spills less than 50 bbl have not been considered. 
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Table 8.6: BOEM GOM LOWC General Spill >= 50 bbl Frequency Characteristics

Production Well 

 Interventions 

Exploration 

Drilling 

Development 

Drilling All Drilling 

197,721 197,721 12,299 19,275 31,574 U.S. GOM OCS 

BOEM >=50 bbl

1980-2011 # 

Frequency

per wellyear # 

Frequency

per wellyear # Frequency

per well # Frequency per well # Frequencyper well 

LOWC 1 5.06E-06 4 2.02E-05 3 2.44E-04 1 5.19E-05 4 1.27E-04 

Table 8.7: BOEM U.S. GOM OCS 1980-2011 LOWC Spill >= 50 bbl Frequency Variability

U.S. GOM OCS Blowout Frequency per Well-Year 

BOEM – Spills >= 50 bbl 

1980-2011 High Low Expected 

Production 1.52E-05 2.59E-07 5.06E-06 

Blowout Frequency per Well High Low Expected 

Exploration Drilling 5.12E-04 6.65E-05 2.44E-04 

Development Drilling 1.55E-04 2.66E-06 5.19E-05 

All Drilling 2.46E-04 4.33E-05 1.27E-04 

Note: 90% confidence that Frequency is in the Low-High Interval 

REF: Offshore Blowouts -Per Holand /page 132 

8.4 U.S. OCS LOWC Spill Volume Distributions (1964-2010) 

The information on U.S. OCS LOWC spill sizes and its analysis utilizing a least squares 

regression [22, Figure 4.3.3-1], forms a reasonable estimate for the relative distribution of 

spill sizes, which can then be applied to the overall frequencies given in Section 8.3. 

Based on the analysis provided in [22], Figure 8.1 provides the cumulative spill size 

exceedance frequency in the top portion, and exceedence percentage in the bottom 

portion. As can be seen, the full range of spill sizes, from less than 1 bbl to almost 10 

million bbl, has been plotted on the horizontal axis. Table 8.8 summarizes the percentage 

distributions based on the figure. Under the column “Full Distribution”, together with the 

normalized partial distribution for spills greater than or equal to 50 bbl on the left hand 

side. Because of the current interest in the largest spills, such as those that have occurred 

with Macondo [23] and earlier in the Mexican portion of the Gulf of Mexico with the 

Ixtox [56] spill, the tabulation of spill sizes has been extended to include the greater than 

1 million size. Although not shown in Table 8.8, the Enormous spill size category has 

further been subdivided into Class A (150,000 to 999,999 bbl) and Class B (>= 1 million 

bbl). The extended percentage distribution is shown in Table 8.9. 
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Figure 8.1: Cumulative Frequency and Percentage Spill Size Exceedance

Table 8.8: U.S. OCS LOWC Spill Distribution Summary (1964-2010)

Spill Range >= 50 bbl 

Distribution 

Full 

Distribution 

Small - Catastrophic 0 bbl - Catastrophic Category 

bbl % % 

Very Small 0-49 n/a 74.82 

Small 50-99 16.31 4.11 

Medium 100-999 38.32 9.65 

Large 1,000-9,999 22.10 5.56 

Huge 10,000-149,999 14.35 3.61 

Enormous >= 150,000 8.91 2.24 

ALL 100.00 100.00 

y=0.0009x‐0.2386

1.00E‐05

1.00E‐04

1.00E‐03

1.00E‐02

0.1 1.0 10.0 100.0 1,000.0 10,000.0 100,000.0 1,000,000.0 10,000,000.0

Spill bbl

FrequencyofSpills exceedingspill Size [pe

y=68.523x‐0.2386

1.00

10.00

100.00

0.1 1.0 10.0 100.0 1,000.0 10,000.0 100,000.0 1,000,000.0 10,000,000.0

Spill bbl

Cumulative

%ofexceedingspi

Spill Size (bbl)

Spill Size (bbl)

Frequency of Spills exceeding spill size (per well) Cumulative

% of exceeding spill size (per well)
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Table 8.9: U.S. OCS LOWC Spill Distribution Summary Including 106 bbl Spills

Spill Range Distribution Category (bbl) Small – Enormous (%) 

Small 50-99 16.31 

Medium 100-999 38.32 

Large 1,000-9,999 22.10 

Huge 10,000-149,999 14.35 

Enormous – Class A 150,000-999,999 5.75 

Enormous – Class B >= 1,000,000 3.17 

ALL 100.00 

8.5 U.S. GOM Drilling Spill Volume Frequencies 

Application of the spill size distributions identified in the above section to the LOWC 

with spills greater than or equal to 50 bbl frequencies identified in Table 8.9, leads to the 

spill size frequency distribution, ranging from small to enormous spills, shown in Table 

8.10, together with the statistical upper and lower 90% confidence intervals. Figure 8.2 

shows a graphical display of the associated full size frequency confidence limits. 

Table 8.10: U.S. OCS LOWC Spill Size Frequency Distribution

U.S. GOM OCS All Drilling Frequency per Well Drilled 

BOEM DATA - SPILLS >=50 bbl (1980-2011) High Low Expected 

Loss of Well Control 2.46E-04 4.33E-05 1.27E-04 

Spill Category bbl Distribution % 

Small 50-99 16.31 4.01E-05 7.06E-06 2.07E-05 

Medium 100-999 38.32 9.41E-05 1.66E-05 4.85E-05 

Large 1,000-9,999 22.10 5.43E-05 9.56E-06 2.80E-05 

Huge 10,000-149,999 14.35 3.52E-05 6.21E-06 1.82E-05 

Enormous – Class A 150,000-999,999 5.75 1.41E-05 2.49E-06 7.28E-06 

Enormous – Class B >= 1,000,000 3.17 7.78E-06 1.37E-06 4.01E-06 

All 100.00 2.46E-04 4.33E-05 1.27E-04 

Figure 8.2: U.S. OCS (1980-2011) Spills Blowout Frequency Variability >= 50 bbl

0.0E+00

1.0E‐05

2.0E‐05

3.0E‐05

4.0E‐05

5.0E‐05

6.0E‐05

7.0E‐05

8.0E‐05

9.0E‐05

1.0E‐04

High 4.01E‐05 9.41E‐05 5.43E‐05 3.52E‐05 1.41E‐05 7.78E‐06

Low 7.06E‐06 1.66E‐05 9.56E‐06 6.21E‐06 2.49E‐06 1.37E‐06

Expected 2.07E‐05 4.85E‐05 2.80E‐05 1.82E‐05 7.28E‐06 4.01E‐06

50‐99 100‐999 1,000‐9,999 10,000‐149,999 150,000‐999,999 >=1,000,000

Small Medium Large Huge Enormous ‐ Class

A

Enormous ‐ Class

B

Spill Frequency per Well Drilled
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From another point of view, it is interesting to simply take a direct large sample statistical 

approach to the occurrence of Enormous Class B spills. Over the period from 1980 to 

2011 (and for that matter, in the entire offshore drilling history in the U.S. GOM), only 

one spill exceeding 1 million bbl of oil has occurred; namely, Macondo [23]. Dividing 

this by the total number of wells drilled (31,574), focusing on the 1980-2011 period, one 

obtains an expected value of 3.17 x 10-5 per well drilled. Although this value is roughly 

one order of magnitude higher than the Enormous Class B spill expected spill value of 

4.01 x 10-6 given in Table 8.10 (obtained as described above), its low 90% confidence 

interval value of 1.62 x 10-6 is higher than the low 90% confidence interval of 1.37 x 10-6

in the distribution in Table 8.10, while its high value of 9.49 x 10-5 (as one could 

anticipate) remains over one order of magnitude higher than the higher 90% confidence 

interval of 7.78 x 10-5 of the value in Table 8.10. Combining the two sets of results, 

averaging the expected value and using upper and lower extremes, one can postulate the 

following ranges of Enormous Class B LOWC occurrence frequencies for U.S. GOM 

well drilling: 

 Expected value: 1.78 x 10-5

 Upper 90% value: 9.49 x 10-5

 Lower 90% value: 1.37 x 10-6

The variability of the large or Enormous Class B spill frequency in Table 8.11 and Figure 

8.3 was assessed on the basis of the single event statistic from a very large population or 

sample size; that presented earlier in Table 8.10 was assessed using a least squares best fit 

method reflecting the influence of smaller spill sizes, and likely represents a better 

historical hindsight evaluation. Further approaches to a better understanding of the 

Enormous Class B and other specific LOWC spill potentials can be generated on the 

basis of risk analysis methods, as described in the next section. 

Table 8.11: Enormous Class B Blowout 

Frequency and Its Variability 

All Wells Drilled 

1980-2011 U.S. OCS GOM 

31,574 

Number of Frequency per well drilled 

Enormous Class B 

Blowouts High Low Expected 

1 9.49E-05 1.62E-06 3.17E-05 

Figure 8.3: U.S. OCS GOM (1980-2011) 

Enormous Class B Spill Frequency 

Variability 

0.0E+00

1.0E‐05

2.0E‐05

3.0E‐05

4.0E‐05

5.0E‐05

6.0E‐05

7.0E‐05

8.0E‐05

9.0E‐05

1.0E‐04

Blowout FrequencyperWellD High 9.49E‐05

Low 1.62E‐06

Expected 3.17E‐05

All WellsDrilled

1980‐2011USOCSGOM

Blowout Frequency per Well Drilled
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8.6 General Description of Largest Gulf of Mexico LOWC Spills 

The largest recorded Gulf of Mexico LOWC spills are the Ixtox blowout [56] which 

commenced on June 3, 1979 in the Mexican portion of the Gulf, and the Macondo 

blowout [23, 27, 30, 49, 51, 55] which commenced on April 20, 2010 in the GOM. 

Ixtox I [56] was an exploratory oil well in the Mexican Gulf drilled by the Sedco I35-F 

semisubmersible in 1979 for Pemex, the Mexican government oil company. The location 

was approximately 50m deep, approximately 100km NW of Ciudad del Carmen. During 

drilling at approximately 3600m below the seafloor, the drill entered a soft stratum and 

mud circulation was lost as it flowed into the formation. Rather than shutting in the well 

using the BOP, Pemex ordered removal of the bit by pulling the drill pipe and running the 

pipe back to the formation to enable pumping cement and material to seal the surrounding 

soft formation. During the drill string pulling, or well swabbing, a kick occurred and 

when shear rams were activated to shut in the well, drill collars were at the shear ram 

elevation, so the drill pipe was not sheared, and the well could not be shut in, resulting in 

the blowout [32, 40]. Formation fluids, oil and gas, continued to flow, ignited, and the rig 

sank. Initial flow rates were estimated at 30,000 bbl per day, and over the 10 month 

period until capping of the well on March 23, 1980 it is estimated a total of over 3.5 

million barrels were spilled into the Gulf. The principal cause of the blowout is improper 

and unsafe procedure used in an attempt to save the well for production. 

The main references used here in outlining the Macondo blowout are those by BOEMRE 

[23] and the U.S.CG [51], while additional views are given in [27, 30, 49, 55]. On April 

20, 2010 the Deepwater Horizon, a Transocean dynamically positioned semisubmersible, 

with BP as the designated operator, was in the process of completing and abandoning the 

Macondo exploration well in a water depth of approximately 1500m, drilled to 

approximately 5600m below the ocean floor. At approximately 10 pm an undetected kick 

occurred, shortly thereafter escalating to a blowout as none of the existing well barriers 

could be used to stop it. The flammable gas component of the blowout resulted in an 

explosion and fire on the rig, killing 11 men, and continuing to burn until the rig sunk, 

likely from firewater overburden on April 22. It is estimated that nearly 5 million barrels 

of oil were spilled into the Gulf during the 87 days it took to cap the well, resulting in the 

largest oil spill in the history of the GOM. The investigations by BOEMRE [23] and the 

U.S.CG [51] concluded that the accident and its aftermath occurred as a result of a 

complex combination of the operator decision to take additional risks to accelerate task 

completion schedules and resultant inadequate and unsafe procedures, failure to follow 

even its own emergency procedures, inadequate operator personnel emergency training 

and emergency drills, numerous incorrect tactical acts and omissions, inadequate 

equipment configurations and inadequate equipment maintenance, and various command 

and control communication failures and procedural deficiencies. It appears the principal 

causes can be summarized as those stemming from deficient safety procedures and 

culture. 
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8.7 Risk Analytic Approaches to Predicting LOWC Spill Frequencies 

and Volumes 

8.7.1 Introduction on Risk Analysis Approaches to Specific LOWC Characteristics 

The value and variability of specific spill volume LOWC frequencies for a specified 

development can be more accurately derived using quantitative risk analysis (QRA) 

approaches including fault and event trees [5]. Accordingly, following a brief review of 

QRA approaches by the author [6, 7, 8] and others [30, 31, 53], an algorithm to assess 

LOWC spill frequencies and characteristics for specified developments using the regional 

frequencies reported in this report as a starting point, and incorporating the principal 

development risk factors is described and illustrated with an example. 

8.7.2 Review of Risk Analysis Approaches to Evaluating Specific LOWC Characteristics 

Application of fault tree analysis (FTA) to evaluate frequencies of systems without 

history (which is adequate to generate failure statistics) such as Arctic offshore oil and 

gas developments, has been extensively developed and applied by the author for BOEM 

(formerly MMS) [13, 14, 15, 16, 17, 18, 19, 20, 21], and others [10, 11]. Essentially, 

historical data such as those generated in the earlier chapters of this report and their 

statistical properties are used as a starting point for fault tree application to oil spill 

indicator quantification for the Alaska OCS, the location without history. In the initial 

fault tree analysis in 2002 [21], data from the GOM OCS were analyzed for the period 

from 1972 to 1999 [2], as well as for earlier periods using world wide data [11]. 

Subsequently, a more refined publication of the data characteristics by the Bureau of 

Safety and Environmental Enforcement (BSEE) [24] has made it possible to conduct a 

more thorough statistical analysis as well as an update of the data and its analysis to 2010

[22]. As described in the following section a similar approach using FTA is taken here to 

the characterization of LOWC properties for specific developments based on regional 

statistics. Some of the methodologies described in Section 8.7.3 are partially adapted 

from the FT applications described and referenced above. 

More LOWC specific QRA studies have recently been published by Norwegian 

investigators, Espen et al [30] and Vandenbussche et al [53]. 

The paper by Espen [30] correctly states that the North Sea blowout frequency studies are 

simply based on average frequencies obtained by dividing the number of blowouts by the 

relevant number of well operations. To provide a better perspective, Espen introduces 

two sets of factors which impact blowout occurrence frequencies; namely Human and 

Organizational Factors (HOF) and Risk Influence Factors (RIF). The factors given are 

described qualitatively and there appears to be some overlap in their definitions – HOFs 

refer to human performance and operations while RIFs deal primarily with the physical 

configuration of the operation including environment, reservoir, and equipment but also 

with operations which are covered by the HOF definition. In any case, Espen’s 

qualitative conclusion that the Macondo blowout and its aftermath were largely due to 

HOF impact is consistent with the BOEMRE [23] conclusions. 

Vandenbussche et al [53] follow a conceptually similar approach to that of Espen [30], 

defining a set of factors which exacerbate individual well blowout probability, more or 
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less covering the same areas as [30], but go a step further by setting up a quantitative 

ranking method which they than use to generate quantitative effects on a base blowout 

probability. Although what they call “risk factors” do not entail risk effects, rather only 

probability effects independent of any volume of spills or other consequences, the 

exercise is useful enough to comment further. A weighing is assigned to the principal 

factors contributing to blowout proclivity as follows: 

 Reservoir and underground conditions: 60%. 

 Rig, riser, and well: 32%. 

 Operational aspects: 8%. 

The authors hereof do not agree with the low weighing given to operational aspects 

which include human performance but do agree that the reservoir should be an important 

contributor (although lower than Espen ranks it) as indeed if the reservoir lacks pressure 

and capacity, a blowout would not occur in the first place. Next, Espen uses the weighted 

result or “score” to multiply a base blowout frequency to obtain one for the specific well 

drilling. Repeating this process for various wells, a graph of frequency versus score is 

plotted with score incorrectly labeled as “well risk level”, when in fact it is merely 

frequency score as no spill volume or other consequence parameter needed to constitute 

risk is given. However, in applying the methodology to specific scenarios, Espen does 

include approximate consideration of both flow duration and rate, providing a tool for 

blowout risk estimation for specific scenarios. In general, the Espen methodology is a 

promising one for the evaluation of LOWC risk from specific developments, but lacks the 

capability to integrate a set of wells as generally used in a production development. 

8.7.3 Fault Tree Analysis Algorithm for Specific Development LOWC Characteristics 

The FTA basic approach to the quantitative characterization of LOWC’s is similar to that 

used for the characterization of oil spill indicators for the Alaskan OCS [12, 14], and 

incorporates some of the concepts proposed by Espen [30] and Vandenbussche [53]. 

Basically, a fault tree is constructed for a set of historically known regional spill 

frequency and volume characteristics and then modified to more accurately depict the 

characteristics associated with a specific development, including the production and 

drilling well configuration in a specified year. 

The main building blocks of the algorithm are as follows: 

 Regional LOWC frequency and its variation with different LOWC events. 

 LOWC spill volume distribution (based on the U.S. OCS) and assumed valid for 

any other locations. 

 Specific development characteristics including location, water depth, reservoir 

characteristics, operator, regulatory regime, and the annual numbers of each type 

of well and well operation. 

We will next describe the algorithm generally, to be followed by a numerical example for 

a specific hypothetical development. 
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Figure 8.4 shows the fault tree (FT) conceptually (without values). As can be seen the 

base events are simply the annual frequencies of LOWC resulting from each well 

operation category for a specified volume spill range (such as <50bbl); namely 

exploration well drilling, development well drilling, production (including oil and 

condensate), and well interventions. These frequencies calculated for the entire numbers 

of wells in each year, are joined through an OR gate or summed, to give the resultant #2, 

which is simply the regional LOWC average resultant for the specified well 

configuration. This regional resultant is then multiplied by the specific development 

factor (analogous to the factors used by Espen) to give resultant #2 the Specific LOWC 

Spill annual frequency for the specified development and spill volume range. 

Figure 8.4: Specific Development LOWC Characteristic Evaluation Conceptual Fault Tree 

Table 8.12 shows the well input table, in this case giving the U.S. GOM >=50bbl LOWC 

spill frequencies, as given earlier in Table 8.6. These are the “Regional Frequencies.” The 

right hand column heading “Year TBA” refers to a generic year only. 

F1

F2

Combined Factor F3

F4

F5

Exploration

Well Drilling

Development

Well Drilling Production Well

Intervention

N per Year N per Year N per Year N per Year

F5 Reservoir

F1 Corporate Safety Culture

F2 Regulatory Regime

F3 Meteorology

F4 Water Depth

1. Specific

LOWC Spill

N per Year

2. Regional

LOWC Spill

N per Year
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Table 8.12: Annual Regional Frequency Input Table 

Region U.S. GOM - Regional LOWC Spills >=50 bbl Year TBA 

# Description 

LOWC Spill 

Frequency 

Unit 

LOWC Spill 

Frequency Unit # of Units LOWC Spillsper Year 

1 Exploration Well Drilling per Well 2.44E-04 Wells 

per Year 1 2.44E-04 

2 Development Well Drilling per Well 5.19E-05 Wells 

per Year 1 5.19E-05 

3 Production per Well-Year 5.06E-06 Well-Years

per Year 1 5.06E-06 

4 Well Intervention per Well-Year 2.02E-05 Well-Years

per Year 1 2.02E-05 

Total Annual LOWC Frequency 3.21E-04 

Table 8.13 then defines the factors, the “Specific Frequency Factors,” used to adjust the 

regional volume frequencies to the specific development. The input values for each factor 

are 1, 3, or 3, with 3 being the worst case. These factors and their weights are as follows: 

 Corporate Safety Culture: 20% 

 Regulatory Regime: 10% 

 Meteorology (eg, hurricanes at specific location): 10% 

 Water Depth (Shelf Range Section 3.5): 10% 

 Reservoir: 50% 

Table 8.13: LOWC Frequency Factor Input Table 

SPECIFIC FREQUENCY FACTORS RANGE: 1 10 

No. Factor Description Weight % Input Factor 

Weight Value 

1 -Average for Region 

F1 Corporate Safety Culture 2 -More Stringent 

3 -Very Stringent 

20 1.0 20 

1 -Regional Prevailing 

F2 Regulatory Regime 2 -Less Stringent 

3 -Lax 

10 1.0 10 

1 -Regional Prevailing 

F3 Meteorology 2 - Some extreme 

3 - Frequent extreme 

10 1.0 10 

1 - Regional Average 

F4 Water Depth 2 - Deep (60m<=600m) 

3 - Very Deep (>600m) 

10 1.0 10 

1 - Normal 

F5 Reservoir 2 - Unknown 

3 - HTHP 

50 1.0 50 

Combined values For Given Region 100 1 100 
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8.14

The algorithm calculates the resultant factor so that given the weightings if all inputs are 

1 (same as regional average), the resultant factor is also 1. In this methodology 

description case, both the specific and regional resultants are the same value as shown in 

Table 8.14. 

Table 8.14: Regional Frequency Resultants Output 

RESULTANTS 

No. Description Value 

1 Specific LOWC Spills per Year 3.21E-04 

2 Regional LOWC Spills per Year 3.21E-04 

Now consider a hypothetical development, say for year 2010, with the annual numbers of 

wells shown in Table 8.15. To rank the development LOWC proclivity (>= 50bbl) 

consider Table 8.16. Then, if the development is in the GOM OCS, the same regulatory 

regime will apply, and we can assume the operator is of a regional average safety culture, 

giving a value of 1 for F1 and F2. Meteorology can also be assumed to be of base value 

so F3 is also 1. Assume water depth is 120 m, F4 is given a value of 2. Now, also assume 

the reservoir is known to be HTHP so F5 is given a value of 3. The fault tree and 

principal resultants for this specific hypothetical but realistic development for year 2010 

are shown in Figure 8.5. The resultants #1 and #2 are shown again in Table 8.17. 

Basically we glean from this that the LOWC spill >=50bbl frequency for this 

development is 5.95 times higher than the average frequency in the GOM OCS. Of 

course we could input frequencies for any spill volume category, and any other region 

and generate analogous information. Also, one can apply the variations of frequencies 

and recalculate, or, to be rigorous, input frequency distributions and run the FT through a 

Monte Carlo simulation as was done by the authors in previous studies for BOEM [14, 

15]. 

Table 8.15: Specific Development Annual Regional Frequency Input Table 

Region U.S. GOM - Regional LOWC Spills >=50 bbl Year 2010 

# Description 

LOWC Spill 

Frequency 

Unit 

LOWC Spill 

Frequency Unit # of Units LOWC Spills per Year 

1 Exploration Well Drilling per Well 2.44E-04 Wells 

per Year 2 4.88E-04 

2 Development Well Drilling per Well 5.19E-05 Wells 

per Year 4 2.08E-04 

3 Production per Well-Year 5.06E-06 Well-Years 

per Year 20 1.01E-04 

4 Well Intervention per Well-Year 2.02E-05 Well-Years 

per Year 20 4.05E-04 

Total Annual LOWC Frequency 1.20E-03 
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8.15

Table 8.16: Specific Development LOWC Frequency Factor Input Table 

Specific Frequency Factors Range 1 10 

No. Factor Description Weight % Input Factor 

Weight Value 

1 -Average for Region 

F1 Corporate Safety Culture 2 -More Stringent 

3 -Very Stringent 

20 1.0 20 

1 -Regional Prevailing 

F2 Regulatory Regime 2 -Less Stringent 

3 -Lax 

10 1.0 10 

1 -Regional Prevailing 

F3 Meteorology 2 - Some extreme 

3 - Frequent extreme 

10 1.0 10 

1 - Regional Average 

F4 Water Depth 2 - Deep (60m<=600m) 

3 - Very Deep (>600m) 

10 2.0 20 

1 - Normal 

F5 Reservoir 2 - Unknown 

3 - HTHP 

50 3.0 150 

Combined For Given Region 100 5.95 210 

Figure 8.5: Quantified Specific Development LOWC Characteristic Evaluation Fault Tree 

7.15E‐03

1

1

Combined Factor 5.95 1

2

3

1.20E‐03

Exploration

Well Drilling

Development

Well Drilling Production Well

Intervention

N per Year N per Year N per Year N per Year

4.88E‐04 2.08E‐04 1.01E‐04 4.05E‐04

F5 Reservoir

2. Regional

LOWC Spill

N per Year

1. Specific

LOWC Spill

N per Year

F1 Corporate Safety Culture

F2 Regulatory Regime

F3 Storm

F4 Water Depth
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8.16

Table 8.17: Specific Development Frequency Resultants Output 

RESULTANTS 

No. Description LOWC/YR 

1 Specific LOWC Spills per Year 7.15E-03 

2 Regional LOWC Spills per Year 1.20E-03 

Finally, it is prudent to provide a note of caution. The algorithm presented, although 

correct mathematically, has not had its frequency factors or weighing factors and their 

significance subjected to rigorous review by other experts, nor has it been applied to 

specific proposed or existing developments. Also, if applied to the largest spill category, 

the Enormous Class B spills, only well drilling should be considered, as production wells 

have to date not produced LOWC spills exceeding 1 million bbl. Accordingly, the 

authors reserve the option to modify aspects of the algorithm following further review 

and application. It is our opinion however that the algorithm promises to fulfill a 

significant BOEM need for assessing LOWC spill frequencies and volumes for specific 

OCS developments, both proposed and existing. 
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9.1

CHAPTER 9 

CONCLUSIONS AND RECOMMENDATIONS 

9.1 General Summary

An extensive study of world wide loss of well control (LOWC) incidents has been carried 

out to support BOEM’s use of the results from the fault tree model generating oil spill 

occurrence rates for oil and gas lease sales and any development projects in the 

Chukchi and Beaufort Sea OCS Planning Areas proposed under BOEM and industry 

planning. 

Specifically, this study included the following principal activities: 

 Update of offshore LOWC frequency information through 2011 for the U.S., 

Canadian and Australian offshore regions, the North Sea, and other areas with a 

comparable regulatory regime, and collation of associated exposure variable 

information that was readily available. 

 Application of statistical procedures to develop LOWC occurrence rates for 

different operational phases and products spilled 

 Evaluation of confidence intervals for LOWC occurrence rates. 

 Provision of statistical measures such as mean and median spill sizes, spill size 

distributions, and as well as provision of methods for possible statistical outliers 

such as the Macondo blowout. 

The principal data sources used were the BOEM/BSEE data and the SINTEF offshore 

blowout database. As access to the full SINTEF database is on a proprietary membership 

basis, the contractor, acquired such membership as part of the work for years 2013 to 

2015. The methods utilized, including data analysis, statistical, probabilistic, and risk 

analysis techniques used, produce results compatible with and applicable to the fault tree 

evaluations of oil spill occurrence estimators used by BOEM. Such an application is 

currently underway and the results generated herein have been found to be compatible 

with the application. 

9.2 General Conclusions 

General conclusions of the work can be summarized as follows: 

 Generally adequate data on LOWC occurrences and their characteristics in 

western waters such as the North Sea and the U.S. GOM, are available from the 

SINTEF database for a sufficiently large exposure for the period from 1980 to 

2011. 

 More detailed data, on LOWC occurrences and their characteristics, including 

spill volumes, for the U.S. OCS are available from the BOEM/BSEE database for 

a sufficiently large exposure for the period from 1980 to 2011. 
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9.2

 The above data are of sufficient quantity and quality to permit the generation of 

statistics, including occurrence rates for different operational phases and 

products spilled, associated confidence intervals, and other statistical measures. 

 Certain data, however, were not available, including spill volumes for locations 

other than the U.S. OCS, well exposure populations by water depth intervals for 

all locations, or detailed characterization of the products spilled from LOWC 

incidents. 

9.3 Specific Conclusions 

Table 9.1 summarizes the key high level LOWC parameters for the principal regions 

studied. Details on these and other results follow in the forthcoming sections. 

Table 9.1: Summary of Principal LOWC Parameters for Key Regions 

EXPOSURE LOWC FREQUENCY LOWC DURATION 

Drilling Production Drilling Production Interventions 50 % 

stopped 

90 % 

REGION stopped 

wells drilled well-years per 1000 

wells drilled 

per 1000 

well-years 

per 1000 

well-years minutes days 

U.S. GOM 31,574 197,721 3.45 0.106 0.314 200 8 

North Sea 13,727 59,141 2.99 0.051 0.355 3 20 

Holland 1,143 2,948 0 0.339 0.339 n/d* n/d 

Australia 2,559 9,589 1.56 0.104 0 n/d n/d 

Canada E Coast 679 3,955 2.95 0 0 n/d n/d 

* n/d = no data 

9.3.1 U.S. GOM OCS 

The following specific conclusions can be summarized from the work detailed in Chapter 

3 for the U.S. GOM OCS: 

 From 1980 to 2011, a total of 31,574 wells were drilled in the U.S. GOM, and a total 

of 197,721 production well-years occurred 

 LOWC frequencies for production wells in the subject period total 1.06 x 10-4 per 

well year, consisting of 81% blowouts and 19% well releases, while frequencies for 

well intervention LOWC’s were 3.14 x 10-4 per well year 

 LOWC frequencies for all well drilling in the subject period total 3.45 x 10-3 per well 

drilled, consisting of 71% blowouts and 29% well releases 

 LOWC durations show that there is approximately a 50% probability that an 

incident will stop within 200 minutes and 90% probability, it will stop within 8 days 

 LOWC incident occurrences by depth indicate that the majority of incidents (58%) 

occur in the shallower <60m water depth range. It should be noted that this value is 

only the number of incidents and not a frequency or rate as the exposure for the 

water depth intervals was not available. 
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9.3

9.3.2 North Sea 

The following specific conclusions can be summarized from the work detailed in Chapter 

4 for the North Sea: 

 From 1980 to 2011, a total of 13,727 wells were drilled in the North Sea and a total 

of 59,141 production well-years occurred 

 LOWC frequencies for production wells in the subject period total 5.07 x 10-5 per 

well year, consisting of 67% blowouts and 33% well releases, while frequencies for 

well intervention LOWCs were 3.55 x 10-4 per well year 

 LOWC frequencies for all well drilling in the subject period total 2.99 x 10-3 per well 

drilled, consisting of 58% blowouts and 42% well releases 

 LOWC durations show that there is approximately a 50% probability that an 

incident will stop within 3 minutes and 90% stop within 23 days 

 LOWC incident occurrences by depth indicate that the majority of incidents (62%) 

occur in the shallower 61 to 152m water depth range. It should be noted that this 

value is only the number of incidents and not a frequency or rate as the exposure for 

the water depth intervals was not available. 

9.3.3 North Sea and GOM 

The following specific conclusions can be summarized from the work detailed in Chapter 

5 for the combined North Sea and U.S. GOM: 

 From 1980 to 2011, a total of 45,301 wells were drilled and a total of 256,862 

production well-years occurred 

 LOWC frequencies for production wells in the subject period total 9.34 x 10-5 per 

well year, consisting of 79% blowouts and 21% well releases, while frequencies for 

well intervention LOWC’s were 3.23 x 10-4 per well year 

 LOWC frequencies for all well drilling in the subject period total 3.31 x 10-3 per well 

drilled, consisting of 67% blowouts and 33% well releases 

 LOWC durations show that there is approximately a 50% probability that an incident 

will stop within 1.5 hours and 90% probability, within 9 days 

 LOWC incident occurrences by depth indicate that a large proportion of incidents 

(48%) occur in the shallower <60m water depth range. It should be noted that this 

value is only the number of incidents and not a frequency or rate as the exposure for 

the water depth intervals was not available. 

9.3.4 Australia, Holland, and Canada 

The following specific conclusions can be summarized from the work detailed in Chapter 

6: 

 Australia 

o From 1980 to 2011, a total of 2,559 wells were drilled and an estimated 

total of 9,589 production well-years occurred. 
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9.4

o LOWC frequencies for production wells in the subject period total 1.04 x 

10-4 per well year, and for interventions, 0 per well year. 

o LOWC frequencies for well drilling in the subject period total 1.56 x 10-3

per well drilled. 

 Holland 

o From 1980 to 2011, a total of 1,143 wells were drilled and an estimated 

total of 2,948 production well-years occurred. 

o LOWC frequencies for production wells in the subject period total 3.39 x 

10-4 per well year, and the same for interventions, 3.39 x 10-4 per well 

year. 

o LOWC frequencies for well drilling in the subject period total 0 as no 

LOWC’s were recorded during drilling. 

 Canada East Coast 

o From 1980 to 2011, a total of 679 wells were drilled and an estimated 

total of 3,955 production well-years occurred 

o LOWC frequencies for production wells and well interventions in the 

subject period are 0 as none were recorded 

o LOWC frequencies for well drilling in the subject period total 2.95 x 10-3

per well drilled 

9.3.5 Regional and Temporal Comparisons 

The following specific conclusions can be summarized from the regional and temporal 

comparison work detailed in Chapter 7: 

 LOWC frequencies from 1980 to 2011 were generated for the North Sea and its 

principal sectors and U.S.GOM for the following principal periods: 

o 1980-2000 

o 2001-2011 

o 1980-2011 

 LOWC frequencies for U.S.GOM showed the following qualitative distributions: 

o 1980-2000-lower production and well drilling LOWC frequencies than 

later 

o 2001-2011- higher production and well drilling LOWC frequencies than 

earlier, but this may be attributable to the requirement for more stringent 

reporting enacted in 2006. 

 LOWC frequencies for North Sea showed the following qualitative distributions: 

o 1980-2000-higher production and well drilling LOWC frequencies than 

later 

o 2001-2011- lower production and well drilling LOWC frequencies than in 

the earlier period 

 The distribution of LOWC frequencies between shallow gas (no BOP) drilling and 

BOP installed drilling were developed and assessed 
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9.5

9.3.6 LOWC Hydrocarbon Spill Characteristics 

The following specific conclusions can be summarized from the regional and temporal 

comparison work detailed in Chapter 8: 

 SINTEF data provides information on the type of hydrocarbon released form LOWC 

incidents and only qualitative information on the degree of pollution and 

consequences thereof 

 BOEM/BSEE data provides detailed LOWC release volume information for 1960-

2011 for the U.S. OCS, which was used to develop a spill size distribution for the 

more recent period of 1980-2011 

 Comparing LOWC frequencies for spills >=50 bbl and LOWC frequencies for all 

LOWC incidents, it was found that LOWC frequencies for spills >=50 were 

approximately 17 times lower than all LOWC frequencies for production and well 

intervention, and approximately 27 times lower for well drilling. 

 Enormous spills of Class B, equal to or exceeding 1 million barrels were considered 

both with a statistical spill volume distribution and as a single valued occurrence. An 

expected frequency of such spills for the GOM was estimated to range from 4 in 1 

million to 3 in 100 thousand wells drilled. 

 An algorithm based on fault tree analysis for more refined estimates of LOWC 

frequencies considering reservoir, water depth, meteorology, regulatory regime, and 

operator safety culture and illustration of its application was provided 

9.3.7 Other Information Generated 

In addition to the principal results summarized above, additional information generated 

and reported herein on LOWC characteristics includes the subdivision and quantification 

of LOWC frequencies among the following categories: 

 Exploration and Development well drilling 

 LOWC flow path subsea or to surface 

 Shallow gas release or drilling without BOP LOWC frequencies 

 Type of product released 

 HC spill volume size distribution for GOM only 

Finally, conclusions were summarized and recommendations for further work were 

provided in this Chapter 9 
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9.6

9.4 Recommendations for Further Work 

The following recommendations based on the work are provided: 

 There appear to be some minor differences between the SINTEF and BOEM/BSEE 

data for the U.S. OCS. It is recommended that these differences be reconciled. The 

authors have made SINTEF aware of this issue and SINTEF may agree to address it 

jointly. 

 There are considerable differences among the regional LOWC parameters displayed 

in Table 9.1. It would be useful to develop a better understanding of the reasons for 

these differences through further review of associated conditions such as those 

postulated as having an effect on LOWC frequencies in Chapter 9 thereof. 

 LOWC duration data for many incidents indicates 0 duration. The significance of this 

datum should be investigated and durations adjusted as it is unlikely that an LOWC 

incident would stop in less than 1 minute. Similarly, the cause of the large differences 

in durations of GOM and North Sea LOWCs should be investigated. 

 The depth interval exposure data are not available for the GOM, resulting in a less 

meaningful assessment of LOWC occurrence variation with depth. As it is, the noted 

variation of LOWC occurrences associated with different depth intervals could be a 

result of simply a variable number of wells in the intervals, a change in LOWC 

frequency, or a combination of the two. Clearly, the variation of LOWC frequency 

and its characteristics with depth would be more instructive. 

 Full exposure data for Australia, Holland, and Canada were not available, and 

accordingly were estimated on the basis of what were assumed to be similar regions 

for which full exposure data were available. It is recommended to obtain exposure 

data from the stated regional administrations, if possible. 

 In earlier editions of SINTEF documentation (pre 2000) spill volumes were included. 

SINTEF has been advised of the usefulness of such information, particularly for 

regions outside the GOM where this information is lacking, and are considering 

options. 

 Surface and subsea LOWC data in SINTEF are not adequately documented to assess 

likelihood of releases into the marine environment. Subsea is defined as a release 

point either at the sea bottom or into another formation beneath the sea bottom. 

Clearly, it is desirable to know which of the subsea incidents release into the sea, and 

this is recommended to be assessed. 

 As indicated in Chapter 9, the algorithm presented, although correct mathematically, 

has not had its frequency factors or weighing factors and their significance subjected 

to rigorous review by other experts, nor has it been applied to specific proposed or 

existing developments. Also, if applied to the largest spill category, the Enormous 

Class B spills, only well drilling should be considered, as production wells have to 

date not produced LOWC spills exceeding 1 million bbl. Accordingly, the authors 

recommend further review of the algorithm with experts, and by application. As 

appropriate, modify aspects of the algorithm following such review and application. 
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