This report presents research strategies aimed at improving the reliability of bolting connections used in offshore subsea oil exploration equipment. The focus is on those fasteners employed in the most critical applications maintaining the pressure boundary of the well, such as blowout preventers (BOPs) which are an essential piece of safety equipment bolted to the wellhead. The overarching objective of the recommendations contained in this report is to reduce the probability of a bolting failure that could lead to a safety issue or cause an unintended release into the ocean environment. To date, even though there have been fasteners failures, (a summary of recent failures is compiled in Appendix E), and near miss failures, no major oil spills have resulted; the overall bolting failure rate is estimated to be in the range of 10−4 to 10−5 based on the total reported failures divided by the number of all fasteners employed in subsea service.1 This “service record” is highly incomplete as there is no industry wide program to find bolts that are failing, or have failed and are just held in place by gravity. What record we have is clearly the result of fortuitously discovered bolt failures during an inspection on the rig, prior to any pressure test, before a major loss of well control event occurred. There is no industry-wide standard or practice that examines systematically and continuously the condition of in-service unfailed bolts. They are discovered when they fail.
1 K. Armagost, Anadarko Petroleum Corp., “Root Cause Failure Analysis In Support of Improved System Reliability,” presented at Connector Reliability for Offshore Oil and Natural Gas Operations Workshop, National Academy of Sciences, Washington, D.C., April 11, 2017.
Complete bolting failures have been historically rare events, but how many near misses and incipient failures remain undiscovered is unknown.
The committee found multiple opportunities for improvement in the engineering design, specification, manufacture, application of these fasteners, and “womb to tomb” oversight of the fasteners. The overall strategy recommended in this report is one of risk management by continued improvement based on analyzing field conditions and fastener performance, and then acting on the results, such as devising roadmaps to conduct and implement research and development in areas that have the potential to improve the reliability of bolts, enhance the safety culture throughout the entire oil and gas industry, increase human factor performance, and institute wide-spread communication of best practices related to bolts throughout the industry, including its supply chain.
A significant amount of crude oil lies under the continental shelf of the United States. Oil is recovered by drilling and transported to shore using barges, ships and pipelines. Underwater drilling off the U.S. coast began in shallow water in 1896 and has progressed to ever greater depths as the underwater drilling technology has evolved.2 The original underwater drilling placed the derrick resting directly on the seabed floor, but as exploration moved to water depths beyond 1,500 ft. (460 m), new high-technology deep-water drilling techniques were developed. Appendix D summarizes the more than 100-year history of subsea oil exploration in the United States.
The U.S. Department of the Interior began regulating the offshore energy and mineral extraction industry in the late 1940s; its jurisdiction was formalized by the Outer Continental Shelf Lands Act (OCSLA) of 1953. After the April 2010 Deepwater Horizon incident in the Gulf of Mexico, the regulatory structure was changed. The Bureau of Safety and Environment Enforcement (BSEE) was established to provide emphasis on safety, enforcement, prevention of oil releases into the environment, and rapid response in case an oil release does occur. Other agencies are charged with Outer Continental Shelf (OCS) oil and gas lease sales, marine safety, and revenue generation.3
2 American Oil and Gas Historical Society, “Offshore Petroleum History,” http://aoghs.org/offshore-history/offshore-oil-history/, accessed March 13, 2017.
4 Bureau of Ocean Energy Management, “Assessment of Undiscovered Oil and Gas Resources of the Nation’s Outer Continental Shelf,” 2016, https://www.boem.gov/2016-National-Assessment-Fact-Sheet.
The preponderance of oil exploration in the OCS occurs in the Gulf of Mexico.
- In 2015, Gulf of Mexico oil production totaled 584 million barrels (9.3 × 107 m3), accounting for 18 percent of total U.S. crude oil production.5
- In 2015 proved oil reserves6 in the Gulf of Mexico were calculated to be 5 billion barrels (8 × 108 m3) of the 40 billion barrels (6.4 × 109 m3) of known reserves for the United States.7
- A 2016 analysis by the U.S. Bureau of Ocean Energy Management estimated undiscovered technically recoverable resources (UTRR) in OCS to be 90 billion barrels (1.4 × 1010 m3) of oil. The UTRR estimate is generated stochastically based on certain assumptions; the values reported here are the mean of the various estimates. This calculation does not include known oil reserves. The offshore distribution of UTRR is shown in Figure 1.2.
- The Bureau of Ocean Energy Management also estimated the amount of undiscovered economically recoverable resources (UERR), which takes into account price-supply considerations. This analysis indicated that if oil were priced at $40/barrel, the UERR would be 40 billion barrels (6.4 × 109 m3), and if oil were priced at $100/barrel, the UERR would be 70 billion barrels (1.1 × 1010 m3).
5 U.S. Energy Information Administration, “Gulf of Mexico Fact Sheet,” https://www.eia.gov/special/gulf_of_mexico/data.php#petroleum_fuel_facts, accessed April 7, 2018.
6Proved reserves are estimated volumes of hydrocarbon resources that analysis of geologic and engineering data demonstrates with reasonable certainty—that is, a probability of 90% or greater—are recoverable under existing economic and operating conditions.
There remains considerable potential for further exploration and recovery of oil in the OCS; the Gulf of Mexico will continue to be an important location for oil exploration and recovery into the future.
Access to these offshore oil and natural gas resources involves the possibility, however remote, of oil spills and subsequent environmental damage. The environmental consequences of an oil spill are more severe underwater than on land as oil from the leak that is almost impossible to immediately stop or capture, and the water eventually disperses the oil over a larger area. As the water depth increases, the water volume exposed increases. As a result, there is an increased consequence of an accidental oil release as the depth increases. Due to concern about the possibility of oil spills, state and federal governments have passed numerous laws restricting oil exploration.8,9 Examples of recent laws passed by Congress include:
- Marine Protection, Research, and Sanctuaries Act, 1972, which prohibits oil and gas drilling in designated sanctuaries.
- North Carolina Outer Banks Protection Act, 1990, which prohibits oil exploration offshore from North Carolina
- Energy Policy Act of 2005 which prohibits drilling on the Great Lakes
- Gulf of Mexico Energy Security Act of 2006 which bans leasing of tracts for oil exploration until 2022 of portions of the eastern and central Gulf of Mexico.
9 E. Kuhr, “To Drill Or Not to Drill—Debate Over Offshore Testing and Drilling in the Atlantic,” Time, January 14, 2014, http://time.com/3249/to-drill-or-not-to-drill-debate-over-offshore-testing-and-drilling-in-the-atlantic/.
Bolted connections are an integral feature of deep-water wells. In any subsea wellhead and marine riser systems, there are thousands of large bolting components (threaded bolts, studs, and nuts), typically between 5 cm and 9 cm in diameter (between 2.0 and 3.5 in.), used in a wide range of applications. Most fasteners in the subsea riser packages hold together components of the systems used daily to drill the well and transport drilling fluids. A relatively fewer number connectors are directly related to holding together critical well control components or maintaining the well pressure boundary mechanical integrity. Typically, up to two thousand bolts are in critical and non-critical applications on safety equipment, such as a BOP. Critical bolts, those that secure the pressure boundary, will number far less than two thousand.
No activity can be entirely risk-free, with risk defined as the product of likelihood of failure and the potential severity of the consequences.10,11,12,13 Quantitative determination of both factors is required for accurate quantification of risk. For low-probability events, conducting risk assessments requires collecting and assessing a significant amount of data. The financial and environmental cost associated with the potential failure of connectors in off-shore petroleum exploration and production also remains to be quantified.14 But the potential consequences of connector failure in some strategic locations could be extremely large: “Federal regulators . . . warned subsea oil drillers and equipment makers that bolt failures in the Gulf of Mexico could result in an oil spill on the scale of the Deepwater Horizon disaster.”15 Thus, even with a low failure probably, the risk of failure for a least a subset of subsea connectors could be significant.
Fundamentally, a fastener failure results when its service load exceeds its remaining strength. Unfortunately, highly variable service loads and in-service material degradation processes that can reduce a fastener’s strength remain to
10 R. Wilson and E.A.C. Crouch, Risk-Benefit Analysis, Harvard University Press, Cambridge, Mass., 2001.
11 National Research Council, Science and Judgment in Risk Assessment, National Academy Press, Washington, D.C., 1994.
12 D. Ropeik and G. Gray, Risk: A Practical Guide for Deciding What’ Really Safe and What’s Really Dangerous in the World Around You, Houghton Mifflin, New York, N.Y., 2002.
13 Rudolph Frederick Stapelberg, Handbook of Reliability, Availability, Maintainability and Safety in Engineering Design, Springer-Verlag, London, U.K., 2009, see pp. 3-21 and 529-545.
14 Costs can always be quantified after the fact; the challenge is to have a realistic estimate before an incident occurs.
15 T. Mann, “U.S. Regulators Warn Drillers to Find Solution to Subsea Bolt Failures,” Wall Street Journal, August 29, 2016, http://www.wsj.com/articles/u-s-regulators-warn-drillers-to-find-solution-to-subsea-bolt-failures-1472490185.
be well understood. Of those subsea fasteners serving a critical role at a pressure boundary or well control device, the likelihood of failure is directly related to both the spectrum of the service stresses and the mechanical strengths in the given environment, neither of which remains constant over time. Even a poorly designed or manufactured fastener can provide decades of trouble free service if the in-service stresses are relatively low; conversely even a well-designed and properly manufactured fastener can suddenly fail if actual environmental conditions move outside the range anticipated by the design, manufacturing and installation, such as excessive cathodic protection voltage potential.
Qualitatively, to a first order, the risk associated with a pressure boundary connector failure is proportional to its distance from the well head. The most important fasteners attach the BOP stack to the wellhead; those next in importance lie in the BOP stack itself and hold together the components such as the blind shear ram necessary for critical functions in well control, and ultimately blowout prevention. The failure of connectors in the Lower Marine Riser Package (LMRP) could result in drilling fluid or hydrocarbon release, or result in loss of means of well control.
The challenges of doing quantitative risk analysis on subsea connectors are illustrated by the voluntary and proactive recall of more than 10,000 bolts after a failure occurred on a subsea hydraulic connector being used on a BOP. This failure led to a spill of approximately 400 barrels of synthetic drilling fluid in the Gulf of Mexico, in which relative minimal environmental damage occurred. This recall was initiated by the manufacturer, motivated by concern that a potentially impacted bolt could cause another release of drilling fluid. A rough estimate of the cost to the industry of this voluntary recall is in the tens of millions of dollars for the global fleet, with the cost of the bolts estimated to be on the order of $1 million to $2 million. Although post-incident analysis has produced some contributing causes, the root cause of the failure has not yet been definitively determined.16,17,18 Industry-led changes to bolting specifications have been made, and a database containing BOP failure information has been established under the auspices of the International Association of Drilling Contractors (IADC)/International Association of Oil and Gas Producers (IOGP) at the urging of BSEE. This database is expected to provide necessary information to the database subscribers conducting failure analysis studies.
16 BSEE, Evaluation of Connector and Bolt Failures—Summary of Findings, QC-FIT Report #2014-01, Office of Offshore Regulatory Programs, August 2014, https://www.bsee.gov/sites/bsee.gov/files/bolt_report_final_8-4-14.pdf.
17 BSEE, Evaluation of Fasteners Failures—Addendum, QC-FIT Report #2016-04, Office of Offshore Regulatory Programs, February 2016, https://www.bsee.gov/sites/bsee.gov/files/qc-fit-nov-bop-bsr-bolt-report-7282017.pdf.
18 BSEE, Evaluation of Fasteners Failures—Addendum II, QC-FIT Report #006, Office of Offshore Regulatory Programs, July 2017, https://www.bsee.gov/sites/bsee.gov/files/qc-fitnov-bop-bsr-bolt-report-7282017.pdf.
Managing risk for low-probability, high-impact events is quite challenging. The root cause of these events is usually difficult to precisely determine and thus eliminate because they occur so infrequently, and measuring success requires large data samples over an extended period of time. The reality is that it is far more straightforward to count the number of failures than to account for the number of failures that have been avoided through proactive actions.
The management of the risk requires improvements in procedures, materials, controls, inspection and maintenance as well as sharing of best practices among/within the oil and gas industry and the government regulator. These actions require expenditure of time and effort that can be challenging to justify without taking the view that reducing low-probability but very-high-cost events is ultimately cost effective. Appendix F summarizes current proactive activities within the U.S. oil and gas industry to improve the reliability of bolting.
The Outer Continental Shelf Lands Act of 1953 was amended in 1978 to include Section 21(b), which states: “In exercising their respective responsibilities . . . the Secretary of the Department in which the Coast Guard is operating, shall require, on all new drilling and production operations and, wherever practicable, on existing operations, the use of the best available and safest technologies which the Secretary determines to be economically feasible, wherever failure of equipment would have a significant effect on safety, health, or the environment, except where the Secretary determines that the incremental benefits are clearly insufficient to justify the incremental costs of utilizing such technologies.”19 This statutory requirement for determining which best available and safest technology options pass an economic feasibility hurdle is a significant challenge for continually improving the reliability of fasteners.
The remainder of this report reviews the critical aspects of fastener design and demonstrated in-service performance, discusses various strategies to further reduce fastener failures, and concludes with potential new approaches to address fastener design and regulatory strategies. Taken together, these recommendations could be used to construct an industry-government action roadmap, aimed at improving fastener reliability for the most critical subsea applications.
- Chapter 2, “Assessment of Critical Subsea Bolting System Design Elements,” reviews the critical design factors and requirements for subsea fasteners
19 BSEE, Statutory Requirements of OCSLA Regarding the Use of BAST, https://www.bsee.gov/what-we-do/regulatory-safety-programs/statutory-requirements, accessed November 13, 2017.
- and summarizes failure modes. The chapter covers fastener design, fastener material selection, variability in loads on fasteners, safety factors for bolting, lifecycle of a fastener, bolt failure modes, cathodic protection and hydrogen uptake, observed cluster failures of fasteners, and options for improving bolting material properties.
- Chapter 3, “Options for Improving Bolting Reliability,” discusses existing fastener standards and specifications and quality assurance options, and presents options for improving government oversight of the fastener lifecycle.
- Chapter 4, “Safety Culture and Human Systems Integration,” describes how human factors can significantly impact the safety culture in preventing fastener failures.
- Chapter 5, Innovation Opportunities,” describes research and development opportunities that could advance fastener performance and reliability. These opportunities fall into the categories of testing protocols, in-situ measurements, hydrogen assisted cracking, coating technologies, new designs, and human systems integration.
- Chapter 6, “Summary of Recommendations,” reiterates the key conclusions and recommendations contained in the report chapters.
In addition, the appendixes contain a significant amount of information that supplements the discussion in the report.
- Appendix A contains the statement of task from the study sponsor, BSEE, which precipitated the study that led to this report.
- Appendix B maps the statement of task to report chapters.
- Appendix C is a list of the acronyms that are used throughout the report.
- Appendix D is synopsis of the more than 100-year effort of subsea oil exploration in the United States.
- Appendix E is a summary of some subsea bolt failures that have occurred in the past.
- Appendix F summarizes recent activities by the oil and gas industry and BSEE to improve bolting reliability.
- Appendix G provides detail to the discussion of the subsea environmental factors that impact fastener design.
- Appendix H is a summary of bolting regulations and standards.
- Appendix I contains details on drilling riser design and describes the various forces that may eventually be places on connectors and bolts.
- Appendix J describes the many different factors that affect bolting preload and the associated conditions of safety factor analysis.
- Appendix K presents the different failure modes experienced by threaded fasteners.