In This Issue
Fall Issue of The Bridge on Social Sciences and Engineering Practice
September 5, 2012 Volume 42 Issue 3

Lessons from the Macondo Well Blowout in the Gulf of Mexico

Tuesday, August 28, 2012

Author: Raymond Wassel

Since the Macondo well blowout, improvements have been made in management and safety systems and in regulatory regimes.

Commercial deep-water drilling involves highly complex and highly risky operations. Companies must coordinate the operation of sophisticated equipment to construct wells in uncertain geologic formations, often under challenging environmental conditions. Despite the impressive capabilities of the technologies, the selection and application of technologies for constructing a particular well are subject to the unpredictability of human decision making, as they were two years ago in drilling the Macondo well in the Gulf of Mexico. The well blowout and subsequent explosions and fire on the Deepwater Horizon drilling rig on April 20, 2010, led to the deaths of 11 workers and at least a dozen serious injuries. It is estimated that nearly 5 million barrels of hydrocarbons were released into the gulf over a period of nearly three months after the blowout (McNutt et al., 2011).

In December 2011, the National Academy of Engineering (NAE) and National Research Council (NRC) released Macondo Well–Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety, a report prepared in response to a request from Secretary of the Interior Ken Salazar.1 The authoring committee of eminent engineers and scientists found that a number of flawed decisions had led to the blowout and explosions, indicating a lack of effective safety management among the companies involved in the disaster. This article provides an overview of the committee’s findings and observations related to the incident and a summary of the recommendations for reducing the likelihood and effects of future well blowouts resulting from offshore drilling.

Challenges in Constructing Offshore Wells

When drilling a well into geologic formations beneath the seabed, rig personnel on the surface pump dense drilling mud into the wellbore to counteract pressure exerted by hydrocarbons or saline water in the pore space of the rocks (referred to as pore pressure). If the pressure exerted by the drilling mud is lower than the pore pressure, fluids may flow into the wellbore (an event known as a “kick”). If the kick is not brought under control, a well blowout can occur.

Because pore pressure usually increases with drilling depth, increases in mud weight may be necessary to prevent kicks as the well is drilled. However, if the pressure exerted by the drilling mud in the wellbore becomes too great, it can cause a fracture in the exposed rock at any point in the wellbore. Drilling mud would then flow from the wellbore into the fracture and could no longer exert sufficient pressure to prevent an influx of reservoir fluids. Like pore pressure, the pressure at which a fracture occurs usually increases with drilling depth, although actual pressures can be either higher or lower than anticipated (Bommer, 2008; Maclachlan, 2007; Zoback, 2010).

If drilling mud alone cannot prevent fluids from entering the well, the rig crew installs steel casing into the wellbore and pumps a cement slurry into the casing. The slurry flows into the annular space outside the casing to seal the bottom portion so fluids from the geologic formations cannot flow up the annulus. Once the cement hardens, the weaker formations above the end of the casing will not be exposed to higher pressures from denser drilling mud as the well is drilled deeper.

If a sufficient amount of hydrocarbons is found and assessments have been completed, exploratory wells are readied for temporary abandonment, typically by sealing them with cemented liners or casings to ensure that there is no hydrocarbon flow. Plugs are also used to provide additional barriers to hydrocarbon flow. Temporary abandonment is a standard practice that allows the operating company to use the expensive drilling rig for activities elsewhere and time to install the infrastructure necessary to transport recovered hydrocarbons to shore.

The Macondo Well

In 2008, BP obtained a 10-year lease to explore and develop a part of the Mississippi Canyon region of the Gulf of Mexico. The lease was owned by BP (65 percent), Anadarko Petroleum (25 percent), and MOEX Offshore (10 percent). The Macondo well was drilled in the leased area approximately 50 miles off the coast of Louisiana to evaluate the potential for producing hydrocarbons and to survey the geologic structures associated with the hydrocarbon reservoirs. As the operating company, BP was responsible for carrying out operations. BP had leased a rig (called Deepwater Horizon) from Transocean, to drill the well.

The Macondo well was drilled to a total depth of more than 18,300 feet below sea level, which included an ocean depth of more than 5,000 feet. The final section of the well passed through several reservoir zones that exhibited decreasing pore pressures with depth.

Figure 1

As shown in Figure 1, the pore pressure decreased from about 14.1 pounds per gallon (ppg)2 in a saltwater-bearing reservoir to about 12.6 ppg in the deepest hydrocarbon-bearing reservoir. Therefore, the drilling mud weight had to be at least 14.1 ppg to prevent the flow of hydrocarbons from the saltwater reservoir. However, the fracture mud weight near the bottom of the Macondo well was only slightly greater than 14.2 ppg (BP, 2010). Under these conditions, there was a very small margin of safety between the fracture mud weight and the mud weight necessary to avoid flow into the well (Figure 2).

Figure 2

The method selected for preparing the well for temporary abandonment was to insert a long-string production casing and cement it in place; the casing extended from the seafloor to the bottom of the well (BP, 2010; Transocean, 2011). In general, cementing operations are potentially problematic because, unless they are carried out properly, channels for fluid flow can form in the annular region outside the casing. In addition, if the pressure exerted by the cement slurry as it is pumped down into the well exceeds the fracture pressure, cement can flow into the fractured formation instead of remaining in the wellbore where it would eventually act as a barrier against high-pressure reservoirs.

To keep from exceeding the formation fracture pressure during the cementing process, the Macondo job was designed to pump a sequence of different slurry mixtures, which included a volume (a term of art that means a specific amount) of low-density cement between two volumes of high-density cement, one intended to fill the bottom section of the casing and one above the volume of low-density cement.

The low-density cement slurry prepared on the rig would have a density of 14.5 ppg when placed at the well bottom where conditions were about 245°F and more than 13,000 pounds of pressure per square inch (psi). To meet that specification, the slurry density was decreased to 6 ppg by adding nitrogen gas to base cement to allow for the substantial compression and heating that would occur as the foamed cement was pumped more than 18,000 feet to the bottom of the well.

The committee found that, because of the great difference in density between the two types of cement used in the pumping sequence, the heavy un-foamed cement pumped in after the foamed cement probably fell into, and perhaps through, the foamed slurry. “This would have had the unintended effect of leaving a tail slurry containing foamed cement in the shoe track at the bottom of the casing rather than leaving the heavy, un-foamed tail cement” (NAE/NRC, 2012, p. 31).

Beginning of the Hydrocarbon Flow That Led to the Blowout

After completing the cementing operation, rig personnel conducted a negative pressure test to assess the integrity of the cemented casing. This is a standard technique that involves intentionally reducing the pressure exerted by drilling mud inside the well to a point below the reservoir pore pressure by displacing some of the drilling mud with less-dense seawater. If the cement has formed an effective barrier, there will be no inflow or pressure buildup during the test.

Several of the negative pressure tests performed on the Macondo well had inconclusive and confusing results (BP, 2010; Transocean, 2011). Nevertheless, even though the negative pressure tests did not demonstrate the integrity of the cement job, rig personnel interpreted the results as indicating that the barriers were effective and proceeded with the temporary abandonment process by continuing to displace the drilling mud with seawater. The report committee concluded, “This was but one of a series of questionable decisions in the days preceding the blowout that had the effect of reducing the margins of safety and that evidenced a lack of safety-driven decision making” (NAE/NRC, 2012, p. 27).

Because the cement and mechanical barriers did not have sufficient integrity, hydrocarbons began to flow into the well. However, personnel on the Deepwater Horizon did not detect the flow until approximately 9:40 p.m., when mud was observed flowing uncontrolled onto the rig floor. At that point, actions were taken to reestablish control over the well, but they were unsuccessful. Flammable gas alarms on the rig sounded at approximately 9:47, followed by two explosions at approximately 9:49 and then a massive fire.

Blowout Preventer System

The blowout preventer (BOP) system is intended to control a well if hydrocarbons unintentionally flow beyond the primary barriers during drilling or other operations. The system can be activated from the rig to seal an open wellbore, close the annular portion of the well around the drill pipe or casing, or cut through the drill pipe and then seal the well.

The BOP system for the Deepwater Horizon (Figure 3) was located at the wellhead on the seafloor. A riser pipe extended from the top of the system to the rig platform so that drilling fluids could circulate between the well and the rig. The uppermost of the five rams of the BOP stack was a blind shear ram (BSR) (see lower half of illustration in Figure 3). The BOP system also had an emergency disconnect system that would enable the rig to move away from the well after the BSR was activated.

Figure 3

As a last resort in a hierarchy of well-control strategies, the two opposing blades of the BSR were designed to slice through the drill pipe and seal the well.3 At the time of the Macondo blowout, rig personnel could not regain control of the well by using the BOP because the BSR did not cut the drill pipe and seal the well. In addition, the emergency-disconnect system failed to separate the Deepwater Horizon from the well. “The BOP system was neither designed nor tested for the dynamic conditions that most likely existed at the time that attempts were made to recapture well control. Furthermore, the design, test, operation, and maintenance of the BOP system were not consistent with a high-reliability, fail-safe device” (NAE/NRC, 2012, p. 71).

The committee also found that although regulations were in place requiring recurrent testing of BOP systems, the testing was not required under conditions that simulated the hydrostatic pressures at which the BOPs were expected to function, or under dynamic flow conditions that could entrain rocks, sand, and cement, thus making cutting a drill pipe more difficult (NAE/NRC, 2012, p. 71).

Nevertheless, there were numerous warnings about potential failures of BOP systems in use prior to the Macondo blowout in reports commissioned by industry operators and regulatory organizations (e.g., West Engineering Services, Inc., 2002, 2004). According to the NAE/NRC committee, “It appears that there was a misplaced trust by responsible government authorities and many industry leaders in the ability of the BOP to act as a fail-safe mechanism” (NAE/NRC, 2012, p. 52).

Industry Management of Offshore Drilling

Companies that conduct offshore drilling operations in the United States assemble a team that includes drilling contractors, service companies, and consultants, each of whom contributes a different type of technical expertise. This complex structure and division of expertise presents a challenge to management in the oil and gas industry, which must conduct an integrated assessment of the margins of safety for a complex well such as Macondo. Various investigations have found that technical failures that occurred while drilling the Macondo well can be traced to inadequate management processes (BOEMRE, 2011; Chief Counsel, 2011; DHSG, 2011; National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, 2011; USCG, 2011).

Based on presentations by industry representatives, the NAE/NRC committee observed that companies involved in offshore drilling tend not to use a comprehensive systems approach for addressing the many safety aspects associated with their operations. One indicator of the lack of an overall systems approach is the limited amount of system safety training provided to personnel (NAE/NRC, 2012, p. 96).

The NAE/NRC report committee found that the actions, policies, and procedures of the companies involved with the Macondo well–Deepwater Horizon disaster did not provide an effective systems approach to safety commensurate with the risks of the well-drilling operation. The lack of a strong safety culture is evident in the multiple flawed decisions that led to the blowout. As the committee stated, “industry management involved failed to appreciate or plan for the safety challenges presented by the Macondo well (NAE/NRC, 2012, p. 101).

Regulation of Offshore Drilling

The Minerals Management Service (MMS) of the U.S. Department of the Interior, the lead regulator at the time of the Macondo blowout, approved the exploration plans for the lease, applications for the permit to drill the well, and subsequent changes to the well drilling plan listed in modifications submitted by BP. The general approach used by MMS for the regulation of offshore drilling was largely prescriptive, that is, the agency developed specific requirements for equipment and procedures and then audited drilling operations to assess compliance with those requirements.

However, using this approach, MMS was unable to keep up with the rapid technological advances in the past few decades that had enabled a large increase in deep-water drilling in the Gulf of Mexico. The agency’s problems were exacerbated by stagnant or decreasing levels of funding and technical staffing over that same period (National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling, 2011).

The NAE/NRC committee found that MMS did not comprehensively assess the risks presented by the well-drilling operation for the Macondo well: “The actions of the regulators did not display an awareness of the risks or the very narrow margins of safety” (NAE/NRC, 2012, p. 113). A rigorous review of test data from the Macondo drilling operation may have revealed that the barriers within the well had not adequately isolated hydrocarbon-bearing formations from the wellbore.

The Macondo well blowout demonstrated the need for a proactive approach to offshore drilling in U.S. waters that integrates all aspects of operations that could affect occupational and system safety (see Leveson, 2011; Rasmussen, 1997; and Rasmussen and Svedung, 2000, for discussions of systems safety). The U.S. Department of the Interior has taken an important first step in that regard by instituting safety and environmental management systems (SEMS) in 30 CFR 250 (Federal Register, Vol. 75, No. 199, Oct. 15, 2010). However, there is still a long way to go before the regulatory system for offshore drilling in the United States achieves the necessary capabilities.

Improving Safety for Offshore Drilling

The committee developed recommendations for industry and regulators, identifying measures that would decrease the likelihood and mitigate the effects of future blowouts. The following paragraphs summarize the committee’s major recommendations.

Because operating companies are the only ones that can view and control all aspects of well integrity, they should have ultimate responsibility and accountability for well design and well construction, as well as for assessing the suitability of the drilling rig and safety equipment. The contractor, who is hired by the operating company to drill the well, should be held responsible and accountable for the safe operation of the offshore equipment. The companies that share an offshore drilling lease should ensure that the operating company conducts activities in a way that keeps risk as low as is reasonably practicable.

As drilling operations are carried out and wells are made ready for temporary abandonment, there are safety-critical points (e.g., determining the integrity of cemented barriers placed in the well) at which poor decisions are likely to increase hazards. Guidelines should be established for incorporating adequate margins of safety into the operating company’s approach to well design.

To improve regulatory effectiveness, the SEMS regulatory program should be expanded to a goal-oriented risk management system that incorporates explicit regulatory review and approval of the safety-critical points in the drilling operation. As offshore drilling operations proceed into deeper waters, the Bureau of Safety and Environmental Enforcement (BSEE)4 and other regulators should identify the safety-critical points that warrant explicit regulatory review and approval before operations can proceed. BSEE should have knowledgeable personnel in place to provide meaningful reviews.

BOP systems should be redesigned, rigorously tested, and maintained to operate reliably under all foreseeable conditions in which they may be deployed. Proper training in the use of these systems in the event of an emergency is also essential. Instrumentation and expert system decision aids should be integrated into the offshore drilling unit to provide personnel with timely warnings of a loss of well control. If the warning is inhibited by personnel or not addressed quickly enough, autonomous operation of the BSRs and other safety systems on the rig should go into effect automatically.

Ensuring sufficient capabilities to mitigate the consequences of a hydrocarbon blowout is an important objective. Two initiatives (the Marine Well Containment Company and the Helix Well Containment Group) have established systems with the capability of containing an underwater well blowout in the U.S. Gulf of Mexico. Industry and other organizations should support further development of these systems.

Industry and regulators should significantly increase the formal education and training in implementing safety systems provided to offshore drilling personnel. Systems should be designed to support the anonymous reporting of safety-related incidents, and companies should investigate each report and convey lessons learned to their personnel and to other companies. Industry should also increase its R&D on improving the safety of offshore drilling (well design, equipment, human operational failures, and management approaches).


Since the Macondo well–Deepwater Horizon incident, some important improvements have been made in the management and safety systems used by companies engaged in offshore oil development and in the regulatory regime. We can be hopeful that these changes are not transitory but signal the beginning of a sustained effort to instill a culture of safety in the industry and government.


BOEMRE (Bureau of Ocean Energy Management, Regulation, and Enforcement). 2011. Report Regarding the Causes of the April 20, 2010 Macondo Well Blowout. Available online at

Bommer, P. 2008. A Primer of Oilwell Drilling, 7th ed. Austin: University of Texas at Austin.

BP. 2010. Deepwater Horizon. Accident Investigation Report. Available online at english/gom_response/STAGING/local_assets/downloads_pdfs/ Deepwater_Horizon_Accident_Investigation_Report.pdf.

Chief Counsel. 2011. Macondo: The Gulf Oil Disaster. Chief Counsel’s Report, National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling. Available online at C21462-408_CCR_for_web_0.pdf.

DHSG (Deepwater Horizon Study Group). 2011. Final Report on the Investigation of the Macondo Well Blowout. Available online at DHSGFinalRepor t-March2011-tag.pdf.

DNV (Det Norske Veritas). 2011. Forensic Examination of Deepwater Horizon Blowout Preventer, Vols. I and II (Appendices). Final Report for U.S. Department of the Interior, Bureau of Ocean Energy Management, Regulation, and Enforcement, Washington, D.C. Report No. EP030842. Available online at, and BOP report - Vol 2 (2).pdf.

Leveson, N.G. 2011. Engineering a Safer World: System Thinking Applied to Safety. Cambridge, Mass.: Massachusetts Institute of Technology Press.

Maclachlan, M. 2007. An Introduction to Marine Drilling. Ledbury, England: Oilfield Publications.

McNutt, M., R. Camilli, G. Guthrie, P. Hsieh, V. Labson, B. Lehr, D. Maclay, A. Ratzel, and M. Sogge. 2011. Assessment of Flow Rate Estimates for the Deepwater Horizon / Macondo Well Oil Spill. Flow Rate Technical Group Report to the National Incident Command, Interagency Solutions Group, March 10. Available online at getfile&Pageid=237763.

NAE/NRC (National Academy of Engineering/National Research Council). 2012. Macondo Well–Deepwater Horizon Blowout: Lessons for Improving Offshore Drilling Safety. Washington, D.C.: The National Academies Press. Available online at

National Commission on the BP Deepwater Horizon Oil Spill and Offshore Drilling. 2011. Deep Water: The Gulf Oil Disaster and the Future of Offshore Drilling. Report to the President. Available online at DEEPWATER_ReporttothePresident_FINAL.pdf.

Rasmussen, J. 1997. Risk management in a dynamic society: A modeling problem. Safety Science 27(2/3): 183–213.

Rasmussen, J., and I. Svedung. 2000. Proactive Risk Management in a Dynamic Society. Karlstad, Sweden: Swedish Rescue Services Agency. Available online at

 Transocean. 2011. Macondo Well Incident. Transocean Investigation Report Vols. I and II (Appendices). Available online at

USCG (U.S. Coast Guard). 2011. Report of Investigation into the Circumstances Surrounding the Explosion, Fire, Sinking and Loss of Eleven Crew Members Aboard the Mobile Offshore Drilling Unit Deepwater Horizon in the Gulf of Mexico April 20–22, 2010, Vol. I. Available online at

West Engineering Services, Inc. 2002. Mini Shear Study for U.S. Minerals Management Service. Requisition No. 2-1011-1003. Dec. Available online at Report.pdf.

West Engineering Services, Inc. 2004. Shear Ram Capabilities Study for U.S. Minerals Management Service. Requisition No. 3-4025-1001. Sept. Available online at West Engineering Final Report.pdf.

Zoback, M.D. 2010. Reservoir Geomechanics. Cambridge, U.K.: Cambridge University Press.



 1 The final version of the report was published in 2012 and is available at A list of committee members can be found in the front matter of the report.

 2 For drilling operations, pore pressure and fracture pressure are commonly expressed as the density of a fluid in the wellbore (in units of pounds per gallon) that would be required to exert an equivalent amount of pressure.

 3 One of the BSR blades had a straight edge; the other was V-shaped. This design is considered to provide inferior shearing performance compared to a configuration with two V-shaped edges (West Engineering Services, Inc., 2004).

 4 In May 2010, Interior Secretary Salazar restructured MMS by reassigning its responsibilities to two newly formed bureaus. The bureau eventually named the Bureau of Safety and Environmental Enforcement (BSEE) is the federal entity now responsible for safety and environmental oversight of offshore oil and gas operations.

About the Author:Raymond Wassel is a senior program officer, Board on Environmental Studies and Toxicology, National Research Council.