During drilling and casing installation, a BOP system is used. In an emergency situation, the BOP system can be activated to seal an open well, close the annular portion of the well around the drill pipe or casing, or cut through the drill pipe with steel shearing blades and then seal the well.
These systems are designed to provide redundant control of the well and prevent unwanted flows from the reservoirs. The integrity of the barriers can be evaluated by pressure tests and by taking measurements with various instruments logging. If there is a delay between finishing drilling operations and commencing completion operations, the well is temporarily abandoned by setting mechanical or cement plugs inside the casing. The Macondo well-Deepwater Horizon incident on April 20, , was not the first major blowout associated with offshore drilling Presidential Commission Staff Past incidents involving blowouts include the following:.
The ultimate release of oil amounted to between 80, and , barrels Kallman and Wheeler A failure to keep the hydrostatic pressure in the well greater than the pore pressure resulted in the flow of hydrocarbons into the well. Attempts to control the well led to blowouts in the immediate surrounding area through several breaches in the geologic formation that extended up through the mud line County of Santa Barbara The formation at the bottom of the well was fractured, causing the loss of mud.
Hydrostatic pressure for control of the well was lost after the drill string was pulled out of the borehole. The BOP failed to secure the well because the thick, large-diameter drill collars were inside the BOP stack and prevented the shear rams from cutting the pipe and the pipe rams from closing around the large-diameter pipe.
The cement in the well and the float equipment failed to prevent flow from the reservoir into the casing. When the temporary well cap was removed to begin completion operations, the BOP was not installed. This left the well open and flow began from the reservoir, eventually reaching the surface where it could not be controlled. The operator estimated that barrels of crude oil were lost per day. The uncontrolled release continued until November 3, , and response operations continued until December 3, Several other major accidents associated with offshore drilling and production that stemmed from causes other than a well blowout are discussed in Chapters 5 and 6.
As the lease operator, BP was the company responsible for carrying out the operations. In addition, because of the well conditions, BP submitted Applications for Permit to Modify that were approved by MMS at various points during the drilling program.
Drilling was halted about a month later on November 8 as the Marianas was secured and evacuated for Hurricane Ida. The Marianas was subsequently removed after sustaining hurricane damage that required dock repairs. After the repairs, the rig was not returned to drill the Macondo well.
The Deepwater Horizon was selected in January to finish drilling the Macondo well. The rig was owned and operated by Transocean and had been under contract to BP in the Gulf of Mexico for approximately 9 years. Subsequent activities leading up to the blowout, explosions, and fire are discussed in the following chapters of this report. Offshore drilling is a safety-critical process that warrants a safety system commensurate with the overall risk presented.
In that light, the committee considered key factors and decisions that may have contributed to the blowout of the Macondo well, including engineering, testing, and maintenance procedures; operational oversight; regulatory procedures; and personnel training and certification. The committee examined the extent to which there were margins of safety to allow for uncertainties in the interactions of equipment, humans, procedures, and the environment under normal and adverse conditions.
The committee developed overall findings of fact related to the incident, observations concerning contributing factors, and recommendations intended to reduce the likelihood and impact of any future well control incidents. The committee also presented more detailed findings, observations, and recom-. To identify causative factors for the blowout, the committee examined the design of the Macondo well, the processes for developing the well design and for making subsequent changes, and the construction of the well.
Particular attention was given to the reported narrow range between pore pressure and fracture gradient BP because of the challenges this presents. Attention was also given to the approach selected to temporarily abandon the well given these conditions. A number of key decisions related to the design, construction, and testing of the barriers critical to the temporary abandonment process were examined and found to be flawed. Recommendations for achieving a more robust approach to implementing and verifying the needed barriers are provided see Chapter 2.
Once the rig crew realized that hydrocarbons were flowing into the well, the BOP system did not recapture well control. The committee tracked the forensic analysis of the BOP arranged by the Marine Board of Investigation 9 and considered key factors that affected the performance of the BOP system during the blowout.
The committee also considered the findings of past evaluations of the reliability of BOP systems under real-world conditions. Chapter 3 reports on the extent to which the design, testing, and maintenance of the Deepwater Horizon BOP system were commensurate with a high-reliability fail-safe mechanism within an overall safety system.
However, there were concerns that aspects of the rig design and operation may have contributed to the. Furthermore, the loss of the rig may have limited options for recapturing control of the well. Overall, neither the companies involved nor the regulatory community has made effective use of real-time data analysis, information on precursor incidents or near misses, or lessons learned in the Gulf of Mexico and worldwide to adjust practices and standards appropriately. On the basis of its investigation of the Macondo well— Deepwater Horizon disaster and discussions with industry operating in the United States and the North Sea and with regulators from the United States, the Republic of the Marshall Islands, Australia, the United Kingdom, and Norway, the committee has developed a series of recommendations that it believes would materially improve the safety of future operations in the Gulf of Mexico.
Given the critical role that margins of safety play in maintaining well control, guidelines should be established to ensure that the design approach incorporates protection against the various credible risks associated with the drilling and completion processes.
Recommendation 2. All primary cemented barriers to flow should be tested to verify quality, quantity, and location of cement. The integrity of primary mechanical barriers such as the float equipment, liner tops, and wellhead seals should be verified by using the best available test procedures. All tests should have established procedures and predefined criteria for acceptable performance and should be subject to independent, near-real-time review by a competent authority.
BOP systems should be redesigned to provide robust and reliable cutting, sealing, and separation capabilities for the drilling environment to which they are being applied and under all foreseeable operating conditions of the rig on which they are installed. Test and maintenance procedures should be established to ensure operability and reliability appropriate to their environment of application. Furthermore, advances in BOP technology should be evaluated from the perspective of overall system safety.
Operator training for emergency BOP operation should be improved to the point that the full capabilities of a more reliable BOP can be competently and correctly employed when needed in the future. Recommendation 3. Instrumentation and expert system decision aids should be used to provide timely warning of loss of well control to drillers on the rig and ideally to onshore drilling monitors as well.
If the warning is inhibited or not addressed in an appropriate time interval, autonomous operation of the blind shear rams, emergency disconnect system, general alarm, and other safety systems on the rig should occur. Recommendations 3. Efforts to reduce the probability of future blowouts should be complemented by capabilities of mitigating the consequences of a loss of well control.
Industry should ensure timely access to demonstrated well-capping and containment capabilities. Recommendation 5. The United States should fully implement a hybrid regulatory system that incorporates a limited number of prescriptive elements into a proactive, goal-oriented risk management system for health, safety, and the environment. Recommendation 6. Department of the Interior and other regulators should identify and enforce safety-critical points during well construction and abandonment that warrant explicit regulatory review and approval before operations can proceed.
A single U. Operating companies should have ultimate responsibility and accountability for well integrity, because only they are in a position to have visibil-. Operating companies should be held responsible and accountable for well design, well construction, and the suitability of the rig and associated safety equipment.
Notwithstanding the above, the drilling contractor should be held responsible and accountable for the operation and safety of the offshore equipment. Recommendations 5. Such efforts should encompass well design, drilling and marine equipment, human factors, and management systems. These endeavors should be conducted to benefit the efforts of industry and government to instill a culture of safety. Industry, BSEE, and other regulators should undertake efforts to expand significantly the formal education and training of personnel engaged in offshore drilling to support proper implementation of system safety.
Industry, BSEE, and other regulators should improve corporate and industrywide systems for reporting safety-related incidents.
Corporations should investigate all such reports and disseminate their lessons-learned findings in a timely manner to all their operating and decision-making personnel and to the industry as a whole. A comprehensive lessons-learned repository should be maintained for industrywide use. This information can be used for training in accident prevention and continually improving standards.
Industry, BSEE, and other regulators should foster an effective safety culture through consistent training, adherence to principles of human factors, system safety, and continued measurement through leading indicators. On the basis of the available evidence, the committee has identified the principal causes of the incident, as summarized above and described in the report in greater detail. The plan included drilling and temporary abandonment of two exploration wells over the prospect.
Deepwater Horizon caught fire when it was drilling at Macondo in April The rig sank into water after the accident and is currently lying over the sea floor, around 1,ft northwest of the well centre and away from subsea pipelines. At the time of the accident, it was located at 52 miles southeast of the Louisiana port of Venice. The operator, US Coast Guard and other agencies are engaged in the execution of an oil spill response plan to recover the oil.
A regional shallow hazards survey and study was carried out at the project area by KC Offshore in Mapping of the block was carried out by BP America in and The rig is a fifth-generation deepwater semi-submersible rig and is owned by Transocean.
The construction of rig started in and was completed in In a 2-D view, this implies flow from Galapagos towards Macondo. It is well recognized that in many basins, regionally connected high-permeability aquifers at a nearly constant overpressure are encased in low-permeability overpressured mudstone such as is illustrated here in the M56 21 , 22 , 23 , A key question is, what controls the aquifer overpressure in these systems?
One common interpretation is that there is a leak point where the aquifer pressure equals the least principal stress. At this leak point, the pore pressure bleeds off through fractures and the aquifer pressure is fixed to the least principal stress 19 , Cooler colors indicate lower overpressure and warmer colors indicate higher overpressure. Arrows are normal to overpressure contours and record the flow direction of pore water within mudstone.
The black dashed line approximates the flow divide: pore water flows upward above this line and downward below it. The vertical axis shows depth increasing relative to the seafloor. White lines approximate mudstone pore pressures at each well location and become dashed below well control.
Key locations i—v : i M Macondo ii M Galapagos iii M deepest mapped depth below seafloor iv Top of Macondo pore pressure regression contour reversal v Potential leak-off point subsalt Fig. Overpressure calculations use a hydrostatic gradient of 0. The pore pressure regression hindered the drilling and temporary abandonment of the Macondo well. To illustrate this, we express the downhole pressures and stresses with an equivalent mud weight EMW or density plot see Methods section Fig.
Within the exposed borehole, a single mud weight is used to maintain the borehole pressure 1 below the fracture pressure to avoid the loss of drilling fluid mud through fractures into the formation and 2 above the pore pressure to prevent flow from the formation into the borehole. The difference between the equivalent mud weight necessary to cause fractures anywhere in the exposed borehole and the EMW that equals the formation pressure anywhere in the exposed borehole is the drilling window.
Along this segment, the drilling window was extremely narrow Fig. Lost mud events record the lower and upper bounds of the fracture pressure brown triangles, see Methods ; the formation integrity test FIT, brown square records a lower bound of the fracture pressure.
The APD is the annular pressure while drilling as recorded on the drill string. The MW brown line records the static pressure from drilling mud weight measured at surface conditions. To prevent influx of M57 pore fluids c , green arrows , the static borehole pressure had to be kept above However, to avoid fracturing the M56 c , brown arrows , the dynamic pressure had to be kept below The zone in orange shows the range of pressures that had to be maintained the drilling window.
The two gray lines represent the static pressure that would be induced by a foamed cement left, Caliper measurements record borehole shape. Cement is pumped through the bottom of the casing and up the annulus. White circles differentiate the foamed tail cement from the traditional unfoamed lead and shoe cement pumped before and after. Arrows indicate flow direction if the exposed borehole pressure deviates from the operating window green, hydrocarbon kick; brown, mud loss.
This narrow drilling window created challenging drilling conditions. Gas flowed into the well from the M57 Fig. Furthermore, on three occasions, mud was lost into the formation Fig. In fact, these events constrain the drilling window. The two mud-loss events into the M56 document a lower fracture pressure within this interval than in the upper half of the well segment Fig. This drop in fracture pressure least principal stress is most likely a result of the reduced pore pressure, but could also be due to different mechanical properties in sands relative to mudstones The narrow drilling window impacted the approach used to cement the production casing in place.
To maintain the pressures along the cement column within the drilling window Fig. The particular foam cement mixture was shown to be unstable during testing prior to and after the blowout 28 , The Macondo well penetrates a complicated hydrogeologic system. A sedimentary section of near lithostatic pressures overlies the lower pressured M56 sand, the exploration target Fig.
To drill and produce this hydrocarbon target required a delicate balance to keep the borehole pressure above the pore pressure present and below the fracture pressure. The technical challenges associated with drilling and cementing this complicated hydrodynamic system contributed to the ultimate blowout of the Macondo well. The overburden stress is calculated by integrating the weight of the water column and the weight of the overlying sediment.
We combine density log data from nearby wells in portions of the Macondo well where no density data were acquired. Logs are corrected to account for borehole washout and for the presence of hydrocarbons. Where no density data are available, a velocity-to-density transform is used If neither density nor velocity data are present, an exponential interpolation between density above and below the interval is used Industry routinely measures pore pressure and takes fluid samples from relatively permeable formations with wireline tools e.
These MDT measurements are corrected to the Macondo well location assuming continuous stratigraphy parallel to the seafloor We also constrain pore pressure from fluid influxes into the borehole kicks and elevated gas levels detected in the incoming drilling mud. Kicks and high gas occur when pore pressure exceeds hydraulic pressure from the drilling fluid in the exposed borehole.
Six such events occurred during drilling operations Figs 2 , 3 and 5 , open triangles. Using drilling information prior-to, during, and after an event, we estimate the location and pore pressure. Drilling information includes the location of sandstones, length of exposed borehole, gas content of the incoming mud, surface mud weight, equivalent static density, equivalent circulating density, and shut-in drill pipe pressure.
The equivalent mud weight is another way of expressing pressure using the average density of the drilling fluid from the drill floor to a location in the borehole. The equivalent static density is the downhole pressure expressed as an equivalent mud weight when the mud pumps are off and thus, there is no circulation. The equivalent circulating density is the downhole pressure expressed as equivalent mud weight as while the drilling fluids circulate.
The circulating density is greater than the equivalent static density because of friction caused by fluid circulation. The fracture pressure is the borehole pressure necessary to hydraulically fracture the formation. It is commonly close to the regional least principal stress but can be affected by stress perturbations due to the borehole geometry and the cohesive strength of the rock.
The downhole static and dynamic drilling pressures leading up to, during, and after each lost mud event are used to bracket the fracture pressure interpretations Fig. We define the upper bound of the fracture pressure with the equivalent circulating density when the losses began and the lower bound from the highest static or dynamic pressure at which the well is stable before or after the loss event see ref.
It is generally accepted that the in-situ stress of mudstone is higher than that of sandstone 25 , so the loss location is assumed to occur in the sandstone nearest to the bit at the time of the loss event. After drilling out of the cemented liner shoe, pressure on the exposed formation was increased to above overburden stress without experiencing fluid loss.
This test result provides further evidence that the subsequent losses occurred deeper, in the M56 reservoir interval. Rapid deposition of this low permeability material is the primary source of overpressure in the Gulf of Mexico It is not practical to directly measure the pressure within these low permeability mudstones.
Instead, mudstone pore pressure is commonly estimated from the compaction state porosity of the rock, which is typically measured by resistivity, density, or velocity 34 , In deepwater Gulf of Mexico Neogene sediments, pore pressure is not accurately described by a single compaction curve.
This is because deeper, hotter, and older mudstones have undergone more compaction than shallower mudstones at the same effective stress. More illitic material has a lower porosity at a given effective stress than a more smectitic material 39 , We follow ref. The left side of Eq.
The right side of Eq. We calibrate the model by determining the effective stress within mudstones adjacent to where pressure has been measured in sandstones. Next, we determine the mudstone porosity at each location from the velocity log after 42 :.
The shallow locations with cooler in-situ temperatures have a higher porosity for a given effective stress than the deeper and warmer locations Fig. Mudstone porosity vs. Color-coded symbols denote in-situ temperature for each mudstone porosity-effective stress calibration point. The points are corrected for clay-bound water porosity open symbols and then are used to calibrate Eq. Porosity is estimated from velocity Eq. Based on these assumptions, the clay-bound water porosity is:. We then use least-squares regression to constrain Eq.
We apply this method calibrated at Macondo to estimate the mudstone pressure at Fig. The close match between the estimated mudstone pressures and the measured sandstone pressures, independent of local calibration, supports the accuracy of our method within this region.
Mudstone sonic porosities are similar in both wells, but the temperature gradients are different. The Macondo well has an average temperature gradient of
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