An Interim Report on causes of the Deepwater Horizon oil rig blowout and ways to prevent such events by the committee for the analysis of causes of the Deepwater Horizon explosion, fire, and oil spill by the National Academy of Engineering; National Research Council mentions the following:
"1.The incident at the Macondo well and Deepwater Horizon MODU was precipitated by the decision to proceed to temporary abandonment of the exploratory well despite indications from several repeated tests of well integrity [the test type known as a negative (pressure) test] that the cementing processes following the installation of a long-string production casing failed to provide an effective barrier to hydrocarbon flow (Sections II and III).
2. The impact of the decision to proceed to temporary abandonment was compounded by delays in recognizing that hydrocarbons were flowing into the well and riser and by a failure to take timely and aggressive well-control actions. Furthermore, failures and/or limitations of the BOP, when it was actuated, inhibited its effectiveness in controlling the well (Sections III and IV).
3. The failures and missed indications of hazard were not isolated events during the preparation of the Macondo well for temporary abandonment. Numerous decisions to proceed toward abandonment despite indications of hazard, such as the results of repeated negative-pressure tests, suggest an insufficient consideration of risk and a lack of operating discipline. The decisions also raise questions about the adequacy of operating knowledge on the part of key personnel. The net effect of these decisions was to reduce the available margins of safety that take into account complexities of the hydrocarbon reservoirs and well geology discovered through drilling and the subsequent changes in the execution of the well plan (Section VI).
4. Other decisions noted by the committee that may have contributed to the Macondo well accident are as follows:
• Changing key supervisory personnel on the Deepwater Horizon MODU just prior to critical temporary abandonment procedures (Section VI);
• Attempting to cement the multiple hydrocarbon and brine zones encountered in the deepest part of the well in a single operational step, despite the fact that these zones had markedly different fluid pressures (because of the different fluid pressures, there was only a small difference between the cement density needed to prevent inflow into the well from the high-pressure formations and the cement density at which an undesirable hydraulic fracture might be created in a low pressure zone) (Section II);
• Choosing to use a long-string production casing in a deep, high-pressure well with multiple hydrocarbon zones instead of using a cement liner over the uncased section of the well (Section II);
• Deciding that only six centralizers would be needed to maintain an adequate annulus for cementing between the casing and the formation rock, even though modeling results suggested that many more centralizers would have been needed (Section II);
• Limiting bottoms-up circulation of drilling mud prior to cementing, which increased the possibility of cement contamination by debris in the well (Section II);
• Not running a bond log after cementing to assess cement integrity in the well, despite the anomalous results of repeated negative-pressure tests (Section II);
• Not incorporating a float shoe at the bottom of the casing as an additional barrier to hydrocarbon flow (Section II); and
• Proceeding with removal of drilling mud from the well without installing the lockdown sleeve on the production casing wellhead seals to ensure the seals could not be shifted by pressure buildup under the seals (Section II).
5. Available evidence suggests there were insufficient checks and balances for decisions involving both the schedule to complete well abandonment procedures and considerations for well safety (Section VI).
6. The decisions mentioned above were not identified or corrected by the operating management processes and procedures of BP or those of their contractors or by the oversight processes employed by the Minerals Management Service (MMS) or other regulators (Sections VI and VII).
7. Currently, there are conflicting views among experts familiar with the incident regarding the type and volume of cement used to prepare the well for abandonment.There are also conflicting views on the adequacy of the time provided for the cement to cure. These factors could have had a material impact on the integrity of the well (Section II).
8. The BOP did not control—or recapture control of—the well once it was realized that hydrocarbons were flowing into the well. Also, both the emergency disconnect system designed to separate the lower marine riser from the rest of the BOP and automatic sequencers controlling the shear ram and disconnect failed to operate (Section IV).
9. Given the large quantity of gas released onto the MODU and the limited wind conditions, ignition was most likely. However, the committee will be looking into reports (such as testimony provided at the MBI hearings) that various alarms and safety systems on the Deepwater Horizon MODU failed to operate as intended, potentially affecting the time available for personnel to evacuate (Section V).
10. The various failures mentioned in this report indicate the lack of a suitable approach for anticipating and managing the inherent risks, uncertainties, and dangers associated with deepwater drilling operations and a failure to learn from previous near misses(Section VI).
11. Of particular concern is an apparent lack of a systems approach that would integrate the multiplicity of factors potentially affecting the safety of the well, monitor the overall margins of safety, and assess the various decisions from perspectives of well integrity and safety. The “safety case” strategy required for drilling operations in the North Sea and elsewhere is one example of such a systems approach (Section VII)
Read the full report in this link.
"1.The incident at the Macondo well and Deepwater Horizon MODU was precipitated by the decision to proceed to temporary abandonment of the exploratory well despite indications from several repeated tests of well integrity [the test type known as a negative (pressure) test] that the cementing processes following the installation of a long-string production casing failed to provide an effective barrier to hydrocarbon flow (Sections II and III).
2. The impact of the decision to proceed to temporary abandonment was compounded by delays in recognizing that hydrocarbons were flowing into the well and riser and by a failure to take timely and aggressive well-control actions. Furthermore, failures and/or limitations of the BOP, when it was actuated, inhibited its effectiveness in controlling the well (Sections III and IV).
3. The failures and missed indications of hazard were not isolated events during the preparation of the Macondo well for temporary abandonment. Numerous decisions to proceed toward abandonment despite indications of hazard, such as the results of repeated negative-pressure tests, suggest an insufficient consideration of risk and a lack of operating discipline. The decisions also raise questions about the adequacy of operating knowledge on the part of key personnel. The net effect of these decisions was to reduce the available margins of safety that take into account complexities of the hydrocarbon reservoirs and well geology discovered through drilling and the subsequent changes in the execution of the well plan (Section VI).
4. Other decisions noted by the committee that may have contributed to the Macondo well accident are as follows:
• Changing key supervisory personnel on the Deepwater Horizon MODU just prior to critical temporary abandonment procedures (Section VI);
• Attempting to cement the multiple hydrocarbon and brine zones encountered in the deepest part of the well in a single operational step, despite the fact that these zones had markedly different fluid pressures (because of the different fluid pressures, there was only a small difference between the cement density needed to prevent inflow into the well from the high-pressure formations and the cement density at which an undesirable hydraulic fracture might be created in a low pressure zone) (Section II);
• Choosing to use a long-string production casing in a deep, high-pressure well with multiple hydrocarbon zones instead of using a cement liner over the uncased section of the well (Section II);
• Deciding that only six centralizers would be needed to maintain an adequate annulus for cementing between the casing and the formation rock, even though modeling results suggested that many more centralizers would have been needed (Section II);
• Limiting bottoms-up circulation of drilling mud prior to cementing, which increased the possibility of cement contamination by debris in the well (Section II);
• Not running a bond log after cementing to assess cement integrity in the well, despite the anomalous results of repeated negative-pressure tests (Section II);
• Not incorporating a float shoe at the bottom of the casing as an additional barrier to hydrocarbon flow (Section II); and
• Proceeding with removal of drilling mud from the well without installing the lockdown sleeve on the production casing wellhead seals to ensure the seals could not be shifted by pressure buildup under the seals (Section II).
5. Available evidence suggests there were insufficient checks and balances for decisions involving both the schedule to complete well abandonment procedures and considerations for well safety (Section VI).
6. The decisions mentioned above were not identified or corrected by the operating management processes and procedures of BP or those of their contractors or by the oversight processes employed by the Minerals Management Service (MMS) or other regulators (Sections VI and VII).
7. Currently, there are conflicting views among experts familiar with the incident regarding the type and volume of cement used to prepare the well for abandonment.There are also conflicting views on the adequacy of the time provided for the cement to cure. These factors could have had a material impact on the integrity of the well (Section II).
8. The BOP did not control—or recapture control of—the well once it was realized that hydrocarbons were flowing into the well. Also, both the emergency disconnect system designed to separate the lower marine riser from the rest of the BOP and automatic sequencers controlling the shear ram and disconnect failed to operate (Section IV).
9. Given the large quantity of gas released onto the MODU and the limited wind conditions, ignition was most likely. However, the committee will be looking into reports (such as testimony provided at the MBI hearings) that various alarms and safety systems on the Deepwater Horizon MODU failed to operate as intended, potentially affecting the time available for personnel to evacuate (Section V).
10. The various failures mentioned in this report indicate the lack of a suitable approach for anticipating and managing the inherent risks, uncertainties, and dangers associated with deepwater drilling operations and a failure to learn from previous near misses(Section VI).
11. Of particular concern is an apparent lack of a systems approach that would integrate the multiplicity of factors potentially affecting the safety of the well, monitor the overall margins of safety, and assess the various decisions from perspectives of well integrity and safety. The “safety case” strategy required for drilling operations in the North Sea and elsewhere is one example of such a systems approach (Section VII)
Read the full report in this link.