Mistakes in cement design, testing, quality assurance and risk assessment top the list of eight critical problems underlying the fatal Deepwater Horizon blast, an internal BP investigation has concluded.
In analyzing the April 20 disaster that killed 11 workers and spilled more than 170 million gallons of oil into the Gulf of Mexico, BP’s team cited “a complex and interlinked series of mechanical failures, human judgments, engineering design, operational implementation and team interfaces” for causing and escalating the accident.
But problems with cement chemistry and application led what BP called “eight key findings.”
The day before the explosion, BP was sealing the well beneath the Deepwater Horizon rig with cement so that the well could be temporarily abandoned. BP has abandoned about 600 wells in the Gulf of Mexico, according to government data.
In this case, however, the annulus cement barrier did not prevent the hydrocarbons from entering the wellbore from the reservoir, BP reported.
“The annulus cement that was placed across the main hydrocarbon zone was a light, nitrified foam cement slurry,” BP said.
“This annulus cement probably experienced nitrogen breakout and migration, allowing hydrocarbons to enter the wellbore annulus. The investigation team concluded that there were weaknesses in cement design and testing, quality assurance and risk assessment [emphasis by BP].”
A Failure ‘of Particular Concern’
Nitrogen breakout allows nitrogen to channel through the cement, which can compromise the cement seal.
“The failure of the cement barrier is of particular concern because similar ones are installed in all wells,” Dr. Richard Pike, former BP executive and current chief executive of the Royal Society of Chemistry, said in an article on the Society’s web site.
Once the cement barriers had failed, Pike added, the subsequent events “exposed other shortcomings in the management and operating procedures of all the companies involved.”
“While it is important that these failings are addressed,” he said, “working out why the cement barrier failed—whether that is down to the cement chemistry, the process, or both—is crucial to making sure this kind of thing doesn't happen again.”
BP listed seven other critical problems:
• The shoe track barriers did not isolate the hydrocarbons.
• The negative-pressure test was accepted although well integrity had not been established.
• Influx was not recognized until hydrocarbons were in the riser.
• Well control response actions failed to regain control of the well.
• Diversion to the mud gas separator resulted in gas venting onto the rig.
• The fire and gas system did not prevent hydrocarbon ignition.
• The Blow-Out Preventer (BOP) emergency mode did not seal the well.
Three methods for operating the BOP in the emergency mode were unsuccessful in sealing the well, BP said. The explosions and fire very likely disabled the emergency disconnect sequence, which was designed to seal the wellbore and disconnect the marine riser from the well.
‘Not Completely Convinced’
The investigation team developed a series of recommendations to address each of its key findings.
BP noted that the report was wholly the product of its own investigation and that the findings had not been reviewed by or discussed with any investigating agency before the document was released.
Indeed, federal prosecutors have announced that they will be testing cement used in the blown-out BP well, to verify BP’s contention that the cement was faulty.
Greg McCormack, director of the petroleum extension service at the University of Texas in Austin, is among several experts who expressed doubt about the report.
McCormack “is not completely convinced that the problem was with the cement chemistry,” the Royal Chemistry Society reported. “'The other challenge is to make sure all the cement gets to the right place with no voids or weak spots,'” he said.
McCormack noted statements in the BP report regarding “significant differences between the method and cement composition proposed by the design team and what was implemented by the contractors on the rig,” RSC’s report said.