Page 64 - Well Control for Completions and Interventions

Introduction and Well Control Fundamentals 55

involved in the incident.” 12 Professor Andrew Hopkins, writing in his book
“Disastrous Decision” reached a similar conclusion, noting that the causes
of the accident were “mundane, involving a series of human and organisational
13
factors similar to those identified in other high-profile accidents.”
Professor Hopkins lists several contributory factors that led to the
blowout, starting with an observation that the concept of “defense-in-
depth” failed. One element of the critical well control defenses, was the
9 5/8v 3 7v production casing. Once cemented, the casing would form
part of the primary well control envelope. It is now known that the
cement failed to isolate the reservoir. Crucially, the crew misinterpreted
the results of the inflow test performed to confirm the integrity of the
cement. They concluded that the barrier was intact. It was not. During
the inflow test, drill pipe pressure built to over 1400 psi (had the barrier
been intact there would have been no pressure increase). The crew bled
off the pressure, only for it to build up again. These clear warning signs,
indicating that reservoir fluids were entering and pressurizing the casing,
were ignored by the crew; or rather, they were explained away. Although
there was justified criticism of BP for having inadequate inflow test pro-
cedures, it is still hard to understand how those at the well site could
rationalize such obvious anomalies. The explanation provided by
Professor Hopkins (and others) is that the group was “subject to powerful
confirmation bias because the cement job had already been declared a success.”
The Oxford Dictionary of Psychology defines confirmation bias as,
“the tendency to test one’s beliefs or conjectures by seeking evidence that might con-
firm or verify them and to ignore evidence that might disconfirm or refute them.” 14
The concept is not new, in 1620 Francis Bacon noted that, once a human
intellect has adopted an opinion (either as something it likes or as something gener-
ally accepted), it draws everything else in to confirm and support it. Even if there
are more and stronger instances against it than there are in its favour, the intellect
either overlooks these or treats them as negligible. 15
For the crew on the Macondo well, the cementing operation had
been declared a success. The belief that the cement was good was rein-
forced by a successful positive pressure test on the casing (2700 psi for 30
minutes). They expected the inflow test to confirm the cement integrity;
and they believed it had despite evidence to the contrary.
Thinking that the inflow test had confirmed the integrity of the cement,
the crew subsequently failed to properly monitor fluid returns during the
displacement to underbalanced fluid. As the heavy mud was displaced by
the much lighter seawater, underbalance was lost, allowing reservoir fluid to