https://phys.org/news/2020-06-lessons-worst-oil-disaster.html
[My oversimplified conclusion: oil exploration where natural gas is
present in deep cold waters may be near impossible to make safe based on
standard industry practice.]
June 30, 2020
New lessons from the worst oil spill disaster ever
by Nancy Bazilchuk, Norwegian University of Science and Technology
Ten years ago, the Deepwater Horizon accident in the Gulf of Mexico
killed eleven men and resulted in the largest accidental oil spill in
history. Years of investigations concluded that the drilling crew missed
critical warning signals that would have stopped the problem. A new
analysis suggests that wasn't the case.
The magnitude of the Deepwater Horizon accident is almost impossible to
fathom. On April 20, 2010, eleven men died when the drilling rig
exploded. An estimated 507 million litres of oil spilled into the Gulf
of Mexico over 87 days, coating nearly 1000 km of coastline with sticky
black goo. Birds and marine life took a beating, and shrimpers who
relied on the Gulf of Mexico were deeply affected when fishing grounds
were closed.
Years of investigations and legal proceedings found many reasons for the
accident, including that the crew itself had missed critical information
which, had they noticed in time, would have allowed them to address the
problem before it exploded.
But a new analysis of data from the drilling platform paints a very
different picture of what has previously been found, said Dag Vavik, a
Norwegian engineer with 30 years in the industry. Vavik recently
defended his Ph.D. on the accident at the Norwegian University of
Science and Technology.
"In previous investigation reports… we have been told how the drilling
crew failed to observe that the well was flowing during the last 20
minutes before the explosion," Vavik said. "However, real time data and
witnesses from the Deepwater Horizon tell a different story."
Questioned industry standard
Vavik has nearly 25 years' experience designing offshore floating
drilling units, like the Deepwater Horizon, and was well aware of the
problems these rigs could face.
His experience made him question an industry practice recommended in
2001 for separating natural gas from drilling mud. Vavik felt the
recommendation could result in an uncontrolled release of mud and gas
onto the rig.
The Deepwater Horizon's mud gas separator system was based on this
recommendation. The problem was that the system was designed to allow
gas and mud to return from the well by being routed directly to the mud
gas separator without any restrictions, Vavik said.
His concerns about the industry practice led him to warn clients and
alert his colleagues to the problem. Ultimately, he ended up designing a
new system for handling the mix of mud and gas for deep water drilling
ships commissioned by Petrobras in Brazil. He ultimately patented the
design.
Vavik became deeply interested in the Deepwater Horizon accident after
reading BP's own report on the disaster. The company found that one of
the main problems contributing to the explosion was the design of the
mud-gas separator system—the exact issue that Vavik himself had flagged
years before, when the industry instituted its 2001 practice.
"When I read this in the investigation report, I blamed myself for not
having done more than I did… to get the industry to change the industry
practice with having a mud gas separator directly connected to the
diverter system," he said. "At the time I promised myself to do whatever
I could to prevent such a disaster ever happening again."
Years later, however, Vavik found out that the drilling crew probably
didn't use the mud gas separator system during the accident, and likely
tried to divert this fluid directly overboard, which is what the written
instructions said they were supposed to do.
"In one way this was a relief," he said. "On the other hand, this meant
that something else must have caused this accident."
And that is what Vavik really wanted to find out.
Previously overlooked data
What launched Vavik on his Ph.D., however, was the discovery in 2014
that some information from the Deepwater Horizon had simply been
dismissed as improbable.
To understand what Vavik found—and why it matters—you need to first
understand what the drilling crew would have been looking for—and what
they found.
The Deepwater Horizon drill ship was an exploration ship, looking for
oil and gas. It was not designed to produce oil and gas, just to find
it. Once the find at this particular drill site had been confirmed, the
crew sealed off the well so it could be later developed for production.
If all had gone according to plan, the drill ship would have moved on.
But things didn't go to plan. The well wasn't actually sealed off
properly, and instead, there was a huge build-up of gas in the well's
piping system in the days before drilling stopped and as the crew tried
to seal off the well. This gas exploded on April 20 and caught fire.
Gas influx is a known problem, Vavik said, and the Deepwater Horizon had
two independent sensors that should have detected it. In fact, the two
sensors actually showed that there was no flow in the system until right
before the explosion.
But somehow investigators decided that the crew hadn't detected the
problem. In their accident assessment report, BP wrote that the "rig
crew did not recognize the influx and did not act to control the well
until hydrocarbons had passed through the BOP (blow out preventor) and
into the riser." Vavik says that's not quite right.
Data suggests well was plugged
Using data from the sensors and a series of simulations in the lab,
Vavik says that part of the problem was that the system was plugged with
gas hydrates, which can form when natural gas encounters cold water and
freezes into a kind of natural gas ice.
The plug of natural gas hydrates means that there was no way for the
crew to know exactly what was going on until right before it happened.
Vavik said BP's investigations and simulations predicted that thousands
of gallons of fluid were coming up from the well every minute during the
last 30 minutes before the explosion. However, he said, the two flow
sensors showed that there was no return flow from the well until right
before the accident.
"Several witness statements support what the recovered flow meter data
was telling us. The situation developed extremely fast," he said. "Flow
from the riser started to come back only a couple of minutes before the
first explosion."
Furthermore, Vavik said, some of the actions known to have been taken by
the crew just before the explosion suggested that they knew there was a
plug in the system.
The crew was troubleshooting and investigating what may have caused the
anomalies they had detected when the hydrate plug suddenly loosened,
Vavik said.
"This caused rapid gas expansion and pressure built up underneath the
gas hydrate plug, allowing the plug to move like a "bullet" in a gun
barrel," he said. "Then it was too late to avoid the accident."
A forensic study
Among Vavik's dissertation opponents was Jerome Schubert , an associate
professor in petroleum engineering at Texas A&M University.
"Your dissertation was like a forensic study, step-by-step," he said
during the defence. "You used simulations to back up your ideas. I liked
your work, and it was great work."
Schubert said it was important for the industry to have a better
understanding of what can go wrong in deep water drilling situations,
and that Vavik's findings did just that.
"That is the value of your work," Schubert told Vavik. "There were a lot
of questions there (in the accident) that no one had the answer to. You
offer potential reasons as to why things didn't look normal."
Among the recommendations Vavik offered based on his research was that
the industry needs a better way to detect influx of gas and gas hydrates
earlier than was done on the Deepwater Horizon. The explosion couldn't
have happened without tons of natural gas entering the drilling system
undetected until it was too late, he said.
"The people who can tell the real story of what happened are not here
anymore," he said. "I hope that the research work I have done will
contribute to give the families and colleagues of the eleven men a
better understanding of what really happened in the last 45 minutes
before the explosion."
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