We are addicted to oil: we consume
about 30,000,000,000 barrels of it per year (a volume of 4.8 cubic
kilometres). The problem is that it's getting harder and harder to
find. Back in the 19th century there were places in the world where
it just oozed naturally out of the ground. If you wanted an oil
well, all you had to do is pick up your shovel and dig a hole!
Those days are long gone. Now we have to drill for oil in more and
more hostile places like the deserts of Africa and the frozen
arctic wastes of Alaska.
Keep on drilling...and
drilling...
Drilling for oil in these places is
quite risky as it's possible to hit an overpressure at any
time.
That blows -
literally!
An overpressure is a high-pressured
pocket of oil or gas. The pressure can be so high that it can
squirt the drill pipe clean out of the hole with enough force to
take the whole drilling rig with it. This is known as a
blow out and can be a bit unnerving to say the
least.
To stop this happening, a very dense
mixture of heavy minerals known as mud is poured into the hole. The
weight of the mud provides a massive pressure designed to balance
the pressure of the oil or gas pushing upwards. This is usually
enough to contain it. If it isn't the next, and often the last,
line of defence is the blowout preventer (BOP).
Bop it!
The BOP is a giant stack of 'shut-off
valves' that close if the weight of the mud cannot contain the over
pressure. They can be huge structures weighing up to 500 tonnes. If
they're working properly, giant hydraulic rams seal the well a few
seconds later.
The drilling
challenge!
As oil gets scarcer and more valuable,
oil companies are drilling in even more challenging environments
such as the deep ocean. This is the most challenging of all.
Britain pioneered ocean drilling back in the 1970s. The off shore
fields of the North Sea still provide Britain with a thousand
million barrels of oil per year.
Back then drilling in 300ft of water
needed all the technology and expertise Britain could throw at it.
Even today it's still not an easy environment to work in. Some of
the largest and most elaborate structures made by man are oil
platforms working in the North Sea. Back then it was easy peasy
compared to where oil companies are working now.
In the Gulf of Mexico rigs are drilling
in seas over 5000ft deep. That's a mile of water before you even
start drilling into the earth! So how do the engineers get the blow
out preventers past all that water in time? They have to be on the
sea bed on these rigs. At this depth, the environment is more
hostile than even the surface of the moon.
Why is it so hostile down
there?
- The pressure that deep down under the water is 147 times more
than what it is at the surface
- No human divers can operate there, the pressure is lethal
- Only remote controlled submarines can survive
- It is pitch black that deep underwater
- No light can penetrate that far
- It is really cold down there. Very icy waters!
So what happens when things go
wrong?
The Deepwater Horizon rig in the Gulf
of Mexico was all over the headlines because it hit an overpressure
and the weight of the mud could not contain it. But that's not all.
What else happened?
- The BOPs failed
- A supersonic blast of oil and gas raged up the hole expanding
and accelerating all the time
- The explosion and fire that happened killed 11 people
- It started one of the worst oil spills in history
The question everyone's asking is, why
did the BOPs fail?
How was the leak
stopped?
This is the 64 billion dollar question.
The key to understanding why it was so hard to plug the leak is to
appreciate how hostile it is down there. No one seems to have
contemplated the prospect of catastrophic failure with a rig
operating in such deep water. It was very difficult to control the
leak because the end of the riser pipe that is used to go from the
BOP on the sea bed to rig on the surface was 5000ft underwater and
gushed about 200,000 gallons of crude oil into the sea every
day.
First...slow the leak
down
The first attempt to slow down the leak
was to prop an enormous steel and concrete funnel over the main
leak and channel the oil up a pipe to the surface where it could be
pumped into a tanker and taken to shore. But that didn't work. The
very low temperature of the water at that depth caused the oil to
congeal into a kind of frozen sludge that simply blocked the
funnel.
What else was
done?
The next attempt involved feeding a new
tube into the old pipe and pumping in a goopy mixture of old tyres
and concrete to try and block the pipe. This also failed.
What next?
The final option was to drill a relief
well. It was a long drawn out, very difficult job. To succeed in
doing this engineers dropped 5000 feet of drill string down to the
sea bed. Then they drilled a further 18,000ft through solid rock.
Finally, they drilled sideways into a target pipe a few metres
across. Easy.
NOT!
This took around three months. An awful
lot of oil found its way into the environment by that time and the
Deepwater Horizon spill ended up being the largest acciedental
marine oil spill in the history of the petroleum industry. The
effect on the nearby marine habitats and the Gulf of Mexico's
fishing and tourism industries was devastating.
