Gravity base structures
are among the biggest and most impressive feats of engineering
in the oil and gas industry.
Three of the Brent platforms are supported by reinforced concrete,
gravity base structures.
They were built in the 1970s
to withstand 200 mile an hour winds and waves 25 metres high.
They've served their purpose effectively,
but now, their robust design is a challenge to decommission.
Gravity base structure, or GBS as we call it,
is made up of a steel skirt,
which anchors the structure to the bottom of the seabed.
It's made up of some cells for storage and legs.
These gravity base structures are made of heavily reinforced concrete.
Each GBS weighs around 300,000 tonnes.
Shell has investigated completely removing the structures from the sea.
When these GBSs were positioned out off shore in the 1970s,
they would have been towed out, filled with ballast
to ensure they were grounded on the seabed.
To refloat the gravity base structures,
this installation would need to be reversed.
Cells would need to be emptied, sealed, and filled with air to induce buoyancy.
The base would need to be excavated
and hydraulics used to prize it from the sea-floor.
And a method of altering the buoyancy would need to be found
for its journey to the surface.
Independent studies suggest that
successfully refloating the GBS would prove extremely difficult.
Nobody's ever done this before.
You can have a scenario called "pop-up",
where you would lose control of the GBS structure.
Attempting to lift the structures from the seabed could also collapse them.
It is technically extremely high risk of failure,
and also a very high safety risk,
far higher than we would accept in any other part of our operations.
Based on 30 years of data gathered on North Sea operations
that would indicate that we have at least one fatality
per gravity base structure being refloated.
And that is a risk which I am not prepared to entertain.
So, having done some deep technical studies
and dialogue with our Independent Review Group,
we've concluded that we won't carry that option forward.
Shell has also considered
leaving the GBS in place and adding navigation aids,
or cutting the legs 55 metres below sea level,
in line with International Maritime Organisation guidelines.
To better understand the complexity around cutting concrete
we have done some trials on shore where we've formed reinforced concrete.
And we've also compressed it to try and replicate the conditions
that are out there actually on the GBSs.
While they've achieved the cut in a test on land,
there's concern it may not work in practice.
As you may appreciate, making a cut without trapping that wire
with 6,000 tonnes of dead weight bearing down on that cut surface,
in a marine environment where you're dealing with the effects of
waves, wind, a dynamic situation, is certainly extremely challenging.
The cost of this partial removal would be very high,
which is something Shell is required to consider.
The UK tax payer will be funding part of the cost of decommissioning,
so it's really important
that the operators are looking
to carry out the work in the most cost effective manner.
Brent's profits were taxed in excess of 70%.
It paid £20 billion, in today's money, in taxes.
Shell can now claim a 70% reimbursement on the cost of decommissioning,
from the taxes it previously paid.
And while the final costs of decommissioning are not yet known,
Brent will remain an overwhelmingly positive contributor to the UK treasury.
Having weighed up all the options,
Shell is not recommending the partial removal of the legs.
Our recommendation, based on ten years of study,
and independent review and analysis by our Independent Review Group,
is to leave the concrete gravity base structures with the legs up, in place.
This would involve capping the legs with concrete
and installing navigation aids.
The structures would be marked on maritime charts
and in FishSafe, the fishermen's GPS warning system.
One of our key contractors came to the conclusion that, actually,
the collision hazard was about one in every 10,000 years.
So well under any accepted standard.
Experts estimate that the GBS legs will degrade to below sea level
in around 250 years.
Then their position would be marked with buoys.
The option to leave the gravity base structure in place
really mirrors what's already happened in the Norwegian sector,
where the structures have been left in place with navigation aids on there,
to aid shipping and fishing.
As a general principle,
the fishing view is, if it's going to be there
and going to cause an obstruction, let's be able to see it.
Legs up, is the choice of the fisherman.