About 320 miles from Aberdeen
is the Brent oil and gas field.
It's one of several large fields
which came on stream during the global oil crisis of the 1970s.
Brent helped keep the lights on in the UK
when energy was in short supply.
At the field's peak,
it produced enough energy to run half the UK's homes.
But now almost all of its recoverable oil and gas reserves have been depleted.
Its next step is decommissioning.
The Brent oil and gas field is made up of four platforms:
Delta, Charlie, Bravo and Alpha.
Between them they have drilled 154 wells.
Three of the platforms have "gravity-base structures" -
giant concrete legs that weigh 300,000 tonnes,
about the same as New York's Empire State Building.
The fourth has a steel support,
or "jacket".
From seabed to the top of the platforms,
they are as tall as the Eiffel Tower.
I think it's fair to say that these are an engineering marvel,
designed, commissioned, constructed and put in place
in a very short period of time.
They were definitely built to last
and built to take the enormous punishments which they are required to.
But at the time they were built,
little thought was given to how they might eventually be decommissioned.
The focus at the time was about getting oil and gas out of the ground
safely and efficiently.
Decommissioning wasn't high on the agenda,
and, indeed, it wasn't necessarily in society's mind at the time as well.
But since 2006, a team at Shell's offices in Aberdeen
has been trying to find the best way to decommission this remote field.
There's enormous challenges associated with the project.
The biggest responsibility is doing it safely.
I think decommissioning is best thought of
as a natural part of the oil and gas lifecycle.
At the end of their life,
we don't want to be leaving hazards behind,
so it's part of being a responsible operator.
Before looking at methods of decommissioning,
the team considered whether these platforms might have an alternative use.
We've looked at wind, carbon capture and storage.
The age and remoteness of the facilities
tends to make those all unviable.
It's a very, very hostile environment.
Decommissioning is now underway.
Shell has started by permanently sealing the wells.
Those wells go from the seabed,
through thousands of feet of rock to the reservoir.
We need to seal off the reservoir from the surface,
so we do that by setting typically 150ft or more of cement plug,
so that we're sure that we have a barrier that is there forever.
Part of Brent's decommissioning involves an extraordinary
twin-hulled, purpose-built vessel.
Its task is to lift the 24,000-tonne topside of Brent Delta
and sail it to shore.
It will be the heaviest lift ever undertaken at sea.
The Pioneering Spirit itself
I think is the largest ship ever by displacement.
It's nearly a million tonnes.
The ship will ballast down, so it will sink down in the water,
will move into position around the platform,
very carefully positioning itself
using dynamic positioning and sensors on the platform.
Then the hydraulic-lift system
will lift the platform free of the legs.
The topside will be sailed to Hartlepool, in the North East of England,
to be dismantled and recycled.
We think that we can recycle at least 97% of the Delta topsides.
Most of it is steel -
carbon steel, stainless steel, some exotic alloys -
so all of that can be recycled.
Lifting the topside will be a major feat of engineering,
but that's just the start of the decommissioning challenge.
Shell has to recommend a way to decommission the steel jacket
and concrete gravity-base structures.
More than six times heavier than the topsides,
it may be unsafe
or unviable to remove them.
What you're aiming for is the complete removal of everything,
but that's not always possible.
There's no silver bullet to some of the problems we face,
and it's required us to generate new, exciting, novel technology
to overcome the challenges that we find.
We've got the anchor hub installed
so we're now installing the top of this double-isolation-valve block.
Shell must also remove oil
trapped inside the gravity-base structures,
behind metre-thick reinforced concrete.
In those bases are storage cells
that were used for the storage of oil before it was evacuated.
The cells contain sediment, water
and a thin layer of oil floating on top.
We call that "attic oil".
It's trapped in the attic of the cells.
And as a responsible operator
we want to remove that oil,
but also it's required by regulation.
There are also 28 pipelines
that need to be removed,
dug into trenches, or safely covered.
And Shell needs to scientifically assess
and deal with piles of rock shards,
or "drill cuttings",
left on the seabed when the wells were drilled.
There's also debris around the field,
which we're duty-bound under regulation to recover.
These are things like scaffolding that's been washed off during storms.
Our plan would be, at the end of the project,
to do a sweep of all that debris
and recover those items.
This will ensure that the field is left in a condition that meets requirements.
In 1992,
15 countries signed the OSPAR Convention.
It sets out legislation
to protect the marine environment of the North-East Atlantic Ocean,
stretching from the coast of Greenland,
through the North Sea,
and right the way down to the Straits of Gibraltar.
Decommissioning activity has to comply with that legislation.
And to ensure that it does,
the Department of Energy and Climate Change
enforce strict regulations in the UK
on how decommissioning takes place.
We have to represent a decommissioning programme
which goes through public consultation,
through OSPAR consultation,
and ultimately to Secretary of State for approval.
In order to reach a recommendation
for how to decommission the different parts of the Brent field,
Shell is completing a comparative assessment process.
The comparative assessment is a huge undertaking.
Our process starts with first determining the decommissioning options,
and from there going through a lengthy exercise of data gathering,
both from science and environment.
Scientific and technical data
has been gathered into over 300 reports.
But before those reports are relied on,
they are subjected to the utmost scrutiny.
Every few months,
a group of esteemed academics meet in London
to ensure the data in the reports is accurate.
They form the project's Independent Review Group.
Our job is to make sure that this big operation is done right,
that the decisions that are made are based on solid evidence
that is as reliable as it possibly can be.
The IRG process is a thorough process.
We submit technical documents to them,
which they give really critical and robust review.
They generate a number of comments, which we then have to formally respond to
before we get to actual acceptance and then formal close out.
I really feel very strongly indeed
that having truly independent review
is a vital part of this process.
Once the IRG is satisfied a report is full and accurate,
Shell then uses it to help inform its decommissioning recommendations.
It compares the different decommissioning options
for any piece of infrastructure against five criteria
to assess how safe they are,
their impact on the environment
and society,
how economically viable they are,
and their technical feasibility.
We weigh all the different options up
and come out with what we think is a balanced, recommended option.
As well as working with the Independent Review Group,
Shell has been engaging with interested parties, or "stakeholders",
to hear their views about decommissioning.
We're inclusive rather than exclusive in who we engage with -
from academics, to environmental NGOs, to fishermen -
so a whole swathe of organisations
represented in that broad sweep of stakeholders.
Stakeholder engagement has been underway since 2007.
Shell has spoken to over 180 organisations,
held more than 300 meetings
and published e-news updates.
Shell recognises that its stakeholders hold a wide range of views and priorities.
This is an incredibly complex decommissioning project.
Much of what we're doing has never been done,
and in many respects we are groundbreaking.
The goal of stakeholder engagement is to find a decommissioning solution
which meets as many of our stakeholders' needs and aspirations as possible.
The Brent decommissioning programme
will be submitted to the Department of Energy and Climate Change.
Once this has been reviewed,
a 60-day public consultation will take place,
so that anyone with any interest can comment on the recommendations.
It's the beginning of the final phase
in the life of the Brent field,
and one that will prove to be as technically challenging as its first.