Brent Alpha has a steel jacket support structure
which stands in a water depth of 140 metres and weighs 31,000 tonnes.
At the base of its eight legs are "footings",
giant steel piles filled with cement, which pin the structure to the sea-floor.
Shell is removing the platform or “topside”,
and the steel jacket structure, above the footings.
Together, this will allow for 25,000 tonnes of steel to be recycled.
Shell has considered different options for decommissioning the footings themselves.
It looked at whether it was feasible to cut
and completely remove the footings in a single lift, with the steel jacket.
The positive elements of complete removal
is you are leaving the seabed in the same state that you found it in,
so you have removed all hazards from the seabed.
Any option for complete removal
requires excavation of the cuttings around the legs of the jacket,
so that you can then cut below the mud-line,
using some sort of technique like diamond wire-cutting
or high-pressure water abrasive cutting.
Sounds straightforward, but, in reality, that's a very complex operation,
which has a high degree of safety risk of those involved,
either through remotely operated vehicles, or through divers,
to actually allow that cut to take place.
The team also considered removing the jacket
and then removing the footings separately.
But both these options are complicated
by the existence of piles of drill cuttings around the footings.
These shards of rock, discarded during the drilling process,
contain some contaminants.
So no matter how you go about trying to remove all the footings,
you would have some kind of disturbance effect on the drill cuttings,
which is undesirable and does have a negative environmental impact.
The team also considered a third option.
Removing the jacket, but leaving the footings in place.
It commissioned independent studies to assess the consequences.
DNV have had to take into account
what is the impact of the jacket degrading over the next 300-500 years,
versus the massive amounts of energy and effort and CO2 emissions
that would have to take place to remove those footings.
The outcome of that for the jacket, from environmental aspects,
is very low impact if we leave those footings in place.
Our recommendation for the Alpha jacket is to take away
the top two thirds of the jacket, to bring that ashore and recycle that
and then leave the bottom third of the jacket in place.
The regulations allow for this, if it's the most appropriate method.
Other operators have done the same.
In the northern North Sea, where the Brents are,
BP's Northwest Hutton platform, for example,
left the footings and the cuttings behind.
Murchison CNR's platform, also quite close to the Brents,
will be leaving their footings and cuttings behind as well.
Shell recognises that leaving the footings
would create a snagging hazard for fishermen.
It would ensure the footings are marked on maritime charts
and entered into the FishSafe system.
There's a FishSafe system which is a computerised system
on all fishing vessels in the North Sea, or UK fishing vessels,
that have an audible alarm in the cockpit so that fishermen know to avoid the area.
We are prepared to live with that
provided all the necessary trials
to allow us to fish as close as sensibly possible
to anything that's left, happen.
To be absolutely frank, a hazard on the seabed is not new.
Wrecks and natural features exist already.