How to Decommission Brent: Alpha Jacket


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.

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