Every oil and gas field has a life cycle,
from discovery of the field and construction of the platform
to operation, maintenance and finally, decommissioning.
In the UK sector of the North Sea,
decommissioning takes place under a mature regulatory process.
Responsibility for ensuring decommissioning complies
with UK regulations
rests with the Department of Energy and Climate Change.
After 40 years of service, decommissioning of Shell's Brent field is underway.
The Department of Energy and Climate Change, or DECC,
stipulates certain activities that must be done.
In line with these regulations,
Shell is plugging and making safe all the wells in the Brent field,
removing all four of the platform's topsides,
recovering seabed debris
and removing the attic oil,
a layer of oil trapped at the top of the subsea storage cells.
For those parts of the field that aren't stipulated by regulation,
an operator is required to complete a detailed process of comparative assessment
to identify and examine the various options for decommissioning.
The process assesses and compares the pros and cons of each option
in a verifiable and transparent manner.
The objective is to show on balance which option offers the best solution
and provide a significant reason why any option other than full removal
is preferable.
Shell is completing comparative assessments
for drill cuttings,
pipelines,
the alpha steel jacket,
options for the three gravity-based structures...
...and contents of the storage cells.
Although there is no specific requirement in the regulation
for a comparative assessment of the content in the storage cells,
Shell has opted to follow the same process to be consistent.
The outcome of a comparative assessment
may suggest leaving a structure or supporting infrastructure in place.
If it does, the OSPAR Convention requires that an operator must show,
in a fully evaluated and reasoned manner,
that there are significant reasons why sea disposal or leaving in place
is preferable to reuse, recycling or final disposal on land.
OSPAR, the Oslo Paris Convention,
was set up to protect and conserve
the marine environment of the North-East Atlantic.
OSPAR recognises that there may be particular difficulties and risks
associated with removing large steel structures
or the concrete gravity bases of some platforms.
So OSPAR provides for operators decommissioning offshore structures
to make a case for exemption from the general rule of complete removal.
They call this exemption "derogation".
If an operator identifies any cases requiring OSPAR to approve a derogation,
DECC asks the operator to provide the comparative assessments
in derogation case documents.
These are part of the decommissioning programme submitted for approval,
which DECC then submits to OSPAR for consideration.
Ultimately, DECC will make the final decision.
DECC stipulate there are five main criteria
that must be used in the comparative assessment process.
Each item to be decommissioned must be examined against these criteria.
They are Safety,
Environmental,
Societal,
Economic
and Technical.
As a starting point,
Shell decided to weight each of the five main criteria equally at 20%.
Then, each of the criteria was reviewed
and further split into appropriate sub criteria
to enable a more comprehensive comparison between the different options available.
For example,
Safety was broken down to consider the risks
that a particular option could pose to offshore personnel,
onshore personnel
and other users of the sea, like fishermen.
On closer inspection,
some options were identified as not being technically feasible.
After consultation with stakeholders and external experts,
these were discarded early in the process.
Studies commissioned from independent experts, Shell analysis
and industry norms
were then used to inform the assessment of each sub criteria.
Shell then analysed the performance of each option in each sub criteria,
and with the help of independent experts,
transformed all these different measures into a score.
The scores for each sub criteria were then weighted
and a total weighted score obtained for each of the options
for each of the structures.
This allowed Shell to develop a single measure
that could be used to compare one option with the other.
As part of this rigorous process,
Shell also conducted sensitivity testing.
One criteria would be increased to 40% weighting,
with the other criteria evenly reduced.
And in one case, cost was removed altogether
to understand if it would change the outcome of the comparative assessment.
Over the last nine years,
Shell and a team of expert consultants have completed a wide range of studies
to inform the comparative assessment process.
Each of these studies has been reviewed by an independent group
of recognised scientific, technical and environmental experts
to ensure they are robust.
The results of the comparative assessments
form the basis of the decision-making process.
Broader considerations, like stakeholder feedback
and other non-technical elements, are also taken into account
for arriving at the appropriate recommendations
for the decommissioning programme.
Whilst the comparative assessment will inform Shell's final decision,
each recommendation will be described in a document
that will explain all the considerations taken into account,
the matters assessed and reasons for the recommendations.
Decommissioning has many challenges and difficulties
which need to be overcome.
The comparative assessment is a useful tool
in ensuring that all the options available are identified, assessed, weighed up
and the best possible outcome is reached in a consistent and transparent manner.