Shell has conducted Comparative Assessments for the Alpha steel jacket, drill cuttings, pipelines, gravity base structures (GBS) and cell contents. The objective of these assessments is to show, on balance, which option provides the best solution.

Comparative Assessment Criteria

In accordance with the requirements of OSPAR Decision 98/3 and the DECC Guidance Notes (issued by regulator the Department for Business, Energy and Industrial Strategy), technically feasible options for the Brent field have been studied and subjected to a formal CA.

Technically feasible options were assessed using the five main Guidance Notes criteria, namely:

  • Safety
  • Environmental
  • Technical
  • Societal
  • Economic

We used the advice provided in the Guidance Notes which lists those matters which are to be considered during a CA of feasible management options. These include but are not restricted to:

  • Technical and engineering aspects
  • Timing
  • Safety
  • Impacts on the marine environment
  • Impacts on other environmental compartments
  • Consumption of natural resources and energy (and climate change)
  • Other consequences to the physical environment
  • Impacts on amenities and the activities of communities
  • Economic aspects

In line with this guidance, therefore, we assessed each option’s performance by dividing that criterion into more specific sub-criteria. For example, the main criterion ‘Environment’ encompasses both the potential environmental impacts arising during the work programme (which is likely to be on a timescale of a few months) and the potential environmental impact arising from the long-term presence of materials, which is likely to be on a timescale of years.

By evaluating these different impacts as separate sub-criteria we were able to properly assess the performance of options in these two measures and examine how the environmental impacts changed with different options.

We decided that ‘Safety’ should be assessed using three sub-criteria, ‘Environmental’ using four sub-criteria and ‘Societal’ using three sub-criteria; the criteria ‘Technical’ and ‘Economic’ were each assessed by one sub-criterion.

DECC Main Criterion



Sub-criterion Description
Safety risk to offshore project personnel An estimate of the safety risk to offshore personnel as a result of completing the proposed offshore programme of work
Safety risk to other users of the sea An estimate of the safety risk to other users of the sea from the long-term legacy of the structure after completion of the proposed programme of work
Safety risk to onshore project personnel An estimate of the safety risk to onshore personnel as a result of completing the proposed offshore programme of work



Sub-criterion Description
Operational environmental impacts An assessment of the environmental impacts that could arise as a result of the planned operations offshore and onshore
Legacy environmental impacts An assessment of the environmental impacts that could arise as a result of the long-term legacy effects of the structure or facility after completion of the proposed programme of work
Energy use An estimate of the total net energy use of the proposed programme of work, including an allowance for energy saved by recycling and energy used in the manufacture of new material to replace otherwise recyclable material left at sea
Gaseous emissions An estimate of the total net emissions of CO2 from the proposed programme of work, including an allowance for emissions from the manufacture of new material to replace otherwise recyclable material left at sea



Sub-criterion Description
Technical feasibility An assessment of the technical feasibility of being able to complete the proposed programme of work as planned




Sub-criterion Description
Effects on commercial fisheries An estimate of the financial gain or loss compared with the current situation that might be experienced by commercial fishermen as a result of the successful completion of the planned programme of work
Employment An estimate of the man-years of employment that might be supported or created by the option
Impact on communities An assessment of the effects of the option on communities and onshore infrastructure



Sub-criterion Description
Cost An estimate of the total likely cost of the option, including an allowance for long-term monitoring and maintenance

Global Scales

We elected to use a method of assessment that uses global scales as a way of i) providing a unit-less scale on which to compare different sub-criteria (for example, Safety Probability Loss of Life (PLL) and environmental impact of operations) and ii) providing a way to compare the performance of the options across all of the facilities within the Brent Decommissioning project. The procedure for generating the global scales involved the following three steps:

  1. The data for each option, facility and sub-criterion was generated using the same method of calculation within a sub-criterion. For example, if the cost estimate for a Brent Alpha jacket option had been generated using current vessel day rate estimates and ignoring any effect of inflation that might be expected to occur between now and the execution of the work, then the cost of an option for a gravity base structure (GBS) was calculated using these same assumptions.
  2. Considering each sub-criterion in turn, the ‘best’ and ‘worst’ data from any option and for any facility was used to fix the top and bottom of the scale for that sub-criterion. For example, the option with the highest PLL is the least desirable and therefore marks the bottom of the scale and is therefore ‘0’ on the scale. The option with the lowest PLL is the most desirable and is therefore ‘1’ on the scale. This resulted in a ‘global scale’ spanning the whole data range for each sub-criterion.
  3. We then mathematically transformed the data for all other options onto these global scales. Thus, a single global scale for each sub-criterion could be used and applied consistently in all of the CAs for all of the facilities. This process of transformation converted the different sub-criteria into a common measure which then allowed us more easily and robustly to examine and compare the overall performances of the options.

The scores from the global scales for each sub-criterion were multiplied by the standard weights and then summed to derive a total weighted score for each option. The option with the highest total weighted score was identified as the ‘CA-recommended option’.

Sensitivity Analysis

The OSPAR Framework for CAs states that the CA shall be ‘sufficiently comprehensive to enable a reasoned judgement on the practicability of each disposal option’, and that ‘the conclusion shall be based on scientific principles…….and linked back to the supporting evidence and arguments’. DECC Guidance Notes also state ‘it is unlikely that cost will be accepted as the main driver unless all other matters show no significant difference’.

To examine the sensitivity of the CA recommended option, therefore, we applied five ‘selected weighting scenarios’ to the scores, to generate new total weighted scores for each option. The selected weighting scenarios were derived after a consideration of the relative values in the global scales, and reflect our view, informed by feedback from meetings and dialogue, of the importance of the various criteria and sub-criteria to all our Stakeholders.

We then examined the total weighted scores in each scenario and assessed how the scores changed, and determined if the order of the options changed in some scenarios. This resulted in the identification of the option that was the ‘Emerging recommendation’. It should be noted that this option may have been so identified because, although not necessarily always the best option in every scenario, overall it performed well in a number of the scenarios.

Identifying the Recommended Option

We used the above assessments and sensitivity analyses - and wider business and corporate considerations - to compare and contrast the performances of options in order to identify our ‘Recommended option’.

Brent Field Recommendations


The OSPAR Convention provides the framework for protecting and conserving the north-east Atlantic (including the North Sea). Within OSPAR, 15 governments and the EU co-operate to protect the marine environment of the North-East Atlantic.

Within its Decision 98/3, the OSPAR Convention recognises that there are difficulties in removing major installations, such as concrete gravity base structures and large steel jackets. In these instances, operators may make a case for exemption – called ‘derogations’ – from the general rule of complete removal from the sea. These have been granted to North Sea operators in the past allowing them to leave structures similar to parts of the Brent infrastructure, in place.

Find out more about the OSPAR Convention