These cells are each over 60m high and almost 20m wide – taller than Nelson’s Column. The cells are made of almost 1m thick concrete and reinforced with steel. Inside, they contain a mixture of attic oil, water, and a layer of sediment which has settled at the base.

Brent GBS during construction in 1975

Shell’s commitment to stakeholders

Shell made a commitment to stakeholders that a sample of the sediment would be obtained prior to submitting the Decommissioning Programme to validate modelling assumptions. In 2014, after several attempts over the previous years, sediment from three of the cells was obtained. The collection of the samples was independently witnessed, verified and analysed before the findings were shared with stakeholders.

The sediment in the cells is a sticky layer made up of a combination of hydrocarbons, sand and water. It was deposited as a result of operational activities during the latter part of the platform’s production life.

Comparative Assessment

Although not a regulatory requirement, Shell carried out a Comparative Assessment for the cell contents to ensure there was consistency in the process. The assessment determined what, on balance, would be the best solution for the cell contents.

Recommendations

The recommendation is that the cell contents be left in place. Shell believes that this is the best solution based on robust modelling and data. The technical difficulties, safety issues and cost of the cell sediment removal, along with treatment and disposal, would be disproportionate to any benefit of removal. The studies show that leaving the cell content in place does not present a significant environmental hazard.

Over time, even as the legs slowly degrade, the GBS will contain the cell sediment. Modelling shows that any exposed cell sediment will disperse very slowly into the marine environment and not extend further than the existing drill cutting piles. This will not have a significant effect on marine organisms or people. In agreement with Department for Business, Energy and Industrial Strategy (BEIS), Shell will implement a monitoring programme for the Brent Field.

Cell Management Stakeholder Task Group

The Cell Management Stakeholder Task Group (CMSTG) was formed in 2011 to allow a focused group of interested and highly-engaged stakeholders to review the dilemmas around the management of the cell sediment.

Forty-two of the 64 cells around the base of Brent Bravo, Brent Charlie and Brent Delta have been used for oil storage and separation, and they contain some legacy oil residue, water, ballast, and sediment.

The Project acknowledged that managing the cell sediment at the bottom of the oil storage tanks presented a considerable challenge, and that there was also a high level of stakeholder interest in this issue. Cell sediment management options were originally discussed with stakeholders at engagement events in 2010.

Proposed cell sediment treatment options included:

  • Remove and re- inject into the reservoir via a new well
  • Remove and take onshore for treatment
  • Leave in Place and cap the sediment
  • Leave in Place with Managed Natural Attenuation (MNA)
  • Leave in Place.

In 2011 discussions on a decision strategy for cell sediment management at stakeholder events included a proposal by stakeholders to set up a small, dedicated group to look at the issues in more detail. The aim was to help inform the owners’ decision-making on cell management, and to ensure wider stakeholder confidence in the choice of a cell management option, or options.

Subsequently 15 organisations and individuals representing a range of different stakeholder perspectives were invited to participate in the CMSTG.

CMSTG members are experts in their respective fields. Over the years we have met them collectively and individually at regular intervals to present our proposals and learn their views and opinions. Their input has been invaluable. They have raised important questions and helped us to refine our approach at each stage of the Comparative Assessment process. They, and we, share the same objective of finding the best possible way to manage the cell contents safely and effectively.

Read about the Cell Management Stakeholder Task Group and their role

Nasa Sonar Spheres

Collaborating with NASA

Since 2010, Shell has been working with NASA to develop a technique to access the cells to gain images of the content. NASA specially designed “sonar spheres” – a bowling ball-sized satellite – to access the cell through existing pipework.

The sphere took sonar images of the cell sediment so that Shell could identify its physical characterisation. Images were successfully obtained from Brent Bravo in May 2016.

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