What are Cells

Three of the four Brent platforms were built using steel reinforced concrete and comprise large concrete legs, between 150m and 165m tall, which are anchored to the seabed by a concrete base. They are called gravity base structures (GBS), because they stay in place thanks to their sheer size and weight – each structure weighs around 300,000 tonnes.  Each base comprises a cluster of storage tanks called cells. There are 64 cells in total, which were designed for oil storage, most of which are 60m high and 20m in diameter, taller than Nelson’s Column. They are made of concrete, just under 1m thick, reinforced with steel bars.

Brent GBS during construction in 1975

What were the Cells used for

Whilst the cells were designed for oil storage, over the operating life of the field only forty-two of the sixty four cells were used for oil storage and separation at some point during production activities. The storage cells provided operational flexibility to allow the platform to produce and store oil when maintenance or other activities prevented direct export of oil to market via pipeline or tanker. They also served as a means of primary separation of oil and produced water. Those cells not used for oil storage contain water for ballast or cooling. All of the cells provide structural support to the legs and additional ballast to secure the platform to the seabed.

During the first 20 years of production, Brent primarily produced oil with associated gas.  From the mid-1990s the field was redeveloped to produce mainly gas with associated fluids (water and oil). During this latter phase of operation the storage cells served as a means to settle out produced sediment and again for separation of oil from the produced water.  When production at Delta and Bravo stopped, seawater was pumped into the cells to replace the oil during final bulk export runs.

What do the Cells contain

After more than 30 years of production, the cells used for storage now contain (from top of the cell to bottom):

  • a layer of crude oil called attic oil, estimated to be 1m deep at the top of most of the cells where attic oil is present;
  • a 1m thick layer of stable emulsion of oil and water called interphase material;
  • a large intermediary layer of water;
  • a layer, 4m on average, of sediment at the bottom of each cell, this is a mixture of oil, sand particles and water;
  • a 1m concrete baseplate supporting the layer of sediment; and
  • typically a 19m layer of sand ballast.

How do we know what is in the Cells

One of our commitments to stakeholders was to sample the sediment prior to submitting our decommissioning programme. We had previously estimated the volume and composition of the cell contents from our historical operating records and computer modelling studies, but wanted to be able to confirm our assumptions about volume and composition with sediment samples.

Cell Sampling Process

Taking samples within the cells had never been done before. The technologies and know-how did not exist. It took eight years to develop the techniques and expertise to overcome the inherent complexity associated with this challenging task. Thousands of man hours were devoted to the sampling project and multiple concepts examined and tested. Two ideas progressed beyond the drawing board to become offshore projects in 2008 and 2012, but eventually both had to be abandoned due to feasibility challenges. 

In 2014 another attempt to obtain samples was made offshore. Crucially this concept was based on creating a new subsea access point in the top of the cell, and integrating five different technologies which had been tested extensively onshore. Three storage cells on Brent Delta were chosen for sampling as they could be accessed using the topside crane. 

Working at 80 metres depth, divers first installed base plates on each of the three cells and bolted them onto the concrete surface. Phase two was carried out from the Brent Delta topside using the platform cranes, supported by a platform-based, remotely-operated underwater vehicle (ROV). A drilling tool was attached to the base plate so it could core into the 0.9m thick concrete tank tops and a sampling tool was subsequently deployed to collect the samples. 

In August 2014, during a period of calm weather, we were able to collect between one and three kilogrammes of sediment from each of the three cells accessed – as well as water samples. A 3D sonar device was also successfully launched in each cell to measure the sediment volume and its surface topography.

Independent analysis

In order to give extra assurance, the samples were collected in controlled conditions, with the offshore operation witnessed throughout by independent observers, Bureau Veritas, a global leader in testing, inspection and certification. Each sampling canister was temperature controlled, sealed and signed to ensure it could not be compromised before onshore analysis.

Once back onshore, a specialist independent laboratory carried out chemical and physical analyses of the sample. The results were shared with the Cell Management Stakeholder Task Group, a group of 15 stakeholders brought together to look specifically at the cell sediment issue.

The analytical programme was very comprehensive. It was approved by Shell and Esso and reviewed by the Independent Review Group. Stakeholders were also asked to contribute to the analytical programme which resulted in the inclusion of alkylated phenols, and other phenol compounds, in to the cell sediment sample analysis.

The programme tested for 156 chemical compounds and analysed six physical parameters for each sediment sample.

What do the samples reveal

As a result of the sampling, we were able to confirm the average volume of sediment per cell to be 1,044 m3 versus our assumption of 1,080m3.

Chemically, the sediment contains no significant amounts of non-biodegradable compounds. Trace levels of naturally occurring radioactive materials and heavy metals were found, but no traces of polychlorinated biphenyls, glycols or organotins. This is positive and in line with our assumptions.

Physically, the upper layer of sediment is a viscous mixture of about 50% water, 25% oil and 25% sand. The higher shear strength detected indicates the sediment is harder to mobilise and therefore will take more effort to recover or conversely is less likely to spread if it comes into contact with the surrounding marine environment.

The findings were mostly in line with the assumptions used throughout the fate modelling. However the water within one of the cells contained a higher hydrocarbon content than expected. Additional water samples taken from different cells since then were below our assumptions. As such, we are continuing to investigate why the hydrocarbon content in water differs between cells and as part of the process to recover the attic oil on Delta, we will carry out further water sampling.

What Are The Decommissioning Options For The Contents Of The Cells

Option 1: Remove the contents and re-inject into subsea wells

The cell contents would be pumped out of the cells into tankers and transported to an offshore location where they would be injected into specially drilled waste disposal wells.  The advantage of this option is that the sediment and contents would be removed from the cells and hence the risk of sediment entering the marine environment when the GBS start to disintegrate, several hundred years from now, would be eliminated. There are however numerous disadvantages. Technically, it would be very challenging to cut a large hole into the 1-metre thick concrete cell top 120m below the surface of the North Sea and safely extract the contents.

Option 2: Remove to shore for treatment and disposal

The cell contents would be pumped out of the cells into tankers and shipped to an onshore waste treatment plant capable of handling large volumes of hydrocarbon slurry. There the slurry would be separated into oil, water and solids. The solids would be treated and disposed of in a landfill, the water would be cleaned and returned to the sea, and the oil reused. The pros of this option are the same as with removing the contents and re-injecting it into subsea wells: the contents would be removed from the cells and the risk of hydrocarbon slurry impacting the marine environment when the GBS disintegrate would be eliminated. On the negative side, the technical challenges and risks of accessing the cells and transferring the slurry from the cells into tankers, and from the tankers to the waste treatment plant are high with only a minor benefit in terms of legacy environmental impact.

Option 3: Leave in place with bioremediation

Bioremediation is a treatment that uses naturally occurring organisms to biodegrade and reduce the amount of organic substances in the sediment. At Brent it would mean adding nutrients to stimulate natural bacteria to break down the oil. Because of the low temperature and the limited amount of available nutrients, the bacteria would not be very active. Treatment by this method is only likely to affect the surface of the sediment, as the nutrients cannot diffuse much deeper than 20 to 30 cm into the sediment. Overall, this option would require substantial quantities of nutrients to be added, but only affect the upper layer of the sediment; it would also take a long time and have no guarantee of success.

Option 4: Leave in place capped

Capping material, such as sand, would be deposited into the cells to form an inert layer, approximately 1 metre thick, on top of the sediment. This would form a protective barrier between the sediment and the sea when the GBS start to disintegrate. Chemicals would also be added to the cell water to promote the biodegradation of hydrocarbons within the water. This option could be challenging to execute; due to the lack of bearing capacity of the sediment (75% of the sediment are fluids it would therefore likely require a structural agent such as gravel to be injected into the cells to support the capping material). This structural agent would need to be added in higher volume than the capping agent itself. The capping material would also only provide a limited amount of protection when the GBS eventually collapse. The technical challenges and cost associated with implementing this option are assessed as disproportionate to the limited benefit from an environmental legacy standpoint.

Option 5: Leave in place

The GBS will continue to provide physical containment to the cell sediment as the legs and caisson slowly degrade so one of our options is, to leave the cell contents in place, untouched and untreated, but effectively contained within the structure. The GBS are expected to remain largely intact on the seabed for hundreds of years. Eventually, they will start to disintegrate or collapse. Studies show that when this happens and the sediment slowly but progressively comes into contact with the marine environment, the impact on the environment is likely to be restricted to a small area close to the cells. Independent scientific analysis of the legacy impact of the cell water and sediment on the marine environment found it would be unlikely to induce any measurable effects at the population level. Effects on the water column would be restricted to local and transient effects close to the release point. A worst case sediment release would result in an impacted area around each platform that is a little larger than the area already impacted around each platform by the historic drill cuttings. Furthermore the impact of low levels of bio accumulating substances would also be localised and so are not expected to induce any measurable effects at the population level in the area surrounding the Brent Field. Compared with the technical challenge of removing the contents and either injecting them in waste disposal wells or transporting them to shore for treatment, the potential impact of the leave in place option is minor and transient.

Which option does our comparative assessment recommend?

The option recommended by our Comparative Assessment for the cell content is to leave the sediment in place, effectively protected by the GBS.

Although removing the cell content would create jobs in the short-term, this is not an area of the decommissioning industry with much capacity for growth given the limited number of similar structures – just 9 GBS are operational in the UK and 27 in the wider OSPAR region compared to 470 installations overall in UK waters.

As we have seen, there are significant technical difficulties involved with removing the cell sediment and this type of operation on such a scale has never been attempted before. All removal options would also carry increased safety risk to personnel.

The cost of removing the cell sediment for onshore treatment and disposal is disproportionate to any environmental legacy benefit. Modelling shows that protection of the sediment will be provided from the cells. This outer structure is predicted to last for hundreds of years.

Our long-term fate modelling studies and environmental impact assessment – both of which were undertaken by independent organisations – show that little will happen to the sediment if it is left untreated inside the cells. Any exposed cell sediment would disperse very slowly into the marine environment, with no significant effect on marine organisms or risk to higher trophic levels or people, and the sediment has a higher shear strength than expected, which means that its ability to flow and spread would be limited.

Studies show the impact of the cell water on the environment will be small, gradual and local. Some of the water will rise to the surface and form a short-lived film, while a third of it will evaporate. We feel that we have applied a high level of conservatism to our modelling.

In arriving at the Comparative Assessment recommended option we carried out sensitivity tests by increasing the weighting of each of the five Department of Business, Energy and Industrial Strategy (BEIS) – formerly the Department of Energy and Climate Change – criteria (Safety, Environmental, Technical Feasibility, Societal and Economic) in turn to see if it would impact the score. It didn’t. Even when we removed cost as a criterion the overall outcome remained the same, with leave in place ahead as the best option.

In addition to the comparative assessments, which form the basis of the evaluation for the decision making process, we also consulted extensively over several years with representatives of our Cell Management Stakeholder Task Group (CMSTG). While some continue to prefer removal, on balance we believe the technical difficulties associated with removal, treatment and disposal are disproportionate to any environmental legacy benefit of removal.

Who Are The Cell Management Stakeholder Task Group

Cell Management Stakeholder Task Group

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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