Typically, they consist of one or more concrete legs that support offshore oil and gas platforms above a base of oil storage cells. The GBS are made of thick concrete reinforced with steel bars and are ballasted down during installation, anchoring them securely to the seabed.
GBS were designed to overcome the challenges of producing oil and gas in the North Sea during the 1970s. Inherently strong and robust, they could support the unusually heavy processing facilities for production from North Sea wells, and withstand the extreme operating conditions of the stormy northern seas. At that time the North Sea pipeline network was still in its infancy, so oil had to be stored temporarily in cells on the seabed then pumped into tankers for transport to market.
There are 42 concrete gravity base structures in the world, 27 of which are in the OSPAR Convention region of the North Sea and north-east Atlantic. There are 12 GBS in the UK sector of the North Sea – nine of these are currently in operation and three have already been decommissioned.
Size and scale of the structures
Of the four production platforms at Brent, three are gravity base structures: Bravo, Charlie and Delta. Bravo and Delta are of a similar design.
Each structure has either three or four concrete legs almost 20 metres in diameter and up to 165m tall.
There are 64 storage cells in the three concrete bases, 42 of which were used to store oil. These cells are 60m in height, up to 20m in diameter, with concrete walls almost 1m thick. They are taller than Nelson’s Column.
Each of the Brent Field’s three GBS weighs approximately 300,000 tonnes, around the same as the Empire State Building in New York.
To this day, Brent is still one of the largest oil and gas fields ever developed in the world and the structures required to unlock its energy reflect that.
What are the decommissioning options?
At the time of their construction, decommissioning was not at the forefront of people’s minds and was not a prominent design consideration. Since then, legislation and technology have changed, and offshore facilities installed in the north-east Atlantic after 9 February 1999 must now be designed to be completely removed.
For the Brent structures to be removed they would have to be detached from the seabed, floated to the surface and towed to shore for dismantling in a deep-water inlet or fjord. Over the past decade, Shell has carried out exhaustive technical feasibility studies into this option. Due to the age and design of the structures, the way they are secured to the seabed and the unpredictable manner in which they could rise to the surface, the project believes the risk is too great for a refloat operation to be performed. The probability of technical failure and the risk to human life and the environment are unacceptably high.
We have therefore focused our attention on two decommissioning options and examined each one in detail. The two options are: 1) partial removal of the legs, and 2) leave in place.
Option 1: Partial removal of the legs
After the topside (platform portion of the structure) has been removed, the upper part of the legs of the gravity base structures would be cut at a point 55m below sea level, in line with International Maritime Organisation (IMO) guidance. The rest of the structures would be left in situ on the seabed. The location of the remaining structure would be clearly marked on charts and documented in FishSAFE, a database that helps fishermen avoid oil and gas structures in UK waters. We would apply for a continuation of the existing safety zone around the gravity base structures.
Option 2: Leave in place
Once the topsides have been removed, the gravity base structures would be left wholly in place. As with option 1, the location of each structure will be marked on all charts documented in the FishSAFE database, and protected by a safety zone. On each GBS, one of the legs will be fitted with aids to navigation.
Left in place, it is predicted that the legs would degrade slowly as the seawater penetrates the concrete and the steel bars begin to corrode. According to our analysis, this slow process of degradation and corrosion would have no measurable impact on the environment. Small pieces of inert concrete will separate from the legs and fall to the seabed or on top of the cells over time.
How and when the legs will eventually collapse is difficult to predict, but our studies suggest it is likely that the visible part of the legs would remain in place for 150 to 250 years. Once the visible part has degraded, the subsea section of the legs is expected to last for another 300-500 years. Despite being punctured and damaged, the oil storage cells themselves may remain largely upright for at least 1,000 years.
What does Shell’s comparative assessment recommend?
Although OSPAR Decision 98/3 generally prohibits the disposal of offshore installations at sea, it does recognise that the dismantling of large concrete structures like those at Brent could pose a significant risk to safety and the environment. OSPAR Decision 98/3 therefore provides for the possibility of a derogation (exemption) from the general rule on removal, reuse, recycling and final disposal on land in appropriate cases using a process known as ‘Comparative Assessment’.
In accordance with these requirements, the topsides of all three platforms will be removed, transported to shore, dismantled and recycled.
After using the comparative assessment process to assess the decommissioning options in terms of the five criteria of: technical feasibility, effects on society, environmental impact, risk to safety, and cost, Shell’s recommendation is to leave the GBS in place.
Cutting and removing the upper portion of the concrete legs would be technically challenging and very costly. Each upper section has a diameter of almost 20 metres and weighs more than 5,000 tonnes. It would have to be cut, raised to the surface and towed to shore for disposal, a process that has never been attempted before.
There would be significant safety risks in cutting and removal – risks that have been assessed at a rate which is 40 times higher than the industry accepted safety level for offshore operations – and with very little environmental benefit.
Shell believes the minimal risk to other users of the sea from leaving the legs in place is manageable and could be minimised by the appropriate navigation and warning measures.
Although removing the GBS would support jobs, these would only be on a short-term basis.
Shell’s conclusion is that the cost, technical feasibility and safety risk of removing the upper part of the legs is disproportionately high, compared to our recommended approach of leaving the legs in place.
The UK is a signatory to the OSPAR Convention. Signatories to the convention agree to specific rules designed to limit the impact of human activity on the marine environment of the North East Atlantic. Under the Convention, it is expected that oil and gas infrastructure will be removed from the marine environment during the decommissioning process. However, binding decisions of the parties to the Convention allow for infrastructure to be left in place in certain circumstances, but only where the competent authority of the State is satisfied that an assessment shows there are significant reasons why this approach is preferable. Reasons could include, for example, that it would be technically difficult to remove the infrastructure, or that the process of removing the infrastructure could be more harmful to the marine environment than leaving it in place.
In the Draft Brent Decommissioning Programme, and after extensive analysis and consultation, Shell proposes to leave some of the existing infrastructure in place – this consists of the concrete gravity based structures and the storage cells on Brent Bravo, Charlie and Delta, in addition to the footings of the Steel Jacket on Brent Alpha. Once the public consultation period has ended, and the Department for Business, Energy and Industrial Strategy (BEIS) is satisfied that there are sufficient grounds to recommend that this infrastructure be left in place, BEIS will initiate consultations with other signatories to the OSPAR Convention to seek their approval to leave this infrastructure in place. This process is known as ‘seeking a derogation’ and can take up to eight months to complete.
More information about the OSPAR Convention, and the institution that was created to implement the OSPAR Convention (the OSPAR Commission) can be found here:
This approach is not without precedent. Five GBS have already been decommissioned in the North Sea to date and all five were granted a derogation and left wholly in place. In addition to this, there are many war-time structures, such as the Maunsell Forts in the Thames and Mersey Estuary, which have been decommissioned and the structures left in place.
What are our conclusions based on?
Since 2007 Shell has examined the options for decommissioning the GBS exhaustively, carrying out more than 70 studies and trials to assess each option in detail. These studies are comprehensive in scope and include:
- analysing the technical challenges and feasibility of detaching the structures from the seabed and refloating them;
- examining how the legs could be cut underwater, raised to the surface, towed to land and disposed of;
- testing the techniques and equipment required to cut and lift the legs;
- calculating how long it might take for the structures to degrade and collapse; and
- evaluating the safety risk to other users of the sea if the structures are left in place.
These studies were conducted by independent contractors, including the company that originally designed the Bravo and Delta structures, consultancies and academia, as well as by our own experts, who draw on Shell’s long experience in offshore oil and gas installations worldwide.
As referred to in Option 1, studies have been conducted focussing on the feasibility of partially removing the legs, the parts of the GBS which extend above the cells and support the platform topside. Removing the upper portion of the legs to a depth 55 metres below sea level would be required to comply with the International Maritime Organisation (IMO) recommendation on the partial removal of structures.
In order to test a method for cutting through the reinforced concrete legs, onshore cutting trials have been conducted. Each trial used a test piece of concrete that was designed to replicate the concrete geometry and was scaled to a ratio of 1:20 to the GBS. A compression frame was used to apply 500 tonnes of force to create conditions similar to the offshore cut of the legs below sea level. These test parameters allowed the team to further develop knowledge and techniques.
The trials examined how diamond wire cutting could be used to make the cuts. In the first test, only diamond wire was used. In the second, hydraulically inflatable wedges called “shims” were inserted as the diamond wire cut progressed, to keep the cut open and stop the wire from being jammed deep in the cut.
Whilst the leg was successfully cut using the method with diamond wire and shims, the difficulties of scaling this test up in a harsh offshore marine environment and then conducting the cut underwater, across a larger diameter under greater compression would be considerable and not without significant technical and safety risk.
After cutting, the legs would need to be transported to shore for recycling. This involves:
- Attaching lifting strops to the cut portion of the legs;
- Lifting and transporting the legs; and
- Receiving the reinforced concrete legs onshore, dismantling and disposing.
As with cutting the legs, these activities have not been undertaken before and would require new technologies and procedures. The legs (which each weigh around 6,000 tonnes) would need to be lifted and secured to preserve their integrity during transportation to shore by heavy lift vessel for dismantling and recycling.
Although the tested cutting method was successful, and preliminary lifting concepts and means of transport to shore were identified, we believe that the forecasted low-level, very long term potential risks to other users of the sea are outweighed by the very tangible risks to which project personnel would be exposed by operations involved with the cut, lift, transportation and final onshore recycling of the legs if removal was selected. We therefore recommend that the GBS legs remain in place.