Brent Field Gravity Base Structures
What is a gravity base structure?
Gravity base structures (GBS) are among the biggest and most impressive feats of engineering in the oil and gas industry.
How to Decommission Brent: Gravity Base Structures
Title: How_to_Decommission_CUT_DOWN_1_Gravity_Based_Str_FINAL_MASTER-_211216_
Duration: 5:41 minutes
Description:
The challenges and solutions for decommissioning of the gravity base structures of three of the Brent platforms.
[Background music plays]
Instrumental music with synthesised effects, at times with softer tones – some delicate, some eerie and suspenseful – and at other times building to a stronger rhythm.
[Graphic]
Computer generated imagery of the outline of the gravity base structure at frame-left against a dark greenish background with lighter streaks descending from top of frame representing rays of light piercing the dark underwater environment.
[Text displays]
Gravity Base Structures
[Narrator]
Gravity base structures are among the biggest and most impressive feats of engineering in the oil and gas industry.
[Video footage]
Low angle footage of an oil platform, planted atop very tall concrete legs in the ocean, seen against the background of blue sky. In the foreground, three vessels are anchored in the ocean around the platform. Bird’s eye view of the platform and the vessels below, seen against the background of the ocean and some land. Wide view of the platform’s concrete legs rising from the ocean.
[Narrator]
Three of the Brent platforms are supported by reinforced concrete gravity base structures.
[Video footage]
Bird’s eye view of one of the Brent platforms surrounded by blue ocean waters. Black and white low angle view of the concrete legs of the gravity base structure under construction. Rear view close-up of a worker wearing safety gear using a cutting torch on a part of the structure.
[Narrator]
They were built in the 1970s to withstand 200 mile an hour winds and waves 25 metres high.
[Video footage]
High angle view of wet cement pouring into a mould. Panning bird’s eye view of the tops of three concrete legs, water visible below and land also visible at the top of frame. High angle view of four concrete legs, rough waters swirling around them. Footage of a vessel rising and dipping in rough seas, the Brent platforms visible in the distance.
[Narrator]
They’ve served their purpose effectively, but now their robust design is a challenge to decommission.
[Video footage]
High angle panning footage, showing the base of the topside structure and the tops of the legs below, rough waters swirling around them. Footage of workers dressed in hardhats and safety gear descending from the topside and down one of the concrete legs using a series of ropes and pulleys.
[Text displays]
Gravity Base Structures
Interview with Katy Lin
[Title]
Decommissioning Project Engineer
[Text displays]
Katy Lin / Decommissioning Project Engineer
[Katy Lin]
Gravity Base Structure, or GBS as we call it, is made up of a steel skirt, which anchors the structure to the bottom of the seabed. It’s made up of some cells for storage and legs.
[Video footage]
Mid-view footage of Katy Lin as she speaks, seen against the slightly out of focus background of an office environment.
[Animated sequence]
Computer generated imagery depicting a panning wide view of one of the Brent platforms seen against the background of grey skies above and grey swirling waters below.
An outline graphic appears in place of the previous imagery. The light blue shading in the upper frame depicts the sky, and is separated from the dark blue shading of the underwater environment in the lower frame by a white line. An outline of the gravity base structure is seen anchored to the bottom of the seabed. The storage cells and legs are highlighted in orange and yellow respectively as the speaker mentions them.
Interview with Alistair Hope
[Title]
Brent Decommissioning Project Director
[Alistair]
These gravity base structures are made of heavily reinforced concrete.
[Animated sequence]
Computer generated imagery of the gravity base structure continues as just described.
[Text displays]
Alistair Hope / Brent Decommissioning Project Director
[Alistair]
Each GBS weighs around 300,000 tonnes.
[Video footage]
Close-up of Alistair Hope as he speaks, seen against the slightly out of focus background of an office environment.
[Narrator]
Shell has investigated completely removing the structures from the sea.
[Animated sequence]
Low and high angle underwater footage panning the length of the concrete legs of the gravity-base structure.
[Katy Lin]
When these GBS’s were positioned out offshore in the 1970’s, they would’ve been towed out, filled with ballast to ensure they were grounded on the seabed.
[Video footage]
Mid-view footage of Katy Lin as she speaks, seen against the slightly out of focus background of an office environment. Extreme wide view of the tall legs of the gravity base structure under construction, seen against a pale blue-grey sky. Low angle footage of the tops of the legs, seen against the sky. High angle view of the top of one of the concrete legs, filled with ballast. High angle view of the ocean and towing vessels below, as from the point of view of the tops of the legs, the storage cells of the base structure also visible below.
[Narrator]
To refloat the Gravity Base Structures, this installation would need to be reversed:
[Animated sequence]
Computer generated imagery depicting a panning wide view of one of the Brent platforms seen against the background of grey skies above and grey swirling waters below.
Again, the outline graphic appears in place of the previous imagery. The light blue shading in the upper frame depicts the sky, and is separated from the dark blue shading of the underwater environment in the lower frame by a white line. An outline of the gravity base structure is seen anchored to the bottom of the seabed.
[Narrator]
Cells would need to be emptied, sealed and filled with air to induce buoyancy…
[Animated sequence]
The continuing computer generated imagery now zooms to a close-up of the storage cells, now shaded in yellow and darkish blue, depicting the sediment in the bottom of the storage cells and the water filling the rest of the cells respectively. This shading drains out of the storage cells, to be replaced by a small red layer at the top of the storage cells, and then a larger light blue below denoting the filling with air.
[Narrator]
The base would need to be excavated and hydraulics used to prize it from the sea-floor…
[Animated sequence]
The continuing computer generated imagery zooms slowly out again, showing the base rising slowly, then thick yellow vertical lines below each of the storage cells extend upwards to show the lifting of the base structure.
[Narrator]
…and a method of altering the buoyancy would need to be found for its journey to the surface.
[Animated sequence]
The continuing computer generated imagery shows the yellow lines dissipating, but the structure continues to rise.
[Narrator]
Independent studies suggest that successfully refloating the GBS would prove extremely difficult.
[Video footage]
Wide footage of Katy Lin and James Blackburn seated at a table and engaged in discussion, a laptop and documents on the table in front of them. Footage of the laptop screen and Katy’s hand pressing keys, causing the onscreen graphics to change, while James’ face is partially visible in profile at frame-right.
[Katy Lin]
Nobody’s ever done this before. You can have a scenario called “pop up” where you would lose control of the GBS structure.
[Video footage]
More footage of Katy and James in profile, both focused on a document in front of them on the table. Mid-view footage of Katy as she speaks, seen against the slightly out of focus background of an office environment. Close-up of the laptop screen containing graphics of the gravity base structure.
[Narrator]
Attempting to lift the structures from the seabed could also collapse them.
[Video footage]
Mid front view of Katy typing on her laptop while James, seated next to her, looks on. Low angle close-up of Katy panning to a close-up of James’ face, both of them focused on the laptop screen in front of them.
[Alistair]
It is technically extremely high risk of failure, and also a very high safety risk, far higher than we would accept in any other part of our operations.
[Video footage]
Slightly out of focus front view footage of Katy and James looking over documents on the table in front of them, cutting to a high angle view of the documents on the table under Katy’s hands, James’ face again partially visible in profile at frame-right. Close-up of Alistair Hope as he speaks, seen against the slightly out of focus background of an office environment
Interview with Duncan Manning
[Title]
Brent Decommissioning Asset Manager
[Duncan Manning]
Based on 30 years of data gathered on North Sea operations…
[Video footage]
Footage of workers dressed in hardhats and safety gear on the topside of one of the Brent platforms, preparing to descend.
[Text displays]
Duncan Manning / Brent Decommissioning Asset Manager
[Duncan Manning]
…that would indicate that we would have at least one fatality per Gravity Base Structure being refloated. And that is a risk which I am not prepared to entertain.
[Video footage]
Close-up of Duncan Manning as he speaks, seen against the slightly out of focus background of an office environment. More footage of the workers dressed in hardhats and safety gear descending from the topside and down one of the concrete legs using a series of ropes and pulleys. Close-up of Duncan as he speaks, seen against the slightly out of focus background of an office environment.
Interview with James Blackburn
[Title]
Brent Decommissioning HSE Manager
[James Blackburn]
So having done some deep technical studies…
[Video footage]
Wide footage of James and other colleagues seated either side of a long table in a conference room, their attention on the screen against the wall beyond on which an infographic is projected.
[Text displays]
James Blackburn / Brent Decommissioning HSE Manager
[James Blackburn]
…and also dialogue with our Independent Review Group, we’ve come to the conclusion that we won’t carry that option forward.
[Video footage]
Close-up of James Blackburn, as he speaks, seen against the slightly out of focus background of an office environment. High angle footage of James and colleagues seated either side of the long conference table, this time the windows and white walls behind James forming the background.
[Narrator]
Shell has also considered: Leaving the GBS in place and adding navigation aids, or cutting the legs 55 metres below sea level, in line with International Maritime Organisation guidelines.
[Video footage]
Rear view close-up of one of James’ colleagues seated at the table, and of another seated on the other side of the table, his attention on the laptop in front of him.
[Animated sequence]
Computer generated imagery depicts the tops of the legs of the gravity base structure seen against the background of swirling grey waters, panning around to a closer view of the tops of the three legs. One of the legs is seen to have a navigation aid planted on it, a red blinking light emitting from it.
The imagery zooms out and down to show the outline of the gravity base structure anchored to the bottom of the seabed, and a white line separating the dark blue shading of the underwater environment and the light blue shading of the sky above where the tops of the legs appear. As it is mentioned by the narrator, the legs are shown to be cut short, the top sections no longer visible in the outline.
[James Blackburn]
To better understand the complexity around cutting concrete is, we have actually done some trials onshore where we’ve formed reinforced concrete and we’ve also compressed it to try and replicate the conditions that are out there actually on the GBS’s.
[Video footage]
Close-up of James Blackburn, as he speaks, seen against the slightly out of focus background of an office environment. Rear view close-up of a worker dressed in hardhat and safety gear facing a block of concrete. A close-up cutting to a high angle view of a jet of water pounding the concrete alongside the drill bit cutting into the block of concrete.
[Narrator]
While they’ve achieved the cut in a test on land, there’s concern it may not work in practice.
[Video footage]
Close-up in profile of a seated John Gillies, panning up to the man leaning alongside him. Close-up of the computer screen in front of them, John’s hand pointing to one of the onscreen infographics.
Interview with John Gillies
[Title]
Brent Decommissioning Execution Manager
[Text displays]
John Gillies / Brent Decommissioning Execution Manager
[John Gillies]
As you may appreciate, making a cut without trapping that wire with 6,000 tons of dead weight bearing down on that cut surface, in a marine environment where you're dealing with the effects of waves, wind, a dynamic situation, is certainly extremely challenging.
[Video footage]
Close-up of John Gillies, as he speaks and points to a model of the base structure to his right, visible at frame-left. Rear view close-up of John and the base structure imagery on the screen he is pointing to. This cuts to a wider view of John and his colleague leaning alongside him, their attention on the onscreen imagery. Side view, cutting to a tilted front view, of a vessel rising and dipping in rough seas.
[Narrator]
The cost of this partial removal would be very high, which is something Shell is required to consider.
[Video footage]
Aerial view of one of the platform topsides and the dark blue seas below, pulling back to a wider view of the same. Low angle view of part of a platform topside and the tower crane alongside, seen against a pale blue sky.
Interview with Roger Esson
[Title]
Chief Executive, Decom North Sea
[Text displays]
Roger Esson / Chief Executive, Decom North Sea
[Roger Esson]
The UK tax payer will be funding part of the cost of decommissioning, so it’s really important that the operators are looking to carry out the work in the most cost effective manner.
[Video footage]
Close-up of Roger Esson as he speaks, seen against an out of focus background of an office environment. Mid-view footage of workers dressed in hardhats and safety gear aboard a platform, blue seas and skies visible in the background. Wide high angle view of a platform topside seen against the background of sea and sky, a helicopter seen approaching in the distance.
[Narrator]
Brent’s profits were taxed in excess of 70%. It paid £20 billion, in today’s money in taxes.
[Video footage]
Low angle view of the helicopter landing on the topside against the background of a cloudy blue sky. Slightly panning bird’s eye view of one of the Brent platforms surrounded by blue ocean waters.
[Narrator]
Shell can now claim a 70% reimbursement on the cost of decommissioning, from the taxes it previously paid. And while the final costs of decommissioning are not yet known, Brent will remain an overwhelmingly positive contributor to the UK treasury.
[Video footage]
Bird’s eye view of workers dressed in safety gear working to attach ropes to one of the many containers aboard the platform. Footage of an elderly worker dressed in safety gear, walking along the platform, inspecting equipment. Extreme wide aerial view of workers dressed in safety gear aboard the platform topside, guiding components which are being hoisted off the platform via a pulley system. Bird’s eye view of a red container being similarly hoisted against the background of ocean waters. Mid view footage of workers dressed in safety gear guiding the red container into place.
[Narrator]
Having weighed up all the options, Shell is not recommending the partial removal of the legs.
[Video footage]
Low angle footage in profile of Duncan and Alistair seated at a desk. Duncan picks up a document off the desk while Alistair looks at the screen in front of him. Close-up in profile of Alistair looking down, the back of Duncan’s head only slightly visible. Rear view footage of the men seated at the desk, their attention now on the computer screen which contains an infographic related to the topside and gravity base structure. A close-up of the infographic also shows Alistair’s hand as he points with his pen to various components displaying on the screen.
[Alistair]
Our recommendation based on 10 years of study, and independent review and analysis by our Independent Review Group, is to leave the concrete gravity base structures with the legs up, in place.
[Video footage]
Close-up of Alistair Hope as he speaks, seen against the slightly out of focus background of an office environment.
[Animated sequence]
Computer generated imagery depicts a panning view of the tops of the legs of the gravity base structure seen against the background of swirling grey waters, the red light of the navigation aid atop one of the legs blinking against the dark background.
[Narrator]
This would involve capping the legs with concrete and installing navigation aids. The structures would be marked on maritime charts and in FishSafe, the fishermen’s GPS warning system.
[Animated sequence]
The continuing computer generated imagery highlights the concrete caps of the legs in yellow and the navigation aid in red, as they are respectively mentioned by the narrator. A FishSafe map fills the screen displaying symbols and lines that chart the offshore surface and subsea oil and gas structures.
[Duncan Manning]
One of our key contractors came to the conclusion that actually the collision hazard was about 1 in every 10,000 years, so well under any accepted standard.
[Video footage]
Close-up of Duncan Manning as he speaks, seen against the slightly out of focus background of an office environment. Wide view of numerous large vessels moored in a harbour, glistening waters in the foreground, a city skyline and cloudy skies in the background.
[Narrator]
Experts estimate that the GBS legs will degrade to below sea level in around 250 years. Then their position would be marked with buoys.
[Video footage]
Close-up of a screen displaying infographics related to the base structures a finger pointing to one of the graphics.
[Animated sequence]
Low and high angle underwater footage panning the length of the concrete legs of the gravity-base structure.
[Roger Esson]
The option to leave the gravity base structure in place, really mirrors what’s already happened in the Norwegian sector where the structures have been left in place with navigation aids on there, to aid shipping and fishing.
[Video footage]
Aerial view of one of the Brent platforms seen against the background of the ocean. Wide view of one of the Brent platforms seen against the background of sea and sky. Close-up of Roger Esson as he speaks, seen against an out of focus background of an office environment. Side view of a vessel moored in the harbour, buildings in the background, and glistening waters and sea birds in the foreground.
Interview with Bertie Armstrong
[Title]
Chief Executive, Scottish Fishermen’s Federation
[Text displays]
Bertie Armstrong / Chief Executive, Scottish Fishermen’s Federation
[Bertie Armstrong]
As a general principle, the fishing view is, if it’s going to be there and going to cause an obstruction, then let’s be able to see it. Legs up, is the choice of the fisherman.
[Video footage]
Close-up of Bertie Armstrong as he speaks, seen against an out of focus background of an office environment. Wide view of Duncan Manning standing at the front of a darkened room next to a projector screen on which infographics related to the base structure are displayed. The backs of people’s heads are visible in the foreground.
[Text displays]
With thanks to CUT UK
[Text displays]
Shell.co.uk/BrentDecomm
The Brent Field’s equal partners are Shell UK Ltd and Esso Exploration and Production UK Ltd
[Audio]
Shell jingle.
[Graphic]
Shell Pecten centred on a white background with text displaying below.
[Text displays]
© Shell International Limited 2016
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.
Ospar Derogation
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:
Precedent
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.
Leg Cutting
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.
Cutting trials
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.
Next steps
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.
Outcome
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.
Brent towout
A close up of the Brent GBS and topside
Brent GBS during construction in 1975
Looking up at the Brent Bravo GBS
Brent Bravo GBS through the clouds
Brent Bravo getting towed to position
Brent Charlie GBS in the stormy weather
The Charlie GBS during tow-out
Construction of a gravity base structure
Tow-out of a gravity base structure
Construction of the GBS - starting with the cells
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