Preparing for the topside lift
The 24,200 tonne ’topside’ of the Brent Delta platform is to be removed in one piece by a single-lift vessel. Using this single lift technique marks a major departure from traditional decommissioning methods where the topside is usually taken apart piece by piece in situ offshore.
After detailed technical and engineering studies, this technique was chosen by the Brent owners for three compelling reasons: it will substantially reduce the safety risk, cost and environmental impact of the operation compared with other methods.
Alistair Hope, Project Director for Brent Decommissioning, said: “Removing the topside in a single lift requires much less work offshore than traditional modular dismantling. There’s less cutting and lifting offshore, which in turn means a reduced chance of accidents.
“But while the lift itself will take seconds, there are months of preparation work ahead to make sure everything goes to plan. The first Brent topside is due to be removed and taken to shore for dismantling and recycling, and it could happen as soon as this year. It will be the heaviest lift for the oil and gas industry that the North Sea has ever seen.”
Summer work underdeck
One of the major activities during summer 2015 was the installation of eight specially designed cruciforms at the exact locations under the platform where the vessel’s lifting arms will make contact with the main deck. These cruciforms will allow the vessel’s lifting arms to be positioned accurately on pre-strengthened lift points.
Eric McWilliam is project engineer for the underdeck work: “This type of lift has never been attempted before which means we can’t just take equipment off the shelf and follow established procedures. Each stage needs to be meticulously planned and often requires bespoke equipment such as the eight cruciforms. And as always with offshore work – particularly if you are working underdeck - you can only work when the weather allows.”
The eight steel cruciforms were manufactured at a specialist fabrication yard in Dundee before being sailed out to the Brent Delta platform. Each 15 tonne cruciform was then lifted by crane onto the deck ready for installation. Offshore workers used a runway system, wires and chains to manoeuvre each cruciform into position before checking its precise positioning with 3D laser survey equipment.
Over a period of five months, a highly skilled team including scaffolders, abseilers and welders worked to fix the cruciforms in place. The team of 36 welders operated in shifts throughout the day and night in order to maintain continuity of the heating and welding operations. In total, the team completed more than 10 km of manual welding.
Eric said: “One of the most time-consuming aspects of the project has been getting the access scaffold in place to allow us to work under the platform. Essentially we have had to build a network of scaffolding underneath the entire deck. End to end the beams and boards would stretch more than 40 km.
“And as each cruciform is fixed into place, the access scaffolding then needs adjusting and rejigging to accommodate the next piece of work. It’s like an endless jigsaw puzzle.
“As reported on our website in Brent e-news in December, our aim was to install all the cruciforms by the end of 2015, and even though we were hampered by bad weather last summer, we managed to achieve this. But we are learning a huge amount as we go. All told, the first lift point took 53 full working days to install. The latest one took just 35 days. We are starting to review lessons learned and can already see ways to do this better on the other Brent Platforms.”
Winter work schedule
With all of the work offshore to prepare for the topside lift, work has recently been underway onshore to fabricate shear restraints – these are steel ring attachments that will be added to the inside of the legs ahead of the cut and lift taking place. A contract was awarded for this work in early November and activity has progressed steadily to prepare each individual section for fit-out which will take place later in 2016.
Offshore, throughout the Christmas and New Year period, work has continued with cutting and removal of the final well conductors, as well as deck strengthening and leg clearance scopes. Through the early part of 2016 further strengthening and sea-fastening of the topside modules will be performed in preparation for the lift.
- The Brent field, located north east of Shetland was discovered in 1971. It has provided oil and gas to the UK for almost 40 years but is now reaching the end of its life and, in compliance with legal requirements, is to be decommissioned.
- The field has produced around 10% of all UK North Sea oil and gas and generated more than £20bn of tax revenue for the UK since production began in 1976.
- Brent Delta ceased production on 31 Dec, 2011. All 40 of its wells have been plugged and made safe.
- Plans to decommission the field infrastructure are being submitted in two parts. The first decommissioning programme, approved in July 2015, addressed the Brent Delta topside only.
- The second decommissioning programme for the remaining Brent field infrastructure is currently being developed but has not yet been submitted.
Did you know?
- The Brent Delta topside measures 72m long by 47m wide and weighs more than 24,000 tonnes, equivalent to 2,500 London buses.
- Between March and October 2015, up to 100 construction workers were working offshore at any one time.