ROYAL Dutch Shell has unveiled plans for decommissioning one of the biggest fields in the North Sea in a landmark move that should provide a major boost to the Scottish economy in coming years.

The oil and gas giant is launching a public consultation on removing one of the production platforms used on the massive Brent field north east of Shetland. This is a key step in the process for removing the production facilities for the field, which is expected to last more than 10 years and involve an outlay of billions of pounds.

Shell will have to remove four production platforms, 60 miles of pipelines and plug 140 wells on the field.

The size of the facilities means this will be the largest decommissioning project Shell has completed in the North Sea.

Hundreds more North Sea fields will run dry in coming years, although giants including Shell are working on huge developments off Shetland.

The director of the Brent decommissioning project Alistair Hope said the bulk of the budget will be spent in Scotland.

"I would say most of the decommissioning work, 85 per cent, is in the UK and most of that is in Scotland," he said.

"A lot of the work is for the plugging and making safe of the wells, most of that's done through Aberdeen," added Mr Hope. Aberdeen-based Wood Group was appointed to lead on the decommissioning of the first platform that will be removed, Brent Delta.

Shell said work on decommissioning Brent will support thousands of full-time UK jobs, many highly skilled, for years to come.

The prospect of such heavy investment in decommissioning work in Scotland will be welcome as the North Sea oil and gas industry reels under the impact of the slump in crude prices since June.

Shell last year cut 250 jobs from the exploration and production operation it runs out of Aberdeen. BP recently announced plans to axe 300 jobs.

Shell noted that the Brent field is just one of 470 fields North Sea fields that will require decommissioning over the next 30 to 40 years.

Industry body Oil & Gas UK reckons the bill could top £40bn.

Shell said: "As one of the first major fields to be decommissioned, Brent will enable UK companies to develop specialist skills and gain invaluable expertise... This knowledge and experience will give the UK an opportunity to become a leader in decommissioning projects, both in the North Sea and worldwide."

Shell has been working on plans to decommission the Brent field for years.

The work will involve closing down a field that has produced around four billion barrels of oil equivalent since it came onstream in 1976. This is almost 10% of all the oil and gas produced in the UK sector of the North Sea.

After deciding options such as reusing the production facilities for schemes in areas like carbon capture and storage were not viable, because of the age and location of the infrastructure, Shell decided to follow a two stage decommissioning process on Brent.

The first will involve removing the upper section, or topside, of the Brent Delta production platform. Shell will use lessons learned from this in finalising plans to decommission the remaining facilities in the second stage.

The plans need UK Government approval and must comply with the Oslo/Paris international convention.

The company will launch a 30 day public consultation on the first phase on February 16. The phase will involve using a giant vessel to lift the 23,500 tonne topside of the platform, the weight of 2,000 London buses, in one piece off the metal legs it stands on. These are in 140 metres of water.

Shell expects to complete the lift in 2016.

The topside will be transported to the Able UK yard on Teeside where around 97 per cent of the material will be recycled. Shell considered 130 yards across the UK before selecting the Able facility.

The removal and transportation process is expected to take 36 hours.

Shell has plugged all 40 wells linked to Brent Delta. It will spend the summer readying the topside to be cut free from the legs it stands on.

The second programme will include Brent Delta's legs, three other sets of topsides and legs, around 140 wells and 28 pipelines. Shell said it will submit proposals when the company is confident they are safe, technically achievable, environmentally sound and financially responsible.

Mr Hope highlighted the challenge Shell will face in dealing with the giant concrete legs that the three remaining platforms sit on, and associated subsea storage facilities. Removing such gravity base structures has not been attempted in the North Sea before.