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Location
Ayrshire
Main Contractor
Value
£750k
Project Length
Introduction

The Hunterston site is situated immediately south of Fairlie on the Ayrshire west coast. One very large Wind Turbine and associated Metrological Mast remained on the site and scheduled for demolition by SSE following clearance of other structures progressively over the years.

The Hunterston site is situated immediately south of Fairlie on the Ayrshire west coast. One very large Wind Turbine and associated Metrological Mast remained on the site and scheduled for demolition by SSE following clearance of other structures progressively over the years. The wind turbine was bigger than most onshore turbines, being in fact an offshore turbine built on the site as a test facility, with the following impressive statistics:

  • Nacelle 100 metres above ground level
  • Blades 77 metres long – longer than the wing span of a Jumbo Jet.
  • Tower 6.0 metres diameter at the base
  • Tower shell high grade steel, 69mm thick at ground level
  • Combined weight of tower, nacelle and blades nearly 1000 tonnes

At 177 metres to the tip of the blades at the highest point, the wind turbine was a very visible navigation reference for sailors in the Firth of Clyde Estuary. It had however come to the end of its experimental life.

Although outwardly relatively isolated, the site and demolition proposals had two major constraints to deal with:

  • Magnox and EDF nuclear plants located approximately 1 kilometer to the south. Vibration levels arising from the demolition were accordingly a potential issue.
  • The surrounding Clyde Estuary shoreline is an important breeding site for migrating birds – the breeding season leaving a narrow window for the demolition.

Keltbray Decommissioning was appointed by SSE as Principal Designer and Principal Contractor for the project. Following comparative risk assessment analysis, the use of explosives proved to be the safest demolition methodology for both structures.

Specialist companies were drafted in to the Decommissioning team:

  • USA based Controlled Demolition Inc. (CDI) as specialist explosives subcontractor due to their vast experience in the use of Linear Shaped Charges (LSC’s) proposed by Keltbray Decommissioning as a mechanism to initiate the controlled collapse of both the wind turbine and met mast.
  • Wentworth House Partnership to carry out a finite element analysis of the turbine single leg tower during the pre-weakening works.
  • Vibrock to predict vibration levels – air overpressure and ground vibration – generated by the demolition event, and to monitor vibration on the day itself. This was particularly important to provide the required reassurance to the nuclear site operators and to the Office for Nuclear Regulation (the nuclear equivalent to the HSE).

The demolition proposal for the wind turbine was to cut out a birdsmouth wedge from the circular steel tower and replace for temporary stability with large steel columns. Calculations were prepared to prove that on LSC blasting of the replacement columns, the remaining heel section of the tower would fail – resulting in a controlled rotational collapse.

The met mast was of triangular form in plan. The proposal for this was to blast out the front leg to initiate rotation about the remaining two rear legs.

With all stakeholder approvals in place, and a Police Scotland Sea Patrol vessel keeping the Clyde Estuary 750 metre Exclusion Zone clear of all shipping and other vessels, the demolition took place on the morning of 26th September 2019.

The demolition was a success, both structures falling as designed, with the relatively strong cross wind causing the turbine to deviate by no more than two degrees from the planned alignment, well within the normal design parameters of plus or minus 10 degrees. A relatively soft landing was achieved with the blades breaking up and debris spread limited. Inspection of the column stubs confirmed the LSCs had performed as designed by CDI. The site successfully returned to SSE in early in December after all materials had been processed and removed.

It is worth noting that Keltbray Decommissioning’s risk-based approach on wind turbine projects has yielded three different methodologies to date – pulling, blasting and dismantling – with each project assessed on its individual merits. On this occasion blasting was used, resulting in a reduction in workforce risk by a factor in excess of 3 when compared to crane dismantling.