We supported E.ON (shortly to be named RWE Renewables) in 2000 with the drilling of the UK’s first offshore wind farm in Blyth. Returning 19 years later we’ve assisted with the decommissioning of the same site, safely removing the turbines with minimal impact on the environment which demonstrates our vital contribution to the full life cycle of the asset.
The site consisted of two 95 m high, 2 MW wind turbine generators each mounted on a 3.5 m diameter monopile fixed in a pre-drilled grouted socket. E.ON’s challenge was to safely remove the turbines for recycling with minimal impact on the environment. The irregular seabed and ravines of sandstone outcrop at the site called for a specialist, low impact approach.
Our solution for minimising environmental impact was based on Excalibur’s cruciform capped jacking legs. Low impact legs are one of the many innovations found on our jack-ups and ensure a highly mobile, light footprint approach for work on wind turbines and cable arrays in nearshore areas and shallow transition zones.
Our jack-up barge Excalibur equipped with a 230 t crane was deployed to decommission the two turbines and foundation structures. Given the monopile weight and the need to remove it from beneath the seabed, we needed to provide a robust technical solution for the removal to overcome not only the weight of the pile but also any embedment friction accumulated during the construction phase and environmental impact over the last 20 years.
Our award-winning solution utilised a load-bearing platform located on Excalibur’s existing aft moon pool reaction beams. Jacking stools, equipped with hydraulic cylinders, exerted 600 t of hydraulic lift to successfully remove the monopile in a safe and controlled manner with minimal impact on the marine environment.
To remove the monopile, we cut the structure 0.5 m below the seafloor using a water-jetting tool. The jack-up’s integral reaction beams were then used to lift the monopile free before transferring it to Excalibur’s deck and onward to recycling. We then filled the remaining socket level to the seafloor with 250 t of ballast stone to return the installation site to a natural condition.
A 1.4 km length of heavy-duty cable was disconnected, terminated and safely earthed to discharge any residual current. Following termination, a Multicat vessel winched the now-redundant cable onboard for eventual disposal. Where the cable ran through an offshore submerged sand bank, a mass-flow excavation tool was used to expose the cable to continue recovery. Where the cable lay in an intertidal location, we recovered it using two 15-tonne 360° excavators. Varying our methods according to the topography through which the cable ran was key to our success and resulted in safe and efficient operations, and full cable recovery.
We won the Innovation Showcase Award in the Renewable Energy category at the 2019 European Commercial Marine Awards.
Fugro’s innovative offshore wind farm (OWF) foundation jacking solution provides cost and safety advantages to owners and operators tackling the challenges of wind farm decommissioning. Our approach is based on robust engineering and efficient operation, which removes the cost and deployment challenges of using bigger cranes and jack-up barges.
Fugro provided E.ON with surety of delivery through our intimate knowledge of the project. In 1998, Fugro conducted the overwater site investigation to inform the design of Blyth’s monopile foundations. Two years later, we returned to drill and grout both the foundation piles. The decommissioning marked the completion of the asset’s life cycle and exemplifies Fugro’s commitment to deliver.
Seeing Excalibur return 19 years later to assist in the decommissioning of the same turbines demonstrated our vital contribution to the full life cycle of the asset and was a memorable moment for our team.
The Blyth decommissioning brought its own set of unique challenges, just as it did when the turbines were constructed almost 20 years ago. The site’s proximity to shore and the fact that it was the UK’s first [offshore wind] decommissioning project generated a lot of interest, which added additional pressure for positive completion.
I would like to thank Fugro; their expertise, professionalism and knowledge were key factors that enabled the successful on-time decommissioning of the UK’s first offshore wind farm without incident.
As a result of Fugro’s work on this project, the benchmark has now been set for future decommissioning activities.
Patrick Rainey, E.ON Offshore Logistics Manager