BLOG – 21 MARCH 2023
Floating wind and the need for multiple ‘stepping-stone’ projects to deploy and demonstrate a wide range of critical technologies
Alex Gauntt, Supply Chain Director
It’s National Renewable Energy Day in the States so a good opportunity for me to reflect on what’s happening in the world of floating off-shore wind as campaigners raise awareness of the benefits of using renewable energy sources.
March 2019 was a bittersweet moment for the offshore wind sector in the UK. The two turbine offshore wind farm, Blyth, at the time owned and operated by E.ON (now RWE) started its decommissioning journey. The project, which was built in 2000 by a consortium made up of Border Wind, Powergen Renewables, Nuon UK and Shell Renewables, was the UK’s first offshore wind project, kick-starting an industry with incredible scale and ambition. Less than 2 km from the Northumberland shore, in less than 10m water depth on average (and not to be confused with the onshore Blyth Harbour Wind Farm along the East Pier of the Port of Blyth commissioned in 1993), the Blyth project consisted of two Vestas 2 MW machines, capable of producing sufficient power for more than 3,000 homes, situated on the UK’s first wind farm monopile foundations.
We cannot state for certain whether the fixed-bottom offshore wind projects in the UK we are seeing announced now, with projects in excess of 1 GW (250 times the size of Blyth) consisting of upwards of 15 MW per turbine and in water depths of 50m or more, would have been possible without the pathfinder aka ‘stepping-stone’ projects such as Blyth; but the industry certainly learnt a lot from such projects, lessons which have materially shaped the development and supply chain strategies of offshore wind farms ever since.
In terms of foundations, the UK was introduced to the concept of a monopile through such a project. A single, rolled and welded mono-tube, driven into the seabed, topped with a transition piece and turbine tower on which the turbine nacelle and rotor is perched, the monopile is bejewelled with secondary steel such as ladders, fenders and cable j-tubes, is a relatively simple device and suits shallow waters. However, this technology swiftly found it had an economic and feasible limit for deployment (limits which were often argued about and debated…and still are to this day) so a new technology was needed. Along came the ‘jacket’ foundations which are lattice towers of multiple rolled tubulars and cross braces to save steel (for cost) and weight (for installation complexity) for deeper water sites. Using primarily oil & gas technology and design codes, the first full-scale deployment of jacket offshore wind foundations in the UK was at the catchily named Distant Offshore Windfarms with No Visual Impact in Deepwater (DOWNVInD) project. Situated adjacent to the Talisman operated Beatrice oil field around 22 km from shore in water depths of up to 45 m, this project was developed by Talisman Energy and Scottish and Southern Energy under a partially EU-funded research project, and fully commissioned in 2007. It consisted of two 5 MW RePower turbines perched atop transition towers on transition pieces, affixed to the forementioned jackets which also held the secondary steel components required.
And that’s it pretty much for fixed bottom offshore wind foundations (except for gravity base type foundations – but that’s a whole story in and of itself and doesn’t feature in the UK very much). Almost every fixed bottom offshore wind farm in the UK is perched atop some sort of monopile or some sort of jacket foundation. 50-60m was generally considered as the economically feasible limit for deploying offshore wind farms in the UK…and then came floating.
As many of you will be are aware, there is a finite amount of relatively shallow water. Although the UK has been abundantly blessed with this asset, it does not equally service all geographies and regions. In areas such as the Southern Celtic Sea, or offshore North Scotland where wind abounds, but the ground is mostly out of reach for fixed-bottom foundations, the answer is floating.
However, floating offshore wind poses a particular challenge, in that there is no clear monopile or jacket (or indeed gravity base) equivalent and there are clear benefits (and challenges) to multiple technology types. The technologies currently deployed at ‘stepping-stone’ scale in the UK, consist of semi-submersible – a sort of floating tubular lattice structure with ballast and buoyancy – and spar-buoy-type foundations – a floating, upright ballasted tube – which both pose their own challenges to the industry and have yet to be deployed at fixed-bottom wind cost-competitive scales.
Semi-submersible foundations are inherently less stable than other types, unless over-sized to create additional stability, meaning that the turbine on top has to deal with a lot more movement than in the fixed wind environment. Meanwhile spar-buoy type solutions typically require extremely deep construction port facilities just to build the foundations, and there just aren’t that many deep-water ports capable of handling these requirements in the UK.
More technologies are required to find the best possible solution to the floating wind challenges in the UK of balancing supply chain capabilities with economic requirements. One exciting technology not yet deployed in the UK is the tension-leg platform (TLP), a smaller, simpler steel structure than a semi-submersible that has tensioned, typically vertical, moorings affixed to the seabed directly underneath the foundation.
The TLP technology offers potentially the most stable of floating wind turbine foundations but prefers deeper water deployment due to the additional cost and complexity of engineering the deployment and anchoring systems. Another exciting opportunity is in concrete, there are huge concrete semi-submersible designs, and concrete barge technologies, neither of which have yet been deployed in the UK at commercial scale.
Each of these technologies offers a particular opportunity to the UK supply chain where certain areas of the UK are well equipped to support concrete production and deployment, while others favour steel, and each have a different economic and engineering proposition to offer the developers and owner/operators of floating wind farms. But until we deploy examples of all the technologies in stepping-stone projects, on a competitive basis, we are not going to sufficiently de-risk them for the next generation of ‘industrial scale’ floating wind projects resulting in a limited choice of technology options potentially unoptimized for the specific UK requirements. After all the first monopile and jacket wind farms weren’t a hundred turbines each for many reasons, not just the cost…