Larger turbines and deeper waters will create extra foundation challenges. (Image credit: Kruwt)

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As offshore wind developers expand into new sea regions and install larger turbines, developers are trialling alternative fixed-bottom foundations in search of new efficiency gains.

Competitive tenders are spurring developers to install larger, higher efficiency turbines while maximizing operational lifespans, to reduce the levelized cost of electricity (LCOE).

Foundation costs represent around 20 to 25% of total project capex. Steel monopiles offer the lowest capex and represent 82% of installed capacity in Europe, but growing project dimensions are accelerating the deployment of suction bucket designs and jacket structures.

Suction buckets use a large, steel open cylinder which is planted and fixed into the seabed fixed using negative pressure.

Last year, Vattenfall became the first developer to install a commercial scale wind farm using suction buckets and jacket structures. Vattenfall installed nine 8.4 MW MHI Vestas turbines and two 8.8 MW models at its European Offshore Wind Deployment Centre (EOWDC) in Aberdeen Bay, north-east Scotland.

“One of our 1,800 tonne suction bucket jacket foundation was installed in what we believe is a world record of two hours and 40 minutes from the time the installation vessel entered the offshore site until deployment was complete,” Adam Ezzamel, EOWDC project director at Vattenfall, said.

Orsted, the world's largest offshore developer by capacity, installed 20 suction bucket foundations at the Borkum Riffgrund 2 offshore wind farm in Germany last year. Siemens Gamesa and Denmark’s Aalborg University will also test a 1,000 ton bucket foundation and steel jacket as part of a 20 million euro offshore wind research project co-funded by the European Union.

Suction buckets require no driving into the seabed and can be installed with a single lift, reducing offshore installation times and costs, Ioannis Lessis, Wind team leader at engineering consultancy Mott MacDonald, told New Energy Update.

The relatively simple design of suction bucket foundations offers potential weight savings, and placement emits minimal noise, unlike traditional monopile installation, Lessis said.

"Savings will come from reduced weights (avoiding the lengthy monopile embedments), reduction of installation time and complexity," he said.

Projects needed

Limited manufacturing and deployment experience of suction buckets in the offshore wind industry means that costs are yet to compete with traditional monopile designs.

                      Offshore wind foundations installed in Europe by end 2018

Source: WindEurope

The principles of suction bucket technology mean they are highly sensitive to soil conditions and unsuitable for shallow water environments while the dynamic loading properties of suction bucket designs remain relatively unproven.

Wider commercial deployment will help developers reduce risks and improve design and installation efficiency.

Growth in monopile foundations has allowed manufacturers to build out efficient production lines and suction bucket developers will need to use economies of scale to make costs more competitive, Lessis said.

"Standardization is another factor, with multiple suction bucket concepts currently being explored," he said.

In one example, Orsted chose monopile foundations over suction bucket jackets at the 1.2 GW Hornsea One project in the UK due to "overall project timeline considerations and limitations of serial production capacities" for suction bucket jackets, it said.

Larger units

Jacket foundations are set to become more prevalent as developers install larger units in deeper water sites, placing greater strain on foundations.

The steel lattice design of jackets offers lightweight and stiff structures which can be modified according to site geometry and has relatively low dependency on soil type.

Jacket developers can also leverage decades of operational expertise in the oil and gas industry. Jackets have already been used on some large offshore wind projects, including the 350 MW Wikinger offshore wind farm in Germany and the 588 MW Beatrice and 714 MW East Anglia One projects in the UK.

               Offshore wind foundations installed in Europe in 2018

                                                            (Click image to enlarge)

Source: WindEurope

The smaller steel tubulars which make up the jacket structure can be fabricated in multiple locations and transferred to the assembly side, providing some logistics savings, Lessis said.

However, manufacturing is more complex and labour-intensive, incurring extra costs, he noted. A lack of standardization also means that manufacturing cannot benefit from automation gains used in monopile fabrication.

Developers should use oil and gas manufacturing infrastructure and economies of scale to reduce the cost of jackets for offshore wind applications, Lessis said.

"The main European fabricators are well-equipped to cope with jacket serial production and have significant experience with this type of foundation," he said.

Driving innovation

Going forward, rising turbine capacities and deeper waters will spur new improvements in foundation efficiency.

"Increased turbine sizes and deeper waters will lead to exploring more economic and technically innovative solutions. Jacket foundations will likely be able to fill this role, with suction buckets deployed in more sites, but restricted by the sensitivity of the technology to the soil conditions," Lessis said.

Innovations in monopile and hybrid material designs could also impact demand for alternative foundation types.

Monopile designers are developing wider diameter units and pushing the limits of manufacturing and installation to accommodate growing project dimensions.

Monopile suppliers have widened pile diameters to 8.0m and are developing widths beyond 10m. Designers are targeting water depths of 40 to 45m, compared with just 30m only five years ago.

Demand for jacket designs in deeper waters may be threatened "if the 10-12m diameter XL monopiles currently in conceptual phase become a reality," Lessis said.

By Neil Ford