Foundations account for a significant part of the cost of an offshore windfarm – making them more cost-effective can help meet ever-lower cost targets, and jackets rather than monopiles might provide the solution
Foundations can account for 30–50 per cent of the total cost of development of an offshore windfarm, so their design, construction and installation is an incredibly important area on which the industry needs to focus in order to continue to reduce the levelised cost of energy from offshore wind.
Continuous improvement and a fair amount of innovation will be required in a number of areas, not least because the size of turbines is growing all the time. 8–9 megawatt (MW) turbines are being offered by OEMs now, and 12 MW – even 15–20 MW units in the longer term – are on the drawing board.
However, as experts at Atkins in the UK told the Offshore Wind Energy 2017 conference in London in June, foundations capable of supporting 10–20 MW turbines are perfectly feasible.
Atkins has been reviewing foundation design, looking at potential future trends between now and 2025, including fixed and floating structures. It notes that foundation design will need to evolve quickly to meet the demands that new, larger, more powerful turbines will place on them.
Atkins believes that monopile foundations will continue to predominate in shallow water (0–40 m), but the shift to larger turbines of 8+ MW will favour the use of jacket foundations. The use of gravity base foundations is also gaining momentum for offshore windfarms with challenging ground conditions, and the use of floating structures is a great option for water depths exceeding 60 m.
The results of the work that Atkins did are reassuring in as much as the company believes that reducing jacket weight by up to 10–15% is possible. This was done by reviewing the requirements for installation, fabrication and design parameters such as corrosion allowance and local jacket joint design.
Having carried out extensive market research on foundations, Atkins believes it has identified a number of design drivers that can help reduce costs. It believes it has also identified cost-saving opportunities for standardisation – a view shared by ST3 Offshore in Poland, which believes that, although jackets are relative newcomers in the offshore wind space, there is significant potential for design optimisation. It also believes that jackets can be an attractive alternative to the monopile cost wise.
Working closely with Salzgitter and a number of major developers, ST3 Offshore has developed a standardised jacket foundation that it anticipates is adaptable to the majority of seabed types, water depths and turbines.
Norman Skillen, managing director of ST3 Offshore UK Ltd, noted that, with the downward forces on a foundation increasing with turbine size, these forces must be distributed through the foundation. “For large turbines and expected turbine development, jacket foundations present a technical and commercial alternative to building larger monopiles,” he told delegates.
He explained that ST3 Offshore’s standardised jacket foundation is built using prefabricated nodes, standard pipes and serial production techniques. The pre-fabricated nodes are prepared using robotic welding and automated manipulation as part of the series production process.
“ST3 Offshore has developed the design of a number of standardised jackets and found that they offer developers a real alternative in terms of a significant reduction in the levelised cost of energy and weight reduction. The jacket solution is also a more environmentally friendly solution, particularly when considered within the lifecycle cost of offshore windfarms,” Mr Skillen said.
Jesper Møller, head of offshore concepts and solutions, told the Offshore Wind Energy 2017 conference that, prior to the formation of Siemens Gamesa, Siemens Wind Power had been working on a gravity jacket concept since 2012.
“The governing thought in designing the jacket was to make it as simple as possible,” said Mr Møller. “From sourcing to assembling to installation, why use customised material if standard pieces will do? Why design a foundation that requires special installation equipment? Getting a high number of players involved around a jacket foundation can ensure that the design is optimised to manufacturing, transportation and installation.”
Mr Møller said internal and external projects that Siemens Gamesa has been running the last few years had provided valuable lessons on where the company and the industry as a whole needs to focus in order to achieve the desired cost levels.
“In addition, current standards for jacket construction are conservative and leave room for improvement,” he explained. “One way to achieve this is robot-welded nodes, which make it possible to reduce the likelihood of mistakes and enable more accurate, smoother welding.”
Mr Møller noted that jacket-type foundations have been used in the offshore industry before, as have standardised components, but have not yet been used in the offshore wind industry. However, Siemens is currently involved in a number of projects with partners to define new manufacturing standards for robot-welded nodes.
Among upcoming projects is one recently awarded by Innovation Fund Denmark that will bring together a developer, academia, welding firms and a manufacturer of foundations.
Mr Møller said that conceptual design of standardised jackets demonstrates that cost savings of up to 40% are realistic. “Working together along the value chain has provided valuable insights and benefits in design for manufacturing and transportation,” he said. “Fatigue tests of nodes have shown that robot-welded nodes are feasible, and the feedback from DNV has been very positive.
“Using standard components from an existing supply chain and ensuring flexible manufacturing, transportation and installation are crucial to ensure long-term cost reductions,” Mr Møller said. “Modularisation of jacket structures is important in order to avoid large upfront investment, which significantly reduces bankability. Modularisation can provide the necessary flexibility to deal with fluctuating market situations. In finding fast and effective assembly methods, a bolted jacket solution is one of the major contributors as it will work in and around the splash zone. Siemens Gamesa will work on this and potential other solutions,” Mr Møller concluded.
Representatives of ODE in the UK told delegates that, to date, most solutions for deeper water had focused on floating technology, but ODE believes that another type of fixed foundation, the articulated wind column (AWC), has much to recommend it.
They described the AWC as a ‘crossover’ technology with 40 years of use in the offshore oil and gas industry that ODE had re-engineered for mid- to deepwater environments (50–200 m). “Designed to accept turbines of 8 MW and above, field demonstration sites are now being sought with a 6–8 MW deployment in the range of 90–100 m water depth,” they explained, noting that the AWC provided a cost-effective solution combined with ease and flexibility of construction, ease of installation and removal and the ability to support very large turbines. Constructed from steel or concrete, the AWC has an articulated joint at the foundation’s base that enables it to be a robust, stable and durable solution for larger turbines in deep water.
“Engineering and model testing demonstrated that the AWC is economically and technically feasible for deepwater, large turbine applications when modelled with an 8 MW turbine in 90 m of water,” they told delegates. “Modelling demonstrated a tilt of more than 6 degrees occurred 0.002% of the time, well within parameters set by the turbine supplier. Looking for a demonstration site, it was found that the AWC increases the UK offshore wind market size,” they claimed, “and opens much of the north, west and southwest to offshore wind, which would be of particular relevance to Scotland, where only around 1 gigawatt of offshore power is accessible by conventional technology but deepwater technology would open up over 60 GW of potential.”
Updated standard to be published
Representatives of the University of Massachusetts, DNV GL and Dong Energy have described the development of an updated standard for bottom-fixed foundations. They explained that the International Electrotechnical Commission standard for bottom-fixed offshore wind turbine design (IEC 61400-3) has been significantly updated to reflect advances in offshore wind turbine technology since the standard was first published in 2009. To increase international applicability of the standard, more guidance was provided on ice loading of wind turbine structures and new guidance added on the prediction of tropical cyclone loading. Additionally, a number of changes were made to the standard to align it more closely to the latest updates to the IEC standard for onshore wind turbine design 61400-1.
A review of the standard was carried out by a team of 40 international experts from industry, academia and certification bodies. Ten meetings of the maintenance team were held over a period of four years, at which the requirements of the standard were discussed, the work of subgroups was presented and updates to the final text of the standard were agreed.
The starting point for the work of the maintenance team was a set of recommendations from IEC TC88 national committees following review of edition 1 of the IEC 61400-1 standard. Members of the maintenance team added their own experiences of using the first edition of the standard and suggestions for improvement. A committee draft for voting was circulated to national committees in 2017.
Significant updates include simplifications to the design load case specifications to make the standard simpler to apply, a greater focus on the assessment of site-specific environmental conditions, additional guidance on ice loading of offshore wind turbines and their support structures and new guidance on extreme loads that result from tropical cyclone events. The standard has also been aligned with the latest updates to the onshore wind turbine design standard IEC 61400-1.
“Edition 2 of IEC 61400-3 contains significant updates that embody the latest best practices of the offshore wind industry and make the standard easier to apply,” they explained, noting that edition 2 of IEC 61400-3 will be reviewed by national committees in 2017 and is expected to be published later this year. A separate technical specification for the design of floating offshore wind turbines is also expected to be issued in 2017.