Inroads are being made by jackets and gravity-based foundations, but monopiles will remain the foundation of choice for offshore wind turbines for the foreseeable future
According to a report from Renewables Consulting Group (RCG), Offshore Wind Foundations: A European Overview, by 2022, the number of fully commissioned offshore foundations in Europe is expected to approach 6,000, and the greatest number of these will be monopiles rather than jackets or gravity-based foundations.
“The market share of each technology group is not expected to change dramatically. However, RCG does expect the share of monopiles to reduce slightly compared to that of jackets and gravity-based structures,” said the consultants.
“This is expected to be driven by the deployment of jackets at Wikinger (Germany), East Anglia 1 and Beatrice (UK), as well as gravity-based structures at the likes of Blyth (UK), Tahkuluoto (Finland) and Fécamp (France).”
RCG said a key reason why alternative foundation solutions have not had greater impact on the market, apart from the slow development of the required industrialised processes for manufacturing and installation, is the constant development of monopiles.
It noted that the maximum diameters and weights associated with monopiles continue to evolve, and the technology (including their method of installation) has kept pace with both the move towards deeper waters and the increase in turbine capacities (and weight).
Monopiles have been deployed successfully in water depths of up to 41 m, and RCG expects the technology to continue to be a competitive solution for projects in water approaching this depth where soil/ground conditions are suitable.
“The market may yet see the solution utilised at depths beyond this. However, recent improvements in the industrialised manufacturing and installation process for alternative solutions and the impending introduction of next-generation turbine technology is expected to intensify the level of competition at this depth range,” RCG said.
The choice of foundation type at any given project is governed primarily by the water depth and soil conditions encountered at the site, while the lack of a suitably industrialised manufacturing and installation process for some solutions remains a key consideration. Likewise, the size and specification of the selected turbine plays a significant role in the final design of the structure.
The overwhelming majority of offshore wind turbines use monopile foundation technology. The remaining commercial fleet comprises a relatively equal mix of jackets, gravity-based structures and tripods, while a handful of single installations at demonstration sites complete the picture and include innovative suction bucket, hybrid jacket and floating substructure technologies.
RCG’s report summarises trends in the development of different foundation types, including projections for the commissioned fleet of turbines. The key features of each foundation concept are summarised, together with the main advantages and relative disadvantages of each type.
In the UK, Atkins is seeking the most efficient monopile design for the Triton Knoll offshore windfarm. The company is working as a subcontractor to a Smulders Sif Steel Foundations joint venture that is looking for a monopile foundation that will be more efficient than any similar units in order to help reduce the overall cost of the project. The foundations will support a total of 90 MHI Vestas V164-9.5 MW turbines and two offshore substations.
The windfarm will be located 32 km off the Lincolnshire coast. It is sited in an area where Atkins has extensive experience in XL monopile and transition piece design engineering. The project is owned by innogy and has a planned installed capacity of 860 MW.
innogy’s foundations package manager for Triton Knoll Richard Hughes said the company was seeking a monopile foundation that has the potential to be lighter than any currently installed in comparable site conditions, so helping to deliver cost reductions at Triton Knoll.
“We’ve worked very closely with Atkins and our preferred foundations supplier Smulders Sif Steel Foundations JV to share our own experiences and knowledge and help support the delivery of real innovation. innogy and Triton Knoll are key members of the industry-wide Pile Soil Analysis Group (PISA), which aims to find ways of reducing costs across the sector by implementing new methods of designing monopile foundations. We expect Triton Knoll to be one of the first projects to see the results of these improvements delivered on the ground.”
Andy Thompson, market lead for offshore engineering at Atkins, explained that “larger turbines create different engineering challenges”. Given that Triton Knoll is the first offshore windfarm to use the V164-9.5 MW turbine, the company is taking a unique approach to addressing the design questions posed by the project, drawing on previous offshore experience in both wind and oil and gas.
“Our UK-based team has many years of experience working on offshore windfarms around the world, and we’re looking forward to putting that expertise into action on the project,” said Mr Thompson. “Our industry continues to learn at an extraordinary pace, and Atkins’ innovative work designing and engineering various parts of an offshore windfarm has been key to helping lower costs for industry and, ultimately, the consumer.
Atkins’ design contract will advance technological capability in the UK market. For much larger turbines, efficient design supporting turbine infrastructure is critical, and as turbines get bigger, more effective and efficient designs for weight, fabrication and installation, as well as increased collaboration across the supply chain, has been crucial to the success of the industry.
The project was awarded a contract for difference in September 2017 and expects to trigger a capital expenditure investment of around £2Bn (US$2.8Bn) into much-needed UK energy infrastructure. This will enable the delivery of some of the lowest cost energy generation for UK consumers.
As highlighted above, monopiles predominate in the offshore wind industry but are not the only option. The Netherlands-based SPT Offshore, which specialises in suction pile anchors and foundations for the offshore oil and gas industry, has unveiled a new type of foundation for offshore wind turbines.
The Ørsted-led PISA research project highlighted by Mr Hughes, which demonstrated that monopile foundations for turbines can be reduced in size and made less expensive, won an award from the British Geotechnical Association at the end of 2017.
The project won a Fleming award from the British Geotechnical Association – an award that recognises excellence in the practical application of geotechnics in a project or part of a project and demonstrates excellence in geotechnical design and construction.
The PISA project concluded that monopiles don’t need to be as long as previously assumed in order to withstand the forces a turbine’s rotor blades are exposed to. Shorter foundations will make it less expensive to install a turbine, which means that the PISA project could help reduce the cost of energy from offshore wind.
Ørsted’s senior manager Jesper Skov Gretlund said “The PISA method is already becoming popular in offshore design. It’s one of the many initiatives that are helping to realise our ambition of making green energy cheaper than energy from fossil sources.”
The new monopile design is the result of collaboration between 11 industry partners as well as the University of Oxford, Imperial College London and University College Dublin. The 11 partners are Ørsted, SSE, Statoil, RWE, Statkraft, Iberdrola, Vattenfall, Alstom, Van Oord, EDF and E.ON. The collaboration was organised and operated under the Carbon Trust Offshore Wind Accelerator, which has specialised in cross-industry collaboration within offshore wind power.
The design methodology used for the PISA project originated from the offshore oil and gas industry and was developed in the 1970s. A new design methodology was developed in two stages. First, the academic work group, led by Oxford University and including Imperial College London and University College Dublin, developed the new model. Subsequently, the model was tested by the Ørsted-led team of industry players at two sites (Cowden, England, and Dunkirk, France) to assess and validate the new design method.
SPT Offshore said its suction pile foundation concept combines the advantages of monopile foundations with the ease and speed of installation of suction pile foundations. The company said they are easy to fabricate and easy to install and remove should the need arise.
The new foundation combines three, potentially four, suction piles and a mono-tubular that sits atop a star-shaped transition piece.
“The mono-tubular has the advantage of being inexpensive to fabricate,” said SPT Offshore, which believes that it is three times less expensive than a jacket-type foundation. “Suction piles have the inherent advantage of fast installation (around three hours) and are virtually noise free (compared to piling of monopiles),” said the company.
Another advantage of the foundation compared to monopiles is that installing suction piles does not transmit vibration or shock to the foundation. This means that the suction pile foundation can be installed with the work platform attached.
“Steel has the benefit of being light and simple to build and the design process is more flexible and allows a shorter fabrication schedule,” said SPT Offshore. “On top of the suction pile and on top of or inside the transition piece, there is room for added ballast, which can be pre-installed or combined with scour protection after installation.
“Installation of a suction pile foundation in dense sand can typically be completed in hours, compared to the days required for a pre-piled jacket and ancillary installation. This reduces the offshore construction spread required in the field significantly.
“At the end of its lifetime, if an offshore wind turbine needs to be removed, it can be done by simply reversing the piling operation. No steel will be left behind, unlike hammered piles, which can only be cut, leaving thousands of tonnes of metal in the seabed.”
The company believes that suction piles are suitable for use in 99% of the North Sea and most shallow-water areas worldwide that have sandy, clay or layered soils.
“We are detailing the design right now and are working towards a demonstration in the next 12 months,” the company concluded.
Gravity-based foundations for Kriegers Flak
The massive foundations for the offshore substations for the Kriegers Flak project in Denmark – one weighing 10,000 tonnes and the other 8,000 tonnes – left the Port of Ostend in Belgium destined for Denmark on 15 January 2018. They are being transported to Denmark on a barge.
Jan De Nul Group and Smulders joined forces to build the gravity-based foundations. Each consists of a concrete part with a steel structure on top. Jan De Nul Group was responsible for the design and construction of the concrete gravity-based foundation. Smulders was responsible for design and construction of the steel structures and decks they will carry. Jan De Nul will also be responsible for the installation of the foundations, ballasting and scour protection for them.