Improved design has helped slash the cost of offshore wind farm foundations but further innovation and new solutions are needed as the industry ramps up to meet expected government targets.
The UK is a leader in offshore wind with more capacity than the rest of the world combined and demand is expected to increase further, creating both challenges and opportunity for geotechnical innovation. The offshore wind industry is in discussions with the UK government to secure a Sector Deal and that will bring a commitment for a third of the UK’s energy to come from wind power by 2030.
Despite the world-leading position, the commitment is a big step up from the 8.5% of the UK electricity generation supplied by wind power in the first quarter of this year and will present new design, cost and supply chain challenges. Foundations are a major cost on offshore windfarms and account for around a third of the total costs so the pressure will be on to deliver solutions for larger turbines in deeper water for less costs than ever before.
Ørsted geotechnical design and install team lead Miguel Pacheco Andrade believes that offshore wind energy could be subsidy free in the near future, which is making it an ever more attractive solution.
“Offshore wind is reducing costs at a faster pace than expected,” he says. “It is now 40% lower that the nuclear strike price.”
Capacity is also increasing - Ørsted is currently working on the Hornsea One offshore wind farm, which will be the world’s largest when it is completed. Plans are already underway for Hornsea Two, which combined with the first phase of the project will deliver a capacity of 3GW.
“That’s as much as nuclear power plant,” says Pacheco Andrade.
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According to Ørsted asset projects project manager Ulrik Hemmingsen, the industry is maturing and is benefitting from over 25 years of in use performance data that is helping current design to be more accurate.
Hemmingsen says that Ørsted has just decommissioned its first offshore windfarm at Vindeby in Denmark, which was installed in the early 1990s using gravity based foundations, “The company has over 1,000 monopiles installed worldwide,” he adds.
In addition to industry experience, Pacheco Andrade adds that the drivers of design have also changed. “We used to look at ultimate limit state but now we consider ultimate service state and fatigue to be a better method of design,” he says.
Projects like the Pile Soil Analysis (Pisa) Project Team formed by Oxford University, Imperial College London, Ørsted, Carbon Trust and Socotec UK have helped to drive down the cost of monopile solutions through greater understanding of foundation performance.
According to Pacheco Andrade, the foundation design innovations delivered under Pisa have been a major contributor to driving down the cost. The cost of the turbines has also been falling, which is helping to drive down the cost of offshore wind.
“There have been some big changes in the sector in recent years as monopile design has moved away from the conventional closed form solution as a result of technology transfer from oil and gas sector,” says Pacheco Andrade.
“We used to use big, slender piles with a diameter to length ratio of one to 50. Now we’re are more focused on a performance design and we have realised how far off our predictions in earlier designs were. We were many orders of magnitude away from reality and we were designing foundations that were far stiffer than necessary.
“Today the standard diameter to length ratio is closer to two to five.
“In the past for turbines with a 2MW to 4MW output, we were using 4m to 6m diameter piles with penetration depths of 20m to 40m and pile lengths of 30m to 70m. Today we are installing foundations for much larger turbines but using 10m to 12m diameter piles that are 50m to 100m in length but we don’t need greater penetration than 20m to 40m.
“Originally we could only work in 35m deep water but now we are looking much deeper.”
The challenges are about to get harder too as larger turbines start to be specified.
“At the moment we are working with 8MW turbines but we will be installing 12MW to 15MW turbines in the near futures,” says Pacheco Andrade. “These have a massive overturning moment.
“The maximum accumulated rotation of a foundation over 25 years is just 0.5°. This is quite a challenge with the massive wind and wave loading these structure experience in the field. However, the tolerance for installation is 0.25° so that actual maximum rotation for design purposes is 0.25°.”
Ørsted says that it is now working with Oxford University to look at cyclic loading in more detail and it hopes to leverage the same design improvements gained by understanding the stiffness under the Pisa project.
“Pisa was looking at the static issues but this project takes us forward to the next stage,” says Pacheco Andrade.
Another challenge that the sector needs to address is noise, which is a big concern.
“You can put mitigation measures in place but these are expensive so we are trying to innovate to remove the issue,” says Pacheco Andrade.
Research is currently underway using Blue Piling technology which uses water pressure to drive piles rather than impact energy.
The bigger turbines and monopiles also mean bigger barges which means that the supply chain needs to step up and invest. “There is a political aspect to this in that the supply chain needs to have confidence in the market and a Sector Deal will help secure that,” says Pacheco Andrade.
Both Hemmingsen and Pacheco Andrade admit that monopiles are not the solution for every site. Ørsted has just worked with NGI to install 20 suction bucket foundations on the Borkum Riffgrund 2 offshore windfarm and it is also installing pile jackets for a project in Taiwan.
However, Pacheco Andrade says that floating foundations could be the ultimate solution of the future. “The floating turbine solutions are the Holy Grail for the sector as it will move us away from the current water depth considerations,” he says.
“At the moment we are restricted to 30m to 60m water depth but Stat Oil’s Hywind demonstrator projects show that the floating approach is possible.
“However the technology needs to go through the same maturation process that fixed foundations have been through so it maybe around 15 years before floating turbines are viable.
“At the moment the economic drivers mean that bottom fixed turbines are a better solution so incentives are needed for floating turbines to be used.”
Whatever the solutions created to meet the demand, it is clear that the offshore wind market will continue to be a growth area for geotechnical expertise both in the short and long term.
Geosea Borkum Riffgrund 2
While the focus is currently on the rise and rise of the offshore wind farm, there will come a time when existing facilties will reach the end of their design lives and need to be decommissioned.
However, Hemmingsen believes that this is not a major challenge and advances in design will aid redevelopment of existing offshore wind farm sites.
“Decomissioning is part of the business case for projects,” he says. “With monopiles they would be cut off 2m below seabed level and suction buckets can be completely removed.”
Ørsted had to undertake a suction bucket removal demonstration as part of the Borkum Riffgrund 2 project.
The presence of old monopile foundations would not prevent an area being redeveloped with a new wind farm, according to Pacheco Andrade. “The larger diameter of modern monopiles means that the new pile can be installed concentrically over the top,” he says.
Hemmingsen says that the choice of foundation is more about the ground conditions and noise issues than the decommissioning. Where decommissioning is influencing design is in the move from grouted connections to bolted ones for the transition pieces.