The offshore wind industry stands ready to take advantage of a new generation of much larger turbines that are being developed, for which new types of foundation may be required. But what kind of foundations will these mega-turbines be founded on – monopiles or jackets?
In the First Quarter 2019 issue of OWJ, BVG Associates senior consultant Andy Logan argued that there is a ‘false dichotomy’ between bottom-fixed and floating foundations for offshore wind.
“Floating wind has long been seen as the next generation of offshore wind and as a competitor to bottom-fixed concepts but cannot yet compete with it on cost,” he said. “But as both technologies continue to mature, they each gain improved viability at intermediate water depths and the choice of foundation type then becomes an organisational or regional preference.”
In like fashion, it is sometimes argued that because turbines are getting bigger and water depths increasing that the monopile foundation has had its day. But as Cameron Dunn, US offshore wind leader at Arup explains, there is still plenty of mileage in the monopile concept.
“As the demand for – and the scale of – offshore windfarms has grown, a consensus had started to form about the limitations of monopile foundations,” he noted recently. “Though with room for cost-effective improvements, should the monopile be written off just yet?
“As someone who had been designing jackets for offshore oil and gas platforms and being keen to apply my knowledge in the renewables market, this got me interested. The transition to jackets seemed to be underway, with US projects like Block Island leading with jacket foundations, setting a precedent for the nascent American sector. Key European wind projects were also evaluating and selecting jacket foundations. Jackets were clearly the future.”
So why, he asked, as recently as 2015, were 97% of the new wind turbine foundations built and installed still monopiles? And how did the average water depth increase from 22 m (2012) to 29 m (2016) and average turbine size reach 6.8 MW by 2018?
Indeed, why have major suppliers like EEW promoted the 10-m diameter XL pile and Sif been commissioned to deliver piles for the Borssele projects in the Netherlands where the water depth is around 38 m? And why, as highlighted elsewhere in this issue, have companies seeking to develop the next round of offshore windfarms in the UK sought permission to use 12-m diameter monopile foundations rather than the 10-m foundations they originally proposed unless they anticipate that this type of foundation still has room for growth?
So, what determines the performance limitations of a monopile for use as an offshore wind turbine foundation? Mr Dunn asked.
In reality there are two key issues: the stiffness of the installed foundation and the nature of the installation procedure. “On both issues, the monopile still represents a good design option and although monopiles will have limits, we simply have not hit them yet,” he suggested.
Offshore wind turbines are by their very nature, dynamic. Each is a rotating, spinning, pitch-altering blade, in essence an ever-changing machine. For optimum long-term performance the entire turbine and foundation ‘system’ needs to maintain a natural frequency, or stiffness, that is specific to each turbine type and size.
Engineers push the performance of the design by varying the steel material, pile thickness, pile diameter, pile profile and pile driven depth. As the engineering of monopile behaviour has improved, the response of the dynamic turbine can be better predicted and designed for. This has enabled designs with deeper water and larger turbines.
“The exciting part is we continue to innovate on the understanding of how monopiles work by combining our expert knowledge of the soils, the monopile and the turbine to develop ever-more efficient and robust analytical tools,” said Mr Dunn. “Most importantly, operational data from existing offshore windfarms is being analysed in order to validate current engineering assumptions and the behaviours of our models and designs.
“As designers and engineers of renewable energy infrastructure, we are currently collaborating with some of the biggest names in offshore wind development. This type of collaboration and proactive engineering design research is yielding key insights into the behaviour of soils under cyclic loading as well as providing the basis for new methods to design larger pile systems. We believe the limits of monopile design will continue to be pushed beyond the horizon.”
Although it is possible to design larger and larger piles and make them work from a performance and operational point of view, this assumes we can also get them fabricated, transported and driven to depth.
As Mr Dunn noted, larger sizes – particularly larger diameter monopiles – have proven to be more difficult to install as a wider pile (thickness to diameter ratio) is more prone to damage during pile driving.
But advances are underway. “Our own soil-structure interaction analysis is enabling the installation of increasingly large monopiles,” Mr Dunn said.
“As a result, the leaders in the monopile supply chain are already planning larger rolling mills, increased thicknesses and handling equipment in preparation for ever-increasing pile sizes, and owners of offshore installation vessels are also planning for offshore cranes that can reach 5,000 tonnes for these next generation of piles.”
The monopile design has proven itself time and again as the cost-effective solution for offshore wind, and even with deeper waters and larger turbines coming soon, it is clear the monopile has some life left in it yet, Mr Dunn concluded.
Gravity base structure has concrete boots
Monopiles may continue to be widespread, but the use of jacket-type foundations is undoubtedly growing and new types of jackets continue to be developed.
Among them is an innovative solution described by Ingecid, a Santander, Spain-based engineering company. It has developed a gravity base structure with four legs, each of which has a concrete ‘boot’.
Unlike other gravity base structures, the boots sit on the seabed and are constructed offshore, using preplaced aggregate concrete. The foundation is intended to replace conventional piles of suction buckets, installation of which Ingecid believes is costly and potentially problematic in certain types of terrain. Conventional gravity base foundations are usually built and completed onshore before being towed into place and sunk.
The company said it believes its jacket concept has a number of potential advantages, among them that it does not require expensive preparation of the seabed or large marine equipment. The aggregate that forms the boots is poured into the structure on the seabed and the structure of the jacket fitted into them.
The approximate dimensions of the feet for a site in a water depth of 40 m are a diameter of 15 m and a height of 9 m. The volume of concrete in each would be around 200 m3.
Working on a base case of a windfarm with around 60 turbines, the company estimates that jackets of this type would cost around €3.0M (US$3.4M) each.
Semi-submersible floater could be cost-effective
Engineers at the University of Stuttgart in Germany unveiled a new type of foundation for a floating turbine at the Wind Europe conference in Bilbao in April 2019.
They believe the foundation has significant cost reduction potential, firstly from the hull shape, which cancels out wave forces on it, reducing loads on the foundation and hence on the turbine; and secondly through using a lightweight cable structure that connects the columns of the semi-submersible platform.
The foundation has a central column with outer columns held in place by radial struts. The radial struts take up compressive forces stresses and cables take up tensile stresses. They claim that simulations have shown that tower-base loads in the new design are comparable to those on an onshore turbine.