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As the demand for – and the scale of – offshore windfarms has grown, a consensus had started to form about the limitations of monopile foundations. 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 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 22m (2012) to 29m (2016) and average turbine size reach 6.8MW (2018)? Indeed, why have major suppliers like EEW promoted the 10m diameter XL pile and SiF been commissioned to deliver piles for the Borseele projects with 38m of water depth?
Knowing the limits
What determines the performance limitations of a monopile for use as an offshore wind turbine foundation? 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 haven’t hit them yet.
Stiffness: an enduring strength
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 behavior 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 ”
Cameron Dunn Associate Principal, Americas South Operations Director
Most importantly, operational data from existing OWTF systems are being analyzed by our Arup teams to validate current engineering assumptions and behaviors 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 efficient proactive engineering design research is yielding key insights into soils behavior 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.
Installation: meeting bigger ambitions
While we can 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. Larger sizes, particularly larger diameter, 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 larger monopiles. 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. Offshore installation vessels are also planning for offshore craneage that can reach 5000te 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.
