Monopiles – the offshore wind industry’s go-to foundation concept for the vast majority of its projects to-date – might be manufactured and installed much more cheaply than in the past, according to convention-questioning new research published by UK government-industry body the Carbon Trust.
Studies looking at the interactions between the cylindrical steel design and clay and sand sea-bottoms commonly found at many offshore wind farm farm sites, point to engineers having “under-estimated the support that the seabed soil provides, resulting in over-conservative design calculations”.
Re-worked estimates coming out of the Carbon Trust’s £3.5m ($4.5m) Pile Soil Analysis (Pisa) project, led by developer Orsted and involving researchers at Oxford University, Imperial College London and University College Dublin, suggest that “more accurate methods” could cut steel requirements for monopiles by up to 30% percent resulting in “significant cost savings”.
“We believe these new methodologies will have a significant impact on the way industry understands the interaction between soil conditions and monopile foundations. It is the culmination of many years of collaborative work between industry and academia,” said Jan Matthiesen, director of the Carbon Trust’s offshore wind division.
Matthiesen notes that until now offshore wind turbine monopile designs – which have grown to ‘XL’ concepts measuring over seven metres in diameter and weighing up to 1,300 tonnes – were based on old offshore oil & gas platform engineering methods, where the foundation piles driven into the seabed to support the foundation “are smaller in diameter, longer and must resist different loading conditions during use”.
The Pisa design methods were tailored larger diameter monopile geometries and specific wind turbine loading conditions, making “bespoke optimisation” of foundations and structures for different geographic locations possible.
“Replacing models which were originally developed for the oil & gas sector is a critical step in reducing costs even further,” said Matthiesen.
University of Oxford profressor of engineering science, Byron Byrne, who was Pisa’s principal investigator, stated: “This] is a fantastic example of industry and academia working closely together to solve engineering problems of real significance to the offshore wind industry”.
He noted that the eight papers coming out of the project being made available on the UK Institute of Civil Engineers website would “facilitate a wider and more rapid uptake of the new knowledge, not only for wind farm design by the industry but also as a basis for new research initiatives in academia”.
The data used in the Pisa project was drawn from field testing at locations in Cowden, in the UK, and Dunkirk, France, with characteristics similar to those found in many areas of the North Sea.
Orsted, which is the leading developer of offshore wind project globally with 15GW in its project pipeline including 6.3GW either operating or under construction in British waters, has already begun using the new methods for monopile design for its Hornsea wind farms, a cluster of gigawatt-plus developments off the UK.
“The Pisa framework is leading to even more efficient and cost-effective monopile foundations for offshore wind farms across the globe,” said Orsted’s Miguel Pacheco Andrade, who was technical manager on the project.
“[It] represents a major paradigm shift in offshore geotechnical design. Not only has it led to significant steel savings for monopile foundations, but the use of Pisa allows greater confidence in our design calculations.”
Historically, monopiles were been restricted to waters of 30 metres or less – after which their size became too unwieldy to manufacture, transport or pile in, making their economics not measure up to the steel jackets and new-generation concretre gravity base ( CGB) designs.
But in last five years, XL concepts were proven and chosen for a number of deeper water projects, with developments such as the 600MW Gemini off the Netherlands and the 400MW Rampion and 336MW Galloper wind farms off Britain pushing back the boundaries for monopiles to waters closer to 40 metres, well beyond the previous thresholds of the technology.