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Carbon Trust project aims to improve fatigue standards for jacket foundations

Fri 24 Feb 2017

Carbon Trust project aims to improve fatigue standards for jacket foundations
The trend towards deeper water and larger wind turbines, such as on the Wikinger offshore windfarm, presents an opportunity to optimise jacket foundation design

The Carbon Trust’s Offshore Wind Accelerator has announced a £2.4 million project, Improved Fatigue Life of Welded Jacket Connections, that aims to cut the cost of offshore wind by optimising the design of jacket foundations.

Via the Improved Fatigue Life of Welded Jacket Connections or ‘JaCo’ project, the Offshore Wind Accelerator (OWA) hopes to reduce costs through improved fatigue standards and validation of faster testing and fabrication methods. Jacket foundations are set to become a more dominant design solution, as monopiles are unlikely to be practical for many future offshore windfarms located in deeper water sites and with larger turbines.

The JaCo project will develop a better understanding of fatigue performance by testing full-size jacket nodes made from existing manual and novel automated welding processes. Coupled with improved standards, it is estimated that a weight reduction of 10 per cent can be achieved if the fatigue resistance (strength) is enhanced by 10–20 per cent through optimised design.

The project will facilitate close collaboration between developers, supply chain, research organisations and government. Leading offshore wind developers Dong Energy, EnBW, Scottish Power Renewables, Statoil and Vattenfall are supporting the project together with funding from the Scottish Government.

Belgian material research centre OCAS will use unique and efficient testing techniques developed inhouse to reduce the time needed to complete fatigue tests. Investigation of fatigue performance of full-scale jacket node designs will encompass a larger number of individual tests than would otherwise be possible with the constraints of conventional testing and the project’s duration. The Carbon Trust, as project co-ordinator, will work with OCAS and industry partners to ensure programme delivery including the accelerated testing, numerical analysis and sourcing of nodes. The project will run over a three-year period and will help to drive the use of jackets as one of the leading fixed offshore wind foundation types in the future. 

The industry trend towards deeper water and larger wind turbines, such as on the Beatrice, Baltic 2 and Wikinger offshore windfarms, presents an opportunity to optimise jacket foundation designs. Jacket foundation technology consists primarily of welded tubular space frames fabricated in carbon steel. Each welded connection point is termed a node. Fatigue at these welds is an important design consideration that often proves to be the limiting factor.

The current group of fatigue classes used in most design standards were originally developed in the 1970s and 1980s and may not necessarily reflect advances and improvements in new welding methods, inspection techniques, quality standards and materials. It is expected that accounting for those factors that prove important to fatigue strength will result in improved fatigue performance of welded nodes.

The industry uses standardised S-N curves for the fatigue design of structures. These experimentally based curves describe a design relationship between the applied stress and lifetime for a given type of weld. Full-scale test data for nodes is naturally limited compared to the general weld database, because conventional large-scale tests based on servo-hydraulic loading methods use a frequency close to or lower than 1Hz – the duration of a fatigue test running to 10 million cycles will exceed 100 days. New testing is therefore costly, and the database for nodes largely consists of older data from the offshore oil and gas industry and does not take into consideration the anticipated beneficial effects of new techniques applied in modern welding and its associated technologies.

OCAS’s novel testing method is based on resonance testing at an increased test frequency of typically 20Hz, significantly speeding up the delivery of results to within just 14 days for a full-scale jacket node. The JaCo project aims to validate the use of what it is anticipated will be less-conservative fatigue S-N curves for offshore wind turbine jacket foundations compared to current practice.

The ability to test a larger number of both manually and automatically welded nodes, combined with improvements in manufacturing techniques and optimised fatigue curves, is expected to lead to reduced weight of jacket foundations and lower capex and installation costs. The project will also be guided by a panel of independent experts, including Cranfield University, DNV GL, Bureau Veritas and BAM (Federal Institute for Materials Research and Testing).

Jan Matthiesen, director of offshore wind at the Carbon Trust, said, “With an increasing trend of larger turbines that will be installed in deeper waters, we will see a growth in the use of more economic jacket structures. We have launched this project to drive cost reductions by optimising such structures. In addition, this project will aim to demonstrate the performance of robotic welding, which will allow the supply chain to fabricate at industrial scale. Working with our partners, innovators and wider stakeholders, the JaCo project is another example on how the collaborative R&D approach of the OWA is accelerating the use of new technology to deliver cost reduction for the offshore wind industry.”

 

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