OCAS’s unique fatigue testing technique drives new Carbon Trust programme

The Carbon Trust’s Offshore Wind Accelerator announced a new project, Improved Fatigue Life of Welded Jacket Connections (JaCo), to cut the cost of offshore wind by optimising the design of jacket foundations, through improved fatigue standards and validation of faster testing and fabrication methods.
The Carbon Trust, as project coordinator, will work with OCAS and industry partners to ensure program delivery including the accelerated testing, numerical modelling analysis and overall sourcing of nodes. The JaCo project will last for 3 years and will help to drive the use of jackets as one of the leading fixed offshore wind foundations in the future.

Jacket foundations are set to become a more dominant design solution, as monopiles are unlikely to be practical for many future offshore wind farms 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 percent can be achieved if the fatigue resistance is enhanced by 10-20 percent through optimised design.The project will facilitate close collaboration between developers, supply chain, research organisation 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.

OCAS will use unique techniques developed in-house to test the fatigue performance of full-scale jacket node designs faster allowing a larger number of tests to be carried out during the duration of the project.

The industry uses S-N curve for the design of structures, which measures the minimal stress required to cause a fatigue failure over a certain number of cycles. However evidence of component level fatigue properties is limited as testing methods are slow using a frequency close to or lower than 1 Hz, which means a standard test at 10 million cycles will exceed 100 days. Therefore, this limited evidence base largely consists of old data and does not take into consideration the new techniques applied in modern welding processes.

OCAS’s proprietary testing method is based on an increased frequency of 20 Hz, significantly speeding up the delivery of results to within just 14 days for a full-scale jacket node.  The JaCo project will validate the use of using less conservative fatigue S-N curves for offshore wind turbine jacket foundations in comparison to current practices.

The ability to test a larger number of both manually and automatically welded nodes, combined with improvements in manufacturing techniques and optimised fatigue curves will lead to reduced weight of jackets foundations, 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).

MVDS

“Through a better understanding and accelerated testing of a larger number of manual and automatic welded nodes in an industrial manner, the objective is to upgrade the fatigue curves for offshore jacket structures enabling a reduction in weight, CAPEX and installation costs,” Marc Vanderschueren, Senior Business development manager Metal Structures R&D partnerships, OCAS