Carbon Trust OWA to challenge jacket foundation fatigue standards

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 (strength) is enhanced by 10-20 percent 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 in-house to reduce the time needed to complete fatigue tests.  This 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 coordinator, will work with OCAS and industry partners to ensure program delivery including the accelerated testing, numerical analysis and sourcing of nodes.  The JaCo project will run over a 3-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 wind farm projects, 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 are naturally limited in number 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 1 Hz – 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 & gas industry and does not take into consideration the anticipated beneficial effects of new techniques applied in modern welding and its associated technologies.

OCAS’ novel testing method is based on resonance testing at an increased test frequency of typically 20 Hz, 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 are 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 commented: "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."

Jan Pedersen, Chief Specialist Foundations, DONG Energy: "For larger water depths combined with the large WTG’s already on the market and the ones to come in the future the jacket foundation is one of the most cost efficient offshore wind foundation concepts. JaCo will help to further lower the cost of this foundation type by allowing fatigue life optimization of jacket nodes through derivation of fatigue design parameters for modern fabricated nodes. It will also give a push for further industrialization through utilization of robot welded nodes, which will have a further cost reduction effect."

Nikolaus Elze, Director Technology Generation Technology, EnBW: "By participating in the JaCo project EnBW invests into the engineering basis for industrialised offshore jacket foundation manufacturing processes in order to open up the market for innovative foundation designs. Our clear ambition is to further reduce costs in the deployment of the future competitive European offshore wind industry."

Dariusz Eichler, Senior Lead Structural Engineer, Vattenfall: "With Vattenfall’s winning tender price for Kriegers Flak offshore windfarm, a new way of thinking and innovation is more than ever necessary in order to bring the cost down. In this context, a jacket foundation solution comes into play whose main challenge however is the high price. A solution to this challenge is called robotic welding of jacket nodes which we believe will be an industry game-changer and will make jacket foundations much more competitive. But before such a new technology can be implemented in the industry, quality of the robotic weld and its durability have to be validated. Vattenfall therefore participates in the JaCo project whose main task is conducting of a series of fatigue tests and thus demonstrating adequate performance of the robot welding as well as formulating improved fatigue design criteria for jacket nodes."

Richard Eakin, Chief Offshore Civil/Structural Engineer, ScottishPower Renewables: "ScottishPower Renewables are very excited to be involved of this joint industry project which combines next generation fabrication processes with advanced testing techniques. We hope that the impact this project has on jacket node fabrication will further contribute to the reduction in the cost of energy."

Halfdan Brustad, Manager of New Energy Solutions, Wind Projects in Statoil: "Statoil is looking forward to continue our efforts on bringing down the cost of offshore wind - this time focus is on improving the fatigue design of jacket nodes. Working together with the industry, is a key factor to improve technology and increase deployment of renewable energy."