Offshore Wind Players to Class Up Jacket Node Fatigue Tests

The Carbon Trust’s Offshore Wind Accelerator announced a new GBP 2.4 million project aiming 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.

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 Carbon Trust said.

The goal of The Improved Fatigue Life of Welded Jacket Connections (JaCo) project is to 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, according to The Carbon Trust.

The project will facilitate 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 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 is expected 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 Carbon Trust said.

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, speeding up the delivery of results to within 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).

”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,” Dariusz Eichler, Senior Lead Structural Engineer at Vattenfall, said.

”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.”