Better understanding of fatigue performance

The CRONOS system sets a new standard for future large-scale fatigue testing activities.

Marc Vanderschueren

Better understanding of fatigue performance

Offshore wind foundations are prone to fatigue damage, mainly through the loading of the wind turbine. With increasing turbine size and installations moving to deeper waters, CAPEX is rising. Still, we are using industry design rules dating from the 80s. Both steel production and welding techniques have evolved quite a bit in the meantime, so it was time for an update of the current performance vs. the standards.

ACCELERATED TESTING ON FULLSCALE NODES

Responding to this market need, OCAS proposed a new approach, which has led to the start of the Jacket Connection (JaCo) project. This industrial research joint industry project aims to develop a better understanding of fatigue performance by accelerated testing of full-size jacket nodes joined by existing manual and novel automated welding processes. ‘Full-size’ is important as this is the only way to get the results into the design standards without extra conservatism and unnecessary cost. The acceleration is important as traditional fatigue testing, by means of hydraulic rigs, is a very slow process. As the certification bodies require many data points, OCAS designed, engineered, and installed its unique CRONOS fatigue test bench, capable of accepting large specimens and testing them much faster than the traditional systems do. Under the auspices of the Carbon Trust Offshore Wind Accelerator (OWA) and eight leading offshore wind operators, OCAS is performing important testing and simulation work.

QUALIFICATION ACHIEVED

In 2021, the CRONOS bench was qualified to deliver fully equivalent test results compared to hydraulic testing rigs. This means that the offshore wind industry – in the form of the Carbon Trust OWA, leading operators ORSTED, VATTENFALL, SHELL, EQUINOR, IBERDROLA, SSE, SIEMENS GAMESA, and ENBW, and the certification bodies DNV and BUREAU VERITAS – now all recognise that fatigue testing can be performed on full-scale specimens at least 20 to 40 times faster without compromising results. Such recognition opens a new field of applications for OCAS in terms of testing geometries for different applications – whereas, in the past, multi-year testing campaigns were required. This encompasses fields such as validating new technologies

for joining tubular elements into nodes, qualifying new robot welding techniques towards optimised fatigue design, upgrading the performance of welded beams, validating the allowable tolerances in bolted flanges, and more. All these topics aim to either reduce the energy cost or extend the lifetime of critical metal components. Apart from the benefit for the industry players, such an approach places OCAS at the forefront when it comes to validating and qualifying technologies to manufacture more fatigue-resistant metal components. Whilst we cannot yet divulge the test results on jacket nodes of the individual manufacturers, several automated welding technologies could offer nice possibilities towards a component fatigue life upgrade. The JaCo project aims to come to completion soon, whereafter the certification bodies will take positions on the conditions for implementing the results into their guidelines. This will be key for operators who want to use those guidelines to extend the life of their foundations or to create a cost-reduced design. Going forward, we are aiming to set up a new project on further fixed-bottom offshore components to use the IP-protected CRONOS testing system. In parallel, we will start up our efforts to look at the next generations of foundations for floating wind, which will see a major deployment by 2030. The bold move OCAS took of developing and commercialising a unique and novel largescale test system in a conservative market is paying off and paving the way for more industrial research service projects in the years to come.