Although, resistance to repetitive loading or fatigue might not be so well known as material degradation term, it is an important material property.
Fatigue?
You may have read about Havilland Comets, world’s first commercial jet airliner, crashing for unknow reasons in the early 1950-ies. The problem was found to be fatigue crack growth from a rivet hole, stressed by the multiple loading and unloading cycles of the cabin pressure. The same type of repetitive loading cycle occurs in offshore wind farm foundations: they are cyclically stressed by the rotating wind turbine. As today, we understand fatigue much better, standards are ensuring that the weakest joints, typically welded connections, withstand this repetitive loading. OCAS has proven that adapted welding procedures can provide a significant improvement in fatigue performance versus the standard practice. However, it takes a very long time to prove that these results are valid on a full-size component. Using hydraulic actuators applying a cyclic loading to the critical component to be tested at a frequency of around 1Hz, it takes on average between 3 and 5 years of testing to obtain a significant number of data points to ensure sufficient statistical relevance. Adding sample generation and preparation to this testing, plus data analysis and conclusions and it is easy to understand that the required effort goes beyond normal R&D project timelines and cost.
Fatigue!
OCAS has come up with a radically new testing bench whereby either a cylindrical, an elliptical or a one-directional loading bending moment can be applied to test specimen joints. The innovation combines “resonant bending” principles, speeding up fatigue testing 20 times faster than the traditional method and this on a full scale critical component joint. This increased testing speed enables to generate fatigue data in weeks rather than months. As such, it now becomes possible to generate loads of data in a normal R&D project timeline without affecting the results. Moreover, the certification bodies accept this OCAS test method for the qualification of fatigue performance even for full size components and allows for crack initiation monitoring. The accelerated fatigue testing method is being used in a first stage to qualify critical, welded leg-bracing connections of offshore wind farm jacket nodes but also other specimen or connection types can be tested. The combination of new, faster welding technology in the foundation supply chain and faster fatigue testing methodology allows the fatigue standards, established in the ‘70’s and ‘80’s on older steel types and manufacturing methods, to be challenged and ultimately updated within a reasonable timeline. The cooperation with the certification bodies allows to use the results on full-size foundations in live offshore wind farm projects of electricity producers and developers. OCAS initiated the JaCo (Jacket Connection) project under the auspices of the Carbon Trust Offshore Wind Accelerator together with industry partners Vattenfall, Orsted, Equinor, Iberdrola, ENBW and Siemens Gamesa as well as certification bodies and is supported by major partners in the supply chain. The aim is hereby to reduce ultimately the cost of an offshore wind farm jacket foundation without increasing risk.
“Cyclic loading patterns impact the structural integrity performance of critical components significantly over their lifetime. Therefore, it is important to predict structural performance through representative, full-scale accelerated testing.”
