Abstract
This paper presents the methodology, execution, and several key findings of the Structural Lifecycle Industry Collaboration Joint Industry Project, which aimed at improving the understanding of fatigue behaviour in butt-welded thick steel plates used in offshore wind monopile foundations. This was a comprehensive programme, coordinated by major offshore wind operators that generated statistically robust S-N fatigue data from 59 large-scale uniaxial tests on 50 mm thick S355ML double-V multi-pass circumferential welds fabricated by three leading monopile manufacturers.
The paper details the rationale for the test programme, the details of the test specimens, the test procedure pursued and discussion of the main results. The tests were conducted on specimens in both as-welded and flush-ground conditions and included blind independent repeat tests at different laboratories. The as-welded tests informed an important change in the inverse slope of the monopile-specific D curve in the latest revision of the DNV-RP-C203 to a value greater than “3″, while the flush-ground tests demonstrated significantly higher fatigue resistance than the existing C1 curve.
The new test data are based on evidence from samples representative of current offshore wind monopile structures, tested within a single, coherent programme. The tests were conducted in statistically sufficient numbers, with independent fabrication and testing, resulting in a high overall degree of confidence. In addition, the specimens were extensively characterised before and after testing to better understand the influence of weld parameters and quality on the fatigue behaviour of thick welded monopile structures.
The results are discussed in terms of optimising specimen design to minimise the number of invalid tests in large-scale fatigue programmes and highlighting existing knowledge gaps that should be addressed in future work to further improve understanding of fatigue behaviour in offshore wind monopile support structures. The methodology presented here can serve as a useful base-line for future studies aiming to accelerate the acceptance or otherwise with respect to fatigue of non-standard structural details, materials and manufacturing methods.
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