Comparison of methodologies to combine strain cycle histograms to an accumulated damage figure at a point of interest along the circumference of an offshore wind support structure

This work is part of the research activities by OWI-lab within MAXWind project on
the subject of data-driven lifetime assessment of offshore wind support structures (OWSS). To
this stage, the recorded strain signals are converted into fore-aft (FA) and side-side (SS)
bending moments to calculate the corresponding damage metrics, such as Palmgren-Miner
damage, damage equivalent load (DEL) or damage equivalent moment (DEM). A refined
lifetime analysis requires the damage/DEL/DEM calculation at a specific heading rather than in
the FA and SS directions, which would be achieved either by directly rotating the strain signals
at the required heading or through a yaw transformation of the averaged fore-aft and side-side
bending moments. Given the amount of data that OWI-lab has collected over the years,
performing such operation has multiple disadvantages, as it easily becomes computationally
prohibitive and highly storage demanding. The lack of flexibility of such approaches is just as
important, in fact changing the heading would mean starting the processing from scratch. The
research aims at finding a low-computational cost yet accurate methodology to derive the
accumulated damage at any point along the circumference of the support structure without
reprocessing the data. That way, the lifetime of the critical sections/welds could be accurately
and quickly evaluated based on real-life strain measurements on an OWSS. This study
compares different methodologies based on FA and SS bending moments to quantify the
accumulated damages at any desired point. A reference method based on individual strain
gauge signals is used to validate the current methodologies.