Definitions and characteristics of loading patterns on single and shared marine anchors for floating offshore wind turbines

The growth of floating offshore wind has prompted new attention on the behaviour of anchors, including the concept of anchor sharing among multiple mooring lines. A critical design interface is between the mooring line loads and the geotechnical design. Typically ten to a hundred design loading conditions must be rationalised into the critical contributions that govern the geotechnical response of the anchor. This paper provides a framework to bridge between the mooring system analysis and the geotechnical design. Firstly, a methodology to translate a general time history of anchor loading into a characteristic ‘fingerprint’ is set out. This fingerprint has three components – (i) a ‘heatmap’ defining the normalised distribution of the cyclic loading across the three spherical dimensions of the resultant load, (ii) a vector of the relative cyclic frequency in these directions, and (iii) two scalar quantities required to unpack these normalised parameters to recover the original time series characteristics. Secondly, measures of similarity are defined, that allow two fingerprints to be quantitatively compared. This provides a new measure to compare the idealised patterns of loading used in model testing, field testing or laboratory element tests with the more complex patterns that are found in real data of anchor loading or from numerical simulations of floating systems. The framework is illustrated by demonstrating the varying similarity evident between example mooring line loads and the common loading patterns applied in geotechnical testing. In summary, this new framework allows anchor loading histories to be distilled into simple fingerprints and similarity measures, providing to tool to accelerate identification of critical design cases and allow more efficient idealisation for geotechnical modelling and design.