Long term effect of operating loads on large monopile-supported offshore wind turbines in sand

Offshore wind turbines (OWTs) are known to experience modal property and foundation tilt changes during operation. These may negatively influence their serviceability and can lead to failure. To avoid these, a comprehensive characterisation of the behavioural patterns of OWTs, especially emergent large ones supported on monopiles, is necessary. In this study, environmental data collected from the proposed prototype location over a 10-year duration are logically grouped into three varying amplitude load cases based on the magnitudes of wind speeds while preserving the accompanying wave loads and percentage occurrence of each. The forces and moments acting on a prototype 10 MW OWT are estimated. Using the relevant scaling laws, an experimental set-up of laboratory offshore wind turbines and loadings are developed and used to perform long term model tests under High Cycle Application loading in sand. Accelerometers and displacement sensors are used to monitor the structural response throughout the experiment. Based on the results, commonly used fixed amplitude cyclic loads may underestimate the change of natural frequencies and/or tilt by up to 20%, with 2% large loads contributing up to 50% of change. This study has the potential to bridge the gap hindering the comprehensive characterisation and prediction of OWT behaviours.