Author(s): Zihao Tang; Bruce Melville; Asaad Y. Shamseldin; Dawei Guan
Linked Author(s): Bruce W. Melville
Keywords: Riprap protection; Vibrating monopile; Scour reduction; Subsidence failure; Offshore wind turbine
Abstract: With the thriving of ocean renewable energy, offshore wind energy is considered the most applicable ocean renewable energy and has been rapidly implemented worldwide. Local scour induced instability of offshore wind turbine monopiles is one of the critical problems. To overcome the scour induced structure failure, various kinds of countermeasures were proposed and tested, such as armouring methods and flow-altering methods (Melville and Coleman 2000; Tang et al. 2022). Riprap rocks or concrete-made artificial riprap are the most applicable bed armouring methods, which have been widely used and studied. Chiew (1995) has summarised the riprap failure mechanisms as shear failure, winnowing failure and edge failure under clear water conditions. This study aims to investigate the performance of riprap armour subjected to monopile vibration, with a focus on the newly observed phenomenon of subsidence failure. The effectiveness of riprap was evaluated under various vibration amplitudes and frequencies, using three sizes of natural riprap rock. There is a strong correlation between vibration amplitude and subsidence depth in front of the monopile, with larger riprap sizes proving more effective at resisting subsidence. Scour topographies were also examined under various vibrational regimes and riprap sizes. New empirical equations were proposed for estimating edge scour depth, subsidence depth, and maximum downstream scour depth, and a novel method is presented for assessing riprap failure under combined vibration and clear water conditions. The influence of angles between the vibration direction and the oncoming flow was also studied. Different vibration angles could change the distribution of the failed riprap and the local scour depth, and the existence of the vibration angle has a significant influence on the local scour topography.
Year: 2024