This paper presents a series of large-scale wave flume experiments on the scour protection damage around a monopile under combined waves and current conditions with model scales of 1:16.67 and 1:8.33. The main objective is to compare the damage data obtained from these large-scale models with existing monopile scour protection design approaches, which were proposed based on small scale wave flume experiments, and to study the applicability of the existing approaches. The static stability (onset of motion and bed shear stress) and the dynamic stability (three-dimensional damage numbers) of the scour protection are investigated. Both results show that the existing design approaches can be conservative when applied to large scale models, which highlights the need of further investigations on scale and model effects. In addition, this paper also analyses the scour protection damage depth. It is observed that damage depths of the scour protection layer under low Keulegan–Carpenter number (KC) conditions are smaller than predictions. The study provides valuable large scale experimental data for future research on the monopile scour protection design.

This study aims to improve the design of scour protection around offshore wind turbine monopiles, as well as future-proofing them against the impacts of climate change. A series of large-scale experiments have been performed in the context of the European HYDRALAB-PLUS PROTEUS (Protection of offshore wind turbine monopiles against scouring) project in the Fast Flow Facility in HR Wallingford. These experiments make use of state of the art optical and acoustic measurement techniques to assess the damage of scour protections under the combined action of waves and currents. These novel PROTEUS tests focus on the study of the grading of the scour protection material as a stabilizing parameter, which has never been done under the combined action of waves and currents at a large scale. Scale effects are reduced and, thus, design risks are minimized. Moreover, the generated data will support the development of future scour protection designs and the validation of numerical models used by researchers worldwide. The testing program objectives are: (i) to compare the performance of single-layer wide-graded material used against scouring with current design practices; (ii) to verify the stability of the scour protection designs under extreme flow conditions; (iii) to provide a benchmark dataset for scour protection stability at large scale; and (iv) to investigate the scale effects on scour protection stability.

The present work introduces an analysis of the measurement and model effects that exist in monopile scour protection experiments with repeated small scale tests. The damage erosion is calculated using the three dimensional global damage number $S_{3D}$ and subarea damage number $S_{3D,i}$. Results show that the standard deviation of the global damage number $\sigma \left(S_{3D}\right)=0.257$ and is approximately 20% of the mean $S_{3D}$, and the standard deviation of the subarea damage number $\sigma \left(S_{3D,i}\right)=0.42$ which can be up to 33% of the mean $S_{3D}$. The irreproducible maximum wave height, chaotic flow field and non-repeatable armour layer construction are regarded as the main reasons for the occurrence of strong model effects. The measurement effects are limited to $\sigma \left(S_{3D}\right)=0.039$ and $\sigma \left(S_{3D,i}\right)=0.083$, which are minor compared to the model effects.