Optimization of Scour Protection Measures for Offshore Wind Turbines: An Experimental Study

Offshore wind energy is considered as one of the most promising resources of clean and renewable energy to replace fossil fuels. Offshore wind turbines (OWTs) normally operate in harsh ocean environments, which could impact their structural integrity. Scour erosion around the foundations of OWTs can substantially threaten the overall performance of these structures, shorten their lifetime and increase the cost during their life cycle, which consequently restricts the further growth of offshore wind energy. However, to date, the vast majority of existing research has tended to focus on single erosion protection measures and considered less about their ecological impacts and potential benefits. In this context, two aspects that can contribute to this field are presented in this thesis: a systematic literature review to analyse the latest advances and provide their prospects and future challenges, and a physical model investigation of innovative scour protection systems. According to the insights from the recent twenty years’ research, the optimisation of scour protection measures was proposed to combine a riprap scour protection with a flow-alternative protection method. This could lead to a significant performance improvement in resisting scour and thus a smaller environmental footprint. It is found that using the circular collar as a stand-alone solution has very limited protection efficiency, but there is great potential for improvement when combining it with riprap protection. Thus, innovatively, combined scour protection systems of riprap protection and collar protection on the riprap scour protection were designed and evaluated comparatively through a series of small-scale flume experiments. Based on the previous studies, 8 different collar designs are proposed, which could be divided into two main groups: conical collars and inverted conical collars. Their protection performances on riprap scour protection around monopile foundations are investigated under moderate to extreme wave conditions. The measurements of this experiment included the vertical velocity profiles, the 2D damage images after every 500 waves, the incident and transmit wave conditions. To analyse the experiment data more efficiently, an innovative 2D particle experiment method and its corresponding analysis methods, including the data extraction method, the division method of the riprap scour protection area, and the damage definition method, were designed and proposed to evaluate the scour protection effect of design collars quantitatively. Furthermore, the impacts of their geometric characteristics on scouring was quantified using image analysis and water velocity measurements. The scour protection effect of the designed collars was analysed through the damage images of the riprap scour protection below, which was able to indirectly reflect scour protection ability of the proposed scour protection system. The study found that the scour protection effect of the conical collars was better than the inverted conical collars except the case of 45° collar angle. The 7.5° conical collar performed better under moderate wave conditions and the 15° conical collar demonstrated a better scour protection ability in the critical scour protection region under both moderate and extreme wave conditions. Besides, the utilization potentiality of the Acoustic Doppler Velocimetry (ADV) was preliminarily explored in this study. The wave attenuation ability of the monopile with collars was found to be very low. Before conducting this study, the scour protection system and the ecological considerations for scour protection measures was largely unstudied. The results of this experimental work will serve as a valuable reference for designing future ecological and efficient scour protection systems, ultimately contributing to the development of offshore regions.