The offshore wind is the sector of marine renewable energy with the highest commercial development at present. The margin to optimise offshore wind foundations is considerable, thus attracting both the scientific and the industrial community. Due to the complexity of the marine environment, the foundation of an offshore wind turbine represents a considerable portion of the overall investment. An important part of the foundation’s costs relates to the scour protections, which prevent scour effects that can lead the structure to reach the ultimate and service limit states. Presently, the advances in scour protections design and its optimisation for marine environments face many challenges, and the latest findings are often bounded by stakeholder’s strict confidential policies. Therefore, this paper provides a broad overview of the latest improvements acquired on this topic, which would otherwise be difficult to obtain by the scientific and general professional community. In addition, this paper summarises the key challenges and recent advances related to offshore wind turbine scour protections. Knowledge gaps, recent findings and prospective research goals are critically analysed, including the study of potential synergies with other marine renewable energy technologies, as wave and tidal energy. This research shows that scour protections are a field of study quite challenging and still with numerous questions to be answered. Thus, optimisation of scour protections in the marine environment represents a meaningful opportunity to further increase the competitiveness of marine renewable energies.

The R&D project Oracle – offshore risk analysis for climate change adaptation and lifetime extension – is focused on optimising the design of scour protection schemes for the extended lifetime of offshore foundations. This project proposes an optimisation of scour protections by combining reliability-based techniques with the novel concepts of dynamic and wide-graded armour layers. The ultimate goal is to develop a decision support system (DSS) for scour protection design and risk and reliability analysis to be used by professionals and researchers dealing with fixed foundations applied to marine renewable energy projects (e.g. wind, wave and tidal). However, a meaningful implementation of such a DSS implies prior analysis of several knowledge gaps related to scour protection in the offshore environment, which are yet to be fully addressed in the literature. This paper provides a brief review of the scientific and technical challenges in the research field of reliability of offshore scour protection. On the basis of identified challenges, the discussion of future research lines is performed with a particular focus on the aim of the Oracle project. The recent findings related to risk and reliability analysis of offshore scour protection are addressed and framed within the existing knowledge gaps in the literature.

The present paper provides a reliability assessment of scour protections applicable to both the static and dynamic stability design. As a case study, Horns Rev 3 hindcast data is used to simulate different failure criteria for an exemplary scour protection suitable for an offshore monopile foundation. The results show that the probability of failure is influenced by several factors, namely the wave friction factor, the definition of the acceptable damage number or the formulations used to calculate the bed shear-stress. The reliability assessment also indicates that annual probabilities of failure, associated to each criterion, might be comparable with the values presented in reliability standards for marine structures. Based on the results, this paper highlights future recommendations to improve the reliability-based design and analysis of scour protections for offshore foundations.

Offshore foundations, namely for offshore wind, wave and tidal applications, often require the use of scour protections. Rip-rap scour protections are an important element of the foundation to ensure that the natural frequency stays within the design limits. Scour protection design still presents a remarkable empirical nature, which typically leads to uncertainty on their behaviour under extreme met-ocean conditions. Therefore, reliability assessment of scour protections has been seen as a possibility to account for design uncertainty and to optimise the scour protections. However, the definition of a suitable limit state function is still a matter of research focus, namely, regarding the proper definition of the acceptable damage level for dynamic scour protections. This research provides a brief review on the recent studies related to both the limit state function and the calculation of damage numbers through bathymetric data. A discussion is raised on how the methodologies for calculating the damage number may influence the limit state function and a theoretical example is given to assess the effects on the probability of failure. Results have shown that the acceptable damage number requires a clearer definition, which should be based on the number of layers of rock material and the area of filter exposure. In addition, this research highlights the need for alternative ways to assess damage.

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