**5. Conclusions**

This paper presents a comprehensive look at the future trends in European offshore wind energy. First, it was shown that offshore wind in Europe is currently moving more towards direct-drive turbines, with each individual installed turbine having a higher rated power. Overall capacity of new sites is also increasing on average. To examine the implications of this change, the stop rates for direct-drive and gear-driven turbines were compared between 39 farms across Europe and South America. It was found that there was some differences, and in particular the top components by stop rate and downtime were different from previous papers that had examined failure rates of wind turbines. In the future, it would be of value to develop a way of mapping stop rates to failure rates, thereby allowing a more direct comparison to be made.

Finally, this paper presented a framework for analysing how well published fault detection models transfer between geared and direct-drive turbines. For this, two decision trees were created to enable a quantitative score to be placed on both the required input channels and failure mode respectively. Overall transferability could then be assessed by considering both metrics together. It was found that components, or assemblies, that were ubiquitous across turbine configurations, such as the pitch system, were more transferable. Whereas, as expected, the gearbox was the least transferable component. The generator, shafts and bearings were somewhat transferable; however, in general, these would require some level of feature engineering to improve the potential performance. While this paper has examined over 25 papers in testing of the proposed framework, not all turbine components or fault conditions were presented. Further study may be needed to apply this framework to these components. Additionally, this paper has focused mostly on SCADA data from wind turbines; however, this technique could be applied to papers that have utilised high-frequency CMS data.

Based on market trends, it is important for researchers to focus their efforts on developing fault detection techniques for the most critical components related to large direct-drive technology. Since no such reliability study exists in the literature on large direct-drive wind turbines, for the time being, previously observed reliability rates must be used and adapted where appropriate. Based on these studies, the most common critical components stated with extended downtime are related to the controller, pitch system, generator and gearbox. Based on the transferability scores presented in this paper, the components that would

require further work are those related to the direct-drive generator. Additional work is required in this area in order to utilise, adapt and improve existing fault detection methods.

**Author Contributions:** Conceptualisation, A.T. and C.M.; methodology, A.T. and C.M.; validation, A.T., C.M. and J.C.; formal analysis, A.T. and C.M.; data curation, A.T. and C.M.; writing—original draft preparation, A.T. and C.M.; writing—review and editing, J.C. and A.M.; visualisation, A.T. and C.M.; funding acquisition, J.C. and A.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by EPSRC gran<sup>t</sup> number EP/T031549/1 and Supergen ORE hub 2018 gran<sup>t</sup> number EP/S000747/1.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Conflicts of Interest:** The authors declare no conflict of interest.
