The manuscript analyzed the operational costs between different configurations of 15 MW turbines by summarizing existing literatures, performing simulations, and conducting sensitive analyses on various scenarios.

Overall, the topic and analysis of the manuscript is of a great interest of the wind energy science/industry. The procedure and results of the research is well organized and presented. The conclusion of “a medium speed turbine may have lower operational costs than a direct drive turbine if the failure rates are below a certain threshold” will be a good guidance for wind farm engineers to plan the future and brings benefits and efficiency to the wind energy industry.

However, I have several comments that will improve the manuscript if addressed before publication:

• In line 50: it would be better to provide a 1-2 sentences brief introduction of each configuration.
• I found that the Figure 1 is not very informative since the two figures are very similar and the figure description is not providing any extra information other than describing the figures. It would help readers to have a better understanding if the figure can focus on the parts (i.e., the gearbox and the direct drive) with differences.
• The 5.2 Key Takeaways session can be reworked to bullet points, which helps readers to get a clearer view of the numbers and comparisons.
• The current conclusion session is a bit lengthy and unstructured. It would help the reader to get a clearer view of the important conclusions of this study. For example, the conclusion session can be easily rewritten into different paragraphs such as motivation of the study, conclusion of the simulation work, performed sensitive analyses and corresponding observations under different scenarios, and the importance of the findings and outlook, etc.
• Small formatting comments:
• The unit “MW” should not be italic in item 2 above line 115.
• BVG is not mentioned with full name in the manuscript when first occurring above line 220.

I believe addressing these comments will improve the readability of the manuscript and enhance its contribution to the field. Overall, the research work has done a great contribution to the wind turbine industry.

The manuscript entitled "Operations and Maintenance Cost Comparison Between 15MW Direct-Drive and Medium-Speed Offshore Wind Turbines" deals with a very interesting topic, which is very important for the near future development of wind turbine technologies.

In a nutshell, the authors analyze the O&M costs of 15 MW direct-drive and medium-speed offshore wind turbines. An O&M model named StrathOW (which I guess has been developed at the University of Strathclyde) is employed to simulate scenarios and the input of the model is given by reliability data for the various sub-components which are taken from the literature. A sensitivity analysis is conducted and extreme cases are contemplated as well. 

The quality of the presentation is very good and my comments to the paper are minor.

1) I would appreciate some more details about the application of the O&M model. It would be interesting for me to understand more in deep how much representative the results are.

2) Summarizing drastically, the result of the paper is that the former or the latter technology is slight more advantageous depending on a series of factors, which the authors discuss. I think that the wind turbine practitioners community might appreciate some more concrete guidelines, or at least criteria. Thus, I recommend to make an effort on this point.