# Decision Support Models for Operations and Maintenance for Offshore Wind Farms: A Review
## 3 Maintenance Strategy  
[PERSON] [43] presents three different categories of issues in offshore wind farm maintenance planning. These are technical, strategic, and operational issues. The technical issues are covered by the failures. Operational issues are comprised of availability of vessels, crew, and spare parts, the weather and other external and economic factors. For the strategical issues, choosing the optimal maintenance strategy is the challenge wind farm operators are facing in their work. Therefore, any maintenance scheduling model should be able to incorporate and evaluate different maintenance strategies. Most models compare some fixed input scenarios or optimize certain aspects of the strategy, such as fleet size and grouping of repairs. All these approaches have the drawback that the operator or researcher using the model must already have an idea of how the maintenance can be improved and be familiar with the factors that influence the scheduling of maintenance. A maintenance model that can also optimize the maintenance strategy without input from the operator or a researcher, does not yet exist. However, since a well-developed model could be used by persons who are not familiar with the problems of offshore maintenance scheduling, the development of such a model is highly encouraged. [PERSON] [69] wrote that an optimal O&M strategy should strive to maximize the total overall equipment effectiveness, which is the product of the planning factor and the overall equipment effectiveness. The overall equipment effectiveness is the factor between the valuable production time and available production time (equal to productivity). The total overall equipment effectiveness is the actually valuable production time with respect to the theoretical production time, so it includes e.g., downtime losses.  
An overview over the different models and which kinds of maintenance they include is presented in Table 3. More details about the different kinds of maintenance are presented in the following subsections.  
\begin{table}
\begin{tabular}{l c c c c} \hline \hline
**Publication** & \begin{tabular}{c} **Prevenive** \\ **Maintenance** \\ \end{tabular} & \begin{tabular}{c} **Condition** \\ **Monitoring** \\ \end{tabular} & \
\begin{tabular}{c} **Condition-Based** \\ **Maintenance** \\ \end{tabular} &
\begin{tabular}{c} **Corrective** \\ **Maintenance** \\ \end{tabular} \\ \hline [PERSON] and [PERSON] [46] & ✓ & & & & ✓ \\ [PERSON] and [PERSON] [67] & ✓\({}^{1}\) & ✓\({}^{1}\) & & & \\ [PERSON] and [PERSON] [10] & & & ✓ & ✓ \\ [PERSON] et al. [30] & ✓ & ✓\({}^{1}\) & ✓ & ✓ \\ [PERSON] and [PERSON] [70] & & & ✓ & ✓ \\ [PERSON] et al. [12] & & & & ✓ \\ [PERSON] and [PERSON] [16] & ✓ & & ✓ & ✓ \\ [PERSON] et al. [17] & ✓ & & & ✓ \\ [PERSON] and [PERSON] [32] & ✓ & & & ✓ \\ [PERSON] et al. [19] & ✓ & & ✓ & ✓ \\ [PERSON] et al. [71] & & ✓ 2 & & \\ [PERSON] and [PERSON] [21] & ✓ & & & ✓ \\ [PERSON] et al. [20] & ✓ & & & ✓ \\ [PERSON] et al. [22] & ✓ & & & ✓ \\ [PERSON] et al. [34] & & & ✓ & ✓ \\ [PERSON] and [PERSON] [35] & & & ✓\({}^{2}\) & \\ [PERSON] et al. [63] & & ✓ & & \\ [PERSON] and [PERSON] [36] & ✓ & & & ✓ \\ [PERSON] et al. [72] & & ✓ & & \\ [PERSON] et al. [73] & & ✓ & & \\ [PERSON] et al. [37] & ✓ & & & ✓ \\ [PERSON] et al. [74] & & ✓ & ✓ & \\ [PERSON] et al. [23] & ✓ & & & ✓ \\ [PERSON] et al. [25] & ✓ & & & ✓ \\ [PERSON] et al. [26] & & & & ✓ \\ [PERSON] et al. [75] & & & ✓ & \\ [PERSON] et al. [40] & & & ✓ & ✓ \\ [PERSON] et al. [76] & & ✓\({}^{2}\) & & \\ [PERSON] and [PERSON] [27] & & & & ✓ \\ [PERSON] et al. [42] & ✓ & & & ✓ \\ \hline \hline \end{tabular}  
* \({}^{1}\) Cost only. \({}^{2}\) Review.  
\end{table}
Table 3: This table summarizes the use of different types of maintenance in the literature.