# Decision Support Models for Operations and Maintenance for Offshore Wind Farms: A Review
## 2 Operation and Maintenance: Influential Factors
### Vessel, Personnel and Spare Part Logistics  
To repair the defects and failures occurring in the offshore wind farm, there are three main requirements. The availability of maintenance workers that are trained to conduct the respective maintenance tasks is crucial. Many repairs need not only a repair team and tools, but also specific spare parts. The availability of those poses another restriction to performing the maintenance and should therefore be included in any maintenance model. To transport both workers and spare parts to the offshore location, transport vessels are required. Therefore, the availability of those presents another part of maintenance analysis and should be included in any model. Additional to the transport vessels, some repairs require jack-up barges or crane vessels. This includes gearbox replacements and blade repair. Most existing models include at least one of the factors. [PERSON] et al. [12] consider in their decision support model two different vessel types, with specific weather restrictions. Additionally, the crew size varies for different turbine components and each vessel has a fixed crew capacity. [PERSON] et al. [13] present a model for an offshore wind farm. With this model, they optimize the number of maintenance technicians, length of shifts, location of accommodation and the choice of transfer vessels and helicopters. In the presented case study, an offshore accommodation with technicians available at any time was favored. For the optimal transport of the workers to the turbines, a crew transfer vessel with an improved access system was chosen. To find the optimal vessel fleet size and composition is the goal of [PERSON] et al. [15]. They present a mixed-integer program and compare 15 different instances. [PERSON] and [PERSON] [16] consider different combinations of vessel types, with different abilities and access restrictions, as well as different ownership models (purchasing the vessel, chartering the vessel). Spare parts are always considered available, with a fixed lead-time and cost. In the logistics module of [PERSON] and [PERSON] [31] an equipment analysis, spare part analysis and repair class analysis is included. [PERSON] et al. [17] include a deterministic waiting time for spare parts depending on the failure type. The number of technicians and vessels needed is also determined by the failure \begin{table}
\begin{tabular}{p{113.8 pt} p{113.8 pt} p{113.8 pt} p{113.8 pt}} \hline \hline
**Publication** & **Failure Modeling** & **Degradation Modeling** & \begin{tabular}{c} **Number of** \\ **Components** \\ \end{tabular} \\ \hline \multirow{4}{*}{\
\begin{tabular}{} \end{tabular} } & & time-based degradation \\  & & (electrical) \\  & Poisson process & discrete deterioration (mechanical) \\  & & continuous deterioration (structural) \\ \hline \multirow{2}{*}{
\begin{tabular}{} \end{tabular} } & Poisson process, periodic failures & constant failure rate, exponential increase, linear increase \\ \hline [PERSON] et al. [38] & & linear fatigue accumulation \\ \hline \multirow{2}{*}{[PERSON] et al. [26]} & Poisson process, Weibull distributed & adjusted failure rates (based on maintenance, fault category, environmental conditions) \\ \cline{2-4}  & & & \\ \hline [PERSON] et al. [39] & 9 different distributions & 12 \\ \hline [PERSON] et al. [40] & & stochastic degradation process \\ \hline [PERSON] and [PERSON] [27] & time-based failure rate & Weibull distribution & 6 \\ \hline Stock-Williams and Swamy [41] & operational data & \\ \hline [PERSON] et al. [42] & & Weibull distributed lifetime \\ \hline \hline \end{tabular}
\end{table} 
Table 2: _Cont._type. Only one type of technicians is considered. Three different vessel types can be chosen in the model, with different charter contracts. In [PERSON] et al. [19] a deterministic optimization of the vessel fleet size and the decision whether to purchase or charter vessels is presented. They also consider different locations for the maintenance base. In their model, [PERSON] et al.