# Improving the Strategy of Maintaining Offshore Wind Turbines through Petri Net Modelling
## 5 Simulation Results and Discussions
### Impact of Maintenance Strategy
#### 5.2.1 The Influence of the Time Interval of \"Advanced Service\"  
When the time interval of \"advanced service\" is three months, four months, six months, one year, two years, three years, four years, and five years, the average number of faults detected by the CMS and found in the full inspections and the average number of failures \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Subsystems** & \(\mathbf{n_{1}}\) & \(\mathbf{n_{2}}\) & \(\mathbf{n_{3}}\) \\ \hline Rotor & 1.97 & 1.20 & 0.10 \\ Drivetrain & 1.56 & 0.64 & 0.02 \\ Power system & 4.40 & 0.95 & 0.14 \\ Bearing system & 4.12 & 1.49 & 0.26 \\ Braking system & 2.42 & 0.56 & 0.04 \\ Structures & 1.18 & 1.63 & 0.12 \\ \hline
**Sum** & **15.65** & **6.48** & **0.68** \\ \hline \hline \end{tabular} Notes: \(n_{1}\)—number of faults detected by the CMS; \(n_{2}\)—number of faults found during “advanced service” and full inspection following recovering from subsystem failures; \(n_{3}\)—number of subsystem failures that will cause the shutdown of the turbine.  
\end{table} 
Table 3: The average number of subsystem failures and subsystem faults. causing the shutdown of the wind turbine are calculated. The corresponding calculation results are shown in Figure 9.  
From Figure 9, it is seen that with the increase of the time between \"advanced service\", the average number of faults detected by the CMS and found in the full inspections decreases gradually. This implies that the wind turbine will have more chances to run with a fault or faults. However, this does not necessarily mean an increased availability of the turbine, because \"running with a fault or faults\" will increase the probability of subsystem failure and thereby increased number of unexpected shutdowns of the turbine, as indicated by the increasing tendency of the average number of failures in Figure 9. The unexpected shutdown due to subsystem failure is often associated with a long downtime, especially in offshore wind farms. To understand the trade-off between the availability and maintenance costs, the downtime and the maintenance cost caused by the faults and failures in Figure 9 are calculated using the PN model developed. To demonstrate the benefit of taking Measure (2) proposed in Section 1, the downtime and maintenance costs before using the new measure are also calculated for comparison. The results of downtime obtained before and after taking Measure (2) are shown in Figure 10.  
From Figure 10, it is found that  
Figure 10: Downtime against the time interval of “advanced service”. Figure 9: Number of turbine faults and the number of failures causing shutdowns. * With the increase of the time between the \"advanced service\" maintenance, the downtime of the turbine caused by the subsystem faults and failures decreases first and then increases after the time interval is greater than one year. This means that although \"advanced service\" is beneficial in decreasing the chance of subsystem failure and the consequent shutdown of the turbine, too frequent \"advanced service\" is not good either because it may cause a waste of time on unnecessary inspections. On the other hand, too infrequent \"advanced service\" may result in more unexpected shutdowns of the turbine due to the increased number of subsystem failures (see Figure 9);
* The minimum downtime is observed when the time between \"advanced service\" is one year regardless of whether Measure (2) is taken. So, from the perspective of downtime, 1 year is the optimal time interval for carrying out \"advanced service\";
* The longer the time between \"advanced service\" maintenance, the more benefits of using Measure (2) are observed. This proves the positive contribution of Measure (2) to the availability of the wind turbine. However, the contribution of Measure (2) to the economy of the wind farm is not only dependent on its contribution to downtime reduction but is also dependent on its contribution to maintenance costs. Therefore, the maintenance costs before and after taking this measure are also calculated.