The first step to evaluate the maintenance strategies is to define a lifetime for the integrated systems and the times in which replace of system must be carried out. The five individual systems have different expected lifetimes, as it can be seen in the picture below:
The lifetime of the integrated systems was set to 100 years. It is worth mentioning that after 60 years the replacement of nuclear power plant, railway power supply and wind turbines must be done.
When it comes to maintenance interventions in civil systems that are inter-related, it is crucial to assess two main factors:
- To what extent the intervention on one system affects the others
- How a breakdown of one system can impact current human activities, so as to define strategies to reduce this impact
As can be seen in the diagram below the operation of the railway system requires energy supply and the existence of two bridges over the channel.
The energy supply can come from two different sources: the offshore wind turbine system and the nuclear power station form the energy system in and around Brunsbüttel. Together they provide energy to the infrastructure like railway and bridges. Therefore, the Maintenance Planing (MP) will be integrated for both systems to perform a life cycle analysis. Through this analysis, the availability of systems and downtime of their interventions will be optimized using generic algorithm.
In the annual timeline displayed below, the coordination between the maintenance interventions of the two energy systems is emphasized through the separate consecutive intervals of the carried out maintenance in both systems. During maintenance activities, the energy output for the Lift Bridge and the Railway System can vary between 1185 MW (in case of a complete shutdown of Wind Turbine) and 288 MW (in case of a complete shutdown of the Nuclear Power Plant). Due to the governing bad weather conditions in the city of Brunsbüttel during winter, maintenance activities on wind turbine need to have summer time intervals (as seen in the displayed timeline). This complement the fact that interventions in nuclear power plant are mainly not controlled by weather conditions and can be operated within the facility.
The diagram below shows which level of integration at the same time will reflect on the duration of the maintenance. The best configurations of maintenance interventions in these systems balancing the maximum distance between interventions and the minimum duration of interventions will be presented later with the pareto frontier.
In the following pareto frontier diagram, the length of the intervention in days is plotted over the distribution of the interventions. It was considered a 6-year parameter to control the design space: The blue graph below indicates the 3 best configurations of maintenance interventions in these systems balancing the maximum distance between interventions and the minimum duration of interventions. This indicates that an overlap between the two energy systems is possible, in which up to 4 parallel maintenance activities are occurring. However, the optimum of such approach need to be properly examined, as it is shown in the multi objective optimization chapter.
The graph below shows the optimal configuration considering all the interventions carried out at the same time, which would take approximately 270 days. As we do not want to overlap the maintenance of both systems, the maintenance activities in one system will be conducted after the other system is complete.
On the other hand, if the railway power supply system is being repaired, the railway lines will not be available for operation, so is the perfect time to make interventions also in the bridges. The second maintenance strategy is, therefore, to match the interventions of those three systems as much as possible.
In the following diagram, the length of the intervention in days is plotted over the distribution of the interventions. It was considered a 4-year parameter to control the design space:
If the priority is to minimize the duration of the intervention, the best option is shown below. In the sub-integrated system (infrastructure system), two simultaneous interventions can be carried out with a duration of approximately 95 days.
