# Improving the Strategy of Maintaining Offshore Wind Turbines through Petri Net Modelling
## 1 Introduction
To meet this need, a new simulation model of OWT maintenance is developed using Petri nets (PNs) in this paper. The reason for choosing to use PNs is that, in contrast to conventional Discrete Event Simulation (DES) models, the PN model provides an intuitive graphical representation of a system of interest and it has been proven to be more flexible and computationally efficient, especially when modelling complex systems [15; 16; 17]. Recently, the PN has also demonstrated success in simulating the degradation and maintenance of wind turbines [18; 19]. Hence, it is adopted in this study to simulate the following three new potential O&M measures, which have been identified from the literature review as important are studied:  
Measure (1)--Carry out periodic maintenance of different wind turbine components at different frequencies according to their specific reliability features. The study of this measure will determine how often periodic maintenance should be conducted. Different wind turbine components develop faults at different periods. However, this phenomenon has not been taken into account in both current maintenance practice and the available mathematical maintenance models. To use maintenance resources (e.g., the size of the maintenance team, different kinds of vessels, service time, etc.) more efficiently and minimise unnecessary investigations, two levels of services, i.e., \"basic service\" and \"advanced service\", are designed in the new PN model. The two levels of services are applied to different wind turbine components according to their reliability features. In the model, \"basic service\" refers to the checking and fixing of issues that may often occur but are difficult to monitor and may accelerate the degradation of key wind turbine components if are not fixed, such as the looseness of bolts, insufficient lubrication, etc. The key turbine components, such as main bearing, pitch bearings, gears, blades, generator, converter, tower structure, etc., will not be inspected during the period of \"basic service\". By contrast, \"advanced service\" requiring more expensive maintenance resources will involve more tasks than \"basic service\". In addition to the work that will be conducted during \"basic service\", the key wind turbine components will be inspected during \"advanced service\"to find those faults that are not detected by the CMS. Almost all OWTs in operation are equipped with CMS to detect incipient faults occurring in key components and prevent the sudden failure of them [20]. Once one or more faults are detected, the corresponding maintenance will be arranged and conducted. Herein, it should be noted that unless the OWT is hit by external extreme events, the faults detected in the OWT assemblies (or subsystems) are usually repairable. In the modelling process of this paper, it is assumed that after the defective subsystems are repaired, their functions will be fully restored. In addition, it should be noted that in existing maintenance models, it is assumed that all minor faults can be fixed in one time of visit [5; 14]. This is inconsistent with current industrial practice because the minor faults to be repaired are not known before the visit, so it is not known whether these minor faults can be repaired during the current visit. This discrepancy has been reduced in the new PN model developed in this paper, which allows an inspection and reporting of any minor faults first and then scheduled maintenance that takes dedicated actions to repair all the detected minor faults. Therefore, the PN model developed can more accurately describe the practical maintenance process of OWTs than previous studies. Measure (2)--Conduct a full inspection of the entire wind turbine system following a major repair (i.e., the recovery from failure). Studying this measure will enable the utilisation of maintenance resources more effectively.