## 1 Introduction  
The hydrocarbon supply chain (HCSC) comprises oil and gas networks. The oil network involves oil fields, oil and gas separation plants (GOSPs), primary storage facilities, oil processing plants, refinery plants, secondary storage facilities, and demand nodes. In comparison, the gas network consists of gas fields, storages facilities, gas plants, fractionation plants, secondary storage facilities, and demand nodes. The two networks overlap in some entities and share some products. Few researchers have studied the optimization of operational planning under the assumption of plant availability with full capacity, yet this assumption is practical. It is crucial to consider interruptions or plants' sudden shutdown when planning for operation. In the real world, plants might be totally or partially unavailable due to periodic maintenance, shutdown, etc. Thus, production capacities will be reduced or reach zero. To restore the plants to their full capacities and keep them in good condition, plants should be maintained regularly and routinely to prevent unplanned failures and increase production rates. Maintenance activities reduce the sudden breakdown and prolong the unit's life. Thus, operation planning models need to be more realistic and should consider the maintenance schedule. To prevent huge losses in production and eventually satisfy customer needs, maintenance jobs should be performed during times of low demands. Besides, connections between upstream and downstream plants need to be considered before deciding to perform maintenance activities, especially in complex networks such as HCSC. Thus, operation planning models must be more realistic and take into account maintenance tasks. Several studies have addressed the integration of operation and maintenance planning activities within the scope of a single machine in different areas and applications, such as the inventory control area. According to the author's knowledge, no work has been done to integrate operation planning and maintenance scheduling in the supply chain. The study aimed to develop a mixed integer programming (MIP) model to coordinate the operation and maintenance activities of the oil supply chain plants, taking into account the best use of available human resources and maintenance resources, while satisfying the maintenance and operational constraints. The expected outputs of the proposed model are the production volumes, flow quantity, exportation volume, inventory level, as well as maintenance schedules for influential HCSC plants while maximizing profit. Maximizing profit is necessary for any organization to meet financial obligations and support the country's budget. The utility of the model is demonstrated using a real case (Saudi Arabia's oil and gas supply chain). The model is expected to have a positive impact on the future management of this important component of the energy sector. A valuable and practical sensitivity analysis was conducted to study the effect of the key parameters on the obtained results. Some managerial insights will be derived and are expected to help and guide decision-makers as they build their decisions. The integration of operation and maintenance activities have not been studied adequately, specifically in the supply chain context. In the area of the hydrocarbon supply chain, no work has tackled production planning and maintenance scheduling simultaneously. Therefore, the maintenance scheduling models of petroleum plants and integrated models in the area of inventory control will be reviewed in this section. In the HSCS, operation and maintenance schedules have been treated independently. For example, [1] developed a multi-objective model for the operational planning of the Hydrocarbon supply chain under the assumption of the entities available during the planning horizon. Few studies have studied the maintenance scheduling of single entities of a petroleum network such as the refinery plant.