## 3 Results
#### Sensitivity Analysis  
A valuable sensitivity analysis was conducted to show how the model was sensitive to the changes in the key parameters. In this study, the impact of changes in selling prices, demand, and store capacity on model output will be evaluated. Figure 8 shows the variation in profit with changes in selling price. It is noted that as the selling price increases, the total profit linearly increases.  
Figure 9 shows the fluctuations in profit with variability in demand. As demand increases, profit decreases because plant and storage capacities are kept constant; therefore, lost sales increase, which leads to customer dissatisfaction and ultimately lower profitability.  
Figure 8: Profatibility changes with variation in demand.  
Figure 7: Fractionation plants inventory level and storage capacity.  
The impact of storage capacity variability on profitability is demonstrated in Figure 10. It is revealed that as storage capacity increases, profit increases due to a decline in lost sales and the satisfaction of most customer needs. Reducing store capacities by more than 55% will generate non-profitable plans.  
The impact of upstream storage capacities on the maintenance schedule and arrangement is investigated in Figure 11.  
Figure 10: Profitability changes with variation in store capacity.  
Figure 9: Profitability changes with the variation in demand.  
The subsequent investigations can be derived from the above Gantt chart after reducing the storage capacity of upstream petroleum plants by 50%:  
* The maintenance activities were distributed uniformly and perfectly among HCSC plants and over the study period.
* The majority of petroleum plants are taken for maintenance during the quarters of low demands.
* Due to the reducing storage capacities of upstream plants, most of the connected plants have been taken for maintenance at the same time because the storage capacity of the upstream units is not sufficient to feed downstream units. For instance, the Jiddah refinery plant and Khurasaniya oil processing plant have been maintained in the same period. Similarly, both Riyadh and PetroRabigh refinery plants are maintained with Khurais oil processing plant.
* Likewise, RasTunura and Yanbu refinery plants were maintained exchangeably; Rastunura is maintained during the first two weeks of the Abqaiq oil processing plant's maintenance time, and then Yanbu is maintained during the last two weeks of the Abqaiq oil processing plant's maintenance period because during the first two weeks of maintaining the Abqaiq plant, inventory capacity is sufficient to feed only RasTunura refinery plant and vice versa for Yanbu refinery plant.
* For natural gas petroleum plants, Jaymah fractionation plant is recommended to be maintained with Uthmaniya and Berri gas plants. Similarly, the Yanbu fractionation plant and Yanbu gas plants are maintained at the same time.  
To determine the significance of integration between operations and scheduling activities during modelling, the model was resolved without integration (only maintenance model). The obtained maintenance schedule is depicted in Figure 12.  
Figure 11: Maintenance schedule for all HCSC plants (reducing the storage capacity of upstream petroleum plants by 50%); Color code: Orange: oil processing plants; Purple: Refinery plants; Red: Gas plants; Light Blue: Fractionator plants.  
It appeared that the HCSC units were taken for maintenance on a random basis during the planning period without taking into account the seasons of high or low demand, which would inevitably result in a huge loss of production and ultimately does not satisfy most customer needs.