## 2 Materials and Methods
### Risk-Based Maintenance  
Risk-based maintenance management is an approach that manages and prioritizes maintenance activities based on risks associated with equipment failures. It involves assessing the probability and consequences of equipment failures and allocating maintenance  
Figure 1: The relations between emerging practices, existing practices, and risk-based maintenance management. resources accordingly. The goal is to focus maintenance efforts on critical assets or failure modes that have the highest possible potential consequence on safety, production, and operating expenses. The NORSOK Z-008 standard was first published in 1996 by the Norwegian petroleum industry to provide guidelines for criticality classification. Since 2011 and in its third revision, it provides guidelines for risk-based maintenance. The NORSOK Z-008 standard [52] is applicable to the entire asset life cycle: (1) the design phase to establish initial maintenance manning requirements, identify hidden failures in critical equipment, and select critical spare parts; (2) preparation for the operation phase to develop maintenance programs in maintenance management systems and select ordinary spare parts; and (3) the use phase to prioritize maintenance work orders and optimize existing maintenance programs. NORSOK Z-008 provided a management framework or loop, as shown in Figure 2, that starts with the requirements and the building of the maintenance program until execution, and then the reporting and analysis to feedback on the continuous improvement process. It should be mentioned that the maintenance management loop was adapted from the Maintenance Baseline Study [53] as a self-assessment tool and published in 1998 by the Norwegian Petroleum Directorate. It is important to highlight that the mindset behind the risk-based maintenance program which adapts seven principles is as follows:  
1. Risk assessment (likelihood and consequence) shall be used as a guiding principle for maintenance decisions such as to prioritize, to select, and to optimize. Assets with higher criticality are prioritized for more frequent and rigorous maintenance activities. 2. RBM is a generic maintenance program that supports both corrective and preventive maintenance concepts and integrates equipment-specific maintenance concepts such as safety integrity for technical barrier elements, risk-based inspection for static process equipment, and reliability-centered and condition-based maintenance for production equipment. 3. RBM is a customized maintenance program based on the overall company policy, e.g., HSE, production, cost. 4. RBM is a circular maintenance program in which data flow (starts with estimates, reporting measures after executions, and generating insights after analysis) completes the entire management loop. 5. RBM is an active learning program that converts historical data and experiences into insights and improvements. 6. RBM is a proactive maintenance program that encourages several engineering analyses at the design phase to determine failure causes, mechanisms, and modes, and engineer dependable assets (design out, redundant, maintainable asset, support systems). 7. RBM is a functional and requirement-based maintenance program that enables modifications and allows upgrades. Figure 2: Management framework of risk-based maintenance according to NORSOK Z-008, adapted from [53]. Over the decades, several methods have been developed and integrated into RBM programs to provide complete information to maintenance decision makers [54].