# A Machine Learning Framework for Condition-Based Maintenance of Marine Diesel Engines: A Case Study
### The Shift to Proactiveness in Maintenance  
This revolution has influenced many fields and applications, among which maintenance has radically changed. Maintenance is defined as \"any activity carried out on an asset in order to ensure that the asset continues to perform its intended functions, or to repair the equipment\" [8]. Many types of maintenance strategies have been proposed in the literature. Some are comparable, some have similarities, some are clearly distinct. Moreover, different authors may define the same strategy differently. In general, maintenance is classified as reactive (also called passive or corrective) or (pro)active (which is sometimes indicated as preventive) [9; 10; 11; 12; 13]. Reactive maintenance (RM) is applied once a complete failure occurs, after breakdown, and requires fixing equipment problems and repairing faults as they arise; this is also described in software and automation systems [14]. In more detail, industry can define it as breakdown maintenance if an asset is working anymore and needs to be repaired; as run-to-failure maintenance if an asset is used until it breaks down or requires replacement or repair; as corrective maintenance if it provides a temporary solution; or as emergency maintenance if a sudden failure occurs in a critical asset which needs to be repaired as soon as possible, which makes this type of maintenance expensive. The approach of RM is simple, with no costs for planning and scheduling regular maintenance tasks. However, especially in the maritime industry, long-term costs are high because of the emergency repairs and related expensive rush charges for parts and because of the downtime, which is unpredictable. Moreover, RM decreases the equipment lifespan, can introduce safety risks, and determines a poorer asset utilization with reduced performance. So, RM is considered for non-essential equipment, when the replacement cost is low, and when the equipment is close to its end of life. Proactive maintenance prevents failures. The terms proactive and preventive are often used as synonyms, but there are differences. Proactive maintenance aims to anticipate failures or faults and stop them before they occur. It consists in periodic rigorous inspections and condition monitoring, in collecting data from sensors, and in replacements at scheduled intervals, regardless of whether a failure has occurred. Then, it considers data to determine when maintenance should occur. Basically, proactive actions try to remove the root causes of failure (wrong or degraded materials, unexpected operations, human errors, etc.) to extend the equipment life and let it function at its best without unexpected breakdown. Proactive maintenance can save costs by avoiding unnecessary maintenance (e.g., repairs or replacements when nothing is wrong), so it can be convenient with respect to preventive maintenance, which is defined below. However, it requires monitoring technology and data analysis. Finally, if the equipment is proactively kept safe, then the risk of accidents is reduced as well as the consequent cost of repairs or replacements. Preventive maintenance (PvM) schedules audits, inspections, and replacements to guarantee the continuous operation of the asset, with the purpose to prevent failures before their occurrence [15; 16]. The aim is to reduce the chances of equipment failure and unplanned downtime and to increase safety. Regular and routine inspections are scheduled with periodicity on a time basis or on the basis of the equipment's iterative utilization,according to a pre-established plan of tasks, while the equipment is still working to prevent unexpected breakdowns. This maintenance approach has advantages (reduced downtime, extended equipment life, saving costs due to emergency situations, improved safety, increased efficiency, and energy saving) but also presents various drawbacks and problems (initial investment costs, unnecessary maintenance with waste of time and resources, dedicated personnel, pause of equipment operation, complex scheduling of maintenance tasks).