Using reliability block diagrams and fault tree circuits to develop a condition based maintenance model for a vessel's main propulsion system and related subsystems

Merchant shipping has undergone a great transformation over the past three decades. The shipping market is highly competitive, which coupled with high crewing and fuel costs, leads to high operational costs. One of the paramount factor involved in vessel operation is the Maintenance cost and there is a dire need to keep this cost to a minimum. Fortunately the earlier policy of repair only maintenance in commercial shipping has been done away with, and was replaced by the policy of preventive maintenance. Planned Maintenance System was introduced by ship management companies in the early 90’s. Planned Maintenance offered benefits over the repair only policy, but has its own demerits. Many a time machinery equipment is opened up for routine maintenance after a specified time interval, irrespective of the need. This could lead to potential failures, which is explained by the fact that preventive maintenance resulted in meddling of a well set piece of machinery equipment, leading to its subsequent failure. This is where Condition based maintenance or CBM steps into prominence. CBM monitors the health of the machinery equipment, analyses the condition and helps you in decision making. The Main Propulsion system forms the heart of a vessel and we need to ensure its reliability, together with the reliability of its associated sub-systems. The entire system can be represented by reliability block diagrams, to show the interdependence of various components comprising the system. This helps in the decision making process of CBM whereby ship’s engineer may decide to stop the running machinery equipment, open and overhaul the same, else postpone the overhaul for a later safe date.