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

Abstract

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 factors 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, replaced by the policy of preventive maintenance. The Planned Maintenance System was introduced by ship management companies in the early 1990s. Planned Maintenance offered benefits over the repair-only policy, but has its own demerits. Often machinery equipment is opened up for routine maintenance after a specified time interval, irrespective of the need. This can lead to potential failures, which is explained by the fact that preventive maintenance results in meddling of a well set piece of machinery equipment, leading to its subsequent failure. This is where condition-based maintenance (CBM) steps into prominence. CBM monitors the health of the machinery equipment, analyses the condition and helps in decision making. The main propulsion system forms the heart of a vessel and its reliability needs to be ensured, 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 a ship’s engineer may decide to stop the running machinery equipment, open and overhaul the same, or postpone the overhaul for a later safe date.