Development of a condition based maintenance model for a vessel’s main propulsion engine

Due to ever increasing demand of maritime transportation in the commercial shipping world vessel sizes are getting bigger and bigger. Generally, the type of the vessels are large bulk carriers, crude oil tankers, liquefied natural gas (LNG) carriers and mega container vessels, that are used to fulfil the demand of cargo transportation. Majority of these vessels are propelled by large slow speed main propulsion engines, directly coupled to the propeller, hence the on-board propulsion system plays a key role in maritime transportation. However, Australian Transport Safety Bureau (ATSB), Marine Accident Investigation Bureau (MAIB), U.K, Japan Transport Safety Board, National Transport Safety Board, U.S.A, identified that numerous accidents happened in the past due to the failure of main propulsion engine. The failure of main propulsion engine may lead to disastrous consequences, resulting in huge financial losses and crew fatality on ships. Therefore, it is required to ensure the safety and reliability of the main propulsion engine to ensure safe and reliable maritime transportation. This can be can be ensured by adopting an efficient and effective maintenance regime. Currently, the main propulsion engines on ships have a Planned Maintenance System (PMS), as required by the International Safety Management (ISM) code, under the directive of the International Maritime Organisation (IMO). In PMS system, the maintenance is carried out on ship’s machinery based on regular intervals, according to engine and component manufacturer’s advice and experience of ship’s Chief Engineer and/or Master. However, studies from literature review undertaken, shows that the PMS is not the best form of maintenance regime. Literature reveals the fact that sister transport industries like railways, aircraft industry, and other process industries like chemical and oil and gas, use operational components similar to those used on ships. Such industries have adopted a Condition Based Maintenance regime (CBM), where maintenance of the system components is carried out based on the condition of the equipment, which is detected by measuring various useful parameters during the operation of the engine. All sister transport industries have reaped huge benefits by employing CBM in terms of maintenance cost and at the same time ensuring high levels of reliability Hence, employing CBM on ships could result in avoidance of wasteful resources in terms of manpower, spare parts and money. Shipping industry is lagging far behind in terms of employing CBM. This has been the foundation and motivation of this study to develop a CBM model for a vessel’s main propulsion engine. The main propulsion engine is dependent on several sub systems to perform a safe and reliable voyage. In this study, CBM model for the main propulsion system is developed. This model first evaluates the reliability of each subsystem and followed by evaluation of reliability of the main propulsion engine. The novelty of the CBM model developed for the vessel’s Main Propulsion Engine is Reliability Centred Maintenance (RCM) model, which will assist to reduce the maintenance cost. This thesis consists of seven chapters. The first chapter is an Introduction, highlighting the general structure of the thesis. In Chapter 2, the authors have put efforts to evaluate the development of large slow speed engines over the past four decades and the changes in system design which besides improving technical efficiency and contribution to combat the environment pollution, also looked at the economic factors and how this could be correlated to reliability of the main propulsion engine. The results conclude, development of turbochargers will play a major role in complementing and improving the overall efficiency and reliability of the main propulsion engine. The details of development of a CBM model for vessels’ main propulsion system and related subsystems is presented in Chapter 3. This chapter also highlighted various tools used in the determination of reliability for the main engine subsystems. The results of this chapter identified that only following a PMS regime on-board vessel could lead to a machinery failure, resulting in stoppage of a vessel at critical juncture. Thus, changing from PMS to CBM is justified in merchant shipping. Fault Tree Analysis and Reliability block diagrams are utilised as important tools in this thesis. The Chapter 4 considered the basic steps involved in determining the reliability of the lubricating oil system, which is one of the subsystem of main engine. This chapter also includes methods of determining the reliability of main engine’s fuel oil system and its impact on the reliability of the main propulsion engine. The results of this chapter demonstrates that use of additional components in the lubricating oil system could provide improvement in the component reliability leading to improved reliability of the main propulsion engine. To determine the cost benefit for using the additional component in the lubricating oil system, the incremental reliability for the differential cost should be compared with the base reliability to cost ratio. Utilizing the least failure rate of the fuel oil system component, as an identical value of failure rate for all components in the fuel oil system, the overall reliability of the main engine fuel oil system could be improved considerably. In Chapter 5 the authors developed a hybrid model to determine reliability of the main engine using Markov modelling and Weibull distribution. This chapter also considered a holistic approach to reliability and safety of a main propulsion engine in a harsh working environment. In Chapter 6 the authors studied data gathered from experienced sea going marine engineers and analysed. The final chapter provides overall conclusions of this study along with some recommendation and direction for future research