Reliability and availability of pod propulsion systems

The application of pod propulsion in a number of vessel types has been increasing steadily over the last two decades. This increase is attributed to the great advantages being offered by pod propulsion systems such as high manoeuvring capability, low noise and vibration, low fuel consumption, etc. Despite these advantages, operators are cautious due to the encountered teething problems, especially in the very competitive passenger ship market, where reliability and the maintainability of the service are crucial. This paper presents a reliability assessment methodology and its application to a combined four-pod propulsion system on a vessel equipped with two fixed- and two rotating-pod units. The assessment methodology made use of Failure Mode and Effect Analysis, Fault Tree Analysis (FTA) and Markov Analysis complementarily. In the FTA, minimal cut set, reliability importance measure and availability analyses were also considered. From the quantitative reliability assessment, the calculated reliabilities of each fixed- and rotating-pod unit, their components' reliabilities as well as the reliability of the combined four-pod propulsion system showed good agreement with the acceptable reliability criteria suggested by the pod manufacturers/operators based on the service experience.