Power and Propulsion Systems on Board Unmanned Naval Vessels

Abstract

With the growing shortage of seafaring personnel, on­board systems need to be more reliable to beable to sail without. Multiple efforts are done to be able to sail with unmanned systems (systems thatdon’t need maintenance while sailing), but for large unmanned surface vehicles, the P&P system isone of the systems that is not reliable enough. The goal of this thesis research was to study the pos­sibilities of adjusting an existing P&P system to make it reliable enough for unmanned sailing. Theresearch resulted in creating a reliability model that gives insight in the direct impact maintenance hason reliability to study the possibilities of removing maintenance personnel from board. Therefore, themain research question of this thesis is:What adjustments to the power and propulsion system of a Navy vessel are necessary to be able tooperate for a given period of time, consisting of several missions, without any on­board maintenancepersonnel?To answer this research question, a study regarding the current system was done first with an functionaland physical decomposition. This gave insight in what functions the P&P system needs to fulfill to pro­vide the ship with power and propulsion, what systems the P&P system contains and what preventivemaintenance actions are currently done to prevent failure. With the data and knowledge acquired bystudying the system, performing a failure mode and effect analysis (FMEA) showed that data currentlyavailable is not able to show the impact of maintenance and thus it is unclear how reliable an unmannedsystem would be.To be able to do so, a fault tree analysis (FTA) model was created to calculate the reliability and showthe direct impact of maintenance for the P&P system on board a Holland class ocean going patrolvessel of the Royal Netherlands Navy. The model made use of an exponential distribution that was im­plemented with a time­dependent Weibull failure rate function while only using the mean time betweenfailure (MTBF) and the maintenance frequency for preventive maintenance actions.As a result, it has been found that either making the 20 weakest components of the P&P system re­dundant, or changing the maintenance strategy from condition based to predictive and making the 5weakest components redundant, the reliability of a P&P system that receives no maintenance during50­day missions can be as reliable as a manned P&P system. Furthermore, several sensory equip­ment is added to replace inspections currently done by maintenance personnel on board.However, even though the reliability of a 50­day unmanned system can be as reliable as a mannedsystem mathematically, the model does not take into account corrective maintenance. An expansionto this model including corrective maintenance should be researched to show the opportunities of anunmanned hybrid propulsion system. Other recommendations for further research include identifyingfailure mechanisms to adjust the failure rate progression and implementing repair times and cost tostudy the feasibility of adjusting the current P&P system and maintenance strategy for unmanned sail­ing.