The impact of redundancy on reliability in machinery systems on unmanned ships

Unmanned and autonomous cargo ships may transform the maritime industry, but there are issues regarding reliability of machinery which must first be solved. This paper examines the effect of voyage length on the reliability of machinery with redundancy on unmanned ships. The limiting effects of dependent failures on the improvement of reliability through the use of redundancy is also explored. A strong relationship between voyage length and probability of independent failures in systems with redundancy is shown. Increased redundancy can easily counteract this negative effect of long unmanned voyages on reliability. Dependent failures, however, are not affected by increased redundancy. The contribution of dependent failures on the total probability of failure is found to easily exceed the contribution from independent failures if even a slight proportion of the failures is dependent. This has serious implications for unmanned ships where the possibility of corrective maintenance is very limited and the consequences of mechanical failures on, e.g. the propulsion of the ships can therefore be expected to be more severe than on conventionally manned ships. Redundancy in itself may not be enough to provide the reliability of machinery systems required for unmanned operation and other solutions must therefore be found.