Reliability Case Notes No. 8. Risk and reliability analysis of Autosub 6000 autonomous underwater vehicle

Brito, M.P.. 2015 Reliability Case Notes No. 8. Risk and reliability analysis of Autosub 6000 autonomous underwater vehicle. Southampton, National Oceanography Centre, 79pp. (National Oceanography Centre Research and Consultancy Report, 50)

Before downloading, please read NORA policies.

Text
NOC_R&C_50.pdf
Restricted to NERC registered users only
Download (2MB) | Request a copy

Abstract/Summary

Autosub 6000 is an autonomous underwater vehicle developed and operated by the National Oceanography Centre, Southampton. The vehicle development was funded by the United Kingdom’s Natural Environment Research Council(NERC). The credibility of Autosub 6000 to perform science missions is directly related to its reliability and availability. The aim of this report is to provide reliability and availability analysis of Autosub 6000 based on the vehicle operational history. This report considers the operational history from September 2007 to September 2012, during which seven campaigns were conducted and a total of 68 missions were carried out by Autosub 6000. Eight of the missions ended prematurely. The two most frequently observed failure modes in its fault history were GPS and battery failures. Both failures raised design errors that have since then been addressed. The report concludes that Autosub 6000 failure rate is correlated with two factors: 1. the maximum operating depth and 2. whether or not new technology is being used. Operations at depth greater than 3000 metres are 92% more likely to have an abort than operations at water depth lower than 3000 metres. Adding new technology to the vehicle increases the failure rate by approximately 200%. Of the 110 faults that emerged during the seven campaigns fifty five were mitigated through design corrections, this resulted in a 10% increase in reliability for a 60 km mission, the reliability of which is 0.7. Autosub 6000 software reliability was assessed using two models, the time‐related Poisson and the Littlewood. Using both models we can conclude that Autosub 6000 has a reliability of 95% for a 20 hr mission. Lithium‐Ion battery reliability was quantified using the FIDES approach. An Autosub 6000 battery pack contains approximately 400 battery cells. The probability of at least one out of four battery packs failing, in 1 year, is 0.20. We concluded that one extra battery pack should be onboard of the vehicle to have at least 96% confidence that the mission will be completed successfully.