Reliability Case Notes No. 2. PAP 2007: DOMS mooring loss report

A new multidisciplinary mooring DOMS (Deep Ocean Monitoring Station) was deployed at the PAP time series observatory in June 2007 with a design specification duration of 18 months. Various features failed and the mooring ultimately parted after 47 days. The top part of the mooring was recovered by R/V Pelagia in September 2007. This report summarises the problems experienced and makes recommendations for future mooring design and deployment. • Mooring failure: Detailed analyses have been carried out using data on the mooring behaviour and on the recovered structure. The root cause of the failure was that the design allowed 550m of line to float on the surface under certain slack conditions. This line then became entangled with the surface buoy on day 192 (11th July) causing stresses and strains for which the mooring was not designed. Probably associated with this change in configuration was the parting of a shackle at 1000m depth following which the top section of the mooring drifted away from its anchor on day 221 (9th August). This failure has prompted a redesign of the mooring so that entanglement can not occur again but in addition it has stimulated a change in the way complex moorings are designed. Such organisational changes have now been made and will be an essential feature of all future design processes. • Data transmission: Although the bi-directional satellite telemetry link to the buoy functioned correctly throughout, the inductive telemetry link to the underwater sensors experienced problems from the outset and ultimately failed. The reasons for this have been identified and solutions recommended. • Sensor failures: Diverse problems were encountered most of which have been identified and solutions found. • Corrosion: This was more serious than expected and mitigation strategies are being developed. • Navigation light failure: The problem has been identified and a solution found.