Fault detection in vibration systems: Identifying damaged moorings.

Offshore platforms are anchored to the ocean floor using moorings to prevent excessive drifting, and these moorings need to be monitored for damage, an expensive process. Is it possible to detect a change in moorings stiffness by measuring the motion of the platform under random wave forcing? The platform's response is strongly dependent on the wave spectrum and direction of forcing, this forcing is random, and the measurements are indirect, so it seems unlikely. To examine the feasibility we examine a much simpler but analogous spring-plate-table system with table rotation mimicking wave action. We find that by using a modal analysis of the underlying system one can unscramble the plate's response and thus determine spring stiffness changes under random forcing, however, as one would expect, the forcing has to have components in an ‘active’ frequency and direction range. In principle the same type of analysis can be used for real moorings. Whilst the spring-plate-table system model was developed with moorings in mind, such fault detection issues often arise under circumstances in which direct fault detection is impossible but the basic underlying system is ‘known’. The simple spring-plate system examined here is perhaps the simplest non-trivial example of this situation. [ABSTRACT FROM AUTHOR]